Spawning of the Snapper, Chrysophrys auratus Forster in the Hauraki Gulf

Transactions and Proceedings of the Royal Society of New Zealand, Volume 84, 1956-57, Page 309

Spawning of the Snapper, Chrysophrys auratus Forster in the Hauraki Gulf By R. Morrison Cassie* Previously of the Fisheries Laboratory, Marine Department. [Received by Editor, September 19, 1955.] Abstract The distribution of the pelagic eggs of the snapper was studied during the summers of 1950–51 and 1951–52, using a high-speed surface plankton sampler. Samples of snapper were taken with the commercial trawl and their sexual condition examined. An apparatus for sub-sampling and counting planktonic eggs is described, and the sources of error both in sampling and sub-sampling are discussed. Eggs tended to be aggregated in patches and occurred at times in all portions of the Gulf which were adequately sampled, with the exception of the enclosed waters to the south of Rangitoto and Waiheke Islands. Egg density apparently reached its maximum for the year in November, 1951, the lateness of the second season being attributed to the lower water temperatures. Probably the main spawning does not take place until a surface water temperature of about 18 degrees Centigrade is reached. It is believed that the schooling of snapper at 10 fathoms to the north and east of Tiritiri Island may be attributed to the presence of a thermocline at about that depth, the fish rising into the warmer water to spawn. The average condition of snapper gonads from trawl samples progressed from “mature” to “ripe” to “spent” during each of the two spawning seasons, but there was no clear indication of delay in development in the colder 1951–52 season. Although the average sex ratio for all seasons tends toward unity, at certain seasons there may be a significantly greater number of one sex in the catches. The high degree of variability and aggregation of the planktonic eggs suggests that before any detailed interpretation could be made of quantitative data, a very much more intensive sampling survey would be required. Introduction It has for many years been common knowledge to Auckland fishermen that, in the summer months, snapper aggregate and spawn in the Hauraki Gulf, particularly on the grounds between Kawau and Tiritiri Islands. The late Captain C. Daniel, of the Marine Department, during the course of his duties as District Inspector of Fisheries, made a study of the occurrence of spawning snapper and of snapper eggs in the plankton. Unfortunately this information has not been published and, since the death of Captain Daniel, many of his records have been lost. However, it is known that snapper eggs were artificially fertilised and were identified in the plankton in the summer of 1925–26. In the following year Mr. M. W. Young, then biologist, Marine Department, took charge of snapper investigations which were continued for some years (Hefford, 1928–33). Figure 1 summarises some of the data collected by Young and Daniel. Floating snapper eggs were recorded within the larger stippled area to the west in November 1931 to January 1932, December 1933 to January 1934, and in December 1934, while in the smaller area off the coast of Coromandel Peninsula they were recorded in December 1926 to February 1927, November 1931, and December to January, 1932. The greater part of the Hauraki Gulf is to-day closed to trawlers and Danish seiners, but there is a considerable fleet of line-fishing boats operating in the Gulf waters. In the summer these boats are often concentrated in or about the larger of the two areas indicated in Figure 1. Lines are commonly set, not on the bottom as at other times of the year, but at a depth of about 10 fathoms, the average depth

of the water being a little over 20 fathoms. Usually the fish caught at this depth are in a ripe or spawning condition and are of the class known as “school fish” or “schoolers”. They have an iridescent silvery lustre, usually with a decided reddish tinge which contrasts with the less conspicuous grey-blue coloration often found in other snapper. Many fishermen believe that the school snapper is a distinct race of fish which enters the Gulf from outside waters for spawning purposes only. It is also claimed that, owing to their pelagic feeding habits, the school fish have sharper, less worn teeth than the bottom-feeding snapper usually resident in the Gulf. Unfortunately neither coloration nor tooth form are sufficiently definite characters for this theory to be readily tested Marking experiments have so far been unsuccessful, while condition factor and morphometric data have not revealed any significant racial differences. It would seem unlikely that any genetic differentiation exists, since ripe fish of both classes are often found together, and on at least one occasion eggs from a female school fish were successfully fertilised from a “non-school” male. During the summer the outer waters of the Gulf are invaded by great numbers of the “chain jelly”, Thallia democratica (Fuller 1953) which often forms the principal food of the schooling fish. It would not appear that any appreciable nourishment is obtained therefrom, since, although the entire alimentary tract is often tightly packed with Thallia, those discharged from the anus are usually entire and show little signs of any digestive action. The act of spawning has been observed by various people. Naturally only instances where this takes place near the surface would be seen The fish, whether male or female, is usually lethargic and drifts near the surface with the minimum of movement, rolling over on its side to discharge clouds of eggs or milt into the water. Fig 1—Reported occurence of snapper eggs in Hauraki Gulf, 1926–32. In a previous paper (Cassie, 1956) the developmental stages of the snapper have been described. From this information it has been possible to identify snapper eggs and larvae when they appear in the plankton In the surveys made by the author in the summers of 1950–51 and 1951–52 only one sample was obtained containing appreciable numbers of larvae, so the present discussion will be limited to the distribution of eggs. The majority of the field work was conducted from the fisheries research vessel “Ikatere”, but in the later part of December, 1951, to February, 1952, owing to a mechanical defect in “Ikatere”, operations were transferred to the patrol vessel “Ocean Star”.

Field Technique Preliminary investigations in the summer of 1949–50 had shown that snapper eggs occurred in irregular patches and that these patches seldom persisted for more than one day. The short duration is not surprising since the egg stage from fertilisation to hatching usually lasts less than 48 hours. For this reason it was desirable to employ a sampling technique which covered the greatest possible area in the time available. From vertical closing net samples it was determined that the majority of eggs were to be found at or near the surface, so that surface sampling should give at least an approximate index of the relative abundance. Three successive models of high speed nets were designed and employed during the course of the work As each new type was brought into use it was calibrated against the previous type by towing the two nets together over the same course and making a volumetric comparison of catches It was found that differences in catching power between nets either of the same or different models were negligible. Model I, which was employed in 1950–51, was based on a design by Mr. A. S. Fuller (then of the Zoology Department, Auckland University College). This consisted of a sleeve of bolting cloth 180 cms long, attached in front to a cylindrical brass mouthpiece 6 cms in diameter by 6 cms long, and behind to a removable brass cup 5 cms in diameter by 7 cms long. On opposite sides of the mouth two brass lugs served for attachment of a steel wire bridle The brass cup was fitted with a wire gauze draining panel so that, except when catches were particularly large, the volume of the sample could be reduced sufficiently to be stored in a four by one inch glass specimen tube Two grades of bolting cloth were used in different instruments of Model I: 16 and 30 meshes per cm. The coarser mesh was sufficient to retain all snapper eggs while releasing smaller organisms and hence facilitated counting, but the finer mesh was found to tear much less readily. In 1951–52 Model II was introduced This had the same mouth and collecting cup, but was reduced in length to 90 cms, and the bolting cloth was replaced by brass gauze, 16 meshes per cm. Mouthpiece and cup adapter were joined by two struts of 0.65 cm diameter brass rod. This model was found more durable, though its length made it somewhat unwieldy, and there was still a tendency for the net to tear about the middle of its length after prolonged use Model III, which is illustrated in Fig. 2, was brought into use in 1953 and has not as yet been employed in any intensive survey. However, it has proved more durable and convenient to use, and samples just as efficiently as Model II, so that it would undoubtedly be preferred for any further surveys of similar nature. The length of the filtering gauze has been reduced to 45 cms, while greater rigidity and protection is gained by the use of four struts. Fig. 2—High-speed surface plankton sampler, Model III Total length 60 cms (approximately) The high-speed sampler was towed over the stern of the ship from the port quarter. When a second net was used this was towed from the starboard quartes, so that the two would be about 3 metres apart The one metre wire bridle was attached by a heavy galvanised swivel to a 40-metre towing warp of 2 ½ cm circumference Phormium rope, which was found to be relatively free from kinking and coiled well on the deck. During plankton sampling the ship travalled at full speed (approximately 8 knots.) The course was plotted by three-point sextant fixes, but

to eliminate the effect of tides and currents on the volume of water filtered, length of tow was recorded from the patent log. As a rule the standard length of tow was four nautical miles, but when dense concentrations of Thallia democratica were encountered it was found necessary to reduce this to 2, 1, or 0.5 miles in order to reduce the volume of the sample to manageable proportions. At least once a week a shot was made with the commercial trawl to observe the condition of snapper, and at times to obtain eggs and milt for artificial fertilisation. A sample of snapper, usually about 40, was measured, weighed and dissected to determine the sex and state of gonads. In October, 1951, a number of such samples were taken by the crew of “Ikatere”, and the author was advised by telegram of the results. By this means some indication was obtained as to when it was advisable to proceed to Auckland to commence plankton sampling. Unfortunately the breakdown of “Ikatere” in early December of that year prevented further trawl sampling. Some line fishing was done from “Ocean Star” in December, but only one sample of useful size was obtained. Throughout these operations a record of surface temperature (taken by bucket and the thermometer) was kept, at least one reading being taken for every trawl shot or plankton tow. Laboratory Technique For the purposes of sorting and counting, samples were divided into two categories; those with less than 200 (approximately) and those with more than 200 spherical teleostean eggs. For the smaller samples a complete count was made. The entire sample was placed in a rectangular counting dish, 50 × 30 mm and 5 mm deep, the bottom of which was ruled lengthwise with numbered parallel lines 2 mm apart (i.e., slightly less than the width of the field of a 16 mm microscope objective) Using a mechanical stage, it was then possible to scan the entire sample line by line, identifying and counting snapper eggs. (Other species were also counted, but the results are not relevant to this discussion.) The principal character employed in identification was the diameter of the egg capsule, which was determined with an eyepiece micrometer. Very few eggs were found in the range 0.81 to 1.02 mm which showed any characteristics other than those of snapper. It is, of course, not impossible that some of the eggs identified as snapper could have belonged to other species. After weighing all available evidence (Cassie, 1956) it has been concluded that this is unlikely, but until the eggs of all species spawning in the Gulf are known there can be no absolute certainty in identification. The frequency distribution of measurements tended to have two or (occasionally) three or more modes. Since the size of eggs taken from individual fish had been found to be approximately normally distributed, it was suspected that two populations (or even two species) of eggs might be present. However, on analysis of a number of such distributions on probability paper (Cassie 1954) it was found that the two apparent size groups could not be matched from sample to sample, so that it is doubtful whether any significance can be attributed to them. In the preserved state, eggs could be divided into two distinct categories according to whether the yolk sac was clear or opaque However, it was found that artificially fertilised eggs from a single fish also exhibited this differentiation, which is therefore assumed to be an artifact. The larger samples, sometimes containing several thousand eggs, could not conveniently be counted directly in this manner, and for these a sub-sampling apparatus as shown in Fig. 3 was constructed. The sample container, A, is a 250 millilitre beaker, and the stirrer, B, is an annulus of 16 gauge brass, 5.6 cms outside and 3 cms inside diameter attached to a brass handle. A straight glass sub-sampling tube, C, 2.7 mm in diameter and 100 cms long, has a scale, D, calibrated in ml near its upper extremity. The sub-sampling tube communicates through a stop-cock, E, with a mercury aspirator consisting of two glass tubes, F and H, 13 mm in diameter and

30 cms long, connected by a rubber pressure tube, G. The top of the open arm has a constriction, K, which damps the oscillation of the mercury. The open arm is attached to a slide, L, which can be raised through a distance of 20 cms and fastened in the upper position by the peg, M. It was originally hoped that direct counts of snapper eggs could be made while they were in the sampling tube. This was eventually found to be impracticable, since snapper eggs could not always be reliably distinguished by the unaided eye from those of other species of similar size. However, it was found possible to make counts of all eggs (disregarding species) with considerable speed and accuracy, so that a convenient test of the validity of the sub-sampling technique could be made by this method. This test will be described in the next section. Fig. 3.—Apparatus for sub-sampling fish eggs from plankton samples. The detailed procedure for operating the sub-sampling apparatus was varied as more experience was gained, but the following scheme was ultimately developed as standard, although in some cases it was slightly modified to cope with the variable nature of the samples. The sample is diluted with tap water to a volume of 100 ml

and poured into the beaker, B, which is raised on a wooden block so that the end of the sub-sampling tube is immersed. The tap, E, is then opened and the slide raised to its upper position. The tap is closed and the slide lowered, thereby creating a negative pressure between the tap and the right-hand column of mercury. The sample is agitated by moving the stirrer rapidly up and down, and at the same time the tap is opened, allowing a sub-sample to be drawn into the sub-sampling tube. The volume of water was then recorded from the scale and the sub-sample allowed to drain into the counting dish. An egg count was then made in the same manner as for the smaller samples. Each sample was sub-sampled a sufficient number of times to give a minimum count of 200 snapper eggs. The total number, N, of snapper eggs in the sample was then estimated as: N = 100 n/v where n = total snapper egg count, all sub-samples. v = total volume of all sub-samples (ml) It will be shown in the next section that, although the volume of sub-samples has a slight variation, little error would be introduced by taking it as constant and employing a mean value of v in the above calculation. Before using the sub-sampling apparatus it was often necessary to remove larger plankton which would not pass into the sampling tube. Whenever possible this was accomplished by washing through a brass wire gauze sieve with approximately 4 meshes to the centimetre. Organisms which could not be removed in this way were picked out by hand. Copepods and other small animals were left in the sample as they could be easily distinguished from eggs when counting. Some difficulty was encountered when Thallia was present, since eggs tended to adhere to the salps and thus could not readily be separated. However, since the tows were of shorter duration, such samples rarely contained large numbers of eggs, and it was possible to make a direct count, removing eggs individually with a pipette. Error and Bias of Sampling It will be seen below that, in the assessment of seasonal production, the variability of the distribution of eggs is so great that only gross changes in sample counts will be of any significance. Nevertheless, since some of the techniques described may be adaptable at a later date to estimation at a higher level of precision, it is of some value to consider as far as possible the errors and biasses which may arise either from the initial sampling or from sub-sampling. No attempt has been made to interpret the sampling data in absolute terms* Assuming complete filtration, the volume of water sampled in a tow of one nautical mile would be approximately 5,800 litres. but it is believed that the surface samples give at least an approximate index of the total number of eggs in the area represented. It is known from laboratory studies that the live snapper egg tends to float at or near the surface, and sub-surface hauls have confirmed the fact that eggs are rare or absent below a depth of 5 fathoms. It seems probable, in fact, that by far the greatest number are concentrated in the surface layer and that, in calm weather, eggs (except those recently released and still finding their way to the surface) would be so close to the surface that nearly all would be susceptible to capture by the high-speed sampler. Variable bias might arise with increasing roughness of the sea, since eggs would then tend to be distributed to a greater depth. If this were so, a horizontal sample would give a lower count owing to the greater vertical dispersal of the eggs. However, since the sampler was towed directly behind the ship it is probable that any variable turbulence due to weather conditions would be negligible compared with the constant turbulence caused by the wake of the ship. To test this hypothesis the data for November, 1950, in which a variety of weather conditions were experienced, has been tabulated for size of catch against state of sea in Table 1.

Table 1—Sample Count and State of Sea. State of Sea  Smooth Slight Moderate Total Eggs per mile. f f χ2 f f χ2 f f χ2 f 0–9 10 11 49 0.193 14 17 23 0 606 14 9 28 2 401 38 10–99 9 6 65 0 830 11 9 98 0.104 2 5 37 2.115 22 100–999 6 6 05 0.000 10 9.07 0.095 4 4 88 0.159 20 1000- 1 1 81 0 362 4 2 72 0.602 1 1.47 0.150 6 26 26.00 1.385 39 39.00 1.407 21 21.00 4.825 86 Total χ2 = 7.617, d. f. = 6, P = 0.2–0.3. Against the frequency, f, of catches of given size is given the frequency, f, which would be expected if catch were independent of state of sea, while χ2 gives a measure of the probability of greater differences between f and f occurring by chance. Individual values of χ2 are all small and total χ2 indicates a probability of between 20 and 30 per cent that greater deviations would occur by chance, so that there is no reason to believe from this evidence that sample counts are influenced by state of sea. As regards variations in catching power between different samplers, either of the same or different models, it was invariably found that if two nets were towed simultaneously the catches when preserved settled to the same volume as far as could be determined by a graduated cylinder reading to the nearest 0.1 ml. Although such paired samples were compared only volumetrically, it was clear that such differences as did exist between counts was unlikely to be greater than would be expected by chance in a randomly distributed population. For instance, a sample of 5 ml volume (which was about average size) would contain about 10,000 snapper eggs. If these were taken from a randomly distributed population the standard deviation of sample counts would be equal to the square root of the mean—i.e., 100. Since a Poisson distribution with such a large mean approximates closely to normal, one in 20 samples would deviate from the mean by more than 200 eggs, the volume of which is about 0.1 ml. Any marked over-dispersion would be expected to result in differences greater than this occurring fairly frequently. This conclusion is an interesting one since it is a well-known fact that plankton, particularly when the density is high, are often non-randomly distributed even to the extent that two nets on opposite sides of the same ship may take entirely different samples. It would appear that if this is the case with snapper eggs the mixing effect of the screw produces a random distribution within the wake of the ship.* This phenomenon may merit further attention in that it provides an independent approach to one of the fundamental problems of plankton sampling: When net samples are taken from an apparently homogeneous water mass, is the non-random distribution of sample counts due to a natural aggregation of the plankton, or to variations in the volume of water sampled? Barnes and Marshall (1951) have used a pump technique to demonstrate fairly conclusively that aggregation does in fact occur, though there still exists the possibility that the pump is also subject to sampling variation. The comparison of net or pump samples in a water mass which has been artificially “randomized” might possibly dispel any remaining doubt on this question. Such an effect could make an appreciable reduction in the sampling error in an over-dispersed population of eggs since it increases both the width and the depth of the body of water for which the sample is truly representative. The above discussion, although it does not exhaust all possible sources of error, reveals no reason to doubt that the samples give an unbiassed estimate of the relative density of eggs in the horizontal plane. The sub-sampling technique, being more amenable to direct control, can be examined more critically. Throughout the examination of samples the usual statistical precautions were taken to ensure that sub-samples provided unbiassed and suitably accurate estimates

of the samples. The following two experiments will indicate that these requirements have, in most cases, been fulfilled. In the first experiment the eggs, regardless of species, in an entire sample were counted by eye in the sub-sampling tube. Since the reduced volume toward the end made adequate stirring impossible, only the first ten sub-samples are recorded separately. The results are shown in Table 2. Table 2.—Sub-sample Counts, All Eggs. Volume of Subsample (millilitres) Subsample Count 5.9 142 6.0 172 6.1 166 5.9 164 5.8 130 6.0 156 6.0 150 5.7 146 6.1 140 6.0 161 59.5 1,527 Remainder 40.5 988 100.0 2,515 For deviation of counts from the observed mean (152.70) χ2 is 10.479 with 9 degrees of freedom, giving a probability of about 0.3. If allowance is made for the slightly different sample counts expected owing to variations in volume, χ2 is reduced to 8.909 with a probability of nearly 0.5. In neither case is there any significant departure from a Poisson distribution of counts and the variation in sub-sample volume could probably be ignored without introducing any appreciable error. Since the total count for 100 ml is 2515, the expected mean count for the mean sub-sample volume (5.95 ml) will be 149.64 (which does not differ greatly from the observed mean, 152 70). For departure of sub-sample counts from the expected mean χ2 is 11 319. With 10 degrees of freedom there is a probability of a little over 0.3 of a higher departure occurring by chance. In the second experiment a smaller sample was sub-sampled and then completely enumerated in species (snapper and other) using both the sub-sampler and the counting dish. The results are shown in Table 3. Table 3—Subsample Counts, by Species. Snapper Other Species Observed Expected Observed Expected Total 180 187.77 36 28 23 216 155 155.60 24 23.40 179 173 170 38 23 25.62 196 187 180.81 21 27.19 208 Remainder 103 103.44 16 15.56 119 798 798 00 120 120.00 918 The proportion of snapper eggs for the sample is 86 93 per cent. From this figure the expected frequency of the two categories is computed and compared with the observed frequency. χ2 is 4.421 with 4 degrees of freedom, giving a probability of between 0.3 and 0.4. From the two experiments and from other similar checks made during the investigation, it is believed that the sub-sample counts are randomly distributed and

give an unbiassed estimate of the true sample mean. In the typical case where at least 200 snapper eggs were counted, the coefficient of variation of the sub-sampling estimate may be computed as follows: Where n = total sub-sample count for snapper eggs. s = standard error of n C = coefficient of variation of n = s/n Since snapper egg counts are distributed as a Poisson series. s2 = n C = 1/√n Since n ≥ 200. C ≤ 1/√200. = 7% (approximately) This maximum value of the co-efficient of variation will be applicable to about 80% of the samples recorded. The remainder will have a co-efficient of variation of an order seldom exceeding 10%, with the exception of a few cases where snapper eggs formed only a small proportion (less than 25%) of the total of all species. In such cases the laborious procedure of sorting over 800 eggs would have been required in order to obtain 200 snapper. Since such samples were few and had little effect on the distribution pattern the standard of accuracy has been relaxed in these cases. In both stages the sub-sample counts are relatively large figures, so that their distribution will approximate closely to normal, and fiducial limits or tests of significance based on the co-officient of variation are appropriate. Distribution of Eggs Since the egg stage of the snapper lasts approximately 45 hours (at 18° C.) it is clear that any one concentration of eggs will not exist for more than two consecutive days unless it continues to be renewed by the parent fish. In practice it was never possible to trace the same group of eggs for more than a day. This may be because the fish normally spawn at some depth (probably 10 fathoms or more) and the time taken for eggs to reach the surface reduces their term of availability to little more than a day. The early blastoderm stages representing the first 14 hours of life were rarely identified in plankton samples, and in most cases where eggs were kept alive the majority in any sample could be hatched out in less than 24 hours. In effect it was found that each day's samples bore little or no relation to those of the previous or the following day An area which on one day had a clearly delimited and dense patch of snapper eggs might be entirely barren on the next. The records of the 1925–32 investigations may perhaps be misleading in that, although they indicate where eggs were found, they seldom indicate where eggs were absent. Possibly, once the two likely areas shown in Fig. 1 had been located, the investigators tended to return to those places rather than to others where they had previously been unsuccessful It is seen from Figs. 4–9 that eggs may be found in very nearly all parts of the area investigated. There is perhaps some tendency for spawning to be concentrated in the western region, but evidence here is not altogether conclusive since time and resources did not permit such an intensive survey of the eastern and particularly the north-eastern section of the Gulf. In nearly all areas which were studied at all intensively, snapper eggs were detected at some stage of the investigation, the only exceptions being the enclosed water south of Rangitoto and Waiheke Islands which are not included in the figures. There is no one zone which seems to be particularly favoured as a spawning region. Although it has in most cases been possible to draw contours of egg density for any one day, little would be gained by showing these in detail, both because of the variation in pattern from day to day and because it was not possible to sample the entire Gulf each day. In order to give an overall picture of the distribution it

Fig. 4.—Density of snapper eggs, Hauraki Gulf, November, 1950. Fig. 5—Density of snapper eggs, Hauraki Gulf, December, 1950.

Fig 6.—Density of snapper eggs, Hauraki Gulf, November, 1951. Fig. 7.—Density of snapper eggs, Hauraki Gulf, December, 1951.

Fig. 8.—Density of snapper eggs, Hauraki Gulf, January, 1952. Fig. 9.—Density of snapper eggs, Hauraki Gulf, February 1–15, 1952

seemed desirable to divide numerical estimates into a relatively small number of categories which when plotted would give a visual impression of the average density throughout each month. Since the frequency distribution of sample counts is highly skewed with a very large number of zero or near-zero counts and a small number of very high counts it was found that a logarithmic interval was most suitable to give approximately an equal number of samples in each category. The four different sizes of sample, each with an upper class limit ten times the previous one, are indicated by lines of varying thickness along the track of the ship in Figs. 4–9. The information in these Figures is further summarised in Figure 10, which gives the proportion of samples of each density for the entire area of each month. The figure in the white sector represents in each case the total number of samples taken. Since the pattern of sampling varied somewhat from month to month these diagrams are only approximately representative, so that only gross changes should be taken into account. Note also that only one count of over 1000 was recorded in each of the last three months, so that the varying size of the black sector probably signifies little more than a difference in the number of samples. Fig. 10.—Summary of snapper egg density, Hauraki Gulf, 1950–51 and 1951–52. The spawning patterns for the two years are by no means identical. In 1950–51 the density of eggs had already reached its maximum in November, and declines steeply through December and January. Unfortunately it is not known whether the November spawning commenced suddenly or was built up slowly in the previous month. In 1951–52 the November density is relatively low, the maximum being in December, with a much slower decline through January and February. It would appear then that the peak of the spawning was reached at least one month later in the 1951–52 summer, but the evidence is scarcely adequate to show which season produced the greater number of eggs. While no one month in the second season equalled November, 1950, it is possible that the more protracted spawning compensated for the lack of intensity at any one time. Sex and Condition of Fish The “condition factor” or weight-length relationship of snapper will be discussed in detail in a further paper (Cassie, in press). Although there is a distinct seasonal fluctuation in the condition factor, the variation between individual fish is so

great that the changes cannot be readily detected except by statistical means.* For any one sample of snapper the relationship between length, L, and weight, W, may be described by the equation: W = KLb the exponent, b, has a value of approximately 2.7, while the constant, K, may be used as a measure of condition factor in place of the usual factor, C, where: C = W/L3 No correlation has been detected between the sexual condition of the gonad (as classified in Table 4) and the condition factor of the fish. Although no specific investigation of palatability was carried out, fish taken during the investigation were regularly eaten aboard the ship. It was found that snapper which had apparently finished spawning were seldom in any way inferior eating to those with full, ripe gonads. Both these observations seem to conflict with the belief that snapper, immediately after spawning, are normally in a thin condition, with soft flesh easily damaged by handling, and often unattractive or unpalatable. The term “spent” is commonly applied to fish in such condition, apparently in the belief that depletion of the gonads is concurrent with physical deterioration, perhaps in analogy with the Pacific salmon which dies very shortly after spawning. It is, of course, obvious that the discharge of eggs or milt would result in some loss of weight (seldom more than 5 per cent, of the total weight of the fish), but on the other hand this loss may either be replaced almost immediately by additional food, or be negligible compared with variations due to other causes. Thus, although the “spent” fish is no doubt a reality to the fish merchant and may occur more commonly after the spawning season, it is none the less clear that physical deterioration and loss of weight do not inevitably follow after spawning. In order to record the condition of the gonads in samples dissected, the code shown in Table 4 was employed. The degree of certainty with which the different classes can be determined is variable. Condition IV could always be identified with little margin for doubt, as could Condition III in the case of females. Ripe eggs are readily seen through the membrane of the ovary, being transparent and about twice the size of the opaque Table 4.—Gonad Condition Code. Code Brief Description. Female. Male. I Immature Ovary small and immature (usually a young fish). Testes small and immature (usually a young fish). II Mature Ovary firm and well developed but with no ripe eggs. Testes well developed but milt not discharged by pressure on abdomen of fish. III Ripe Ripe eggs present but cannot be discharged by pressure on abdomen of fish. IV Running Eggs readily discharged by gentle pressure on abdomen of fish. Milt readily discharged by gentle pressure on abdomen of fish. V Spent Ovary small and watery in appearance. Testes small and watery in appearance. Table 5.—Monthly Distribution of Sexual Condition Year. Month I II III IV V N I II & III IV V N 1950 November 9 76 13 2 45 71 29 21 December 6 44 16 6 27 81 7 72 18 3 61 1951 January 7 7 87 45 24 76 17 October 21 30 49 100 12 88 140 November 5 18 65 10 1 17 4 87 10 82 December† Taken by handline. N = Total number of fish examined. – 17 61 22 18 15 77 8 13

yellow immature eggs. Conditions I and V were sometimes indistinguishable, and in cases of doubt were classed as V. The results are shown in Table 5. Catches were, for the most part, taken in the western portion of the Gulf, north of Tiritiri Island, but other catches taken from various other regions have been included in the monthly average since they do not appear to differ in their characteristics to any marked degree. Although the data are not sufficiently comprehensive to give a detailed picture of change in sexual conditions, there appear to be several features worthy of mention. As might be expected, there is a preponderance of mature or ripe gonads at the beginning, and of spent gonads toward the end of the season. There is, however, a relatively small number of running fish, and this group is never the dominant one in the catch. There are three possible reasons for this. First, that the time during which eggs or milt may be discharged is of relatively short duration (although it is noticeable that the proportion of running males is usually greater than that of. Table 6.—Sex Ratio. Date. Males. Females χ2 P 29 11.50 21 27 0.750 0.3 - 0.5 5.12.50 28 25 0 170 0.5 - 0.7 12.12.50 3 4 0 143 0.7 - 0.8 14 12.50 18 25 1.140 0.2 - 0.3 15.12.50 12 27 5.769 0.01 - 0.02 4. 1.51 3 20 12.565 < 0.001 5. 1.51 14 25 3.103 0.05 - 0.1 12. 1.51 39 41 0.050 0.8 - 0.9 17 1.51 7 26 10 939 < 0.001 19. 1.51 9 7 0 250 0.5 - 0.7 31. 1.51 16 21 0.676 0.3 - 0.5 13 13 0.000 0.9 5. 3.51 7 13 1.800 0.1 - 0.2 14. 3.51 15 5 5.000 0.02 - 0.05 15 3.51 13 13 0.000 > 0.9 1. 851 18 10 2.286 0.1 - 0.2 29. 8.51 22 18 0.400 0.5 - 0.7 6. 9.51 19 14 0.758 0.3 - 0.5 27. 9.51 21 18 0.231 0.5 - 0.7 (b) 23.10.51 22 18 0.400 0.5 - 0.7 24.10.51 24 16 1.600 0.2 - 0.3 25.10.51 18 22 0.400 0.5 - 0.7 25 10.51 23 17 0.900 0.3 - 0.5 30.10 51 27 13 4.900 0.02 - 0.05 31.10 51 26 14 3.600 0.05 - 0.1 1.11.51 20 20 0.000 > 0.9 1.11 51 18 22 0.400 0.5 - 0.7 14 11.51 20 19 0.026 0.8 - 0.9 28 11.51 24 16 1.600 0.2 - 0.3 18 12 51 8 10 0.222 0.5 - 0.7 18. 8.52 18 20 0.105 0.7 - 0.8 4.9 52 16 21 0.676 0.3 - 0.5 23.10 52 21 19 0.100 0.7 - 0.8 30.11.52 15 25 2.500 0.1 - 0.2 Total (d.f. = 34) 63.459 < 0.001 Pooled (d.f. = 1) 598 624 0 553 0.3 - 0.5 Interaction (d.f. = 33) 62 906 < 0.001 d.f. χ2 P Total 10 34.805 < 0.001 (a) Pooled 1 14.011 < 0.001 Interaction 9 20 794 < 0.02 Total 10 15 475 0.1 - 0.2 (b) Pooled 1 9.473 0.002 Interaction 9 6.002 0.7 - 0.8

females). Secondly, that the spawning snapper may tend to rise into midwater (out of range of the trawl) for the act of spawning but remain near the bottom at other times. Thirdly, that rough treatment received in the trawl may cause the ripe sexual products to be discharged before landing so that running fish are no longer recognisable when examined. Some support is given to the last two possibilities by the fact that in the December, 1952 sample (the only one taken by handlines), 77 per cent of the males and 22 per cent, of the females are running. About half of these fish were taken in midwater, and the line usually subjects the fish to less rough handling than the trawl. Comparison between the two seasons must be treated with caution, since the only two sets of data taken under strictly comparable conditions are those for November. Even making due allowance for the obvious deficiencies of the data, it is difficult to reconcile Table 5 with the frequency of eggs as shown in Fig. 10. Thus, although spawning was earlier in 1950–51, the condition of female fish as judged by columns II and III seems to be less advanced in the months of November and December. It is also surprising that spent fish do not predominate until January, 1951, two months after the principal burst of spawning. Although a more comprehensive collection of data might shed more light on these apparent anomalies, it is likely that neither trawl nor handline will give a truly representative sample of the entire spawning population if fish in the different stages have a different behaviour pattern—e.g., migration, vertical distribution, susceptibility to capture, etc. From the fish samples described above, together with data taken at other times of the year, estimates of the male-female ratio were made. Table 6 gives this data together with a χ2 test which gives a measure of departure from the expected ratio of unity. For the pooled data the overall departure from expectation is not significant, but both the total and interaction χ2 are significant at the 0.1 per cent. level, indicating heterogeneity in the sex ratio between samples. The high level of significance is contributed principally by the two samples, 4/1/51 and 17/1/51. If these are removed from the table, neither total nor interaction χ2 is significant, suggesting that the two samples are in some way aberrant. However, there is further evidence of heterogeneity in the two periods (a) and (b). In the first of these (December, 1950, and January, 1951) there is a consistently greater number of females, while in the second (August to October, 1951) males predominate. The separate analysis at the bottom of the table shows that in both cases the ratio of the pooled sex totals deviates significantly from unity (χ2 = 14.011, 9.473), but the periods differ in that (a) shows heterogeneity in the sex ratio (χ2 = 20 794) while (b) does not (χ2 = 6.002). Although no permanent disparity between male and female snapper is evident from Table 6, there is apparently either a differential mortality or (far more probably) a difference in behaviour which results in a greater number of one or other other sex being caught in certain seasons. Correlation with Hydrographic Conditions Although single surface water temperatures are in themselves a somewhat unreliable index of the underlying water mass, by averaging these temperatures in ten mile squares (each square overlapping adjacent squares by five miles) it has been possible to obtain mean monthly temperatures from which isotherms have been drawn to depict at least the gross features of monthly temperature variations (Fig. 11) (Broken lines represent isotherms where the data are less adequate) It is believed that these mean isotherms are approximately representative of the water mass temperature in the upper ten fathoms, since those taken in November, 1951, are in substantial agreement with a series of bathythermograph soundings taken in the last week of that month by N.Z. Oceanographic Institute.

It will be seen that there is a marked difference in surface temperature in the Gulf between the two seasons. In November, 1950, there is a gradient from 18 5° C. near Rangitoto Channel to 17 5° C. across the mouth of the Gulf. In December the gradient remains the same, but the temperature has increased approximately one degree throughout. In November, 1951, the temperatures are at least a degree cooler than at the same time the prvious year, while in December the temperature gradient of the previous year is reversed in the inner portion of the Gulf, with a pocket of cold water lying along the western shore. It is not until January that the mean temperatures reach 18.0° C. in the western part of the Gulf. Table 7 summarises the general meteorological and hydrographic conditions for the two seasons Water temperatures are taken from the mean isotherms midway between Kawau and Tiritiri Islands, while the remaining data are from meteorological records for Auckland. Fig. 11a.—Mean monthly isotherms, Hauraki Gulf, November, 1950. Fig. 11b.—Mean monthly isotherms, Hauraki Gulf, December, 1950. Fig. 11c.—Mean monthly isotherms, Hauraki Gulf, November, 1951. Fig. 11d.—Mean monthly isotherms, Hauraki Gulf, December, 1951. Fig. 11e.—Mean monthly isotherms, Hauraki Gulf, January, 1952.

Table VII.—Water Temperatures and Meteorological Records Mean surface water temp. Degrees C. Mean air temp Degrees C. Rain days Sunshine hours Remarks November, 1950 18.0 17.4 10 168 Mild easterly conditions December, 1950 19 0 19.0 9 233 Warm westerly conditions January, 1951 19 7 11 179 Mild settled weather November, 1951 16.7 16.2 15 205 Unsettled westerly December, 1951 17.0 16.7 16 204 Cool and changeable January, 1952 18.0 17.9 14 239 Changeable westerly It is seen from these records that water and air temperatures are closely correlated and that in general the weather was warmer and more settled in the summer of 1950–51. Since the main bursts of spawning occurred in November, 1950, and January, 1952, it would appear that 18° C. may be a critical water temperature and that until the surface temperature rises to this level the majority of fish will not spawn. It will be noted, however, that the temperatures quoted are means about which a considerable variation may occur, even though the main water mass may be more constant. There is no apparent correlation between egg counts and temperatures for individual samples. Unless the snapper spawns at the surface it will obviously be dependent on subsurface rather than surface temperature. However, the bathythermograph soundings mentioned above indicate that approximately the same temperature prevails from the surface to between 10 and 15 fathoms, an abrupt thermocline of up to 3° C. separating the warmer upper from the colder lower water mass. Since the “school snapper” in the early part of the spawning season are usually reported at depths of approximately 10 fathoms it would seem that the fish do indeed seek this upper layer about the time it reaches the necessary minimum temperature of 18° C. In future investigations it may be possible to amplify this hypothesis by correlating the depth of the thermocline with the depth at which snapper are taken. If, as seems likely, the fish rise only until they reach the lower boundary of the warm water and no more, such information might be of considerable use to the line fisherman in determining what length of line to set Cousteau (1953) describes similar conditions where the change in temperature is so sharp that an aqualung diver swimming in the warmer layer may extend his hand a few inches and readily feel the colder layer beneath. It would perhaps be possible for a fish to adjust its hydrostatic balance to float without effort on the thermocline. Discussion A study of the spawning of any species of fish exploited by a commercial fishery has two important aspects. In the first place it supplies part of the necesasry background of knowledge of the behaviour of that species. Such information may be of direct benefit to the fisherman if it enables him to obtain a better catch in proportion to his efforts. Thus, if a simple means of locating the thermocline could be developed, it is possible that the line fisherman would be provided with a more efficient means of locating the school snapper.* It is perhaps a debatable question whether more efficient exploitation, particularly of spawning fish, is desirable. However, provided exploitation is kept to a suitable level, there seems to be no reason why greater efficiency should be adverse to the long-term interests of the fishery.A knowledge of behaviour may also help to solve some points of doubt or dispute, thereby clearing the path for future investigations. For instance, at the time Captain Daniel's investigations were carried out, the mere fact that the eggs of the snapper were found to float on the surface immediately dispelled the popular contention that trawl and Danish seine nets being dragged along the bottom were destroying the “spawn” (Hefford 1929).

Beyond this general knowledge of behaviour, a spawning investigation may sometimes be adapted to give quantitative estimates for two of the variables which describe the structure of the fish population First, under certain circumstances the number of eggs in a given area may provide a measure of the potential recruitment of young fish to the stock in later years Secondly, if the average number of eggs produced by one female fish is known, the total number of female fish spawning in that area may be computed. Since the sex ratio is approximately unity, twice this figure gives the total population of spawning fish, which is an estimate of the stock available to commercial exploitation. To be of value, estimates of this type must not only be made to a sufficient degree of accuracy to detect any changes which occur from year to year, but must also be made in such a way that the magnitude of sampling error can be determined. A perusal of Figs. 4–9 shows that, not only is the distribution of eggs at any one time highly aggregated, but that a very large proportion of the total quota is provided by comparatively infrequent dense patches, the omission of any one of which might contribute a considerable error to the total estimate In some cases these patches may be of considerably smaller width than is indicated by the 4-mile unit sampling lines It is indeed possible that all the difference between a prolific and a meagre spawning season might be made by one very dense patch of eggs, occurring for one day only and in one small area. Contributing to this difficulty is the fact that the snapper egg develops comparatively rapidly and is thus available for estimation for no more than two days as compared with say, the North Atlantic cod which, at temperatures near freezing, remains unhatched for over a month It is thus clear that any truly accurate estimate will require a pattern of samples repeated every day and covering every part of the Gulf It is not possible at present to state the appropriate sampling interval, but it would seem that this should not be greater than five miles. Assuming that the present high-speed plankton sampler gives a completely adequate sample for this purpose, such a programme would require nearly 24 hours towing at 8 knots each day for several months, a working intensity which cannot be approached with present resources. Even then, the time allotted has made no allowance for random sampling which would be necessary to obtain an estimate of sampling error. It would seem unlikely, therefore, that any useful quantitative estimate of total egg production could be made without employing several vessels. Similarly, some caution is necessary in interpreting the data presented above. Although the relative lateness of the 1950–51 spawning compared with that of the previous year appears to be established with a reasonable degree of likelihood, it is by no means impossible that bursts of spawning which remained undetected could have made a substantial difference to the interpretation of results. Acknowledgments The data presented in this paper were collected while the author was Marine Biologist in the Fisheries Branch, Marine Department. Assistance and information has been given by many officers of the Department, in particular the late Captain C. Daniel (many of whose records were made available by Messrs Young and Gilliver), Captain A Duthie, Mr. E. W. Gilliver, Miss M. K. McKenzie, Mr. N. Pijl, Mr. H. D. White and Mr. M. W. Young Thanks are also due to Messrs. G. T. Russell and D. M. Garner, of New Zealand Oceanographic Institute, for the use of unpublished bathythermograph data, and to the author's wife and Mr. K. R. Allen who, by their constructive criticism have assisted greatly in the preparation of the manuscript. References Barnes, H., and Marshall, S. M., 1951. On the variability of replicate plankton samples and some applications of contagious series to the statistical distribution of catches over restricted periods. Jour. Mar. Biol. Assoc. 30, pp. 233–263.

Cassie, R. M., 1954. Some uses of probability paper in the analysis of size-frequency distributions. Aust. Jour. Mar. Freshw. Res. 5, pp. 513–522. —— 1956. Early development of the snapper, Chrysophrys auratus Forster Trans. Roy. Soc. N.Z. 83, pp. 705–713. —— In Press. Condition factor of the snapper, Chrysophrys auratus Forster in the Hauraki Gulf. N.Z. Jour. Sci. Tech. (B). Cousteau, J. Y., 1953. The Silent World. London. Fuller, A. S., 1953. Seasonal variation in the plankton and salinity of the Hauraki Gulf, New Zealand. Nature 171, p. 525. Hefford, A. E., 1928–33. Report on Fisheries. Ann. Repts. N.Z. Marine Dept. (Years ending 31st March, 1928–33). R. Morrison Cassie N.Z. Oceanographic Institute D.S.I.R. Wellington