Early Development of the Snapper, Chrysophrys auratus Forster

Transactions and Proceedings of the Royal Society of New Zealand, Volume 83, 1955-56, Page 705

Early Development of the Snapper, Chrysophrys auratus Forster By R. Morrison Cassie* Previously of The Fisheries Laboratory. Marine Department, now The N. Z. Oceanographic Institute, D.S.I.R, Wellington. [Received by the Editor, May 20, 1955.] Abstract Developmental stages from fertilisation to the end of the yolk sac stage in the larva are described. At 18° C. the egg hatches 45 hours after fertilisation and the yolk sac is completely absorbed in about three days. Identification in plankton samples may be made from the diameter of the egg membrane (0.8-1.0 mm) and oil globule (0.25 mm), from the bright yellow surface pigment, and from the general features and dimensions illustrated. Introduction The following information has been collected during a study of the snapper fishery in the Hauraki Gulf commenced in 1948. One of the many factors which it is desirable to know in assessing the potential production in a commercial fishery is the annual “recruitment” of young fish to the population being exploited. It is thus necessary to be able to identify the species concerned not only in its adult form but also in the egg and larval stages. This work is designed primarily not as an embryological study, but rather to supply a ready means of identifying early developmental stages. It is believed that snapper eggs were first artificially fertilised and hatched, and identified in the plankton by Captain C. Daniel (District Inspector of Fisheries, Marine Department) in the summer of 1925-26 (Hefford, 1929). Although no detailed records of this work have been published, it has since become quite generally known that a considerable number of snapper spawn in summer near Tiritiri Island in the Hauraki Gulf, and that, in common with the majority of Teleostii, fertilisation is external and the egg planktonic. The author repeated Captain Daniel's experiment in November, 1950. Preserved material, notes and sketches were collected during November and December of that year and brief observations were also made by the author in the summer of 1951-52, and by Miss M. K. McKenzie, in 1952-53. In November and December, 1954, a 35mm macro-camera was setup in Auckland to supplement the previous information with photographic records. Plates 23-26 have been prepared from these photographs. The usual spawning season in the Hauraki Gulf is apparently from mid-November to mid-January, though low water temperatures may delay its onset until mid-December, while it may continue until the end of February or even March at times. At Awanui, in the north, spawning may commence several weeks earlier, while in Marlborough Sounds and Tasman Bay it is correspondingly later, suggesting that a certain minimum temperature is necessary.

Technique Material for photography was obtained principally in the neighbourhood of Tiritiri Island. Fish were caught in a commercial trawl by the Fishery Research Vessel Ikatere, and immediately the catch was landed it was sorted for “running” snapper. The “running” condition is readily recognised by gently stroking the abdomen of the fish between fingers and thumb in a tailward direction. The eggs or milt if ripe are thus expressed through the cloaca. For the earlier developmental stages several fish were kept alive in a tank of running water until port was reached. Otherwise eggs were expressed and fertilised at once. Eggs and milt were collected in separate two-pound jars which had been previously quarter filled with clean sea water. After making certain that no faecal matter or immature eggs were included in either jar, the contents of the two were mixed. Sea water was added nearly to the top of the jar, which was then covered with number 40 grit gauze. Ashore the samples were allowed to stand for about 15 minutes (except when early cleavage stages were required) until all viable eggs were floating at the surface. It was then possible to draw off a suitable quantity of eggs with an eye-dropper pipette and place them in a jar of freshly filtered sea water. Thus dead eggs and extraneous material which had sunk to the bottom were eliminated, thereby removing likely sources of bacterial contamination as well as foreign bodies which might mar otherwise acceptable photographs. It was found that any attempt at aeration was unnecessary, and in fact undesirable, since the disturbance of the water was liable to produce premature mortality, particularly in the larval stages. Thus, provided the water remained clear, no further attention was given other than protection from dust or extremes of temperature, and the removal of dead eggs or larvae. Any cloudiness or unpleasant odour was immediately eliminated by pipetting off all healthy material and placing in clean water. In order to prevent the spread of any bacterial contamination it was found desirable to use a separate pipette for each jar and to boil all used glassware before fresh material was introduced. Water temperature was taken at hourly intervals, though it was not always convenient to make a complete 24-hour record. Material required for examination was removed by pipette and placed on a glass slide in a drop of filtered sea water, and when necessary irrigated until all particles of dust were removed from the camera field. The specimen was oriented by a current of water from the pipette, or by fine glass needles. The majority of photographs were taken without a cover glass and with the camera vertical. In the case of the lateral views, however (Plate 25). the whole apparatus was tilted to a horizontal position and the specimen contained in a flat-walled rectangular glass cell constructed by cementing a 2 × 1 ¼ inch coverslip with 1 mm spacers to a 3 × 1 ½ inch slide, leaving a space at one edge for introducing the egg or larva. In addition to trawl catches, a series of plankton samples at all depths were made, in an effort to secure eggs and larvae in their natural state. Although eggs in all stages were relatively easy to obtain, the only larvae found in numbers were in a single vertical haul taken at night in about five fathoms at Bostaquet Bay on the south coast of Kawau Island. These were all in the same stage of development and had obviously hatched only an hour or two before being taken Owing to the scarcity of planktonic material and the apparent inability of larvae

to survive under natural conditions after the yolk sac had been absorbed, it has not yet been possible to describe the later larval and postlarval stages of the snapper. Time of Development The time taken for various developmental processes is, as might be expected, dependent largely on water temperature. Since the average surface temperature under natural conditions is about 18°C. this has been taken as the standard of reference, and times given for the various stages are based as closely as possible on this figure. It has been possible to do this for the egg up to time of hatching with an error seldom exceeding 5 per cent., but the times for larval stages are more approximate. It is not unlikely that larvae are demersal in habit, at least after the first day, in which case the water temperatures concerned would be appreciably lower than 18°C. The most precise and easily identified moment in early development is the time of hatching. Although this may vary a little in different eggs, the majority in any one batch usually hatched within an hour of each other. Table I gives three records of time and mean temperature from fertilisation to hatching. Table I. Hatching Date Hatching Time Mean Temperature 22.11.51 45 hours 18.0°C. 30.11.51 41 hours 19.4°C. 2.12.54 36 hours 21.0°C. In will be noted that these figures are in exact agreement with Van't Hoff's law (i.e., that the speed of a chemical reaction doubles with each rise in temperature of 10°C.) although, considering the probable experimental error, this apparent goodness of fit may be largely coincidental. Tunes of development for various recorded stages of the egg have been corrected for a temperature of 18°C. by applying the equation: tn = ta Tn/Ta Where: tn = expected development time at 18°C. ta = actual development time. Tn = hatching time at 18°C. = 45 hours (Table 1). Ta = actual hatching time. This calculation is independent of the exact nature of the time: temperature relationship, provided the temperature remains reasonably constant. Development The process of development may best be followed by reference to the figures in Plates 23 to 26. Since this work deals primarily with identification, the description is mainly concerned with those structures which can be seen in the live material under an ordinary stereoscopic microscope. However, the interpretation of certain details has been amplified by sectioning fixed material. When discharged by the female the eggs are transparent, colourless spheres about 0.9 mm in diameter, presenting in mass the appearance of very fine boiled sago. The yolk which fills the thin, smooth egg membrane is clear and undiffer-entiated, the only visible inclusion being a clear oil globule about 0.25 mm in diameter lying close to the membrane. Owing to its high refractility this globule

appears black toward its periphery in the photographs. In still water the eggs float to the surface owing to the buoyancy of the oil glouble which occupies the upper pole of the sphere. Half an hour after milt has been added to the eggs (Plate 23, Figure 1) the protoplasm is differentiated from the yolk to form a circular germinal disc, 0.5 to 0.6 mm in diameter around the lower pole. The egg membrane, which is seen in optical section, appears as a dark outline outside and concentric with the germinal disc, and is not to be confused with the edge of the circular field of view commonly shown in photomicrographs. In the photograph the camera is vertical and is focused on the lower surface of the egg, so that the oil globule appears slightly out of focus. By adjusting the convergence of the illumination or throwing it slightly off centre the presence of minute inclusions may be detected in the yolk and germinal disc, particularly the latter. Plate 25, Figure 1, shows the same stage from an oblique view point so that the germinal disc is seen in profile. Although the germinal disc has been displaced toward the lower edge of the photograph in this and other oblique views taken with the vertical camera, the oil globule retains its position in the centre of the circle described by the egg membrane. It would appear from this that the yolk is in a semi-fluid state, so that when the egg is rotated round a horizontal axis the oil globule is able to move to the uppermost point of the sphere, provided the angular distance travelled is not greater than about 80 degrees. After an hour (Plate 23, Fig. 2) the first cleavage has been completed, giving two distinct elliptical blastomeres. In this figure the minute inclusions in the protoplasm may be seen. These become less evident as development proceeds. In 1½ hours the second cleavage has taken place at right angles to the first (Plate 23, Fig. 3). Both this and the following figure show clearly the tendency of adjacent walls of blastomeres to coalesce along the cleavage plane, so that the most recent cleavage may be identified by the deeper indentation at the edge of the germinal disc. The first two cleavage furrows pass more or less vertically through the blastoderm, so that the four blastomeres are very clearly defined, although still incompletely separated in their deeper portions. A third cleavage parallel to the first has taken place after 2 hours, producing an 8-celled stage (Plate 23, Fig. 4), while in 2½ hours a fourth cleavage parallel to the second gives 16 blastomeres (Plate 23, Fig. 5). The furrows by this time are less clearly defined, since the third and fourth cleavage planes curve inward beneath the surface of the blastoderm to intersect the first and second furrows, thereby splitting the blastoderm into two layers. In Plate 25, Fig.2. an oblique view shows the edge of the blastoderm at 2⅔ hours in the process of cleavage between 16 and 32 cells. At 3 hours the surface pattern of the 32-celled stage (Plate 23, Fig.6) is becoming obscure owing to the two-layered structure, the deeper layer being still a continuous sheet of protoplasm, while the superficial layer is divided into distinct cells. By 3½ hours (Plate 23, Fig.7) the blastoderm, now with 64 cells, is becoming roughly circular, while the regularity of the cleavage pattern is being lost. Cell division now continues at approximately half-hour intervals until at 6 hours (Plate 23, Fig. 8) the blastoderm has become a multiple-layered cellular plate, still undifferentiated and with individual cells still visible after about 11 cleavage. A lateral view of the same stage (Plate 25, Fig.3) shows the position of the egg as it floats in the water with the oil globule uppermost and

the blastoderm beneath. The blastoderm is still of approximately the same dimensions as the original undivided germinal disc. Ten hours after fertilisation (Plate 23, Fig. 9) the blastoderm, which is still radially symmetrical, has spread over the surface of the yolk till its diameter is about 0.8 mm. while the germ ring first appears as a slight thickening in the deeper layer at the periphery of the blastoderm. One hour later (Plate 23, Fig. 10) the germ ring is more clearly defined, and part of it is beginning to spread toward the centre of the blastoderm, forming the embryonic shield at what will later become the posterior extremity of the egg. At 12 hours (Plate 23, Fig. 11) the embryonic shield has enlarged still more, while the blastoderm edge opposite is extending towards the equator of the egg. Plate 25, Fig. 4 is an oblique view of the same stage showing how the shape (but not the volume) of the original disc has changed. In another hour (Plate 23, Fig. 12) the blastoderm has spread beyond the equator of the egg and is almost hemispherical. In Plate 24, Fig. 1. at 15 hours the blastoderm is still spreading rapidly until at 17 hours (Plate 24, Fig. 2) the surface of the yolk is almost completely enclosed, leaving exposed only a circular yolk plug about 0.5 mm in diameter. The embryo now appears as a slight folding and thickening of the blastoderm in the lower hemisphere of the egg, while at 18 hours the head has begun to take shape, although the caudal extremity is still little differentiated (Plate 24, Fig. 3). Plate 25, Fig. 5 is a posterior view of the egg at the same stage, the camera being focused on the yolk plug, so that the head on the far side of the egg is slightly out of focus, though it is clearly seen projecting up toward the centre of the photograph. The blastodermal rim is circular in shape except where it is interrupted by the undulating edge of the caudal swellings. Figure 6 is a lateral view showing the thickened edge of the blastoderm and the pro-truding yolk plug immediately above the caudal extremity of the embryo. At 20 hours (Plate 24, Fig. 4) the embryo is distinct throughout its length, with the optic lobes just beginning to appear on the head, while the caudal swellings show prominently on either side of the posterior extremity. At 22 hours (Plate 24. Fig. 5) the optic lobes have increased in size and the myotomes are beginning to form, while the caudal swellings are no longer apparent. There is little change in the general appearance of the embryo at 24 hours (Plate 24, Fig. 6). beyond a general clarification of all parts seen in the previous figure. Optic and olfactory lobes have become more prominent, the myotomes more distinct and the tail is beginning to extend beyond the edge of the blastoderm. Stages similar to this, because they remain superficially unchanged for a relatively long period, are by far the most common in plankton samples. The head with the rudiments of the visceral arches is becoming more differentiated by 26 hours (Plate 24. Fig. 7) while the tail is growing away from the yolk sac and curving slightly to the right (i.e., the left in the photograph since the embryo lies ventral side uppermost). The rudiment of the heart may be seen just below the olfactory lobe, curving forward from right to left in the figure A lateral view of a slightly later stage (30 hours) is shown in Plate 25, Fig. 7. The tail still has a blunt, rounded extremity. The heart shows as a slight projection on the ventral surface immediately behind the head, and begins to beat irregularly very soon after this stage. By 39 hours (Plate 24, Fig. 8) the tail has increased in length and has a more pointed tip than previously. The granular yellow pigment which is characteristic of the young larva is now plainly seen,

the heart beats fairly regularly, and occasional convulsive movements are seen in the embryo. The embryo is ready to hatch at 45 hours (Plate 24, Fig. 9). The eyes, as yet lacking retinal pigment, have a clearly differentiated lens, while auditory capsules can be plainly seen on either side of the posterior extremity of the head, about 0.3 mm behind the centre of the eye. The tail is fully developed and terminates in a point. Yellow pigment spots are visible, particularly at the anterior and posterior edges of the eye and on either side of the tail (between 8 and 9 o'clock in the figure) though smaller spots occur at intervals along the body. This pigment, black in the photograph, is bright yellow by reflected light, and is probably one of the most useful diagnostic characters of the species at this stage. The heart beats regularly, and movements of the embryo within the egg membrane become more and more frequent toward hatching time Plate 25, Fig. 8 shows a lateral view of the same stage. Since the camera is focused on the more distant head region the photograph gives the illusion of reversing the caudal flexure. The embryonic median fin and myotomes in the tail may be seen clearly. An umbilicus near the right-hand side of the figure passes from the anal region of the embryo into the yolk sac. Plate 24, Fig. 10, shows the same embryo commencing to hatch. This is accomplished by a periodic thrashing of the tail, which gradually distorts the egg membrane and eventually ruptures it, usually in the vicinity of the head From this point emergence may take place in a few minutes, though it is not unusual for up to an hour to elapse before the massive yolk sac is finally forced through the broken membrane and the larva escapes, leaving the empty egg shell behind as in Plate 24, Fig. 11. The tail flexure is retained for some time, often as long as 12 hours, and gives the larva a tendency to swim in circles. A lateral view of the same larva is shown in Plate 26, Fig. 1. Although, for the purposes of comparison, it is represented with dorsal side uppermost, the actual swimming position is normally inverted, and at rest the body lies at an angle of about 45 degrees to the horizontal, with the tail downward and the relatively buoyant yolk sac uppermost. It is only after about 24 hours that larvae eventually “right” themselves. Under laboratory conditions the newly-hatched snapper tend to float or swim in clusters near the surface of the water. When viewed with the naked eye, the bright yellow granular surface pigmentation is characteristic. The three most prominent patches are on the head (particularly about the eyes), in front of the vent, and midway along the tail. This last patch may spread irregularly over several myotomes, but it is almost invariably centred on the 18th myotome behind the cephalic region. A further patch of variable size usually occurs immediately behind the head, while others still smaller are scattered along the dorsal surface and the ventral region behind the vent. Black or brown pigment is usually absent at this stage, but occasional melanophores, usually stellate in form may occur on the yolk sac or oil globule. The eye, still lacking retinal pigment, has all its principal components including a crystalline lens fully developed. The posterior extremity of the gut with a vestige of post-anal gut is plainly visible, while the mid-gut, though less obvious. may be seen from sections to have a distinct circular epithelial wall. The two-chambered heart may be seen beating regularly and lies obliquely across the ventral surface with the ventricle to the right. The auditory capsules are situated on either side of the hind-brain, where they appear as two transparent ovals, each

with a pair of dark spots near the centre. The median fin is continuous round the tail from the back of the head dorsally to the anus ventrally. Being thin and transparent, it is not always clearly visible by transmitted light, and is best seen when the illumination is slightly off centre. Fin rays are at this stage either obscure or absent. After the first 24 hours (Plate 26. Fig. 2) the larvae usually prefer the bottom of the container, either resting head down, tail inclined at an angle of 45 degrees, or swimming in short bursts in the horizontal position as in the figure. The main characteristics seen at hatching are still present, but the body has increased in length from 2.1 to 2.7 mm, mainly by growth of the tail. At the same time the yolk sac has been reduced to about a quarter of its previous volume and is separated by about half its length from the anus. Yellow pigment has persisted but has become fibrous rather than granular, having an arborescent appearance where it lies on the surface of an organ such as the eye, the gut or the oil globule. After two days (Plate 26. Fig. 3) the yolk sac is still more reduced in size and the body elongated to 3.2 mm. The rudiments of jaws have begun to form, though there is still no functional mouth. The alimentary canal is lengthening and becoming convoluted. Yellow pigment is still present, but is reduced in area, while small black melanophores are beginning to appear on various parts of the body. The retina of the eye is beginning to darken, but the lens is still distinctly visible. Small pectoral fins have commenced to develop. After three to four days (Plate 26, Figs. 4 and 5) the larva appears to have reached the maximum development possible without external sources of food. Although there has been little gain in length (at least under laboratory conditions) considerable structural modification has taken place. The yolk sac has been completely absorbed, while the oil globule has disappeared. The jaws are completely developed and functional, and the gut has increased in size and become more convoluted. The retina is now pigmented a deep black, against which the lens can no longer be distinguished in side view, though from above it may be seen projecting into the space between retina and cornea. The pectoral fins are enlarged and fully functional and can be seen clearly from above. Yellow pigment has by now largely disappeared except for a few traces such as that on the tail in Figure 5. In its place black pigment has become more conspicuous as in the regularly spaced granules on the undersurface of the tail and arborescent patches on the surface of the gut and other organs. The median fin is still thin and transparent, but clearly developed fin rays may be readily distinguished. Development ordinarily proceeds up to this stage in the absence of food, and in fact, for convenience of photography, several batches of larvae were raised entirely in filtered sea water. It has not as yet been possible to produce the environmental conditions necessary for development to proceed beyond this point. Obviously, suitable food is an essential, but the addition of plankton taken with a fine-meshed net in the vicinity of spawning grounds has so far not proved successful, perhaps partly owing to the fact that dead organic matter almost invariably accumulates in the container and is difficult to remove without undue disturbance of the larvae. Specimens have been kept alive for as long as 12 days from hatching, but beyond the fourth day development appears to be arrested and no significant changes in form are noted. Although food is an important factor in development at this stage, it is probable that other aspects

of the natural environment have not been sufficiently reproduced. For instance, while some range of temperature is tolerated, any sudden rise or fall in water temperature is frequently fatal, and in the absence of suitable regulating apparatus such an eventuality is almost impossible to avoid over periods of more than a week. Dannevig and Hansen (1952) have found that after the yolk sac is absorbed, the larvae of many fish may be adversely affected or even killed by bubbles of gas in the body cavity or intestine. This may be avoided by reducing the percentage gas saturation of the water, either by removing part of the gas or by increasing the hydrostatic pressure of the water. It is not unlikely that the snapper larva normally becomes demersal soon after hatching, and thus is exposed to water of higher than atmospheric pressure. This sensitivity to dissolved gases may also be the reason why artificial aeration of the water is almost invariably fatal to snapper larvae. Identification In the absence of a complete collection and identification of all species of eggs and larvae which may be present in the plankton, it is not possible, of course, to state with certainty that no other species will present some or all of the same characteristics as those described above. Indeed, the early developmental stages are conspicuously devoid of any prominent characters which immediately place them in any one restricted category. The eggs, for instance, belong to a very large group of pelagic teleostean eggs which have a smooth spherical membrane, a small perivitelline space, a single oil globule, and a clear, almost featureless, yolk. They lack the characteristic elliptical shape of the anchovy (Engraulis), the cellular yolk and large perivitteline space of the pilchard (Sardinops), or the multiple oil globule of the yellow-eyed mullet (Agonostomus forsteri). The description of the larval stages could also apply with only minor modifications to a great number of genera. Perhaps the most useful general characteristic, particularly in the case of the egg, is size. By this means the eggs of a great number of other species may be immediately eliminated from any sample. Table II gives the diameters of various spherical eggs which have been collected in the Hauraki Gulf during the snapper spawning season. With the exception of Sardinops all eggs have been taken from the female of the species named, and thus are positively identified. The pilchard eggs were taken from the plankton and identified by their close resemblance to other species of the genus in Australia and South Africa (Munro 1945, Davies 1954). All measurements have been taken from material fixed in 5 per cent, formalin. At least in the case of the snapper, shrinkage of the egg membrane is so slight that the live material is not likely to deviate appreciably from these figures. Table II. Species m s Sm N n Agonostomus forsteri 0.870 0.023 0.023 7 1400 Chrysophrus awatus 0.914 0.034 0.026 40 2000 Chelidonichthys kumu 1.33 0.056 0.042 4 500 Sardinops neopilchardis 1.43 0.11 — — 20 Zcus faber 1.95 0.03 — 1 100 m = mean diameter s = average standard deviation within samples sm = standard deviation between sample means. N = number of samples, n = total number of eggs measured.

The snapper egg, vertical views. X 37 1—½ hour 2-1 hour 3—½ hours 4—2 hours 5—2 ½ hours 6—3 hours 7—3 ½ hours 8-6 hours 9-10 hours 10-11 hours 11-12 hours 12-13 hours

The snapper egg, vertical views X 37. 1—15 hours. 2—17 hours 3—18 hours 4—20 hours 5—22 hours. 6—24 hours 7—26 hours 8—39 hours 9—45 hours. 10—Commencing to hatch 11—The larva newly hatched.

The snapper egg oblique and side views X 37 1—½ hour oblique view 2—2 ¾ hours oblique view 3—6 hours side view 4—12 hours, oblique view 5—13 hours end view 6-18 hours side view 7—30 hours, side view 8-45 hours, side view

The snapper larva X 37. 1—Newly hatched 2—1 day. 3—2 days 4 and 5—3 to 4 days.

Of this list, only one, on the basis of size, could be confused with the snapper— i.e., Agonostomus, which is readily distinguished by its multiple oil globules which may be up to ten in number. From the above statistics it is safe to say that only one in every hundred snapper eggs will fall outside the diameter range 0.81-1.02 mm. Although there may be some slight change in diameter between fertilisation and hatching, this variation is small compared with that between individual eggs. In all the 40 samples of snapper eggs the length of the parent fish was recorded (ranging from 26 to 60 cms), but there was found to be no significant correlation between size of fish and size of egg. The diameter of the oil globule is also constant within narrow limits, not only in the egg, but also in the larva up to the second or third day. From a sample of 200 preserved eggs this was found to be 0.25 mm with a standard deviation of 0.06 mm. The position of the globule is typically at the posterior extremity of the yolk sac in the larva (cf. the tarakihi, Cheilodactylus macropterus, where it is anterior). Owing to the relatively rapid growth rate. other dimensions change too rapidly to be more than a general guide to identification. The scale given at the bottom of Plate 26 is applicable to all figures. Apart from size, the most obvious feature of the second day egg and of the larva up to three days is the yellow pigment, visible even to the naked eye as three distinct patches, one on the head, one in front of the anus, and one midway along the tail. Under the microscope the more detailed pattern shown in Plate 26 may be distinguished, the more prominent patches maintaining a fairly constant position in relation to myotomes and other organs. Acknowledgments The field work for this paper was carried out while Marine Biologist to the Fisheries Branch, Marine Department. Thanks are due to all officers of the Department who have assisted in this work, particularly Captain A. Duthie and Miss M. K. McKenzie. References Dannevig, A. and Hansen, S., 1952. Factors involved in Hatching and Rearing Fish Eggs and Larvae. Fiskeriderectoratets Skrifter. Ser. Havund. 10 (1), 36 pp. Davies, D. H, 1954 The South African pilchard (Sardinops ocellata). Development, occurrence and distribution of eggs and larvae. 1950-51. Div. Fish. Invest. Rept. 15, 28 pp. Hefford, A E., 1929 Report on the fisheries of the Hauraki Gulf, with special reference to the snapper fishery and to the effects of “Power” fishing (trawling and Danish seining). N.Z. Mar. Dept. Rept. on Fisheries, 1929, pp. 30-61. Munro, I. S., 1945. Postlarval stages of Australian fishes Queensland Museum 12 (3). pp. 136-153. R. M. Cassie, M.Sc., 51 Poole Crescent, Wainui-o-mata.