The Food of the Black Shag (Phalacrocorax carbo novaehollandiae) in Otago Inland Waters

Transactions of the Royal Society of New Zealand : Biological Sciences, Volume 11, Issue 2, 3 December 1968, Page 9

The Food of the Black Shag (Phalacrocorax carbo novaehollandiae) in Otago Inland Waters

K. W. Duncan

By

Zoology Department, University of Canterbury. [Received by the Editor, January 19, 1968.]

Abstract

Analysis of 74 stomachs and 90 pellets of the black shag ( Phalacrocorax carbo) showed that, in Otago, brown trout ( Salmo trutta), perch ( Perea fluviatilis) and crayfish ( Paranephrops zealandicus ) were the main food items. Adult birds select small fish and eat approximately 300 to 400 g of fish at one time. The stomachs of nestlings are useless for quantitative food analysis. There is no evidence that the birds eat small invertebrates, any present in the stomachs having been released from the stomachs of fish eaten by shags.

Introduction

Since the introduction of freshwater game fishes a century ago, there has been much controversy over the effect of predation by black shags (Phalacrocorax carbo) on these fishes, especially the two trout species— Salmo trutta and S. gairdnerii. The factual basis of this controversy is limited as very few studies have been made of the shags' diet and none on the effect of its predation on trout.

Williams (1945) collected 2,833 analyses of shag stomach contents made by numerous individuals. This data showed that brown trout (S. trutta) was the main food species of shags in Otago inland waters. Eels were a very minor part of the diet. Stokell (1952) has criticised this work because Williams did not count the small invertebrates present in the stomachs.

Falla and Stokell (1945) analysed 62 stomachs taken from various localities at different times of the year. They found that trout and eels (Anguilla spp.) were the main fish species eaten. These results have often been extrapolated to situations not covered in their work. In particular they have been used to criticise Williams' conclusions.

Dickinson (1950) analysed 29 stomachs taken from the Rotorua-Taupo district during July. The native bully (Gobiomorphus sp.), carp (Carassius sp.) and crayfish were the main food items, while rainbow trout (Salmo gairdnerii; Dickinson's S. irideus) was very infrequent.

It is obvious that none of these studies meet the requirements for assessing the effects of predation outlined by Duncan (1967). They are either not extensive enough (in time or numbers) or do not provide essential information on the population dynamics of the prey.

The present work attempts to provide a partial analysis of the interactions of predator (shags) and prey (fish) for Lake Mahinerangi. Information on one of the prey species, perch {Perea fluviatilis) , has already been published (Duncan, 1967). Aspects of the trout population and the results of an experiment on the regulation of the fish populations by shags will be published on a future occasion.

Methods

The birds sampled were shot in pursuance of the Otago Acclimatisation Society's shag destruction policy.* Each bird was opened almost immediately after death; the oesophagus and duodenum were tied off and the stomach injected with 20cc of four percent formalin to both preserve it and to reduce post-mortem digestion.

During the breeding season the nestlings were used to lure the adults into gunshot range. In consequence they were starved for most of the day and so were useless for analysis.

The method of removing and counting the stomach contents was standardised as much as possible. The stomachs were slit lengthwise and the distribution and degree of digestion of the various food items was noted. Large, easily recognised pieces such as fish, were picked out, identified and measured. Particular note was taken of whether or not the contents of the fishes’ stomachs had been liberated into the lumen of the shag’s stomach. The remaining material was inspected under a low powered stereoscopic microscope. The identification of fish remains was made possible by comparing them with a reference collection of bones, otoliths and scales.

The degree of digestion of each food item was estimated using the digestion index outlined in Table I.

Regurgitated pellets from the Luella rookery at Lake Mahinerangi were examined to supplement the stomach analyses from this area. These pellets are the

indigestible remains of prey bound in a mucous envelope which are periodically regurgitated when the bird is at roost.

Stomach contents were recorded and assessed by counting each individual of each food species (Hartley, 1948). The live weights of ingested trout and perch were estimated indirectly by using the appropriate regressions of weight on length.

Results

1. Population census

Falla (1937) considered a population survey to be one of the prime requisites of any study of the shag, but lack of time and transport severely limited any census work during this study. An approximate survey of the Otago inland shag population was compiled for the year beginning October, 1959 (Table II), partly from personal observation and partly from Acclimatisation Society records.

Total Number of Nests* = 229. Estimated Adult Population (at 2/ nest)** = 458. Estimated Total Population (at 6/ nest) + = 1,374. Percentage Mortality of Young (chicks and nestlings) = 39% =+=. Percentage Mortality of Adults (breeding season) = 55%. Total Percentage Mortality Throughout Year = 71.8%.

2. Areas Sampled and Condition of the Stomachs

Great difficulty was experienced in obtaining large enough samples. The low numbers in the samples reflects the small population of shags in Otago due to the local Acclimatisation Society’s shag destruction policy during that period. Table 111 gives information on sampling places and dates. Many more rookeries were visited than those listed but most of these had been deserted.

Most of the adults' stomachs collected were full because the birds were shot flying either into their nests or into roosting spots (Table IV).

3. Stomach Analyses

A summary of the stomach analyses is presented in Table V and the raw data are given in the Appendix. The figures for caddis and molluscs must be treated with caution. Caddis cases fragment in the stomachs so that the numbers entered in the table represent particles, not necessarily whole cases. Molluscs tend to break up rapidly during digestion so the recorded numbers are probably an underestimate.

4. Size of Prey and Amount of Food Eaten

The fork lengths of perch and trout found in the stomachs are shown in Figure 1 as percentage frequency distributions. Also shown in this figure are the fork lengths of perch and trout taken by seining and angling from Lake Mahinerangi. The mean lengths (arithmetic) of perch and trout eaten by shags are 14.85 cm and 12.69 cm respectively.

The weight of food eaten at one time was estimated from the lengths of the fish found in the stomachs. Regressions of weight on length were established by least squares for trout and perch from Lake Mahinerangi and for trout from the Teviot River. Table VII shows the total weight of fish in those stomachs which had all of

Average number of organisms per stomach = 107.8. The ratio of invertebrates to fish is 20.7:1; excluding nestlings this ratio becomes 7.5:1.

the ingested food in a measureable condition. The average total weight of trout per stomach is 316 g while the greatest weight found was 880 g. The average weight of perch per stomach was 378 g.

5. Origin of the Small Invertebrates found in the Stomachs

The results of the arbitrary rating of the degree of digestion for each item found in the stomachs are shown in Table VIII.

No small invertebrates were found in the oesophagi or non-glandular region of the stomachs and they were usually associated with well-digested fishes.

These observations suggest that invertebrates are not eaten by the shags but come from the stomachs of ingested fishes. To investigate this further the following observations were made. A sample of 10 shags and 18 trout were taken at the same time of day from Lake Onslow on 18/12/1961 (Appendix No. 62 to 71). The stomach contents of both species were analysed and compared (Table IX). Four shags which showed evidence of having fed in the Teviot River (presence of Olinga,

Ephemeroptera, Helicopsyche, and Hydropsyche in their stomachs) have been excluded from the comparison. The number of trout present which had released their stomach contents into the shags 5 stomachs were totalled for the remaining six shags. The number of invertebrates present were then compared with the number expected due to release from the stomachs of the ingested trout —the “ expected number 55 being derived from the stomach analyses of the trout sample.

Table IX shows that the only really divergent item is Potamopyrgus; it being much less numerous in shags' stomachs than in trout. Statistical comparison of the other entries in Table IX shows that there is no significant difference between them (P ( x»> = 0.2).

A similar comparison for Lake Mahinerangi is presented in Table X, but here the comparison is less valid than that in Table IX because:

(1) Perch show a major diet change from plankton-feeding to benthos-feeding when they reach 11 to 13cm in length (Allen, 1935; Duncan, 1967) —a large number of the perch eaten by Shags are smaller than this. It was impossible to count the planktonic organisms in the shags' stomachs and so, because the data for the " perch " column are derived solely from benthos-feeders, the " expected number " of invertebrates is far higher than the observed.

(2) The samples were not taken at the same time.

(3) The means of perch lengths differed for the two samples. Perch taken by seining had a mean length of 19.8 cm while perch caught by shags had a mean length of 14.85 cm. Because of their smaller size the amount of food must be less in those perch caught by shags than in the perch taken by seining. However, Table X does show that the temporal occurrence of the different species of invertebrates in shags follows the same seasonal pattern as the diet of perch.

6. Pellet Analysis

The pellets consisted mainly of bones, scales and otoliths of trout and perch and ectoskeletons and gastroliths of Paranephrops (Scott and Duncan, 1967). Very few

invertebrate remains, other than Paranephrops, were. present. The results of the analyses are shown in Table XI, together with the stomach analyses of Mahinerangi shags.

The figures quoted for mean numbers per pellet in Table XI are unreliable as some of the pellets were disintegrating and only a part of these may have been collected.

Discussion and Conclusions

1. Population Census

The data presented in Table II should not be regarded as accurate as many approximations and assumptions have been made in its compilation. Furthermore, only about one half of the rookeries were visited by the author —the rest of the data coming from Acclimatisation Society records. It seems likely, however, that it is as reliable as those censuses recorded by Williams (1945). Comparison of his data with Table II shows that the population in 1959-60 was very much smaller than earlier.

Year Source Number of nests 1926-27 Williams (1945) 1,273 1936-37 „ 428 1942 „ 1,753 1959-60 present work 228

The decrease in numbers is the result of intensive shag destruction. In view of the high mortality it is probable that the inland population was not self-maintaining in 1959-60 but depended upon recruitment from maritime rookeries. These recruits may have been young birds following the spawning migrations of trout, eels and lampreys.

Since the change in the Acclimatisation Society's policy on black shags the population on Lake Mahinerangi has increased rapidly. This will be discussed in a future publication together with an assessment of the effect of shag predation on fishes.

2. Food of the Shag

The diet of shags, as shown in the present work, is very similar to that listed by Williams (1945) with trout being the most important food species in rivers and perch in lakes. Eels are an insignificant part of the diet of Otago inland shags. The author has seen only one eel taken by shags during this study —it was a half-digested 18 inch specimen found at the Luella rookery in 1964. A small number of crayfish are eaten as are native fishes.

The agreement of these results with those of Williams are also reasonable with respect to quantity of fish taken (4 to 7), and length of fish selected, so that the criticisms of Williams' results are discounted. However, the author contends that Williams' conclusions are not justified by his data because he has only shown that the black shag eats a certain number of prey species. This is not sufficient evidence to justify a value judgment as there are numerous cases in which predation is beneficial (Duncan, 1967).

The amount of food per stomach (Table VII) agrees fairly well with the values published by Mattingley (1927), Madsen and Sparck (1950), van Dobben (1952) and McNally (1957) being about 350 g per fishing period. Ward (1924), Collinge (1927) and Williams (1945), however, all consider that the shag takes between five and ten times as much per day. The maximum amount of fish present was 880 g which agrees well with Madsen's (1950) and van Dobben's (1952) estimate of about 770 g.

3. Origin of the Invertebrates Found in the Stomachs

There is no evidence that black shags eat small invertebrates in Otago. The observations leading to this conclusion are listed below.

1. No small invertebrates were found in the oesophagi or non-glandular region of the stomachs.

2. Any invertebrates found in the shags' stomachs were well digested. The results of rating the degree of digestion for each item (Table VIII) suggests that some degree of predigestion of invertebrates has occurred.

3. There is no significant difference between the number of invertebrates found in shags' stomachs and the number expected due to release from the stomachs of ingested fish.

4. The seasonal occurrence of invertebrates in the Mahinerangi birds follows the dietary pattern of perch. If shags were eating small invertebrates there would be no such correlation as their winter food pattern would be much the same as in summer. This is not so for perch as their habit of migrating to deeper water in winter causes a change in their diet —they can utilise the restricted (in species) bottom fauna of deeper waters only.

5. The stomachs of birds shot flying in to feed their young contained many fish but only a few invertebrates (e.g., Appendix No. 2) while those shot off the nest after a period of incubation or feeding their young have no whole fish present and numerous invertebrates (e.g., Appendix No. 1).

The high number of invertebrates present in nestlings (noted also by Falla and Stokell, 1945) suggests that young birds cannot rid themselves of undigested remains for some time or can only do so at infrequent intervals. The rare occurrence of recognisable fish remains in the stomachs of nestlings is not surprising in view of the way the adult feeds its young (Oliver, 1955: 214) and the selective shooting methods usually employed. The young are fed on predigested fishes from which the heads, containing all the easily recognisable parts, may have dropped off. So these recognisable parts are not likely to be transferred from parent to young. Amalgamation of the results from nestling stomachs with analyses of adult stomachs (as is done in Table V), will, therefore, give a biased picture by under-estimating the number of fish per stomach.

Appendix

Details of the stomach contents

Location—Shag Valley Rookery Date—27/10/1960 No. Collected—Two Adults Contents — (1) Adult Helicopsyche .. 50 Olinga feredayi 19 Pycnocentria 52 Coleoptera '■% 7 Unidentified insects 6 Stones 6 Peat -2 (2) Adult Salmo trutta 3 Hydropsyche 1 Pycnocentria 16 Coleoptera 3 Ameletus . 1 Adult Trichoptera 19

Ephemeroptera 7 Unidentified insects 4 Location—Lake Luella Date—l3/11/1960 No. Collected—Nine Adults, one Nestling Contents—(3) Adult Perea fluviatilis 1 Salmo trutta 1 Limnaea 1 Coleoptera 1 Hydropsyche 5 Olinga 11 Nematode 1 Otoliths 1 pr. (4) Adult Perea fluviatilis ...... 7 Pycnocentria 3 Olinga ...... 2

Adult Trichoptera ...„. ...... ...... 1 Nematode i ....; ...... 1 (5) Adult Perea, fluviatilis ...... ....... 4 Otoliths — „„•/ 5 prs. Total Perea ...... ..L ....... ...... 6 Pyronota ■ festiva . • i.:..;, „ , 1 Unidentified insects 3 (6) Adult Enjpty (7) Adult Salmo trutta ...... 1...':3 Paraneophrops \ 1 Olinga ...... ....... ...... 4 19 Pycnocentria •...... .....i ...... ...„. : 4 Pyronota ■. ' ...... 1 Unidentified insects ./... .:.f 3 ...... 1 (8) Adult Perea ■:]....... ....... ...... , .„... , .'. . 3 Pycnocentria V-It ...... 7.V."..r f 1 Olinga ...... 1 Unidentified insects ;..... / .’ 1 Otoliths ...... ....... 7 prs. (9) Adult Perea ...... .... .I'.. ...... 4 Otoliths—perch ...... ; ;..... ...... 14 prs. Total perch •; ’.;.... ■' • ■ 20 Unidentified fish ...... 1 Pycnocentria ...... ...... . ...... ’ L... ...... 3 Olinga ',;.J ...... ...... 4 Coleoptera ■ .:.... ...... " 1 Stick ■ , . ' • ■ : ...... •...... 1 (10) Nestling Pycnocentria ...... ...... ...... ............... 4 Trichoptera ...... ...... „.... ...... 4 Pine stems : '| ...... ;L../■ ...... ;• 20 Pieces of bark ' t ... 27 Nematode ...:.. • ; / 2 (11) Adult Perea g...... ■ ' t.v..; ; : ? 4 Ephemeroptera ...... ..... ..:... 1 Pycnocentria . ...... 15 Olinga ...... ...... ; ..;... .;.... 2 Sheep droppings , r.” >|! 2 Otolithsperch t.;...- :.... • > 5 prs. Total perch ...... 6 (12) Adult Perea ///../ ...... .;....• ...... ...... 2 Pyronota ...... ...... ...... ...... ...... 1 Olinga ..!... 3 Pycnocentria ....... ...... ...... ...... 5 Adult Trichoptera t;;/' : ...... ’ 2 Unidentified insects ...... ...... ...... 2 —Perea ...... ...... .....v 10 prs. , Total' Perea 11 LocationN.W. Corner, Lake Waihola , Date—6/12/1960 No. Collected—Four Adults Contents—(l3) Adult Perea ...... ! ...„ 1 Otoliths ...... 1 pr. Coleoptera ...... .• ...... 1 Unidentified insects ...... .- . 2 Quantity sand and mud

(14) Adult Salmo trutta - 1 Otoliths ...... kkiJkf:::.: 2 prs. Pycnocentria - ; ...... ; 55 Olinga - r j... : .....: ...... > 4 Pseudonema amabilis kk::.:.k\kkkk^y : ' 1 Corixid ...... ....... ...... .. ...... 1 Archichauliodes ....'* ...... 1 Pyronota estiva ...i.. ...... ...... ...... 1 Odontria • kk:'■rZlikX. 1 Odonata larva . .....7 - ...... 1 Planorbis '..... -ii.L', 1 Potamopyrgus ' Si ....:. ...... 15 Limnaea ' .....: •-r..V.fc;' 'k. 6 (15) Adult Perea 225! 2 Chironomid larva ...... ...... 6 Coleoptera fi;,. •‘ .1... ....;. 1 Simulid * Y .’ ...... 1 Unidentified ...... : 1 Otoliths Perea .....; ( 2 prs. Total Perea , ...... ...... ‘ ’ ...... ...... ' ...... 2 (16) Adult Salmo trutta ~ .•. » ...... 1. Perea .... ’. L... : k... .-... 1 Otoliths —Perea ..... 4 prs. Total Perea , T- •• •- ...... 5 Pycnocentria ...... ■ .Vi.;.'. J.; 25 Odontria : : 1 ■ Potamopyrgus ‘ 1 Location— River; one mile down from Wall 8-12/12/1960 , No. Collected—l 7 Adults* eight Flying Young, ten Nestlings Contents . (17) Flying Young Salmo trutta k.k. ..:.../...... k.k ...... ; 1 Olinga 23 Pseudonema amabilis '■k...:u 1 ...kk. 3 Pycnocentria i 69 Rhantus .... ..... ...... ...... 1 * Unidentified Coleoptera ...... ..../ ...... 3 Grass stem ...... ...... 10 Stones .; 33 Large quantity sandy grit Antiporus i. !.... ' .;.v.. ■' 2 (18) Adult Otolith (unidentified) ...... ' 1 Olinga ... ..,, 57 Pycnocentria ...... ......’ 74 Antiporus ':....k ...... I 3 Rhantus 1 Bronscini ( Mecodema ?) : 1...- ...... , 1 Unidentified Coleoptera ...... 1 Grass stems /..u, _ : v......' .. ...... 5 Stones ....;: ...... 66 Large quantity sandy grit (19) Flying Young , , Otoliths (Trout) ../7 2 prs. Olinga ...... ‘/...1 ...... 3 prs. Pycnocentria ...... .....; ....... ......,...... 40 Odontria .’ 1 Antiporus / kkk : r.\ ...... 1 ...». ‘ 2 Unidentified Coleoptera 3 Green capsules ...... ...... ...... ...... 3

Stones 24 Sandy grit (20) Adult Salmo trutta 1 Otoliths {Salmo) 1 pr. Olinga 10 Pycnocentria 23 Archichauliodes ; 1 Antiporus 1 Unidentified insects 3 Grass seed head Stones 19 (21) Adult Salmo trutta ...... 1 Otoliths (Salmo) 1 pr. Pycnocentria 191 Pseudonema amabilis • 17 Hydropsyche ...... 1 Potamopyrgus 11 Peat 1 Quantity of sand (22.) Nestling Olinga 38 Rhantus ...... 5 Broscini Beetle » 2 Antiporus 2 Potamopyrgus 1 Stones 29 Sand Plant Capsules 3 (23) Nestling Empty (24) Nestling Olinga . ;. 24 Pycnocentria 48 Rhantus 22 Stones 35 Quantity of sandy grit (25) Flying Young Salmo trutta 1 10 Unidentified Maxilla ( Gobiomorphus ?) 1 Stenoperla 1 Unidentified beetles 2 Adult Ephemeroptera 1 Parnid larva ...... 5 Hydropsyche 1 Pycnocentria 3 Diptera .-. 2 (26) Nestling Piece of GaCOs (Otolith) 1 Pycnocentria 246 Olinga .:. • ....’.. 181 Odontria ...... 2 Antiporus 1 Unidentified beetle 1 Floral bud of Sophora tetraptera 1 Stones 12 Small amount of sand Peat ...... 1 (27) Nestling Olinga 104 Pycnocentria 98 Antiporus , 1 Rhantus pulverosus 3 Pieces of wood ...... 2

(28) Adult Salmo trutta 6 Otoliths (Salmo) 4 prs. Pyronota festiva 199 Potamopyrgus .. 47 Odontria ... ...... 2 Wood borer beetle 6 Carabidae 1 Adult Ephemeroptera 1 Dytiscidae 1 Unidentified insects 4 Olinga 1 Pycnocentria ! 4 (29) Adult Salmo trutta -2 Otoliths (Salmo) 3 Pycnocentria 12 Pseudonema amabilis ;. .;.... 18 Potamopyrgus 4 Simulid larva 1 Stones (30) Adult Salmo trutta ...... ; 18 Otoliths (Salmo) 12 prs. Olinga 3 Pycnocentria ...... ' 14 Pseudonema .„... ...... „ 11 Tipulid larva 26 Potamopyrgus 2 (Nematodes . v 7 ) Stones ...... 2 (31) Adult Salmo trutta ...... 19 Unidentified fish 2 Paranephrops ...... ...... 1 Pycnocentria 70 Olinga „.... 3 Gorixid 3 Ghironomid larvae 15 Potamopyrgus - ..!... 5 Stones : 5 Small quantity of sand (32) Adult Galaxias sp 27 Pycnocentria ...... 17 Olinga 10 Potamopyrgus ...... ....:. 1 Gorixid ’* ...... 2 Unidentified insects ...... 5 Green plant matter (33) Adult Salmo trutta .!.... ...... 3 Otoliths (Salmo) ....;; 4 prs. Salmonid fish 1 Olinga 8 Pycnocentria 5 Ameletus ...... 3 Hydropsyche 2 Adult Trichopteran 1 Ant (with wings) 1 Antiporus 1 Rhantus ...... 1 Schist ...... ...... 3 Quartz 4 Shot 1 (Nematode ..«*• ...... 1)

(34) Flying Young Salmo trutta . . 22 Unidentified fish 5 Paranephrops 1 Olinga ...... ; , 8 Pycnocentria ...... .......C 43 Pseudonema .. ....... .;.J 16 Rhantus ...... 2 Antiporus f. ...... ...... ■ 1 Ant !fS®| ...... 1 Ghironomid adults ...... ...... < ...... 3 Parnid larvae •■..l' 1... ■ 2 Potamopyrgus 29 Odonata larva "'. • ...... 1 Sticks (Manuka and Matagouri) Grass stems ...... ...... ...... ...... 5 Mass of grass roots (Nematodes >..„ 2SI 1) Small amount of sand Otoliths {Salmo) ...... 3 prs. (35) Adult Salmo trutta ...... ’...!* 2 Otoliths 2 prs. Unidentified fish ...... ...;.. 1 Pseudonema ...... 5 Pycnocentria .............. ; ...... 4 Potamopyrgus ...... ' 8 Unidentified Trichoptera adult 1 Unidentified Trichoptera larva v;.:.;. 1 Small stones (36) Adult Salmo trutta ...... 3 Olinga ...... ...... 2 Pycnocentria . ....;. 6 Borer ...... ...... ...... 2 Dytiscid ...... ..1. ...... ...... ...... 1 Potamopyrgus 10 Peat ...... 2 Otoliths (Salmo) 1 pr. (37) Adult Salmo trutta 2 Otoliths (Salmo) 1 pr. Simulid adults ...... 12 Simulid larvae ...... 2 Pamid larvae i...... ...... 2 Droppings , ...... ......' 2 Stone , 1 Grass seed head (38) Flying Young Salmo trutta ...... 2 Unidentified fish ...... 2 Otoliths 5 prs. Pseudonema amabilis 31 P. obsoleta Pycnocentria ;. . 49 Olinga 9 Odontria ......;. 3 Rhantus 1 Parnid larvae —••• ,■: 2 Limnaea ; ...:.. 1 Potamopyrgus 24 Unidentified insects ...... 1 Sticks and grass stems . 6 Ceratophyllum ' . 2 pieces Droppings ' " 2 Stones 5 Small quantity sand

(39) Nestling Pycnocentria ...... ...... 199 Olinga ...... ,1.1 'prfV.f..' A 102 Oxythira albiceps :..... m ill ? l 3 Pyronota festiva ...... ; 1 Rhantus ...... 1 Hydora '..ll 1 . ...... 1 Odontria 2 Unidentified Coleoptera 1:': ....„ ; 3 Twigs ...... ...... ...... 4 Large amount grass roots (40) Nestling Salmo trutta ...... ...... ...... 2 Otoliths {Salmo) ....„ 2 prs. Pycnocentria 581 Olinga 11 ...... 228 Pseudonema amabilis ;; Ill'll 17 Odontria ...... ...... 111, ...... 7 Rhantus ...... ...... ’ ...1: 1 Pyronota festiva ,Ill'll:/ 11;', 6 Unidentified Coleoptera ,;H'. 15 Archichauliodes diversus ...... 1 Lepidoptera ...... 1 Arachnida .....: ...... ...... ...... 1 Potamopyrgus ...1 ...... ...... ...... 24 Nematodes ; ...... 6 Pieces of peat 2 Twigs (cases. of Pseudonema) 1.*.. 4 Grass stem and roots Stones ....... ...... .....; ■ 49 Sand (41) Nestling Pycnocentria ...... ...... ...... 681 Olinga ...... r/'1... 397 Pseudonema amabilis \ 1 Oxythira albiceps ...... 70 Odontria .;....• 4 Hydora ...... ...... ...... 9 Pyronota ...;.. j...... l::fi ...... 1 Unidentified Coleoptera ...... ;, 6 Rhantus ...... ...... ...... 1 Twigs 1 ...... ............ 1 Clump of roots Sand (42) Nestling Salmo trutta ...... 1.1" inf 1 Nerve cord of ? ; ...... 4 Pycnocentria ...... .-11 795 Olinga : : J ' 1.1. 'll. 305 Pseudonema amabilis 1 : 'll 6 Oxythira albiceps ...... v 1... 38 Odontria. ...... ...1 ...... 16 Pamids (Hydora 2) ...... < 5 Pyronota festiva ...... ...... 1 Unidentified Coleoptera ...... ...... 3 Grass stems ...... ....... ...... 1.;..... 3 Stones ....... ...... ...... 1.1..-i1.;.; 120 (43) Flying Young Pycnocentria ...... ...... ...... ,„.... 24 Dytiscid ...... ...„. ...... ...... Ill", 1 Quantity of coarse sand (44) Flying Young Pycnocentria ...... ...„. ..1.1;. ••••••• 17 Unidentified insect ...1 ...... 1.11.1 1 Potamogeton (1 piece) Few small stones

(45) Adult Salmo trutta , 2 Pycnocentria 46 Pseudonema amabilis ... 8 Oxythira albiceps 3 Amphipod 1 Unidentified insect 1 Potamopyrgus 5 Twig 1 Plant stem Small stones 5 (46) Adult Salmo trutta 1 Otoliths of above 1 pr. Potamopyrgus '.. 1 Odontria 25 (47) Adult Salmo trutta 2 Pycnocentria 27 Ameletus (?) 1 Coccinella 1 Potamopyrgus 2 Dipteran 1 Large amount of feathers (shag?) (48) Adult Salmo trutta ...... - K Otoliths (Salmo) 3 prs Total Salmo 5 Antiporus wakefieldi 37 Pycnocentria 24 Olinga 30 Helicopsyche 24 Pseudonema amabilis 58 Planorbis 1 Potamopyrgus 4 Limnaea 6 Chironomid #„.... 1 Unidentified insects 3 (Nematodes 6) (49) Nestling Pycnocentria ...... ...... 365 Olinga ! 39 Helicopsyche 2 Oxythira albiceps 44 Pseudonema amabilis 23 Twigs (cases of P. obsoleta?) 7 Archichauliodes diversus 1 Small amount of plant matter Rhantus 1 Hydora Feather fly 1 (50) Flying Young Salmo trutta .: 2 Pycnocentria 49 Olinga 10 Helicopsyche 10 Dytiscus ; 1 Hydora 2 Unidentified larva 1 Potamopyrgus ... t 12 (51) Adult Salmo trutta 3 Pycnocentria 79 Pseudonema amabilis 10 Odonata nymph 1 Rhantus 2 Dytiscidae 1

Ichneumonidae ...... ..Z / 1 Potamopyrgus ....;' 1 Potamogeton 1 Twig Location Luella Date—2l/5/1961 Contents—(s2) Adult Perea ...... ...... ...I.. 1 ...... 10 Otoliths Perea ...... 6 prs. Total Perea 13 Potampyrgus V ...... /.J.' ...... 1 Dytiscid 1 Twig (Triplectides case) S:: 1 (53) Adult Perea ...... i.:.. ...... 11 Otoliths Perea 10 prs. Total Perea 11 (54) Adult Perea ...... 3 Otoliths Perea jSSSI : 3 Total Perea , ....../ 3 (55) Adult Perea "... 1 ...... ;..... ■ 3 Otoliths Perea 3 Total Perea 4 (56) Adult Perea ' 5 Otoliths Perea .....: 5 Total Perea 5 (57) Adult Perea .. ......,: 2 Otoliths Perea ...... ...... 2 Total Perea ....'. 2 Pycnocentria ....;. ..... 2 Amphipod ; 1 (58) Adult Perea ...... ...... ...... 7 Otoliths Perea 5 prs. Total Perea ....;. . ..... 7 (59) Adult Perea 6 Otoliths Perea ( W 3 prs. Total Perea ' ! :. •' ; / 6 (60) Adult Perea r ... ;.../. | 9 Otoliths Perea ...... ...... ...... 6 Total Perea i 9 (61) Adult Perea ...... ...... 3 Otoliths Perea 9 Total Perea 10 Potamopyrgus ...... 1 Location—Lake Onslow Date—lß/12/1961 Contents—(62) Male Salmo trutta 7 Oxythira albiceps : .../ 5 Pseudonema amabilis 5 Olingaferedayi 6 Pycnocentria (evecta?) ' 1.;., ||%....?214 Pyronota festiva 1 Odontria sp ...... 1

Dytiscid 15 Aeschnid .„... 1 Moth larva 1 Moth adult ...... 1 Neuroptera (Archicauliodes sp.) 1 Otoliths Salmo 2 prs. Total Salmo trutta 7 Ranunculus tip (63) Female Salmo trutta ; 3 Olinga 1 Pycnocentria ; ...... 24 Pseudonema amabilis ...... 3 Dytiscid ...... 4 Odonata nymphs ....... 2 Ichneumon .....'. 1 Potamopyrgus ...... - 6 Paranephrops zealandicus 1 (64) Female Salmo trutta 1 Otoliths Salmo ...;. ....... 1 Total Salmo trutta ...... 1 Odonata nymphs 2 Odontria sp ...... 1 Potamopyrgus sp 24 Pseudonema amabilis 35 (65) Male Salmo trutta 2 Olinga feredayi ...... . v „. 25 Pycnocentria evecta 97 Oxythira albiceps ...... 2 Coleoptera .’.... ...... 6 Potamopyrgus sp ' ;,.... 20 Peat Nitella (66) Female Odontria sp. 2 Pycnocentria evecta ...... 5 Pseudonema obsoleta 2 Parnid ...... , .„... 1 Oxythira albiceps ...... ....... ■ 1 Twigs (Pseudonema cases?) 3 Nostoc (67) Female Salmo trutta ...... ■ 8 Unidentified fishes (trout?) 3 Pyronota {estiva 1. V 1 Caedicia olivacea ;L, ...... 1 Triplectides amabilis 6 Dytiscid : 1 Weevil 1 Neuroptera wings ....;. 2 prs.

Parnid ...... 2 Unidentified insects 5 (68) Male Salmo trutta ....„ 2 Otoliths Salmo - 3 Total Salmo trutta 3 Pycnocentria evecta 288 Olinga feredayi ....... 30 Helicopsyche sp 2 Hydropsyche sp 1 Archichauliodes ...... 1 Pyronota festiva ...... 6 Odontria sp 4 Odonata 2 Potamoprgus 10 (69) Male Salmo trutta 4 Otoliths Salmo 3 prs. Total Salmo trutta 4 Pyc7iocentria evecta 45 Dytiscid 4 Prenolepsis longicornis 1 Austrolperla cyrene 1 Plecoptera 2 Liodessus plicatus larva 1 Hemiptera i 1 Unidentified insects 12 (70) Female Salmo trutta ...... ...... 1 Unidentified fish 1 Dytiscid ‘ ...... 6 Potamopyrgus 4 Unidentified insect ..... ; 1 Odontria sp. ...... 1 Plecoptera * „.... 1 Pycnocentria evecta .... ; 3 (71) Adult Salmo trutta : 1 Location—Lake Luella Date—29/5/1962 Contents—(72) Adult Perea ...... 11 Chironomid heads present (73) Adult Perea ...... 8 Chironomid larvae heads present (74) Adult Perea 16 Chironomid larvae heads present

Acknowledgments

The author wishes to express thanks to the Council, Staff and Members of the Otago Acclimatisation Society for their financial and field assistance without which this study would not have been possible, Dr D. Scott for his encouragement and criticisms and Dr V. M. Stout for reading and criticising the manuscript.

Literature Cited

Boud, R.; Eldon, G. A., 1960. Investigation of large black shags in North Canterbury. Freshwater Fisheries Advisory Council (N.Z. Mar. Dept.), Investigation Dept., 21. (Unpublished.)

Collinge, W. E., 1927. the Food of Some British Wild Birds. 2nd. Ed., New York.

Dickinson, P., 1951. Stomach contents of New Zealand inland shags. Aust. J. mar. Freshwat. Res., 2(2): 245-253. Dobben, W. H. van, 1952. The food of the cormorant in the Netherlands. Ardea, 40: 1-63. Duncan, K. W., 1967. The food and population structure of perch (Perea fluviatilis L.) in Lake Mahinerangi. Trans. R. Soc. N.Z., Zool., 9(5): 45-52.

Falla, R. A., 1937. The distribution and feeding habits of freshwater cormorants in New Zealand. Rep. Australas. Ass. Advmt. Sci., XXIII, 136-137. (Abstract only.) Falla, R. A,; Stokell, G., 1945. Investigation of the stomach contents of New Zealand freshwater shags. Trans. R. Soc. N.Z., 74(4): 320-331.

Hartley, P. H. T., 1948. The assessment of the food of birds. Ibis, 90(3): 361-381.

Madsen, F. J.; Sparck, R., 1950, The feeding habits of the southern cormorant {Phalacrocorax carbo sinensis Shaw) in Denmark. Dan. Rev. Game Biol., 1(3): 45-75.

Mattingley, A. H. E., 1927. Cormorants in relation to fisheries. Condor, 29(4). McNally, J., 1957. The feeding habits of cormorants in Victoria. Fish. Contr. Viet., 6: 1-36. Oliver, W. R. 8., 1955. New Zealand Birds. A. H. and A. W. Reed, Wellington. (2nd. Ed.) Scott, D.; Duncan, K. W., 1967. The function of freshwater crayfish gastroliths and their occurrence in perch, trout and shag stomachs. N.Z. J. mar. Freshwat. Res., 1(2): 99-104.

Stokell, G.j 1952. The New Zealand freshwater research. N.Z. Fishing and Shooting Gaz., 19(9): 4-8. Ward, F., 1924. The fishing cormorant. Country Life, Nov., 1924. Williams, H. G., 1945. The Shag Menace. (Printers: Coulls Somerville and Wilkie, Dunedin.)

K. W. Duncan, Zoology Department, University of Canterbury, Christchurch, N.Z.

* This policy has now been revised and the bounty on shags removed (Otago Acclimatisation Society; Annual Report for 1965).

* Possibly inaccurate as it is often difficult to determine whether a nest has been occupied earlier in the season or if it has been deserted all season. Inaccuracies in this value will affect the percentage mortalities.

** Assuming no double-nesting or multiple mating.

+ Assuming an average clutch of four eggs.

== Obviously an underestimate as it is unlikely that young (or eggs) whose parents have been killed will survive. The true value will lie between this and 89 percent —the latter being derived from the number of nests " shot" times 4, divided by the total number of nests times 4.

* Fragmentation of the cases made numeration difficult—only posterior ends were counted.

** Molluscs appear to digest very quickly and leave no recognisable remains.

*** Excluding trout which had not released their stomach contents

* None of these were lake fish. The invertebrates present in the shags' stomachs were river dwellers (see Appendix No. 3 and 7), indicating that the shags had been feeding in the surrounding rivers. It is possible that the trout recorded in pellets were also river fish but there is no way of checking this point.

Index Fish Crustacea Trichop ter a Larvae Coleoptera Mollusca A Intact Intact Intact Intact , Intact Skin of “ Loose Loose “ Loose “ Loose B Skin of head digested “ Loose jointed ” “ Loose jointed ” “ Loose jointed ” “ Loose jointed ” Bones of Flesh Case Head, Shell head expartly firmthorax, hard— posed. - digested flesh abdomen flesh G Bones of head exposed. Stomach contents released into shag’s stomach Flesh partly digested Case firmflesh gone Head, thorax, abdomen separated 1 Shell hard— flesh gone Otoliths, Joints Case Elytra Shell D Otoliths, tail-pieces, bones only Joints separated Case soft Elytra intact Shell “ soft Otoliths, Fragments Fragments Fragments Fragments E Otoliths, bones only Fragments and gastroliths Fragments Fragments Fragments

Table I.—Rating system for the degree of digestion.

Location Number of . Nests* Number of Adults Killed . Number of Young Killed Number of Eggs Killed . Total Number Killed BIRDS SHOT AT ROOKERIES Deep Stream 1 1 2 0 4 6 2 4 2 3 12 17 3 4 3 0 16 19 Upper Waipori 1 11 0 33 0 33 River 2 15 18 14 18 50 Meggatburn 35 38 37 0 75 Shag Valley 20 12 20 0 32 Loganburn 14(?) 7 24 4 35 Mullocky 5 9 0 20 29 Bullock Creek 8 2 20 18 40 Pomahaka R. 1 2 3 0 5 Maclennan R. 50 100 25 0 125 Waikaia R. 12 10 29 14 53 Martins Greek 9 21 29 2 52 Manuherikia 15 25 . 6 4 35 Subtotals 204 251 • 243 112 606 BIRDS FROM ROOKERIES NOT SHOT Gatlins R. 14 Teviot R. 4 Luella 7 (Lake Mahinerangi) 7 jjaivu iviauiiici cuigi y Subtotal , 25 BIRDS SHOT AT ROOST OUT OF BREEDING SEASON ■ Totals 1 381 381381 381 Totals229 1 229632 632243 243112 112987 987

Table ll.—Census of Inland Black Shags from October, 1959, to June, 1960.

Date Locality Number of Stomachs Collected 27.X.1960 Shag Valley 2 13.XI.1960 Luella (Lake Mahinerangi) 10 6.XII.1960 Lake Waihola 4 8-12.XII.1960 Teviot R. 35 21.X.1961 Luella (Lake Mahinerangi) 10 18.XII.1961 Lake Onslow 10 29.V.1962 Luella (Lake Mahinerangi) 3 Subtotal 74 Number of Pellets Collected 21.XII.1962 Lake Mahinerangi 17 30.XI.1963 99 10 9.X. 1964 99 18 25.XI.1965 99 13 18.X.1966 99 11 17.X.1967 99 21 Subtotal 90

Table 111. —Sampling Sites and Number of Samples Collected.

Number of Number Number No. Containing Stomachs Empty Containing Inverterbrates Fish Only Adults 55 1 52 2 Flying young 8 0 6 2 Nestlings 11 1 3 7 Total 74 2 61 11

Table IV.—Condition of the Stomachs.

Species Total No. Found No. of Stomachs in Which Found Highest No. in One Average No. Per Total No. of Stomachs Average No. Per No. of Stomachs in Which Found Salmo trutta 151 37 22 2.0 4.1 Perea fluviatilis 173 24 20 2.3 7.2 Galaxias sp. 27 1 27 — — Unidentified fishes 21 10 5 0.3 2.1 Paranephrops zealandicus 4 4 1 0.5 1.0 Trichoptera larvae 6,820 56 1,244 92.2 12.2 Other aquatic insects 260 46 26 3.5 5.7 Terrestrial insects 241 39 40 3.3 6.2 Molluscs 280 24 47 3.8 11.5

Table V.—Analysis of the Important Elements found in the Stomachs of all Birds Sampled.

Species (Average- No. Per Bird) Shag R. Lake Waihola Teviot R. Adults Nestlings Lake Onslow Lake Mahinerangi Salmo trutta 1.5 0.5 4.4 0.6 3.0 0.13 Perea fluviatilis 0 2.5 0 0 0 6.73 Galaxias sp. 0 0 1.1 0 0 0 Paranephrops 0 0 0.08 0 0.1 0.05

Table VI. —Regional Variation in Diet.

Stomachs Containing Perch Stomachs Containing Trout Number of Fish Weight Number of Fish Weight Found Weight (g) Number of Fish Found Weight (g) 8 442 3 880 4 418 10 170 5 245 6 290 10 339 18 360 5 306 3 215 2 90 2 88 7 244 3 , 111 1 250 7 300 1 214 2 220 3 80 1 78 7 / U 258 3 385 Mean 378 316

Table Vll.—Weight of Food in Shag Stomachs.

Species A B G D E Salmo trutta 3 19 23 16 39 Perea fluviatilis 17 18 23 23 20 Paranephrops 25 25 50 Gaddis 1 39 60 . Goleoptera 2 29 67 Mollusca 32 68

Table Vlll.—Percentage Frequencies of the Index of Digestion for each of the Food Species.

Species Shags Trout Pseudonema 43 39.4 Odonata 3 4 Pycnocentria 31* 29.3 Dytiscidae 16 8.7 Other insects 22 18 Potamopyrgus** 28 94.6 Number of trout 12*** 12

Table IX.—Comparison of Invertebrates present in Shag and Trout Stomachs.

Species Shags Perch caddis 38 214 57 perch Pyronota festiva 2 37 from 10 Summer Water beetles 1 14 stomachs Odonata 5 64 Unidentified insects 10 2 fig perch Pycnocentria 2 0 from 13 Winter chironomids numerous 2,400 stomachs

Table X.-—Comparison of Invertebrates present in Shag and Perch Stomachs.

Year Season No. in Sample Mean No. Per Stomach or Pellet Stomachs Pellets Trout Perch Crayfish 1960-61 Breeding 19 0.21* 6.73 0.16 1962 Winter 3 0 11.6 0 1962 Breeding 17 0 1.24 0.12 1963 Breeding 10 0.2 7.1 • 0.3 1964.. Breeding 18 0.06 5.6 0.2 1965 Breeding 13 0 7.4 0.15 1966 Breeding 11 0.18 3.7 0.09 1967 Breeding 22 0.045 5.05 0.045

Table Xl.—Pellet and Stomach Analysis of Adult Shags from Lake Mahinerangi.