ICES C.M. 1995 , ,GM 1995/G:17Demersal Fish Committee

•

FEEDING HABITS OF JOHN DORY, Zeus faber (L1NNAEUS, 1758) OFF THEPORTUGUESE CONTINENTAL COAST (ICES DIV. D(A)

Alexandra Si/va .Institute P6rtugues de Investigac;äo Maritima

Av. Brasilia, 1400 Lisboa, Portugal

.' ,

ABSTRACT

The feeding habits of John Dory (Zeus (aber Linnaeus, 1758) were studied,based on the analysis of stomach contents of fishes sampled in five groundfishsurveys carried out off the Portuguese coast in different seasons, between 1990and 1992.

The rmiin aspects of feeding biology analysed in this paper are: dietary shiftswith the size of fish, seasonal and annual variations in the food camposition andin tl1e feeding intensity.

Multivariate methods were used to investigate ontogenetic diet shifts and nNOmain length groups were identified: 8.0-24.9 cm fishes, feeding mainly onCallionymus spp. and Gadiculus argenteus and those witl1 25.0-55.9 cm whosediet was mainly composed of Micromesistius poutassou and Macroramphosussc%pax. It was also observed a change from smaller prey species With a morepronounced benthic behaviour t6 larger sized pelagic prey species, suggestingthat John Dory changes to a more pelagic behaviour. throughout its development.Transitional length groups with a mixed food composition were observed. .

Seasonal and annual variations in the relative. imp0rtance of the main prey inJottn Dory 's diet were observed and were associated either with variations ofprey abundance in the".environment or witl1 variations in the depth distribution ofpredator and prey.

A consistemt seasonal and/or annual pattern of feeding iritEmsity was not found.

Key words: Zeus (aber, John Dary, feeCling habits, dietary shifts, feedingintensity, Portuguese waters

•

INTRODUCTION

The study cf feeding habits of fishes contributes to the knowledge of intra andinterspecific trophic relationships and thus to understand the structure and dynamics ofmarine communities. When commercially exploited species are involved, in the role ofpredators and/or as main prey species, the study of feeding habits of fishes are a basicstep to multispecies assessment approaches therefore, being important to the definitionof fishery management options.

The t:ophic status of a fish in the community may change during its development:ontogenetic diet shifts are a ccmmon feature, in the Iife histo,y of most fish species.Among piscivores the most weil documented. ontogenetic diet shift is. probably thetransition from invertebrate feeding as juveniles to fish feeding as adults (see Gibsclnand Ezzi, 1987; Winemiller, 1989; Smale, 1992). The terms "feeding stanzas"(Paloheimo and Dickie, 1965)and "threshold lengths" (Parker and Larkin, 1959) areoften used in the literature to refer respectively the size groups with common diets andthe length at which dietary changes are observed.

Some of the factörs which may determinate these shifts include changes in the mouthand gut morphology (e.g. Ross, 1978), seasonallity of prey in relation to the occurrenceof juvenile stages (e.g. Winemiller ei a/., 1989), intra· and interspecific competition (e.g.Robb and Hislop, 1980) and energy maximisation (s.g. Stergiou arid Fourtouni, 1991).Energy maximisation has been associated to John Doi"y's (Zeus (aber) shift from azooplankton and small benthic fish diet to a demersal fish diet at a threshold length of14 cm (Stergiou and Fourtouni, 1991).

The study of food habits and dietary shifts, also contributes to the knowledge ofphysiological and behavioural changes during fish development.

John Dory is a demersal fish occurririg off the Portuguese coast in moderateabundance (Silva, 1992a). It is exploited by the trawl arid artisanal fishing fleets andalthough annual landings seldom exceed 450 tons it has a high market pnce similar tothat of Hake (Merluccius merluccius) and of Monkfish (Lophius spp.) (Silva, 1992b).Knowledge on the biology and ecology of this species is scarce, apart from generalinformation given in textbooks. Franca ei a/. (1970) reported some results on thedistribution and population structure of Johr, Dory off the coast of Angola and Janssen(1979) provided some information on its population structure off the Dutch coast. Thereis some Iimited information on the abundance and distribution (Silva, 1992a) and on theexploitation of this species (Silva, 1992b) in the Portuguese waters.

The only study on the (eeding biology of John Dory was developed by Stergiou andFourtouni (1991) which analysed the feeding habits, ontogenetic diet shifts andselectivity of this species in the Eastem Mediterranean.

In this paper, data is presented on the food habits and on the dietary shifts cf JohnDory in the Portuguese waters as weil as some prelimiria,y information on temporalvariations in the food composition and feeding intensity of this species. In the study ofdietary shifts, a statistical methodology similar to that used by Stergiou and Fourtouni(1991) was followed in orderto make the results more comparable.

1

MATERIALAND METHODS

John Oory stomaehs were sampled during five groundfish surveys carried out off thePortuguese eontinental eoast on board ofthe research vessel "Noruega" from InstitutoPortugues de Investigac;äo Maritima (IPIMAR). Sampling dates were Oetober/Oeeember1990, Oetober/November 1991 and FebruarylMareh, July, Getober/November 1992. Inthe analysis of temporal variations in food eomposition, these surveys will bedesignated respeetively, Autumn 90, Autumn 91, Spring 92, Summer 92 and Autumn92. The general methodology followed in these surveys is deseribed in Anon. (1991).

In the 1990 SUrvey, stomaehs were eollected aeeording to a stratified sampling scheme,based on the definition of demersal fish eommunities in a previous study (E. Serräo,unpublished data). This resulted in a large number of stomaehs being .collected in onlya few sampling stations. In the other surveys, stomaehs were eollected from all the fishcaught.

In eaeh station, the fish were sorted by 1 em length c1asses and weighed. Afterwards,fish were proeessed individually: total length was reeorded and the stornach dissectedand immediately frozen at -18°C. Oata on sex and maturity stage was also eolleeted inthe 1992 surveys.-Inthe laboratory, the stomaehs were thawed, empty stomaehs werereeorded and for stomaehs eontaining food, their contents were biotted dry withabsorbent paper and total stomaeh weight was determined. Prey were identified to thelowest possible taxonomie level, eounted, weighed and measured (total length of fishand carapace length of decapod erustaeeans) whenever possible.

A total of 777 stomaehs were examined, 323 were empty (42%) and 454 eontainedfood.

To investigate dietary shifts with the size of fish, sampies from all surveys were pooled.This prevented a detailed study of seasonal ehanges in diet but was required to ensurean adequate sampie size. Fish were grouped in 17 length c1asses (codes L1 to L17)and the number of full stomaehs analysed per length elass ranged between 16 and 46(Table I).

From the full list 'of prey obtained (Table 11), some were grouped in higher taxonomielevels (e.g. Phylum, Class, Family or Genera levels) to inerease the chances of finding •similarities between the different predator length c1asses. Prey groups whieh oeeurredonly onee (Anthozoa, Paracentrotus lividus and Ammodytes sp.) were exeluded fromthe analysis. Unidentified fish remains were also exeluded from the analysis to preventlength groups being eonsidered similar.

14 different prey groups.were established (Table 11): molluses (MOL), erustaeea (CRU).fish larvae (LAR), Sardina pilchardus (SAR), Argentina sphyraena (ARG).Macrorhamphosus scolopax (MAC), Gadiculus argenteus (GAO), Micromesistiuspoutassou (MIC). Cepo/a macrophta/ma (CEP), Trachurus spp. (TRA), Boops boops(BOO), Callionymus spp. (CAL), Pleuroneetiforms (PLE) and Gobidae (GOB).

The number and the weight of each prey group in each of the 17 predator lengthelasses were eomputed and then normalised using a log(x+1) transformation.Oissimilarity matriees based on prey numbers and on prey weights were eomputedusing the Bray-Curtis index (Bray and CurUs. 1957).

2

•

•

A c1assification method (cluster analysis) and an ordination method (rTlUltidimensionalscaling, MDS) where applied to each matrix in order to identify groups with similar diets(Legendre and Legendre, 1979; Field et a/., 1982).

Cluster analysis was performed using tha UPGMA algorithm and an arbitrary level of60% dissimilarity was used to separate groups.

The number of dimensions for multidimensional scaling was determined by theexamination of the plot of stress versus number of dimensions and following theempirical decision method referred by Kruskal and Wish (1981). Three dimensions,giving a stress value of 0.07, were used for both prey numbers arid prey weightsanalysis. A PCA (principal components analysis) was run to obtain the initialconfiguration for MDS.

The use of multiple regression analysis is one of the possible ways to carry. on theinterpretation of MDS results (Kruskal and Wish, 1981); one (or more)charaeteristics ofthe ordinated objeets may be associated with their positions in the MDS configurationby regressing the values of that charaeteristic over the coordinates of the objeets.

This tachnique was not used in Ülis study because the coordinates of the lengthc1asses in the MDS configuration (in this.case the independent variables) did not meetthe normality assumptions of regression analysis. Consequently, anexploratoryapproach was followed: 5 variables - meaii prey number in stomachs (MPN), meanprey weight in stomachs (MPW), mean prey length in stomachs (MPL), diet breadth (B)and proportion of pelagic prey in the diet (PPNI PPW) - ware considered for theinterpretation of MDS results. Each series of length group coordinates in the MDSdimensions was plotted together with one of the variables; when a relationship wasapparent, a Spearman rank order correlation coefficient was computed to judge itssignificance. Spearman R's were estimated for the pairs DIM1-MPL, DIM1-PPN/PPW,DIM2-B. The mean prey weight in stomachs (MPW) did not show an obvious relationwith any of the variables but its logarithm followed c10sely the variation in Dimension Iscores. So, a correlation coefficient betWeen 10g(MPW) and Dimension I scores wascomputed.

Only flsh prey were included in the estimation of mean prey length in stomachs.Furthermore, Cepo/a macropthtalma and Ammodytes sp. were excluded from thisestimate due to their particularly elongate body morphology.

Diet breadth of the length groups was estimated using Levin's index (Leviris, 1968).

B = 1/L: p?

where Pi is the proportion of each prey group, in terms of numbers.

Prey were classified as pelagic or benthic (Table 11) accordingto the descriptions ofFischer et a/. (1981) and Whitehead et a/. (1984) andUia proportion of pelagic prey instomachs wascomputed both in terms of number (PPN) and iri terms of weight (PPW)after ttie logarithmic transformation.

Temporal variations in John Dory~s diet and teeding intensity were investigated tor thegroups identified by multivariate analysis.

3

The percentage of each prey, in number and weight, in each survey was compared withthe corresponding values estimated from the pooled surveys data. For prey groupswhose importance in the diet seemed to present temporal variations, an index ofabundance in the study area was estimated and the respective depth range distributioncalculated from data obtained in the same surveys.

The mean weight of stomach contEmts and the percentage of empty stomachs in eachsurvey were used as indices of feeding intensity.

RESULTS

Food composition and dietary shifts with the growth of fishTwo main groups of length classes were identified by cluster analysis (Figure 1a,b):Group A formed by 8.0 to 24.9 cm fishes (Iength classes L1 tci L6) and Group B formedby 25.0 to 55.9 cm fishes (Iength classes L7 to L17). These groups are Iinked at highdissimilarity values (about 80%) both on prey numbers (Figure 1a) and on prey weights(Figure 1b) analysis.

A similar picture 1s clear in two of the MDS bidimensional plots (Dimension I vs.Dimension 11 and Dimension I vs. Dimension 11I, Figure 2a,b): two main groups separatealong the direction of Dimension I axis both on prey numbers (Figure 2a) and on preyweights analysis (Figure 2b). Exactly the same cluster, identified as Group A in thedendrograms, is formed. On the other hand, some of the elements of cluster analysisGroup B occupy now a transiticinal position. In the prey numbers analysis, fishes with25.0-30.9 cin and 35.0-36.9 cm (Iength classes L7 to L9 and L12) are located betweenthe main groups (Figure 2a). These length classes are also Iinked in a sub-group,branching trom Group B in the prey numbers dendrogram. In the prey weights analysis,only L7 and L8 (fish with 25.0-28.9) appear as transitional length classes in theDimension I direction. This axis thus represents the gradient of the predator length.

In the plot of Dimension I vs. Dimension 11 from prey weights analysis (Figure 2b),another group, formed by length classes L10 (31.0-32.9 cm) and L16 (44.0-46.9 cm),separates from main Group B but in the direction of Dimension 11 axis. In the preyweights dendrogram, these two length classes together with class L8 form a sub-groupwhich links with the other elements of Group B at 50% dissimilarity level. •

Plots of the length classes scores in Dimension 11 vs. Dimension 11I axis (not presented)show a large dispersion and no clear group separation.

Table 11I presents the pefcentage in number (%N) and in weight (%W) of the prey in thediet of the two predator groups identified trom the pooled surveys data. Callionymusspp. (Dragonets) and Gadiculus argenteus (Silvery pout) represent about 50% of GroupA prey, both in numbers and in weight. In a number basis, Crustacea, Gobidae andPleuronectiforms are also important food items. Crustacea become relatively lessimportant in terms of weight while Cepola macrophtalma (Redband fish) acquires someimportance but is represented by only two individuals.

For Group 8 fishes, Micromesistius poutassou (Blue whiting) and Macroramphosusscolopax (Snipefish) are the most important prey in a number basis (37.1% and 25.8%

4

•

•

respeetively). In terms of weight, Snipefish is replaced by Sardina pilchardus (Sardine).Trachurus spp. (Horse mackerel and Jack mackerei) and Argentina sphyraena(Argentine) are also moderately important prey for John Dory of this length group.

Table IV shows de Spearman Rank Order Correlations between the scores in the MDSfinal configurations and the tested explanatory variables.

Scores in Dimension I are significantly correlated with the proportion of pelagic prey instomaclis both in terms of numbers (PPN) arid weights (PPW),. indicating that JohnDory ct!anges from a diet dominated by benthic prey to a diet dominated by pelagicprey. This change is not gradual but rather abrupt from length class 23.0-24.9 cm(PPN=0.09, PPW=0.10) to length cJass 25.0-26.9 cm (PPN=0.61, PPW=0.76). WithinGroup A fish the proportion of pelagic prey (which corresponds mainly to Silvery pout)increases up to length class 19.0-20.9 cm (PPN=0.48, PPW==0.46) and theri declinesuntil length class 23.0-24.9 cm. This variable presents a contimious increase withlength in Group 8 fish attaining values around 0.90.

Dimension I is also correlated with the mean prey length (MPL) and with the logtransformed mean prey weight (MPW) in stomachs (Table IV). 80th these variablespresent a slightly increasing trend with length within each of the groups and a steepincrease between them. This reßeets the rapid charige from a diet dominated by smaJlsized and lighter fish like Dragonets to adiet domiriated by larger sized and heavier fist!Iike 81ue whiting.

The index of prey.diversity estimated for each length class (8) shows a large variabilitywith no cJear trend with the predator length. Within Group A fish, it presentsan increasefrom length class 8.0-12.9 cm (8=2.42) to length class 19.0-20.9 cm (8=5.38) and thena decrease untillength class 23.0-24.9 cm (8=2.49). The largest diet breadth values inGroup 8 are observed in lengths 25.0-30.9 cm (8 varies betWeeri 5.30 and 6.41). Fishgreater than 31 cm present low prey diversity indices (around 2.0). The diet breadthindex showed a variation simiJar to the length class coordinates in Dimension 11 axis butthis apparent agreement is not statisticaJly significant at the 5% level.

The mean prey number in stomachs (MPN) presents a narrow range of variation withthe predator length: from a minimum of 1.1 prey/stomach in 8.0-12.9 cm fish to amaximum of 2.6 prey/stomach in fish with 44.0-46.9 cm. It shows, however a generalincreasing trend with the predator length. No relationship was noticed between themean prey numbers and any of the MDS dimensions.

Scores in dimension' 111 could not be associated with any of the variables c6nsidered forMDS interpretation.

Seasonal and annual variations in food compositionThe percentage of each prey group in the diet of the predator groups identified bymultivariate analysis is presented in Figure 3a (number basis) and in Figure 3b (weightbasis). For Group A fish, Dragonets are considerably more important in Summer andSpring than in Autumn. Crustacea present the opposite pattern although less clearly onEi weight basis. Silvery pout is particularly important in the Autumn 91 survey and showsa very low percentage in the Spring 92 survey.

5

Blue whiting, identified as the main Group B prey trom pooled data, disappears in theSummer and in the Autumn 92 surveys, being replaced by Sardine and Snipefistr inSummer and exclusively by the latter in Autumn.

Variations in the dietary importance ot Dragonets and SiI'/ery pout tor Group A JohnDory seem to be associated both with their absolute and with their relative abundance.The estimated index of abundance of Dragonets is generally higher in Summer arid inAutumn and lower in Spring (Table V), what is in agreement with the findings of Safran(1990) for the French coast. Thus, the greater proportion of Dragonets in John Dory·sdiet in Summer seems to reflect their availability in that season. In spite of their lowabundance in Spring, Dragonets are also very important in John Dory's diet in thisseason what may be a consequence of the decrease in the relative abundance ofSilvery pout. This decrease seems to be mainly a consequence of the non-overlappingdepth distributions of Silvery pout and John Dory. According to Albert (1993), Silverypout exhibits seasonal variations in depth distribution oCGurring in deeper waters inWinter, shallower waters in Summer and even shallower in Autumn off the NOrWegiancoast. With the exception of the Autumn 92 survey, the depth distribution of Silverypout in the Portuguese waters is also shallower in the Autumn and deeper in the Winter •(taking into account that the Spring 92 survey took place in FebruarylMarch, Le. lateWinter). When Silvery pout and John Dory occur at the same depths, the percentage ofthis prey in the diet also reflects its abundance.

The proportion of Blue whiting and Snipefish in Group B John Dory's diet is not limitedby their depth distributions since in both cases a large overlap is observed in allsurveys. For Blue whiting, a gradual abundance decrease is observed from Autumn 90to Autumn 92 surveY. Its importance in John Dory's diet also decreases gradually untilSpring 92 and suddenly drops to zero in the Summer and Autumn of the same year.The abundance of Snipefish shows an almost 10 fold increase in these two surveyswhich is reflected in its dietary importance particularly in the Autumn survey.

Feeding intensityThe mean weight of stornach contents is, in general, 10 times higher in Group B fishthan in Group A fish (Table VI) showing higher values in Autumn, intermediate values inSummer and lower values in Spring for both groups. Preliminary results indicate thatJohn Dory spawns in late Winter and early Spring, off the Portuguese coast (Silva, •1992a; Silva, unpublished results). The lower feeding level in the Spring 92 survey is inagreement with the generalised observation of a decrease in feeding intensity duringgonad development and spawning in fish (Hoar, 1969).

On the other hand, the variation in the percentage of empty stomachs gives no supportto this hypothesis since neither a seasonal trend nor a consistent pattern between thepredator groups is observed (Table VI).

DISCUSSION

The results of this study indicate that John Dory undergoes a major dietary changethrough its growth, around a length of 25 cm. From adiet characterised by small fishspecies Iike Dragonets, Silvery pout, Gobidae and crustacean species (mainlyPandalidae), it changes gradually to a diet dominated by larger fish spedes Iike Bluewhiting and Sardine. Snipefish, although a relatively small fish, is also an important prey

6

•

•

for larger John Doiy, in numelical temis. Probably, this is more a consequEmce of thegreat abundarice of this species in the Portuguese waters than the reflex of a true foodpreference.

Stergiou and Fourtouni (1991) studied the food habits a:1d ontogenetic diet shifts ofJohn Dory in the eastern Mediterranean. Based on a similar range of fish lengths, theyidentified a single dietary shift around 14 cm, corresponding to the transition from a dietdominated by Mysids, Callionymus maculatus and Denteltosteus quadrimaculatus to adiet dominated by Cepola macrophta/ma, Spicara flexuosa arid Serranus hepatus. Theyfurther divided the smaller length group in a zooplanktophagous stage (5.1-8.0 cm fish)and in a transitional stage to piscivori (8.1·14.0 cm fish). In a sampie of 9 individuals inthe length range 2.7-11.7 cm from the West Coast of Scotland , Gibsan and Ezzi(1987) also found Mysids ar'ld small henthic fish as the main John Dory prey.

Our results did not show any stage with a dominance of zooplanktonic prey (only 3Euphausiaceae where observed in a fish with 18 cm) what may be due to the smallnumber of stomachs sampled from the lower lerigth classes (only 6 stomaehs from fish8.0-9.9 cm).

The feeding spectra of John Dory <14 cm in the Eastern Mediterranean (Stergiou andFourtouni, 1991) closely resembles that of individuals up to 25 cm analysed in thisstudy, Le. main prey species correspond to small benthic fish and crustacea. The samegenera and species are even ohserved in the tWo areas. On the other hand, the diet oflarger fish is very different in the two areas: while in the Eastern Meditemineari JohnDory changes to ci diet characterised by larger demersal fish among which Cepolamacrophtalma is largely dominant, in the Portuguese waters we observe a shift tolarger pelagic fish where Blue whiting dominates.

The classification of prey as benthic or pelagic followed here represents a simplificationand imposes a rather rigid division in a behavioural trait which is seldom very clear. InfaCt, some Pandalid crUstacear'lS, usually classified as benthic in generill teXtbooks areknown to have strong swimming activitY occupying the whole water column adjacent tothe sea f100r (e.g. see Dauvin et a/., 1994); Silvery pout, ger'lerally designated a pelagicfish is also found close to the sea f100r (Priede et a/., 1994). Even in the case of Bluewhiting, Sardine and Snipefish, considered typical pelagic species, ttie classification isnot fully descriptive of their behaviour. Both Blue whiting and Sardine exhibit verticalmigrations forming dense schools close to the bottom in some circumstances (Anon.,1989; Bailey, 1982). The Snipefish is caught in large quantities by bottom trawl(Cardador et a/., 1994) what indicates that it also occurs in the lower water layers.

Nevertheless, this s1mplified c1assification enables the eXtraction of a general pattern ofJohn Dory behaviour t./lrough its groWth, whicti suggests that youriger individualsdepend mainly on bottom-dwelling or benthopelagic prey, living close to the bottom andfrom 25 crn onwards they hegin to explore a wider range of the water column, preyingmainly on schooling pelagic species. In a study of trophic interactions in fishcommunities of South Africa, Smale (1992) refers that numerous demersal fish speciesexhibit vertical migrations to exploit pelagic prey. He inchJded in this category asouthern relative of John Dory, the Cape Dory Zeus capensis.

The observed threshold lerigth (25 cm) does not seem to correspond to any importantchange in John Dory's mouth gape or overall body morph610gy (Stergiou and

7

Fourtouni, 1991). On the other hand, it is coincident with the beginning of the firstreproductive development in this species; data from the same sampies which providedthe stomachs analysed in this study show that sexually active individuals have lengthsgreater than 24 cm although only beyond a length of 36 cm the numbers of pre- andpost-spawning fish are significative (fish in spawning condition are always scarce)(Silva, unpublished results).

The energy expenditure of the maturative process is widely studied in the context ofannual reproductive cycles of fish (Hoar, 1969; Love, 1970). The first maturationpossibly represents the beginning of a Iife history stage involving much higher energeticdema:lds which may request a considerable change in the food habits. John Dory'sdietary shift doesn't involve an increase in the number of prey consumed, so greaterenergetic needs seem to be satisfied by the ingestion of larger and heavier prey. Thesharp increase in the mean weight of John Dory stornach contents observed with thedietary shift supports the idea of a disproportionate increase in energetic needs.

The process of sexual maturation and spawning has also been associated to anontogenetic diet change in Cod (paz et a/., 1993).

The reason why John Dory shifts to a pelagic species diet instead of starting to feed onlarger benthicldemersal species like in the Mediterranean remains unsolved and furtherinformation on prey selection could give some insight to this question. It is possible,however, that the availability and accessibility of certain species in the environmentlargely determinates their importance in the diet of John Dory: the main pelagic speciesconsumed by mature John Dory (Blue whiting, Sardine and Snipefish) are veryabundant in the Portuguese waters and usually form dense sctlools, a behaviour knownto increase significantly the benefiUcost ratio of prey consumption. The change to a dietdominated by schooling prey mayaiso contribute to the observed feedingspecialisation.

An opportunistic behaviour also seems to emerge from the analysis of temporalchanges in John Dory's diet. The observed seasonal and annual variations in theimportance of the main prey for mature and immature John Dory seem to be mainlydetermined by their availability in the environment.

•

Predation of John Dory, on the stocks of other commercial fish species is potentially •important in the case of Blue Whiting. The individuals found in John Dory stomachs aremostly immatures (Iength < 18 cm), indicating that some effect on this component ofBlue Whiting population could be verified. However, the relatively low abundance ofJohn Dory off the portuguese coast, in comparison to that of other Blue Whitingpredators like Hake" (Merluccius merluccius) (H. Cabral, unpublished data), suggeststhat this effect is probably negligible in relative terms.

Within the various species which co-occur with John Dory in the Portuguese waters,competition seems more likely to happen with Hake: the diets of the two speciesoverlap in a large extent (Guichet, 1995; H. Cabral, unpublished data) and Silvery poutand Blue whiting are respectively important prey for younger and older individuals ofboth species.

Nevertheless, the feeding habits of other fish predators Iike Monkfish, Dogfish and raysin the Portuguese waters have not been studied.

8

•

Further studies, with more detailed information namelyon the variations of John Doryfood habits with depth, area and season, are needed to investigate interspecificinteractions Iike predation and competition.

9

•

Length c/asses Mean length No. full stomaehsCode Length range (ern) (ern)

L1 8.0 - 12.9 11.0 28L2 13.0 - 16.9 15.1 26L3 17.0 - 18.9 17.6 46L4 19.0 - 20.9 19.8 38L5 21.0 - 22.9 21.9 28L6 23.0 - 24.9 23.9 31L7 25.0 - 26.9 26.0 26L8 27.0 - 28.9 27.5 16L9 29.0 - 30.9 29.8 31

L10 31.0 - 32.9 31.9 21L11 33.0 - 34.9 34.0 24L12 35.0 - 36.9 35.9 30L13 37.0 - 38.9 36.8 22L14 39.0 - 40.9 39.5 22L15 41.0 - 43.9 42.4 28L16 44.0 - 46.9 44.7 17L17 45.0 - 55.9 50.8 21

Table I - Length classes, mean John Dory length and number of full stomaehs ineach length class •

10

Prey group PreyAnthozoaParacentrotus /ividusSepia sp.

MOL Sepia e/egansLOLIGINIDAEAI/otheutis subu/ata

Benthic/Pelagic

88PP

Number Weight (g)o 1.01 0.41 1.52 5.42 0.21 3.2

CRU

!.AR

SAR

ARG

MAC

GAD

MIC

CEP

TRA

800

CAL

PLE

G08

CrustaceaEuphausiaceaPANDALIDAECARIDAEPanda/ina sp.Panda/ina brevirostrisP/esionika sp.Plesionika heterocarpusParapanda/us sp.

Fish

IFish larvae

ISardina pi/chardus

IATgentina sphyraena

IMacroramphosus scolopax

IGadiculus aTgenteus

IMicromesistius poutassou

ICepo/a macrophta/ma

ITrachurus trachurusTrachurus picturatus

IBoops boops

Ammodytes sp.

Cal/ionymus sp.Callionymus IyraCal/ionymus macu/atusCa/lionymus reticu/atus

PleuronectiformsLepidorhombus sp.LepidOrhombus whiffiagonisCitharus macro/epidotusAmoglossus sp.

GOBIDAEGobius nigerPomatoschistus minutusDenteltosteus quadrimaculatus

Unidentified remains

B

8B88888

P

P

P

P

P

P

8

PP

8

BBBB

B8888

8BBB

435o263

112

10

2

20

22

63

47

89

8

121

2

2

1728

122

11111

10

32

101

o

1.70.03.01.50.12.21.3

21.41.6

915.6

0.1

944.8

276.5

312.4

145.3

1727.4

83.8

329.093.0

86.0

14.1

11.949.1

8.240.6

39.33.5

10.838.667.5

18.82.1

10.33.3

47.1

•

Table 11 - Number and weight of prey identified on John Dory stomachs,c1assification of prey as benthic or pelagic and grouping of prey for theanalysis of dietary shifts.

11

•

Group A= 8.0-24.9 cm Group B= 25.0-55.9 cmPrey group %N %W %N %W

MOL 2.3 0.5 0.8 0.2CRU 17.6 6.1 2.1 0.2LAR 1.1 0.0 0.0 0.0SAR 0.6 5.4 7.9 23.5ARG 2.8 6.4 7.1 6.4MAC 0.6 0.2 25.8 7.9GAO 23.9 28.0 2.1 0.7MIC 0.0 0.0 37.1 44.0CEP 1.1 10.7 2.5 1.0TRA 0.0 0.0 5.4 10.7BOO 0.0 0.0 0.8 2.2CAL 34.7 22.3 2.9 0.4PLE 6.3 12.1 5.4 2.8GOB 9.1 8.3 0.0 0.0

Table 111 - Percentage of prey in number (%N) and weight (%W) in the length groupsidentified by multivariate analysis.

Analysis Pairs Spearman R p-Ievel

OlM 1- PPN 0.93 < 0.001Prey numbers OlM 1- MPL 0.80 < 0.001

OlM I - LOG(MPW) 0.85 < 0.001OlM 11 -B 0.13 0.619OlM I-PPW 0.90 < 0.001

Prey weights OIMI-MPL 0.79 < 0.001OlM 1- LOG(MPW) 0.84 < 0.001OlM 11 - B 0.45 0.058

Table IV - Spearman rank correlations between the MDS configuration scores andthe explanatory variables. PPN - proportion of pelagic prey in numbers;PPW - proportion of pelagic prey in weight: MPL - mean prey length;MPW - mean prey weight; B - diet breadth.

12

CAL

Group A: 8.0 - 24.9 cm

GAD

15-224 40-567 34-518 41-320 38-302 10Q-590 125-546 298-558 106-511 225-550

34-300 60-208 73-244 41-355 56-242 34-300 60-208 73-244 41-355 56-242

% Prey in the diet

Prey abundance in the environment(mean numberlhour)

Prey depth range distribution (m)

John Dory's depth range distribution (m)

Aut 90 Aut 91 Spr 92

6.9 12.5 58.9

3.8 1.9 1.6

Sum92 Aut92

61.5 15.4

3.4 6.7

Aut 90 Aut 91 Spr 92 Sum 92 Aut 92

24.1 58.3 2.7 Z3.1 15.4

6.2 8.7 0.9 1.8 0.2

% Prey in the diet

Prey abundance in the environment(mean numberlhour)

Prey depth range distribution (m)

John Dory's depth range distribution (m)

Group B: 25.0 - 55.9 cm

MIC MAC

Aut90 Aut91 Spr92 Sum92 Aut92 Aut90 Aut91 Spr92 Sum92 Aut92

56.8 52.2 37.3 0 0 20.4 20.3 17.3 27.3 62.5

3700 2700 1800 1500 1400 ooסס1 42000 12000 98COO 1CXXXXJ

68-708 50-700 86-706 65-558 63-680 73-265 85-241 76-255 44-244 70-242 e39-242 50-240 54-244 43-355 46-180 39-242 50-240 54-244 43-355 46-180

Table V - Percentage prey in the diett index of abundance in the environment anddepth range distribution of the main pray of group A and group B JohnDory in each sUlVey. Depth range distribution of John Dory in eachsUlVey.

Survey Mean 'vVeight of % empty stomachs

stornach contents (g)

Autumn 90 2.6 44GroupA Autumn 91 1.1 52

Spring 92 0.4 32 •Summer 92 1.5 32Autumn 92 2.2 30Autumn 90 21.1 52

Group B . Autumn 91 12.5 42Spring 92 6.7 43SU'!lmer 92 10.5 32Autumn 92 17.2 23

Table VI - Feeding intensity indexes of Group A and Group B John Dory in eachsurvey.

13

lilC1l

"'Co~(J)C1l(J)(J)(1l

U~-ClCC1l

...I

L1L4

L2

L5

L6

L3

L7

L9

L12

L8

L10L11

L16L15L17

L13L14

(a) Prey numbers

I,I I

I I

I

I I, II

~,

I >-I

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Bray-Curtis dissimilarity index

eightsj

(b) PreywI

I II

I ~

ltI--

i

I

I

L1L2

L5

L6

L3

L4

L7L9

L12

L13

L11

L14

L15L17

L8

UOL16

lilC1l

"'Co~(J)C1l(J)(J)(1l

u~-ClCC1l

...I

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Bray-Curtis dissimilarity index

Figure 1 - Dendrograms tor prey numbers (a) and prey weights (b) analysis.

14

(a)

06

OS

l.8.l.8• L12.

-1i 1i~ l.9.- ~~ .E ECi Ci L7

-{J'.

L7 L9.-<l6 •-{JS

-1 -<l6-12 -<lS -<l.4 0 0.4 OS 1.2 ·12 -<lS -<l.4 0 04 os 12

Dmer1SIQ'l1 Dimer1SlQ'l1

(b)

OS

06

04 L1.•

02L1/~1 L15

5 L13 •0;c:'"ECi .<l2

-{J4

-{J6

-<lS-16 -12 -<lS -<l4 0 04 OS 12

Dimension I

1.2

O.S

0.4

-c0...c"Ei5 -<l4

-<lS

·1216 -16 -12

L.6•

l.3•

-<l4

Dimension I

0.4 O.S 12 16

Figure 2 - Bidimensional plots ot MDS final configuration tor prey numbers (a) and preyweights (b) analysis. •

15

•

%N

70

60 A •0 •SO c

40

x30

201iiI

0 x• •10 • • 0• • C AIl i8 0

0 •0 0 11

LAR GOB MOL CAL CRU GAD PLE CEP ARG SAR MIC MAC TRA BOO

A Prey groups

.POOled surveys c Autumn 90 • Autumn 91 0 Spring 92 A Summer 92 • Autumn 92 x Pooled surveys c Autumn 90 • Autumn 91 0 Spring 92 A Summer 92 • Aulumn 92

"ioN

70 r-------------------------,

Prey groups

c

11R

•x

cg

••X

o

8 lio ..-__-II-~a___I_ _lAI__lI__lII_---:;.~_i:I___I---c-_-_iix

LAR GOB MOL CAL CRU GAD PLE CEP ARG SAR MIC MAC TRA BOO

(b)

"IoW-

70

60

50

40

30

20

10A0

•c

•

Q

~A •

0 A0 A

C • iAI0 g i • tS )(

II! 11 X ~ D • " !

GOB MOL CAL CRU GAD PLE CEP ARG SAR MIC MAC TRA BOO

Prey groups

40

20

10

30

60

so

X Pooled surveys c Aulumn 90 • Autumn 91 0 Spring 92 A Summer 92 • Autumn 92 X Pooled surveys C Autumn 90 • Autumn 91 0 Spring 92 A Summer 92 • Autumn 92

Figure 3 - Percentage ot prey groups in numbers (a) and weight (b) in each survey and torpooled surveys data.

16

•

•

REFERENCES

Albert, O.T., 1993. Distribution, population strueture and diet of silvery pout (Gadiculus argenteusthori J. SCHMIDT), poor cod (Trisopterus minutus L.), four-bearded rockling (Rhinonemuscimbrius L.) and Vahl's eelpout (Lycodes vahlii gracilis REINHARDT) in the Norwegian Deep.Sarsia, 78(2): 141-154.

Anon., 1989. Report of the Planning Group for Acoustic Surveys in ICES Sub-areas VIII and IX.ICES C.M. 19891H:6, 12 p.

Anon., 1991. Report of the Study Group on the Coordination of Bottom TralM surveys in sub­areas VI, VII, VIII and division IXa. ICES C.M. 19911G:13, 33 p.

Bailey, RS., 1982. The population biology of Blue Whiting in the North Atlantic. Adv. Mar. BioI.,19: 257-335.

Bray, J.R; Curtis, J.T., 1957. An ordination of the upland forest communities of southernWisconsin. Eco/ogical Monographs, 27: 325-349.

Cardador, F; Silva, A; Pestana, G.; Borges, F.; Azevedo, M.; Martins, M.; Caramelo, A, 1994.Distribui«äo e abundäncia de peixes na costa Portuguesa. Poster apresentado no 1° Simposiosobre a Margem Continentallberica Atläntica, Lisboa, 28-30 Novembro 1994.

Dauvin, J.-C.; Iglesias, A; Lorgere, J.-C., 1994. Circalittoral suprabenthic coarse and sandcommunity from the western English Channel. J. Mar. Biol. Ass. V.K., 74(3): 543-562.

Field, J.G., Clark, K.R & War"Mck, RM., 1982. A practical strategy for analysing multispeciesdistribution patterns. Marine Eco/ogy-Pragress series, 8: 37-52.

Fischer, W.; Bianchi, G.; Scott, W.B. (Eds.). 1981. Fiches FAO d'identification des especes pourles besoins de la peche. At/antique centre-est. Vois. 1-7, pag. var.

Franca, M.P.L.; Costa, F.C.; Franca, P., 1970. Contribui«äo para 0 conhecimento da biologia dosZEIDAE de Angola. Notas Centra Bio/. aquat. trop., 22, 55 p.

Gibson, RR; Ezzi, I.A., 1987. Feeding relationships of a demersal fish assemblage on the Westcoast of Scotland. J. Fish BioI., 31: 55-69.

Ghichet, R, 1995. The diet of European Hake (Merluccius merluccius) in the northern part of theBay of Biscay. ICES J. mar. Sei., 52: 21-31.

Hoar, W.S., 1969. Reproduction. In: Hoar, W.S.; RandalI, D.J. (Eds.), Fish Physiology, Vol. 11I,Academic Press, London, p. 1-72.

Janssen, G.M., 1979. The occurrence of Zeus taber (Linnaeus, 1758) in the coastal waters ofThe Netherlands. BuH. Zoo/. Mus., 6(20): 153-158.

Kruskal, J.B.; Wish, M., 1978. Multidimensional scaling. Sage University Paper senes onQuantitative Applications in the Social Sciences, 07-011. Beverly Hills and London, SagePublications.

Legendre, L.; Legendre, P., 1979. Eco/ogie numerique. Tome 2. La structure des donneeseco/ogiques. Collection d' Ecologie, n° 13, Masson, Paris et Les Presses Universitaires duQuebec, 254 p.

17

Levins, R, 1968. Evolution in Changing Environments: Some Theoretical Explorations.Princeton, N.J., Princeton University Press.

Love, RM., 1970. The chemieal biology ot fishes. Academic Press, Landon, 547 p.

Paloheimo, J.E.; Dickie, L.M., 1965. Food and growlh of fishes. J. Fish. Res. Bd. Can., 22: 521­542.

Parker, RR; Larkin, P.A, 1959. A concept of growlh in fish. J. Fish. Res. Bd. Can., 16: 721-745.

Paz, J.; Casas, J.M.; Perez-Gfmdaras, G., 1993. The feeding of Cod (Gadus morhua) onFlemish Cap, 1989-90. NAFO Sei. Coune. studies, 19: 41-50.

Priede, I.G.; Bagley, P.M.; Smith, A; Creasey, S.; Merrett, N.R, 1994. Scavenging deepdemersal fishes of the Porcupine seabight, North-east Atlantic: observations by baited camera,trap and trawL J. Mar. Biol. Ass. U.K., 74(3): 481-498.

Robb, AP.; Hislop, J.RG., 1980. The food of five gadoid species duting the pelagic Q-Groupphase in the North Sea. J. Fish. BioI., 16: 199-217.

Ross, S.T., 1978. Trophic ontogeny of the leopard searobin (Prionotus scitulus ) Pisces: _TRIGLlDAE). Fish. Bull., 76(1): 225-234.

Safran, P., 1990. Spatio-temporal variability in the structure of a nectobenthic fish nursery: adescriptive study. Oeean. Acta, 13(1): 97-106.

Silva, A., 1992a. Distribution and abundance of John Dory (Zeus taber, Linnaeus 1758) in thePortuguese continental waters (ICES Div. Ixa) during 1986-1991. ICES C.M. 19921G:68, 33 p.

Silva, A, 1992b. Contribuic;äo para 0 conhecimento da pesca de Peixe-galo (Zeus taber,Linnaeus 1758) na costa continental Portuguesa. Relat. Tee. Cient. INIP, 56, 19 p.

Smale, M.J., 1992. Predotory fish and their prey - an overview of trophic interactions in the fishcommunities of the West and South-West coasts of South Africa. In: Payne, AI.L.; Brink, KH.;Mann, KH.; Hilbam, R (Eds.), Benguela Trophic Functioning, S. Afr. J. mar. Sei., 12: 803­821.

Stergiou, KI.; Fourtouni, H., 1991. Food habits, ontogenetic diet shift and selectivity in Zeustaber Linnaeus, 1758. J. Fish BioI., 39: 589-603.

Whitehead, P.J.P.; Bauchat, M.L.; Hureau, J.-C.; Nielsen, J.; Tartanese, E. (Eds.). 1984. Fishesofthe North-Eastem At/antie andthe Mediterranean. Vois. 1-111, UNESCO, Paris, 1473 p.

Winemiller, 0., 1989. Ontogenetic diet shifts and resource partitioning among piscivorous fishesin the Venezuelan Idahnos. Env. Biol. Fishes, 26: 177-199.

18

•

'.