Feeding habits of the Pacific bearded brotula Brotula clarkae Hubbs, 1944 (Ophidiidae) along the Pacific coast of Costa Rica,Central America

The present study analyzed the diet composition, ontogenetic shifts and dietary overlap of Brotula clarkae in relation to stage of maturity and sex. Samples were collected from the trawling fishery along the Pacific coast of Costa Rica (2011–2012) at depths ranging from 41.4 to 168.3 m; however, over 80% of the sampled fish were obtained at depths between 50 and 75 m. Size ranged from 14.4 to 98.4 cm total length. Of the 323 analyzed stomachs, 44.3% were from males, 86% were from immature individuals, and 49.8% had at least one prey item. According to the prey‐specific index of relative importance (P_SIRI), decapod shrimps were the most important prey (57.6% P_SIRIi) followed by teleosts (28.2% P_SIRIi), stomatopods (10.8% P_SIRIi), and crabs (3.3% P_SIRIi). Male and female B. clarkae exhibited a high dietary overlap (C_H = 0.94). Immature B. clarkae consumed primarily shrimps and crabs (71.5% of stomachs from immature specimens contained shrimps, which accounted for over 66.0% P_SIRIi); mature individuals consumed a large proportion of teleosts and stomatopods, which together contributed to over 91.0% P_SIRIi. Both immature and mature B. clarkae overlapped spatially with the commercial trawling fishery grounds along the Pacific coast of Costa Rica. However, juveniles feed predominantly on shrimps, suggesting that immature B. clarkae may be subjected to high fishing pressure as by‐catch, making them particularly vulnerable to overexploitation.     

Feeding habits of ocellate spot skate,Okamejei kenojei (Müller & Henle, 1841), in coastal waters of Taean,Korea

The feeding habits of Okamejei kenojei were studied using 592 specimens collected in the coastal waters of Taean, Korea from April 2008 to March 2009. O. kenojei is a bottom‐feeding carnivore that consumes mainly shrimp, fishes, and crabs. Its diet also includes small quantities of amphipods, mysids, cephalopods, euphausiids, copepods, isopods, and polychaetes. The total length (TL) of individuals in this study ranged from 8.2 to 49.0 cm. Cluster analysis based on %IRI (index of relative importance) identified three size classes. Group A (< 20 cm TL) ate primarily caridean shrimp and amphipods; group B (20–30 cm TL) ate exclusively shrimp; and group C (> 30 cm TL) ate penaeoidean shrimp, fishes, and crabs. O. kenojei showed ontogenetic changes in feeding habits. Although shrimps were the primary food consumed by all size groups, the proportion of shrimp in the total diet decreased and the consumption of fishes and crabs gradually increased with the body size of O. kenojei. Size of the prey organisms also increased. Smaller individuals fed mainly on small prey, such as amphipods, mysids, and small shrimp, whereas larger individuals preferred larger prey, such as larger shrimp, fishes, and crabs. The size‐related diet breadth and the percentage of empty stomachs were significant; the diet breadth gradually increased with body size, whereas the percentage of empty stomachs decreased. Seasonal changes in the O. kenojei diet were not significant, but shrimp constituted 97.3% of the summer diet by %IRI. Seasonal changes in diet breadth and the percentage of empty stomachs were not significant.     

Catch composition,reproductive biology and diet of the bramble shark Echinorhinus brucus (Squaliformes: Echinorhinidae) from the south‐eastern Arabian Sea

Fishery and biological data are presented for the poorly known bramble shark Echinorhinus brucus (Squaliformes: Echinorhinidae), from the deep waters of the south‐eastern Arabian Sea. A total of 5318 individuals from by‐catch landings of deep‐water bottom set longlines, gillnets and shrimp trawl fisheries operating at depths of 200–1200 m were recorded between January 2008 and December 2011 at the Kochi Fisheries Harbour (Kerala). A total of 431 individuals, from 46 to 318 cm total length (L_T) and 0·8 to 132 kg total mass (M_T), were examined to determine biological data for this species. The L_T at which 50% were mature (L_T50) for females and males was estimated at 189 and 187 cm L_T. Litter size ranged from 10 to 36 and size at birth was between 42 and 46 cm L_T. Dietary analysis of stomach contents revealed E. brucus feeds on a variety of prey including crustaceans (69% index of relative importance, I_RI), teleosts (25·8% I_RI), cephalopods (1·7% I_RI) and elasmobranchs (0·7% I_RI). This study provides the first detailed biological data for this species and also highlights the extent of the by‐catch fishery for this species in Indian waters.     

Diet and feeding strategy of blackmouth catshark Galeus melastomus

Diet and feeding strategy of the blackmouth catshark Galeus melastomus in the deep waters of the eastern Ionian Sea were investigated. Sampling was carried out using experimental bottom longline fishing at depths ranging from 300 to 855 m in summer and autumn 2010. Diet variability with fish size, season, area, sex and depth zone was tested and only season was found to significantly affect the diet of the species. Of the 870 stomachs examined, only 1·4% were empty. Cumulative prey curves showed that the sample sizes were adequate to describe the main prey items of the diet for both seasons. Prey identified belonged primarily to three major groups: fishes, cephalopods and dendrobranchiatan and caridean shrimps. In autumn, the above three major groups were found as principal prey, whereas in summer cephalopods followed by fishes were the principal prey and shrimps were found as secondary prey. SIMPER analysis indicated high dissimilarity between seasons and highlighted that Sepiolidae, Myctophidae, fishes, cephalopods, shrimps and other crustaceans contributed to seasonal differences in the diet of G. melastomus. The prey diversity index was higher in autumn than in summer. A high dietary overlap was observed between the two seasons. Galeus melastomus behaved as an opportunistic predator with a variety of unimportant prey in its diet. Feeding strategy indicated that G. melastomus holds a generalist niche with a considerable specialization at the individual level. This strategy seems to be an adaptation to a food‐scarce environment, as typified in deep‐water habitats.     

Feeding habits of the speckled guitarfish Rhinobatos glaucostigma (Elasmobranchii: Rhinobatidae)

The stomachs of 464 speckled guitarfish Rhinobatos glaucostigma were sampled from the south‐eastern Gulf of California (GC) to determine diet composition. Numerical indices and prey‐specific index of relative importance (%I_PSIR) were used to determine the feeding strategy of the species. An analysis of similarity (ANOSIM) was used to determine differences in diet with respect to sex, season (dry or rainy) and maturity stages (immature or mature). The diversity and niche breadth (by sex, season and maturity) and a general trophic level were determined. The overall diet was dominated by shrimps (%I_PSIR = 43·47), amphipods (%I_PSIR = 18·89) and crabs (%I_PSIR = 18·07). ANOSIM demonstrated differences in the diet by maturity and season, but not by sex. Rainy and dry season diets were dominated by shrimps and amphipods, respectively. Immature specimens fed mainly on amphipods, whereas mature fish preferred shrimps and crabs. Rhinobatos glaucostigma showed a narrow niche breadth with an intermediary trophic level (T_L = 3·72) and can be considered as a secondary consumer in the soft‐bottom demersal community of the south‐east GC. Understanding the feeding habits and trophic level of R. glaucostigma is vital to help identify the segments of the population vulnerable to overfishing by artisanal and industrial fisheries, and to aid in conservation and management of this elasmobranch.     

Biological observations on the bristly catshark Bythaelurus hispidus from deep waters off the south‐west coast of India

Biological data are presented for the poorly known bristly catshark Bythaelurus hispidus based on specimens collected from the by‐catch of the commercial deep‐sea shrimp trawl fishery operating in the Arabian Sea at depths of 200–500 m off the south‐west coast of India. One hundred and sixty‐two individuals, which ranged from 120 to 366 mm total length (L_T), were collected for this study. Size‐at‐maturity (L₅₀) for females and males was estimated at 252 and 235 mm L_T, respectively. The reproductive mode of B. hispidus was aplacental viviparity, which is the rarest reproductive mode within the Scyliorhinidae and is considered to be the most advanced of the three reproductive modes occurring within this family. Dietary analysis of stomach contents revealed B. hispidus feeds on a variety of prey, primarily fishes.     

Optimization of foraging and diet by the piscivorous Othos dentex (Serranidae)

The aim of this study was to determine the dietary characteristics and mouth morphology of Othos dentex and to use these data, together with in situ observations of feeding behaviour, to elucidate how foraging and diet are optimized by this piscivorous serranid. Seasonal spear and line fishing over reefs in south‐western Australia yielded 426 O. dentex (total length, L_T, 183–605 mm), among which the stomachs of 95 contained food. The food in the stomachs of 76 fish was sufficiently undigested to be seen to contain, almost invariably, a single fish prey, which was typically identifiable to family and often to species. The prey of O. dentex, which were measured (L_T), represented 10 families, of which the Labridae and Pempheridae constituted nearly two‐thirds of the prey volume. Two‐way crossed analysis of similarities of volumetric data for stomach contents showed that the dietary compositions of the different length classes of O. dentex in the various seasons were significantly related to length class of prey, but not to prey family, length class within the various prey families or season. Furthermore, an inverse (Q‐mode) analysis, including one‐way analysis of similarities, showed that the patterns in the prey consumed by the different length classes of O. dentex in the various seasons were related more strongly to length class than prey family. The former trend is exemplified in a shade plot, by a marked diagonality of the length classes of prey with increasing predator size. The ingestion of typically a single teleost prey, whose body size increases as that of O. dentex increases, reduces the frequency required for seeking prey, thus saving energy and reducing the potential for intraspecific competition for food. The ability of O. dentex to ingest large prey is facilitated by its possession of a very large gape, prominent recurved teeth, dorsal and independently‐moveable eyes, cryptic colouration and effective ambush behaviour. Othos dentex has thus evolved very cost‐effective mechanisms for optimizing its foraging and diet.     

Fish by‐catch in shrimp beam trawls in the northern South China Sea

The by‐catch of shrimp beam trawl fisheries in the northern South China Sea were estimated during the 2005–2006 and 2012–2013 fishing seasons. A total of 98 hauls from three important shrimp fishing grounds were used in the analyses. A total of 119 by‐catch species were captured, compared with only 15 target species. The ratio of by‐catch to shrimp ranged from 1.01 to 13.90. The dominant standard length (SL) range of fish by‐catch was 51–100 mm. The percentage of juveniles in the total catch of some fish species was high (e.g. 100% for A. aneus).     

Changes in the diet of Nile perch,Lates niloticus (L), in the Mwanza Gulf,Lake Victoria

Samples of Nile perch (Lates niloticus L.) were collected for stomach analysis from trawl catches conducted in the Mwanza Gulf from September 1986 to September 1988. Initially haplochromine cichlids formed the main food item for the Nile perch. Despite their decline, haplochromines still formed the major part of its diet in 1986. After the virtually complete disappearance of the haplochromines in 1987 and 1988, the benthic shrimp Caridina nilotica, the pelagic cyprinid, Rastrineobola argentea and juvenile Nile perch became the main food of Nile perch. Twenty-four trawl sessions were conducted to determine the relative quantities of respective prey taken. When present, Caridina is the major prey, mainly taken by day. If absent, juvenile Nile perch constitute the main part of the diet. Seasonal differences in diet composition probably reflect seasonal fluctuations in the abundance of the main prey species.     

Modelling gastric evacuation in gadoids feeding on crustaceans

A mechanistic, prey surface‐dependent model was expanded to describe the course and rate of gastric evacuation in predatory fishes feeding on crustacean prey with robust exoskeletons. This was accomplished by adding a layer of higher resistance to the digestive processes outside the inner softer parts of a prey cylinder abstraction and splitting up the prey evacuation into two stages: an initial stage where the exoskeleton is cracked and a second where the prey remains are digested and evacuated. The model was parameterized for crustaceans with different levels of armour fed to Atlantic cod Gadus morhua or whiting Merlangius merlangus and recovered from the stomachs at different post‐prandial times. The prey species were krill Meganyctiphanes norvegica; shrimps and prawns Crangon crangon, Pandalus borealis, Pandalus montagui and Eualus macilentus; crabs Liocarcinus depurator and Chionoecetes opilio. In accordance with the apparent intraspecific isometric relationship between exoskeleton mass and total body mass, the model described stage duration and rate of evacuation of the crustacean prey independently of meal and prey sizes. The duration of the first stage increased (0–33 h) and the evacuation rate of both stages decreased (by a half) with increasing level of the crustacean armament in terms of chitin and ash. A common, interspecific parameterization of the model within each of the categories krill, shrimp and crab can probably be used if the contents of chitin and ash are similar among prey species per prey category. The model offers a simple way for estimating evacuation rates from stomach content data in order to obtain food consumption rates of wild fishes, provided that information about digestion stage of crustacean prey is available.     

Feeding dynamics and respiration of the bottom-dwelling caridean shrimp Nauticaris marionis Bate, 1888 (Crustacea: Decapoda) in the vicinity of Marion Island (Southern Ocean)

The feeding dynamics and oxygen uptake of the bottom-dwelling caridean shrimp Nauticaris marionis were studied during the April/May 1984, 1996 and 1997 cruises to Marion Island (Prince Edward Islands, Southern Ocean). N. marionis is thought to have an opportunistic feeding mode. Prey composition varied considerably between the years and sites investigated. Overall, benthic (mainly hydrozoans and bottom-dwelling polychaetes) and, at times, pelagic (largely euphausiids and copepods) prey items dominated in the stomachs of N. marionis both by occurrence and by volume. Generally, pelagic prey contributed more to the diets of smaller shrimps, while benthic prey was a more important component in the guts of larger specimens. Wet, dry and ash-free dry weight were determined for specimens used in respiration experiments. The respiration rates of N. marionis females with carapace length 6.6–11.1 mm ranged from 80 to 250 μl O₂ individual⁻¹ · h⁻¹, or from 0.588 to 2.756 μl O₂ · mg⁻¹ dry weight h⁻¹. Regression analyses showed highly significant correlations between oxygen consumption and carapace length for N. marionis. Daily ingestion rates estimated using an in situ gut content analysis technique (4.4% of body dry weight) and an energy budget approach (average 4.7% of body dry weight, range 2.0–7.5%) showed good agreement with each other. Accepted: 29 July 1998     

Ontogenetic dietary shifts and feeding ecology of the rasptail skate Raja velezi and the brown smoothhound shark Mustelus henlei along the Pacific coast of Costa Rica,Central America

Stomachs from 511 Raja velezi and 340 Mustelus henlei captured as by‐catch in the commercial trawling fishery (2010–2012) were analysed to examine diet composition, ontogenetic shifts and degree of dietary overlap between species life stages in the Pacific Ocean of Costa Rica. Shrimps were the most important prey categories in the diet of R. velezi, while teleosts and cephalopods dominated the diet of M. henlei. Diet comparisons between different stages of R. velezi and M. henlei revealed clear ontogenetic dietary shifts: crustaceans (mainly shrimps, crabs and stomatopods) dominated the diet of immature individuals, and adults had a higher proportion of teleosts. The results suggest that R. velezi is an epibenthic predator that specializes in shrimps during early life stages, and to a lesser extent, teleosts as it matures, while M. henlei is an opportunistic predator with a highly diverse diet consisting of teleosts, cephalopods, shrimps and stomatopods. This study also found little evidence of dietary overlap between species or life stages and suggests that intra‐ and interspecific competition between R. velezi and M. henlei may be reduced by: (1) diet specialization in immature stages of R. velezi, (2) ontogenetic dietary shifts between immature and mature individuals, (3) prey‐size selectivity in larger individuals of R. velezi and (4) differences in depth utilization in overlapping geographical regions.     

Diet and feeding strategy of smooth butterfly ray Gymnura micrura in northeastern Brazil

The purpose of the present study was to analyze the diet of the smooth butterfly ray Gymnura micrura using qualitative and quantitative approaches to describe the feeding spectrum as well as ecological aspects related to feeding and niche width. The rays were obtained through monthly sampling as by‐catch from bottom trawl fisheries in northeastern Brazil from August 2007 to July 2008. A total of 286 stomachs were sampled, of which 176 contained food items. G. micrura is a predator in the region that feeds intermittently on large whole prey items, followed by a long period of digestion with simply little or no feeding activity. A low diversity of food items was found in the G. micrura diet, with an absolute predominance of teleosts, especially the roughneck grunt Pomadasys corvinaeformis. It is suggested that the species has a narrow feeding niche and a high degree of specialization. This presumed high degree of feeding specialization is discussed and the findings are compared in light of similar studies for closely related species, allowing to propose the family as primarily constituting piscivorous rays.     

An assessment of bycatch reduction devices in Kuwait's shrimp trawl fishery

Three bycatch reduction devices (BRDs), i.e. the Turtle Excluder Device (TED), Square Mesh Codend (SMC) and Fisheye (FE), were tested to evaluate their feasibility in the Kuwait shrimp‐trawl fishery. All three BRDs could be fitted to a shrimp trawl and normal fishing activities could be conducted both on an industrial trawler and an artisanal dhow boat. The comparisons of the catch from a net equipped with a BRD on one side and a standard net (exact same net configuration but without a BRD device) on the other side of a double‐rigged shrimp trawler (steel boat) showed different results in the three BRD types. The net fitted with a TED performed well without capturing any turtles or large animals, and did not significantly reduce shrimp catch or fish bycatch (124.4: 117.0 and 852.0: 905.8, both P > 0.05). Although the SMC reduced both shrimp catch and bycatch in general, the Wilcoxon's signed rank test showed a significant difference only among bycatch (1034.2: 1147.3, P < 0.05). Chi‐square tests showed that a net fitted with SMC caught significantly larger shrimp (more large and fewer small sizes) than a standard net for all three major shrimp species (33.5: 32.5; 26.9: 25.0; 20.3: 19.3, all P < 0.01). This indicates that the SMC reduced the catch of small size shrimps as well as smaller size fish. A net fitted with a fisheye significantly reduced both the shrimp catch and bycatch (137.4: 174.6; 997.5: 1377.8, both P < 0.01); the shrimp loss was probably due to an inappropriate location of the FE in the trawl.     

The vertical distribution of fish in the open water area of a deep temperate mesotrophic lake assessed by hydroacoustics and midwater trawling

The fish stock of a deep temperate, mesotrophic lake was sampled at different depths using a fixed‐frame fry trawl, during two nights in mid‐September 2009. Additionally, horizontal and vertical hydroacoustics were used simultaneously to evaluate fish abundance and biomass estimates obtained by the trawl. Roach Rutilus rutilus and smelt Osmerus eperlanus were the dominant species of young‐of‐the‐year (YOY) fish in the trawl catches from the surface layers (0–9 m). Bleak Alburnus alburnus dominated the catch of older fish in the upper part of the surface profile (0–6 m). Around the thermocline (9–13 m) smelt dominated the catches of both the YOY and older fish. Beneath the thermocline (13–36 m) vendace Coregonus albula dominated the catch of YOY fish, and smelt was the only species of older fish in the trawl catches. Species composition, abundance and biomass of the YOY and older fish were heterogeneous throughout the depth profiles of the lake, but only abundance differed significantly between the layers. The hydroacoustics gave relatively similar estimates of abundance and biomass to those obtained by the trawl in all the depths sampled. Our results indicate that there is a clear separation of small fish of different species along the vertical profile of a deep temperate lake during the night, and an unequal vertical distribution of fish abundance and biomass. The similarity of the trawl and hydroacoustics estimates of abundances and biomass indicated that the trawl sampling did not cause important avoidance reactions of small fish during the night in this deep temperate lake (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of sea temperature and initial marine feeding on survival of Atlantic salmon Salmo salar post-smolts from the Rivers Orkla and Hals, Norway

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid‐norway (1 sea‐winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post‐smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post‐smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R_F) nor other prey groups in post‐smolt stomachs in the two fjord systems. In the Altafjord, the post‐smolts fed mainly on pelagic fish larva (70–98%) and had a stable R_F (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R_F (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post‐smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post‐smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post‐smolt phase.     

Size selectivity of three diamond mesh codends for the European hake (Merluccius merluccius) and the tub gurnard (Trigla lucerna) in the Sea of Marmara, Turkey

The effects of an increase in the codend mesh size from 44 to 56 and 80 mm on size selectivity of the two by‐catch species (the European hake Merluccius merlucius and the tub gurnard Trigla lucerna) commonly captured in the shrimp fishery in the Sea of Marmara, were evaluated. The parameters of the logistic selectivity model were estimated by maximum likelihood. Length at 50% retention, L₅₀, was found to increase with mesh size for both species. The L₅₀ values of 44, 56 and 80 mm were, respectively, 19.8, 21.5 and 33.4 cm for European hake, 17.9, 17.7 and 25.6 cm for tub gurnard. The main conclusions are that 44 mm and 56 mm diamond mesh codends retain a considerable amount of undersized hake and tub gurnard as by‐catch in the shrimp fishery. With 80 mm mesh size codend, most juvenile European hake are released. For the tub gurnard, however, even this mesh size does not provide a good enough selection. From the biological point of view, it is concluded that the legal 44 mm diamond mesh codend does not provide satisfactory selection for European hake and the tub‐gurnard in trawl fisheries of the Sea of Marmara.     

The ecological significance of the zooplanktivores, snipefish Macroramphosus spp. and boarfish Capros aper, in the food web of the south-east North Atlantic

The diets of slender snipefish Macroramphosus gracilis, longspine snipefish Macroramphosus scolopax and boarfish Capros aper, three very abundant species on the Portuguese coast, were studied from samples collected between July 2002 and April 2003. Variations in the diet with fish size, season and area, as well as diet overlap and diversity, are explored in this study. The diets of slender snipefish and boarfish were mainly composed of copepods, with the main prey being Temora spp. and Calanus spp., respectively. Mysid shrimps were the most important food item in the diet of longspine snipefish. During the summer season, when the availability of different prey items was highest, the two species of snipefish and the boarfish fed on different prey. Temora spp. were the most important prey in the stomachs of slender snipefish in the summer on the south‐west coast, while Calanus spp. started appearing in the stomachs of boarfish in the spring in the north, increasing their abundance in the summer on the south and south‐west coasts. The abundance of mysids appeared to increase in the autumn on the south‐west coast, being at that time an important food item for both longspine snipefish and boarfish. For slender snipefish and boarfish, the differences in stomach species diversity were explained firstly by the season and then by the area and fish size. For longspine snipefish, the area did not explain the species diversity variability, season being the first variable determining the differences. Of all three species, slender snipefish was the one with highest diversity of stomach contents, particularly in spring and summer on the north and south‐west coasts. Diet overlap between species was very low. No predation on eggs and larval stages of fishes was found for any of these fish species. This work is the first to address the diets of snipefish species and boarfish in the south‐east North Atlantic, in a large spatio‐temporal coverage. These species are important prey for many commercial species on the Portuguese coast and, given their abundance, may have a great impact on zooplankton communities, thus assuming a pivotal position in marine food webs.     

Analysis of the diet of the tadpole shrimp, Triops sinensis, in paddy fields of Shouchang River watershed

The tadpole shrimp, Triops sinensis, was discovered in the paddy fields of the Shouchang River watershed. The gut contents of the shrimp from these field populations were analyzed in order to quantify their dietary patterns. Some species of tadpole shrimps were found to prey on mosquito larvae as well as uproot weeds. Mosquito larvae and weeds were seldom found in the paddy fields which the tadpole shrimp inhabited in the Shouchang River watershed. The results further showed that the larvae of these tadpole shrimp mainly feed on algae and rice lamina. The tadpole shrimp could feed on animal food including protozoa, nematodes, rotifers, crustaceans, and aquatic insect larvae, though the animal food was subsidiary.     

Trophic ecology of yellownose skate Zearaja chilensis,a top predator in the south‐western Atlantic Ocean

The diet and trophic level (T_L) of the yellownose skate Zearaja chilensis in the south‐western Atlantic Ocean (35°–54° S), and how these varied in relation to body size, sex, maturity stage, depth and region were determined by analysis of stomach contents. From 776 specimens analysed, 671 (86·5%) ranging from 180 to 1190 mm total length (L_T) had prey in their stomachs. The diet was dominated by fishes, mainly the notothenioid Patagonotothen ramsayi and the Argentine hake Merluccius hubbsi. The consumption of fishes and crabs increased with increasing predator size, and these preys were more important in the north than in the south. Isopods and other crustaceans were consumed more in the south and their consumption decreased as the size of Z. chilensis increased. The T_L of Z. chilensis increased with L_T from 4·29 to 4·59 (mean 4·53), confirming their ecological role as a top predator. The small and large size classes exhibited a low diet overlap and the highest spatial segregation, whereas medium and large specimens had higher co‐occurrence and dietary overlap indices. A clear distinction in tooth shape was noted between sexes in adult specimens, with males having longer cusps. This sexual heterodonty may be related to reproductive behaviour, increasing the grasping ability of males during courtship, because there were no differences in diet between the sexes.