Trophic size‐structure of sailfish Istiophorus platypterus in eastern Taiwan estimated by stable isotope analysis

To examine trophic dynamics over different size classes, an isotopic study of sailfish Istiophorus platypterus life‐history stages was carried out. Samples were collected from eastern Taiwan and the South China Sea during April 2009 and February 2012. A total of 263 samples (111–245 cm, lower jaw fork length, L_LJFL) were examined for changes in trophic structure in relation to L_LJFL by using stable isotope analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N). The δ¹⁵N values for I. platypterus ranged from 7·51 to 14·19‰ (mean ± s.d . = 12·06 ± 1·16‰) and the δ¹³C values ranged from ?22·04 to ?15·48‰ (mean ± s.d . = ?17·62 ± 1·10‰). The δ¹⁵N values were positively dependent on L_LJFL (r² = 0·377), whereas δ¹³C were negatively dependent on L_LJFL (r² = 0·063). There were significantly different seasonal changes in nitrogen and carbon isotopic concentration, but no significant differences in concentrations between eastern Taiwan and the South China Sea were reported. The trophic level (T_L) of each L_LJFL class was correlated, starting from 2·84 T_L for size class I (L_LJFL < 140 cm) and reaching 5·03 T_L for size class VI (L_LJFL > 221 cm). The mean ± s.d . T_L was 4·43 ± 0·19 for all samples. The results reveal that I. platypterus occupies a wide range of trophic levels and different size classes occupy different trophic positions in the pelagic ecosystem.     

Ontogenetic shifts in morphology and resource use of cisco Coregonus artedi

Two previously described lacustrine cisco Coregonus spp. morphs [i.e. a small (<300 mm fork length, L_F), low‐gillraker (≤44) morph and a large (≥300 mm L_F), high‐gillraker (≥45) morph] from Great Slave Lake, NT, Canada, were found to be synonymous with cisco Coregonus artedi. Geometric body shape did not differ between the two size classes nor could they be differentiated by 24 size‐corrected linear measurements, indicating that the two groups had similar phenotypes. Strong, positive correlations between all linear characters and geometric centroid size (a composite variable of fish body length, mass and age) suggested that body morphology changed with age as fish grew. Total gillraker number (N_GR) increased with L_F according to: N_GR = 36·3 + 0·034L_F. Differences in gillraker number and phenotype with age and size were explained by shifts in habitat and trophic resource use. Relative abundance within 0–30, 30–60, 60–90 and >90 m depth strata differed between size classes suggesting that morphology changed when fish shifted their habitat as they grew older. Large C. artedi had lower δ¹³C and slightly higher δ¹⁵N, indicating greater reliance on pelagic prey resources (i.e. more or larger zooplankton, such as Mysis spp.), compared to small C. artedi, which relied slightly more on benthic prey. Gillraker shape and number have always been used as key diagnostic characters in coregonine taxonomy; based on the findings presented here, ontogenetic shifts should be accounted for in resulting classifications.     

Prey composition and ontogenetic shift in coastal populations of longnose gar Lepisosteus osseus

Longnose gar Lepisosteus osseus were collected from May 2012 to July 2013 in the Charleston Harbor and Winyah Bay estuaries (SC, U.S.A.). This study examined trends in stomach fullness, described major prey components and their importance in the diet of L. osseus, compared stomach content‐based trophic level estimates with the stable‐isotope‐based proxy: δ¹⁵N and tested for the occurrence of an ontogenetic diet shift using stomach content analysis and stable C and N isotopes (δ¹³C and δ¹⁵N). Dominant prey families were Clupeidae, Sciaenidae, Penaeidae, Fundulidae and Mugilidae, with the highest consumption rates in autumn. Trophic levels calculated using stomach contents did not correspond to δ¹⁵N (P > 0·05). Stomach contents and stable‐isotope signatures indicate ontogenetic prey composition shifts from low trophic level benthic prey (fundulids) to higher trophic level pelagic prey (clupeids) as the fish grow between 400 and 600 mm in standard length. Due to their biomass, abundance and top predator status, L. osseus play a significant ecological role in the estuarine community composition, although this effect has often been overlooked by past researchers and should be considered in future estuarine community studies.     

Resource use of great hammerhead sharks (Sphyrna mokarran) off eastern Australia

Great hammerhead sharks Sphyrna mokarran are the largest member of Sphyrnidae, yet the roles of these large sharks in the food webs of coastal ecosystems are still poorly understood. Here we obtained samples of muscle, liver and vertebrae from large S. mokarran (234–383 cm total length; L_T) caught as by-catch off eastern Australia and used stable-isotope analyses of δ¹⁵N, δ¹³C and δ³⁴S to infer their resource use and any associated ontogenetic patterns. The results indicated large S. mokarran are apex predators primarily relying on other sharks and rays for their diet, with a preference for benthic resources such as Australian cownose rays Rhinoperon neglecta during the austral summer. Teleosts, cephalopods and crustaceans were not significant components of S. mokarran diets, though some conspecifics appeared to rely on more diverse resources over the austral summer. Ontogenetic shifts in resource use were detected but trajectories of the increases in trophic level varied among individuals. Most S. mokarran had non-linear trajectories in ontogenetic resource-use shifts implying size was not the main explanatory factor. Stable isotope values of δ¹³C and δ³⁴S in muscle suggest S. mokarran span coastal, pelagic and benthic food webs in eastern Australia.     

Diet composition of starry smooth-hound Mustelus asterias and methodological considerations for assessing the trophic level of predatory fish

The stomach contents of 640 starry smooth-hound Mustelus asterias from the north-east Atlantic were examined. The diet was dominated by crustaceans (98.8% percentage of index of relative importance, %IRI), with the two main prey species being hermit crab Pagurus bernhardus (34% IRI) and flying crab Liocarcinus holsatus (15% IRI). Ontogenetic dietary preferences showed that smaller individuals [20–69 cm total length (L_T) n = 283] had a significantly lower diversity of prey than larger individuals (70–124 cm L_T, n = 348); however, 18 prey species were found exclusively in smaller individuals and eight prey taxa were found exclusively in larger individuals. Larger commercially important brachyurans such as edible crab Cancer pagurus and velvet swimming crab Necora puber were more prevalent in the diet of larger individuals. Specimens from the North Sea ecoregion had a lower diversity of prey types for a given sample size than fish from the Celtic Seas ecoregion. Whilst cumulative prey curves did not reach an asymptote, this was primarily due to the high taxonomic resolution utilized and 95% of the diet was described by just seven crustacean taxa. The trophic level (TL) was calculated as 4.34 when species-level prey categories were used. This fine-scale taxonomic resolution resulted in a TL estimate close to a whole level above that estimated using wider taxonomic groupings. This large bias has important methodological implications for TL studies based on categorized prey data, particularly those of predatory fish.     

Feeding ecology of pelagic fish larvae and juveniles in slope waters of the Gulf of Mexico

Stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) were used to investigate feeding patterns of larval and early juvenile pelagic fishes in slope waters of the Gulf of Mexico. Contribution of organic matter supplied to fishes and trophic position within this pelagic food web was estimated in 2007 and 2008 by comparing dietary signatures of the two main producers in this ecosystem: phytoplankton [based on particulate organic matter (POM)] and Sargassum spp. Stable isotope ratios of POM and pelagic Sargassum spp. were significantly different from one another with δ¹³C values of POM depleted by 3–6‰ and δ¹⁵N values enriched by 2 relative to Sargassum spp. Stable isotope ratios were significantly different among the five pelagic fishes examined: blue marlin Makaira nigricans, dolphinfish Coryphaena hippurus, pompano dolphinfish Coryphaena equiselis, sailfish Istiophorus platypterus and swordfish Xiphias gladius. Mean δ¹³C values ranged almost 2 among fishes and were most depleted in I. platypterus. In addition, mean δ¹⁵N values ranged 4–5 with highest mean values found for both C. hippurus and C. equiselis and the lowest mean value for M. nigricans during both years. Increasing δ¹³C or δ¹⁵N with standard length suggested that shifts in trophic position and diet occurred during early life for several species examined. Results of a two‐source mixing model suggest approximately an equal contribution of organic matter by both sources (POM = 55%; pelagic Sargassum spp. = 45%) to the early life stages of pelagic fishes examined. Contribution of organic matter, however, varied among species, and sensitivity analyses indicated that organic source estimates changed from 2 to 13% for a δ¹³C fractionation change of ±0·25‰ or a δ¹⁵N fractionation change of ± 1·0‰ relative to original fractionation values.     

Marine trophic niche use and life history diversity among Arctic charr Salvelinus alpinus in southwestern Greenland

Life history strategies and potential marine niche use of Arctic charr Salvelinus alpinus (n = 237, 84–652 mm, total body length, L_T) were determined during the ice-free season (2012) at three different watercourses in south-western Greenland. All Arctic charr were collected from freshwater habitats. Based on stable isotopes of δ³⁴S, the Arctic charr were categorized as either marine- or freshwater-dependent feeders. The use of time-integrated trophic tracers (stable isotopes of δ¹³C, δ¹⁵N, δ³⁴S) suggested that several trophic groups of Arctic charr operate alongside within each fjord system. The groups suggested were one group that specialized in the marine habitat, in addition to two freshwater resident morphs (small-sized resident and/or large-growing cannibalistic individuals). Stomach contents consisted entirely of freshwater and terrestrial prey (i.e., insects), indicating that marine-dependent feeders also fed in freshwater habitats after return from their marine migration. Growth and maturity patterns further supported variable life history strategies within each watercourse. The life history strategy patterns and marine trophic niche use were consistent across the watercourses along several hundred kilometres of coastline. This study represents the first ecological baseline for partially anadromous populations of Greenland Arctic charr.     

Foraging habits in a generalist predator: Sex and age influence habitat selection and resource use among bottlenose dolphins (Tursiops truncatus)

This study examines resource use (diet, habitat use, and trophic level) within and among demographic groups (males, females, and juveniles) of bottlenose dolphins (Tursiops truncatus). We analyzed the δ¹³C and δ¹⁵N values of 15 prey species constituting 84% of the species found in stomach contents. We used these data to establish a trophic enrichment factor (TEF) to inform dietary analysis using a Bayesian isotope mixing model. We document a TEF of 0‰ and 2.0‰ for δ¹³C and δ¹⁵N, respectively. The dietary results showed that all demographic groups relied heavily on low trophic level seagrass‐associated prey. Bayesian standard ellipse areas (SEA_b) were calculated to assess diversity in resource use. The SEA_b of females was nearly four times larger than that of males indicating varied resource use, likely a consequence of small home ranges and habitat specialization. Juveniles possessed an intermediate SEA_b, generally feeding at a lower trophic level compared to females, potentially an effect of natal philopatry and immature foraging skills. The small SEA_b of males reflects a high degree of specialization on seagrass associated prey. Patterns in resource use by the demographic groups are likely linked to differences in the relative importance of social and ecological factors.     

Trophic role of demersal mesopredators on rocky reefs in an equatorial Atlantic Ocean island

The decrease in the number of sharks around Saint Peter and Saint Paul Archipelago (SPSPA) may impact food web structure. We investigated trophic relationships in the shallow rocky reefs of the SPSPA using stable isotopes and stomach contents with a particular focus on three abundant mesopredators: Caranx lugubris, Enchelycore nigricans and Muraena pavonina. Food web structure was described using samples of the most abundant basal resources, fishes and invertebrates, which were collected in April and October 2012. Individuals of the three focal species (n = 138: C. lugubris, n = 56; E. nigricans, n = 18; M. pavonina, n = 64) were collected during four expeditions to SPSPA (April and October of 2011 and 2012). Results suggest that this shallow water food web is supported by trophic pathways originating from benthic resources. Stable isotope data suggest potential competitive interactions between the whitespot moray and the other two mesopredators. Conversely, stomach content data suggest little niche overlap in the three focal species, but these data must be interpreted carefully because of the small sample sizes and restricted temporal sampling windows. All three mesopredators have a significant, albeit weak, relationship between body size and δ¹⁵N, suggesting ontogenetic diet shifts. These data contribute baseline information to assess shifts in food web structure that may stem from top predator decline in this unique ecosystem.     

Spatial,ontogenetic and interspecific variability in stable isotope ratios of nitrogen and carbon of Merluccius capensis and Merluccius paradoxus off South Africa

General linear models (GLMs) were used to determine the relative importance of interspecific, ontogenetic and spatial effects in explaining variability in stable isotope ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C) of the co‐occurring Cape hakes Merluccius capensis and Merluccius paradoxus off South Africa. Significant GLMs were derived for both isotopes, explaining 74 and 56% of observed variance in Merluccius spp. δ¹⁵N and δ¹³C, respectively. Spatial effects (west or south coast) contributed most towards explaining variability in the δ¹⁵N model, with Merluccius spp. off the west coast having higher (by c. 1·4‰) δ¹⁵N levels than Merluccius spp. off the south coast. Fish size and species were also significant in explaining variability in δ¹⁵N, with both species showing significant linear increases in δ¹⁵N with size and M. capensis having higher (by c. 0·7‰) δ¹⁵N values than M. paradoxus. Species and coast explained most and similar amounts of variability in the δ¹³C model, with M. capensis having higher (by c. 0·8‰) δ¹³C values than M. paradoxus, and values being lower (by c. 0·7‰) for fishes off the west coast compared with the south coast. These results not only corroborate the knowledge of Merluccius spp. feeding ecology gained from dietary studies, in particular the ontogenetic change in trophic level corresponding to a changing diet, but also that M. capensis feeds at a slightly higher trophic level than M. paradoxus. The spatial difference in Merluccius spp. δ¹⁵N appears due to a difference in isotopic baseline, and not as a result of Merluccius spp. feeding higher in the food web off the west than the south coast, and provides new evidence that corroborates previous observations of biogeographic differences in isotopic baselines around the South African coast. This study also provides quantitative data on the relative trophic level and trophic width of Cape hakes over a large size range that can be used in ecosystem models of the southern Benguela.     

Diets and trophic guilds of small fishes from coastal marine habitats in western Taiwan

The diets and trophic guilds of small fishes were examined along marine sandy beaches and in estuaries at depths <1·5 m in western Taiwan, Republic of China. Copepods were the most frequently identified item in fish guts, indicating they are key prey for the fish assemblages studied. Piscivore, crustacivore, detritivore, omnivore, zooplanktivore and terrestrial invertivore trophic guilds were identified. The zooplanktivore guild contained the most fish species. Maximum prey size consumption was positively correlated with standard length (L_S) in seven species and at the assemblage level and negatively correlated with L_S in a single detritivorous species. The diet data and trophic guild scheme produced by this study contribute to an understanding of coastal marine food webs and can inform ecosystem‐based fisheries management.     

Ontogenetic diet shift of age‐0 year Pacific bluefin tuna Thunnus orientalis

Stomach contents of 437 age‐0 year Pacific bluefin tuna Thunnus orientalis (20·3–59·4 cm fork length, L_F) caught in the Tsushima Current and the Kuroshio regions around Japan were examined to investigate their ontogenetic diet shift. Prey compositions were diverse and different between regions. Although the seasonal growth patterns were different between regions, ontogenetic diet shifts shared a common pattern. In the Tsushima Current region (Sea of Japan), small T. orientalis (20–25 cm L_F) preyed upon small squid (juvenile Enoploteuthis chunii), and larger ones (25–35 cm L_F) gradually shifted their diet to mesopelagic fish (Maurolicus japonicus). In the Kuroshio region (Pacific Ocean), small T. orientalis (20–25 cm L_F) preyed upon small zooplankton (mostly crustacean larvae), and larger ones (25–40 cm L_F) shifted to epipelagic fishes (Etrumeus teres, Sardinops melanostictus and Engraulis japonicus). The observed data suggest that T. orientalis switch to a diet more based on fish prey items, which have more body mass and greater swimming ability than small squid and zooplankton, after they reach a L_F of 25 cm.     

Quantitative diet analysis of four mesopredators from a coral reef

The diets of four common mesopredator fishes were examined in the back‐reef habitat of a subtropical fringing reef system during the summer months. Quantitative gut content analyses revealed that crustaceans, represented >60% of ingested prey (% mass) by the latticed sand‐perch Parapercis clathrata, brown dottyback Pseudochromis fuscus and half‐moon grouper Epinephelus rivulatus. Dietary analyses also provided insights into ontogenetic shifts. Juvenile P. fuscus ingested large numbers of crustaceans (amphipods and isopods); these small prey were rarely found in larger individuals (<1% of ingested mass). Fishes also made an important contribution to the diets of all three species representing 10–30% of ingested mass. Conversely, the sand lizardfish Synodus dermatogenys fed exclusively on fishes including clupeids, gobies and labrids. Differences in the gut contents of the four species recorded were not apparent using stable isotope analysis of muscle tissues. The similarity of δ¹³C values in muscle tissues suggested that carbon within prey was derived from primary producers, with comparable carbon isotope signatures to corals and macroalgae, whilst similarities in δ¹⁵N values indicated that all four species belonged to the same trophic level. Thus, interspecific differences between mesopredator diets were undetectable when using stable isotope analysis which suggests that detailed elucidation of trophic pathways requires gut content analyses.     

Variation in the diet of the Patagonian toothfish with size, depth and season around the Falkland Islands

The ontogenetic and seasonal variations in the feeding spectrum were studied in 756 specimens of the Patagonian toothfish Dissostichus eleginoides (16–159 cm total length, L_T) collected on the shelf, continental slope and bathyal waters (67–1960 m, depth range) around the Falkland Islands between April 1999 and August 2002. On the shelf, small toothfish (<40 cm L_T) were active predators taking mostly one relatively large prey item at a time (mainly near‐bottom Patagonotothen ramsayi and Loligo gahi). Medium‐size toothfish (40–60 cm L_T) fed on the same prey, but the number of prey items increased to 1–2 items per fish. Large toothfish (>60 cm L_T) switched their diet to other large pelagic fishes occurring near the bottom (Macruronus magellanicus and Micromesistius australis australis), again taking mostly one prey item at a time. The diet of medium‐size D. eleginoides on the shelf varied seasonally depending on the abundance and migrations of the major prey species. Patagonotothen ramsayi was abundant in the diet throughout the year, whereas L. gahi appeared only from February to October during its offshore seasonal migrations to the depth range of D. eleginoides. During November to January, L. gahi migrated inshore to spawn and disappeared from the toothfish diet, being substituted by M. australis australis which dispersed on the shelf after spawning. After its ontogenetic descent to the lower part of the continental slope (500–1000 m depths), toothfish took less active (than on the shelf) fishes such as Antimora rostrata whilst also feeding on active near‐bottom macrourids and skates. In their deepest habitat (>1000 m depths), toothfish became a typical opportunistic predator, feeding mainly on relatively small and inactive fishes, squids and prawn‐like crustaceans Acanthephyra pelagica and Thymops birsteini. Decrease in hunting activity with depth could be related to a specific adaptation to keep neutral buoyancy by increase of lipid content in white muscles of D. eleginoides with size.     

Questioning assumptions of trophic behavior in a broadly ranging marine predator guild

We evaluated whether existing assumptions regarding the trophic ecology of a poorly‐studied predator guild, northwest (NW) Atlantic skates (family: Rajidae), were supported across broad geographic scales. Four hypotheses were tested using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope values as a proxy for foraging behavior: 1) species exhibit ontogenetic shifts in habitat and thus display a shift in ¹³C with differential use of the continental shelf; 2) species exhibit ontogenetic prey shifts (i.e. from smaller to larger prey items) and become enriched in ¹⁵N; 3) individuals acquire energy from spatially confined local resource pools and exhibit limited displacement; and 4) species exhibit similarly sized and highly overlapping trophic niches. We found some evidence for ontogenetic shifts in habitat‐use (δ¹³C) for thorny and little skate and diet (δ¹⁵N) of thorny and winter skate and hypothesize that individuals exhibit gradual trophic niche transition, especially in δ¹⁵N space, rather than a clear and distinct shift in diet throughout ontogeny. Spatial isoscapes generated for little, thorny, and winter skate highlighted distinct spatial patterns in isotopic composition across the coastal shelf. For little and thorny skate, patterns mimicked expected spatial variability in the isotopic composition of phytoplankton/POM, suggesting limited displacement and utilization of spatially confined resource pools. Winter skate, however, exhibited a much narrower range of δ¹³C and δ¹⁵N values, suggesting individuals may use resources from a more confined latitudinal range. Although high total trophic niche overlap was observed between some species (e.g. little and thorny skate), sympatric species (e.g. little and winter skate) exhibited a degree of trophic niche separation. These findings offer new insight into the trophic dynamics of a poorly‐studied, vulnerable group of predators, and highlight a need to re‐examine assumptions pertaining to aspects of their ecology.     

Stable-isotope analysis reveals sources of organic matter and ontogenic feeding shifts of a mangrove-dependent predator species,New Granada sea catfish,Ariopsis canteri

To gain a better understanding on the trophic ecology of New Granada sea catfish, Ariopsis canteri, and their linkage to mangroves, nitrogen and stable carbon isotopes (δ¹⁵N and δ¹³C), as well as Bayesian mixing models, were used to explore trophic dynamics and potential ontogenic feeding shifts across different size classes: class I (8–20 cm), class II (21–32 cm) and class III (>32 cm). The study area was the estuary of the Atrato River Delta, where information about fish ecology is scarce. The δ¹³C of size class I was lower (mean ± s.d . = −24.96 ± 0.69‰) than that of size classes II (−22.20 ± 0.90‰) and III (−22.00 ± 1.96‰). The δ¹⁵N of size class I was lower (mean ± s.d . = 8.50 ± 0.67‰) than that of size classes II (9.77 ± 0.60‰) and III (10.00 ± 0.66‰). Body size was positively and significantly correlated to δ¹⁵N and δ¹³C. Individuals with L_T > 32 cm presented the highest estimated trophic position (3.8). Five-source mixing models indicated that for class I, the mean estimated contribution of macroalgae was the highest (6%–57% c.i. ), and for classes II and III, the mean estimated contribution of macrophytes was the highest (3%–53% c.i. and 4%–53% c.i. , respectively). Ontogenetic feeding shifts of A. canteri were confirmed evidencing decreasing intraspecific competition between small and large individuals. Results suggest that mangroves are a nursery and feeding ground habitat for this species and that mangroves support A. canteri mainly due to the substrate/habitat that supports sources in the food webs. These results can be used in ecosystem-based fishery management focused on the protection of extensive mangrove areas in the southern Caribbean Sea.     

Changes in trophic level of Squatina guggenheim with increasing body length: relationships with type, size and trophic level of its prey

The occurrence of changes in the trophic level (TL) of sharks with growth has not been quantified until now. Here length-related changes on Squatina guggenheim Marini trophic level were determined, and shifts in type, size and trophic level of its prey were analysed. Sampling took place during five bottom trawl surveys conducted in the Argentine–Uruguayan Common Fishing Zone during spring (December/1995, October/1997) and fall (March/1997, March–April/1998, May–June/1998), using an Engel bottom-trawl net to capture the sharks. Three length groups were defined based on diet composition and using a cluster analysis (group I, 23–60 cm; group II, 61–80 cm; group III, 81–91 cm L _T). An ANOSIM procedure detected significant differences (P < 0.05) in the diet spectrum between the three length groups. The smallest sharks (group I) ingested fish prey ranging from 5 to 21 cm L _T, medium sharks (group II) fed on fish prey between 11 and 35 cm L _T, and largest sharks (group III) preyed on fish between 13 and 40 cm L _T. Diet structure of length groups were discriminated by almost the same prey taxa that characterized them. The increase of S. guggenheim body length promoted a decrease in the relative importance of small pelagic fishes. Contrarily, prey as medium benthopelagic fishes, medium pelagic squid and medium benthopelagic fishes showed an inverse tendency, indicating a broad diet spectrum of adults. Predator-length and prey-length relationship indicated a trend where 44.8% of S. guggenheim diet was integrated by prey <20% of their own body length and 32.8% of their diet was composed by prey >30% of their own length. The increase of mean prey weight was associated with the increase of predator weight and length. Smallest sharks (group I) were identified as secondary consumers (TL < 4) whereas medium sharks (group II) and largest sharks (group III) were placed as tertiary consumers (TL > 4). The study revealed an increase in S. guggenheim TL with shark growth as a consequence of changes on type, size and TL of prey ingested.     

Spatio‐temporal variability in the cannibalistic behaviour of European hake Merluccius merluccius: the influence of recruit abundance and prey availability

Cannibalistic behaviour of European hake Merluccius merluccius was studied through the analysis of 49 836 gut contents belonging to individuals from 6 to 82 cm in total length (L_T). Samples were collected every autumn between 1993 and 2009. The results showed that the consumption of conspecific individuals was consistent over space and time. The abundance, spatial distribution patterns and L_T structure of M. merluccius recruits were the main variables involved in M. merluccius cannibalism. A geographical pattern was found since increasing cannibalism was observed in areas of recruit aggregations. The L_T spectrum of recruits in autumn was also a key factor and dependent on the spawning period. When adults spawned from late spring to summer, an increasing cannibalism trend was found in autumn, due to the ideal size structure of the prey (M. merluccius recruits) for predators. Depth was also a significant variable, and a cannibal peak was detected at depths ranging between 50 and 200 m, coinciding with a spatial overlap of predator (pre‐adults) and prey (recruits). The cannibalistic behaviour of M. merluccius began at c. 12 cm, although 75% were mid‐sized individuals between 16 and 30 cm. Additionally, 90% of prey‐hake were recruits <18·5 cm. Some biological and ecological aspects such as growth rate, spawning period, predation and competition interactions and the effect of some oceanographic events on M. merluccius cannibalism are also discussed.     

Trophic shift in young‐of‐the‐year Mugilidae during salt‐marsh colonization

This study investigated the trophic shift of young‐of‐the‐year (YOY) thinlip grey mullet Liza ramada and golden grey mullet Liza aurata during their recruitment in a salt marsh located on the European Atlantic Ocean coast. Stable‐isotope signatures (δ¹³C and δ¹⁵N) of the fishes followed a pattern, having enrichments in ¹³C and ¹⁵N with increasing fork length (L_F): δ¹³C in fishes < 30 mm ranged from ?19.5 to ?15.0‰, whereas in fishes > 30 mm δ¹³C ranged from ?15.8 to ?12.7‰, closer to the level in salt‐marsh food resources. Large differences between the δ¹⁵N values of mugilids and those of food sources (6·0‰ on average) showed that YOY are secondary consumers, similar to older individuals, when feeding in the salt marsh. YOY mugilids shift from browsing on pelagic prey to grazing on benthic resources from the salt marsh before reaching 30 mm L_F. The results highlight the role of European salt marshes as nurseries for juvenile mugilids.     

Feeding across the food web: The interaction between diet,movement and body size in estuarine crocodiles (Crocodylus porosus)

The estuarine crocodile (Crocodylus porosus) is an apex predator across freshwater, estuarine and coastal environments. The impact of a changing C. porosus population upon the ecosystem is unknown, but due to large ontogenetic changes in body mass (>1000‐fold) their impact may be wide reaching and substantial. Here we investigated the relationship between diet, movement and body size in a population of C. porosus inhabiting a tidal river in northern Australia. Subcutaneous acoustic transmitters and fixed underwater receivers were used to determine the activity space and movement patterns of 42 individuals (202–451 cm in total length). There was no size‐related spatial partitioning among different sized crocodiles. Large individuals (snout–vent length (SVL): 160 cm < SVL < 188.5 cm) did, however, exhibit a much larger activity space than other size classes. Diet and individual specialization was assessed using the composition of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in tissues with different turnover rates. There was a quadratic relationship between body size and δ¹⁵N, suggesting that medium‐sized individuals (110 cm < SVL < 160 cm) incorporated a greater proportion of high trophic prey into their diets than small (SVL < 110 cm) or large individuals (SVL > 160 cm). Tissue δ¹³C composition on the other hand was positively correlated with body size, indicating that different size classes were trophically linked to primary producers in different habitats. Individual‐level analyses showed that small crocodiles were generalist feeders while medium and large size classes specialized on particular prey items within the food webs they fed. The findings further our understanding of ontogenetic variation in C. porosus diet, and suggest that change in C. porosus population size or demographics may be influential at various levels across the local food web.