Prey-specific Index Of Relative Importance Research Articles

Feeding ecology and reproductive biology of small coastal sharks in Malaysian waters.

Small coastal demersal sharks form a major proportion of the sharks landed in Malaysia. However, little is known about their feeding ecology and reproduction. This study sought to elucidate the dietary patterns, role of ontogeny in prey consumption, and reproductive biology of four dominant small demersal shark species in Malaysian waters: the Hasselt's bamboo shark, Chiloscyllium hasseltii; brownbanded bamboo shark, C. punctatum; spadenose shark, Scoliodon laticaudus; and Pacific spadenose shark, S. macrorhynchos. Dietary analyses revealed a high overlap in prey taxa consumed; clear resource partitioning among co-occurring species based on the percentage Prey-specific Index of Relative Importance (%PSIRI), with higher fish %PSIRI for Chiloscyllium hasseltii, higher cephalopod %PSIRI for C. punctatum, and higher crustacean %PSIRI for both Scoliodon species; and an ontogenetic diet shift, seen through changes in prey size. Based on the examination of reproductive organs, the results showed larger sizes at maturity for males compared to females for all four species; no obvious reproductive cycles, based on hepatosomatic and gonadosomatic indices for all species; female bias in the sex ratio of the embryos of Scoliodon species; and increased reproductive output (number of eggs or embryos and size of eggs) with larger female size for C. hasseltii and Scoliodon species. The partitioning of food resources minimizes direct competition for food and supports coexistence within shared coastal habitats. The reproductive strategies of these small coastal sharks appear to be favorable for supporting short-term population productivity; although a reduction in fishing pressure, especially from bottom trawlers, is essential for the long-term sustainable use of these sharks.

Feeding ecology of the shortfin mako shark, Isurus oxyrinchus, in the Ecuadorian Pacific Ocean

Abstract Shortfin mako shark, Isurus oxyrinchus, is listed as an endangered species with declining global population. Thus, studies regarding its biology and ecology are important to recommend fishery management and conservation measures. This study aimed to determine the diet composition and feeding habits of I. oxyrinchus in Ecuadorian waters. Samples were obtained from Santa Rosa fishing port (Ecuador). The total length (LT), sex and sexual characteristics were recorded, and stomach contents were collected. A total of 142 individuals were recorded, comprising 81 females (104–295 cm LT) and 61 males (127–245 cm LT). A total of 24 prey species were identified, including crustaceans, cephalopods, teleosts and cetaceans. According to the Prey-Specific Index of Relative Importance (PSIRI), the main prey taxa were the ommastrephid squid, Dosidicus gigas (42.57%) and Sthenoteuthis oualaniensis (21.04%), followed by fish from the family Hemiramphidae (11.85%). Isurus oxyrinchus is a specialist predator that preferred a low number of prey (Bi = 0.25), both by sex (Bi; females = 0.29 and males = 0.34) and life stages (Bi; juveniles = 0.27 and adults = 0.37). The trophic overlap was medium for sexes (J = 0.54) and biological cycle phases (J = 0.42). Trophic level (TLk) was 4.47, indicating that I. oxyrinchus is a tertiary predator. This information will help in fisheries management based on an ecosystem approach, where this species fulfils an ecological role, and its interactions with other species allow us to understand how the flow of nutrients and energy occurs within an ecosystem.

Small but voracious: invasive generalist consumes more zooplankton in winter than native planktivore

In recent years, Lake Constance has experienced an invasion and domination of three-spined stickleback (Gasterosteus aculeatus) in the pelagic zone, which has coincided with a decline in the native whitefish (Coregonus wartmanni) population. Similar massive invasions of sticklebacks into pelagic zones have been recognized also in marine areas or small lakes worldwide. However, their diet overlaps with native species is rarely evaluated, especially in the winter season, which often presents a bottleneck for fish survival. In this study, we compared the diet of pelagic sticklebacks with the diet of the substantially larger native whitefish in different seasons, to evaluate the threat of the recent stickleback invasion on whitefish populations. By monthly sampling of zooplankton and both fish species diets, we could demonstrate that sticklebacks select similar prey throughout most of the year and consume more prey than whitefish during the winter. With relations between prey availability and prey selection, interspecific and intraspecific seasonal diet variability and indices like a prey-specific index of relative importance, we discuss the importance of zooplankton species traits and abundance for whitefish and stickleback predation. This study shows that sticklebacks, despite their small size, represent a serious potential diet competitor to native planktivorous fish. Sticklebacks quickly adapt to new environments, and thus we advocate precautions regarding their introduction into similar lakes as Lake Constance, as this could cause irreversible ecological changes.

Comparative trophic ecology of two sympatric guitarfishes Pseudobatos (Chondrichthyes, Rhinobatidae) from Southeast Brazil, southwestern Atlantic.

The present study compares the trophic ecology of two guitarfishes (Pseudobatos percellens and Pseudobatos horkelii) from the continental shelf of the São Paulo State, Brazil, caught by the bottom pair trawls between 2007 and 2009. These two sympatric species are under different threat categories, "Vulnerable" and "Critically Endangered," respectively, according to Brazilian agencies. Thus, any study considering trophic ecology parameters is pivotal in understanding the trophic ecology role of such species in the ecosystem. The authors analysed 500 stomachs of P. percellens and 108 of P. horkelii, quantifying with dietary indexes: numerical (%N), gravimetric (%W), frequency of occurrence (%FO) and the prey-specific index of relative importance (%PSIRI). For P. percellens and P. horkelii, 26 and 14 different prey items were found, respectively. Crustacea and Teleostei were the most important prey items for both species, indicating a specialist feeding behaviour due to their low niche amplitude. The results provide evidence related to ontogenetic dietary shift in P. percellens and individual-level diet specialization in both species. These two benthic-demersal elasmobranchs are important mesopredators (3,7 for P. percellens, and 3,5 for P. horkelii) in the study area, with high trophic-level values.

Habitat-Specific Foraging by Striped Bass (Morone saxatilis) in the San Francisco Estuary, California: Implications for Tidal Restoration

Non-native predatory fish strongly affect aquatic communities, and anthropogenic habitat alterations can exacerbate their effects. Loss of natural habitat, and restoration actions that reverse habitat loss, can modify relationships between non-native predators and prey. Predicting how these relationships will change is often difficult because insufficient information exists on the habitat-specific feeding ecology of non-native predators. To address this information gap, we examined diets of non-native Striped Bass (Morone saxatilis; 63–671 mm standard length; estimated age 1–5 y) in the San Francisco Estuary during spring and summer in three habitat types—marsh, shoal, and channel—with the marsh habitat type serving as a model for ongoing and future restoration. Based on a prey-specific index of relative importance, Striped Bass diets were dominated by macroinvertebrates in spring and summer (amphipods in spring, decapods and isopods in summer). In spring, diets were relatively consistent across habitats. In summer, marsh diets were dominated by sphaeromatid isopods and shoal/channel diets by idoteid amphipods and decapods. Striped Bass consumed a variety of native and non-native fishes, primarily Prickly Sculpin (Cottus asper) and Gobiidae. The highest importance of fish prey was in the marsh in spring (~ 40% prey weight), and fish prey comprised less than 25% prey weight in all other season/habitat combinations. Linear discriminant analyses suggested that marsh foraging was prevalent in Striped Bass collected in other habitats, mostly because of the predominance of marsh-associated invertebrates found in the stomachs of individual Striped Bass collected outside the marsh. Striped Bass diets differ across habitats, with marsh foraging important to Striped Bass regardless of collection location. This information can be used to forecast the potential utilization of restored habitats by this non-native piscivore.

Diet of the rasptail skate, Rostroraja velezi (Rajiformes: Rajidae), off Piura, Peru

Batoids are among the least studied group of vertebrates and one of the most vulnerable to fishing. The rasptail skate, Rostroraja velezi, is a bycatch species in northern Peru for which there are no studies on its diet. This research aims to document the diet of R. velezi on the northern Peruvian coast and to determine if there are sex-based differences in the diet. A total of 74 R. velezi stomachs were collected, 59 of which had stomach contents. The cumulative prey curve showed reliability in the number of samples collected. According to the prey-specific index of relative importance, diet was dominated by Acanthephyra sp. (31%), other unidentified Caridea (15%), and unidentified teleosts (17%). A low significant difference in diet between sexes was found (R = 0.21, P < 0.01). The trophic position of R. velezi was 3.9 ± 0.7. This species displays bentopelagic feeding behavior and it is a secondary predator in northern Peru.

Dietary ecology and trophic level of adults of the Chilean angel shark Squatina armata Philippi 1887 in the Central-Eastern Pacific Ocean

As top predators, sharks play an important role in the regulation of marine ecosystems. This study provides information on the feeding ecology and trophic level of the Chilean angel shark Squatina armata in the central-eastern Pacific Ocean. A total of 320 sharks were caught by the artisanal fishery in the Ecuadorian Pacific Ocean and landed in Manta (Ecuador) between November 2003 and October 2004, in which such specimens measured 66 cm to 98.5 cm of total length (TL). The prey included bony fish, elasmobranchs, mollusks, and crustaceans. According to the Prey-Specific Index of Relative Importance (%Psiri), the kolibri shrimp Solenocera agassizii, lizardfish Synodus spp., and peanut rock shrimp Sicyonia picta were the main prey in the diet of S. armata. The sharks which belonged to size I (65–80 cm TL), consumed mostly drum Larimus spp., S. agassizi, and bignose conger Rhynchoconger nitens; while for size II (80–95 cm TL) and size III (95–109 cm TL), sharks fed mostly on S. agassizi, Synodus spp., and S. picta. The results showed that S. armata feeds on the same prey items all year round and it does not display dietary changes. This shark species is a selective predator with functions of a mesopredator (trophic level = 3.20–5.20). It could be classified as mainly piscivorous and carcinophagus, in which this shark species searches for food mainly in benthic and coastal areas. This study highlights the scarcity of knowledge regarding the biology and ecology of S. armata in the central-eastern Pacific Ocean. Additionally, it underlines the need to produce information that will allow adequate management and conservation measures to be implemented for this species by understanding their important feeding spots in coastal areas of Ecuador.

Diet of invasive cats, rats and tegu lizards reveals impact over threatened species in a tropical island

Vertebrates introduced in non-native habitats have contributed to several extinctions in the modern era, with direct effects mainly over birds, mammals and reptiles on islands. Fernando de Noronha Archipelago, in tropical Atlantic Ocean, is a World Heritage natural site, holding the most diverse breeding seabird community off Brazil, in addition to endemic landbirds and reptiles. It also holds invasive black rats (Rattus rattus), tegu lizards (Salvator merianae) and feral cats (Felis catus), among the 26 exotic species reported in the archipelago, all of which are species with potentially high impact upon native fauna. Aiming to assess the role of exotic vertebrates on the fauna, we investigated their diets through stomach content and scat analysis, and stable isotope analysis (SIA) followed by isotopic mixing models. The main food items for tegu lizards were fruits, with relative importance (Prey-specific Index of Relative Importance – %PSIRI) of 41.3%, and the endemic Noronha skink (Trachylepis atlantica; 19.6%) and for black rats was Noronha skink (30.3%). The diet of feral cats was composed by rodents (31.6%), birds (28.6%) and Noronha skink (18.8%). SIA and Bayesian mixing models provided evidences that invasive species use marine matter to synthesize tissues, probably derived directly or indirectly from seabird colonies. This study demonstrated that exotic species feed on other exotic, as well as endemic species. The most heavily impacted species was the endemic Noronha skink. We demonstrated the predation pressure of exotic fauna upon endemic vertebrates, and strongly recommend the implementation of an invasive species control and eradication plan.

Effects of age, maturity stage, sex and seasonality on the feeding strategies of the diamond stingray (Hypanus dipterurus) in the southern Gulf of California

The diet of the diamond stingray (Hypanus dipterurus) was quantified based on individuals collected monthly from October 2013 to December 2015 on Espiritu Santo Island in the Bahía de La Paz, México. Of the 473 stomachs analysed, 211 (44.6%) contained food. Analysis of the prey-specific index of relative importance indicated that the diet of H. dipterurus was based on infaunal and epibenthic invertebrates, especially the bivalve Solemya spp. (42.2%) and the stomatopod Nannosquilla raymanningi (7.7%). Statistical analyses of the diet considering sex, age, maturity stage and interactions between sex, age and season indicated that older females consumed more polychaetes in the cold (November–April) season and that younger females consumed more stomatopods and bivalves in the warm (May–October) season. The feeding strategy of the species is specialised on three prey categories (bivalves, polychaetes and stomatopods), with low trophic niche breadth values. The calculated trophic level indicated that the diamond stingray is a secondary consumer and mesopredator. We conclude that the diamond stingray does not maintain a redundant trophic function with sympatric elasmobranchs in the study area, and is therefore likely a key prey density-regulator functioning to link energy derived from lower to upper trophic levels within the southern Gulf of California.

Feeding habits variability of Lutjanus synagris and Lutjanus griseus in the littoral of Campeche, Mexico: an approach of food web trophic interactions between two snapper species

The study of the feeding habits variability (spatial and temporal scales) allows us to evaluate the trophic interactions between species, thus, the short and long-term effects of the removal of different species by the presence of different phenomena. In this study, we carried out stable isotope (δ13C and δ15N) and stomach content analyses to infer the trophic linkages between two snapper species (Lutjanus synagris and Lutjanus griseus) captured in the littoral of Campeche, Mexico. In total, 781 specimens were collected (528 L. synagris and 253 L. griseus) and based on relative importance index [%PSIRI] both snapper species consumed based on the prey-specific index of relative importance (PSIRI), Penaeidae family (PSIRI = 23.41%) was the most important species in the diet of Lutjanus synagris, while Callinectes sapidus (PSIRI = 21.45%) was the primary prey of Lutjanus griseus. The isotopic analyses indicated that both snapper species feed in the coastal-marine (δ15N: 10.6 to 12.1‰ and δ13C: -15.7 to -12.7‰); however, according to PERMANOVA, low diet similarity was found between snapper species (R = 0.07, P < 0.01), also, significant differences was detected in the δ15N and δ13C values between L. synagris, and L. griseus. Therefore, despite both snapper species presents similar trophic positions (L. synagris: 4.2 ± 0.2; L. griseus: 3.9 ± 0.1) and feeding behavior (according to SIBER = opportunistic predators), a low trophic overlap was observed, probably associated with the differential use of habitat in the coastal zone; where L. synagris is ecologically fed in areas of seagrass, while L. griseus is probably intermittent between pasture and mangrove areas, therefore, the role of each species is crucial in the dynamics of coastal-marine ecosystems as predators and potential structuring of the populations of their prey.

Feeding habits of the fishes Euthynnus lineatus and Scomberomorus sierra (Perciformes: Scombridae) in the Eastern Tropical Pacific

ABSTRACT. Introduction: Black skipjack, Euthynnus lineatus, and Pacific sierra, Scomberomorus sierra, are of great economic importance in the small-scale fishery of the Tropical Eastern Pacific and in particular in the study area. Objectives: The objectives of the present study were to assess the diet components of E. lineatus and S. sierra, as well as to evaluate the niche width, trophic level, and trophic overlap between the two species, by size and season. Methods: Biological samples were obtained weekly from small-scale fishery catches in Bahía de Acapulco, Mexico. The E. lineatus sampling period occurred from October 2016 to October 2017, whereas S. sierra was sampled from October 2016 to June 2018. The Prey-specific Index of Relative Importance (% PSIRI) was used to evaluate the importance of each prey item in the diet of the predator. Levin´s index (Bi) was used as a measure of niche width. The Morisita-Horn index was used to evaluate diet overlap by size (juveniles or adults), by season (dry or wet), and between the two species. The trophic level (TL) of predators was calculated using the Cortés´ method. Results: A total of 262 E. lineatus stomachs were analyzed (42 % were empty and 58 % contained food) and a total of 209 S. sierra stomachs were analyzed (74.6 % were empty and 25.3 % contained food). Of the total E. lineatus specimens sampled, 107 were juveniles and 155 were adults. Black skipjack juveniles fed on five food items and adults fed on 13 food items. Of the total S. sierra specimens sampled, 75 were juveniles and 134 were adults. Pacific sierra juveniles and adults fed on five food items. The diet of the two species comprised fish, mollusks, and crustaceans. The two species showed narrow niche width. The diet of juvenile and adult E. lineatus presented differences with growth, contrary to what was observed for juveniles and adults of S. sierra. There were no significant differences in diet by season for both species. The trophic overlap obtained between the two species was low. The trophic level calculated for both species indicated that are tertiary consumers. Conclusions: Black skipjack and Pacific sierra are tertiary predators and present a specialist behavior, feeding mainly on fish from the families Engraulidae and Clupeidae. Although a degree of similarity in the dietary food components of these species was observed, there was not a high degree of interspecific competition for food.

Diet shifts of Rhizoprionodon terraenovae from the southern Gulf of Mexico: possible scenario by temperature changes

The current study examined the stomach contents of the Atlantic sharpnose shark (Rhizoprionodon terraenovae) in the southern Gulf of Mexico during 2015 to understand the relationship between diet and changes in sea surface temperature (SST). Prey-specific index of relative importance (%PSIRI), diet breadth (Bi), trophic level (TrL), and trophic overlap (PERMANOVA) were calculated between sexes, body size, and climatic seasons (dry, rainy and winter storm). The lowest temperature recorded in the area was during February (23.9°C), and the highest was during August (29.1°C). A total of 124 stomachs were analyzed, with 54.84% containing food. The trophic spectrum was composed of 32 identified prey, with demersal fish (Haemulon plumierii; %PSIRI = 22.82) and pelagic fish (Sardinella aurita; %PSIRI = 12.83) being the most important. According to the diet breadth (Bi = 0.002), Costello's graph, and trophic level (TrL = 4.2), R. terraenovae is a specialist tertiary consumer. PERMANOVA indicated significant trophic differences between sexes (F = 32.22; P < 0.05), body size (F = 13.68; P < 0.05), and among climatic seasons (F = 23.86; P < 0.05). Spearman's correlation indicated a negative relationship between the diversity of prey consumed by R. terraenovae and sea surface temperature (r = -0.75; P < 0.05). Therefore, diet for R. terraenovae is associated with SST, allowing for the development of possible scenarios related to climatic phenomena like climate change.

Trophic preferences of three pelagic fish inhabiting the Galapagos Marine Reserve: ecological inferences using multiple analyses

There is a great need to understand how resource interactions alter the functioning of ecosystems, where the selective elimination of pelagic fishes can lead to changes in food web structure. This work analyzes the trophic niches of three species of commercial importance in the Galapagos Marine Reserve, yellowfin tuna, Thunnus albacares (TA), skipjack tuna, Katsuwonus pelamis (KP), and wahoo, Acanthocybium solandri (AS), via multiple analyses. According to the prey-specific index of relative importance, the most important prey for TA was Dosidicus gigas, while for AS it was unidentified pelagic fish. Interspecific differences were found between the isotopic signatures of AS and those of TA and KP. The isotope mixing model provides evidence for some predominance of D. gigas in the diets of TA and KP, while the fishes Selar crumenophthalmus and Paranthias colonus contribute to the diet of AS. The stable isotope Bayesian ellipses show a high overlap between TA and KP, suggesting a similar use of resources and feeding areas, while the ellipse of AS does not overlap with that of the other species. Both AS and TA were present around the islands more commonly during the day, with a peak in detections in the morning for AS and a greater presence of TA throughout the afternoon; there were only a few detections of KP in the days immediately after tagging. In summary, the results of this study suggest a pelagic foraging strategy with differential consumption of prey between AS and the other species.

Artisanal trawling impact over prey availability and diet of estuarine megabenthic organisms in southern Brazil

Trawling fisheries cause in their path uncounted impacts, including direct and indirect effects. This study aimed to evaluate the impact of artisanal trawls on the food availability and its possible impact on the diet of two commercial crustacean species, the pink-shrimp Penaeus paulensis and the blue crab Callinectes sapidus. Two mesocosms were constructed (control and simulated fishing), where sediment samples were collected to analyze the zoobenthos, sediment grain sizes and the percentage of organic matter. Sediment samples were separated into two strata (0–5 and 5–10 cm depth) to evaluate the vertical structure of the fauna, the availability of prey and possible shifts in these patterns after trawling. The Prey-Specific Index of Relative Importance was used in stomach contents analyses. Significant decreases in the percentage of organic matter were observed before and after trawling. Perturbations caused by trawling activities resulted in alterations on the superficial layer, with significant changes in zoobenthos total abundance induced by resuspension and settlement of organisms in other places. Changes in the abundance and richness of zoobenthic organisms caused by trawling were reflected in the diet of pink shrimp and blue crab. A decrease in the main ingested organisms and an increase in the ingestion of more refractory and less energetic items (mainly plants) occurred. Overall, there was a significant effect of trawling on benthic organisms, in particular cases reflected in the predators stomach content, further confirming the impact of trawling on estuarine trophic webs.

What matters for intraspecific diet changes: the dietary differences between different areas or the increase in body size? The case of the searobin Prionotus punctatus in a tropical bay

Resource use diversity occurs when a population is composed of ecologically heterogeneous individuals who use only a subset of the population’s resource availability. The aim of this study was to examine the diet of juveniles, sub-adults and adults of Prionotus punctatus (40 to 190 mm Total Length - TL) to assess the importance of factors driving intraspecific changes, such as spatial differences in the diet or individual body size changes. The intraspecific trophic strategy was also described. We examined the stomach contents of 210 individuals collected in two zones of a tropical bay (outer and inner) grouped into three size classes ( 140 mm TL, adults). The Prey-Specific Index of Relative Importance (PSIRI) indicated that Peneidae, Amphipoda (excluding Caprellidae), Mysidacea, Teleostei and Copepoda were the most important food items. The change in the diet during fish growth differed between the two bay zones. In the inner zone, the juveniles fed mainly on Copepoda and Amphipoda (excluding Caprellidae), whereas the adults fed mainly on Teleostei and Peneidae. In the outer zone, the juveniles fed mainly on Mysidacea, whereas the adults fed mainly on Peneidae and Brachyura, with Amphipoda (excluding Caprellidae) being an important prey for individuals in all size classes. Significant differences were detected in the diets among size classes (Pseudo-F = 5.52; P = 0.003) but not between the two zones (Pseudo-F = 2.20; P = 0.113) according to PERMANOVA. Niche breadth decreased during ontogeny, and the niche overlap among the size classes was low ( 0.80) in both zones. Together, these observations suggest that the feeding niche in the larger-sized individuals of this species tends to overlap irrespective of the dietary differences between different areas, which can indicate some degree of narrowing in morphological and behavioural features. The increase in body size rather than spatial dietary differences seems to be a major determinant for intraspecific changes in feeding habits. Therefore, diet partitioning along growth, rather than dietary spatial change, seems to be the main mechanism used by this species to decrease intraspecific competition.

Diet composition of New Zealand’s endemic rough skate, Zearaja nasuta

ABSTRACTDiet analysis can provide an insight into the structure and function of an ecosystem, and can be used in ecosystem-based frameworks to inform management and conservation decisions. Diet composition of rough skate (Zearaja nausta) from the south east coast of New Zealand, was investigated for the first time. We examined 35 stomachs from three trawls between March and June 2017. Prey importance was assessed by a prey specific index of relative importance (PSIRI). This population of rough skates (n = 32) was found to be specialised feeders, primarily preying on one species, Nectocarcinus antarcticus. This study provides the first insight into rough skate trophic interactions in the surrounding marine community.

Differences in trophic resources and niches of two juvenile predatory species in three Pangani estuarine zones, Tanzania: stomach contents and stable isotope approaches

BackgroundEstuaries are primary habitats that serve as feeding and nursery grounds for most juvenile marine fish. However, estuaries have been used as fishing grounds by the artisanal fishers in Tanzania. The slow-growing predatory fish at juvenile and sub-adult stages are among the most frequently caught species that functionally enhance multiple linkages of energy pathways within the food web. Stomach contents and stable isotopes (δ13C and δ15N) were used to describe the nutritional sources and trophic niches between the co-existing benthic, predatory species, Carangoides chrysophrys and Epinephelus malabaricus in the Pangani estuary, Tanzania.ResultsThe findings indicated significant inter-specific variations in dietary composition (PERMANOVA, p = 0.001, pseudo-F = 15.81). The prey-specific index of relative importance (%PSIRI) indicated that juvenile shrimps (%PSIRI = 51.4) and Teleostei (%PSIRI = 26.5) were the main diets of C. chrysophrys while brachyura (%PSIRI = 38.8), juvenile shrimps (%PSIRI = 25.6) and Teleostei (%PSIRI = 23.3) were important diets of E. malabaricus. The isotope mixing models indicated that the predatory fish species accumulate nutrients derived from similar autotrophic sources, microphytobenthos, seagrass and macro-algae via consumption of small fish, including clupeids and mugilids. Yet, they significantly showed different isotopic niche width with varying degree of niche overlap across the longitudinal estuary gradient. This situation was justified by the presence of basal food sources among the estuarine zones that isotopically were different.ConclusionThe reliance of both predators on clupeids and mugilid preys that are trophically linked with estuarine and marine basal food sources, is an indication of low estuarine food webs’ connectivity to the fresh water related food web. This situation is most likely threatening the stability of the estuarine food web structure. Management strategies and plans in place should be cautiously implemented to ensure the balanced anthropogenic freshwater use in the catchment and fishing activities, for the maintenance of the Pangani estuarine ecosystem health.

Trophic ecology of Magellanic penguins (Spheniscus magellanicus) during the non-breeding period

Some top predators couple their migration with the migration of their main prey. Magellanic penguins, Spheniscus magellanicus, migrate north from their Patagonian colonies during their non-breeding period, supposedly following the Argentine anchovy, Engraulis anchoita, a keystone species and their main prey item in the southwestern Atlantic Ocean. However, previous studies based on stranded carcasses suggest that cephalopods are the main food item for the penguins at their Brazilian wintering grounds. Moreover, anchovy stocks are in the early stages of commercial exploitation in southern Brazil. This study aims to investigate the importance of the Argentine anchovy in the diet of Magellanic penguins, including penguins of different age classes, both healthy and debilitated individuals, and in different parts of their annual cycle, using stomach content analysis and stable isotope analysis of multiple tissues, representing different time windows. Juvenile and adult penguins were collected (n = 54), either stranded on the beach or incidentally killed during gillnet fishing in adjacent waters. Penguins collected at sea had higher body mass indexes compared to stranded, demonstrating that they were healthy individuals. Among adult penguins from both areas (n = 21), fish was the main food item in their stomachs (prey-specific index of relative importance, PSIRI = 86%), with high contribution of Argentine anchovy. Cephalopods were the main food item (PSIRI = 71%) for juveniles (n = 20), with a predominance of the squid Doryteuthis sanpaulensis. In liver, muscle and feathers of penguins, mixed models based on δ13C and δ15N values demonstrated large contribution of the Argentine anchovy in the diet of the juveniles (95% credibility interval = 46–98.6%) and adults (39.8–98.9%), despite the high importance of cephalopods in the juvenile diet as identified in the stomach content analysis. Adults and juveniles presented isotopic niche overlap for all the tissues analysed, despite the differences in isotopic niche areas (= niche width) among the age classes. In 29% of the stomach contents, plastic debris was also found. Our results highlight the importance of using complementary techniques to study trophic ecology, as stomach content analysis of individuals found dead on beaches may not provide reliable information on diet. The strong reliance of penguins on the Argentine anchovy as a key resource throughout its annual cycle suggests that the stocks of this small pelagic fish should be exploited sustainably, ensuring the minimal forage biomass needed not only for seabirds but also for other consumers.

Striped marlin (Kajikia audax) diet variability off Cabo San Lucas, B.C.S., Mexico during El Niño–La Niña events

The stomach contents of 1261 striped marlins caught from 2007 to 2010 by the sport-fishing fleet of Cabo San Lucas, Baja California Sur, Mexico, were analysed, taking into account sea surface temperatures estimated using satellite images of the study area and the Oceanic Niño Index (ONI). The analysis allowed the identification of three anomalously cold periods during 2007, 2008 and 2010 and a warm period in 2009. The stomach content results showed 89 different prey items eaten during the study period. Of the specimens captured, 868 (69%) stomachs contained food items. The most important prey items were cephalopods (Dosidicus gigasandArgonautaspp.) and fish (Scomber japonicus,Selar crumenophthalmusandSardinops caeruleus). Despite having a broad trophic spectrum, the striped marlin was classified as a specialist consumer, withBi = 0.03. Based on the diet breadth value and the prey-specific index of relative importance (PSIRI), the jumbo squid,D. gigas, was the main prey species, coinciding with cold periods. On the other hand, the fishesS. caeruleusandS. japonicusshowed the highest PSIRI values during the 2009 warm anomalies. Analysis of similarity showed the formation of four groups in relation to PSIRI values. According to a non-metric multidimensional scaling ordination, these groups were formed with a similarity of 30% and a stress of 0.17. These results demonstrate inter-annual variation in the diet as well as in the availability and abundance of prey items.

Do abiotic and ontogenetic factors influence the diet of a generalist predator? Feeding ecology of the Pacific spiny dogfish (Squalus suckleyi) in the northeast Pacific Ocean

Stomach contents from 1221 Pacific spiny dogfish (Squalus suckleyi) were collected from the Gulf of Alaska over 2004–2006 and analyzed to determine predominant prey species. Pacific spiny dogfish in the Gulf of Alaska have a variable diet suggesting that they are generalist feeders. Of the 68.30% of stomachs with prey contents, the most important prey groups based on the percent prey-specific index of relative importance (%PSIRI) were shrimp (27.06%), cephalopods (17.16%), and forage fish (17.11%). Remaining components of the diet were inconsistent. Commercially valuable species, such as salmon, Oncorhynchus spp., and rockfish, Sebastes spp. constituted only 2.37% and 1.51% of the diet of Pacific spiny dogfish, respectively. Diet diversity was not significantly influenced by spatial, temporal or ontogenetic factors; however, trophic level of prey and average prey item weight varied both interannually and with ontogeny. With increasing size, Pacific spiny dogfish incorporate larger prey items into their diet, and diet composition may be driven more by seasonal availability and prey size than any other factors.