Effects of body size, age and maturity stage on diet in a large shark: ecological and applied implications

Ontogenetic diet shifts are a widespread phenomenon among vertebrates, although their relationships with life history traits are poorly known. We analyzed the relative importance of body size, age and maturity stage as determinants of the diet of a marine top predator, the copper shark, Carcharhinus brachyurus, by examining stomach contents using a multiple-hypothesis modeling approach. Copper sharks shifted their diet as size and age increased and as they became sexually mature, incorporated larger prey as they grew, and had a discrete shift in diet with body size, with only individuals larger than ≈200 cm total length able to prey on chondrichthyans. Body size was the most important trait explaining the consumption of chondrichthyans, while age determined the consumption of pelagic teleosts. Pelagic teleosts were consumed mostly by medium-aged sharks, a result, probably, of a risk-reducing feeding strategy at young ages coupled with either a senescence-related decline in performance or a change in sensory capabilities as sharks age. Copper sharks of all sizes were able to cut prey in pieces, implying that gape limitation (i.e., the impossibility of eating prey larger than a predator’s mouth) did not play a role in producing the diet shift. Our results suggest that, contrary to the current practice of setting minimum but not maximum size limits in catches, any plan to conserve or restore the ecological function of sharks, through their predatory control of large prey, should aim to maintain the largest individuals.     

Size, Sex And Geographic Variation in the Diet of the Tiger Shark, Galeocerdo Cuvier, From Western Australian Waters

Stomach contents from tiger sharks, Galeocerdo cuvier, caught on lines off the central coast of Western Australia were analysed to investigate variations in the diet due to sex, size and geographic location. Stomachs from 84 specimens contained food, while 26 had empty stomachs and 66 had regurgitated. Twelve prey groups were identified, the most common being turtles, sea snakes, teleost fishes, dugongs and sea birds. Dietary overlap was high between males and females. An ontogenetic shift was observed in the diet. Smaller prey (e.g. cephalopods, teleosts and sea snakes) were more common in small individuals, while the occurrence of larger prey (e.g. turtles, dugongs and elasmobranchs) increased with increasing shark size. Differences in the diet were observed between four regions along the central Western Australian coast. The ability to catch and consume large prey, prey availability, prey density, and prey profitability were identified as factors influencing the diet. The high level of occurrence of dugongs and turtles in the diet of G. cuvier, relative to their abundance, suggests that shark predation may play an important role in regulating populations of these species.     

Food habits and ontogenetic changes in the diet of the sandbar shark, Carcharhinus plumbeus, in Hawaii

The sandbar shark, Carcharhinus plumbeus, is a wide-ranging coastal species in tropical and temperate regions, and it is the most common species of shark in Hawaii, as in many locations where it occurs. Information on the diet and feeding habits of this species in the Pacific Ocean are extremely limited. For this study we quantified the diet of sandbar sharks in Hawaii based on records collected during the Hawaii Cooperative Shark Research and Control Program from 1967 to 1969. During this program a total of 565 stomachs were examined, of which 265 contained food. Sharks ranged in size from 59 to 190 cm total length. Teleosts were the most common prey group, but both cephalopods and crustaceans also occurred frequently. Ontogenetic changes in diet of sandbar sharks were apparent, with crustaceans forming a greater proportion of the diet of smaller sharks. Both cephalopods and elasmobranchs increased in importance with increasing shark size. Prey diversity also increased with size, with large, mobile, and reef prey species found more commonly in the diet of larger sharks. Mature male and female sharks appeared to segregate by depth, though major differences in the diet between the sexes were not apparent. However, there was some evidence of dietary differences between sharks caught in different depths and seasons. The results of this study suggest that sandbar sharks in Hawaii and throughout the world, are primarily piscivores, but also consume a variety of invertebrate prey, and that their diet varies with geographical location and stage of development.     

Influence of teleost abundance on the distribution and abundance of sharks in Florida Bay, USA

Understanding the factors that influence the distribution and abundance of predators, including sharks, is important for predicting the impacts of human changes to the environment. Such studies are particularly important in Florida Bay, USA where there are planned large-scale changes to patterns of freshwater input from the Everglades ecosystem. Studies of many marine predators suggest that links between predator and prey habitat use may vary with spatial scale, but there have been few studies of the role of prey distribution in shaping habitat use and abundance of sharks. We used longline catches of sharks and trawls for potential teleost prey to determine the influence of teleost abundance on shark abundance at the scale of regions and habitats in Florida Bay. We found that shark catch per unit effort (CPUE) was not linked to CPUE ofteleosts at the scale of sampling sites, but shark CPUE was positively correlated with the mean CPUE for teleosts within a region. Although there does not appear to be a strong match between the abundance of teleosts and sharks at small spatial scales, regional shark abundance is likely driven, at least partially, by the availability of prey. Management strategies that influence teleost abundance will have cascading effects to higher trophic levels in Florida Bay. Electronic Supplementary Material is available for this article at     

Feeding ecology and trophic segregation of two sympatric mesopredatory sharks in the heavily exploited coastal ecosystem of the Adriatic Sea

The ecological roles and trophic interactions of two commercially important mesopredatory shark species, Squalus acanthias and Mustelus punctulatus that co‐occur on the continental shelf of the north‐central Adriatic Sea were investigated. Both shark species are dietary specialists, with a significant dietary overlap recorded only during the spring season. They showed different patterns of feeding as they grew: S. acanthias extended its trophic niche with an increase in size, while M. punctulatus developed a more specialized diet. These two sharks partition food resources and reduce niche overlap by foraging at different trophic levels. Mustelus punctulatus is a crustacean feeder, specialized in foraging on scavenging malacostracans frequently found along trawl tracks or on discards in the Adriatic fishing zone. Conversely, S. acanthias prefers small pelagic fishes, which are commercially exploited and in decline. The different foraging strategies adopted by these two species suggest that they should be managed separately. Dietary specialization, direct competition with humans for prey and their higher intrinsic vulnerability make S. acanthias particularly susceptible to the effects of anthropogenic perturbations.     

Diet of Pacific sleeper shark, a potential Steller sea lion predator, in the north-east Pacific Ocean

Pacific sleeper sharks Somniosus pacificus were captured near Steller sea lion Eumetopias jubatus rookeries during the period when Steller sea lion pups are most vulnerable to Pacific sleeper shark predation (first water entrance and weaning). Analysis of stomach contents revealed that teleosts were the dominant prey in August and cephalopods were the dominant prey in May ( n = 198). Marine mammals were found in 15% of stomachs regardless of season, but no Steller sea lion tissues were detected. Molecular genetic analysis identified grey whale Eschrichtius robustus and harbour seal Phoca vitulina remains in some Pacific sleeper shark stomachs. Most mammals were cetacean and at least 70% of the cetaceans were probably scavenged. Although Pacific sleeper shark and Steller sea lion ranges overlapped, so predation could potentially occur, the diet study suggested that predation on Steller sea lions is unlikely, at least when pups first enter the water or during weaning. Harbour seals were infrequent prey and may have been consumed alive. Pacific sleeper sharks consume fast-swimming prey like Pacific salmon Oncorhynchus sp., most likely live animals rather than scavenged animals. Pacific sleeper sharks appeared to be opportunistic consumers of the available prey and carrion, feeding both on the bottom and in the water column, and their diet shifted to teleosts and cetacean carrion as the fish grew larger.     

Ontogenetic dietary shifts and feeding behavior of the tiger shark, Galeocerdo cuvier, in Hawaiian waters

Synopsis Stomach content data from 281 tiger sharks caught during shark control programs in Hawaii between 1967 and 1969, and during 1976 were analyzed to examine feeding habits and ontogenetic shifts in diet. As sharks increased in size, prey diversity and frequency of occurrence of large prey items increased. The percent occurrence of teleosts and cephalopods in stomachs decreased as sharks increased in length, while occurrence of elasmobranchs, turtles, land mammals, crustaceans, and undigestible items increased. Comparisons between the diets of tiger sharks from Hawaii and other locations indicate that ontogenetic shifts are universal in this species and that tiger sharks may be opportunistic feeders that prey heavily on abundant, easy to capture prey. Small tiger sharks may be spatially segregated from medium and large sharks and appear to be primarily nocturnal, bottom feeders. Large tiger sharks feed near the bottom at night, but also feed at the surface during the day. Prey, similar in size to humans, begin to occur in the diet of tiger sharks approximately 230 cm TL, and therefore sharks of this size and larger may pose the greatest threat to humans. Ontogenetic shifts in diet may be attributed to increased size of sharks, expanded range and exploitation of habitats of larger sharks, and/or improved hunting skill of larger sharks.Deceased 1974     

Diet of two commercially important shark species in the United Arab Emirates: milk shark,Rhizoprionodon acutus (Rüppell, 1837), and slit‐eye shark,Loxodon macrorhinus (Müller & Henle, 1839)

The diets of the milk shark, Rhizoprionodon acutus, and the slit‐eye shark, Loxodon macrorhinus, landed from the artisanal fishery in the Arabian Gulf waters of the United Arab Emirates were investigated to determine their dietary preferences. Stomach contents from 57 milk sharks and 53 slit eye sharks were collected from Abu Dhabi (R. acutus, n = 23), Dubai (R. acutus, n = 5; L. macrorhinus, n = 15) and Ras Al Khaimah (R. acutus, n = 29; L. macrorhinus, n = 38) during fishery surveys from January to May 2012. Prey items were identified to the lowest possible taxonomic level, grouped into five categories including ‘teleost fish’, ‘cephalopods’, ‘crustaceans’, ‘invertebrates’, and ‘other’. The diets of both species were described using the numeric, frequency and weight methods, and the index of relative importance (IRI). The majority of stomachs for both species had food, with 66.6% of milk shark stomachs and 90.5% of slit‐eye shark stomachs containing prey items, both dominated by small teleosts. Rhizoprionodon acutus fed on a wide variety of teleost species, primarily Engraulidae (anchovies) (28%), Gerreidae (mojarras) (5.6%) and Carangidae (jacks) (1.6%) with occasional crustacean and cephalopod prey (8%). On the other hand, L. macrorhinus seemed to have a preference for one species in terms of teleosts (anchovies) (35.1%) and fed on a wider variety of crustaceans and cephalopods (22.6%). There was little overlap in the diets of these two species, suggesting that they may either be using different habitats or that in these waters, the milk shark is a generalist species while the slit‐eye is a specialist feeder.     

Modulation in Feeding Kinematics and Motor Pattern of the Nurse Shark <Emphasis Type="Italic">Ginglymostoma cirratum</Emphasis>

Synopsis Studies of feeding in bony fishes have almost universally demonstrated the ability of individuals to modulate their method of capture in response to differing stimuli. Preliminary evidence indicates that morphologically specialized inertial suction feeding sharks are the most likely fishes to lack inherent modulatory ability. We examined the ability of the nurse shark, Ginglymostoma cirratum, to modulate its feeding behavior based on different food types and sizes. G. cirratum is an inertial suction feeding fish that is apparently stereotyped in its food capture behavior. Electromyography showed no statistical difference between feeding motor patterns based on food type (squid or fish) or size (gape width or twice gape width), although there were slight inter-individual differences in the onset of muscle firing for some muscles. Kinematic analysis showed a statistical difference in variables associated with durations for different food types, with the durations for all variables being faster for squid bites than fish bites, but no difference based on the size of the food item. This apparent lack of modulation may be associated with specialization of the morphology and behavior of G. cirratum for obligate suction prey capture. This functional specialization constrains the method in which G. cirratum captures prey but does not appear to result in dietary specialization. An unusual post capture spit-suck manipulation allows this shark to handle and ingest large prey.     

Protection from illegal fishing and shark recovery restructures mesopredatory fish communities on a coral reef

The recovery of communities of predatory fishes within a no‐take marine reserve after the eradication of illegal fishing provides an opportunity to examine the role of sharks and other large‐bodied mesopredatory fishes in structuring reef fish communities. We used baited remote underwater video stations to investigate whether an increase in sharks was associated with a change in structure of the mesopredatory fish community at Ashmore Reef, Western Australia. We found an almost fourfold increase in shark abundance in reef habitat from 0.64 hr^?1 ± 0.15 SE in 2004, when Ashmore Reef was being fished illegally, to 2.45 hr^?1 ± 0.37 in 2016, after eight years of full‐time enforcement of the reserve. Shark recovery in reef habitat was accompanied by a two and a half‐fold decline in the abundance of small mesopredatory fishes (≤50 cm TL) (14.00 hr^?1 ± 3.79 to 5.6 hr^?1 ± 1.20) and a concomitant increase in large mesopredatory fishes (≥100 cm TL) from 1.82 hr^?1 ± 0.48 to 4.27 hr^?1 ± 0.93. In contrast, near‐reef habitats showed an increase in abundance of large mesopredatory fishes between years (2.00 hr^?1 ± 0.65 to 4.56 hr^?1 ± 1.11), although only smaller increases in sharks (0.67 hr^?1 ± 0.25 to 1.22 hr^?1 ± 0.34) and smaller mesopredatory fishes. Although the abundance of most mesopredatory groups increased with recovery from fishing, we suggest that the large decline of small mesopredatory fish in reef habitat was mostly due to higher predation pressure following the increase in sharks and large mesopredatory fishes. At the regional scale, the structure of fished communities at Ashmore Reef in 2004 resembled those of present day Scott Reefs, where fishing still continues today. In 2016, Ashmore fish communities resembled those of the Rowley Shoals, which have been protected from fishing for decades.     

Food habits of the shortfin mako, Isurus oxyrinchus, off the southwest coast of Portugal

The shortfin mako, Isurus oxyrinchus, is caught in the eastern North Atlantic as a regular bycatch of the surface-drift longline fishery, mainly directed towards swordfish, Xiphias gladius. Stomachs of 112 shortfin mako sharks, ranging in size from 64 cm to 290 cm fork length, showed teleosts to be the principal component of the diet, occurring in 87% of the stomachs and accounting for over 90% of the contents by weight. Crustaceans and cephalopods were also relatively important in this species’ diet, whereas other elasmobranchs were only present in lower percentages. Meal overlap was observed in half of the sampled sharks. No clear trend of prey size selectivity was found, despite smaller individuals seeming incapable of pursuing larger and faster prey. The retention of small prey was also observed in the diet of all sizes of shark. Seasonality in food habits was in accordance with the current availability of food items. The observed vacuity index of 12% is comparable to foraging ecology studies using gillnetting and appears not to be influenced by baited longline gear. Morphological relationships of the digestive system might add important information to the foraging ecology studies and to ecosystem modelling.     

Ontogenetic diet shifts and prey selection in nursery bound lemon sharks, Negaprion brevirostris, indicate a flexible foraging tactic

Ontogenetic variations in shark diet are often qualitatively inferred from dietary analysis and hindered by high levels of unidentified prey or small sample sizes. This study focused on nursery bound lemon sharks (Negaprion brevirostris, n?=?396), enabling some control over the confounding variables of prey choice associated with ontogeny. Nursery bound lemon sharks exhibited weak ontogenetic variation in dietary composition with high levels of dietary overlap. Variation in prey preference of lemon sharks with ontogeny was complex, but revealed a continuous shift from predominantly opportunistic benthic foraging as neonates to more selective piscivory with increasing shark size while in the nursery. Lemon sharks demonstrated a discrete ontogenetic shift in the number of prey consumed and stomach content weight (Kruskal-Wallis tests p?<?0.01), as well as prey size (ANOVA, p?<?0.001). All sizes of sharks exhibited positive size selection of prey (Mann?CWhitney U tests, p?<?0.01). However, the lack of size preference by all but the largest lemon sharks for their major prey (yellowfin mojarra, Gerres cinereus), suggests neonate sharks, while capable of occasionally foraging on large prey, are relatively inept opportunistic foragers. This was evident in high diet breadth, low diversity of consumed prey and lower trophic level than larger sharks. This study represents the first quantitative analysis of ontogenetic variation in prey preference and size selection in sharks, indicating a flexible foraging tactic in lemon sharks and the importance of hunting ability and predator size in prey choice.     

Brain morphology in large pelagic fishes: a comparison between sharks and teleosts

A quantitative comparison was made of both relative brain size (encephalization) and the relative development of five brain area of pelagic sharks and teleosts. Two integration areas (the telencephalon and the corpus cerebellum) and three sensory brain areas (the olfactory bulbs, optic tectum and octavolateralis area, which receive primary projections from the olfactory epithelium, eye and octavolateralis senses, respectively), in four species of pelagic shark and six species of pelagic teleost were investigated. The relative proportions of the three sensory brain areas were assessed as a proportion of the total 'sensory brain', while the two integration areas were assessed relative to the sensory brain. The allometric analysis of relative brain size revealed that pelagic sharks had larger brains than pelagic teleosts. The volume of the telencephalon was significantly larger in the sharks, while the corpus cerebellum was also larger and more heavily foliated in these animals. There were also significant differences in the relative development of the sensory brain areas between the two groups, with the sharks having larger olfactory bulbs and octavolateralis areas, whilst the teleosts had larger optic tecta. Cluster analysis performed on the sensory brain areas data confirmed the differences in the composition of the sensory brain in sharks and teleosts and indicated that these two groups of pelagic fishes had evolved different sensory strategies to cope with the demands of life in the open ocean.     

Diet composition of smooth-hound shark, Mustelus mustelus (Linnaeus, 1758), in the Gulf of Gabès, southern Tunisia

The diet of the smooth-hound shark, Mustelus mustelus , from the Gulf of Gabès (southern Tunisia, central Mediterranean Sea) was investigated with respect to fish size and season. Stomach contents were analyzed from 540 specimens with total lengths ranging from 34 to 158.5 cm. Of the total number of stomachs examined, 63 were empty (11.67%). Smooth-hound shark fed mainly on crustaceans, fishes and cephalopods. Sipunculids, polychaetes and echinoderms were occasional preys. No differences were found between the diets of males and females. Ontogenetic changes in diet of M. mustelus were apparent, with crustaceans forming a greater proportion of the diet of smaller sharks. Both teleosts and molluscs increased in importance with increasing shark size. Consumptions of polycheates, sipunculids and echinoderms were not related to predator size. Prey diversity also increased with size, with large and mobile prey species found more commonly in the diet of larger sharks. The limited overlap in the dietary compositions of juveniles, subadults and adults suggests the possibility of resource partitioning. Seasonality in food habits was in accordance with the dynamics of the predator and the prey species.     

Dietary variations within a family of ambush predators (Platycephalidae) occupying different habitats and environments in the same geographical region

This study has determined the extents and basis for variations in the composition of the prey ingested by the abundant species of a family highly adapted for ambush predation, i.e. Platycephalidae, in a region (south‐western Australia) where that family is found in different habitats and environments. Dietary data were thus collected for Leviprora inops and Platycephalus laevigatus from seagrass in marine embayments and for Platycephalus westraliae from over sand in an estuary. These were then collated with those recorded previously for Platycephalus speculator from over sand and in seagrass in an estuary and for Platycephalus longispinis from over sand in coastal marine waters. While crustaceans and teleosts together dominated the diet of all five species, their percentage volumetric dietary contributions varied greatly, with those of crustaceans ranging from 7% for L. inops to 65% for P. speculator and those of teleosts ranging from 29% for P. longispinis to 91% for L. inops. For analyses, the data were separated into two sets. The first comprised the 17 dietary categories of invertebrates and all identified and unidentified teleosts collectively, while the second consisted of the 23 identified teleost families, both of which were subjected to permutational analysis of variance (PERMANOVA), analysis of similarities (ANOSIM) and a new (two‐way) version of the RELATE procedure. The diets of three species changed seasonally, when using invertebrate dietary categories and teleosts collectively, but with only one species, when employing identified teleost families, probably reflecting a greater tendency for invertebrate than teleost prey abundance to change during the year. On the basis of dietary data for invertebrate taxa + teleosts collectively, the diets of three of the five species changed serially with body size, with a fourth species feeding, throughout life, predominantly on the carid Palaemonetes australis. Based on identified teleost families, the diets of the three species that fed predominantly on teleosts underwent serial size‐related changes. Although L. inops and the co‐occurring P. laevigatus both consume large volumes of teleosts, the former ingests larger, less demersal and more mobile prey, e.g. the labrids Haletta semifasciata and Neoodax balteatus, than the latter, e.g. the scorpaenid Gymnapistes marmoratus, reflecting the possession by L. inops of a far longer head and larger buccal cavity. Circumstantial evidence suggests that the large differences in the volumes of crustaceans and teleosts consumed by each platycephalid species are related to differences in the relative availability of these prey in the different habitats or environments of each species.     

New insights into the trophic ecology of blacktip sharks (Carcharhinus limbatus) from a subtropical estuary in the western Gulf of Mexico

As environmental change persists, understanding resource use patterns is of value to predict the consequences of shifting trophic structures. While many sharks are opportunistic predators, some exhibit prey selectivity, putting them at higher risk compared to species with greater trophic plasticity. In the Gulf of Mexico (GOM), Clupeids and Sciaenids comprise 69% of blacktip shark (Carcharhinus limbatus) diets, which is consequential considering potential responses of these prey groups to disturbance and over harvesting. We assessed if blacktips exhibit selectivity for Clupeids and Sciaenids in the western GOM based on stomach contents from sharks in coastal Texas. Clupeids comprised <2% of diets, while striped mullet (Mugil cephalus) and red drum (Sciaenops ocellatus) comprised >70% of identifiable prey. Ontogenetic shifts from smaller (Clupeids, small Sciaenids) to larger, higher trophic level (Ariidae, Elasmobranchii) prey fits our understanding of foraging among coastal sharks, and suggests our regional understanding of blacktip trophic ecology may be limited by the sizes of sampled sharks. Observed increases in blacktip densities coupled with declines in prey (Mugilids, Sciaenids) is concerning if blacktips have limited diet plasticity. Yet GOM blacktips may be more generalized than previously thought, which is promising for conservation and management.     

Ontogeny of head and caudal fin shape of an apex marine predator: The tiger shark (Galeocerdo cuvier)

How morphology changes with size can have profound effects on the life history and ecology of an animal. For apex predators that can impact higher level ecosystem processes, such changes may have consequences for other species. Tiger sharks (Galeocerdo cuvier) are an apex predator in tropical seas, and, as adults, are highly migratory. However, little is known about ontogenetic changes in their body form, especially in relation to two aspects of shape that influence locomotion (caudal fin) and feeding (head shape). We captured digital images of the heads and caudal fins of live tiger sharks from Southern Florida and the Bahamas ranging in body size (hence age), and quantified shape of each using elliptical Fourier analysis. This revealed changes in the shape of the head and caudal fin of tiger sharks across ontogeny. Smaller juvenile tiger sharks show an asymmetrical tail with the dorsal (upper) lobe being substantially larger than the ventral (lower) lobe, and transition to more symmetrical tail in larger adults, although the upper lobe remains relatively larger in adults. The heads of juvenile tiger sharks are more conical, which transition to relatively broader heads over ontogeny. We interpret these changes as a result of two ecological transitions. First, adult tiger sharks can undertake extensive migrations and a more symmetrical tail could be more efficient for swimming longer distances, although we did not test this possibility. Second, adult tiger sharks expand their diet to consume larger and more diverse prey with age (turtles, mammals, and elasmobranchs), which requires substantially greater bite area and force to process. In contrast, juvenile tiger sharks consume smaller prey, such as fishes, crustaceans, and invertebrates. Our data reveal significant morphological shifts in an apex predator, which could have effects for other species that tiger sharks consume and interact with. J. Morphol. 277:556–564, 2016. © 2016 Wiley Periodicals, Inc.     

New insights on the trophic ecology of blue (Prionace glauca) and shortfin mako sharks (Isurus oxyrinchus) from the oceanic eastern South Pacific

The blue shark (Prionace glauca) and the shortfin mako shark (Isurus oxyrinchus) are two large and highly migratory sharks distributed in most oceans. Although they are often caught in the south Pacific Ocean long-line fisheries, their trophic ecology is poorly understood. Stable isotopes with Bayesian mixing and dependence concentration models were performed to determine the diet and trophic differences between the two species in the South-eastern Pacific Ocean. According to the mixing models, fishes are the most important prey of these sharks. Dolphin calves and remains were found in the stomachs of both species, which represents a novel finding in trophic ecology of South Pacific sharks. Intra-specific differences were found in P. glauca, but not in specimens of I. oxyrinchus. The two sharks showed a high degree of diet overlap (73%), primarily over mackerel and dolphin carcasses. Our results indicate that blue and shortfin mako sharks have a generalist feeding strategy in the eastern Pacific Ocean, with a strong preference for teleost fishes and also for dolphin carcasses. Therefore, trophic studies are useful to understand energy flow through the food web, and the trophic position of key species.     

Biology of the Galapagos shark,Carcharhinus galapagensis, in Hawai'i

Synopsis Catch records from the Hawai'i Cooperative Shark Research and Control Program, which operated in Hawai'i from 1967–1969, were examined and data on the Galapagos shark,Carcharhinus galapagensis were analyzed. A total of 304 Galapagos sharks was caught, predominantly with longlines. More female sharks were caught than males, and the catch was skewed geographically. On the island of O'ahu the highest catch rates occurred along the north and south coasts. High catch rates also occurred near points of land, where longshore currents converge. Average depth of capture was greater for juveniles (45.1 m) and mature males (60.2 m), than for subadults (38.8 m) and mature female sharks (34.2 m). Males appear to reach maturity between 205 and 239 cm total length, and females between 215 and 245 cm. Litter size ranged from 4 to 16 pups, with an average of 8.7. In Hawaiian waters Galapagos sharks are born at just over 80 cm total length. Mating and parturition apparently occur early in the year, and gestation is estimated to be about 12 months. Stomach contents consisted mainly of teleosts and benthic prey, and ontogenetic changes in diet occurred as sharks increased in size. Sharks consumed a smaller proportion of teleosts and more elasmobranchs with increasing size. Dietary diversity also increased with increasing size of shark.     

Gape-limitation,foraging tactics and prey size selectivity of two microcarnivorous species of fish

Summary Patterns of prey size selectivity were quantified in the field for two species of marine microcarnivorous fish, Embiotoca jacksoni and Embiotoca lateralis (Embiotocidae) to test Scott and Murdoch's (1983) size spectrum hypothesis. Two mechanisms accounted for observed selectivity: the relative size of a fish in relation to its prey, and the type of foraging behavior used. Juvenile E. jacksoni were gape limited and newborn individuals achieved highest selectivity for the smallest prey size by using a visual picking foraging strategy. As young E. jacksoni grew, highest preference shifted to the next larger prey sizes. When E. jacksoni reached adulthood, the principal mode of foraging changed from visual picking to relatively indiscriminant winnowing behavior. The shift in foraging behavior by adults was accompanied by a decline in overall preference for prey size; sizes were taken nearly in proportion to their relative abundance. Adult E. lateralis retained a visual picking strategy and achieved highest selectivity for the largest class of prey. These differences in selectivity patterns by adult fish were not explained by gape-limination since adults of both species could ingest the largest prey items available to them. These results support Scott and Murdoch's (1983) hypothesis that the qualitative pattern of size selectivity depends largely on the range of available prey sizes relative to that a predator can effectively harvest.