Standardized diet compositions were derived for 97 species of marine mammals from published accounts of stomach contents as well as from morphological, behavioural and other information. Diet was apportioned among eight categories of prey types (benthic invertebrates, large zooplankton, small squids, large squids, small pelagic fishes, mesopelagic fishes, miscellaneous fishes and higher invertebrates). Trophic levels were estimated for each species of marine mammals and compared with published estimates derived using stable isotope ratios. Trophic levels ranged from 3.2-3.4 in baleen whales and sea otters, to 3.8-4.4 in most pinnipeds and odontocete whales, to 4.5-4.6 in killer whales. Such information can be used for ecosystem modelling and related studies.

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Diet composition and trophic levels of marine mammals

Preface.Acknowledgements.Introduction.Form and Function.Origin and Evolution.Nautilus: The Survivor.Biodiversity and Zoogeography.Life Cycle.Growth.Physiological Ecology.Reproductive.From Egg to Recruitment.Coastal and Shelf Species.Oceanic and Deep-Sea Species.Cephalopods as Predators.Cephalopods as Prey.Fishing Methods and Scientific Sampling.Fishery Resources.Fisheries Oceanography.Assessment and Management.Conclusions.Appendix A: Classification of Living Cephalopod Families.Appendix B: Synopsis of Living Cephalopod Families.References.Index.

Cephalopods: Ecology and Fisheries

Two forms of killer whale (Orcinus orca), resident and transient, occur sympatrically in coastal waters of British Columbia, Washington State, and southeastern Alaska. The two forms do not mix, and differ in seasonal distribution, social structure, and behaviour. These distinctions have been attributed to apparent differences in diet, although no comprehensive comparative analysis of the diets of the two forms had been undertaken. Here we present such an analysis, based on field observations of predation and on the stomach contents of stranded killer whales collected over a 20-year period. In total, 22 species of fish and 1 species of squid were documented in the diet of resident-type killer whales; 12 of these are previously unrecorded as prey of O. orca. Despite the diversity of fish species taken, resident whales have a clear preference for salmon prey. In field observations of feeding, 96% of fish taken were salmonids. Six species of salmonids were identified from prey fragments, with chinook salmon (Oncorhynchus tshawytscha ) being the most common. The stomach contents of stranded residents also indicated a preference for chinook salmon. On rare occasions, resident whales were seen to harass marine mammals, but no kills were confirmed and no mammalian remains were found in the stomachs of stranded residents. Transient killer whales were observed to prey only on pinnipeds, cetaceans, and seabirds. Six mammal species were taken, with over half of observed attacks involving harbour seals (Phoca vitulina). Seabirds do not appear to represent a significant prey resource. This study thus reveals the existence of strikingly divergent prey preferences of resident and transient killer whales, which are reflected in distinctive foraging strategies and related sociobiological traits of these sympatric populations. 1471

Dietary specialization in two sympatric populations of killer whales (Orcinus orca) in coastal British Columbia and adjacent waters

Long-term declines in oxygen concentrations are evident throughout much of the ocean interior and are particularly acute in midwater oxygen minimum zones (OMZs). These regions are defined by extremely low oxygen concentrations (<20–45 μmol kg−1), cover wide expanses of the ocean, and are associated with productive oceanic and coastal regions. OMZs have expanded over the past 50 years, and this expansion is predicted to continue as the climate warms worldwide. Shoaling of the upper boundaries of the OMZs accompanies OMZ expansion, and decreased oxygen at shallower depths can affect all marine organisms through multiple direct and indirect mechanisms. Effects include altered microbial processes that produce and consume key nutrients and gases, changes in predator-prey dynamics, and shifts in the abundance and accessibility of commercially fished species. Although many species will be negatively affected by these effects, others may expand their range or exploit new niches. OMZ shoaling is thus likely to hav...

Oceanographic and biological effects of shoaling of the oxygen minimum zone.

Summary 1 Empirical testing of optimal foraging models for breath-hold divers has been difficult Here we report data from sound and movement recording DTags placed on 23 short-finned pilot whales off Tenerife to study the foraging strategies used to catch deep-water prey 2 Day and night foraging dives had a maximum depth and duration of 1018 m and 21 min Vocal behaviour during dives was consistent with biosonar-based foraging, with long series of echolocation clicks interspersed with buzzes Similar buzzes have been associated with prey capture attempts in other echolocating species 3 Foraging dives seemed to adapt to circadian rhythms Deep dives during the day were deeper, but contained fewer buzzes (median 1), than night-time deep dives (median 5 buzzes) 4 In most deep (540‐1019 m) daytime dives with buzzes, a downward directed sprint reaching up to 9 m s ‐1 occurred just prior to a buzz and coincided with the deepest point in the dive, suggestive of a chase after escaping prey 5 A large percentage (10‐36%) of the drag-related locomotion cost of these dives (15 min long) is spent in sprinting (19‐79 s) This energetic foraging tactic focused on a single or few prey items has not been observed previously in deep-diving mammals but resembles the high-risk/high-gain strategy of some terrestrial hunters such as cheetahs 6 Deep sprints contrast with the expectation that deep-diving mammals will swim at moderate speeds optimized to reduce oxygen consumption and maximize foraging time at depth Pilot whales may have developed this tactic to target a deep-water niche formed by large/calorific/fast moving prey such as giant squid

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C E Nauen

Papers

FAO species catalogue. v.3: Cephalopods of the world. An annotated and illustrated catalogue of species of interest to fisheries.