Exploring variability in the diet of depredating sperm whales in the Gulf of Alaska through stable isotope analysis

Sperm whales interact with commercially important groundfish fisheries offshore in the Gulf of Alaska (GOA). This study aims to use stable isotope analysis to better understand the trophic variability of sperm whales and their potential prey, and to use dietary mixing models to estimate the importance of prey species to sperm whale diets. We analysed tissue samples from sperm whales and seven potential prey (five groundfish and two squid species). Samples were analysed for stable carbon and nitrogen isotope ratios, and diet composition was estimated using Bayesian isotopic mixing models. Mixing model results suggest that an isotopically combined sablefish/ dogfish group, skates and rockfish make up the largest proportion of sperm whale diets (35%, 28% and 12%) in the GOA. The top prey items of whales that interact more frequently with fishing vessels consisted of skates (49%) and the sablefish/dogfish group (24%). This is the first known study to provide an isotopic baseline of adult male sperm whales and these adult groundfish and offshore squid species, and to assign contributions of prey to whale diets in the GOA. This study provides information to commercial fishermen and fisheries managers to better understand trophic connections of important commercial species.Data were collected in collaboration with Cascadia Research Collective, Scripps Institution of Oceanography, Alaska Sea Life Center, Alaska Longline Fishermen’s Association and the Sitka Sound Science Center. SEASWAP co-PIs were all integral in making this project happen: Linda Behnken, Dan Falvey, Victoria O’Connell, Aaron Thode and Russ Andrews. John Calambokidis and Greg Schorr collected biopsy samples used in this project. Kelly Robertson and Gabriela Serra-Valente archived samples at Southwest Fisheries Science Center. Special thanks to the commercial longline fishermen who donated fish and squid that they caught: Frank Balovich and Cale Laduke (F/V Carole D), Paul Ipok (F/V Myra), Walt Cunningham and Jeff Farvour (F/V Christi-Rob), Ryan Nichols (F/V Nekton), Stephen Rhoads and Nick Nekeferof (F/V Magia), Phil Wyman and Kevin Johnson (F/V Archangel), Lucas Skordahl (F/V Tyee), Tyrus Moffitt and Alek Dyakanoff. NMFS GOA longline survey, bottom trawl survey and ecosystem assessment cruise personnel collected specimens: Chris Lunsford, Cindy Tribuzio, Pete Hulson, Dana Hanselman, Cheryl Barnes, Nancy Roberson, Jamal Moss and Wes Strasburger. Laboratory and analysis assistance provided by Illiana Ruiz-Cooley, Todd Miller, Casey Clark, John Logan, Andrew Parnell, Ellen Chenoweth, Madison Kosma, Mike Sigler, Corey Fugate, Matt Rogers, Kate Hauch, Michelle Parke, Kristina Long, Nevé Baker, Emily Whitney and Annie Masterman. Jen Cedarleaf archived historical samples and managed the database. The Inter-Library-Loan folks with the UAF Rasmussen library found all kinds of crazy whaling documents. Finally, special thanks to the Alaska Stable Isotope Facility team of MatWooller, Tim Howe and Norma Haubenstock for their work running bulk isotopes for all of these samples.Ye

What Happened to Gray Whales during the Pleistocene? The Ecological Impact of Sea-Level Change on Benthic Feeding Areas in the North Pacific Ocean

Gray whales (Eschrichtius robustus) undertake long migrations, from Baja California to Alaska, to feed on seasonally productive benthos of the Bering and Chukchi seas. The invertebrates that form their primary prey are restricted to shallow water environments, but global sea-level changes during the Pleistocene eliminated or reduced this critical habitat multiple times. Because the fossil record of gray whales is coincident with the onset of Northern Hemisphere glaciation, gray whales survived these massive changes to their feeding habitat, but it is unclear how.We reconstructed gray whale carrying capacity fluctuations during the past 120,000 years by quantifying gray whale feeding habitat availability using bathymetric data for the North Pacific Ocean, constrained by their maximum diving depth. We calculated carrying capacity based on modern estimates of metabolic demand, prey availability, and feeding duration; we also constrained our estimates to reflect current population size and account for glaciated and non-glaciated areas in the North Pacific. Our results show that key feeding areas eliminated by sea-level lowstands were not replaced by commensurate areas. Our reconstructions show that such reductions affected carrying capacity, and harmonic means of these fluctuations do not differ dramatically from genetic estimates of carrying capacity.Assuming current carrying capacity estimates, Pleistocene glacial maxima may have created multiple, weak genetic bottlenecks, although the current temporal resolution of genetic datasets does not test for such signals. Our results do not, however, falsify molecular estimates of pre-whaling population size because those abundances would have been sufficient to survive the loss of major benthic feeding areas (i.e., the majority of the Bering Shelf) during glacial maxima. We propose that gray whales survived the disappearance of their primary feeding ground by employing generalist filter-feeding modes, similar to the resident gray whales found between northern Washington State and Vancouver Island

Stable isotopes indicate population structuring in the Southwest Atlantic population of right whales (Eubalaena australis)

From the early 17th century to the 1970s southern right whales, Eubalaena australis, were subject to intense exploitation along the Atlantic coast of South America. Catches along this coast recorded by whalers originally formed a continuum from Brazil to Tierra del Fuego. Nevertheless, the recovery of the population has apparently occurred fragmentarily, and with two main areas of concentration, one off southern Brazil (Santa Catarina) and another off central Argentina (Peninsula Valdés). This pattern suggests some level of heterogeneity amongst the population, which is apparently contradicted by records that traced individuals moving throughout the whole geographical extension covered by the species in the Southwest Atlantic. To test the hypothesis of the potential occurrence of discrete subpopulations exploiting specific habitats, we investigated N, C and O isotopic values in 125 bone samples obtained from whaling factories operating in the early 1970s in southern Brazil (n = 72) and from contemporary and more recent strandings occurring in central Argentina (n = 53). Results indicated significant differences between the two sampling areas, being δ13C and δ18O values significantly higher in samples from southern Brazil than in those from central Argentina. This variation was consistent with isotopic baselines from the two areas, indicating the occurrence of some level of structure in the Southwest Atlantic right whale population and equally that whales more likely feed in areas commonly thought to exclusively serve as nursing grounds. Results aim at reconsidering of the units currently used in the management of the southern right whale in the Southwest Atlantic Ocean. In the context of the current die-off affecting the species in Peninsula Valdés, these results also highlight the necessity to better understand movements of individuals and precisely identify their feeding areas