Dear MARMAM community, On behalf of my co-authors, I am pleased to share with you our two recent publications, published as companion papers in Marine Ecology Progress Series.
Bury, SJ, Peters, KJ, Sabadel, AJM, St John Glew, K, Trueman, C, Wunder, MB, Cobain, MRD, Schmitt, N, Donnelly, D, Magozzi, S, Owen, K, Brown, JCS, Escobar-Flores, P, Constantine, R, O’Driscoll, RL, Double, M, Gales, N, Childerhouse, S, Pinkerton, MH (2024) Southern Ocean humpback whale trophic ecology. I. Combining multiple stable isotope methods elucidates diet, trophic position and foraging areas. Marine Ecology Progress Series 734:123-155 - DOI: https://doi.org/10.3354/meps14532 ABSTRACT: Southern Ocean humpback whales Megaptera novaeangliae are capital breeders, breeding in the warm tropics/subtropics in the winter and migrating to nutrient-rich Antarctic feeding grounds in the summer. The classic feeding model is for the species to fast while migrating and breeding, surviving on blubber energy stores. Whilst northern hemisphere humpback whales are generalists, southern hemisphere counterparts are perceived as krill specialists, but for many populations, uncertainties remain regarding their diet and preferred feeding locations. This study used bulk and compound-specific stable isotope analyses and isoscape-based feeding location assignments to assess the diet, trophic ecology and likely feeding areas of humpback whales sampled in the Ross Sea region and around the Balleny Islands. Sampled whales had a mixed diet of plankton, krill and fish, similar to the diet of northern hemisphere humpback whales. Proportions of fish consumed varied but were often high (2-60%), thus challenging the widely held paradigm of Southern Ocean humpback whales being exclusive krill feeders. These whales had lower δ15N values and trophic position estimates than their northern hemisphere counterparts, likely due to lower Southern Ocean baseline δ15N surface water values and a lower percentage consumption of fish, respectively. Most whales fed in the Ross Sea shelf/slope and Balleny Islands high-productivity regions, but some isotopically distinct whales (mostly males) fed at higher trophic levels either around the Balleny Islands and frontal upwelling areas to the north, or en route to Antarctica in temperate waters off southern Australia and New Zealand. These results support other observations of humpback whales feeding during migration, highlighting the species’ dietary plasticity, which may increase their foraging and breeding success and provide them with greater resilience to anthropogenically mediated ecological change. This study highlights the importance of combining in situ field data with regional-scale isoscapes to reliably assess trophic structure and animal feeding locations, and to better inform ecosystem conservation and management of marine protected areas. Owen K, Thompson, RM, Donnelly, D, Noad, M, Bury, SJ, Pinkerton MH, Dunlop R (2024) Southern Ocean humpback whale trophic ecology. II. Influence of fasting and opportunistic feeding on skin stable isotope values of migrating whales. Marine Ecology Progress Series. 734:157-171 - DOI: https://doi.org/10.3354/meps14539. ABSTRACT: Many baleen whale species migrate between low-latitude breeding grounds and high-latitude feeding grounds, with increasing evidence that humpback whales Megaptera novaeangliae utilise supplemental feeding sites in temperate regions while migrating. The diet of whales while migrating is often unknown, and the impact that temperate feeding and/or fasting has on biochemical tracers used to investigate diet remains unclear. The aims of this study were to (1) determine whether prey consumption at supplemental feeding sites could be detected by carbon (δ13C) and nitrogen (δ15N) stable isotope analysis of skin; (2) obtain information on diet during migration; and (3) ascertain the impact of potential fasting on stable isotope values of baleen whales. Skin samples were taken from the eastern Australian humpback whale population on Antarctic feeding grounds and 2 sites on the southward migration route (a sub-tropical site and a temperate site) across 2 yr. At the sub-tropical site, δ13C and δ15N were consistent with the last place of foraging 5 mo earlier. One exception was the higher (0.5 ‰) δ15N value in 2011, suggesting that in some years, potentially when blubber reserves are insufficient, δ15N may be influenced by fasting. In both years, skin δ13C and δ15N values at the temperate site were higher than those from the Antarctic and sub-tropical sites, indicating that a feeding signal from temperate zones had likely been incorporated, with whales feeding on fish and krill. Importantly, supplemental feeding while migrating could affect the interpretation of whale diet on feeding grounds if sampled early in the season. Please feel free to contact me or Sarah Bury (sarah.b...@niwa.co.nz<mailto:sarah.b...@niwa.co.nz>) if you have any questions. Kind Regards Kylie Dr Kylie Owen Intendent Populationanalys och -övervakning Naturhistoriska riksmuseet Box 50007 | 104 05 Stockholm +46 (0) 761 701 594 [Logotyp för Naturhistoriska Riksmuseet] [Naturhistoriska riksmuseet, Forskar, Bevarar och Förklarar, NRM.SE]
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