How Many "Big Macs-worth" of Herring Can a Whale Eat?
| Marilyn Sigman, Alaska SeaGrant/MAP
Tags: ALaska Marine Ecosystems, Humpback Whales, Herring, Predation
A suite of studies presented at the 2010 Alaska Marine Science Symposium focused on humpback whales as a potential culprit restricting the recovery of herring stocks in Prince William Sound. The herring population crashed in the early 1990s shortly after the Exxon Valdez oil spill. During the same period, the endangered North Pacific humpback whale population was recovering and is now increasing at a rate of 5-7% each year. Whales have been observed frequently in PWS during the winter in recent years but no one really knew how many herring the whales were eating. (Teerlink et. al., 2010)
A team of nine scientists first determined how many whales were spending the winter in PWS by reviewing historical observations, identifying individuals by their fluke pattern, and marking and “recapturing” whales with more observations during 2008 and 2009. They estimated 250 and 220 whales in PWS during 2008 and 2009, respectively; with numbers peaking in October and November, dropping down to a low in January, and then building in numbers until March. These numbers translated into 230,000 and 235,000 “whale-days” when whales could have been feeding on herring during the winter. (Moran et. al., 2010)
The scientists next determined what the whales were eating. During their cruises, they watch whales feeding and used echosounders that allowed them to distinguish between fish and invertebrate prey (euphasiids) concentrations where the whales were feeding. They observed 199 whales feeding and a total of 255 groups of whales and were able to determine that the prey of 186 of the groups was herring 82% of the time. They further confirmed that whales were eating herring in later summer by taking biopsy samples and analyzing the composition of the whale blubber during the fall. (Straley et. al., 2010)
They then calculated the biomass of herring that the whales could be consuming, based on the number of whales, the number of whale-days of feeding, the energy demands of the whales, and the bioenergetics of whale migration, feeding efficiency, and other energy-related factors. According to their model, a 30-ton whale consumes 1.4-5.4 Gigajoules of energy every day – the equivalent of 660 – 2390 Big Macs. The outcome of their modeling efforts led to the conclusion that the whales could be consuming 18-23% of the total biomass of herring between September and March. Since the herring that survived the winter were the next year’s breeding stock, the scientists concluded that this rate of predation could be the factor restraining the recovery and was greater than the “surplus” of herring that would otherwise be available for commercial or subsistence fishing. (Heintz et. al., 2010)
Nine scientists collaborated on the series of studies whose methods and conclusions were summarized in four posters and one oral presentation. Additional studies are underway as part of an integrated study of disease and predation as significant factors affecting recovery of the herring stock.
Link to 2010 Alaska Marine Science Symposium abstractsLink to 2010 Alaska Marine Science Symposium abstracts
Teerlink, S.F., O. von Ziesegar, J.M. Straley, and T.J. Quinn. Humpback whale predation on Pacific herring Part 1: construction of a time-series of humpback whale abundance in Prince William Sound relative to Pacific herring dynamics. (Poster presentation)
Moran, J.R., J.M. Straley, R.A. Heintz. Whale predation on winter herring Part 2: how many whales are feeding on herring? (Poster presentation)
Straley, J.M., J.R. Moran, R.A. Heintz,L. Schaufler, K. McLaughlin, A. McLaughlin, S. Rice. Humpback whale predation on Pacific herring Part 3: what do humpback whales eat during the fall and winter in Prince William Sound? (Oral presentation)
Heintz, R.A. Humpback whale predation on Pacific herring Part 4: a review of models to predict whale ingestion rates. (Poster presentation)
Heintz, R.A., J.R. Moran, J.J. Vollenweider, J.M. Straley, T.J. Quinn, S.F. Teerlink, S. Rice. Humpback whale predation on Pacific herring Part 5: impacts of whale foraging on three herring populations in Alaska (Poster presentation)