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A Critical Review of the “Regime Shift-Junk Food” Hypothesis for the Steller Sea Lion Decline

A Critical Review of the “Regime Shift-Junk Food” Hypothesis for the Steller Sea Lion Decline. Lowell Fritz and Sarah Hinckley Alaska Fisheries Science Center National Marine Fisheries Service. Regime Shift/Junk Food Hypothesis.

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A Critical Review of the “Regime Shift-Junk Food” Hypothesis for the Steller Sea Lion Decline

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  1. A Critical Review of the “Regime Shift-Junk Food” Hypothesis for the Steller Sea Lion Decline Lowell Fritz and Sarah Hinckley Alaska Fisheries Science Center National Marine Fisheries Service

  2. Regime Shift/Junk Food Hypothesis • 1976/77 regime shift changed the fish community of the North Pacific Ocean from one “dominated” by clupeids-osmerids to gadid-flatfish • Sea lions ate more gadid and less clupeids-osmerids after the regime shift • Gadids are low quality food (“junk”) • Sea lion vital rates (survival, fecundity) were reduced

  3. Things that should have been observed(if this hypothesis is true) • Within the Fish Community: • Gadids low before the late ‘70s and high since • Clupeids and Osmerids (C&O) high before the late ‘70s and low since • In Steller Sea Lion Food Habits • Less gadid consumption prior to the late 1970s • More C&O consumption after the late 1970s • Negative consequences to sea lions/otariids when they eat a lot of gadids

  4. Steller Sea Lion Rookeries, Stock Structure and Population Trends Non-Pup Index Western Stock Eastern Stock

  5. Eastern Bering Sea Pollock Catch and the Regimes

  6. EBS Pollock Recruitment 1964-1998

  7. GOA Pollock Recruitment 1968-2000

  8. EBS Pacific cod

  9. Herring

  10. Eastern Bering Sea Herring Residuals from ln-transformed mean recruitment;Williams and Quinn 2000

  11. EBS Pollock and Herring Age 3+ Biomass 1,000s of mt; Wespestad 1991, 1997; Ianelli et al. 2002

  12. Gulf of Alaska Herring Stocks Residuals from ln-transformed mean recruitment;Williams and Quinn 2000

  13. Historical Perspective • Alexander, A. B. Seal Schooner Observations. 1894-95 • David Starr Jordan, L. Stejneger, et al. 1896, 1898. Pollock and cod were plentiful and formed “by far” the most important part of the fur seals and sea lions diet. • Jordan, D.S. and B.W. Evermann 1902. “Alaskan pollock … isexcessively abundant throughout the Bering Sea, swimming near the surface and furnishing the great part of the food of the fur seal. It reaches a length of 3 feet and is doubtless a good food-fish.” • Kenyon and Wilke 1952 “Evidence from the food remains on the Pribilof rookeries is that fur seals depend to a large degree on the…family Gadidae during their stay in the Bering Sea.” • Fiscus, Baines and Wilke 1962 “Theragra, Mallotus and squid have consistently been the principal food of seals in the Bering Sea” since observations began in the 19th century (N=thousands)

  14. Steller sea lion food habits: Western Stock Pre vs Post Regime Shift

  15. Range in Energy Density of Various Fish eaten by Steller sea lions s

  16. Seasonality of Collection • Energy density directly affected by reproductive cycle • Most species have lowest energy density immediately after spawning • Energy content increases during the summer feeding season, stored as lipid either in liver (pollock) or flesh (herring) • As gonads develop, energy transferred from liver/flesh to gonads • Energy lost at spawning

  17. Range in Energy Density

  18. Captive feeding studies – what do they show? • Single species diets not natural • Short duration • Many sources of variation not controlled for (age, season, reproductive status, etc) • Control of appetite in captive animals not simple (Issues of palatibility, acclimation, feeding rhythms, manner of presentation of food, training) • Byproducts of improper storage (e.g. formaldehyde) may be present in fish – cause of “gadoid anemia”

  19. Other pinnipeds and gadids Species Gadid % FO Pop. Status CA Sea Lion 28-98% Increasing Stellers SE AK 55-90% Increasing Pac NW 62-100% Increasing Harbor seals NW Atlantic 71% Increasing Brit. Columbia 62% Increasing FO = Frequency of occurrence in scat or spewing samples

  20. Conclusions • Gadids and herring had peaks in abundance both before and after 1976-77 regime shift • Data are insufficient to conclude that Steller sea lion diets have changed significantly • No single species is “junk” because of seasonal/ontogenetic variation and need for “balanced diet” • Gadids are common in the diets of pinniped populations around the world, many of which are increasing

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