Abstract
California sea lions increased from approximately 50 000 to 340 000 animals in the last 40 years, and their pups are starving and stranding on beaches in southern California, raising questions about the adequacy of their food supply. We investigated whether the declining sea lion pup weight at San Miguel rookery was associated with changes in abundance and quality of sardine, anchovy, rockfish and market squid forage. In the last decade off central California, where breeding female sea lions from San Miguel rookery feed, sardine and anchovy greatly decreased in biomass, whereas market squid and rockfish abundance increased. Pup weights fell as forage food quality declined associated with changes in the relative abundances of forage species. A model explained 67% of the variance in pup weights using forage from central and southern California and 81% of the variance in pup weights using forage from the female sea lion foraging range. A shift from high to poor quality forage for breeding females results in food limitation of the pups, ultimately flooding animal rescue centres with starving sea lion pups. Our study is unusual in using a long-term, fishery-independent dataset to directly address an important consequence of forage decline on the productivity of a large marine predator. Whether forage declines are environmentally driven, are due to a combination of environmental drivers and fishing removals, or are due to density-dependent interactions between forage and sea lions is uncertain. However, declining forage abundance and quality was coherent over a large area (32.5–38° N) for a decade, suggesting that trends in forage are environmentally driven.
Highlights
A major goal of marine conservation is to protect higher trophic-level species as their removal can have dramatic consequences on entire food webs [1]
We show that food limitation of nursing female sea lions and the consequent malnourishment of their pups is not limited to strong El Niños, but is a result of an increasing population experiencing a period of changing forage composition near the California Channel Island breeding colonies
During the decade 2004–2014, the forage fish eaten by sea lions showed distinct trends that affected both prey abundance and prey quality off central and southern California, and in the foraging range of the sea lions from San Miguel Island
Summary
A major goal of marine conservation is to protect higher trophic-level species as their removal can have dramatic consequences on entire food webs [1]. California sea lions (Zalophus californianus) were historically heavily exploited for their oils, hides and meat from the late 1800s and were later killed by fishery interactions [3], and population sizes were low in the early 1970s. Sea lions eat a wide range of fish and squid, and their diet shows strong interannual variability, but the most common items in their diet are market squid (Doryteuthis opalescens), northern anchovy (Engraulis mordax), Pacific sardine (Sardinops sagax), shortbelly rockfish (Sebastes jordani), juvenile Pacific hake (Merluccius productus), Pacific mackerel (Scomber japonicus) and jack mackerel (Trachurus symmetricus) [5,6,7,8,9], potentially placing them in competition with fisheries. During El Niño conditions, their diet changes to less nutritious and less preferred species, including juvenile rockfish and hake, instead of sardine and anchovy [9]. Melin et al [9] demonstrated lower pup weights when scats from females primarily contain rockfish and squid
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