Food limitation of sea lion pups and the decline of forage off central and southern California

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.

Foraging strategies of a generalist marine predator inhabiting a dynamic environment

“…It also provides insight into how female California sea lions respond to the seasonal, annual, and multi-year changes in prey availability that are characteristics of the CCS. This is particularly important in light of recent significant oceanographic changes in the CCS, including increased sea surface temperatures and reduced primary productivity that have affected the condition and survival of California sea lion pups (Leising et al 2014;McClatchie et al 2016). …”

A fully coupled ecosystem model to predict the foraging ecology of apex predators in the California Current

“…In contrast, more recent information corresponding to a period of lower population abundances for sardine and anchovy suggests that the 2 species contributed less than 20% to diet composition (Hassrick et al 2014, McClatchie et al 2016. Variability in prey importance might be further compounded by 'ex treme' events, as evidenced by data from Monte rey Bay (i.e.…”

“…Consumption was parameterized so that relative diet contributions (in units of biomass) were approximately 33% for anchovy, 17% for sardine, 38% for market squid, and 12% for mackerel, which roughly reflect long-term diet information from southern California (Lowry & Carretta 1999). However, existing data suggest a significant degree of spatial and temporal variability in diet composition, with sardines becoming more dominant (60− 80% of diet) during El Niño conditions (Weise & Harvey 2008) or both sardine and anchovy percentages dropping (< 20% of the diet) during periods of lower population abundances for these species (Hassrick et al 2014, McClatchie et al 2016.…”

“…Consumption was parameterized so that relative diet contributions (in units of biomass) were approximately 33% for anchovy, 17% for sardine, 38% for market squid, and 12% for mackerel, which roughly reflect long-term diet information from southern California (Lowry & Carretta 1999). However, existing data suggest a significant degree of spatial and temporal variability in diet composition, with sardines becoming more dominant (60− 80% of diet) during El Niño conditions (Weise & Harvey 2008) or both sardine and anchovy percentages dropping (< 20% of the diet) during periods of lower population abundances for these species (Hassrick et al 2014, McClatchie et al 2016.Horizontal foraging movement for sea lions is simulated using kinesis (Humston et al 2004) with optimal temperature conditions of 12 ± 2°C corresponding to the signature of recently upwelled, productive waters. Kinesis uses a combination of inertial and random displacements based on the proximity to ideal conditions, whereby inertial or random behaviors are more heavily weighted when temperatures are, respectively, near or far from optimal.…”

“…Sea lions are the most abundant apex predator in the CCLME (Carretta et al 2005), and their foraging behavior has been observed to shift from predominantly nearshore to farther offshore during anomalously warm conditions (Weise et al 2006). Sardine and anchovy are specifically in cluded in the model, as their population abundances exhibit significant interannual and decadal fluctuations (Schwartzlose et al 1999), and they are commonly found in the diet of California sea lions (Lowry & Carretta 1999) although with a significant degree of year-to-year variability (Weise & Harvey 2008, McClatchie et al 2016. The main focus of the present work is therefore to demonstrate that the fully coupled model can be used to track environmental and feeding conditions experienced by sea lions on daily to interannual timescales, and to investigate the complex interplay between environmentally mediated behavioral re sponses and prey-mediated feeding conditions as drivers for yearto-year variations in their foraging patterns and success off of central California.…”