Using predator diets to infer forage fish distribution and assess responses to climate variability in the eastern Bering Sea

Abstract

Forage fishes comprise an integral part of marine food webs in the highly productive ecosystems of the North Pacific. However, significant knowledge gaps exist related to the status of forage fish, their life histories, and how populations may react to future climatic shifts. Standardized bottom trawl surveys are critical to the stock assessment of groundfish but lack the gear and protocols to quantitatively evaluate small pelagic forage fish. Where diet data are available, predators may be used as an indirect method of collecting data on forage fish distribution and relative abundance. We used stomach contents data to infer predator-prey interactions in the eastern Bering Sea and to analyze the distribution of 5 forage taxa over a 34 yr time series (1985-2019). Using 4 dominant groundfish predators, we constructed forage fish and predator depth and temperature habitat profiles and used center of gravity (COG) analysis and global index of collocation to examine predator-prey overlap. Results provide insight on the habitat partitioning and competitive interactions between forage species and the dynamics between predators and prey. Interannual COG analyses indicated recent periods of cooling (2007-2013) and warming (2014-2019) had significant effects on the distribution of forage fish, and suggest differences in the relative resilience of forage fish populations to climate change in this region. Population shifts were particularly evident in recent periods of anomalous warming, highlighting the need to understand how future periods of prolonged warming may affect predator-prey dynamics. Results also demonstrate the importance of predator diet time series and how these data might inform multi-species models and management strategies.