Combining Alaska forage fish data from research surveys and predator diets to assess forage fish response to climatic regime shifts

Abstract

Forage fish play a crucial role in the marine ecosystems of Alaska through the transfer of energy from lower trophic levels to marine predators. Understanding and quantifying Alaskan forage fish responses to rapid climatic shifts is needed to inform adaptive management strategies under a warmer and more variable future in Alaska. However, data on the occurrence and abundance of forage fish in Alaska are limited due to the diversity of life history strategies and patchy distribution that make their capture in fisheries-independent surveys difficult. In this thesis, I address current gaps in the data available on forage fish through the publication of an Alaska Forage Fish Database (AFFD) comprised of data from trawls, beach seines, and the diets of predator consuming forage fish, collected by a variety of contributing agencies and organizations. This data compilation effort resulted in a database containing forage fish abundance data from 461,449 sampling events spanning the years 1953 - 2023. Using the compiled dataset, I then assess large-scale spatial and temporal patterns in the occurrence of Pacific capelin (Mallotus catervarius), Pacific herring (Clupea pallasii), and sand lance (Ammodytes spp.) in the Bering Sea and Gulf of Alaska. Interannual trends in the encounter probability for each species, based on models fit to data from fisheries surveys, groundfish diets, and seabird diets, exhibit coherence in large-scale changes among observation types in the Gulf of Alaska (1972 - 2023) and Bering Sea (1980 - 2023). I compare the distribution of species’ encounter probability across decadal climate stanzas and demonstrate the differences in distribution of species during the 2014 - 2019 heatwaves compared to prior stanzas, as well as reoccurring distribution patterns in response to similar climatic characteristics among stanzas. Findings from this thesis facilitate an expanded understanding of forage fish population dynamics and spatial distribution in response to rapid climatic shifts.