ENERGY DENSITY OF AMPELISCID AMPHIPOD POPULATIONS PREYED UPON BY GRAY WHALES IN THE NORTHERN BERING AND CHUKCHI SEAS

Abstract

The shallow and highly productive waters of the northern Bering and Chukchi Seas in the Pacific sector of the Arctic are able to support numerous upper trophic level predators, including benthic feeders such as the gray whale (Eschrichtius robustus). The Eastern North Pacific population, currently the largest population of the species, migrates thousands of kilometers from calving lagoons in Baja California, Mexico to Arctic foraging grounds to feed on numerous kinds of marine invertebrates. Historically, their primary source of food has been benthic ampeliscid amphipods (Family Ampeliscidae), which can form large, dense mats of tubes in sandy sediments on the seafloor. However, several studies have noted a decline in the biomass of these amphipods since the 1980s, particularly in the Chirikov Basin just south of Bering Strait,where they are the most abundant. Concurrently with this trend, gray whales have experienced two major Unusual Mortality Events (1999-2000 and 2019-2023), as defined by NOAA; observations of emaciated whales during these events have called into question the quality and availability of benthic amphipods as prey for gray whales and their ability to support the gray whale population. To address these questions, the caloric contents of several of the most abundant benthic amphipod genera in this region were measured and compared to similar measurements published in studies going back as far as 1978. As part of this study, a conversion factor between frozen and formalin-preserved amphipod specimens was developed and applied to a time series of amphipod abundance and biomass (1970-2019) to better quantify how energy density of benthic amphipod communities has changed over time. The energetic content of these amphipods has not changed significantly from historical values, but the amount of biomass per square meter of seafloor available has declined drastically. For ampeliscid amphipods in the Chirikov Basin, the maximum mean energy density was 347.08 kJ m^-2 in 1984; the minimum mean energy density was 31.07 kJ m^-2 in 2010, a 90% decline over 26 years. These findings suggest that, while benthic amphipods are still a high-quality food source, the extensive and dense sources of energy they once provided are no longer available. In response, gray whales are venturing further north in search of prey, bypassing formerly sufficient foraging grounds. Two supplementary (S) files of data sets are included with this thesis project, along with descriptions of each of the variables used in the dataset. File S1 consists of all the caloric measurements made for Chapter 2. File S2 is the compiled time series synthesized for Chapter 3. Some variables in the time series dataset (S2) are calculated from values in the Chapter 2 caloric data.