Estimating sizes of fish consumed by ice seals using otolith length-fish length relationships

Abstract

Arctic fishes and ice seals are key components of the Alaskan Arctic ecosystem. Bearded (Erignathus barbatus), spotted (Phoca largha) and ringed (Pusa hispida) seals are consumers of Arctic marine fishes. Little is known about the sizes of fish that ice seals consume because prey items are digested quickly once exposed to stomach acids. Otoliths, fish ear bones, are often the only parts of a fish that remain in a seal stomach. Otolith length relates directly to fish length, making size estimations of consumed fish possible for piscivore diet studies. Otoliths were measured from fishes collected from cruises in the Beaufort and Chukchi seas during 2009 - 2014. Otolith length - fish length and fish length - fish weight relationships were developed for 11 Arctic marine fish species that are commonly consumed by ice seals in Alaska. Otoliths from seal stomachs provided by subsistence hunters to the Alaska Department of Fish and Game were identified to species level and measured for total length. A mixed effects model was used to determine how the variables of seal species, harvest location, seal age class and sex influenced the sizes of fish consumed. Harvest location and seal age class were the primary factors that affected fish size in ice seal stomachs. Estimating length and weights of fishes consumed by ice seals will help further diet and energetics studies that have not previously been possible in the Alaskan Arctic.