Salish Sea Marine Mammal Density in Response to Increasing Sea Surface Temperature and Trophic Level Changes

Abstract

The San Juan Archipelago is situated in the state of Washington, south of the Strait of Georgia and east of Vancouver Island. In this biodiverse region resides an abundance of marine mammal species, including harbor porpoises, harbor seals, and Steller sea lions. Near the top of the trophic food chain, these animals rely on fish for prey while avoiding the predator at the very top: the transient killer whale. Marine mammals such as these pinnipeds act as ecosystem sentinels; their behaviors provide obvious, visible responses to environmental stressors and serve as warnings for underlying problems in the ecosystem. Marine mammal surveys were conducted aboard a research vessel over the course of 17 years, from 2008 to 2024. Species of mammal and number present were recorded. Sea surface temperatures, zooplankton samples, and catch per unit effort of Pacific sand lance were obtained, as well as a dataset from the Whale Museum on transient orca sightings and Pacific herring spawn counts from Fisheries and Oceans Canada herring stock. Our results imply that rising sea surface temperatures are related to decreases in marine mammal density. While prey abundance could also be a factor, the data so far indicates it is unlikely. Whether rising sea surface temperature is impacting lower trophic levels and causing marine mammals to redistribute would entail further study. This study aims to address whether climatic factors, particularly sea surface temperature and the 2014-2015 marine heat wave, and prey abundances affect marine mammal density in the San Juan Archipelago. The report also examines the presence of transient killer whales in relation to pinnipeds over the years 2012 to 2019