Dolphins and herring of the Salish Sea: understanding responses of a top predator to fluctuating prey and human disturbance

• Main Author: Ashe, Erin
• Format: Text
• Language: English
• Published: Western CEDAR 2020
• Subjects: Fresh Water Studies; Life Sciences; Marine Biology; Natural Resources and Conservation; Terrestrial and Aquatic Ecology; Pacific; Oncorhynchus gorbuscha; Pink salmon
• Online Access: https://cedar.wwu.edu/ssec/2020ssec/allsessions/26

Pacific white-sided dolphins (Sagmatias obliquidens) are found reliably in inshore waters of British Columbia, including the northern extent of the Salish Sea. A long-term (>25 year) ecological study on this species provided an opportunity to develop a generalizable framework to predict population consequences of varying prey levels and acoustic disturbance to a top predator in the northeastern Pacific. Dolphin survival and reproductive rates were estimated from a large, long-term photo-ID dataset. We modelled non-calf survival as time-varying functions of two prey species as candidate covariates: Pacific herring (Clupea pallasii) and pink salmon (Oncorhynchus gorbuscha). Pacific herring are critical to food webs in the Salish Sea and for a healthy ecosystem. Herring support seabirds, fish, and marine mammals and are of vital cultural significance, but many stocks, including in the Salish Sea, have declined and failed to recover. We found that Pacific white-sided dolphin apparent survival declined significantly during years of low herring abundance, but did not vary with pink salmon abundance. Our field experiments revealed that dolphins decreased their time feeding in the presence of low-amplitude (126 dB) boat noise. During experimental treatments, the altered activity budgets resulted in a mean proportional reduction in foraging of 0.289 (95% bootstrap CI: 0.163, 0.408) between control and treatment conditions. We constructed a PCoD-like model that allowed us to predict demographic consequences of disturbance by perturbing this prey-demography link. The result is a flexible, generalizable PCoD framework that can be applied to model responses of a marine top predator to naturally varying levels of forage fish abundance, as well as noise-mediated disruption of foraging. We welcome opportunities to derive prey-demography links for similar species and apply the tool to the Salish Sea ecosystem more broadly.