Insights into the migratory patterns and seasonal distribution of one of the world’s rarest whales, the North Pacific right whale

Abstract

The eastern North Pacific right whale (NPRW; Eubalaena japonica) is one of the world’s rarest large whales, with fewer than 35 animals remaining. Foundational data on the distribution and biology of this species is lacking, hampering effective monitoring and conservation. In this dissertation, I used disparate ecological approaches – food web modeling and stable isotope analysis– to broaden our understanding of the distribution and trophic ecology of this rare whale. The right whale’s primary forging ground on the highly productive Southeastern Bering Sea shelf is experiencing a rapid decline in seasonal sea ice extent. Annual fall surveys of zooplankton in this region provides a data-rich resource to explore relationships between shifting environmental and right whale prey – the zooplankton genera Calanus, Neocalanus, and Thysanoessa. The results of these surveys have been used extensively in prior research to study zooplankton dynamics, but few studies have incorporated species interactions. I created a discrete-time Bayesian biophysical food web model of the Bering shelf zooplankton community to jointly model relationships between environmental covariates and individual zooplankton species during a warming period on the shelf (1996-2016). This model framework allowed me to quantify the contribution of density independence and density dependence to zooplankton community dynamics. Similar to the results of prior research, I found that sea ice dynamics drove density-independent growth across zooplankton species, but species interactions contributed only minimally to community dynamics. My results suggest that the presumed preferred prey of right whales, Calanus glacialis, will shift north with the decreasing sea ice cover to stay in cold bottom water conditions. Next, I used carbon and nitrogen stable isotope values in NPRW skin and baleen to study the distribution and foraging ecology of North Pacific right whales. Whale skin provides a seasonal snapshot of the whale's ecology (weeks to months prior to sampling), whereas right whale baleen can be used to reconstruct years of ecology history. I first analyzed carbon and nitrogen isotope values in right whale skin collected on the feeding grounds in recent decades (1997-2021) and modeled these data jointly to look for trends that correlated with sea ice cover and region. As part of this, I estimated the trophic level of NPRWs and constructed a map of baseline stable isotope values in the North Pacific using zooplankton. I used these regional source values in a mixing model of NPRW skin tissue to estimate the primary regional sources in NPRW skin samples. I found that skin biopsies collected on the feeding ground primarily reflected summer feeding, but analysis of the outer skin layer hints at possible spring distribution. I also found I found evidence of shifting distribution and individual foraging strategies with oceanographic conditions. Further I found evidence of shifts in baseline nitrogen isotope values and phytoplankton bloom composition with oceanographic conditions. I used similar methods to study migratory patterns of historical North Pacific right whales using samples of baleen obtained from the commercial whaling era. I analyzed carbon and nitrogen isotope values in six baleen plates collected from 1871 to 1961 to reconstruct the migratory patterns of these specimens. My results include the first documentation of migratory patterns, overwintering areas, year-round foraging ecology, and possible life history using stable isotope ratios. Overall, my dissertation illustrates the power of bringing disparate analytical approaches to the study of an extremely rare and data-limited species of baleen whale. My dissertation provides important new baseline information on the eastern population of North Pacific right whales, which managers can use to implement targeted monitoring programs.