New Zealand Southern Right Whale Eubalaena australis — Foraging Ecology and Distribution

Abstract

Movement patterns reflect migration and foraging strategies and are central to understanding an exploited species recovery. Here we use two complementary tools to investigate movement patterns: stable isotopes and citizen science reporting. Stable isotopes are a valuable tool for tracing patterns of foraging and movement ecology of wide-ranging animals when access to remote habitat is challenging. The diet and locations of current offshore feeding grounds of New Zealand southern right whales (Eubalaena australis, SRWs) are poorly understood. Historically, the population foraged northeast of the mainland around the Chatham Rise and Louisville Trench. However, contemporary data suggest a substantial shift in foraging grounds to the south of Australia. Here we investigated the foraging ecology of SRWs using o13C and o15N isotope values from skin samples (n = 591) with associated genetic profiles (identity, sex, age, reproductive status) collected between 1995 and 2020, with 38 whales sampled in >1 year. We found that niche space has contracted over time and o15N values deplete both with a change in maternal state (from non-lactating adult females to lactating cow; mean = -0.75 ± 1.10) and temporally (1995 - 2020) regardless of demographic class (mean = -0.29 ± 1.00). We provide novel insights into the value of stable isotopes in determining temporal shifts in foraging niche and variation by demography, including the possible physiological effects of reproductive state and the associated cost of gestation, for a recovering population. Using opportunistically collected data, the distribution of SRWs around mainland New Zealand was reassessed to determine the importance of the mainland to SRWs. Specifically, we used sightings reports from social media to supplement sightings data from the Department of Conservation's National Cetacean Sightings Database. The 116 sightings (2011 - 2021) were similar to previous findings (1976 - 2010), suggesting slow recolonisation of the mainland. Our findings suggest that as the population recovers from historical exploitation, we see adaptation to new foraging grounds and variation in prey sources, supporting the continued success of the population. We suggest that citizen science initiatives accompanied by supplementary social media data should continue to understand the spatiotemporal extent of SRW recovery around mainland New Zealand.