Temporal and spatial distributions of baleen whales in central New Zealand, based on passive acoustic monitoring

Abstract

Effective management of wild animal populations relies on an understanding of their spatio-temporal distributions, and how they overlap with threats. Passive acoustic monitoring (PAM) is a non-invasive method to investigate the distribution of free-ranging species that reliably produce sound, and soundscape analysis enables an integrated approach whereby vocal species are sampled concurrently with ambient environmental information. Physical and acoustic overlaps between soundscape contributors can highlight potential interactions and threats. Worldwide, the distributions of baleen whales have proven difficult to study due to logistical constraints imposed by their offshore distributions and limited surface time. However, many baleen whales reliably produce stereotyped sound, and are therefore suitable to study acoustically. In the western South Pacific, weather and sea conditions have particularly constrained understanding of baleen whale distributions. Here, four acoustic recorders were deployed in the marine environment around central New Zealand to elucidate the spatio-temporal distributions of ‘data deficient’ baleen whales that frequent the region. The recorded data featured notable acoustic contributions from several baleen whale species. Seismic surveys, shipping and earthquakes overlapped in time, space, and frequency with baleen whale vocalisations, highlighting potential risks and stressors in New Zealand waters. An acoustic approach enabled pygmy blue whales (PBWs) to be distinguished from ‘critically endangered’ Antarctic blue whales (ABWs). PBWs were detected in the South Taranaki Bight (STB) throughout recording periods, and detection area modelling demonstrated that an aggregation of PBWs likely occurs in the STB; an industrial area. ABWs and humpback whales migrated through central New Zealand seasonally, with both species generally travelling north through Cook Strait and the STB, rather than along the east coast of the North Island. ABWs were also detected during their breeding season. Based on song content, the humpback whales were likely migrating to breed in New Caledonia, or East Australia, and song transmission may occur between the two breeding populations in the New Zealand migratory corridor. This study highlights the utility of PAM to map the distributions of free-ranging species and potential stressors, which can be challenging to study via alternative methods. Such data are essential to underpin conservation and management decisions.