Observations of Hector's dolphins (Cephalorhynchus hectori) associating with inshore fishing trawlers at Banks Peninsula, New Zealand

Abstract

Fishing activities have the potential to alter the behaviour of cetaceans and pose a threat through bycatch. We present observations concerning the influence of inshore trawling on group size and behaviour of Hector's dolphins (Cephalorhynchus hectori) at Banks Peninsula, New Zealand, made during photo-ID surveys from 2003 to 2007. Aggregations of dolphins, typically involved in what we assumed to be foraging behaviour, were observed following small inshore trawlers yearround. Group size was larger for aggregations of dolphins following trawlers (mean ± SE = 22.9 ± 1.3, n = 83) than for non-trawler groups (3.7 ± 0.2, n = 439), and dolphins were typically in a more "excited" state, frequently exhibiting aerial and sexual behaviours. We suggest that trawlers increase the availability of prey for Hector's dolphins and hence foraging behind trawlers is an energetically favourable activity. However, following trawlers probably also increases the risk of being caught in trawl nets, compounding the threats faced by this endangered species.

Hector's dolphin diet: The species, sizes and relative importance of prey eaten by Cephalorhynchus hectori, investigated using stomach content analysis

“…90 s (Slooten and Dawson ). Hector's dolphins have been observed following inshore trawlers in large groups in water 9–32 m deep (Rayment and Webster ), and are likely to be taking fish stirred up, but not caught by the trawl net. The trawl net and steeply descending traces consistent with dolphins diving to the seafloor around the net are regularly observed on the echosounder during these occasions.…”

“…90 s (Slooten and Dawson 1994). Hector's dolphins have been observed following inshore trawlers in large groups in water 9-32 m deep (Rayment and Webster 2009), and are likely to be taking fish Figure 3. Occurrence (%) of prey types in all stomachs examined (n = 63), and those collected from the South Island east coast (SIEC; n = 36) and west coast (SIWC; n = 23).…”

Common Dolphin (Delphinus delphis) Bycatch in New Zealand Commercial Trawl Fisheries

“…Similarly, there has been little (less than 1%) observer coverage of inshore set-net and trawl fisheries operating on the North Island west coast. Inshore fisheries are of concern, as they often overlap with the frequent occurrence of dolphins in shallow coastal waters (e.g., Hector's dolphin Cephalorhynchus hectori , [12]; vaquita Phocoena sinus [50]). Common dolphin in New Zealand waters have been shown to favour inshore waters during spring and summer, and also during warmer La Niña conditions [28].…”

“…A number of cetacean species are attracted to fishing vessels, and have been observed to feed in association with trawlers [11], [12]. One species that features prominently in bycatch reports across different fisheries and regions is short-beaked common dolphin Delphinus delphis Linnaeus, 1758 (hereafter referred to as “common dolphin”) [13]–[16].…”

Impacts of fisheries on the Critically Endangered humpback dolphin Sousa chinensis population in the eastern Taiwan Strait

“…gillnets, trammel nets and trawl nets). This has been documented, for example, for Hector's dolphins (Rayment & Webster 2009) and PRE Sousa, which are known to follow trawlers in unusually large groups (Jefferson 2000). Mothers with calves may be particularly drawn to fishing operations because they have higher energetic demands, putting them and especially their calves at greater risk of entanglement (Fertl & Leatherwood 1997).…”