Trudi Webster

Distribution of southern right whales on the Auckland Islands calving grounds

Abstract Southern right whales (Eubalaena australis) were virtually extirpated from New Zealand by commercial whaling, but are now recovering. Previous research at the Auckland Islands has suggested that Port Ross, a sheltered embayment at the northern end of the islands, is the primary calving ground for right whales in New Zealand. However, an abundance of potentially suitable calving areas exist outside Port Ross, raising the question of whether the growing population might be expanding its distribution to occupy new or former habitats. To address this we conducted the first systematic line-transect survey of potential right whale calving habitat at the Auckland Islands in winter 2011. High densities of southern right whales were sighted inside Port Ross, including all but one of the 21 mother-calf pairs recorded. The survey confirmed that Port Ross is the principal calving area for southern right whales in the Auckland Islands. This conclusion has implications for the estimation of demographic parameters of right whales in New Zealand, which have thus far relied on data gathered largely in Port Ross, and highlights the importance of protecting this habitat.

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 Hectors 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 year‐round. 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 Hectors 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.

Evidence of Sex Segregation in Hector's Dolphin (Cephalorhynchus hectori)

Segregation by sex is evident at a variety of levels in many birds, fishes, and mammals. Segregation has been observed in marine mammals to varying degrees, but it was previously undocumented in Hector’s dolphin (Cephalorhynchus hectori). Forty-three groups (of group size ≤ 5) were sexed using an underwater pole-camera; 91% of groups consisting of two to five individuals (n = 32) were either all male or all female. Sexes were obtained from an additional seven groups containing calves. All of the adults associating with mothers and their young were female. This research suggests that Hector’s dolphin groups are highly segregated by sex. Sex segregation might have implications for reproduction in Hector’s dolphins, including difficulty in finding a mate as local populations decline.

Quantitative analysis of the acoustic repertoire of southern right whales in New Zealand

Quantitatively describing the acoustic repertoire of a species is important for establishing effective passive acoustic monitoring programs and developing automated call detectors. This process is particularly important when the study site is remote and visual surveys are not cost effective. Little is known about the vocal behavior of southern right whales (Eubalaena australis) in New Zealand. The aim of this study was to describe and quantify their entire vocal repertoire on calving grounds in the sub-Antarctic Auckland Islands. Over three austral winters (2010-2012), 4349 calls were recorded, measured, and classified into 10 call types. The most frequently observed types were pulsive, upcall, and tonal low vocalizations. A long tonal low call (≤15.5 s duration) and a very high call (peak frequency ∼750 Hz) were described for the first time. Random Forest multivariate analysis of 28 measured variables was used to classify calls with a high degree of accuracy (82%). The most important variables for classification were maximum ceiling frequency, number of inflection points, duration, and the difference between the start and end frequency. This classification system proved to be a repeatable, fast, and objective method for categorising right whale calls and shows promise for other vocal taxa.

Estimated calving interval for the New Zealand southern right whale (Eubalaena australis)

ABSTRACT Southern right whales (Eubalaena australis) were widespread in New Zealand waters before commercial whaling in the nineteenth century caused drastic declines in their abundance and distribution. Following the cessation of whaling, the population has been recovering and is now slowly recolonising its former range. Estimates of population demographics, including reproductive output, are essential for predicting the trajectory of this population. We gathered photo-identification data on female southern right whales during annual field trips to the Auckland Islands, the principal calving area in New Zealand waters. Forty-five calving intervals were observed between 2006 and 2013 (mean interval = 3.31 years, 95% CI = 3.06–3.57). Incorporating the effects of possible missed calving events produced a plausible range of mean calving intervals from 3.17 to 3.31 years. Our results suggest that the calving interval of New Zealand southern right whales is similar to that found in populations elsewhere.

Hector's dolphin movement patterns in response to height and direction of ocean swell

ABSTRACT A central question to any understanding of ecology is how animals use their habitat, and how habitat use is influenced by temporally changing features of the environment. Previous research on Hectors dolphins at Akaroa Harbour, New Zealand suggested that dolphins leave inshore, harbour environments during or after rough weather. To test this hypothesis, visual sightings (2000–2012) and acoustic detections (2007–2008) of Hectors dolphins in Akaroa Harbour were modelled to test for a relationship with swell height and swell direction. Sighting rates and acoustic detection rates in Akaroa Harbour were significantly lower on days after big swell events and in some linear models after swell events from the south. These results indicate that swell events influence Hectors dolphin movements in and around Akaroa Harbour. Possible reasons for this behaviour are diverse and need further investigation. However, this information can be used both to predict daily dolphin movement for conservation and research purposes, and to suggest how dolphins may react in future if extreme weather events are becoming frequent.