The Ecology of Large Marine Predators in the Hauraki Gulf

Abstract

Knowledge of the distribution and abundance of wildlife populations is essential for the description of a species niche and the development of effective conservation and management strategies. Knowledge gaps exist for many large marine predators, who are often rare, difficult to study, and of high conservation concern. The Hauraki Gulf, New Zealand, is a trophic hotspot that supports a diverse range of large marine predators, yet most of the existing research pertains to single-species or species groups. This study aimed to perform a comprehensive study of the fine-scale distribution patterns of cetaceans, sharks and potential prey patches, and to estimate the local abundance of the common dolphin Delphinus delphis and the nationally endangered Bryde’s whale Balaenoptera eden brydei. Double-observer line transect aerial surveys were conducted in the Gulf from November 2013 to October 2014 to collect distribution data on cetaceans, sharks, and their prey patches, as well as to collect abundance data using mark-recapture distance sampling methods. Platform-of-opportunity and dedicated boat surveys were conducted during the same period to collect photo-identification data to estimate Bryde’s whale abundance with mark-recapture (MR) methods. Bryde’s whale abundance was estimated with a custom-made MR model to minimise bias associated with heterogeneity in capture probabilities resulting from the use of opportunistic data. Boosted regression tree models were applied using sightings data from aerial surveys to model the occurrences of three cetacean species and four shark species to identify biological and environmental drivers of habitat use and to produce monthly prediction maps of suitable habitat with spatially explicit maps of uncertainty. Aerial survey data and mark-recapture distance sampling (MRDS) techniques were used to estimate the abundance of common dolphin and Bryde’s whale populations with a novel, probabilistic approach to reduce biases associated with errors in the identification of duplicate sightings and observer’s judgements of group size. Habitat preferences were consistent with knowledge of these species’ ecological niches, and there was a strong link between the presence of all predator species and the availability of their potential prey. Distribution maps revealed dynamic distribution patterns in time and space but indicated that the presence of predators coincided with major oceanographic processes that boost primary productivity. MRDS methods estimated 2478 individual common dolphins (95% CI = 1598, 3615) and 15 individual Bryde’s whales (95% CI = 6, 30) in the Gulf over the study period. Photo-identification MR methods estimated 69 Bryde’s whales (95% CI = 37, 101) in the population for the same period. This study represents the most comprehensive evaluation of large marine predators in the Hauraki Gulf in both time and space. The results indicate that the Gulf is an important foraging area for large marine predatorssupporting three year-round populations of cetaceans and an influx of sharks during the warm season. This study demonstrated the importance of the Gulf to several shark species, especially as a nursery ground for the hammerhead shark, and identified regions of important habitat, which is a substantial increase in our knowledge of these animals that are most vulnerable to bycatch mortality. With careful management, the Gulf has the capacity to support a biodiverse community of large predators and it is strongly recommended that conservation planning focuses on implementing temporal restrictions of harmful activities within spatially defined boundaries to maximise protection of these large predators. By considering temporal as well as spatial distribution patterns, it is possible to use an evidence-based conservation approach to ensure adaptive marine spatial planning that is dynamic and effective.