The rise and fall of the spiny lobster (Jasus edwardsii) in the Hauraki Gulf, northeastern New Zealand

Abstract

The spiny lobster (Jasus edwardsii) is a critical sea urchin predator on Australasian rocky reefs. In New Zealand, J. edwardsii also supports the country’s most valuable inshore commercial fishery, and subsequently is subjected to intense fishing pressure. While marine reserves have long been shown to be effective in rebuilding lobster populations, declines inside reserves and in fished areas in the Hauraki Gulf, northeastern New Zealand, over the last two decades raise concerns about the plight of this important species. While the recent fisheries assessment suggests improvements in the wider stocks, fisheries-independent assessments are needed to evaluate this. Furthermore, ocean warming has resulted in the expansion of long-spined sea urchins (Centrostephanus rodgersii) in northeastern New Zealand, and it is unknown if this has been facilitated by declines in J. edwardsii populations. The overall aim of this thesis was therefore to investigate the current state of lobster populations in the Hauraki Gulf, and better understand the cause and potential ecological consequences of large-scale declines. This was achieved by carrying out a region-wide dive survey of fished locations and fully protected marine reserves across the Hauraki Gulf, tracking lobster movement in the Cape Rodney-Okakari Point (CROP) marine reserve with a passive acoustic array, and carrying out feeding assays to determine the ability of J. edwardsii to consume C. rodgersii, and its preference for another barren-forming sea urchin, Evechinus chloroticus. Lobster densities in three marine reserves and their adjacent fished locations have not significantly increased following the 2018 catch reduction, and both vulnerable (legal-sized) and spawning stock biomass in the fished population of the Hauraki Gulf are estimated to be ≤5% of reserve biomass. These estimates are in stark contrast to fisheries estimates and suggest the recent stock assessment has grossly overestimated the state of the J. edwardsii population in the Hauraki Gulf. This highlights the value of using marine reserves as a proxy for estimates of unfished biomass to evaluate the status of surrounding fisheries. The passive acoustic array in the CROP marine reserve did not provide adequate simultaneous detections for the signal triangulation necessary for high-resolution data. However, the detections were able to produce weighted positions that allowed tracking of larger-scale movements and diurnal rhythms. The tracking data suggested lobster movements were less extensive than that reported in earlier studies, and there was little evidence to suggest aggregatory behaviour still occurred in contrast to movement data reported from the 1990s. These long-term changes are likely related to declines in overall population size and highlight the inadequate size of the CROP marine reserve in protecting the resident population from the effects of fishing. Feeding assays revealed that J. edwardsii were capable of consuming small C. rodgersii, but they exhibited a clear preference for E. chloroticus. This is likely related to a lobster’s ability to grasp the total diameter of an urchin, which due to longer spines in C. rodgersii translates to lobsters consuming only those with very small test diameters. Nevertheless, the consumptive abilities of J. edwardsii for both barren-forming urchin species in New Zealand reinforces their critical role as an urchin predator and emphasizes the importance of protecting J. edwardsii if urchin barrens are to be controlled or prevented in the Hauraki Gulf. Overall, these findings demonstrate the significant impacts of fishing on population density and behaviour of J. edwardsii and highlight the precarious state of the J. edwardsii population in the Hauraki Gulf. In an uncertain future of climate change stressors, these data strongly suggest a precautionary approach to management of J. edwardsii is necessary if it is to fulfil its functional role within the Hauraki Gulf once again.