Exploring spatial and temporal patterns in New Zealand scampi fisheries ecology

Abstract

New Zealand scampi (Metanephrops challengeri) is a commercially important species, representing one of New Zealand’s most highly valued seafood export products by weight. Despite their economic significance, relatively little is understood of their ecology, and the factors affecting their distribution in comparison to their northern hemisphere counterpart, the Norway lobster (Nephrops norvegicus). This is largely due to the difficulty and expense of gathering data for cryptic deep-sea species such as New Zealand scampi. There remain substantial information gaps in our knowledge with regard to aspects of their habitat ecology, reproductive biology and distribution that are likely to have implications for management and sustainability of the fishery. The research presented in this thesis aims to improve our understanding of patterns of scampi habitat associations, trawl efficiency, maturity, catch distribution and fishing effort to provide insights into the potential for improved fishing efficiency and scampi management. To achieve this aim, the research for this thesis investigated novel aspects of the in-situ biology of New Zealand scampi at a variety of scales, using a combination of field video experimentation, field sampling and analysis of historical catch data. These included: trawl-scale investigations of scampi-habitat associations and the first direct assessments of scampi trawl efficiency using video technology deployed on trawl nets (Chapter 2); assessment of broad-scale patterns in the size at onset of maturity of female scampi and the first estimates of the size at onset of maturity of male scampi (Chapter 3); and the adoption of a range of spatial analyses not previously documented for the New Zealand scampi fishery to investigate drivers of spatial and temporal patterns of commercial fishing effort, catch and catch per unit effort (CPUE), and assess historical catch efficiency of the fleet (Chapter 4). The results of the research suggest a negative relationship between the distribution of scampi burrows (most reliable proxy of scampi abundance) and smooth topography, but a positive relationship with conical mounds (formed by bioturbators) within three fisheries management areas (FMAs) (Chapter 2). These findings indicate that scampi may be patchily distributed over fine scales and potentially predictable in their localised distribution. Scampi trawl efficiency was low, with only 30% of observed scampi passing over the footrope and into the trawl net, indicating that a much lower proportion of emerged scampi are caught during trawling than previously assumed. Catching success decreased further over rough topography (27%), with the presence of conical mounds (23%), and when scampi were partially in their burrows (10%) (Chapter 2). Estimates of the size at onset of maturity for female scampi increased with latitude among FMAs and corresponded with decreases in both water temperature and scampi density. The results also indicate that gonadal widening (morphological maturation) and early-stage gonad maturation (physiological maturation) occurs prior to the bearing of eggs (Chapter 3). Finally, trends in spatial variations of scampi fishing effort and catch within FMAs coincided with changes in fisheries management strategies and were likely influenced by fishers avoiding moulting seasons and strategically targeting larger sized scampi in shallower but less scampi-dense areas. An observed disconnect between the spatial allocation of fishing effort and spatial patterns of CPUE may signal catch inefficiencies and sub-optimal fishing of the scampi fleet (Chapter 4). Collectively, these findings of the research presented in this thesis have the potential to help inform more targeted and sustainable fishing practices, whilst also identifying regional differences in scampi reproductive potential and fishing susceptibility and contribute to more robust stock assessments.