Citation
Summary
In New Zealand, the Quota Management System stocks of alfonsino include both Beryx splendens and B. decadactylus. However, B. splendens makes up the great majority of the catch. Beryx species, including B. splendens, are widely distributed in the world’s oceans, but their population biology is poorly known.
The New Zealand alfonsino fishery became established in the mid 1980s, initially off the southern east coast of the North Island and later on the eastern Chatham Rise. These areas make up the key BYX 2 and BYX 3 fisheries, although alfonsino is caught in smaller volumes widely around New Zealand. Alfonsino is fished by both bottom and midwater trawling and the main fishery areas are often associated with bathymetric features. In this report, spatial density clustering was used to delimit the key areas of the fishery; the areas identified were then used as the spatial strata in analyses of catch per unit effort (CPUE).
The CPUE modelling undertaken was assisted by using a dataset encompassing effort from BYX 2 and BYX 3; in particular this allowed for covariates, including vessel coefficients, to be estimated using a larger dataset from across the fishery. The resulting model had improved diagnostics compared with models using data from smaller areas. However, there was evidence that the temporal trends in CPUE differed between areas. Groups of neighbouring areas were amalgamated into four key fishery regions: a northern and southern area off the North Island southern east coast, and areas to the north and south-east of the Chatham Islands. These areas had differing temporal trends in CPUE and it was not possible to propose an abundance index that was suitable for stock-level monitoring.
The population structure and dynamics of alfonsino is not well understood; most of the directed research on alfonsino in New Zealand was carried out in the early 1990s and the limited sampling of catches since then allows for little new insight. Alfonsino is known to have a long (~1 year) pelagic larval stage with much more limited movement of adult fish after settlement. Nevertheless, there is evidence of size structuring with depth, different age structures on different fishing grounds, and differential growth by sex. It is unclear, however, if the differing trends in the main areas of the fishery arise due to differential recruitment driven by varying oceanic drivers, or as a result of processes—including varying fishing pressure— acting on the recruited populations.
Although CPUE analyses of commercial fishing effort appear to provide a reasonable approach to monitoring regional trends, extensive biological sampling and otolith ageing to provide spatially resolved age frequency distributions over time, will be required to better understand the dynamics of alfonsino populations and develop a fuller stock monitoring approach.