Towards the development of a catch per unit effort (CPUE) index for the purse seine fishery

Purse seine fisheries target an assemblage of pelagic fish on the eastern coasts of the North Island, New Zealand. Key target stocks in this fishery include blue mackerel (EMA 1), jack mackerel (JMA 1), kahawai (KAH 1), trevally (TRE 1) and skipjack tuna (SKJ 1). For stocks targeted predominantly by the purse seine method (including JMA 1 and EMA 1), determining their population status is challenging.

A common metric used to provide information on population trends and abundance in fisheries is catch per unit effort (CPUE). The use of CPUE in purse seine fisheries is problematic, owing to the difficulty in defining the applicable fishing effort. Past determinations of this metric have generally been considered unsuitable for further application in stock assessments and characterisations.

To address shortfalls in the utility of CPUE for purse seine fishing activities, an initial characterisation of the fishery was conducted. Determinations of ‘best available’ CPUE metrics for the fishery complex were then performed using the available statutory fisheries data, with catch rates defined as both catch-per-day and catch-per-set. While some variation and trends were observable in time series of these two catch rates, they are considered to remain of limited utility for incorporation into any analysis of stock abundance.

To improve the utility of the CPUE metric, key considerations associated with its use in the purse seine fishery were addressed. Its application in a mixed species fishery was investigated, whereby the extent to which vessels switch targets during fishing trips was quantified and the prevalence of mixed species catches was determined. The visual method by which fish are targeted, which affords skippers the ability to be selective in their capture of schools, was discussed. An analysis of how frequently spotter planes are used in the fishery was conducted, whereby it was identified that spotter plane use and influence varied between species. Whether constraints associated with the holding capacity of the vessel influence the size of schools caught by skippers was also investigated, and identified to be non-significant.

Determining the search behaviours of the purse seine fleet, which represents an important form of effort undertaken in the fishery, remains the primary knowledge gap limiting the development of a meaningful CPUE index. The use of geospatial information recorded by the vessels’ Geospatial Position Reporting system, and spotter plane Automatic Dependent Surveillance-Broadcast transponders, was therefore explored to identify whether they provide opportunities for determining search activity. Classification of vessel activity by speed and turning angle enabled the inference of stationary, transiting, and searching patterns of motion. Areas for further improvement of these classifications were also identified. Visualisation of select fishing trips identify that spotter planes can provide extensive searching activity in the wider area in which the vessels fish. While searching, spotter planes were also observed to identify schools that are not targeted and vessels often catch schools that were not recorded by the planes. Therefore, only a partial understanding of searching activity in the purse seine fishery can be achieved by utilising current information on either the activities of spotter planes, or vessels, independently.

Further efforts to develop a CPUE index for this fishery should focus on continued development of the geospatial data that can be collected from the vessels and spotter planes, before combining these datasets into a useful determination of the encounter rates of target species by both the plane and vessel. Complementary development of ‘non-fishing’ or ‘non-catch’ information collection systems that record the observations of the skippers and pilots, and the decisions made by skippers when choosing to set upon or forgo a school of fish, would further refine our understanding of the effort undertaken within the purse seine fishery to achieve their catch. A more detailed understanding of the environmental responses of these target pelagic species would also support a more complete understanding of fishery dynamics, including both the catchable component of the stocks and their overall abundance trends.