Characterisation and CPUE standardisation for school shark in New Zealand, 1989–90 to 2018–19

School shark (Galeorhinus galeus) is a medium-sized coastal shark species that is common across the shelf and upper slope in New Zealand, and in temperate waters elsewhere. The New Zealand fisheries for school shark are complex because the species is caught by multiple fishing gears, both as a target species and as bycatch. The main fishing gears that catch school shark are setnet and bottom longline (as target species), and bottom trawl (as bycatch). School shark individuals are highly mobile with frequent movements between quota management areas, and different life-stages use specific areas within their coastal distribution.
This high mobility makes the interpretation of trends in fisheries catch-per-unit-effort (CPUE) data difficult. Previous analyses found conflicting trends in CPUE index series across regions, and sometimes between fishing gears in the same region.

This project summarised size-frequency information for school shark and updated previous fisheries characterisations. CPUE index series were standardised for five monitoring regions spanning New Zealand’s Economic Exclusive Zone. In addition to the previously-used fishing methods of setnet and bottom longline, standardised CPUE index series were also developed for bottom trawl as another fishing method.
For the three fishing methods, candidate index series included at least two levels of effort resolution, for individual fishing events and for daily effort. For most fisheries, a third candidate index series was also developed at the effort resolution of individual fishing trips.

Size composition data for school shark catches in New Zealand were collated across three different sources: Fisheries New Zealand scientific observer programme, research trawl surveys, and voluntary logbook data forming part of a previous Adaptive Management Programme (AMP). The three size composition data sets showed variable patterns in the spatial distribution of sizes over time, but all data sets included a higher prevalence of large individuals in Southland or neighbouring areas. Mature individuals were also more common in commercial fishery samples from the observer programme and AMP samples than in the inshore trawl surveys for all regions where they co-occurred. This finding confirmed that large, mature school shark can be captured by commercial bottom trawl. Given that the AMP was discontinued in 2009, observer samples could provide valuable information on catch composition, gear vulnerability and size distribution across a more representative sample of the population; however, there are currently few years with sufficient observations across all fishing gears to support a broad-scale analysis of these data.

This update of standardised CPUE series included the development of abundance indices at multiple effort resolutions across all series, resulting in a total of 36 index series. In general, the accepted index series were at the daily effort resolution for setnet and bottom longline fisheries, and at the trip effort resolution for bottom-trawl fisheries. The Inshore Fisheries Working Group (INSWG) accepted standardised CPUE series as indices of abundance for Far North & SCH 1E, Chatham Rise (SCH 4) and Lower SCH 3 & SCH 5, and an index of abundance from a research trawl survey for SCH 7, SCH 8 & lower SCH 1W. No index of abundance was accepted for SCH 2 & top of SCH 3, and this result was similar to the previous analysis. Based on the accepted indices, school shark abundance is increasing in Far North & SCH 1E, stable in Chatham Rise (SCH 4), declining in Lower SCH 3 & SCH 5, and stable in SCH 7, SCH 8 & lower SCH 1W, following a potential initial decline in the late 1990s.

Bottom-trawl CPUE index series were included in this analysis for all regions except Chatham Rise. These series were developed in part to elucidate conflicting trends in relative abundance from setnet- and bottom longline-based CPUE index series. The bottom-trawl standardised index series was accepted by the INSWG as one of the three monitoring series for Far North & SCH 1E. The bottom-trawl index series for SCH 2 & top of SCH 3 did not resolve the ongoing conflict between the CPUE series developed from the setnet and bottom-longline fisheries in SCH 2 & top of SCH 3. Instead, this additional series matched the bottom-longline index series in the earlier part of the time series and the setnet index series in the latter part. Dedicated research for this CPUE monitoring unit that considers spatial overlap between fishing methods over time might help to resolve the uncertainty around this discrepancy. The spatial distribution of school shark across life-stages in New Zealand also needs to be better characterised.