Characterisation and CPUE analyses for the SPO 1, SPO 2, SPO 7, and SPO 8 fisheries to 2020–21

The fisheries taking rig (Mustelus lenticulatus) around the New Zealand North Island and South Island are described from 1989–90 to 2020–21, based on compulsory reported commercial catch and effort data held by Fisheries New Zealand. A number of set net (SN) and bottom trawl (BT) fisheries take rig around New Zealand. The set net fisheries tend to be fisheries targeted at rig or, less frequently, at school shark. Smaller rig (usually less than 1 m long) are taken incidentally in mixed target species bottom trawl fisheries off the North and South islands. Detailed characteristics of the landings data associated with these fisheries, as well as the spatial, temporal, target species and depth distributions relative to the catch of rig in these fisheries are presented for all of New Zealand, separated into five SPO Quota Management Areas (QMAs). Annual performance of the landings in each SPO QMA relative to the respective Total Allowable Commercial Catches and some regulatory information are also presented.

Commercial catch-per-unit-effort (CPUE) analyses for six set net (SN) and five bottom trawl (BT) fisheries were evaluated as biomass indices to track population trends in these QMAs. These analyses were also based on the compulsory reported commercial catch and effort data that are collected by Fisheries New Zealand. Seven of the eleven fisheries were extensions of the same analyses that had been done in 2019 when rig were last reviewed. The other four fisheries (two SN and two BT) were created when wider BT and SN fisheries covering the combined east and south coasts of the South Island were split into regions representing separate east and south coast fisheries. This step was taken by the Inshore Working Group (INSWG) because the implied residuals from the component statistical areas pointed to spatial heterogeneity in both regions, particularly for the set net fishery. The CPUE series for the five BT fisheries (SPO 1W BT, SPO 2 BT, SPO 3 BT East Coast, SPO 3 BT Foveaux Strait, SPO 7 BT) and three of the six SN fisheries (SPO 3 SN East Coast, SPO 3 SN Foveaux Strait, SPO 7(038) SN) were deemed to be of High Quality (Research Ranking=1) and consequently could be used for monitoring rig abundance. The CPUE series from the remaining three SN fisheries (SPO 1E(007) SN, SPO 1W(043) SN, SPO 1W(044) SN) were given a Research Ranking of 2 (Medium or Mixed Quality). These CPUE series were downgraded because of concern that the spatial coverage in these fisheries was too restricted to monitor the full area, but the INSWG concluded they could still be used, with caution, for monitoring rig. Because the five BT fisheries were analysed at the trip level due to the large number of trips which landed rig but failed to declare rig in the estimated catches, three of the BT analyses (SPO 1W BT, SPO 2 BT, SPO7 BT) were repeated at the level of an individual tow. The combined east and south coast analysis (SPO 3 BT) was also evaluated in this way. These additional analyses were done by using data from the tow-by-tow forms which were adopted in 2007–08 so that the longer trip-based series could be evaluated for possible bias relative to the more detailed event-based analyses over the overlapping years. The resulting series acceptably matched all four of the longer trip-based series.

Four of the five BT fisheries show similar strong increasing trends in recent years, with only the SPO 1W BT(trip) series remaining steady, but above the series mean, from the early 2010s. The SN fisheries have a mixed response, with the SPO 1W(043) SN and SPO 7(038) SN fisheries showing strong recent increases while the SPO 1E(007) SN and SPO 3 SN East Coast fisheries showed a strong increase in 2020 but fell back to just above the series mean in 2021. The remaining two SN fisheries (SPO 3 SN Foveaux Strait and SPO 1W(044) SN) show a more complex response over the most recent three years, with the Foveaux Strait SN series holding steady or possibly increasing at a level above the series mean while the (044) SN series has dropped to a level near the series mean. The increasing trends in the BT fisheries are interpreted as indicating good recruitment, an observation that seems to be corroborated by the east coast South Island survey but not by the two west coast surveys. Although the SN fisheries give a mixed signal, none show signs of long-term population decline. These contradictory signals are difficult to reconcile and suggest that the fisheries that capture this species need to be closely monitored.

Length analyses indicate that the available monitoring tools (CPUE and trawl survey) sample different parts of the rig population, with the two trawl surveys dominated by small rig that are less than 70 cm in total length, the BT fisheries taking somewhat larger rig which are generally less than 1 m in total length, and the set net fisheries capturing much larger rig (particularly females) up to 1.3 m.