Indicator based analysis of the status of New Zealand blue, mako and porbeagle sharks

Presented as an information paper to WCPFC SC11. Also tabled in 2017 as CCSBT-ERS/1703/BGD 01 and previously in 2015 as CCSBT-ERS/1503/21

Cartilaginous fishes generally have low productivity because of their low to moderate growth rates, and their low fecundity. Despite their vulnerability to over-fishing, a lack of suitable data means that conventional stock assessments are rarely possible. To address that limitation, this report performs indicator analyses for blue, porbeagle and mako sharks − three shark species that are taken primarily as bycatch in the New Zealand tuna longline fishery. The main data sources were the Ministry for Primary Industries (MPI) commercial catch-effort database for the 2005 to 2013 fishing years, and the MPI observer database for the 1993 to 2013 fishing years. Our analyses were restricted to the surface longline fishery, and divided into two regional strata − North region comprising Fisheries Management Areas (FMAs) 1, 2, 8, and 9, and South region comprising FMAs 5 and 7. The following indicators were calculated: high-CPUE (the proportion of half-degree rectangles having unstandardised catch per unit effort (CPUE) greater than a specified threshold); proportion-zeroes (the proportion of half-degree rectangles having zero reported catches in a fishing year); geometric mean index (the geometric mean of the species abundances in catches, for both the catch of all species including teleosts, and the catch of just the three sharks); standardised CPUE (for both commercial and observer data); proportion of males in the catch; and median lengths of males and females.
None of the indicators for the period 2005−2013 suggested that any of the shark species were declining in either North or South regions. In fact, some of the indicators suggested positive trends for all three species. We caution that there are a number of important caveats associated with our indicator analyses, especially relating to data quality and availability, and goodness of model fit in the CPUE analyses. Nevertheless we conclude that there is no evidence that the stocks of blue, porbeagle and mako sharks in New Zealand waters have been adversely affected by fishing at the levels experienced since 2005, and that there are good signs that they are increasing. Observer data, which span a longer time period than commercial fishery data, suggest that blue and mako shark abundance may have declined during the late 1990s and early 2000s, and then increased since the mid 2000s, an interpretation that is consistent with the indicators based on the more recent commercial data. Porbeagle shark abundance may have declined rapidly in the early 2000s before stabilising at a relatively low level. The indicators presented here cover only the most recent portion of a longer fishing history that was characterised by greater effort levels in the 1980s and early 1990s by foreign fishing vessels. There is no information on the effect of this earlier fishing effort, as there are no shark catch data from that period, nor effort data from before 1980. Furthermore, the three shark species are capable of migrating outside the New Zealand Exclusive Economic Zone where foreign fishing may also have impacted on the wider South Pacific stocks of these species. In order to understand trends in the wider stocks, and to quantify their status in relation to management reference points, regional stock assessments are now required.