Recalculation of historical landings of porbeagle shark
The Western and Central Pacific Fisheries Commission is leading the development of a Southern Hemisphere porbeagle shark (Lamna nasus) stock status assessment. A crucial input into any stock assessment is the landings history, which provides a measure of fishing mortality. The current official time series of New Zealand porbeagle landings consists of whole weights aggregated annually by the Ministry for Primary Industries (MPI) from Licensed Fish Receiver Returns (LFRRs) for fishing years 1989–90 to 2001–02 and Monthly Harvest Returns (MHRs) for 2002–03 to 2015–16. Recent work has shown that MPI conversion factors (CFs) applied to shark fin processed weights to determine whole weights were not always appropriate. Most importantly, chartered Japanese surface longliners have been landing porbeagle shark fins in a different state from that used by domestic surface longliners. Japanese vessels land the whole tail whereas domestic vessels land only the valuable lower lobe of the tail. Different CFs should be applied to these two different landed states, but in practice a single CF has been used for both. Furthermore, the CF for porbeagle shark fins changed from 30 to 45 in October 2004. These variations may have produced major errors in the calculated whole weights (because most porbeagle landings consist of fins only), and inconsistent time series. This study generated an improved time series of annual porbeagle shark landings by fisheries within the New Zealand Exclusive Economic Zone. MPI data were used to determine the landed amounts of processed porbeagle shark, and then appropriate CFs were applied to the landings. The mortality of discards and released live sharks was estimated under several scenarios of post- release mortality rates. The most plausible time series of porbeagle fishing mortality comprised the landings corrected by applying appropriate CFs and the medium mortality rate scenario (50% mortality of unspecified discards (a mixture of live and dead sharks) and 30% mortality of live releases) from 1998 to 2016. The new time series differs little from the existing LFRR/MHR series, with only 6% more mortality by weight overall and with annual values varying by –22% to +42%. The relatively small impact of the corrections can be attributed to the negating effect of changes in the nationality of the surface longline (SLL) fleet. Although the new time series differs little from the existing time series, the former is based on more appropriate conversion factors, and includes estimates of mortality of discarded sharks and live releases, so it should be used in any future stock assessments.