This is the final report for the updated basking shark bycatch review 2016-17 and replaces all previous and preliminary reports for it.
Basking sharks are caught incidentally in New Zealand trawl and set net fisheries. They were protected in December 2010, and the last review of bycatch was undertaken in 2012. This report reviews recent international studies on basking shark population structure, biology and productivity, and updates previous reviews of New Zealand bycatch and management measures.
Basking shark bycatch
There is only weak, non-significant genetic structuring of basking sharks at the scale of ocean basins, suggesting the existence of large-scale movement. This is confirmed by new tagging studies that found movements of 3000–4600 km in the eastern Atlantic and northeastern Pacific oceans, supporting earlier research that showed movements of up to 9600 km.
Basking sharks frequently inhabit ocean depths greater than 600 m, and may remain there for months. The familiar aggregations of basking sharks in shallow coastal waters in many parts of their global range appear to represent only part of their complex behavioural repertoire and habitat requirements.
Observed raw catch per unit effort (CPUE) by trawlers has been at or near zero in East Coast (EC) and West Coast (WC) fisheries since the mid 2000s, while CPUE in Southland–Auckland Islands region (SA) has continued to fluctuate around low levels. It is not known whether the low numbers of captures inrecent decades are a result of different operational methods used by the fleet, a change in regional availability of sharks, or a decline in basking shark abundance.
SA region was responsible for 83% of the basking shark captures reported from three key regions in 2011–2016. More than half of the SA captures came from the arrow squid target trawl fishery. Catch rates were greatest in 200−400 m of water, at the deeper end of the squid fishery depth range, and in the silver warehou fishery. Sharks were caught at moderate rates down to depths as great as 800 m, particularly in the hake fishery.
No specific management measures are in place for basking sharks, apart from mandatory reporting of captures and the return of captured sharks to the sea. However, an active mitigation programme has been operated by Deepwater Group to reduce shark captures since October 2013. It is not yet clear whether the mitigation measures have had any effect on basking shark captures, and given the low and variable catch rates of sharks, any effect will be difficult to detect.
A move towards headline heights of less than 4 m, and a reduction of fishing in the favoured depth range of sharks, would probably reduce basking shark captures. However, there may be other unknown factors influencing catch rates, and it is unlikely that basking shark captures can be eliminated.