Characterisation and CPUE analyses for jack mackerels in the JMA 7 fishery up to 2022

Since the early-2000s the jack mackerel fishery in JMA 7 has been primarily undertaken by a small fleet of large midwater trawl vessels, with target effort focussed in the Taranaki Bight. During the 1990s, bottom trawling and purse seining were also used to target jack mackerels in JMA 7, but catches using these methods have been low since the turn of the century.

Jack mackerels, while managed as a single stock within the Quota Management System, comprise three species: Trachurus declivis, T. novaezelandiae, and T. murphyi. It is necessary to use observer sampling data to disaggregate catches to species level in order to undertake species-specific stock assessments. Previously, fully quantitative stock assessments for jack mackerel species in JMA 7 were attempted but were inconclusive due to concerns with observer sampling data.

One concern was that fish may be stratified in the net and pounds, leading to bias if sampling was not undertaken throughout the processing period. However, additional sampling data suggested that jack mackerel catches appear to be sufficiently well-mixed, such that bias in species composition and length estimates is unlikely to be a significant concern even if sampling was not spread throughout catch processing.

However, there has also been concern about the reliability of observer identification of the individual species. This problem is primarily evident from age-length data, where otolith samples collected by observers have subsequently been aged. A range of other data quality metrics were investigated, but it was concluded that the age-length data provided the most reliable approach for identifying trips where some degree of misidentification had taken place. Catch per unit effort (CPUE) analyses were conducted with and without filtering for trips where misidentification was identified, but there was little difference in the resulting trends.

The CPUE analyses for the native jack mackerel species, Trachurus declivis and T. novaezelandiae, were used to undertake partial quantitative stock assessments. The geometric means of CPUE for the period 2004–2011 for T. declivis, and 2006–2012 for T. novaezealandiae, were adopted as the target reference levels. In 2022, T. declivis was assessed as being likely to be at or above the target while T. novaezealandiae was very likely to be at or above the target.