User menu
Different approaches to modelling fishing selectivity can have a profound impact on stock assessments. This exploratory work considers how this selectivity might be better modelled. Interest in this subject was inspired by the length composition changes observed in longlines over 2003/4 and how these changes may be best accounted for. Data used were the longline length frequencies for yellowfin and bigeye tunas within the tropical region (+/- 15 degrees latitude) covering the period from 1952 to 2018 and Regional Observer Program data. All the data are available to the public on IOTC’s website. Results showed that there is a clear break in length frequency patterns around 2003/4, most apparent for yellowfin although a similar pattern occurs for bigeye. This pattern is solely found in the Taiwanese fleet length frequencies that makes up most of the data. It was originally hypothesized that the change in length compositions in 2003/4 could be due to change in discard practices. However, this appears to be inconsistent with the available data which does not show length frequency truncation that might result from size-specific discarding. The data are more consistent with some other cause such as a change in vessel operations which have affected the underlying selectivity. What this change might have been and how these changes may impact yellowfin and bigeye stock assessments remain unclear to date. Ways to proceed with this analysis that may provide further insight are discussed.
