Catch estimates of blue shark in the IOTC area of competence for the 2021 stock assessment | IOTC

Rice J (2021) Catch estimates of blue shark in the IOTC area of competence for the 2021 stock assessment | IOTC. In: IOTC- 17th Working Party on Ecosystems & Bycatch (Assessment). IOTC-2021-WPEB17(AS)-14

Catch histories form an important component of stock assessments and so having a reliable and believable catch series is a key part in gauging the level of stock depletion. In data-limited situations, reported nominal catches are often not considered reliable and so reconstruction of catch histories plays an important role. The first Indian Ocean stock assessment of blue shark took place in 2015, however, due to the amount of uncertainty in the assessments, the conclusion regarding stock status remained as uncertain. The historic catch series was considered to be one of the key sources of uncertainty and so the Working Party requested that participants develop new approaches to reconstructing historic catches to be used as alternate series for assessment. This paper uses the available nominal catch data currently held in the IOTC database and a generalized additive model (GAM) approach to reconstructing historic blue shark catches in the Indian Ocean. Additionally a ratio based method was used to estimate the unreported blue shark catches. The methods described in this paper attempt to account for not reporting of blue shark catches. Based on the methodology used in 2017 GAM using target catches were used to predict the expected catches where there are zero reported catches. The resulting estimated catch series were very similar to the catches estimated in 2017 (same trend and scale) with catches increasing over the time period of the fishery, reaching approximately 50-66,000 t in recent years. With a drop in catches in the recent years that mimics the drop in reported catch. Similar to the work done in 2017 these estimates are prepared for used in the assessment model that has 8 fleets.