Casal2 assessment for Antarctic krill in Subarea 48.1: a pilot model

Preliminary results from an integrated Casal2 assessment of Antarctic krill around the northern Antarctic Peninsula (in the area historically surveyed by the U.S. AMLR Program) and covering the years 1976-2021 are reported here. The assessment includes a 20-year forward projection. Data supplied to the model were fishery catches (1976-2021), acoustic biomass surveys (1996-2020), length-frequencies from the fishery (2011-2019) and length-frequencies from research surveys (1981-2011). Biomass estimates from fishing vessel and research vessel acoustic surveys were combined using Jolly-Hampton weightings (Table 2.4 in WG-EMM-2021-05r1). Unweighted length-frequencies from January and February were used from research surveys and the fishery. The 20-year projection demonstrates how the effect of future fishery catches on the population can be assessed. Two versions of the U.S. AMLR acoustic survey data, one using NASC converted to biomass and the other using NASC attributed to krill but not converted to biomass, produced similar population estimates when U.S. AMLR and fishing vessel survey catchabilities were estimated, process errors were assigned, and synoptic surveys conducted in 2000 and 2019 were assumed to provide estimates of absolute biomass. At the end of the 20-year projection with 620,000 metric tons caught per year, spawning biomass was about 64% of unexploited biomass. The current data, model configurations, and R plotting scripts are available at ‘https://github.com/us-amlr/Casal2-krill-model’. Our results demonstrate that data are available to fit integrated models and make krill stock assessments consistent with assessments in other CCAMLR fisheries. The results also demonstrate that integrated assessment models provide a method to convert NASC estimates to biomass estimates without collecting length-frequency data during every acoustic survey and subsequently applying a target-strength model. We believe the Scientific Committee could design a data-collection plan for the krill fishery that facilitates application of integrated assessment models by combining frequent surveys that simply report NASC with occasional surveys during which length-frequency data are collected using research nets.