Testing of trawl-acoustic stock estimation of spawning capelin 2020 (Toktrapport nr. 2-2020)

Sammendrag

This report describes the second in a series of three trawl-acoustic monitoring surveys of the spawning stock of capelin during the migration to the coast. The survey is a response to a proposal from the industry to evaluate the possibility of using winter monitoring of maturing capelin as an input to the capelin assessment and advice. The timing and geographic coverage of the survey are such that they would be relevant to use for advice given that the output is reliable. Pre-defined areas off the Troms and Finnmark coast were covered using two vessels, Vendla surveying the western part and Eros the eastern part. A stratified random transect design was adopted with two complementary zig-zag grids, the first going in a west-east and the second in an east-west direction over the same strata. The ultimate biomass estimate combines the two coverages, but evaluation about the mobility of the fish can be done by comparing the coverages. The final geographical allocation of survey effort was decided based on information from the scouting vessel Hovden Viking which covered the area a week prior to the main survey. Echo sounders with frequencies from 18-200 kHz were run together with sonars, and target trawls were carried out on significant pelagic aggregations. Capelin abundance was estimated using 38 kHz data. The total biomass of maturing capelin in the coverage area was estimated at 62 298 tons, with a CV of 38%. The 5% lower and 95% upper confidence limits were 26 655 and 104 305 tons, respectively. The result is in accordance with the prediction from the autumn 2019, but the high CV and wide confidence interval show that the survey result is uncertain. The high uncertainty despite the good survey coverage is likely due to the very patchy distribution of the capelin. In addition, in the western coverage area which had most of the capelin recordings, there was a big difference in both estimate and distribution in the return east-west coverage compared to the west-east. This underlines the high mobility of the capelin when it is in this state of migration. The strong dynamic was also evident last year and makes the monitoring of the capelin spawning migration challenging. Mean length of the capelin was 16.2 cm, mean weight 22.3 g, and maturation had progressed further in the western than the eastern area which was the opposite of what was observed last year. Methodological investigations including target strength measurements and testing of the autonomous Sailbuoy were carried out successfully and are described in the report. A thorough evaluation of the survey series and its usefulness as input to the capelin advice will be done after the third survey in this series is completed.