Midwater trawl catchability as an aspect of a quantitative assessment of krill biomass conducted using a trawl census survey
Midwater trawl catchability is a variable which depends on both the behavioural and distributional characteristics of the target species and trawl parameters. Reliability of biomass assessments will improve with better knowledge of trawl catchability and krill distribution patterns. A comparative analysis of trawl catchability for commercial midwater trawls and small research trawls, in this case using an Isaacs-Kidd trawl, demonstrated the futility of using the latter for trawl census survey biomass assessments due to their highly variable catchability over the survey area and also because the biomass estimates thus obtained were characterised by high relative error. A standard method for conducting krill trawl census surveys needs to be developed. Mathematical modelling fishing trawls as well as hydroacoustic research on catchability properties of trawls can facilitate selecting the optimal construction of commercial midwater trawls. Assessments of the total amount of krill entering the trawl should be carried out when selecting trawl construction.