Selectivity of commercial and research trawls in relation to krill
This paper describes the results of fishing a single krill aggregation with a commercial trawl RT 72/308 and an Isaacs-Kidd midwater research trawl (IKMT). The commercial trawl catches contained more large krill (35-58 mm) than catches of the research trawl (30-54 mm). The maximum difference in mean length in catches made by both trawls was ∆L = 6.2 mm. Statistically significant variability in krill length composition was observed between catches made by the research trawl, while in commercial trawl catches krill length composition hardly varied at all. Statistical analysis of catches showed that when it comes to obtaining representative trawl samples, estimates of length composition in krill catches made using the commercial RT 72/308 trawl are more accurate than those obtained using the IKMT research trawl. The two types of trawl have different catchability characteristics: the research trawl is better at retaining small krill, while the opposite is true for the commercial trawl. Consequently, estimates of krill length based on data obtained using either commercial or research trawls will be biased and contain systematic errors, mainly due to the selectivity properties of the trawls. The various selectivity properties of the trawl used in acoustic surveys have different effects on the accuracy of krill abundance and biomass estimates. Estimates of krill mass density (g/m2) and biomass are less dependent upon the selectivity properties of trawls used in acoustic surveys than are estimates of krill numerical density (numbers/m2) and abundance.