The carapace matters: refinement of the instantaneous growth rate method for Antarctic krill Euphausia superba Dana, 1850 (Euphausiacea)

Growth of Antarctic krill Euphausia superba Dana, 1850 is commonly calculated using the Instantaneous Growth Rate (IGR) method based on the difference between the uropod lengths of the moulted exoskeleton and the uropod lengths after moulting. To date, this method has not accounted for sex-dependent differences in body proportion, relying only upon uropod measurements. We measured the carapace, uropod, and total body lengths of gravid females, non-gravid females, males, and juvenile krill from the Indian Sector of the Southern Ocean. Growth rates derived using a combination of the carapace and uropod measurements for gravid females were different from those derived through the traditional uropod-only based IGR, whereas non-gravid females, male, and juvenile growth rates showed no significant difference between methods. The refined method we propose successfully reflects dimorphism in growth between the sexes of krill, with gravid females having enlarged carapaces during the reproductive season. The interaction between growth and reproduction must be considered to improve the reliability of predictions from krill life history models, which is possible through the use of sex-dependent IGR measurements. We propose that, whenever possible, measurements of carapace and total length should be made along with uropod measurements. Together with assessments of maturity stages of krill that did not moult during experiments, these measurements will aid in further informing krill stock assessments.