Variability in the density contrast of Antarctic krill (Euphausia superba): The non-negligible importance of size, season and region

The acoustic method has been widely used for estimating the biomass of Antarctic krill (Euphausia superba), the accuracy of which is often influenced by the parameters of the acoustic scattering model. A constant g-value has been used for this model, potentially introducing biases in the estimates due to effects of ontogeny, season and region. This study investigated the g-value of krill and the effects of size, season and region on this value using the generalized additive model (GAM) and found that the g-values were significantly affected by these variables. The optimal GAM explained 57.3 % of the deviance in the krill g-values, with 'Month' contributing the largest share (24.32 % absolute, 42.43 % relative to the explained deviance). A 0.5 % change in the g-value was found to result in approximately 0.8 dB difference in target strength. This study concluded that the importance of krill standard length, region and season cannot be neglected when considering the g-value as an input parameter in acoustic estimation of krill biomass. The study further suggests that ontogenetic, regional and seasonal variation in the g-value of krill should be incorporated into acoustic biomass estimation models to improve their accuracy and support krill conservation and management efforts.