Interannual variability in the length–weight relationship can disrupt the abundance–biomass correlation of sea scallop ( Placopecten magellanicus )

Fisheries stock assessments often rely on biomass estimates derived from abundance-at-length data and an assumed length–weight relationship (LWR) to establish stock status, which, relative to reference points, informs decisions on harvest rates. While biomass is typically assumed to reflect stock abundance, this assumption can be challenged by abrupt and sustained shifts in the body size of individuals within a stock, such as those arising from environmentally driven changes in the LWR. In the Bay of Fundy, Canada, the sea scallop (Placopecten magellanicus) has recently shown significant interannual variation in its LWR, with corresponding fluctuations in biomass. Using four spatio-temporal generalized additive mixed models, we explore the influence of spatio-temporal and environmental factors on the LWR, shell heights (i.e. size), abundance, and biomass of scallops in the Bay of Fundy, from 2011 to 2023. Results demonstrate that there was a significant shift in the LWR in 2023, which resulted in a large biomass increase (82%), despite only modest increases in shell heights (2%) and abundance (7%). Annual shifts in LWRs can therefore affect the correlation between abundance and biomass. We discuss the broader implications of this finding for stock assessments that depend on a LWR, particularly in the context of climate change and increased volatility in environmental conditions.