The weight-length relationships are of a great interest in fish biology, stock assessment and fishery management, by predicting weight from length information and allowing to estimate the stock biomass. Weight-length relationships of megrim (Lepidorhombus whiffiagonis), an economically relevant flatfish species, are studied for two decades (2001 to 2019) in northern Bay of Biscay and Celtic Sea (ICES stock Div. 7.b-k, 8.abd), two important European fishing areas. More than 22000 specimens collected from commercial landings by AZTI and IEO were sampled. Total fish length, total weight and commercial gutted weight were obtained. Four five-year periods (quinquennium) (2000-04; 2005-09; 2010-14; 2015-19) were defined in the studied time-series. Both the large sample size and time-series available, as well as the collaboration between both institutions collecting complementary information from this stock, allowed obtaining robust somatic parameters and analyzing their temporal variations. A weight-length relationship model was fitted and the quinquennium and semester factors, which are of interest biologically and for assessment purposes, and their interactions were significant in the preliminary model. The weight-length relationships, both for total and gutted weight and for the combined sexes of the most recent quinquennium are presented in this work and considered the most suitable to be used in the oncoming annual assessment process of the status of this stock in northern Bay of Biscay and Celtic Sea in ICES
