Megrim (Lepidorhombus whiffiagonis) in northern Iberian waters: Age determination corroboration, growth, abundance and mortality of the year-classes

Abstract

Megrim (Lepidorhombus whiffiagonis) is a commercially important European flatfish which population assessment requires reliable age and growth information. An extensive sampling of nearly 10000 otoliths over 25 years has been collected during annual bottom trawl surveys in southern Bay of Biscay and Galician waters. Samples were aged and yearly age-length-keys (ALKs) were built. A matrix of abundance indices by age and year was obtained after applying each ALK to their respective length distribution. The current age estimation criterion using otoliths of megrim was corroborated by tracking year-class abundance indices in the surveys. The strength of the year-classes was well tracked and significant correlations between abundance of consecutive age-groups from the same cohort were obtained. Von Bertalanffy growth parameters were estimated for the time-series (Lt = 56 [1-e(-0.12(t+1.82)]), and the growth performance indices estimated were within the range previously found for megrim in Bay of Biscay and nearby waters. These new parameters are representative of the growth of the Atlantic Iberian megrim stock (ICES Div. 8.c and 9.a) and can be used as a proxy of annual direct ageing in the assessment of this stock. Findings indicate that growth and mortality of the year-classes may influence population size/age structure of megrim. Abundant year-classes of megrim reach small mean lengths at the first fully recruited age 1, showing high mortality in the following older age-groups of the year-classes, and vice versa in the scarce year-classes. These results, together with the evidence of the inverse relationship between abundance and mean length by age of the year-classes, have showed, for the first time, evidence of density-dependent growth and mortality in the juvenile stage of megrim. The higher or smaller mean size and abundance of a year-class of megrim is determined by processes that take place in their first life stages and is maintained throughout the older age-groups of the year-class. The existence in consecutive years of cohorts showing small (1990s), or high (2000s) mean lengths-at-age, seems to indicate that growth in the first life stages may be conditioned by multiannual events in which occur favorable environmental and feeding conditions. Abundance is the most significant factor explaining the growth of age 1, whereas the climate index, used as environmental proxy, represented low relevance, and the fishing effort has no explanatory effect.