The discussion on fisheries induced evolution has sprung from the observation that small fish are getting more and more abundant in World’s fish stocks. Well, being observed at a population level this does not tell much about the background of the observed changes: they might reflect 1) demographic changes in the age/size structure of the stock, or 2) plastic or 3) evolutionary shifts in the phenotype distributions. However, a further observation that size specific probabilities of maturation (i.e. probabilitistic reaction norms) have decreased has been considered to support last one of the three hypotheses.
An intensively exploited Atlantic salmon population in River Ason (Cantabria, northern Spain) provided an excellent possibility to explore these questions in detail. Namely, General Franco established a salmon monitoring program to this river already late 1940s, after which each salmon caught at the river has been registered, measured and scale sample collected. The time series from 1949 to 2002 suggests that the size of returning salmon (i.e. size at maturation) has decreased, so the case is similar to those fish stocks in which the fisheries induced evolution has been suspected. However, we went further than just simply asking whether maturation probabilities had changed, and investigated what had happened over the same period in salmon growth. After substantial amount of scale reading and back-calculations of growth trajectories (boosted with real ale tastings) in rainy Wales, a surprising result came out: there were simultaneous changes in juvenile growth and age at maturation. So, something ‘fishy’ was going on… Read more about this study here.