Size-at-Age, Growth and Condition of a Trout (Salmo trutta L.) Population from Central Portugal  ^†

Abstract

The trout (Salmo trutta L.) is a salmonid species that is endemic to European rivers, which has a significant interest from ecological, environmental quality, and socioeconomic (i.e., recreational fisheries) perspectives. Much has been written about its bioecology around the world but there is a lack of information about the populations inhabiting the southern limit of its natural distribution. This study aims to learn more about trout biology in the Mondego River basin, Central Portugal, an area where the species is highly searched for in angling activities, and in need of effective measures for its sustainable management and conservation that consider the specificities of this highly plastic species. From electric fishing campaigns, complemented with the help of local fly-fishing anglers, we captured 104 trout in the Mondego River basin. From these, we collected length and weight data, scale samples for age, and life-history determination, and all the trout were tagged with PIT tags to be identified if recaptured. Observed length varied between 95 mm and 560 mm, and five age groups (1+ to 5+) were identified, showing that the population is well structured. The results also show that growth varies considerably throughout the species ontogeny with a larger annual increment between the 3+ and 4+ classes. Age-at-length relationships that were found for the population from Mondego River are considerably different from others that were obtained for the Iberian Peninsula. The weight–length relationship shows a negative allometric growth. The mean value of Fulton’s condition factor that was calculated for the population was 1.04, indicating the poor condition of the fish, and there was no significant variability between different age classes when considering this parameter. The present study provided basic information about trout population biology at the southern limit of its distribution which is useful to allow comparisons with other populations from other regions, being essential to help fishery biologists and managers developing appropriate and adapted management and conservation strategies for this species.