Sex- and length-dependent variation in migratory propensity in brown trout

In partially migratory species, individuals either migrate at some point(s) in life or reside within their natal habitat throughout life. For salmonid fish, migration creates opportunities for feeding and growth, but it is also associated with increased mortality risk. Such trade-offs likely differ between the sexes, since reproductive output is more closely tied to body size in females than males. However, testing hypotheses on sex-specific migratory behaviour in would-be first-time migratory salmonids is difficult, since sexes are generally morphologically indistinguishable prior to maturation. Previous studies have evaluated the influence of sex on migration based on dissection of migratory juveniles or the sex ratio of returning adults. However, both approaches are potentially biased by differential survival during migration. Here, we utilise advances in minimally invasive genetic sex-determination methods for salmonids to investigate sex-specific, spring out-migration propensity in potamodromous brown trout (Salmo trutta) in a pre-Alpine, central European lake. We show that there are marked differences in migratory behaviour between males and females, with small (~10 cm) females being approximately twice as likely to migrate out of their natal river in spring compared to similarly sized males, which generally migrate for the first time at larger sizes (in similar proportions to larger females). This study highlights how novel genetic sex-determination techniques can provide insight into the sex- and size-specific life-history trade-offs that shape migration propensity. Moving forward, these techniques should become useful tools for ecologists and fisheries managers.