Body size and the adoption of a migratory tactic in brook charr

The presence of a conditional strategy based on size attained before migration and a sex-ratio at migration biased towards females were explored in a population of brook charr Salvelinus fontinalis in the Sainte-Marguerite River, Quebec Province, Canada, where anadromous and resident forms live sympatrically. Seaward migration in the system occurred at 1 and 2 years old. Comparisons between backcalculated size-at-age of migrant and resident fish revealed that smaller fish at age 1 year delay migration to the following year and that bigger fish either migrated at age 1 year or remained resident for the rest of their life-cycle. Slow growth was associated with migration later in life (age 2 years) at a bigger size, which is consistent with the hypothesis of a threshold size for migration. No difference in size at age 1 year between migrant and resident fish suggests that other factors, such as growth efficiency and the presence of heritability of the tactics, are involved. Overall the sex ratio was equal for migrant and resident fish, while an age-specific bias was found: more males migrated at age 1 year and more females at age 2 years. These differences suggest that different tactics are adopted by different sexes.