Migration decisions by Atlantic Salmon smolts in a complex estuary: Should they stay or should they go?
The life history of the Atlantic salmon (Salmo salar) provides options for decisions about the transition from the natal, freshwater habitat to the marine ecosystem (and back). Eastern Canadian salmon populations are in a precarious state, suggesting that many smolts are making bad migration decisions. The Bras d’Or Lake of Cape Breton offers 1,300km2 (28km3) of estuarine habitat between the natal rivers of the watershed and the NW Atlantic Ocean. Local ecological knowledge references “Lake Salmon”. High rates of at-sea mortality may be avoided if smolts remain in the estuary to close their life cycle. To test this hypothesis we established a network of acoustic receivers in the Bras d’Or estuary, tagged 99 smolts leaving the largest river over three consecutive years, and tracked their movements through the estuary and beyond.
A small majority (52%) of the tagged smolts left the estuary through the two channels connecting the Bras d’Or to the NW Atlantic Ocean. The proportion leaving was highest (70%) in the third year. Individual transit times of these fish through the estuary varied greatly from 7 to 52 days (mean 31d), with the diurnal, tidal and interannual cycles accounting for approximately equal proportions of the observed variance in departure rates. Natural mortality of smolts within the estuary was low (10% to 19% depending on the year) compared with rates measured in the ocean. Overall, 21% (17% to 31% among years) of the smolts chose to remain in the estuary at least for the duration of the tag life (approx. 65d). Although they are not land-locked, some Atlantic Salmon spawned in the Bras d’Or watersheds are experimenting with the option of staying there rather than going to sea.