Insights on Mortality Vectors using Acoustic Telemetry: A Case Study Using Atlantic Salmon and Anadromous Brook Trout.

Declines in Atlantic salmon populations along the East Coast of Canada are postulated to be, in part, a result of low marine survival. Previous studies used acoustic telemetry to estimate survival of salmon smolts in estuaries and coastal zones; however, mortality causes frequently are not determined. We used acoustic telemetry to study Atlantic salmon smolts and anadromous brook trout from four rivers on the East Coast of Nova Scotia. The study used both passive and active tracking to determine the fate of tagged fish. Causes of mortality were inferred from the  fate of the fish, modelled survival, and observed behaviour. Tags from salmon classified as dead were infrequently detected in the water via active tracking (84%,  sd =15%, n=47 ) and were presumed to have been  removed from the aquatic environment, suggesting predation or scavenging by either terrestrial or avian piscivores. Seal and avian predation could also be inferred via temperature/pressure tags and movement patterns. Finally, the manner in which covariates influenced modelled survival also provided potential insights to mortality vectors, in particular, highlighting a potential sub-lethal effect of freshwater acidity.