Quantifying exposure and residence time of migrating Atlantic salmon post-smolts at aquaculture sites in a region of extreme tides

Abstract

Aquaculture production now exceeds capture fisheries, but concerns persist regarding environmental impacts. Salmon farms within Atlantic salmon (Salmo salar) migratory corridors may create hotspots of predation, parasites, or disease that could affect out-migrating post-smolts. However, residence time near farm sites remains poorly characterized. We used acoustic telemetry to quantify survival and residency of 60 post-smolts moving through the estuarine and coastal waters of Passamaquoddy Bay, New Brunswick, Canada, an area with concentrated aquaculture and extreme tidal range. Of the 53 individuals reaching marine waters, 23 (43%) were recorded within the ~400 m detection range of at least one monitored farm across 55 events. Median residence time per farm ranged from 4.5 to 32.5 min, with a cumulative median of 31.2 min across all four farms. Tides strongly influenced movements, increasing residence time and farm exposure by reversing seaward migration. Post-smolts were incidentally present near farms, without evidence of attraction or elevated predation. Although indirect or latent effects were not assessed, offshore mortality played a greater role in recruitment than apparent mortality associated with aquaculture sites.