Ocean-entry timing and marine habitat-use of Canadian Dolly Varden: Dispersal among conservation, hydrocarbon exploration, and shipping areas in the Beaufort Sea

Abstract

Conservation and management of anadromous salmonids are enhanced by understanding timing, spatial extent, and occupied depths and temperatures in marine feeding habitats. We examined ocean-entry timing and marine habitat-use, and their association with environmental conditions (e.g., sea-ice and sea-surface temperatures (SSTs)) of anadromous Dolly Varden (Salvelinus malma malma) from the Canadian Arctic using pop-up satellite archival tags (PSAT) and data storage tags. Using this information, we evaluated the extent tagged fish occupied offshore (>5 km) habitats, and their proximity to a marine protected area (MPA) and areas of potential threats in the Canadian Beaufort Sea. Ocean-entry by tagged fish using the western Mackenzie Delta for freshwater migration occurred approximately mid-June (range = 8–26 June) and closely followed landfast sea-ice break-up based on satellite imagery. While at sea, fish predominately occupied surface waters (<2 m) at SSTs of 5–10 °C. PSAT end-locations were 37–152 km offshore, typically near the sea-ice edge, greatly extending previously reported distances from shore in the Alaskan Beaufort Sea. The spatial extent of offshore dispersal by Dolly Varden is likely influenced by SSTs and sea-ice conditions, and the physical properties of the Mackenzie River plume (e.g., turbidity), which extends preferred temperatures farther from shore. In relatively cooler years characterized by later sea-ice breakup and a summer sea-ice margin situated closer to shore, fish spent more time in nearshore than offshore habitats (51.7% vs. 48.3%) compared to warmer years (12.6% vs. 87.5%). Furthermore, fish typically occupied shallower offshore mean depths (2.2 m vs. 3.4 m) of the water column and experienced colder mean water temperatures (5.1 °C vs. 7.4 °C) in cooler versus warmer years. Dolly Varden were found within or adjacent to hydrocarbon lease areas and shipping lanes, and may be vulnerable to threats associated with these activities. Although PSATs reported outside of the MPA boundaries, which are situated adjacent to the Mackenzie Delta, occupancy in spring was inferred during ocean-entry and while transitioning to offshore areas. This first study to describe how environmental conditions influence marine distribution of Canadian Dolly Varden together with their proximity to anthropogenic threats is relevant for assessing impacts of climate change and future development.