Atlantic Salmon Marine Threats Research

Low marine survival of adult Atlantic salmon is widely understood to be the most significant factor causing population declines and preventing population recovery. Research to identify specific sources of marine mortality is complex due to the large areas of marine habitat used by salmon as well as the number and diversity of potential threats. These challenges are further complicated by the limited availability of adult salmon for research, as many populations have severely declined throughout the Maritimes Region.

For these reasons, the Marine Threats Research Unit focuses on smolts in the rivers and estuaries where they face numerous threats. These threats include predation, habitat degradation or those that may cause reduced marine survival, such as poor juvenile growth, health and/or condition of smolts. Identifying riverine, estuarine and near-shore factors affecting survival is also more likely to provide manageable solutions than identifying more distant far-field effects in open ocean habitat.

figure 1 - Map showing Nashwaak River in NB and Gaspereau and Stewiacke rivers in Nova Scotia
Map showing Nashwaak River in NB and Gaspereau and Stewiacke rivers in Nova Scotia

Smolt migration and survival studies

New acoustic technology is allowing scientists to better track Atlantic salmon smolt migration and their predation.

For the Stewiacke and Gaspereau Rivers in Nova Scotia, Atlantic salmon smolts from the endangered inner Bay of Fundy population are captured in smolt traps and fishways as they leave the river for their ocean migration, which is primarily limited to the Bay of Fundy. An acoustic tag is then surgically implanted in the fish. This tag emits pings that are detected by a receiver placed in the river, estuary or Bay of Fundy when the fish swims by. This allows researchers to track the timing of the smolts migration from the river into the estuaries and beyond. Additionally, predation can be tracked, as the implanted acoustic tags are unique, consisting of a special glue that breaks down if the fish is eaten by a predator, which changes the tags signal number. Previously, researchers had to determine if the fish was eaten by a predator based on recorded erratic tag movement patterns not typical of a salmon smolt.

In the Stewiacke River, Atlantic salmon smolts must face many predators on their way out, such as large numbers of spawning striped bass in the tidal reaches and other predators (invasive or introduced) in the upper freshwater areas, such as chain pickerel, smallmouth bass and brown trout. This research is also underway on the Gaspereau River where smolts must navigate hydro-electric installations, such as dams, bypasses, etc., to reach the Minas Passage.

For the outer Bay of Fundy Atlantic salmon population, research is underway on migration and survival rates of Nashwaak River smolts in the Saint John River system in New Brunswick. Previous research suggests that a higher portion of this population’s outgoing migration experience mortality in the riverine and estuarine environments than some other populations.  Ongoing research aims to more precisely identify these areas of mortality and to try to identify specific causes. 

These smolts tend to have a larger ocean range, migrating to areas outside of the Bay of Fundy with some tagged fish being detected as far away as the Cabot Strait between Cape Breton, Nova Scotia and Newfoundland.

Much of this work is done in collaboration with other departmental salmon researchers, the Mi’Kmaw Conservation Group, the Ocean Tracking Network, Vemco, and Dalhousie and Acadia universities.

For more information on these and other Atlantic salmon populations in the Maritimes Region, visit the DFO website.

figure 2 - Atlantic salmon smolts are collected in a rotary screw trap to provide biological data and for tagging.
Atlantic salmon smolts are collected in a rotary screw trap to provide biological data and for tagging.
figure 3 - An acoustic tracking tag is inserted in an Atlantic salmon smolt. When the fish is released, the tag emits pings to receivers in the river and estuary and allows us to track its migration and survival rate.
An acoustic tracking tag is inserted in an Atlantic salmon smolt. When the fish is released, the tag emits pings to receivers in the river and estuary and allows us to track its migration and survival rate.
figure 4 - Receivers are placed throughout the river and estuary to track smolt migration.
Receivers are placed throughout the river and estuary to track smolt migration.