Research Document - 2013/006

Recovery Potential Assessment for Southern Upland Atlantic Salmon: Habitat Requirements and Availability, Threats to Populations, and Feasibility of Habitat Restoration

By Bowlby, H.D., Horsman, T., Mitchell, S.C., and Gibson, A.J.F.

The purpose of this Research Document is to provide background information on the habitat characteristics required by Atlantic salmon in the Southern Upland to complete their life cycle, as well as the stressors and threats impacting those processes.  The document includes information related to:

1. functional descriptions of habitat properties,
2. the spatial extent of areas in the Southern Upland having these properties,
3. the identified threats to habitat, as well as threats to populations that are not habitat-related,
4. the extent to which threats have reduced habitat quality or quantity in the Southern Upland, and
5. the potential for mitigation of identified threats.

Each of these components was requested by the Terms of Reference (TOR) for the Recovery Potential Assessment for Southern Upland salmon. Information is presented for the freshwater and marine (and estuarine where appropriate) environments separately.

Habitat requirements of Atlantic salmon in fresh water include properties such as water quality, substrate composition, discharge characteristics, and accessibility.  Several life stages (eggs, age 0, age 1 and age 2+ juveniles) have specific habitat types that are required to support essential life cycle processes. At the current low population sizes of Southern Upland Atlantic salmon, freshwater habitats are unlikely to be limiting recovery in rivers where a large proportion of accessible area remains. Unfortunately, impassable dams and highly acidic water have reduced freshwater habitat availability by approximately 40% for populations in the Designatable Unit.

Habitat requirements in marine and estuarine environments have not been delineated spatially. However, these are thought to be primarily related to food availability and oceanographic conditions, since individuals require resources and water conditions that support rapid growth. As such, the areas occupied by Atlantic salmon populations from the Southern Upland likely change over time depending on variation in oceanographic environments (currents, temperature, and food availability). Based on tagging data, Southern Upland Atlantic salmon are widely distributed in the marine environment along the Atlantic coast for the majority of the year. Research on population dynamics of Atlantic salmon demonstrate that survival in the marine environment is not resource-limited, so the availability of habitat in marine environments is not limiting population size.

Multiple threats have been identified that are likely to have an effect on Atlantic salmon populations in the Southern Upland, either historically, currently or in the future.  In general, the linkages between threats and changes to Atlantic salmon populations have been established in the scientific literature, but have not been quantified for specific rivers in the Southern Upland. Where possible, the relative magnitude of a specific threat has been quantified among watersheds in the Southern Upland using Geographic Information System (GIS) analyses. In freshwater environments, it is likely that these threats have resulted in an overall reduction in habitat quality. The feasibility of restoring habitats to higher values is likely greater in freshwater environments than in marine because it is possible to quantify the impact of a given threat on a population, and the threats are more localized and tractable to address in the short-term.

Complete document (PDF, 7,367 KB)