Research Document - 2009/081
Review of DFO Science information for Atlantic salmon (Salmo salar) populations in the Southern Upland region of Nova Scotia
By A.J.F. Gibson, H.D. Bowlby, D.L. Sam, and P.G. Amiro
Abstract
The purpose of this document is to review available information for Atlantic salmon populations in the Southern Upland region of Nova Scotia in support of an assessment of the extinction risk by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). As such, annually collected monitoring data on all life stages of salmon from two index river populations, as well as adult and juvenile abundance and distribution data from other populations in the Southern Upland, were assessed relative to conservation requirements and other reference values. Trends in abundance and distribution, changes to life-history parameter values, overall population viability, and threats to population persistence were quantified where possible.
For the two index populations (LaHave and St. Mary’s rivers), current abundance of all life stages is low and populations have undergone declines in excess of 50% over the previous three generations. Mortality rates of immature and adult salmon at sea are high, and there is some indication that freshwater productivity has declined over the last 20 years. Surveys to assess the status of surrounding populations relative to the index populations suggest that current adult and juvenile abundance is critically low in most rivers, and that population extirpations may have occurred. For rivers in which salmon were found, juvenile abundance has declined markedly over the last eight years. Furthermore, estimated declines in adult escapement, as determined by changes in recreational catch or monitoring at fishways, exceed 95% in some rivers.
Estimates of current life-history parameter values for salmon in the index rivers indicate that populations may be viable at low overall population size. However, the maximum reproductive rates of the populations are extremely low, indicating little capacity for a compensatory response to further decreases in population size. This places populations at risk from the cumulative effects of environmental variability or stochastic events. In terms of recovery potential, increases to freshwater productivity have the potential to enhance long-term population viability but will not recover populations to levels above the conservation requirement without an increase in survival at sea.
Acidification is known to have substantially reduced the capacity of rivers in the Southern Upland to produce salmon and populations in highly acidified rivers are likely extirpated. Other threats to Atlantic salmon in the Southern Upland are thought to affect a large proportion of populations, yet their impact on spawner abundance in any specific population tends to be low or uncertain. However, equilibrium analyses suggest that population viability could be enhanced via increased freshwater productivity, although the extent to which habitat restoration efforts can increase freshwater production is not known. Given the current trends in salmon abundance in the Southern Upland, any actions to promote recovery will have to be timely to be effective.
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