Research Document - 2008/010
Equilibrium analyses of the recovery feasibility of four Atlantic salmon (Salmo salar) in Nova Scotia and Southwest New Brunswick
By A.J.F. Gibson, R.A. Jones, and P.G. Amiro
Abstract
Four case studies are presented that illustrate the relationship between threats and recovery potential of salmon in the Atlantic Nova Scotia and Bay of Fundy sub-region. Equilibrium models are used to show the present status of each population, as well as the expected effect of recovery actions on the population. Equilibrium models split the life cycle of a species into two or more parts, and determine the population size at which the rates in each part of the life cycle are balanced, such that the population does not increase or decrease in size. By varying the life history parameters in a manner that represents the expected response to a human activity and examining the resulting change in equilibrium population size, the effects of the activity on the population can be evaluated in a way that places the expected response in the context of other threats to the population. The threats and stressors discussed in each case study are representative of the major types affecting salmon population viability in Bay of Fundy and Nova Scotia Atlantic coast rivers: acidification, hydroelectric development, low freshwater habitat productivity, and low at-sea survival.
The LaHave River (above Morgans Falls) case study was developed using data specific to this population and illustrates the effect of reduced at-sea survival on population viability. A freshwater production curve was derived using estimates of annual egg deposition and estimates of the number and age composition of smolts emigrating from this river. The lifetime egg production by smolts throughout their lives was estimated using return rate data, population-specific fecundities, and estimates of repeat spawning frequency. Return rates to LaHave River have averaged 2.37% (range: 1.09% to 4.33%) for salmon maturing after 1 winter at-sea and 0.48% (range: 0.24% to 0.97%) for salmon maturing after 2 winters at-sea. Presently, only low at-sea survival has been identified as a threat for this population. Opportunities to increase either freshwater productivity or capacity are limited, but could be expected to improve the potential for population recovery. At the lowest observed return rates, the population has an equilibrium size of zero; while at the average return rates, the equilibrium population size is roughly 1.1 million eggs, a value similar to recent egg depositions. If the high return rate observed in 1999 was maintained, the population would be expected to grow to a level above its conservation limit, and could support fishing activities.
The Big Salmon River case study, thought to be representative of the dynamics of endangered inner Bay of Fundy populations, was also based on population-specific data. Similar to the LaHave case study, the primary stressor to populations within this Conservation Unit (CU) is low at-sea survival, although other threats exist. The effect of decreased at-sea survival for the Big Salmon River population is a shift in the equilibrium population size from about 4 times the conservation egg requirement to a population that is not viable (equilibrium of zero). In the absence of human intervention or a change in at-sea survival, these populations are expected to become extirpated. In the past, population size was very sensitive to the amount of freshwater habitat; however, at present, increasing habitat quantity or quality is expected to have little to no effect on population size given present at-sea survival rates. Increasing at-sea survival, such that return rates are 4% for 1SW (1 sea winter) and 0.5% for 2SW (2 sea winters), results in a shift of the population equilibrium to about twice the conservation egg requirement. The case study illustrates how factors influencing one part of the life cycle, (in this case at-sea survival) can limit the effectiveness of recovery actions focused on other parts of the life cycle (freshwater habitat restoration).
Interactions among multiple stressors are illustrated by the other 2 case studies presented. The salmon population in the Tobique River, New Brunswick, is under stress from reduced at-sea survival, but is further impacted by reduced survival of smolts as they migrate downstream past hydroelectric generating stations and through impoundments. Freshwater habitat productivity also appears low based on analyses of egg-to-smolt survival and juvenile abundance data undertaken as part of the case study. Given present freshwater production and at-sea survival, the population is not viable irrespective of the status of fish passage, even at the maximum observed return rates used in this analysis. If marine survival increased to a hypothesized (but plausible) 8% for 1SW and 3% for 2SW salmon, the population is still not viable at present passage survival rates, but a small equilibrium population size exists if fish passage is improved. If freshwater production is increased and marine survival increases, the population equilibrium is greater than the conservation requirement if fish passage survival is increased to 100%, but is less than half the requirement at present passage survival rates. Fish passage mortality. Therefore, has the potential to substantially limit the effectiveness of other recovery efforts, but addressing the fish passage survival issue alone is not expected to be sufficient to produce a viable population in this river.
A similar outcome was found in the West River (Sheet Harbour) case study. This river is impacted by both acidification and reduced at-sea survival. Low equilibrium population sizes can be obtained by addressing either threat, although long-term population viability in a randomly fluctuating environment is unknown. Based on the model results, both at-sea survival and acidification would have to be addressed to affect population recovery. These case studies illustrate that where multiple threats exist for a population, multiple responses are likely required to bring about recovery of the population.
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