Research Document - 2014/005

Recovery Potential Assessment for Eastern Cape Breton Atlantic Salmon (Salmo salar): Population Viability Analyses

By A. J. F. Gibson and A. L. Levy

Abstract

The Eastern Cape Breton (ECB) Designatable Unit (DU) of Atlantic Salmon was assessed as “Endangered” by the Committee on the Status of Endangered Wildlife in Canada in 2010. This document, prepared in support of a recovery potential assessment for this DU, contains information about the dynamics and viability of salmon in two of the 46 rivers thought to support ECB salmon populations. These two populations, found in the Middle and Baddeck rivers, are the only two ECB populations with sufficient data to evaluate population dynamics, and data for these populations, particularly age composition data, are limited. A forward projecting population dynamics model was developed that is specific to the types of data available for these populations. Output from the model includes estimates of smolt age and sea age composition, asymptotic recruitment levels, maximum lifetime reproductive rates, and equilibrium population sizes.

Despite these populations being in close geographic proximity, and subject to uncertainty in the results, model output indicates that the dynamics of these populations differ. Equilibrium calculations, based on current estimates of stage-specific mortality and maturity rates, suggest that abundance should tend toward 50 % of the conservation requirement of the respective river.  However, the estimated maximum lifetime reproductive rate (the maximum number of spawners produced by a spawner throughout its life at very low abundance – a key indicator of extinction risk) for the Middle population is double that of the Baddeck population (values of 3.22 and 1.61 spawners/ spawner, respectively).  Relative to populations in the Southern Upland and inner Bay of Fundy Atlantic Salmon DUs, these dynamics indicate a lower risk of extirpation for these two ECB populations. Trends in the recruitment deviates from the model are suggestive that productivity in the Middle population has not changed during the last 30 years, whereas the productivity of Baddeck population may have declined slowly during this time.

Population viability analyses based on this model indicate that, under current conditions, there is a low probability of extinction or of meeting recovery targets for these two populations if conditions remain unchanged.

There are differences in life history strategies among salmon populations in eastern Cape Breton; one of the most notable is the difference in the proportion of salmon maturing after one winter at sea as observed for Cape Breton Highland populations and for the populations to the southeast.  Given these differences, inferences cannot be made about the dynamics and viability of other ECB populations based on the dynamics of populations in the Middle and Baddeck rivers.

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