Research Document - 2008/057
Population Viability Analysis of Inner Bay of Fundy Atlantic Salmon with and without Live Gene Banking
By A.J.F. Gibson, H.D. Bowlby, J.R. Bryan, and P.G. Amiro
Abstract
The purpose of this Research Document is to provide background information on the population dynamics of inner Bay of Fundy (iBoF) Atlantic salmon in support of recovery planning for this designatable unit (DU). It covers issues in the Terms of Reference (TOR) for the iBoF Atlantic salmon Recovery Potential Assessment (RPA) related to abundance, trends, trajectories, recovery targets, and population viability. Abundance of iBoF salmon, which is thought to have been in the vicinity of 40,000 adults historically, has declined to less than a couple hundred adults presently in the wild. Bayesian analyses of abundance indices indicate a greater than 99% decline rate for these populations during the last 30 years. Electrofishing surveys for juvenile salmon in the early 2000’s indicate that salmon are extirpated from many iBoF rivers. Two different population viability analyses (PVA), one based on trends in the Stewiacke River population and one based on the life history of the Big Salmon River population, indicate a high probability that this DU will become extinct in about one decade, in the absence of human intervention or a change in at-sea survival. At present, the primary intervention is a Live Gene Banking (LGB) program designed to minimize the loss of genetic diversity, such that the populations can be restored when conditions become favorable for their survival in the wild. Within this program, salmon are taken from the wild as they are migrating to sea, grown to maturity, and spawned in captivity. Unfed fry are then released back into the river to maximize exposure to the wild environment. This program thus bypasses the marine phase of the life cycle in which extremely high mortality rates are placing the population at-risk. A life history based PVA that includes the LGB indicates a high probability that the population can be maintained through Live Gene Banking. In addition, this PVA was used to explore scenarios representative of 4 ways in which humans may impact on this DU: bycatch mortality in fisheries, incidental harm of juveniles via activity around rivers, downstream passage mortality at dams, and habitat restoration. In the absence of the LGB, the results indicate that at the present low at-sea survival rates, neither the probability of recovery (near zero) nor the probability of extinction (near one) is very sensitive to low levels of human-induced mortality. Similarly, at high at-sea survival rates, the probabilities of extinction or recovery are not very sensitive to low levels of human-induced mortality, although both the population recovery rates and the size of the recovered populations decrease with increasing mortality. Additionally, low levels of human-induced mortality have little effect on the probability of extinction (near zero) when the LGB is operating, even at very low levels of at-sea survival. A critical period exists when populations are beginning to recover, when both the probability of extinction and the probability of recovery are sensitive to low levels of mortality. The use of the conservation spawner requirement, a limit reference point used for salmon fisheries management, is proposed as a river-specific abundance recovery target. The number of rivers required for long-term persistence of the DU is not known, but the probability of persistence, ecological integrity of river ecosystems in which salmon were found, and human benefits are all increased if salmon are recovered in as many rivers as possible. Re-evaluation of the recovery targets and the effects of human-induced mortality, once populations are showing signs of recovery, is recommended.
Accessibility Notice
This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.
- Date modified: