Research Document - 2016/029

Biological characteristics and population dynamics of Atlantic Salmon (Salmo salar) from the Miramichi River, New Brunswick, Canada

By G. Chaput, S.G. Douglas, and J. Hayward

Abstract

This report summarizes the information on biological characteristics of Atlantic salmon (Salmo salar) from the Miramichi River (New Brunswick, Canada) over the period 1971 to 2014. Emphasis is placed on characteristics of salmon from the Northwest Miramichi in support of a review of the risks to long-term fitness of wild Atlantic salmon of a proposed supplementation program consisting of the captive-rearing in freshwater of wild salmon smolts to the adult stage and release back to the river to spawn. Freshwater dynamics of Atlantic salmon are examined to assist in the evaluation of the risks and /or benefits of the proposed supplementation activity in achieving one of the stated goals which is to increase abundance of adult Atlantic salmon to the river. The Atlantic salmon population from the Miramichi is characterized by complex phenotypic diversity. In any given year, there are typically 6 year classes of immature fish and as many as 9 year classes of mature fish in the combined freshwater and marine ecosystems. There are defined run timing components of the population to the river and headwater areas at higher elevations are primarily utilized by early-run fish. There is an important sex ratio bias between the maiden sea age groups of fish. Increased return rate to a second spawning is an example of phenotypic plasticity in this population responding to changes in the ecosystem. Indices of juvenile abundance for the past four decades indicate that freshwater production of Atlantic salmon increased from low values in the 1970s and peaked in the late 1990s and early 2000s. Abundance indices of fry and small parr are currently well above those of the 1970s to mid-1980s. Preliminary stock and recruitment analyses indicate that the most important density dependent control occurs between the egg and fry stage with modelled theoretical carrying capacity of fry almost realized in the late 1990s. If the egg to fry recruitment dynamic is as severely compensatory as suggested by these analyses, there may be very little gain to be realized in increased smolt production and subsequent adult returns by supplementing the current spawning escapement levels with large numbers of captive-reared adult spawners. Density dependent factors will rapidly adjust the abundances of juveniles to levels which can be sustained by the productive capacity of the freshwater habitat.

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