Cage Depth Overwintering

Project Status:    Complete.

Funded By:    Aquaculture Component of the Research and Development Program, and the Newfoundland Salmonid Growers Association.

Introduction:    Aside from tangible demonstration by the Newfoundland salmonid growers of aquaculture production capability in the temperate, Northwest-Atlantic environment of Bay d’Espoir, in a comparative sense our aquaculture opportunity is unique (Tlusty et al. 2000) for two main reasons. First, being located at approximately 47.83° N, and with the largest freshwater inflow of any small Newfoundland Bay (2.0 x 10⁶ m³ d^-1, MSRL Report 1980), the fjord freezes over during the winter. The Atlantic salmon production cycle, spanning 32 months (20 months estuarine on-growing), makes under-ice cage culture a necessary component of the annual production cycle for this species. While steelhead aquaculture has proven that a marketable product can be achieved in one growing season, speciality markets for large, high-value steelhead (e.g., Japan) also dictate longer ongrowing intervals that entail overwintering. The cold Labrador current, that exerts considerable influence on the entire Newfoundland coastline, subjects Newfoundland’s coastal zone to winter water temperatures that can exceed the lower lethal limit for Atlantic salmon (–0.7° C; Saunders et al. 1983). This makes for less than optimal production conditions since, under such conditions, salmonids exhibit little-to-no growth for three to four months of the year.

The Newfoundland salmonid aquaculture industry is poised for expansion. Considering its track record of environmental studies (Tlusty et al. 1999) and its emphasis on ongoing environmental monitoring (i.e., precautionary industry development) there is little in the way of rational argument to oppose such Newfoundland industry expansion (Garcia 1994). Early emphasis within the local industry on minimizing financial investment, proximity to infrastructure and physical protection from the sea is being supplemented by industry concern about biological and ecological principles as advocated by Rosenthal (1994). Having developed considerable insight into its ongrowing environment, established critical infrastructure in support of production opportunity, and achieved tangible demonstration of its ability to produce quality salmonid products to the international marketplace, it is apparent the Newfoundland industry has progressed well beyond its infancy. However, in a global sense, our present production is at a level (~2500t/year) indicative of an industry only just entering its adolescence.

In recognition of fluctuating environmental conditions, and a critical need for additional, safe overwintering sites, Newfoundland’s salmonid farmers have begun examining alternative sites further afield from the Bay d’Espoir estuary. Industry perceptions are that, if the Bay d’Espoir industry is to achieve economic viability, we must establish a cause-and-effect relation between seasonal and diurnal variation in environmental conditions in the area immediately surrounding Bay d’Espoir aquaculture sites and fish distribution both within and around the cages. Only by comparing fish behaviour both within and outside cages will it become apparent if the net structure is forcing the caged fish into less preferred microhabitat. An important question the salmonid growers wish to address in the immediate term is that of necessary cage depth for optimal salmonid aquaculture performance efficiency during winter (ice cover) periods. This question is posed by the Newfoundland Salmonid Growers Association as one of its two highest-priority research needs.

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