Research Document - 2014/006
Assessment of the Recovery Potential for the Outer Bay of Fundy Population of Atlantic Salmon: Threats to Populations
By C.N. Clarke, S.M. Ratelle, and R.A. Jones
Abstract
This research document provides a compilation of information and review of major threats currently thought to impact the persistence of wild Atlantic Salmon in the Outer Bay of Fundy (OBoF) Designatable Unit (DU). Over 90% of the wild salmon and accessible habitat in this unit lie in the Saint John River (SJR) and its tributaries which, aside from the St. Lawrence, compose the largest river in northeastern North America. Thus, the status of the entire DU population weighs heavily on the status of the SJR. The impacts of threats to Atlantic Salmon vary in dimensions of space, time, and severity. As such, attempts have been made to provide dimensional context for each threat although limitations of available information were encountered. Nearly all threats discussed here have been documented for their effect on salmon populations; however, many have not been directly reported for their effects on OBoF Atlantic Salmon. The information presented here intends to provide only a summary of knowledge on the extent and magnitude each threat poses to this population in either the freshwater or marine environment. Most of this information contributed to the development of a Recovery Potential Assessment (RPA) Science Advisory Report (SAR) for the OBoF Atlantic Salmon population (or DU 16; DFO 2014).
To the extent available data permits, each threat has been assessed a rank of concern relative to other threats to the population. Threats thought to be most limiting population recovery ranked as highest concern. Following past science advice for improving threats assessments for species at risk, a proposed approach is presented for assessing accumulated threat impact on each river.
The threat of highest concern in the freshwater environment, and for the entire DU, stems from the gauntlet of hydro dams facing migrating salmon. The most significant structure is the Mactaquac hydro-generation dam on the SJR, above which lies 36% of the entire DU habitat relying on manual operations to truck salmon above the dam to complete upstream migrations. Aside from the obvious barriers to upstream migration, hydro dams on the SJR affect flow and temperature regimes, which are critical to/for migration cues, alter food web and aquatic community structure, and harbour a growing abundance and diversity of non-native predators to salmon within reservoirs and tailraces. Smolts migrating from systems upstream of Mactaquac Dam have been shown to experience up to 45% additive mortality when passing through three major hydro dams.
Marine threats of highest concern include: depressed population phenomena, salmonid aquaculture operations and shifts in oceanic conditions caused by changes in climate.
Effects of depressed population phenomena are often difficult to quantify but are expected to contribute to the negative effects caused by most other individual threats. Shifts in oceanic conditions may be resulting in unfavorable temperature, current, and predator/prey conditions which contribute to reductions in survival during the salmon’s marine life phase. Aquaculture operations, although less studied in Atlantic Canada than elsewhere, affect wild salmon in several ways including altered pathogen/predator/prey dynamics and through the interactions of wild and escaped farmed salmon.
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