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COSEWIC assessment and status report on the Atlantic Salmon (Inner Bay of Fundy populations) in Canada
- Assessment Summary
- Executive Summary
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Special Significance of the Species
- Existing Protection or Other Status Designations
- Technical Summary
- Acknowledgements and Authorities Contacted
- Information Sources
- Biographical Summary of Report Writers
- Appendix 1. General biology of Atlantic salmon
Freshwater Requirements and Trends
Atlantic salmon rivers are generally clear, cool and well-oxygenated, with low to moderate gradient, and possessing bottom substrates of gravel, cobble and boulder (National Recovery Team 2002). Riffles, rapids, pools and flats are utilized at different life stages. Juveniles grow on invertebrate drift. During summer, water temperatures typically range between 15 and 25°C. Specific freshwater habitat requirements for iBoF salmon are known, and suitability indices exist for both summer and winter. These indices, such as stream gradient, have been used to estimate the productive area and capacities for 22 iBoF rivers (Figure 7). Productive capacity for juveniles varies greatly on a river-by-river basis, but iBoF rivers currently have a variety of habitats and are believed to be well-suited to the production of salmon (Gibson et al. 2004).
There is no evidence of an incremental loss in freshwater habitat that would explain the declines in iBoF Atlantic salmon populations since the late 1980s (National Recovery Team 2002). Beginning in the mid- to late-19th century, freshwater habitat has been impacted by forestry, agriculture and road development. Barriers to salmon migration, such as dams, dykes and causeways, have also impacted many iBoF rivers. For example, the Petitcodiac River once produced an estimated 20% of the iBoF’s salmon (National Recovery Team 2002), but a causeway built in 1968 largely blocked the migration of adults and smolts (Locke et al. 2003). The pH of iBoF rivers is typically above 6.0, and thus acidity levels, a problem for salmon elsewhere (e.g., Lacroix and Knox 2005), are not a factor in continued decline. While there can be little doubt that the removal of access to spawning and rearing habitat has decreased the salmon production capacity of the iBoF region over the course of the past two centuries, the timing of these events does not correspond with the recent demographic collapse (see POPULATION SIZES AND TRENDS). In a recent critical habitat assessment, Trzcinski et al. (2004) concluded that population viability (and recovery to conservation limits) could not be achieved by increasing the quantity or quality of freshwater habitat. They further noted that releases of Live Gene Bank progeny (see EXISTING PROTECTION) appear to survive well in fresh water (i.e., to smolt), but experience heavy marine mortality. This is
Figure 7. Area, productive capacity of age-1 and older parr, and production per unit area for 22 inner Bay of Fundysalmon rivers. Area was estimated from aerial photographs and ortho-photo maps. Productive capacity was estimated using stream gradient (from ortho-photo maps) as a proxy for habitat quality. Col is an abbreviation for Colchester Co., NS. From Amiro et al. (2003).
not to say that there are no freshwater habitat concerns within the iBoF: barriers to passage, water quality issues, and habitat loss due to a variety of factors are present in some rivers. Currently, however, there appears to be an abundance of quality freshwater habitat for salmon within the iBoF region (Amiro et al. 2003; Gibson et al. 2004; Trzcinski et al. 2004), yet salmon numbers continue to decline.
Marine Requirements and Trends
Ocean habitat requirements for iBoF Atlantic salmon are less well known than those for fresh water. One of the only current indicators of marine habitat quality for Atlantic salmon in general is temperature: Atlantic salmon are thought to occupy a marine temperature envelope from 1-13°C, with preference around 8°C (Reddin and Friedland 1993). Due to the infusion of oceanic water, the Bay of Fundy and Gulf of Maine provide this temperature range, and the presence of two important prey species (sand lance and euphausids; National Recovery Team 2002) make occupation possible. However, a recent assessment of habitat based on sea surface temperature shows how seasonally dynamic and potentially limiting the marine environment could become for iBoF salmon (Amiro et al. 2003). A relatively minor change in the marine environment, a restriction in the use of the environment, or the introduction of new stressors (e.g., diseases) in late summer could be particularly detrimental to salmon survival.
A decline in marine survival through the smolt to adult life stage appears to underlie the collapse in iBoF populations. Significant decline in marine habitat quality and abundance may be occurring by at least three mechanisms. First, over 400 tidal barriers have been constructed in the Bay of Fundy, and while their placement pre-dates 1970 (Wells 1999), it is possible that cumulative effects through time have negatively altered the iBoF ecosystem for salmon. Second, a large aquaculture industry has grown in the western Bay of Fundy, northern Gulf of Maine, and southwest region of the Scotian Coast in the past 20 years (Figure 8). Third, primary production is apparently declining in parts of the western North Atlantic (Gregg et al. 2003). This decline might cause dramatic changes in energy flow, fish physiological condition and fish community structure, as recently indicated for the eastern Scotian Shelf (Choi et al. 2004). Potential causes of the decline in primary production include climate change and cooler local temperatures (Drinkwater et al. 2003), and enormous removals of fish biomass by marine fisheries that cannot be matched by net primary production (Choi et al. 2004). Smolt and post-smolt iBoF Atlantic salmon may also be subjected to predation by marine mammals in the Bay of Fundy, such as harbour seals (Phoca vitulina) and grey seals (Halichoerus grypus), although the level of this predation mortality has not been estimated. These trends as they relate to iBoF Atlantic salmon are discussed further under Limiting Factors and Threats.
Figure 8. Map showing the locations of aquaculture facilities in the outer Bay of Fundy and Gulfof Maineas of 2003. All licensed Canadian (closed circles) and U.S. (open circles) farms are shown, but not all facilities are in operation. No salmon farms occur within the inner Bay of Fundy. Map provided by Blythe Chang (DFO, St. Andrews).
Only portions of two iBoF rivers, 77% of the Upper Salmon River and 58% of Point Wolfe River, are within federally protected land (Fundy National Park). These two rivers contain a very small proportion of iBoF individuals, and thus would not represent adequate freshwater habitat protection. All remaining rivers flow through lands that are privately or provincially owned (National Recovery Team 2002). The Fisheries Act, administered by Fisheries and Oceans Canada, provides some responsibilities for the protection of all freshwater and marine fish habitat including alteration, destruction, accessibility, and introduction of deleterious substances into the water.
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