Factors affecting the within-river spawning migration of Atlantic salmon, with emphasis on human impacts

Abstract

We review factors affecting the withinriver spawning migration of Atlantic salmon. With populations declining across the entire distribution range, it is important that spawners survive in the last phase of the spawning migration. Knowledge on the factors affecting migration is essential for the protection of populations, and to increase the success of reintroduction programmes. A number of studies have documented that the upstream migration may be delayed for many weeks at man-made obstacles such as power station outlets, residual flow stretches, dams, weirs and fishways. The fish may also be delayed at natural migration barriers. Often, the magnitude of delay is not predictable; fish may be considerably delayed at barriers that appear to humans to be easily passable, or they may quickly pass barriers that appear difficult. Stressful events like catch-and-release angling may affect upstream migration. Impacts of human activities may also cause altered migration patterns, affect the withinriver distribution of the spawning population, and severe barriers may result in displacement of the spawning population to other rivers. Factors documented to affect within-river migration include previous experience, water discharge, water temperature, water velocity, required jump heights, fish size, fish acclimatisation, light, water quality/pollution, time of the season, and catch and handling stress. How each of these factors affects the upstream migration is to a varying extent understood; however, the effects may differ among different river sections and sites. There are likely a number of additional important factors, and the relationship between different factors is complex. The understanding of general mechanisms stimulating fish within-river migration are still lacking, and it cannot be reliably predicted under which conditions a fish will pass a given migration barrier or which conditions are needed to stimulate migration at different sites. The strong focus on the effects of water discharge in past work may have hampered consideration of other factors. Exploration of the influence of these other factors in future studies could improve our understanding of what controls the upstream migration.

The impact of a small-scale riverine obstacle on the upstream migration of Atlantic Salmon

“…Obstacles directly increase energy consumption, thus fish with low energy reserves may need to reach spawning locations rapidly so as to rest and preserve remaining reserves for spawning. Although the effect of increased energy expenditure on reproductive success remains unknown, it would not be unreasonable to hypothesise that reduced energy reserves will ultimately have a negative impact on reproductive success (Thorstad et al, 2008).…”

“…At large complex obstacles, where fish are delayed and their migration thwarted, passage occurs primarily during daylight Gowans et al, 1999Gowans et al, , 2003Null & Niemela, 2011), whilst at less complex structures and natural by-pass channels passage generally occurs at night (Dunkley & Shearer, 1989;. Light intensity preference for passage requirements appears to be site specific and related to the visual orientation needs at each given obstacle (Banks, 1969;Thorstad et al, 2008). The timing of passage (in either day or night) may be an early indicator of passage difficulty for salmon, with daytime passage potentially indicating higher levels of passage difficulty.…”

“…The timing of passage (in either day or night) may be an early indicator of passage difficulty for salmon, with daytime passage potentially indicating higher levels of passage difficulty. Indeed there is likely to be an interaction between the light required to successfully ascend obstacles and the preference of turbid water or darkness as an anti-predator mechanism (Banks, 1969;Thorstad et al, 2008). Surprisingly, fish in 2012 which were sampled from within the fish pass and subsequently released downstream did not attempt to re-ascend through the pass a second time, instead ascending across the weir face.…”

“…Considerable effort has been made to mitigate the effects of river obstacles through the development of fish passes, which aim to facilitate the upstream and downstream migration of individuals around or through obstacles (Larinier, 1998;Guiny et al, 2005;Bunt et al, 2012). The efficiency of such structures is however often questioned; flow conditions such as water velocity and depth within the pass itself are not always conducive to upstream passage of fish (Thorstad et al, 2008;Noonan et al, 2012;Cooke & Hinch, 2013). For example, the addition of fish screens at the 86-m-high Pitlochry Dam (Scotland) increased the proportion of fish ascending the dam from 45% of fish which attempted (Webb, 1990) up to 100% by guiding fish away from the turbine entrances (Gowans et al, 1999).…”

Adaptation Turning Points in River Restoration? The Rhine Salmon Case

“…Rivers have to meet certain conditions for Atlantic salmon. A detailed description of necessary conditions can be obtained from Thorstad et al [25], and Bardonnet and Baglinière [26]. In short, the following conditions have to be met: (1).…”