Scottish Marine and Freshwater Science Vol 6 No 12: The demography of a phenotypically mixed Atlantic salmon (Salmo salar) population as discerned for an eastern Scottish river

Past Fisheries Management Actions and their Effects

There were appreciable challenges in assessing the effects of oceanic-fisheries' management actions (West Greenland, Faeroes) on the North Esk's salmon populations. Clear trends were not evident. The poorly-realistic marine-mortality models, allied with a high year-on-year variability in the estimated marine survivals, and exacerbated by one particularly extreme (high) survival estimate at the start of our data-series (see Supplementary Material, part J.1, for details of the strong effect of the 1980 cohort), obscured any clear underlying trends in marine survivals, despite the sub-stock decompositions attempted here.

More surprisingly, the effects of management actions which restricted the North Esk's within-river fisheries were also rather unclear, and could not be confidently ascribed to that cause alone. Unfortunately, as PEA abundances are not measured at adjacent Scottish rivers, it is unknown whether similar in-river survival improvements also occurred elsewhere.

It should be noted that some biological mechanisms could potentially cause delayed responses to ameliorative management actions, thus blurring the expected synchrony of cause and effect. Assuming that run-times are heritable, Fishery Induced Evolution effects ( FIE; for brief overviews see Heino and Dieckmann 2009 and Stenseth and Dunlop 2009; for summary reviews see Allendorf et al. 2008; Fenberg and Roy 2008; Jørgensen et al. 2007; Law 2007; Kuparinen and Merilä 2007) could result in salmon populations' responses to previous fishery-selection pressures becoming manifest as delayed changes. Any such prior-induced changes might then have carried over into our study period. However, such FIE effects on run-timing would be expected to also alter the seasonal distribution of run-times observed during our study (within the sub-stocks, to seasons that better matched months with reduced risk from fishing). Examination of the North Esk data (not reported here) failed to find such changes.