Long-term declines in body size and shifts in run timing of Atlantic salmon in Ireland


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Records extending back from three to seven decades from Atlantic salmon Salmo salar fisheries on the Blackwater, Newport and Owenduff River systems in Ireland, were used to test the hypothesis that there have been progressive declines in size and associated shifts in run timing. They showed identical patterns in four respects: 1) peaks of catch, in spring (March to April) and summer (June to July), 2) the largest fish were caught in the spring, with declines in size over the summer and a slight increase in autumn, 3) marked declines over time in average size and in the proportion of the Atlantic salmon taken in spring and 4) reductions in size in most months at all three sites. Correlations were also detected in average fish mass among the rivers over time (especially the Owenduff and Newport systems), showing a slight rise from the late 1960s until the late 1970s, followed by a steep decline. Data from an electronic counter on the Blackwater showed a reduction in the abundance of early migrants. The early fish were apparently much more vulnerable to fishing than later migrants, so the catch records overstated the abundance of early fish. These long-term declines in size, which have been noted elsewhere, may be due to other factors in addition to changes in marine productivity. Specifically, reduction in the abundance of large, spring-running Atlantic salmon may be due to selection resulting from heavy angler exploitation.