Temporal variation in sea trout Salmo trutta life history traits in the Erriff River, western Ireland

The demographic and life history characteristics of sea trout Salmo trutta L. populations can be changed by a range of pressures in both freshwater and marine environments. Few long-term monitoring programmes are in place to assess temporal change in population dynamics. We analysed a 20 yr time series (1985-2004) using 15 sea trout population response variables in the Erriff River, western Ireland. Over this period, when time was considered as a categorical variable comprising 4 sequential periods of 5 yr, important life history changes were observed. The most dramatic of these changes corresponded with the period immediately after the commencement of salmon farming in the local estuary, with significant decreases in the number and length of sea trout kelts, the estimated number of eggs deposited, the sea trout rod catch, the proportion of older (1+ and 2+ sea age) fish and the frequency of repeat spawners. We found a significant positive relationship between the number of salmon lice Lepeophtheirus salmonis in the local salmon farm and the number of lice found on sea trout collected contemporaneously in local rivers. Results of this long-term monitoring programme demonstrate that significant changes in sea trout population structure with respect to quantitative life history traits can occur over a relatively short time period and suggest that the introduction of salmon farming into the local estuary most likely contributed to the observed changes in sea trout population dynamics.