Temporal and environmental influences on the variation in sea trout ( Salmo trutta L.) smolt migration in the Burrishoole system in the west of Ireland from 1971 to 2000

The relationships between a number of environmental variables and the daily sea trout ( Salmo trutta L.) smolt output in the Burrishoole system were examined between 1971 and 2000. The average number of smolts migrating each year in the Burrishoole system decreased significantly from 3814 in the 1970s and 3899 in the 1980s to 1520 in the 1990s. A historic low of 769 smolts was recorded in the year 2000. The annual smolt runs were divided into a number of points, the duration in days to the 5, 50 and 95% points of the run. All of the points took substantially longer to reach in the 1980s and 1990s than in the 1970s, and they occurred earlier in the year. Sea trout smolt runs have shifted from discrete well-defined periods in the 1970s to more spread out less distinct runs in the 1990s. Two sets of environmental variables were identified as having an important role in smolt migration, regulating factors and controlling factors. Regulating factors operate before the smolt run and affect the physiological process of smoltification. The primary regulating factors were photoperiod and temperature. Controlling factors operate within the smolt run, affect the physical process of migration. The primary controlling factor was absolute water level, followed by change in water level, temperature and photoperiod. It is likely that the significant decline in the number of smolts migrating from the Burrishoole system over the period also had some effect on the dynamics of the smolt run.