Studies of the Atlantic salmon (Salmo salar L.) in Scotland
Salmon catch data wee analysed from fisheries in or around the mouths of some rivers on the east coast of Scotland spanning the period from the late eighteenth century to the 1980s. It was found that the numbers and sea-age composition of salmon caught and the time of year that salmon were caught varied in the long-term in a coincident manner in different fisheries. These coincident changes in salmon catch data could not be explained by changes in the operation of fisheries and were therefore the results of changes in the population of salmon returning to the Scottish coast. The observed changes in salmon population parameters often coincided with known hydrobiological events in the North Atlantic area, but simple consistent associations between hydrobiological changes and changes in salmon populations were not found. It was concluded that salmon populations were influenced by marine factors but that relationships may be complex. Since there may be several different mechanisms which can cause changes in the sea-age and time of return of salmon, it is not to be expected that a simple causative relationship should be found. The characteristics of salmon which were caught spawning in different burns within the River North Esk system during the winters of 1989/90 and 1990/91 were analysed. It was found that those fish which spawned furthest upstream tended to have a higher sea-age and smolt-age, a smaller length and entered the river earlier in the year than would have been expected if fish entering the North Esk at different times of the year spawned randomly throughout the North Esk system. Likewise those fish which spawned further downstream tended to be large (for their sea-age), had a low sea-age and smolt-age and entered the North Esk late in the year. Although less strong evidence was obtained, a similar pattern was observed for fish which spawned at different times within the spawning season. Those fish which spawned earliest (late October/early November) tended to have entered the river earliest in the year. Analysis of environmental conditions in different burns during the spawning season of 1990/91 suggested that fish which entered the North Esk at different times of year selected particular tributaries to spawn in - the segregation observed was not the result of the chance occurrence of environmental conditions which were conducive to spawning at different times of the year in different burns. The smolt-ages of the adult fish caught in each burn were similar to that of juvenile fish in each respective burn. This suggests that adult fish were returning to the part of the river system in which they were born. Time of return to freshwater and time of spawning were concluded to be adaptations to different environments within the North Esk system. However these characteristics may not be rigorously important and the constitution of a sub-population may alter as a result of selection processes perhaps in the sea.