Aspects of the Ecology of the Brown Trout (Salmo trutta L.) in Relation to Management of the Fishery on the River Don, Aberdeenshire.

Abstract

1. A range of studies were undertaken regarding the present status of the brown trout in the River Don, Aberdeenshire, in order to formulate a preliminary management plan for the maintenance of this high quality brown trout fishery.
2. Extensive electrofishing and habitat surveys provided information on the distribution and abundance of juvenile salmonids throughout the catchment. Juvenile trout (0+ and 1+) were found predominantly in the tributaries while older trout (=2+) and juvenile salmon (0+ and 1+) were found predominantly in the main river. The effectiveness of the HABSCORE models in predicting juvenile salmonid numbers from habitat and catchment parameters at sites in the Don catchment was assessed.
3. The localised effects of degradation of the physical stream habitat - particularly through grazing and channelisation - upon the salmonid populations were demonstrated. Salmonid densities were generally low in areas affected by these land-uses.
4. The recruitment of juvenile trout from a tributary nursery area into the main river during spring was assessed by trapping. Emigration peaked at age 2. Older trout migrated downstream earlier than younger trout and faster growing trout migrated earlier than slower growing trout of the same age. Predictive models based on environmental parameters accounted for 64 and 78% of the variation in the timing of migration of brown trout and trout smolts respectively.
5. The fecundity of adult sea and brown trout was assessed. Artificial stripping yielded, on average, 92% of the total number of eggs produced by the female. The number of eggs produced was significantly and positively correlated with the fork length and the somatic weight of the female. No significant differences in these relationships were apparent between brown trout and sea trout females although the fecundity at a standardised body length varied between the two populations.
6. Perforated boxes containing 50 eyed trout eggs were buried in spawning gravels to monitor intra-gravel survival in several tributaries throughout the catchment over a three month period. Survival to hatching was high (> 80%) at sites in the upper part of the catchment but was relatively low (< 50%) at sites in the mid and lower catchment. Low survival rates appeared to be related to a high degree of infiltration of fine organic sediments.
7. The movements of 20 large brown trout were monitored for up to 20 days by radio-telemetry, following displacement downstream of the home site. Movements following release involved an initial, relatively brief period of exploratory movement (lasting up to 5 days). This was followed by a period of more restricted, territorial movement which persisted for the remaining duration of the study (15 to 20 days). Two trout returned to the home site within 5 days. The remaining 18 trout appeared to establish new territories near to the release sites within approximately five days of release.
8. Hotel and estate fishing records - going back as far as 1898 - were analysed to determine the historic status of the brown trout fishery. Large numbers of small trout were taken by anglers during the first half of the century. The catches of trout recorded by anglers have decreased since 1980 while the mean weight of the recorded catch has increased.
9. Detailed fishing records from an angling club on the lower Don revealed that the fishing effort was the most important determinant of catch while water temperature also had a significant effect but river flow did not. A small percentage of the anglers accounted for a disproportionately large percentage of the total catch and associated fishing effort. There was evidence for compensatory growth following a period of low stock abundance.
10. Information on the age and size structure and diversity of anglers catches was obtained by creel survey. Catches from the lower river where fishing effort was greatest, consisted of mostly small, young trout (age 3+). Larger, older trout were more frequent in the catches from the upper river, where the fishing effort was relatively low. The age structure diversity of the upper river catch was greater than that of the lower river catch. Voluntary returns of logbooks and scale samples by anglers were very low.