Identification of Environmental Factors that Influence the Upstream Migration of Sea Trout Salmo trutta in Shetland Waters

Abstract: Migrating sea trout Salmo trutta L. in the waters of the Shetland Islands (UK) were once found in abundant numbers but they have been in decline since the mid-1980s. Therefore, it is becoming ever more critical to investigate sea trout behaviour with respect to the prevailing environmental conditions in the unique Shetland habitat so that we can determine how this prized and important species might be re-established to more sustainable levels. Here, during a period of four months (July to October) in 2015, environmental and fish catch rate data were collected for a small sample size of sea trout caught at six Shetland locations. We then looked for trends and correlations in the data that might help account for the migratory behaviour of the fish with respect to the environmental conditions at the time of capture. Our data showed that wind speed and pH correlated to the catch rate. However, no significant correlation was found between the water temperature, air temperature, barometric pressure, or humidity, and catch rate. Multivariate regression showed that wind speed and pH explained 26.3% of the variance in the catch rate. The sex ratio of the sea trout caught was skewed towards females at a ratio of 4.7 : 1.0 and the largest trout caught were all female. Recording biological data and understanding the migration patterns and behaviours of the remaining wild populations of sea trout are important for optimizing management plans to support current efforts to increase the numbers of this valuable anadromous species in Shetland waters.