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.
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This work was conducted when Andrew L. Miller was on a 6-month sabbatical at the NAFC Marine Centre, Scalloway, Shetland, during which time he was appointed as an Honorary Professor of the University of the Highlands and Islands, Inverness, Scotland. We would like to thank Drs. R. Andrew King and Jamie R. Stevens (University of Exeter) for including information derived from the sea trout scales we provided, in their genetic investigation on the origins of sea trout caught in the marine environment of the Shetland Isles, and for their personal communication concerning this matter. Thanks also to Alec Miller (SAA) for his invaluable advice regarding Shetland sea trout, and to Harvey Y.S. Chan, Jacky T. Hung (The Hong Kong University of Science and Technology), and Alan M. Shipley (Applied Electronics LLC), for helping with sample collection. We also acknowledge the technical and logistic support of the NAFC staff who assisted with the project: Beth Mouat, Gregg Arthur, Paul Macdonald, Saro Saravanan, Geoff Young, Kenny Gifford, Callum Tait, and Mark Jones.
The project was supported by a Marine Alliance Science Technology Scotland (MASTS) Visiting Fellowship Scheme, project no. MASTS15SC01; a Scottish Funding Council/Research Grants Council Joint Research Scheme, project no. X-HKUST602/14; and the Hong Kong Research Grants Council General Research Fund, project nos. 16100115 and 16100719. We also acknowledge funding from the Hong Kong Innovation and Technology Commission, project no. ITCPD/17-9.
Conflict of interests. The authors declare that they have no conflicts of interest.
Statement on the welfare of animals. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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Ho, K.E., Webb, S.E., Angus, C. et al. Identification of Environmental Factors that Influence the Upstream Migration of Sea Trout Salmo trutta in Shetland Waters. J. Ichthyol. 61, 738–751 (2021). https://doi.org/10.1134/S0032945221050088