The influence of physiology, life history and environmental conditions on the marine migration patterns of sea trout

Abstract

The marine migration of sea trout (anadromous brown trout, Salmo trutta) is an evolutionary adaption to exploit richer feeding opportunities at sea compared to the freshwater habitat. Over the last decades, there has been a significant decrease in many sea trout populations along the coast of Norway. While at sea, the sea trout commonly reside in coastal areas heavily affected by human activities such as aquaculture, marine traffic, fishing, and establishment of marine infrastructure. The marine migrations are important for the growth of sea trout, and consequently for the recruitment and population dynamics in coastal watersheds. The underlying mechanisms driving this variation are poorly understood. In order to predict the consequences of human activities in marine coastal areas on sea trout and implement effective conservation measures, a better mechanistic understanding of the marine migration patterns of the sea trout is needed.

The aims of this thesis were to study how physiology, previous life history and experienced environmental conditions affect the marine behaviour of sea trout, and to examine how salmon farming can affect nearby sea trout populations in terms of marine growth and utilization of marine versus fresh- and brackish water habitats. The results in this thesis provide new knowledge about how sea trout display behavioural responses to both the individual’s physiological state and environmental conditions, but also show consistency in migratory behaviour among years. The large variation in marine migratory behaviour of sea trout observed, implies that the susceptibility to human activity will not only vary among populations, but also among individuals within populations. The uneven effect of human activities on the individuals within populations can potentially alter population structures by posing increased risk of mortality and/or reduced marine growth on subgroups within the populations with certain behavioural traits.