Do all roads lead home? Straying of anadromous brown trout (Salmo trutta) in a fjord system

Salmonids are a group of iconic species with high cultural and socio-economic significance, making them one of the most studied group of fish. Salmonids are out-standing for their incredible life-histories, which involve significant physiological and morphological transformations, and migrations that can span over hundreds or even thousands of kilometres. One of the most amazing aspects of salmonid life-histories is their ability to return and spawn in the same area where they were born - a life-history strategy referred to as natal philopatry or natal homing. However, not all salmonids return to their natal river, meaning there is often a proportion of individuals that return to non-natal rivers - a behavioural trait referred to as straying. While straying has important implications for different population parameters, its prevalence, connection to other life-history characteristics and spatio-temporal patterns characterising it remain largely unexplored. To address some of these unknowns, this thesis aimed to investigate various aspects of straying behaviour among anadromous brown trout, also referred to as sea trout, originating from rivers Villestrup, Kastbjerg, Valsgaard and Maren Møllebæk, of which all flow into Mariager fjord, Denmark.
The primary objective of this thesis was to determine the prevalence of straying in Mariager fjord over the course of an individual’s life as well as across evolutionary time. This was achieved by PIT tagging juvenile brown trout in river Villestrup during their out-migration in the spring and following their movements from river Villestrup to all of the studied rivers (MS I, II); or by PIT tagging juvenile brown trout in the summer in all of the rivers (MS III) and following their out-migration in the autumn or spring and tracking their return to any of the rivers. Additionally, as PIT telemetry is not applicable in all systems, suitability of otolith microchemistry in quantifying the level of straying was tested; and the level of gene flow was estimated to determine possible implications of straying on the genetic structure (MS IV). As a result, PIT telemetry revealed that on average 37% of sea trout in this system strayed at some point in their life. We further, determined that otolith microchemistry is a viable alternative to PIT telemetry in Mariager fjord. By analysing the otolith microchemistry fingerprints of juvenile brown trout, we were able to assign individuals to their respective river of origin, with an accuracy of 80%. This consequently enabled us to determine that strayers constituted a substantial proportion (43%) of the spawning population in all rivers (MS IV), providing further evidence of the prevalence of straying in this system. Genetics analysis further revealed that such high level of straying has left its mark on the genetic structure, as the results of MS IV determined that there was a high level of gene flow between the rivers, to such an extent that all the rivers genetically form just on population. The use of PIT telemetry in MS I, II and III further provided the possibility to determine spatio-temporal patterns of straying among sea trout during their first and subsequent returns to freshwater. We found that the majority of the strayers that returned for the first time visited non-natal river(s) only, while repeat spawners displayed high variability in their migratory patterns by migrating both to natal and non-natal rivers, with the migratory patterns changing from one year to the next. Lastly, all of the studies within this thesis contributed to identifying factors that are linked to an individual’s likelihood of straying across multiple life stages (juvenile and adult), including developmental status (parr, pre-smolt, smolt) during out-migration (MS I), duration of the marine phase (MS I and III), and individual length upon return to freshwater as an adult (IV).
This thesis provides evidence that straying can be prevalent and is an integral part of many sea trout life-history strategies. Straying has clear effects to the genetic structure and likely to other population parameters, like recruitment. These are important aspects to take into consideration by managers, to ensure the effectiveness of management actions and the sustainability of sea trout populations. While several novel insights into straying were made in this thesis, there are still many aspects that remain unclear and would benefit from additional research, such as fitness related costs, and the prevalence of it in other geographical.