Life stage specific habitat use and mortality in two species of salmonid

Honkanen, Hannele Marjut (2017) Life stage specific habitat use and mortality in two species of salmonid. PhD thesis, University of Glasgow.

Abstract

Despite that the life histories of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) are very well studied, there are still gaps in our knowledge which have consequences for management and conservation of these species. This is of particular importance now that wild populations of Atlantic salmon and anadromous brown trout are decreasing throughout their distribution range. This thesis aims to address some of these gaps in knowledge in life stage specific habitat use and mortality of these two species by bringing together five separate studies.

Regular assessment of stock size is one of the most important aspects of fisheries management. Electrofishing techniques are widely used for the estimation of the size of stream dwelling fish populations for both fishery management and scientific study. In contrast to multiple pass, population depletion methods, single pass catch-per-unit-effort (CPUE) methods are less time consuming and labour intensive. A possible issue with the commonly used, fixed total time CPUE protocol is that it does not differentiate between the time spent actively fishing and the time incurred while not actively fishing, leading to handling error which is likely related to fish density. This was tested in a comparative field study. I showed that this commonly used technique is systematically underestimating juvenile salmonid numbers at higher densities. In addition this technique is failing to adequately determine fish community structure at low population densities. This work indicates a simple modification to the methodology that will reduce systematic error.

Knowledge of the species’ biology and habitat requirements can be linked with habitat data to create a model that predicts the distribution of that species. As salmonids, such as Atlantic salmon, have fairly strict habitat requirements, knowledge of habitat use by salmon can then be used to estimate the potential salmonid production in a given area of certain quality. The accuracy of a habitat grading system currently used by the Loughs Agency was tested. The results suggest that in its current format, the habitat grading system does not accurately describe the abundance of juvenile salmonids. When data was analysed to see which habitat variables best described salmonid density, for 0+ salmon important variables were flow and substrate and for older juveniles the important variables were flow, substrate, depth and percentage of undercut banks.

All populations are controlled by density-dependent and density –independent factors. For a species such as Atlantic salmon that has life stages differing in habitat requirements, the relative contributions of these two factors for population regulation on each life stage are likely to vary. Using a long-term data-set on Atlantic salmon migrants returning to the Foyle catchment, Ireland, the role of density-dependent and life-stage specific environmental factors regulating population size was determined. A Ricker density-dependent model showed that the spawning adult population size significantly predicted variation in the resultant filial generation, however a large amount of variation remained unexplained. It was shown that environmental factors were significant in explaining some of the remaining variance and that these influences were linked to specific life stages. Three life stages – spawning and incubation, fry emergence, and marine survival – were shown to be sensitive to environmental effects resulting in changes to the returning cohort strength. It is concluded that these life stage specific environmental effects are likely to contribute to the stochastic variation in population size resulting from the application of traditional stock-recruitment models.

Atlantic salmon undertake extensive migrations between freshwater and marine habitats. Smolt migration is associated with high mortality and is thus considered a critical life stage in the Atlantic salmon life history. Smolt migration through standing waters is still mostly an unknown process and it is not known what guides migration during lake migration. Using acoustic telemetry, three hypotheses derived from known principles of migration in rivers were tested: i.) smolts will take the shortest possible route, ii.) smolts will display unidirectional movement and iii.) smolts will be continuously moving. None of these three expectations were supported by the results of this study. Instead evidence was found of smolts moving in a seemingly random fashion, displaying counter intuitive migration patterns and spending relatively long periods effectively static during their lake migration.

Brown trout display a great range of life history variation ranging from river residency to anadromy. Unlike Atlantic salmon that migrate hundreds of kilometres to the open ocean to feed, anadromous brown trout often stay within 80-100 km from their natal river in coastal areas. Despite this, very little is still known about their behaviour during this coastal feeding part of the life cycle. It was found that brown trout in the Clyde estuary use a fairly small part of the estuary and seem to move fairly actively. Fish size did not explain any of the movement or residency patterns.

The five separate studies presented in this thesis combine field empirical studies from rivers, lakes and the coastal marine environments and historical data analysis studies on two salmonid species. The results presented in this thesis provide tools for managers of anadromous salmonids.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Salmo salar, Salmo trutta, telemetry, fisheries, conservation.
• Subjects: Q Science > QH Natural history; Q Science > QH Natural history > QH301 Biology; Q Science > QL Zoology
• Colleges/Schools: College of Medical Veterinary and Life Sciences > Institute of Biodiversity Animal Health and Comparative Medicine
• Supervisor's Name: Adams, Professor Colin E. and Boylan, Dr. Patrick
• Date of Award: 2017
• Depositing User: Hannele Marjut Honkanen
• Unique ID: glathesis:2017-8375
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 07 Sep 2017 13:43
• Last Modified: 17 Oct 2017 08:07
• URI: https://theses.gla.ac.uk/id/eprint/8375