Can the river location within a fjord explain the density of Atlantic salmon and sea trout?

Abstract

The rivers that drain into the Hardangerfjord were historically known to have numerous populations of both sea trout (Salmo trutta) and Atlantic salmon (Salmo salar). After a decline in catches during the last decades many of the rivers have been closed for fishing. In this study we use snorkelling observations from rivers and catch statistics from 2004 to 2011 to describe the current situation and analyse the patterns of density of wild salmon, sea trout and escaped farmed salmon in the Hardangerfjord rivers. We hypothesize that some of the variance in density of salmon and sea trout can be explained by the location of the river in the fjord, with fish from rivers with a longer fjord exposure having a lower density. A median number of 3.5 salmon )ha(1 and 14.9 sea trout )ha (1 were observed in the rivers. Farmed salmon were observed in all rivers and constituted on average 23.3% of the total number of observed salmon. For salmon, there was an inverse log-linear relationship between density in the river and migration distance to coast, with a higher density in rivers with a shorter migration distance. For sea trout there was no evident relationship with location within the fjord. We suggest that the spatial patterns observed for salmon and sea trout can be related to the species-specific differences in habitat use within the fjord system.

Timing is everything: Survival of Atlantic salmon Salmo salar postsmolts during events of high salmon lice densities

“…Consequently, Atlantic salmon populations migrating long distances in the fjord are expected to be more seriously affected by lice. For example, in the Vosso River, lice‐induced mortality has been estimated to surpass 30%, which illustrates a more general trend: wild Atlantic salmon populations from the inner part of the fjord have lower population densities (Vollset, Skoglund, et al, 2014). This supports the hypothesis of increased lice‐induced mortality in populations with long fjord migrations.…”

“…In Hardangerfjord, it has been documented that the proportion of returning fish is lower for fish that originate from rivers further away from the coast, i.e. deeper into the fjords, which may be related to longer time periods of exposure to lice or predators in the environment (Vollset, Skoglund, et al, 2014). Moreover, the timing of the migration of the salmon smolt seems to be crucial.…”

Swimming with the fishes: validating drift diving to identify farmed Atlantic salmon escapees in the wild

“…freshwater and marine ecosystems; St. John et al 1990, Beauchamp et al 1994, Toft et al 2007, with ex tensive use in lotic ecosystems to monitor salmo nids (Thurow et al 2006, Orell et al 2011, Vollset et al 2014. Although drift diving has been used for several decades to assess salmonid populations in rivers, some concerns exist (Dolloff et al 1996).…”

Exocrine glands of Lepeophtheirus salmonis (Copepoda: Caligidae): Distribution, developmental appearance, and site of secretion

“…Lice infestation can be detrimental to fish as it disturbs the osmotic balance and stress the fish increasing the susceptibility to other pathogens (Grimnes and Jakobsen ; Finstad et al, ; Tort ). An increased occurrence of resistance to antiparasitic treatments and the consecutive impact on wild salmonid fish (Besnier et al, ; Fjørtoft et al, ; Vollset et al, ; Aaen et al, ), underscores the importance of a better understanding of this parasite to enable the development of new countermeasures.…”