Knut Wiik Vollset

Salmon lice increase the age of returning Atlantic salmon

The global increase in the production of domestic farmed fish in open net pens has created concerns about the resilience of wild populations owing to shifts in host–parasite systems in coastal ecosystems. However, little is known about the effects of increased parasite abundance on life-history traits in wild fish populations. Here, we report the results of two separate studies in which 379 779 hatchery-reared Atlantic salmon smolts were treated (or not) against salmon lice, marked and released. Adults were later recaptured, and we specifically tested whether the age distribution of the returning spawners was affected by the treatment. The estimates of parasite-induced mortality were 31.9% and 0.6% in the River Vosso and River Dale stock experiments, respectively. Age of returning salmon was on average higher in treated versus untreated fish. The percentages of fish returning after one winter at sea were 37.5% and 29.9% for the treated and untreated groups, respectively. We conclude that salmon lice increase the age of returning salmon, either by affecting their age at maturity or by disproportionately increasing mortality in fish that mature early.

The role of prey concentration and size range in the growth and survival of larval cod

Abstract The effects of food quantity vs. food quality may be influenced by complex interactions that are difficult to isolate in trophic studies of fish larvae in the natural environment. Food quantity, or abundance, is mediated by the proportion of suitable prey available, with ‘suitability’ defined variously as size, species, or nutritional content. Using an experimental approach, food consumption, growth, and survival of larval cod, in response to zooplankton abundance and diversity of zooplankton sizes, were tested in a replicated land-based mesocosm (2500 l) system. Northeast Arctic cod (Gadus morhua L.) larvae were reared for two months with a manipulated zooplankton prey field comprised of narrow (N) or wide (W) prey spectrum (range of prey sizes), combined with high (H) or low (L) prey concentration. In the W treatments, cod larvae were fed both small and large sized zooplankton, while in the N treatments they were fed only small zooplankton. Initial prey concentration was 2000 prey l−1 in the H treatments and 200 prey l−1 in the L treatments, decreasing to 500 and 50 prey l−1 during the course of the experiment. Larval cod grew faster at higher prey concentrations. Prey concentration as well as spectrum affected larval feeding patterns with a wider spectrum prey size leading to the consumption of larger prey, but without a significant effect on larval growth. Survival was significantly higher in high prey concentration treatments, and also tended to be better in treatments with wider prey size diversity.

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.

Effects of hydropeaking on the spawning behaviour of Atlantic salmon Salmo salar and brown trout Salmo trutta

An in situ camera set-up was used to study the spawning activity of Atlantic salmon Salmo salar and brown trout Salmo trutta throughout two consecutive seasons in a spawning area affected by hydropower-related pulse flows due to hydropeaking. The purpose was to test whether the flow variation discouraged spawning in shallow areas or motivated spawning into areas with elevated risk of incubation mortality. There were more S. salar observed on the spawning ground during days with high discharge. The presence of S. salar in the spawning grounds was not affected by the hydropeaking cycles of the preceding night. Female S. salar were observed preparing nests within the first hour after water discharge had increased to levels suitable for spawning. In contrast, the number of S. trutta was not correlated with flow and nest preparation was also observed at a discharge corresponding to the lowest discharge levels during a hydropeaking cycle. Survival was generally high in nests excavated the following winter, with only 5·4% suffering mortality due to dewatering. The results suggest that S. salar may respond rapidly to variable-flow conditions and utilize short windows with suitable flows for spawning. Smaller S. trutta may utilize low-flow conditions to spawn in areas that are not habitable by larger S. salar during low flow.

Vertical migration of Norwegian spring-spawning herring larvae in relation to predator and prey distribution

Abstract A diel vertical migration (DVM) pattern of Norwegian spring spawning herring (Clupea harengus) larvae was investigated during 19 and 20 April 2009. Factors influencing DVM included physical and biological properties of the water column. Data on larvae, prey and predators were collected with a depth-stratified multisampling device, inshore of Sklinna bank, close to the Norwegian coast, while light conditions were calculated using a Matlab® algorithm. A type I DVM pattern (i.e. deep during daytime, shallow at night) was observed for herring larvae, mainly occurring above the thermocline. No size-dependent differences were observed for larval vertical positioning. The highest overlap in depth distribution with their main prey was observed during daytime, when larvae were distributed deeper in the water column. From acoustics and macroplankton trawl data, a type I DVM was also observed for krill, although their concentrations were relatively low in the area. Cumulative predator–prey overlap plots suggest that krill most likely forage on copepods and smaller organisms. During the day, larvae concentrate near the thermocline to feed, while they move towards the surface at dusk, possibly to use the remaining light to continuing feeding. During the night, when light levels were too low to feed, larvae spread out in the water column above the thermocline. This migration pattern reduces the overlap between larvae and potential predators such as krill, which also move higher up during nighttime. It is suggested that the pattern of herring larvae DVM is a behavioural response to active pursuit of prey.

Interplay of individual interactions and turbidity affects the functional response of three-spined sticklebacks Gasterosteus aculeatus

The effects of turbidity, size and the presence of conspecifics on the functional response, feeding latency and activity in the three-spined stickleback Gasterosteus aculeatus were examined. A significant interaction between standard length and presence of conspecifics demonstrated an increase in attack rates of larger individuals in the presence of conspecifics. Attack rate was also higher in turbid water. Feeding latency decreased with prey concentration and presence of conspecifics, but was not affected by turbidity. Activity level did not change with prey levels, but increased with turbidity. These results can help to better understand how individual flexibility in the functional response can affect prey mortality according to environmental perturbation and social interaction at the level of the predator.

Interaction between migration behaviour and estuarine mortality in cultivated Atlantic salmon Salmo salar smolts

Migration behaviour and estuarine mortality of cultivated Atlantic salmon Salmo salar smolts in a 16 km long estuary were studied using two methods: (1) acoustic telemetry and (2) group tagging in combination with trap nets. Progression rates of surviving individuals through the estuary were relatively slow using both methods [0·38 LT (total length) s-1 v. 0·25 LT  s-1 ]. In 2012, the progression rate was slow from the river to the estuary (0·55 LT  s-1 ) and the first part of the estuary (0·31 LT  s-1 ), but increased thereafter (1·45-2·21 LT  s-1 ). In 2013, the progression rate was fast from the river to the estuary (4·31 LT  s-1 ) but was slower thereafter (0·18-0·91 LT  s-1 ). Survival to the fjord was higher in 2012 (47%) compared to 2013 (6%). Fast moving individuals were more likely to migrate successfully through the estuary compared to slower moving individuals. Adult recapture of coded-wire-tagged S. salar was generally low (0·00-0·04%). Mortality hot spots were related to topographically distinct areas such as the river outlet (in 2012) or the sill separating the estuary and the fjord (in 2013). At the sill, an aggregation of cod Gadus morhua predating on cultivated smolts was identified. The results indicate that slow progression rates through the estuary decreases the likelihood of smolts being detected outside the estuary. The highly stochastic and site-specific mortality patterns observed in this study highlight the complexity in extrapolating mortality patterns of single release groups to the entire smolt run of wild S. salar.

Salmon lice infestations on sea trout predicts infestations on migrating salmon post-smolts

Salmon lice infestations on sea trout predicts infestations on migrating salmon post-smolts Knut Wiik Vollset,* Elina Halttunen, Bengt Finstad, Ørjan Karlsen, Pål Arne Bjørn and Ian Dohoo Uni Research, LFI Freshwater Biology, Nygårdsporten 112, 5006 Bergen, Norway Institute of Marine Research, PO Box 6404, NO-9294 Tromsø, Norway Norwegian Institute for Nature Research, PO Box 5685 Sluppen, N-7485 Trondheim, Norway University of Prince Edward Island, Charlottetown, Canada *Corresponding author: tel: þ4755584723; e-mail: knut.vollset@uni.no.

Quantitative risk assessment of salmon louse-induced mortality of seaward-migrating post-smolt Atlantic salmon

The Norwegian government recently implemented a new management system to regulate salmon farming in Norway, aiming to promote environmentally sustainable growth in the aquaculture industry. The Norwegian coast has been divided into 13 production zones and the volume of salmonid production in the zones will be regulated based on salmon lice effects on wild salmonids. Here we present a model for assessing salmon louse-induced mortality of seaward-migrating post-smolts of Atlantic salmon. The model quantifies expected salmon lice infestations and louse-induced mortality of migrating post-smolt salmon from 401 salmon rivers draining into Norwegian coastal waters. It is assumed that migrating post-smolts follow the shortest path from river outlets to the high seas, at constant progression rates. During this migration, fish are infested by salmon lice of farm origin according to an empirical infestation model. Furthermore, louse-induced mortality is estimated from the estimated louse infestations. Rivers draining into production zones on the West Coast of Norway were at the highest risk of adverse lice effects. In comparison, rivers draining into northerly production zones, along with the southernmost production zone, were at lower risk. After adjusting for standing stock biomass, estimates of louse-egg output varied by factors of up to 8 between production zones. Correlation between biomass adjusted output of louse infestation and densities of farmed salmon in the production zones suggests that a large-scale density-dependent host-parasite effect is a major driver of louse infestation rates and parasite-induced mortality. The estimates are sensitive to many of the processes in the chain of events in the model. Nevertheless, we argue that the model is suited to assess spatial and temporal risks associated with farm-origin salmon lice.

Out of sync: monitoring the time of sea entry of wild and hatchery salmon Salmo salar smolt using floating passive-integrated transponder antennae

Floating passive-integrated transponder (PIT) antennae and smolt traps were used to study the time of sea entry and relative recapture of wild and hatchery-reared Atlantic salmon Salmo salar smolt released below and above a lake formed in the Vosso River. In total, 8.4 and 4.1% of the tagged wild and hatchery fish, respectively, were detected leaving the river (i.e. sea entry). Wild smolts released below the lake were detected leaving the river 16 days before smolts released above the lake, which also showed a 52% lower probability of detection during out-migration. Hatchery smolts were out of sync with the wild smolts and were detected approximately 2 months later than the wild smolts from both release locations, with an 84% lower likelihood of detection than wild fish. Size selection was evident for wild fish released above the lake, but not below the lake, with an overall likelihood of detection increasing by 2.6% per cm total length (LT ). Wild fish caught in the tributaries and transported to the main river had a 64% lower likelihood of detection than fish caught and released in the main river. This study demonstrates that floating PIT antennae out-performed the traditional rotary screw trap in the ability to detect tagged smolts and that it is an efficient tool for evaluating the time of sea entry of S. salar smolts in a large river system.