Can sea trout Salmo trutta compromise successful eradication of Gyrodactylus salaris by hiding from CFT Legumin (rotenone) treatments?

In this study, 34 anadromous brown trout (sea trout) Salmo trutta were equipped with acoustic transmitters in order to examine whether they performed avoidance behaviour in response to a CFT Legumin (rotenone) treatment in the Norwegian River Vefsna. Migratory behaviour of the S. trutta was monitored by use of 15 automatic listening stations and manual tracking in the lower part of the river, in the estuary and in the fjord. None of the studied S. trutta survived the rotenone treatment and no indications of successful avoidance behaviour were observed.

A concept for improving Atlantic salmon Salmo salar smolt migration past hydro power intakes.

In this study, cost effective (in terms of reducing loss of power production) measures for increasing bypass migration of Atlantic salmon Salmo salar were developed and tested by establishing statistical models for timing of smolt migration and favourable diversion of water to the bypass. Initial tracking of radio-tagged smolts showed very low bypass migration under normal hydropower operations. Bypass migration increased when bypass discharge was experimentally increased and a model was developed that described relationships between total river discharge, bypass diversion and smolt migration route. Further improvements were obtained by installing two strobe lights at the power-production tunnel entrance that increased bypass migration during the night, but not during daytime. According to the behaviour of radio-tagged fish, the implemented measures contributed to increasing the annual percentage of bypass migration from 11 to 64%, and according to model predictions to 60-74% when the hydropower facilities were operated according to the developed models. To ensure correct timing of discharge diversion a smolt migration model was developed based on environmental variables that could successfully predict the general pattern of migration timing. The concept presented for improving smolt migration past hydropower intakes should be applicable in many systems where migration past hydropower installations cannot easily be solved by screening systems.

Early marine migration of European silver eel Anguilla anguilla in northern Norway.

To study the migratory behaviour in wild northern European silver eel Anguilla anguilla during sea entry and early marine migration, 32 individuals were tagged with acoustic transmitters and registered at four automatic listening station arrays from the mouth of the north Norwegian River Alta and throughout the Alta Fjord. The A. anguilla entered the fjord during all parts of the tidal cycle and did not seem to utilize the outgoing tidal currents. They migrated mainly during the night, in both the river mouth and the fjord. On average, they spent 2·7 days travelling from the river mouth to the outermost array, 31 km from the river mouth, corresponding to an average migratory speed of 0·5 km h(-1) . The A. anguilla generally migrated in the central part of the fjord and in the uppermost 10-25% of the water column, but with frequent dives to greater depths. Already 4 km after sea entry, A. anguilla were observed diving deeper than 130 m within 20-30 min periods. Hence, this study demonstrated that A. anguilla may perform an active diving behaviour during the early marine migration. The study took place in a pristine area with a minimum of anthropogenic interventions and by individuals from a population still uninfected by the introduced parasite Anguillicoloides crassus. The results may therefore be used as a baseline for future studies of the A. anguilla early marine migration.

Does river of origin influence the early marine migratory performance of Salmo salar?

The early marine migratory behaviour of two populations of hatchery-reared Atlantic salmon Salmo salar was compared in a common-garden experiment. Post-smolts from a river in a long fjord (Laerdal River, 144 km from the open coastline, n = 79) and a short fjord (Flekke River, 20 km from the open coastline, n = 80) in western Norway were tagged with acoustic transmitters and released during the spring of 2005 and 2006 in the inner part of the Hardangerfjord system (Opo River mouth, 179 km from the open coastline). The migratory behaviour of the tagged fish was monitored by acoustic listening stations in the fjord system up to 167 km from the release site. The Laerdal fish began migrating before the Flekke fish and had higher progression rates in the middle part of the fjord system. A greater number of Laerdal fish was detected along the most direct migratory route and in the outermost part of the Hardangerfjord system, which is indicative of a higher survival. The results from this study demonstrate differences in early marine migratory behaviour between S. salar from two different stocks and suggest that the distance a S. salar population travels to reach the open coastline may influence its early marine migratory behaviour and performance. The selective pressures of marine predation and arrival time at feeding areas in the ocean may be stronger for stocks with a longer inshore migration, creating more efficient migrants over time.

Migratory behaviour and survival rates of wild northern Atlantic salmon Salmo salar post-smolts: effects of environmental factors.

To study smolt behaviour and survival of a northern Atlantic salmon Salmo salar population during river descent, sea entry and fjord migration, 120 wild S. salar were tagged with acoustic tags and registered at four automatic listening station arrays in the mouth of the north Norwegian River Alta and throughout the Alta Fjord. An estimated 75% of the post-smolts survived from the river mouth, through the estuary and the first 17 km of the fjord. Survival rates in the fjord varied with fork length (LF), and ranged from 97.0 to 99.5% km(-1). On average, the post-smolts spent 1.5 days (36 h, range 11-365 h) travelling from the river mouth to the last fjord array, 31 km from the river mouth. The migratory speed was slower (1.8 LF s(-1)) in the first 4 km after sea entry compared with the next 27 km (3.0 LF s(-1)). Post-smolts entered the fjord more often during the high or ebbing tide (70%). There was no clear diurnal migration pattern within the river and fjord, but most of the post-smolts entered the fjord at night (66%, 2000-0800 hours), despite the 24 h daylight at this latitude. The tidal cycle, wind-induced currents and the smolts' own movements seemed to influence migratory speeds and routes in different parts of the fjord. A large variation in migration patterns, both in the river and fjord, might indicate that individuals in stochastic estuarine and marine environments are exposed to highly variable selection regimes, resulting in different responses to environmental factors on both temporal and spatial scales. Post-smolts in the northern Alta Fjord had similar early marine survival rates to those observed previously in southern fjords; however, fjord residency in the north was shorter.

Effects of subchronic exposure to aluminium in acidic water on bioenergetics of Atlantic salmon (Salmo salar).

Atlantic salmon (Salmo salar) were exposed for 36 days to water from the acidified (pH 5.2) Fossbekk River (Norway) with 50 micro x L(-1) of aluminium (Al) added as AlCl(3), or to circumneutral water (pH 6.6) from Ims River (Norway). Food consumption, cardiac output, swimming activity, and growth rate were measured to determine the effects of sublethal levels of Al in acidic water on the bioenergetics of the fish. Food consumption was reduced in acidic water and Al during the first 12 days, but was not significantly different from the control group during the rest of the exposure. A significant decrease in body weight was concomitant with the reduction in food consumption. The fish also kept losing weight during the rest of the exposure despite the return of their appetite to normal. Cardiac output of the fish exposed to acidic water and Al was not significantly different from the control group after 36 days of exposure. Swimming activity remained significantly elevated throughout the exposure to acidic water and Al. These results indicate that sublethal levels of Al can alter the energy budget of Atlantic salmon living in acidified surface waters.