Predation on Atlantic salmon and sea trout during their first days as postsmolts

Radio-tagged smolts of Atlantic salmon Salmo salar and sea trout Salmo trutta were predated heavily by sea birds after crossing the saline limit in the estuary of the River Skjern, Denmark. Most predation took place within the first 9 h after estuarine entry. The field data do not contradict the hypothesis of maladaptive anti-predatory behaviour.     

Migration speeds and orientation of Atlantic salmon and sea trout post-smolts in a Norwegian fjord system

We recorded the observed and actual swimming speeds of Atlantic salmon and sea trout post-smolts in a Norwegian fjord system, and initiated studies on the orientation mechanisms of the post-smolts. We tracked Atlantic salmon and sea trout with acoustic transmitters for up to 14 h after release. The actual swimming speed and direction of a fish relative to the ground is the vector sum of the observed movements of the fish and the movements of the water. We determined actual swimming speeds and directions of the post-smolts, which reflect their real swimming capacities and orientation, by corrections for the speed and direction of the water current. The post-smolts were actively swimming. The observed direction of movement was dependent on the actual movement of the fish and not the water current. Water currents were not systematically used as an orientation cue either in Atlantic salmon or sea trout, as the actual movements were random compared to the direction of the water current. The actual movement of sea trout were in all compass directions, with no systematic pattern. The Atlantic salmon also moved in all compass directions, but with the lowest frequency of actual movement towards the fjord.     

Migration of anadromous brown trout Salmo trutta in a Norwegian river

1. Upstream and downstream migrating anadromous brown trout Salmo trutta were monitored daily in fish traps in the River Imsa in south-western Norway for 24 years, from 1976 to 1999. One-third of the fish descended to sea during spring (February–June) and two-thirds during autumn (September–January).
2. In spring, high water temperature appeared to influence the downstream descent. Large brown trout (> 30 cm, chiefly two or more sea sojourns) descended earlier and appeared less dependent on high water temperature than smaller and younger fish. The spring water flow was generally low and of little importance for the descent.
3. In autumn, the daily number of descending brown trout correlated positively with flow and negatively with water temperature.
4. Brown trout ascended from the sea between April and December, but more than 70% ascended between August and October. The number of ascending trout increased significantly with both decreasing temperature and flow during the autumn. This response to flow appeared to be the result of the autumn discharge which is generally high and most fish ascended at an intermediate flow of 7.5–10 m³ s⁻¹ (which is low for the season).
5. In a river like the Imsa with low spring and high autumn flows, water temperature appears to be the main environmental factor influencing the timing and rate of spring descent, while both water temperature and flow seemed to influence the timing and rate of the autumn descent and ascent. These relationships make sea trout migrations susceptible to variation in climate and human impacts of the flow regime in rivers.     

Keeping close to the river,shore and surface: the first marine migration of brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) post-smolts

Acoustic telemetry was utilized to track 49 brown trout (Salmo trutta) and 37 Arctic charr (Salvelinus alpinus) first-time migrants of wild origin [post-smolts; mean L_F (fork length): 169 and 172 mm] in a large fjord in northern Norway. The S. trutta were registered at sea for more than twice the time of the S. alpinus (medians of 54 and 22 days, respectively). Both species were mostly detected near river mouths (>80% of detections) and almost exclusively spent their time (>95%) within the interior 18 km of the fjord. They were surface oriented, with most detections at <1 m depth and S. trutta deeper on average (median mean depths of 0.7 and 0.5 m, respectively). This study concludes that post-smolts of both species stay closer to the surface and to river mouths than larger veteran migrants. This study emphasizes the importance of river mouths and upper water layers for the survival of both species during their first marine migration.     

Marine depth use of sea trout Salmo trutta in fjord areas of central Norway

The vertical behaviour of 44 veteran sea trout Salmo trutta (275–580 mm) in different marine fjord habitats (estuary, pelagic, near shore with and without steep cliffs) was documented during May–February by acoustic telemetry. The swimming depth of S. trutta was influenced by habitat, time of day (day v. night), season, seawater temperature and the body length at the time of tagging. Mean swimming depth during May–September was 1·7 m (individual means ranged from 0·4 to 6·4 m). Hence, S. trutta were generally surface oriented, but performed dives down to 24 m. Mean swimming depth in May–September was deeper in the near‐shore habitats with or without steep cliffs (2·0 m and 2·5 m, respectively) than in the pelagic areas (1·2 m). May–September mean swimming depth in all habitats was slightly deeper during day (1·9 m) than at night (1·2 m), confirming that S. trutta conducted small‐scale diel vertical movements. During summer, S. trutta residing in near‐shore habitat progressively moved deeper over the period May (mean 1·1 m) to August (mean 4·0 m) and then reoccupied shallower areas (mean 2·3 m) during September. In winter (November and February), individuals residing in the innermost part of the fjords were found at similar average depths as they occupied during the summer (mean 1·3 m). The swimming depths of S. trutta coincide with the previously known surface orientation of salmon lice Lepeophtheirus salmonis. Combined with previous studies on horizontal use of S. trutta, this study illustrates how S. trutta utilize marine water bodies commonly influenced by anthropogenic factors such as aquaculture, harbours and marine constructions, marine renewable energy production or other human activity. This suggests that the marine behaviour of S. trutta and its susceptibility to coastal anthropogenic factors should be considered in marine planning processes.     

Seasonal occurrence of sea lice Lepeophtheirus salmonis on sea trout in two north Norwegian fjords

Seasonal occurrence of the parasitic copepod Lepeophtheirus salmonis (sea lice) was studied from March to December 2001 in two large north Norwegian sill fjords without fish farming activity, the Ranafjord and the Balsfjord. Anadromous brown trout Salmo trutta (sea trout) in both fjords had a low infestation rate during all sampling periods, but followed a seasonal pattern. During early and late winter (November to December and March to April) and spring (May to June), the prevalence varied from 0 to 25% and the abundance was <0·5 sea lice. Adults dominated (92%) during this period, particularly gravid females. In both fjords, the highest prevalence was during September (80–81%, all stages represented). In Ranafjord, the abundance and mean intensity during this month was 6·8 and 8·6 sea lice, respectively, while in Balsfjord it was 3·6 and 4·5 sea lice, respectively. Fish were captured at temperatures down to 1° C and at full strength sea water which is supposed to cause osmoregulatory problems for the fish. This observation has implications for the understanding of high‐latitude sea trout behaviour and can also make the fish more vulnerable to heavy sea lice infestation during this period. It is suggested that winter running sea trout help to maintain a self replicating local population of sea lice within such fjord systems where other possible hosts ( e.g . farmed Atlantic salmon Salmo salar ) are not present during a whole year cycle.     

Strontium content of scales as a marker for distinguishing between sea trout and brown trout

Strontium was determined in trout scales from a river where it is often difficult to distinguish between sea trout and resident brown trout by coloration or other visual marks. Sr values were compared with values in scales from brown trout caught above the anadromous stretch of the same river and in scales from a river where sea trout coloration is typical. In the first river, the Sr concentration was generally low, and as a mean only 50 ppm higher in scales from individuals classified as sea trout from the anadromous stretch than in brown trout scales from the upper stretch. There was no consistency between fish coloration and Sr concentration in scales from presumed sea trout on the anadromous stretch. Individuals with a typical sea trout coloration could have a lower concentration of Sr than individuals that were classified as uncertain sea trout by coloration. Fish weight did not seem to influence Sr levels. The mean Sr concentration in scales from the typical sea trout colored population in the second river was 2.8 times higher than that of the anadromous part of the first river. The high variability of Sr concentration in sea trout scales may be explained by differences in individual and population life history. The Sr levels reflect differences in saltwater exposure, either expressed by length of stay or concentration of salt in marine habitats. The study has shown that fish coloration is an inadequate mean to distinguish between resident and migratory trout. Nor is Sr determination of scales alone sufficient, because of low inter-group and high intra-group variability in some rivers. However, Sr values can give valuable information on individual and population migration on a large scale.     

Differences in susceptibility to sea lice infection between a sea run and a freshwater resident population of brown trout

Two experimental groups comprising mixed Norwegian sea run and freshwater resident brown trout Salmo trutta were infected with sea lice Lepeophtheirus salmonis in replicate tanks. Comparison of mean abundance and louse development between the sea run and resident groups revealed highly significant differences in lice abundance. The resident trout had an average abundance ± of 6·3±0·37 and 6·6±0·43 lice whilst the sea trout had an average abundance of 3·5 ±0·25 and 3·3 ±0·28 lice 29 days post infection at 9° C. No differences in development of lice, of either sex, were detected between the groups. As host groups were naive to sea lice at the start of the experiment, this suggests that there was a significant difference in susceptibility to sea lice infection between them, which may be genetically determined.     

Brown trout Salmo trutta released to support recreational fishing in a Norwegian fjord

One- and two-year-old hatchery reared juveniles of seven freshwater resident and anadromous populations of Scandinavian brown trout were released in the outer and inner part of the Oslofjord and in the River Akerselva, flowing through the city of Oslo. Recapture rates were highest (mean 20.3%) for river released fish and lowest for those released in the outer (16.8%) and inner (12.1%) fjord. In general, recapture rate increased with fish size at release (r=0.76). When released in fresh water, most of the recaptures were from fresh water and when released in the fjord, most recaptures were from the fjord. In general, freshwater resident stocks showed a higher degree of freshwater residency than anadromous stocks. However, mean migratory distance was longer for freshwater resident than anadromous fish. Trout moved longer distances at sea when released in the outer than in the inner fjord. Specific growth rate and size at recapture varied among release sites and stocks; they were highest for fish released in the outer fjord and lowest for river-released trout. There was no consistent difference in sea growth between freshwater resident and anadromous stocks. Estimated total yield was highest for fish released in the outer fjord, whereas there was no significant difference in yields between trout released in the river and the inner Oslofjord.     

Food of anadromous brown trout at sea

The food of migratory Salmo trutta in coastal marine waters along the Norwegian Skagerrak coast varied significantly with age, season and habitat of the fish. The main prey categories in terms of frequency of occurrence were fishes followed by crustaceans, surface insects and polychaetes. Seasonal variation in diet and within habitats was found, supporting the view that brown trout is an opportunistic feeder. An ontogenetic niche shift was observed with post-smolts feeding on inshore and shallow water prey communities, while larger brown trout fed mainly on pelagic fishes.     

Survival and emergence pattern of sea trout fry in substrata of different compositions

The emergence of the sea trout Salmo trutta fry of Gotland, Sweden, was studied in the laboratory. The main limiting factor for survival up to hatching was the interstitial oxygen concentration, which depends on the gravel permeability, therefore on the substratum granulometry. Fry emergence was possible in substrata in which the geometric mean particle diameter ( d _g) was >15 mm and in substrata in which d _g was <6 mm. In the substrata of low d _g, emergence took place earlier than in the other ones. As a consequence the fry were smaller and their yolk sac only partially resorbed. In an unfavourable environment, fry started to emerge as soon as they reacted positively to light, about 2 weeks after hatching.     

Control of lake trout reproduction: role of lipids

Somatic lipid density of lake trout decreased during summer and increased after spawning in October, peaking April, prior to ovarian recrudescence. Non-lipid density increased from June or July until August and remained relatively constant until March. All lake trout ( n =30) reared on high ( n =10, 0·4%), medium ( n =10, 0·15%) and low ( n =10, 0·05%) rations from March to September spawned. In a second experiment for which the treatments commenced in November, the proportion of females maturing in the following October (high=88%, medium=69%, low=28%) was positively related to ration levels.     

Migration of hatchery‐reared Atlantic salmon and wild anadromous brown trout post‐smolts in a Norwegian fjord system

Hatchery‐reared Atlantic salmon Salmo salar ( n  = 25) and wild anadromous brown trout (sea trout) Salmo trutta ( n  = 15) smolts were tagged with coded acoustic transmitters and released at the mouth of the River Eira on the west coast of Norway. Data logging receivers recorded the fish during their outward migration at 9, 32, 48 and 77 km from the release site. Seventeen Atlantic salmon (68%) and eight sea trout (53%) were recorded after release. Mean migratory speeds between different receiver sites ranged from 0·49 to 1·82 body lengths (total length) per second (bl s⁻¹) for Atlantic salmon and 0·11–2·60 bl s⁻¹ for sea trout. Atlantic salmon were recorded 9, 48 and 77 km from the river mouth on average 28, 65 and 83 h after release, respectively. Sea trout were recorded 9 km from the release site 438 h after release. Only four (23%) sea trout were detected in the outer part of the fjord system, while the rest of the fish seemed to stay in the inner fjord system. The Atlantic salmon stayed for a longer time in the inner part than in the outer parts of the fjord system, but distinct from sea trout, migrated through the whole fjord system into the ocean.     

Metazoan parasite community structure of sea trout on the west coast of Ireland

A total of 129 sea trout smolts from 10 locations on the west coast of Ireland were examined for metazoan parasites. Of 11 species identified only four had a prevalence >20%. Two species Hysterothylacium aduncum and Hemiuris communis have a confirmed marine origin; Eubothrium crassum has been shown to exist in both the sea and fresh water. The remaining species were fresh water. Overall the parasite community was dominated by autogenic species with only Diphyllobothrium dendriticum having allogenic transmission. Infracommunity values for mean species richness, and Brillouins index of diversity were low, indicating the presence of an impoverished helminth community with little potential for interspecific interaction. The relationship between marine helminths and sea lice was also examined.     

Feeding pattern and diet of first feeding brown trout fry under natural conditions

The feeding habits of brown trout Salmo trutta fry were studied during the critical first feeding period in a natural spawning and nursery stream. A low proportion of the fry initiated exogenous feeding before emergence from the gravel, and while having nearly 30% of the yolk sac remaining. This probably reflected low feeding motivation or limited feeding opportunities within the gravel environment. The majority of the fry started feeding after emergence, and after most or all yolk was absorbed. Some fry emerged with large amounts of yolk remaining, while others emerged after yolk exhaustion. The degree of stomach fullness revealed that feeding was more efficient after a territory had been acquired. The diets of the young fry were dominated by chironomid larvae, followed by zooplankton and Plecoptera larvae. Fry dispersing downstream and out of the nursery area were significantly smaller than resident fry, indicating displacement due to competition for territories. The majority of the downstream dispersing fry had initiated feeding, and there was at this point no evidence of starvation in any of the fry. It therefore appeared that the later emerging fry actively migrated out of the overpopulated nursery area to find available territories further downstream.     

Experimental measurement of thermal preferenda in sea trout smolts

Experiments were conducted to estimate the threshold water temperature rise (Δt) for avoidance by smolts of sea trout Salmo trutta . In three out of four experiments, the median avoidance threshold was found to be c . +6° C (+5·4 to +6·4° C). In the fourth experiment, the fish were able to detect and avoid temperature rises of < +1° C.     

Temporal variability in marine feeding of sympatric Arctic charr and sea trout

The marine feeding pattern of anadromous brown trout (sea trout) Salmo trutta and Arctic charr Salvelinus alpinus was studied during June to August in 1992–1993 and 2000–2004 in a fjord in northern Norway. In general sea trout fed proportionally more on fishes than on crustaceans and insects (81, 1 and 18% by mass, respectively) by comparison with Arctic charr (52, 25 and 22% by mass, respectively). Herring Clupea harengus dominated the total fish diet of both species, but the Arctic charr also fed significantly on gadoids and sandlance Ammodytes spp. While sea trout became virtually all piscivorous at fork lengths ( L _F) ≥250 mm, the Arctic charr was ≥400 mm L _F before shifting totally to a fish diet. Despite annual variation in diet and forage ratios, there was a clear shift in diet from 1992–1993 to 2000–2004. Sandlance and different crustaceans constituted most of the diet during the initial period with a shift towards gadoids and especially herring during the latter period. This shift seemed to be associated with a high abundance of herring larvae during the latter sampling period, indicating a preferential selection on herring when present, particularly by sea trout. Furthermore, an index indicated dietary overlap in years with intensive feeding on herring of both species, and usually differences in the trophic ecology during years feeding mostly on other prey species. In combination, it was hypothesized that the two species reflect the type of marine prey present within a fjord system over time, and therefore provide an index of variation in the production and biological diversity of their potential prey within fjords.     

The influence of small-scale variation in habitat on winter trout distribution and diet in an afforested catchment

(1) Habitat influences on trout distribution and diet at a reach scale were analysed over a single winter in a conifer-afforested catchment in Ireland, in an area subject to very low levels of atmospheric pollution. (2) A total of 234 salmonids were sampled from which 1194 individual prey items were recovered. Salmonids were sampled at eleven sites over a 2 km stretch of the river Douglas, a third order stream of the river Araglin in the Munster Blackwater, Co. Cork, Ireland. (3) In this winter study, water depth and flow primarily influenced trout distribution. (4) Stepwise multiple regression was used to identify the most influential environmental factors on trout metrics. Trout biomass was positively correlated to maximum depth and the percentage of pool area within sites, but density and condition were not significantly influenced by habitat variation at the reach scale. (5) The results from diet analysis suggest that trout are heterogeneous generalist feeders in winter, preying mainly on Trichoptera and Ephemeroptera species. Prey diversity in the diet of individuals throughout the study was positively correlated to the percentage of deciduous vegetation, undercut banks and coniferous vegetation, whilst the prey sub-population diversity at site scale was not related to any of the habitat variables measured.     

Development of a microtitre plate indirect ELISA for measuring cortisol in teleosts, and evaluation of stress responses in rainbow trout and gilthead sea bream

A microtitre plate indirect enzyme‐linked immunoassay (ELISA) was developed for measuring plasma cortisol levels in rainbow trout Oncorhynchus mykiss, gilthead sea bream Sparus auratus sea bass Dicentrarchus labrax and Senegalese sole Solea senegalensis. Covalink microplates pretreated with disuccinimidyl suberate were coated with bovine serum albumin (BSA) conjugated to cortisol‐3‐carboxymethyl oxime. After blocking with BSA, competition was started by addition of plasma samples and anti‐cortisol antibody raised in rabbit. Goat anti‐rabbit IgG conjugated‐peroxidase was added as second antibody and then incubated with orthophenylenediamine as substrate. Reaction was stopped with 0·1 M HCl and absorbance was read at 450 nm in an automatic plate reader. The standard curve was linear from the lower limit of sensitivity of the assay (c. 0·3 ng ml^?1) to c. 3000 ng ml^?1. Dose‐response inhibition curves using serially diluted plasma samples of four species consistently showed parallelism with the standard curve using cortisol. The ELISA satisfied the strictest criteria of specificity (cross‐reactivity of anti‐cortisol antibody with testosterone, progesterone and 17ß‐oestradiol was negligible, cross‐reactivity with cortisone, corticosterone and 11‐deoxycortisol, was 1·5, 1 and 0·1%, respectively), reproducibility (interassay CV <6%), precision (intra‐assay CV <4%), and accuracy (average recovery >98%). Plasma cortisol concentration in rested fishes was in the range of 5–30 ng ml^?1. To physiologically validate the technique, changes in plasma cortisol concentrations were also measured in plasma of rainbow trout and gilthead sea bream following an acute 15 min chasing or 3 min air‐exposure stress, respectively. In both species plasma concentrations of cortisol, glucose and lactate rose significantly with respect to controls, showing concentrations similar to those reported previously for these species under similar stress conditions. Furthermore, gilthead sea bream chronically stressed by maintaining for 14 days under increased stocking density conditions also showed increased concentrations of plasma cortisol and glucose. These results validate the indirect ELISA technique developed for use in the evaluation of plasma cortisol concentration of at least four fish species.     

Abundance, distribution, reproduction and diet of notacanthid fishes from the north-east Atlantic

Notacanthid fishes were among the 10 most abundant species sampled in research surveys using fine meshed trawls fished to depths of 3000 m in the Rockall Trough and 4500 m in the Porcupine Seabight between 1975 and 1992. Two species, Noctacanthus bonapartei and Polyacanthonotus rissoanus , were dominant but their population structure and certain aspects of morphometric and reproductive biology differed between the two sampling areas. In general, sex ratios were skewed in favour of females, size increased with depth and male and immature individuals were restricted to shallower depth zones. Fecundity was highly correlated with total weight and showed a positive relationship with species' size. Ovaries contained two batches of eggs and the spawning season was protracted, occurring mostly over winter months. Dietary studies showed a dependence on benthic macro fauna though the two dominant species have different dentition and exploit different trophic niches. In the Rockall Trough, peak abundances for N. bonapartei and P. rissoanus occurred in the 750 m and 1250 m zones, respectively, lying within the depth range currently exploited by commercial deep-water trawls (600–1400 m). The probable impacts of commercial operations on notacanthid fishes are discussed.