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.     

Survival and growth of sea trout parr in fresh and brackish water

There was no di.erence in survival or growth rate over 63 days for young-of-the-year sea trout Salmo trutta in fresh and brackish Baltic Sea water (6·7 psu). Hence, such sea trout parr that migrate from the freshwater habitat in which they hatch to the Baltic coastal zone, without smolting, should experience little or no physiological cost in survival and growth.     

Estimating the success of natural spawning of salmonids in streams

A new method is described to estimate the survival of salmonids in streams from fertilization of the eggs to the emergence of the fry. The method is compared to other techniques generally used to estimate the egg-to-fry survival: fry traps, excavation of redds, construction of artificial redds and laboratory experiments. Especially designed boxes filled with substratum and freshly fertilized eggs are buried in the stream bed. Survival is determined at three different embryological development stages: eyed stage, hatching and emergence. Boxes are planted inside the stream bed using a special injector, so that substratum alterations are minimal around the boxes. This method was tested successfully in two different studies, one in Switzerland on the brown trout, the other one in Sweden on the sea trout.     

Dissolved oxygen concentration and emergence of sea trout fry from natural redds in tributaries of the River Rhine

A total of 4289 sea trout Salmo trutta alevins swam up from six of eleven covered redds. They emerged only from those redds where the mean dissolved oxygen level was >6·9 mg l⁻¹ and the Sorting coe.cient, an indicator of intragravel void space, was <5. These results are discussed in relation to field studies using artificially buried salmonid eggs but also in relation to the ongoing programme for the reintroduction of Atlantic salmon in tributaries of the River Rhine.     

Flexible modes of anadromy in Baltic sea trout: making the most of marginal spawning streams

From examination of the ratios of strontium to calcium laid down as a lifetime record in the otoliths of sea trout Salmo trutta from Gotland, Baltic Sea, it was found that: (1) the shortest stream was used mostly by precociously emigrant or coastally hatched spawners; (2) longer streams had more fish that underwent normal smoltification; (3) sea-caught fish were predominantly coastally hatched (presumably near stream mouths). Furthermore, some otoliths showed no evidence of a freshwater history at all, raising the possibility of a contingent of the coastal population that does not depend on riverine spawning. The results emphasize the importance of the coastal zone as natal and early life habitat for sea trout in the Baltic, particularly with respect to a potential change to a warmer climate which may exacerbate conditions within small, ephemeral trout streams.     

Habitat use and dispersal of post-smolt sea trout Salmo trutta in a Scottish sea loch system

Post-smolt anadromous brown trout Salmo trutta , sea trout, from two Scottish west coast rivers, the Balgy and Shieldaig, flowing into adjacent sea lochs were tracked simultaneously using arrays of moored acoustic receivers to determine dispersal patterns and loss rates. Fish tended to stay close to their natal rivers for the first 14 day after entering the sea, during which time about half the fish were lost to the study. Although initially the overall pattern of dispersal was similar for individual fish from both rivers, towards the end of the study the groups had converged into one of the loch basins. There were also pronounced individual differences in habitat use with all those fish detected for >42 days exhibiting different patterns of habitat use. Loss rates were similar between the two rivers despite differences in the range of air-breathing predators to which the fish were initially exposed. These findings suggest that any management of predators or other mortality agents should be targeted towards mouths of rivers during and immediately following smolt emigration.     

Changes in some endocrinological and non-specific immunological parameters during seawater exposure in the brown trout

The influence of autumnal progressive and direct seawater transfers on ionic parameters, plasma growth hormone (GH) and thyroid hormones (TH) and also on the non–specific immune traits phagocytic activity, lysozyme and non–specific cytotoxicity were examined in 45–55 g brown trout ( Salmo trutta ). In both experiments, the seawater transfer induced the same pattern of endogeneous modifications but they were more pronounced and more lasting after the direct seawater transfer than after the progressive one. In seawater–transferred trout, there was a significant transitory increase of the plasma osmolarity, chloride concentration, GH levels and a transient decrease of the TH. The phagocytic activity of the pronephric leucocytes and the lysozyme concentrations were significantly higher in seawater–transferred trout than in controls. Nevertheless, the non–specific cytotoxicity should not be modified after the seawater exposure. Moreover significant positive correlations were observed between plasma GH and chemiluminescence or lysozyme increases. These data support the hypothesis that GH is involved in the salmonids' non–specific immune potential, especially by stimulating the macrophage functions.     

Characteristics of brown trout males influence growth and survival of their offspring

Half-sib groups of juvenile brown trout Salmo trutta (0+ years) having fathers with different characteristics were compared in several respects. Two kinds of experiments were performed, aquarium observations in the laboratory and stocking in a semi-natural stream. It was found in the aquarium experiments that the offspring of fathers with larger adipose fins were more active and were probably better at defending territories. Offspring of males chosen by females had higher growth rate. The feeding rate was higher in juveniles having a less aggressive father or a father that was chosen by the mother. The stream experiments showed that offspring of dominant males had higher growth rates, and that offspring of less aggressive fathers had higher survival. Thus, characteristics of the males (fathers) influences the quality of the offspring, and there seem to be benefits for the females both to choose among potential mates and to spawn with the most dominant male. As these male characteristics are never regarded during the process of artificial breeding in hatcheries, the results indicate a basis for divergence between hatchery and wild populations.     

Spawning characteristics of the anadromous brown trout in a small Swedish stream

The spawning pattern of the anadromous brown trout Salmo trutta was studied in Själsöån, a small stream in Gotland, Sweden, during eight winters between 1992–1993 and 1999–2000. The total length ( L _T) at spawning was normally distributed (185–890 mm) for females and multimodal for males (300, 400 and 550 mm most frequent length classes). Spawning males were significantly younger (2+ to 4+ years) than females (3+ to 5+ years). The sex‐ratio at the beginning and at the end of the spawning season favoured males. The mean ±  s . d . number of spawners was 70 ± 16 individuals per year. Migration into and out of the stream occurred between November and June. The highest number of spawning fish was found in the stream at the end of November or at the beginning of December. Migration mainly occurred during high water flow and at night. The majority of the females entered the stream and spawned the same (29·3% of all the females) or the next night (32·8% of all the females) while males may have stayed for 2 to 3 weeks (21·3% of all the males) in the stream before spawning. Males usually remained much longer in the stream (mean ±  s . d . 45 ± 56 days) than females (16 ± 30 days). Females lost more mass in the stream (mean ±  s . d . 17·3 ± 8·6%) than males (7·7 ± 9·6%). For both sexes, mass loss was positively correlated with the time spent in the stream. Only 7·3% of the males and 5·7% of the females occurred in the stream for >1 year. Spawning took place only during the night.     

Population regulation in adult, but not juvenile, resident trout (Salmo trutta) in a Lake District stream

1. Unlike a neighbouring sea-trout population that showed strong density-dependent survival, a resident trout population ( Salmo trutta L.) showed simple proportionate survival in the early life-stages. However, this persistent population fluctuated within narrow limits. Mature adults, especially during spawning, were the only possible life-stage left in which regulation might occur. 2. An October census, just prior to spawning, was made at five sites (total area 300 m ² ) from 1965 to 1983. Gravel nests (redds) associated with females of known size were excavated outside these sites to obtain a power-function relationship between egg density per redd and female length (range 181–280 mm, n = 26). This relationship and the census data for females (range 186–284 mm) were used to estimate egg densities in each year-class. 3. The census data for the early life-stages (0+, 1+, 2+ trout) confirmed proportionate survival with no evidence for density-dependent regulation. In contrast, the number of spawning females produced in each year-class was strongly density dependent on the initial number of females that laid eggs at the start of the year-class. Similarly, total egg production in each year-class was density dependent on initial egg density. 4 Both relationships were well described by the Ricker and Beverton-Holt stock-recruitment models (P < 0.001) and the goodness-of-fit was similar for both models. This study is probably the first to provide clear evidence for fish population regulation in the adult, rather than the juvenile, stage.     

Relationship between the drift of macroinvertebrates and the activity of brown trout in a small stream

Brown trout Salmo trutta were most active in a small stream at night, dusk and dawn when drift rate was highest, but correlations between hourly drift rates and the trout's activity varied substantially between individuals, between different dates for a single individual, and between different periods of the daily cycle. On some occasions, the trout were responsive to the total drift rate, either at night or during the day, and on others to the largest drifting organisms only (terrestrial organisms, adults of Ephemeroptera, Diptera and Trichoptera). The study supports the idea that trout adapt their activity pattern to the abundance of drifting prey, either as generalists towards any organism, or as specialists towards the largest ones.     

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.     

Reproductive migration of brown trout in a small Norwegian river studied by telemetry

The movement of 34 large (39–73 cm standard length) brown trout Salmo trutta was monitored using radio telemetry for up to 74 days in Brumunda, a small Norwegian river (mean annual discharge 3·3 m³ s⁻¹) flowing into the large Lake Mjøsa. The maximum range of movement in the river was 20 km. No clear relationships existed between individual movement and water discharge, temperature and barometric pressure. Brown trout migrated at all levels of water discharge. At low discharge (<2 m³ s⁻¹) movements were nocturnal. A weir 5·3 km from the outlet restricted ascending brown trout at low ( c . 6° C), but not at high ( c . 8° C) water temperatures. Spawning occurred in September to October and tagged individuals spent 2–51 days at the spawning sites. Mean migration speed from tagging to when the fish reached the spawning area, and from when they left the spawning areas and reached the lake was 1·0 and 2·3 km day⁻¹, respectively. All tagged brown trout that survived spawning returned to the lake after spawning.     

Predation by brown trout: a major mortality factor for sexually mature European minnows

Brown trout Salmo trutta in the subalpine lake, Øvre Heimdalsvatn, showed large temporal variation in the rate of predation on the introduced minnow Phoxinus phoxinus population. Minnows were found in the stomachs of brown trout between 16 and 38 cm L _T. Significantly greater predation was recorded shortly after ice break at the end of June 1999, with frequencies of 9 and 20% within the L _T classes 16–29·9 cm and  ≥30 cm, respectively. Predation on minnows was only occasionally detected during July, August and September. The high level of predation coincided with minnow spawning, and lengths of consumed minnows were equal to those of sexually mature individuals. Accepting a causal link between minnow spawning, which lasted c . 3 weeks, and the contemporary high rate of predation, the estimated annual consumption of minnows by the brown trout population would be 138 kg wet mass. Although most of the annual consumption of minnows by brown trout (90%) occurred within a very short period (3 weeks), it accounted for a significant proportion (60%) of the annual loss in biomass of the sexually mature part of the population.     

Genetic assessment of Atlantic salmon Salmo salar and sea trout Salmo trutta stocking in a Baltic Sea river

Microsatellite DNA variation was used to assess the outcome of stocking Atlantic salmon Salmo salar and migratory trout Salmo trutta in River Sävarå, N Sweden. No information on pre‐stocking genetic composition of S. salar and S. trutta in River Sävarå was available. In 2 year‐classes of S. salar smolt, microsatellite data indicated that post‐stocking genetic composition differed markedly (F_ST= 0·048) from the main donor strain, Byskeälven S. salar, and from other Gulf of Bothnia S. salar stocks (F_ST 0·047 and 0·132). The STRUCTURE programme failed to detect any substructuring within Sävarå salmon. It was concluded that only minor introgression estimated to a proportion of 0·11 (95% CI 0·07–0·16) has occurred in S. salar. Salmo trutta showed overall low differentiation among populations with maximum F_ST of 0·03 making analysis more cumbersome than in S. salar. Still, the SävaråS. trutta deviated significantly from potential donor populations, and STRUCTURE software supported that majority of trout in Sävarå formed a distinct genetic population. Admixture was more extensive in S. trutta and estimated to 0·17 (95% CI 0·10–0·25).     

Intercohort competition causes spatial segregation in brown trout in artificial streams

When together in a flume, 13-cm L _s (age-1) trout appeared to increase their activity and behaved more aggressively, whereas 10-cm L _s (age-0) trout tended to move less and were rarely aggressive in the presence of the larger size class. Both size classes were less mobile and preferred lower water velocities in winter than in summer, and increased their use of instream cover in winter. When both size classes were present, only small trout decreased their use of low water velocities and cover. The results indicate that intercohort competition may cause spatial segregation among size groups of brown trout, especially in winter when trout attempt to minimize their maintenance costs.     

Comparing upriver spawning migration of Atlantic salmon Salmo salar and sea trout Salmo trutta

Radio tagged wild Atlantic salmon Salmo salar(n = 30) and sea trout Salmo trutta(n = 19) were simultaneously released from a sea pen outside the mouth of the River Lærdalselva and their migration to spawning areas was recorded. The distance from the river mouth to a position held at spawning ranged from 2 to 24 km and did not differ between the species (mean ± s .d . 15·9 ± 4·3 and 14·9 ± 5·2 km for Atlantic salmon and sea trout, respectively). The duration of the migration phase, however, was significantly shorter for Atlantic salmon than for sea trout (8–12 days, respectively). All Atlantic salmon migrated straight to an area near the spawning ground, whereas 50% of the sea trout had a stepwise progression with one or more periods with erratic movements before reaching the spawning area. After the migration phase, a distinct search phase with repeated movements up‐ and downstream at or close to the position held at spawning was identified for the majority of the fishes (75%, both species). This search phase was significantly shorter for Atlantic salmon than for sea trout (mean 13–31 days, respectively). Mean ± s .d . length of the river stretch used during the search phase was larger for sea trout (3·3 ± 2·5 km) than for Atlantic salmon (1·2 ± 0·9 km). A distinct holding phase, with no movements until spawning, was also observed in the majority of the Atlantic salmon (80%, mean duration 22 days) and sea trout (65%, mean duration 12 days). For both species, a weak, non‐significant trend was observed in the relationship between time spent on the migration phase, and time spent on the search (r² = 0·43) and holding phase (r² = 0·24). There was a highly significant decrease, however, in the duration of the holding phase with an increase in the time spent on the search phase (r² = 0·67).     

Movement and early survival of age‐0 brown trout

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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.