Performance of juvenile brown trout exposed to fluctuating water level and temperature

Individual daily food intake, mass‐specific growth rate and growth efficiency in groups of juvenile brown trout Salmo trutta were compared in tank experiments with three water level regimes (fluctuating, stable high and low water levels) and two temperature regimes (fluctuating between 10 and 14° C and constant 14° C) to simulate events during hydropeaking in regulated rivers. Fish exposed to high stable water level showed higher food intake and growth rate, and higher or similar growth efficiency than fish exposed to fluctuating or stable low water level. Both groups of slow‐growing and fast‐growing individuals fed less and grew slower at stable low and fluctuating water level than at stable high water level. Furthermore, growth and growth efficiency were lower in brown trout exposed to stable low water level and fluctuating temperature, particularly for groups of fish with slow growth. Temperature did not have any effect at high water level. For groups of fast‐growing fish, there was no difference in growth efficiency between treatments. It is concluded that fluctuating water level and temperature have a potentially detrimental effect on growth in juvenile brown trout and effects are more severe in slow‐ than fast‐growing fish.     

Effect of fluctuating flow and temperature on cover type selection and behaviour by juvenile brown trout in artificial flumes

Brown trout Salmo trutta in a flume used cover more often and were more aggregated at low water temperatures in winter compared with summer. Cover combinations providing the most cover (overhead, visual and velocity cover) were preferred in winter whereas velocity refuges were preferred in late summer; and brown trout rarely used any of these cover structures in early summer. Flow affected cover use especially in winter, when most of the trout moved to shelters during the first high flow period. Movements by brown trout were most frequent in early summer. The much higher aggression rate in summer compared with winter decreased during the first period of high flow in both summer experiments. The results indicate seasonal changes in cover habitat and cover type preferences suggesting that habitat complexity is important, and availability of cover is particularly important during different seasons and fluctuating flow.     

Cortisol and thyroid hormone responses to acid stress in the brown trout, Salmo trutta L.

Individual cannulated brown trout monitored during exposure to acidic water showed increased plasma cortisol after 3 h at pH 4.0 with low (0.05 mm) or high (2.8 mm) calcium (Ca) content, and after 2 days in acidic water with a high Ca content. Most fish did not survive for 2 days in acidic water with a low Ca content. Non-cannulated fish showed a similar increase in mean plasma cortisol after 2 days in high-Ca acidic water (pH 4.0–4.6), but not in acidic water of a low Ca content. After 7 days of exposure to acidic water, plasma cortisol appeared to recover when there was a high Ca content but increased 20-fold when Ca content was low. In cannulated fish severe acid stress resulted in a marked and rapid thyroid response. Plasma thyroxine (T₄) was elevated after 3 h exposure to acidic water of both low and high Ca content and remained elevated for 2 days of acid exposure with high Ca. In non-cannulated fish an increase in mean T₄ was apparent only after 7 days in low-Ca acidic water. Plasma triiodothyronine (T₃) levels were not significantly altered by any of the acid regimes. Plasma glucose of cannulated fish was elevated within 3 h of acid-exposure and remained elevated after 2 days in high-Ca acidic water.     

Movement and mortality of stocked brown trout in a stream

The movement and mortality of stocked brown trout Salmo trutta were investigated using radio telemetry. Four brown trout left the study area whereas the remaining fish were stationary. After 5 weeks, 13 out of 50 tagged brown trout were still alive in the stream. Surviving fish had a significantly lower mean movement per day than fish, which later either died or disappeared. This difference in behaviour was most pronounced 2 to 8 days after release. Predation by the otter Lutra lutra was probably the main cause of the observed mortality.     

Cardiovascular, ventilatory and total activity responses of brown trout to handling stress

Changes in total activity, heart and ventilation rates were observed in 2-year-old brown trout, following handling stress, using non-contact bioelectronic monitoring equipment. Experiments were carried out in laboratory conditions at water temperatures below 4° C, Transfer between tanks as well as 5 min restraint stress increased the total activity of fish for 24 to 48 h, after which it declined to near the pre-stress level. The transfer and struggle both elevated the heart rate for 3 to 4 days. Ventilation rate was elevated to a maximum of about 30% above the nominal level and recovered within 3 to 4 days. Both heart and ventilation rates were higher in feeding fish relative to fasting fish after stress and rates remained higher throughout a 7 day period of recovery. A diel rhythm of lower rates during the night appeared in both heart and ventilation rates within 3 to 4 days after handling stress.     

Endocrine responses of brown trout, Salmo trutta L., to acid, aluminium and lime dosing in a Welsh hill stream

Brown trout caged in an acidic, softwater, Welsh hill stream running through close-canopy conifer forest and in an adjacent, circumneutral, moorland stream were studied after a natural rainfall event and after experimental acid and aluminium dosing of the forest stream. Endocrine (cortisol and thyroxine) and metabolic (glucose) stress responses occurred in fish held in the forest stream. Liming downstream of the acid zones mitigated these responses.     

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.     

Pools as refugia for brown trout during two summer droughts: trout responses to thermal and oxygen stress

In non–drought years (1977, 1985), temperatures and oxygen concentrations from 1 to 14 July at the deepest point in each of five pools in Wilfin Beck were similar with ranges of 12–18° C and 7·8–9·8 mg l^–1. Trout Salmo trutta were present in all pools. In drought years (1976, 1983), temperature increased and oxygen concentration decreased as pool size decreased. In the two smallest pools, they were outside the thermal and oxygen limits for trout (ranges for both pools 24–29° C, 1·2–2·5 mg l^–1), and trout were absent. Values in a medium–sized pool were close to the incipient lethal levels and a few juvenile trout were present in both drought years. The lowest temperatures and highest oxygen concentrations were recorded in the two largest pools (ranges 20–25° C, 3·6–4·8 mg l^–1) and trout of all ages (0+ to adults) were present in both drought years. In these two pools, both temperature and oxygen concentration decreased from the surface to the deepest point in the pool. Trout preferred lower temperatures near the pool bottom rather than higher oxygen concentrations near the surface, but some fish moved towards the surface at night when the pool cooled slightly. These field results were discussed in relation to lethal values recorded for brown trout in the laboratory, and there was general agreement between field and laboratory values. Trout in the drought years occurred at temperatures close to, or below, the incipient lethal value of 24·7° C (+0·5) and also at the highest oxygen concentrations, but only when these were at temperatures below the incipient lethal value.     

Recovery of wild, juvenile brown trout from stress of flow reduction, electrofishing, handling and transfer from river to an indoor simulated stream channel

Stress in wild brown trout Salmo trutta was assessed by sampling blood and measuring the concentrations of plasma cortisol and blood glucose in fish collected by electrofishing and immediately anaesthetized with metomidate. In the River Nidelva, Trondheim, Norway, the resting blood plasma cortisol concentration in the juvenile (0 + year) brown trout was 52 ± 44 nM (mean ± s .d .) in December and 2·3 nM (detection limit) in January. The corresponding blood glucose values were 1·8 ± 0·9 and 1·2 ± 0·2 mM, respectively. After electrofishing, handling and transport to the artificial stream, plasma cortisol and blood glucose levels increased significantly in both experiments. A maximum plasma cortisol level of 239 ± 120 and 71 ± 32 nM and a maximum blood glucose level of 3·9 ± 0·9 and 3·0 ± 0·9 mM were measured in the December and January stream channel experiments after transport, respectively. After introduction to the artificial stream, the blood plasma cortisol level returned to resting values within 24 h in the January stream channel experiment. The blood glucose levels remained at a higher level compared to the reference group throughout the December experiment, while it returned to resting values after 24 h in the January stream channel experiment. The major difference between the December and January experiments was the temperature within the artificial stream, 15–17° C in December and 7–9° C in January. This may have influenced the blood glucose levels. After dewatering of the artificial streambed there was a significant increase in plasma cortisol both in the December and January experiments, and after 24 h the plasma cortisol returned to the resting level in the January experiment. The blood glucose also increased during dewatering, although not significantly.     

Artificial lakes delay the migration of brown trout Salmo trutta smolts: a comparison of migratory behaviour in a stream and through an artificial lake

Juvenile salmonids experience high mortality when negotiating lentic waters during their downstream migration to the sea. The development of artificial lakes and wetlands in streams has become a widely used management tool to reduce nutrient load to coastal areas. Such wetlands may threaten anadromous populations. In this study we quantify net ground speed of downstream migrating brown trout Salmo trutta smolts in equally long stream and lake sections in a Danish lowland stream and artificial lake. This was done by passive integrated transponder telemetry in 2016 and 2017. Mean net ground speed in the stream section was 36.58 and 0.8 km day⁻¹ in the lake section. This decrease of net ground speed through the lake may lead to prolonged exposure to predators and probably contributes to high mortalities threatening anadromous populations.     

Triploid progeny from a female Atlantic salmon × brown trout hybrid backcrossed to a male brown trout

A female Atlantic salmon × brown trout hybrid was backcrossed to a male brown trout. Electrophoretic analysis of diagnostic enzymes showed that the progeny were triploid. However, a few individuals were partially diploid.     

Movement and site fidelity in young brown trout Salmo trutta populations in a southern Irish stream

Populations of brown trout Salmo trutta were monitored at a number of sites within a single stream, using an individual marking technique and recapturing uniquely marked fish repeatedly over a period of 12 months. Individual 1 + and 2 + resident brown trout in the Glenfinish River were found to consist of stationary and mobile component populations. The latter population consisted of a number of individuals observed moving mostly in an upstream direction, within a range of 0.03–2.24 km. On a large spatial scale, individuals in the stationary component population exhibited some degree of home site fidelity within the stream, over a period of 3–4 months, after which the fish tended to move from the site. Within sites, fidelity to either riffle or pool habitats, mostly the latter, was apparent in a proportion of the population. On a smaller scale, fidelity to the exact position with respect to boulders in the stream was also evident in a number of individuals. Home range size was calculated amongst these individuals, with ranges of up to 20 m recorded.     

The role of F-series prostaglandins as reproductive priming pheromones in the brown trout

Electrophysiological studies demonstrated that the olfactory epithelium of mature male brown trout Salmo trutta parr was acutely sensitive to F-series prostaglandins (PGFs) PGF_1α and PGF_2α, with detection threshold concentrations of 10⁻¹¹ M. The olfactory epithelium was also sensitive to the PGF metabolite 15-ketoPGF_2α (threshold 10⁻⁸ m), but did not detect a further metabolite, 13,14,-dihydro-15-ketoPGF_2α Immature brown trout did not detect any of the prostaglandins tested. Exposure of mature male brown trout parr to waterborne PGF_1α and PGF_2α (concentration 10⁻⁸ m), resulted in significant increases in levels of expressible milt and the plasma concentrations of 17,20β-dihydroxy-4-pregnen-3-one, testosterone and 11-ketotestosterone. The olfactory epithelium of both immature and mature male brown trout parr was sensitive to the urine and ovarian fluid from ovulated female brown trout. Exposure of mature male brown trout parr to ovarian fluid resulted in an increase in the levels of plasma 17,20β-dihydroxy-4-pregnen-3-one whilst exposure to urine increased the levels of expressible milt. In addition, PGF_2α was found to be present within both the urine and ovarian fluid of mature female brown trout. It is suggested that the F-series prostaglandins have a role as priming pheromones in male brown trout.     

Retention of visible implant and visible implant elastomer tags in brown trout in an English chalk stream

Retention of both rigid and new soft standard size visible implant (VI) tags in brown trout Salmo trutta after 6 months varied between 42 and 97% for different batches, with no evidence of increased retention of the new type of tag. The 6 month retention of visible implant elastomer tags (VIE) also varied but with a mean of 96%. The VIE retention gradually declined with time up to 42 months.     

Effects of abrupt cold shock on stress responses and recovery in brown trout exhausted by swimming

To simulate swimming in a trawl, age 3 year brown trout Salmo trutta (L.) were made to swim against a flow of 0·5 m s⁻¹ for 60 min. To simulate cold shock, similar to placing them in a chilling tank, the fish were kept for 10 min in a tank containing ice and water. To simulate the combined stressors, the fish were first made to swim followed by a cold shock. The fish were in a comatose state 10 min after cold shock and combined stressors but conscious after swimming only. All the fish survived until the end of the studied recovery period (maximum 24 h). Cold shock after swimming (combined stressors v . swimming only) did not produce higher blood cortisol, lactate or glucose concentrations 10 min after the treatment. The effect of cold shock, however, was evident in the delayed start of recovery in cortisol and glucose concentrations. All the stress indicators used decreased to the levels for undisturbed fish within 24 h, except in the case of glucose after the combined stressors.     

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.     

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.     

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.     

Periodic habitat loss alters the competitive coexistence between brown trout and bullheads in a small stream over 34 years

1. Changes in the population density of juvenile sea trout Salmo trutta L. and bullheads Cottus gobio L. were compared in a small stream over 34 years. Both species have a similar diet and obviously live in the same general habitat. Habitat loss was most marked in seven summer droughts: severest in 1976, 1983, 1984, 1995, and less severe but followed by autumn droughts in 1969, 1989 and 1993. The contrasting effects of habitat loss on the two species were examined. 2. For both species, the Ricker curvilinear model significantly fit (P < 0.001) the relationship between initial egg density and survivor density for successive life stages, even though egg densities were much lower for bullheads than trout. These analyses provided evidence for density-dependent population regulation and also identified extreme outliers, most being for year-classes affected by summer droughts. 3. The variable effects of changes in habitable area (= % wettable area in sampling section) were quantified by using the residuals, each residual being the absolute value expressed as a percentage of the expected value from the Ricker curve. Significant relationships between the residuals and habitable area showed that habitat loss had a marked effect on survivor density, this being negative for 0+ and 1+ trout, and positive for 0+, 1+ and 2+/3+ bullheads. 4. Therefore, during periods of habitat loss in the summer months, bullhead density increased at the expense of trout density. Low flows and a decrease in wettable area were associated with a marked reduction in habitat quality for drift-feeding trout and an increase in habitat quality, and perhaps also quantity, for benthic-feeding bullheads. This case study shows that, during a major perturbation, the relationship between the densities of two species can change markedly in favour of the less numerous species. The competitive coexistence between the two species is therefore a dynamic process that changes through time with periodic changes in the environment.     

Microhabitat use by brown trout, Salmo trutta L. and Atlantic salmon, S. salar L., in a stream: a comparative study of underwater and river bank observations

Two methods, visual observation from the river bank and visual observation underwater by diving, were compared for microhabitat studies in young brown trout and Atlantic salmon in a stream. A wide range of habitat conditions were surveyed. Each method yielded different results with respect to microhabitat use. River bank observations missed small fish under surface turbulence and in deeper waters. Underwater observations missed small fish in shallow areas.