pH preference and avoidance responses of adult brook trout Salvelinus fontinalis and brown trout Salmo trutta

The pH preferred and avoided by wild, adult brook trout Salvelinus fontinalis and brown trout Salmo trutta was examined in a series a laboratory tests using gradual and steep‐gradient flow‐through aquaria. The results were compared with those published for the observed segregation patterns of juvenile S. fontinalis and S. trutta in Pennsylvania streams. The adult S. trutta tested showed a preference for pH 4·0 while adult S. fontinalis did not prefer any pH within the range tested. Salmo trutta are not found in Pennsylvania streams with a base‐flow pH < 5·8 which suggests that S. trutta prefer pH well above 4·0. Adult S. trutta displayed a lack of avoidance at pH below 5·0, as also reported earlier for juveniles. The avoidance pH of wild, adult S. fontinalis (between pH 5·5 and 6·0) and S. trutta (between pH 6·5 and 7·0) did not differ appreciably from earlier study results for the avoidance pH of juvenile S. fontinalis and S. trutta. A comparison of c.i. around these avoidance estimates indicates that avoidance pH is similar among adult S. fontinalis and S. trutta in this study. The limited overlap of c.i. for avoidance pH values for the two species, however, suggests that some S. trutta will display avoidance at a higher pH when S. fontinalis will not. The results of this study indicate that segregation patterns of adult S. fontinalis and S. trutta in Pennsylvania streams could be related to pH and that competition with S. trutta could be mediating the occurrence of S. fontinalis at some pH levels.     

Use of streambed substrate as refuge by steelhead or rainbow trout Oncorhynchus mykiss during simulated freshets

A flume was used to estimate the carrying capacity of streambed substrates for juvenile steelhead or rainbow trout Oncorhynchus mykiss seeking refuge from simulated freshets. The simulated freshets had mean water column velocities of c. 1·1 m s^?1. The number of O. mykiss finding cover within the interstices of the substrate was documented for different substrate sizes and levels of embeddedness. The availability of suitable refuges determined the carrying capacity of the substrate for O. mykiss. For the size of the O. mykiss tested [mean ± s.d. fork length (L_F) = 122 ± 12·6 mm], the number of interstices with depths ≥200 mm measured with a 14·0 mm diameter flexible plastic tube was the best predictor of the number of O. mykiss able to find cover (r² = 0·75). Oncorhynchus mykiss seeking refuge from freshets may need deeper interstices than those seeking concealment at autumn or winter base flows. The availability of interstices suitable as refuge from high flows may determine autumn and winter carrying capacity.     

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).     

Environmental drivers inducing habitat expansion and shift of introduced alien trout in the Himalayan ecosystem and management concerns

There is a concern of the spread of introduced trout Salmo trutta and Oncorhynchus mykiss which might have potential effects on native fish species in the Himalaya. We present the first assessment of current habitat expansion of introduced trout induced by environmental drivers and posing threats to the local fish diversity. Maximum Entropy (MaxEnt) model was used and overlaid with presence-only data onto bioclimatic and environmental layers to characterize the conditions most suitable for the habitat expansion of trout. Mean AUC value for S. trutta was 0.919 and 0.881 for O. mykiss respectively showing that the MaxEnt model was highly accurate and statistically significant. The precipitation of driest quarter (Bio_17) alone accounted for 71.4% habitat expansion of S. trutta across rivers’ length and 61.1% in the case of O. mykiss. The Jackknife test of different environmental variable particularly Bio_17 and the coldest quarter (Bio_19) depicted their potential role in habitat expansion. The occurrences of trout in the Himalayan streams predicted trade-offs between few environmental variables and habitat expansion. The findings suggested that habitat expansion of trout was induced by identified environmental drivers impacting the array of biological and ecological integrity in the new geographic spaces concerning trout invasion.     

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.     

Latitudinal and altitudinal growth patterns of brown trout Salmo trutta at different spatial scales

Spatial variation in growth of stream‐dwelling brown trout Salmo trutta was explored in 13 populations using a long‐term study (1993–2004) in the Bay of Biscay drainage, northern Spain. The high variability in fork length (L_F) of S. trutta in the study area was similar to the body‐size range found in the entire European distribution of the species. Mean L_F at age varied: 0+ years, 57·4–100·7 mm; 1+ years, 111·6–176·0 mm; 2+ years, 155·6–248·4 mm and 3+ years, 194·3–290·9 mm. Average L_F at age was higher in main courses and lower reaches compared with small tributaries and upper reaches. Annual specific growth rates (G_L) were: 0+ to 1+ years, 0·634–0·825 mm mm⁻¹ year⁻¹; 1+ to 2+ years, 0·243–0·342 mm mm⁻¹ year⁻¹; 2+ to 3+ years, 0·166–0·222 mm mm⁻¹ year⁻¹, showing a great homogeneity. Regression models showed that water temperature and altitude were the major determinants of L_F at age variability within the study area. A broader spatial analysis using available data from stream‐dwelling S. trutta populations throughout Europe indicated a negative relationship between latitude and L_F of individuals and a negative interaction between latitude and altitude. These findings support previous evidence of the pervasive role of water temperature on the L_F of this species. Altitude appeared as the overall factor that includes the local variation of other variables, such as water temperature or food availability. At a larger scale, latitude was the factor that encompassed these environmental gradients and explained the differences in L_F of S. trutta. In summary, L_F at age in stream‐dwelling S. trutta decreases with latitude in Europe, the converse of Bergmann's rule.     

Hardness does not affect the physiological responses of wild and domestic strains of diploid and triploid rainbow trout Oncorhynchus mykiss to short‐term exposure to pH 9.5

This study examined the effects of water hardness on the physiological responses associated with high pH exposure in multiple strains of diploid and triploid rainbow trout Oncorhynchus mykiss. To accomplish this, three wild strains and one domesticated strain of diploid and triploid O. mykiss were abruptly transferred from control soft water (City of Vancouver dechlorinated tap water; pH 6·7; [CaCO₃] < 17·9 mg l^?1) to control soft water (handling control), high pH soft water (pH 9·5; [CaCO₃] < 17·9 mg l^?1), or high pH hard water (pH 9·5; [CaCO₃] = 320 mg l^?1) followed by sampling at 24 h for physiological measurements. There was a significant effect of ploidy on loss of equilibrium (LOE) over the 24 h exposure, with only triploid O. mykiss losing equilibrium at high pH in both soft and hard water. Furthermore, exposure to pH 9·5 resulted in significant decreases in plasma sodium and chloride, and increases in plasma and brain ammonia with no differences between soft and hard water. There was no significant effect of strain on LOE, but there were significant differences between strains in brain ammonia and plasma cortisol. Overall, there were no clear protective effects of hardness on high pH exposure in these strains of O. mykiss.     

Short‐term evaluation of visible implant alpha tags in juveniles of three fish species under laboratory conditions

Visible implant alpha (VI alpha) tag‐induced changes in mortality and condition, as well as tag retention and readability, were examined during a 4‐week period for juveniles of three fish species: tiger muskellunge Esox masquinongy × Esox lucius (91 ± 7 mm total length, L_T, mean ± s.d.), Snake River cutthroat trout Oncorhynchus clarki behnkei (84 ± 8 mm) and rainbow trout Oncorhynchus mykiss (85 ± 5 mm). Mortality and condition did not differ between tagged fish and control fish for any species and overall tag retention rates were high (92% for E. masquinongy × E. lucius, 91% for O. c. behnkei and 100% for O. mykiss). Short‐term readability of VI alpha tags was low in juvenile E. masquinongy × E. lucius and juvenile O. c. behnkei. Therefore, it is not recommend to use VI alpha tags in juvenile E. masquinongy × E. lucius or juvenile O. c. behnkei for periods >2 weeks, but VI alpha tags seem to be suitable for juvenile O. mykiss for a period of at least 4 weeks.     

High level of population genetic structuring in lake-run brown trout, Salmo trutta, of the Inari Basin, northern Finland

Rivers draining into (Lake) Inarijärvi, northern Finland, sustain a number of lake‐run brown trout, Salmo trutta, populations but, as with most lake‐run S. trutta systems, the level of population genetic structuring among populations is unknown. To address this and to assist fish stock management in the region, the population genetic structure of S. trutta collected from 28 sampling sites in rivers flowing into Inarijärvi was studied using 13 microsatellite loci. Populations were clustered into three separate groups, largely corresponding to geographic regions, with between‐region F_ST values ranging from 0·11 to 0·16. The significant differentiation observed between most populations within each region also implies that individual populations should be recognized as separate management units and actions to improve, and subsequently maintain, conditions for natural spawning should be prioritized. The results of this study further indicate that the trout from each of these regions may have different biological characteristics, such as local‐lake feeding behaviour among the western populations and strong isolation among the northern stocks. As a consequence, further research is warranted to better understand the level of ecological uniqueness of lake‐run S. trutta populations.     

Temporal stability and rates of post‐depositional change in geochemical signatures of brown trout Salmo trutta scales

This study investigates temporal stability in the scale microchemistry of brown trout Salmo trutta in feeder streams of a large heterogeneous lake catchment and rates of change after migration into the lake. Laser‐ablation inductively coupled plasma mass spectrometry was used to quantify the elemental concentrations of Na, Mg, Mn, Cu, Zn, Ba and Sr in archived (1997–2002) scales of juvenile S. trutta collected from six major feeder streams of Lough Mask, County Mayo, Ireland. Water‐element Ca ratios within these streams were determined for the fish sampling period and for a later period (2013–2015). Salmo trutta scale Sr and Ba concentrations were significantly (P < 0·05) correlated with stream water sample Sr:Ca and Ba:Ca ratios respectively from both periods, indicating multi‐annual stability in scale and water‐elemental signatures. Discriminant analysis of scale chemistries correctly classified 91% of sampled juvenile S. trutta to their stream of origin using a cross‐validated classification model. This model was used to test whether assumed post‐depositional change in scale element concentrations reduced correct natal stream classification of S. trutta in successive years after migration into Lough Mask. Fish residing in the lake for 1–3 years could be reliably classified to their most likely natal stream, but the probability of correct classification diminished strongly with longer lake residence. Use of scale chemistry to identify natal streams of lake S. trutta should focus on recent migrants, but may not require contemporary water chemistry data.     

Basic physico-chemical parameters of milt from sea trout (Salmo trutta m. trutta), brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss)

The aim of the study was to compare the physico‐chemical parameters of milt from sea trout (Salmo trutta m. trutta), brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss). Milt was collected by stripping and spermatozoa concentrations, were determined and compared with sperm motility and spermatocrit values along with seminal plasma indices (pH, osmolality, sodium, potassium, chlorine, calcium, magnesium, glucose and protein concentrations). The highest spermatozoa concentration of 22.3 ± 6.7 × 10⁹ ml^?1 was found in the sea trout milt, and was significantly different of those observed in brook trout (11.9 ± 3.3 × 10⁹ ml^?1) and rainbow trout (10.7 ± 4.4 × 10⁹ ml^?1). The values for pH and K⁺ did not differ significantly among species. The mean pH was 8.0 in the milt of each species and the K⁺ concentrations ranged from 24.8 ± 7.2 to 30.5 ± 7.6 mm L^?1. Considerable differences were determined for the Ca²⁺ ions concentrations. The highest value was found in sea trout (1.7 ± 0.3 mm L^?1), while in the rainbow trout it was 0.7 ± 0.5 and in the brook trout 0.4 ± 0.1 mm L^?1. The most pronounced differences were found in the glucose concentration cause of its unnaturally low concentration in rainbow trout of the mean value of 6.0 ± 15.2 mg L^?1. The mean value in sea trout and brook trout was 185.0 ± 172.4 and 231.2 ± 148.4 mg L^?1 respectively. For all species, protein mean values were below 1.3 g L^?1. The mean osmolality was between 230.6 ± 98.6 and 272.0 ± 26.4 mOsm kg^?1 in the species studied. No correlation was found between any components determined in milt and the spermatozoa motility (P > 0.05). The sperm concentration was positively correlated with the protein content in the milt of the three species studied, other less exhibited correlation was found.     

Radiocaesium concentrations in the muscle and eggs of salmonids from Lake Chuzenji,Japan, after the Fukushima fallout

Approximately 18 months (September to December 2012) after the Fukushima Dai‐ichi Nuclear Power Plant accident, elevated radiocaesium concentrations were measured in samples of muscle and eggs from masu salmon Oncorhynchus masou, kokanee Oncorhynchus nerka, brown trout Salmo trutta and lake trout Salvelinus namaycush from the Lake Chuzenji system, central Honshu Island, Japan (160 km from the station). Mean muscle concentrations were 142·9–249·2 Bq kg^?1 wet mass and mean concentrations in eggs were 38·7–79·0 Bq kg^?1 wet mass. There was no relationship between fork length and muscle radiocaesium concentration in any of the species, but there were significant relationships between individual muscle and egg radiocaesium concentrations from O. masou, S. trutta and S. namaycush.     

Characterization of tetranucleotide microsatellites for Rio Grande cutthroat trout and rainbow trout,and their cross‐amplification in other cutthroat trout subspecies

We describe the isolation and characterization of 12 tetranucleotide microsatellites for Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis) and rainbow trout (Oncorhynchus mykiss), and subsequently investigate their performance in Colorado River cutthroat trout (Oncorhynchus clarkii pleuriticus), greenback cutthroat trout (Oncorhynchus clarkii stomias) and Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri). All 12 loci are polymorphic in all subspecies of O. clarkii examined.     

External fluorescence retention of calcein‐marked juvenile brown trout Salmo trutta raised in natural and artificial environments

The fluorescence retention and intensity of juvenile brown trout Salmo trutta marked during their first summer were monitored in a hatchery and in four natural streams. A handheld detector was used for direct examination. In the hatchery, three marking treatments (T) were compared: 3·5 min in a 0·5% calcein solution (T0·5‐3·5), 7 min in a 0·5% calcein solution (T0·5‐7) and 3·5 min in a 1% calcein solution (T1‐3·5). The fish were raised indoors for 11 months and then outdoors until 18 months. The fluorescence retention rate was 100% in all treatments at 11 months, although T1‐3·5 showed the highest mean fluorescence intensity, followed by T0·5‐7 and T0·5‐3·5. The fluorescence intensity was not correlated with the final total length (L_T) of the fish in two treatments, although it significantly decreased with increasing L_T in T1‐3·5. At 18 months, <30% of the fish were still slightly fluorescent, suggesting a negative effect of sunlight exposure. In stream studies, the fluorescence intensity did not significantly differ according to final L_T; an overall mean ± s.d . retention rate of 70·7 ± 26·6% was measured at 12 months with a decrease to 48·6 ± 24·6% at 24 months. Significant differences amongst streams and within reaches of the same stream were observed. Because of a significant positive effect of the shading index on the fluorescence intensity, the use of calcein should be restricted to streams unexposed to direct sunlight. Consequently, the marking method would be reliable for 1 year monitoring studies in shaded streams.     

Non‐native trout limit native brook trout access to space and thermal refugia in a restored large‐river system

We used direct observation via snorkeling surveys to quantify microhabitat use by native brook (Salvelinus fontinalis) and non‐native brown (Salmo trutta) and rainbow (Onchorynchus mykiss) trout occupying natural and restored pool habitats within a large, high‐elevation Appalachian river, United States. Permutational multivariate analysis of variance (PERMANOVA) and subsequent two‐way analysis of variance (ANOVA) indicated a significant difference in microhabitat use by brook and non‐native trout within restored pools. We also detected a significant difference in microhabitat use by brook trout occupying pools in allopatry versus those occupying pools in sympatry with non‐native trout—a pattern that appears to be modulated by size. Smaller brook trout often occupied pools in the absence of non‐native species, where they used shallower and faster focal habitats. Larger brook trout occupied pools with, and utilized similar focal habitats (i.e. deeper, slower velocity) as, non‐native trout. Non‐native trout consistently occupied more thermally suitable microhabitats closer to cover as compared to brook trout, including the use of thermal refugia (i.e. ambient–focal temperature >2°C). These results suggest that non‐native trout influence brook trout use of restored habitats by: (1) displacing smaller brook trout from restored pools, and (2) displacing small and large brook trout from optimal microhabitats (cooler, deeper, and lower velocity). Consequently, benefits of habitat restoration in large rivers may only be fully realized by brook trout in the absence of non‐native species. Future research within this and other large river systems should characterize brook trout response to stream restoration following removal of non‐native species.     

Bi‐parentally inherited species‐specific markers identify hybridization between rainbow trout and cutthroat trout subspecies

Eight polymerase chain reaction primer sets amplifying bi‐parentally inherited species‐specific markers were developed that differentiate between rainbow trout (Oncorhynchus mykiss) and various cutthroat trout (O. clarki) subspecies. The primers were tested within known F₁ and first generation hybrid backcrosses and were shown to amplify codominantly within hybrids. Heterozygous individuals also amplified a slower migrating band that was a heteroduplex, caused by the annealing of polymerase chain reaction products from both species. These primer sets have numerous advantages for native cutthroat trout conservation including statistical genetic analyses of known crosses and simple hybrid identification.     

Dietary ontogeny and niche shift to piscivory in lacustrine brown trout Salmo trutta revealed by stomach content and stable isotope analyses

The feeding ecology and ontogeny of a large size range of brown trout Salmo trutta in Lake Fyresvatnet, southern Norway, were examined by stomach content and stable isotope analyses. According to the stomach contents, the S. trutta changed their diet at c. 30 cm total length (L_T). The smaller size classes fed on benthic invertebrates and surface insects, whereas larger S. trutta (>30 cm) fed mainly on whitefish Coregonus lavaretus. A similar, but more gradual shift to piscivory in the size range 25–30 cm was found when using the stable isotope mixing model SIAR to reveal dietary ontogeny. The δ¹³C isotopic signature confirmed that S. trutta independent of size predominantly relied upon benthic energy sources, suggesting that the littoral zone was the primary foraging habitat for both invertebrate and piscivorous feeders. The δ¹⁵N values and trophic position increased with predator length, ranging from an average of 3·60 for small‐sized S. trutta (<15 cm) to 4·15 for large‐sized fish (>35 cm). The S. trutta exhibited a relatively slow growth rate during the predominant invertebrate feeding stages up to 7 years of age and 28 cm L_T, whereas fish above this size and age displayed a rapid growth rate of 9–11 cm year^?1, demonstrating the profitability of piscivorous feeding.     

Density‐dependent effects of non‐native brown trout Salmo trutta on the species–area relationship in stream fish assemblages

The spatial scale and density‐dependent effects of non‐native brown trout Salmo trutta on species richness of fish assemblages were examined at 48 study sites in Mamachi Stream, a tributary of Chitose River, Hokkaido, Japan. The density of age ≥1 year S. trutta was high in the upstream side of the main stem of Mamachi Stream. Fish species richness increased with increasing area of study sites (habitat size), but the increasing magnitude of the species richness with area decreased with increasing age of ≥1 year S. trutta density. The relationships between age ≥1 year S. trutta, however, and presence–absence of each species seemed to be different among species. Species richness was also determined by location and physical environmental variables, i.e. it was high on the downstream side and in structurally complex environments.     

Quantification of ventricular β2‐adrenoceptor density and ligand binding affinity in wild sockeye salmon Oncorhynchus nerka smolts using a novel modification to the tritiated ligand technique

A new, image‐based, tritiated ligand technique for measuring cardiac β₂‐adrenoceptor (β₂‐AR) binding characteristics was developed and validated with adult rainbow trout Oncorhynchus mykiss hearts so that the tissue limitation of traditional receptor binding techniques could be overcome and measurements could be made in hearts nearly 14‐times smaller than previously used. The myocardial cell‐surface (functional) β₂‐AR density of O. nerka smolts sampled at the headwaters of the Chilko River was 54·2 fmol mg protein^?1 and about half of that previously found in return migrating adults of the same population, but still more than twice that of adult hatchery O. mykiss (21·1 fmol mg protein^?1). This technique now opens the possibility of investigating cardiac receptor density in a much wider range of fish species and life stages.