Food selection by brook trout in a subalpine stream

The diet and food selection of brook trout (Salvelinus fontinalis) and macroinvertebrate drift are described for a high altitude stream (3 205 m above mean sea level) adjacent to the alpine zone on the Snowy Range in the Medicine Bow National Forest, Wyoming. Diet composition differed between brook trout of two length classes, 150 mm and > 150 mm in total length. The number of prey organisms per stomach generally declined and the proportion of terrestrial prey items increased from July to September, 1985. Fish of both length classes selected for Trichoptera and Coleoptera over the three sampling months, and fish > 150 mm also selected for terrestrial insects in August and September.The Unit is jointly supported by the University of Wyoming, the Wyoming Game and Fish Department, and the US Fish and Wildlife ServiceThe Unit is jointly supported by the University of Wyoming, the Wyoming Game and Fish Department, and the US Fish and Wildlife Service     

Emergence chronology of brook charr,Salvenus fontinalis,alevins in an acidic stream

Synopsis The emergence chronology of brook charr alevins in the Chikanishing River, Ontario, Canada and the concurrent changes in stream chemistry resulting from acidic snowmelt runoff were examined during the spring, 1986–1988. Emergence patterns were similar among years with the most intense emergence (> 70%) occurring during the declining stream discharge following the spring flood. This period coincided with stream pH depressions (minimum pH 5.2) in 2 of 3 years, suggesting emergence behaviour frequently subjects the earliest free-swimming life interval of brook charr to episodic acidification events.     

Summer stream habitat partitioning by sympatric Arctic charr, Atlantic salmon and brown trout in two sub-arctic rivers

Summer habitat use by sympatric Arctic charr Salvelinus alpinus, young Atlantic salmon Salmo salar and brown trout Salmo trutta was studied by two methods, direct underwater observation and electrofishing, across a range of habitats in two sub-arctic rivers. More Arctic charr and fewer Atlantic salmon parr were observed by electrofishing in comparison to direct underwater observation, perhaps suggesting a more cryptic behaviour by Arctic charr. The three species segregated in habitat use. Arctic charr, as found by direct underwater observation, most frequently used slow (mean ±s .d . water velocity 7·2 ± 16·6 cm s⁻¹) or often stillwater and deep habitats (mean ±s .d . depth 170·1 ± 72·1 cm). The most frequently used mesohabitat type was a pool. Young Atlantic salmon favoured the faster flowing areas (mean ±s .d . water velocity 44·0 ± 16·8 cm s⁻¹ and depth 57·1 ± 19·0 cm), while brown trout occupied intermediate habitats (mean ±s .d . water velocity 33·1 ± 18·6 cm s⁻¹ and depth 50·2 ± 18·0 cm). Niche overlap was considerable. The Arctic charr observed were on average larger (total length) than Atlantic salmon and brown trout (mean ±s .d . 21·9 ± 8·0, 10·2 ± 3·1 and 13·4 ± 4·5 cm). Similar habitat segregation between Atlantic salmon and brown trout was found by electrofishing, but more fishes were observed in shallower habitats. Electrofishing suggested that Arctic charr occupied habitats similar to brown trout. These results, however, are biased because electrofishing was inefficient in the slow-deep habitat favoured by Arctic charr. Habitat use changed between day and night in a similar way for all three species. At night, fishes held positions closer to the bottom than in the day and were more often observed in shallower stream areas mostly with lower water velocities and finer substrata. The observed habitat segregation is probably the result of interference competition, but the influence of innate selective differences needs more study.     

Selection on Arctic charr generated by competition from brown trout

We experimentally explored population‐ and individual‐level effects on Arctic charr (Salvelinus alpinus) resulting from resource competition with its common European competitor, the brown trout (Salmo trutta). At the population level, we compared performance of the two species in their natural sympatric state with that of Arctic charr in allopatry. At the individual level, we established selection gradients for morphological traits of Arctic charr in allopatric and in sympatric conditions. We found evidence for interspecific competition likely by interference at the population level when comparing differences in average performance between treatments. The growth and feeding rates did not differ significantly between allopatric and sympatric Arctic charr despite lower charr densities (substitutive design) in sympatric enclosures indicating that inter‐ and intraspecific competition are of similar strength. The two species showed distinct niche segregation in sympatry, and brown trout grew faster than Arctic charr. Arctic charr did not expand their niche in allopatry, indicating that the two species compete to a limited degree for the same resources and that interference may suppress the growth of charr in sympatric enclosures. At the individual level, however, we found directional selection in sympatric enclosures against individual Arctic charr with large head and long fins and against individuals feeding on zoobenthos rather than zooplankton indicating competition for common resources (possibly exploitative) between trout and these charr individuals. In allopatric enclosures these relations were not significant. Diets were correlated to the morphology supporting selection against the benthic‐feeding type, i.e. individuals with morphology and feeding behaviour most similar to their competitor, the benthic feeding brown trout. Thus, this study lends support to the hypothesis that Arctic charr have evolved in competition with brown trout, and through ecological character displacement adapted to their present niche.     

Annual production by a stream brook charr population and by its principal invertebrate food

Synopsis Annual production by the brook charr (Salvelinus fontinalis) population in Valley Creek, Minnesota, over the five-year period 1968–1972, was closely correlated to production by its main invertebrate food, Gammarus pseudolimnaeus, which had been reduced by siltation. Annual production was 163–191 kg ha^–1 (wet weight) in 1968–1969, respectively, and then dropped to a mininum of 79 kg ha^–1 in 1971, the year most seriously affected by decreased invertebrate production and siltation; year-class strength, standing stock, and total year-class (cohort) production followed approximately the same pattern. Total year-class production was highest for the 1968–1969 year classes at 211–178 kg ha^–1, respectively, and lowest for the 1971 year class at 76 kg ha^–1. Annual P/B ratios ranged from 1.0 to 1.9; cohort P/B ratios, for the 1968–1972 year classes, ranged from 5.6 to 7.2.Paper No. 11,384, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota 55108, U.S.A.     

Hybridization between native white-spotted charr and nonnative brook trout in the upper Sorachi River,Hokkaido, Japan

Invasion status and impacts of nonnative brook trout (Salvelinus fontinalis) in a Hokkaido stream were investigated with field surveys and genetic analyses. Nonnative brook trout was detected in nine (41 %) of the 22 sampled reaches in three tributaries of the Sorachi River, Hokkaido, Japan. Based on the external pigmentation, twelve putative hybrids between brook trout and native white-spotted charr (Salvelinus leucomaenis) were collected in two reaches. Microsatellite and mitochondrial DNA data established that 58% of these hybrids were first-generation (F₁) progenies between male brook trout and female white-spotted charr. Our results suggest potential negative impacts of nonnative brook trout on native charr populations in Hokkaido through interspecific interactions.     

Evaluating a science-based decision support tool used to prioritize brook charr conservation project proposals in the eastern United States

Declines in brook charr (Salvelinus fontinalis) throughout its historic eastern United States of America (U.S.) range prompted the formation of the Eastern Brook Trout [Charr] Joint Venture (Joint Venture) whose mandate is to restore these charr by working cooperatively at a range-wide, regional-scale. Joint Venture habitat projects in the U.S. are initiated by local community-based organizations, assisted by fisheries management institutions, and funded through the U.S. Fish and Wildlife Service. In order to assist with ranking these community-based proposals for available funding, the Joint Venture designed a quantitative scoring method as a decision support tool that addressed range-wide threats that were documented to limit brook charr production. These threats include habitat modification (e.g., dams, urbanization, and agricultural landuse) and interactions with antagonistic non-native species [i.e., brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss)]. This article evaluated the prioritization method used by comparing the quantitative rankings of the proposals with the final reports for 12 completed projects using paired t tests for unequal sample sizes and variance. Two of the 12 comparisons revealed statistically significant differences (P < 0.05) between project proposals and final reports; namely, the scoring criteria for the proposals did not accurately reflect the outcome of these projects in relation to brook charr management. Large inter-quartile ranges between reviewer scores were common, indicating that the quantitative scoring method includes qualitative elements. In addition to the science-based criteria, subjective value judgments by individual reviewers have factored into the Joint Venture’s decision making process. By evaluating the decision support tool, this analysis aims to assist the Joint Venture in structuring its approach to conservation and rehabilitation of brook charr populations in the eastern U.S. To better improve the Joint Venture’s ability to address brook charr management on a regional level, we recommend that future proposal prioritization scoring criteria provide more explicit guidance for accurate scoring, separate scoring methods for policy and assessment work, and a weighting factor for larger than sub-watershed projects and for scoring criteria for which information is unknown.     

Seasonal influence of brook trout and mottled sculpin on lower trophic levels in an Appalachian stream

1. In some situations fish have strong top‐down effects in stream communities while in others they seem to be relatively unimportant. Differences in the impact of fish may depend on a variety of factors including the foraging mode of the fish, interactions among fish species and temporal variation in environmental conditions and species interactions. 2. We investigated the effect of brook trout (Salvelinus fontinalis) and mottled sculpin (Cottus bairdi) on lower trophic levels in Appalachian streams and whether or not interactions between these fish changed their influence. Mesocosms were placed in a headwater stream in a randomized complete block design. Within blocks, mesocosms were randomly assigned to one of the following treatments: (i) no fish; (ii) sculpin only; (iii) trout only and (iv) both sculpin and trout. Fish biomass was the same in all three fish treatments. Invertebrate density and algal biomass in mesocosms were determined after 3 weeks. We repeated the experiment in the autumn, spring and summer to test for seasonality of fish effects. 3. The effect of fish on invertebrate assemblages was seasonal and depended on prey identity. Sculpin strongly suppressed grazer abundance in spring while trout had little effect on grazers in any season. The influence of both fish on insect predators was similar and relatively constant across seasons. We found little evidence of an interaction between sculpin and trout that strongly influenced their effect on prey across seasons. 4. None of the fish treatments influenced algal biomass during any of the seasons. Algal growth was also seasonal, with a two‐ to four‐fold increase in algal biomass in spring compared to autumn and summer. 5. Our results indicate that benthic and drift feeding fish differ in their effects on some, but not all prey. Furthermore, fish effects on prey were strongly seasonal for some, but not all prey types. While the temporal context is not commonly considered, our results indicate seasonality can be an important component of predator–prey interactions in streams.     

Stable isotope evidence of trophic interactions between introduced brook trout Salvelinus fontinalis and native brown trout Salmo trutta in a mountain stream of south-west France

The potential trophic impact of introduced brook trout Salvelinus fontinalis on native brown trout Salmo trutta in a mountain stream (south-west France) was investigated using stable isotope analysis (SIA). The isotopic signatures (δ¹³C and δ¹⁵N) of S. fontinalis were similar regardless of the absence or presence of S. trutta , and SIA mixing models revealed that S. fontinalis diet consisted mainly of terrestrial invertebrates. Conversely, a significant shift in S. trutta isotopic signatures (depletion of 1·6‰δ¹³C and enrichment of 0·6‰δ¹⁵N) was observed in sympatry with S. fontinalis ; this may be due to a dietary shift towards terrestrial invertebrates. Contrary to an expected dietary divergence in sympatry, an elevated level of dietary overlap was observed between the non-native and native salmonids when in co-occurrence. This dietary convergence is more likely to be due to behavioural interactions than to variations in food availability or fish displacements.     

Diel and density-related changes in food consumption and prey selection by brook charr in a New Hampshire stream

Synopsis We report the results of a field study testing influences of both density and changes over the diel cycle on food consumption and prey selection by brook charr,Salvelinus fontinalis. Charr density in replicate 35 m long sections of a New Hampshire stream was adjusted to either medium or high levels (relative to natural densities). Diets of charr and the availability of drifting prey were then sampled every four hours for 24 hours. There were no significant diel changes in the weight of prey consumed by charr per four hours, though there was some indication of reduced feeding at night. Chary fed selectively on different prey taxa, showing most preference for cased caddis larvae. Several species of mayflies and stoneflies were selected more strongly during the day than at night. Charr fed selectively on larger prey during the daytime but showed no size-selection at night. The density of charr had no significant effects on either their rate of food consumption or on selection for prey of different taxa or sizes.     

Epidemiology of Renibacterium salmoninarum in wild Arctic charr and brown trout in Iceland

Renibacterium salmoninarum (Rs) is common in wild Arctic charr Salvelinus alpinus and brown trout Salmo trutta in Iceland. Of 22 charr and nine trout populations none were free of Rs antigens. In two charr populations only one fish exceeded the Rs antigen detection limit and in one of these cases the ELISA value was within uncertainty limits of the infection criterion. Mean prevalence of infection was 46% for Arctic charr (range: 3–100%) and 35% for brown trout (range: 6–81%). No infected fish showed gross pathological signs of bacterial kidney disease (BKD). The ubiquity and high prevalences of infection indicated that the bacterium has been endemic for a long time, and is probably a normal, low density resident in the fish. A lack of correlation in mean intensity of Rs antigen and prevalence of infection between sympatricpopulations of Arctic charr and brown trout suggests that the dynamics of infection and internal proliferation of bacteria can be quite independent in the two species even if they live in the same lake. Rs intensity and its coefficient of variation decreased with age in older fish, suggesting a connection between Rs intensity and host mortality. However, this can be caused by other ecological factors that decrease survival, especially low food availability, which simultaneously increase the susceptibility to Rs infection and internal proliferation.     

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.     

Environmental influences on territoriality of juvenile brook charr,Salvelinus fontinalis,in a stream environment

Synopsis Juvenile brook charr, Salvelinus fontinalis, defending territories in a laboratory stream channel changed their defensive tactics when fish numbers or water current velocity were increased. The associated increase in energy costs of territorial defense were apparently reduced by using relatively more lateral displays and fewer chases to repel intruders, and by decreasing territory size. Territory size tended to increase with food ration, although levels of aggression were not affected. These results emphasize the importance of energetic considerations in territoral defense in these fish, and are discussed in terms of current hypotheses regarding the resource base for territoriality.     

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.