Fluctuating densities of juvenile brown trout (Salmo trutta) and Alpine bullhead (Cottus poecilopus) over a 25-year period in the Atna River, Norway

The density, diet and habitat use of brown trout (Salmo trutta) and Siberian sculpin (Cottus poecilopus) were studied in the subalpine River Atna in southeastern Norway in the autumn during a six year period (1986-1991). There was an inverse relationship between the density of brown trout and Siberian sculpin. Diet overlap, as indicated by the Schoener index, was high between the two species, ranging between 0.48 and 0.86. Chironomid larvae and other aquatic insects were the most common food items for both species. Brown trout also consumed substantive amounts of surface insects. Siberian sculpin typically occupied sites with finer substrates and greater water depths than brown trout, even though there was considerable overlap in habitat use between the two species. Because the two species shared similar habitats, we suggest that the potential for species interactions exists, particularly at sites where density of sculpin is high.

Age, growth and density of Siberian sculpin (Cottus poecilopus) and young brown trout (Salmo trutta) within two sections of River Atna; above Lake Atnsjoen [Section 1 at altitudes between 739 and 715 m] and below Lake Atnsjoen [Section 2 at altitudes between 430 and 370 m] was studied during a 6-year period (1986-91). The water temperature was considerably lower in Section 1 than in Section 2, as the number of days with a water temperature above 10 °C (TD> 10 °c ) from spring to August 1 ranged between 2-26 and 26-52 days, respectively. Juvenile brown trout (age 0+) attained a significantly smaller body size in Section 1 than in Section 2; mean length ±SD was 35 ± 8 mm (ranged 27-46) and 43 + 7 mm (range 38-46), respectively. In Section 2, there was a highly positive correlation between the body length of 0+ brown trout and mean water temperature in June (p 5-10°C) as test variables, we found a highly positive correlation between the August 1 body length of 0+ brown trout and TD>9 °C from spring to August 1 in Section 2 (p 7 °C for trout in Section 1 (p = 0.11). Young Siberian sculpin (age 0+ and 1+) also exhibited slower growth in Section 1 than in Section 2, but this was not the case among older specimens. In the year with the lowest temperature measured (1987), no 0+ Siberian sculpin were caught in any of the two sections, indicating that low temperature affects their survival. Both species exhibited large spatial and temporal variation in density. Thus, data on abundance and growth sampled on one occasion at one site can not be regarded as representative for these two fish populations.

Density and composition of benthic invertebrates and the diet of brown trout Salmo trutta and Alpine bullhead Cottus poecilopus were studied at two sites in one Norwegian stream. The sites were separated by an impassable waterfall, and brown trout density was five to 10 times higher at the upper, allopatric site than downstream where it lived in sympatry with the Alpine bullhead. Benthic invertebrate communities did not differ between sites; however, the size distribution of chironomids and trichopterans were skewed towards lighter individuals at the sympatric site. Diet composition suggested that sympatric brown trout foraged more on invertebrate drift and from the surface than allopatric brown trout. Alpine bullhead diet did not differ significantly from brown trout diet, except that the Alpine bullhead fed on heavier individual prey within a few taxa and did not consume chironomid pupae or surface insects. The collected data support the hypothesis that brown trout living in sympatry with Alpine bullhead feed at locations with higher predation risk, which is a probable explanation for their lower population density.

– In‐stream habitat was measured and trout density was estimated in Merrick Brook (105 habitat units) and the Tankerhoosen River (135 habitat units), Connecticut to determine relationships between habitat use of brook trout Salvelinus fontinalis and brown trout Salmo trutta and woody debris. In each habitat unit, woody debris was inventoried, and length, width, depth, area, width : depth ratio and undercut bank area were estimated. Trout abundance was estimated by snorkeling. Multiple regression was used to test relationships between trout density and principal components describing habitat unit variables. In Merrick Brook, habitat unit size and shape explained most of the variability in density of brook trout (<130 and ≥130 mm) and brown trout (<150 mm) among habitat units, although principle components describing large woody debris or fine woody debris contributed significantly to variations in density of brook trout (≥130 mm) and brown trout (<150 and ≥150 mm). In the Tankerhoosen River, fine woody debris explained most of the variability in density of brook trout (<130 and ≥130 mm), followed by habitat unit size and shape. Both large woody debris and fine woody debris contributed significantly to variations in density of brown trout (≥150 mm). These results suggest that woody debris is an important component of wild trout habitat above that provided by habitat unit shape and size alone.

Recovery of young brown trout (Salmo trutta) in acidified streams: What are the critical values for acid-neutralizing capacity?

We examined the relationship between young brown trout ( Salmo trutta) density in lake tributaries, and water chemistry and habitat variables. The study was carried out during the autumn in three acidic, softwater river systems in western and southwestern Norway; Gaular and Vikedal (1987–1993) and Bjerkreim (1988–1993). The streams had mean calcium concentrations of 0.35 mg L-1 (Gaular), 0.52 mg L-1 (Vikedal) and 0.84 mg L-1 (Bjerkreim). The concentration of inorganic Al was generally low, with mean values of 8.40 (Gaular), 22.22 (Vikedal) and 43.36 μg L-1 (Bjerkreim). In multiple regressions that involved different water chemistry variables, brown trout density correlated best with calcium concentration and with a combination of calcium and pH; the Ca2+:H+ ratio. In Vikedal and Gaular, calcium explained 51 and 57%, respectively, of the variability in brown trout densities. Althoug alkalinity exhibited the best correlation with brown trout density in Bjerkreim ( r2=0.33), it was similar to that of the model that included all major ions plus pH. The Ca2+:H+ ratio had a larger effect for variability in brown trout density in Gaular (r2=0.66) than calcium alone. In Vikedal and Bjerkreim, the Ca2+:H+ ratio also correlated with brown trout density, but considerably less than in Gaular. The predictive power of habitat variables was much lower than that of water chemistry; the single most important factors were altitude in Gaular (r2=0.22), mean water temperature in Vikedal (r2=0.11) and depth SD (index of heterogeneity) in Bjerkreim (r2=0.07). Models that included both habitat and water chemistry variables showed that the density of young brown trout was predicted primarily by calcium concentrations in Gaular (r2=0.75) and Vikedal (r2=0.54), as opposed to pH in Bjerkreim (r2=0.25). Habitat had low effect in all three river systems (r2=0.01–0.04). The final model explained 86, 68 and 32%, respectively, of the variability in brown trout density in the three catchments. Thus, water chemistry variables seem to be factors that limit the density of young brown trout in acidic softwater streams.

In freshwater streams, flooding is a typical source of natural disturbance that plays a key role in the dynamics of animal populations and communities. However, habitat degradation and fish stocking might increase the severity of its impact. We tested the effects of a flash flood on the abundance of three size classes of headwater dwelling Alpine bullhead, Cottus poecilopus, in the streams of the Carpathian Mountains in the Czech Republic, that are stocked with hatchery-reared brown trout, Salmo trutta. We showed that the overall abundance of Alpine bullhead was highest at the sites with the least degraded habitat (i.e., natural habitat) and we caught almost no Alpine bullhead at the sites with the most degraded habitat. The flash flood had a strong negative effect on the abundance of the largest individuals of Alpine bullhead. Abundance of small and medium size Alpine bullhead was negatively affected by the abundance of adult stocked brown trout before as well as after the flash flood. However, negative effect of adult brown trout abundance on abundance of large Alpine bullhead was not significant before the flash flood, and it became significant after the flash flood. This could indicate an accumulation of negative impacts of trout stocking and flash flood on this size class. Overall, our results suggest that stocking of hatchery trout and habitat degradation can reinforce the impact of flash floods on the population of Alpine bullhead in the streams of the Carpathian Mountains.

The reproductive success of brown trout Salmo trutta in White River, Arkansas, tailwater reaches is highly variable, resulting in the need for supplemental stocking. A better understanding of the physical and biotic factors affecting reproduction and survival would enhance the contribution of wild fish. We compared fecundity, reproductive chronology, physical habitat, water quality, trout density, food availability, diet, predation, and competitive interactions among four tailwater reaches to identify factors influencing brown trout reproductive success. The fecundity and condition factor of prespawning brown trout were significantly lower at Beaver Tailwater, a reach known for reproductive failure, than at other sites, among which no differences were found. Brown trout spawning was observed from 11 October to 23 November 1996, and juvenile emergence began on 28 February 1997. Significant among-site differences were detected for spawning and juvenile microhabitat variables, but the variables fell within or near suitable or optimal ranges reported in the literature for this species. Age-0 brown trout density differed significantly among sites, but growth and condition did not. Predation by Ozark sculpin Cottus hypselurus on trout eggs or age-0 trout of any species was not observed among the 418 stomachs examined. Ozark sculpin density and diet overlap with age-0 brown trout were highest and invertebrate food availability and water fertility were lowest at Beaver Tailwater relative to the other reaches. Our findings indicate that differences in trophic conditions occur among tailwater reaches, and a lower system productive capacity was identified at Beaver Tailwater. We suggest that management efforts include refining the multispecies trout stocking regime to improve production efficiency, enhancing flow regulation, and increasing habitat complexity to increase invertebrate and fish productivity. Such efforts may lead to improved natural reproduction and the increased abundance and quality of wild trout in this system and other regulated rivers.

The density of juvenile trout is influenced by many factors, including water temperature, discharge, substrate composition, and the presence of aquatic vegetation. The objective of this study was to determine habitat conditions that influence densities of age‐0 brown trout Salmo trutta and brook trout Salvelinus fontinalis in the Au Sable River, Michigan. Electrofishing was conducted with a 250‐V DC tote barge at 12 sites throughout the Au Sable River watershed from June through August of 2000 and 2001. Densities of age‐0 brown trout and brook trout varied significantly among sites located on the main stem as well as in tributaries. Analysis of covariance was used to evaluate annual interactions between age‐0 trout densities and habitat variables. Simple and forward stepwise linear regressions were used to model brown trout and brook trout densities in relation to habitat variables. Percent gravel substrate and percent emergent vegetation accounted for 62% of the variance in age‐0 brown trout density. In two separate analyses, mean minimum water temperature accounted for 31% of the variance in the density of age‐0 brook trout, whereas mean daily water temperature fluctuation accounted for 28% of the variance. Once habitat conditions that influence age‐0 fish production are identified, stocking programs should focus on areas where age‐0 fish would be able to survive. If no suitable juvenile habitat is available, managers should instead invest effort in enhancing or creating nursery habitat. Such efforts will not only be valuable to the growth and survival of stocked individuals but will also have the benefit of enhancing the potential for the fishery to be sustained through natural reproduction.

Population densities, size and condition factor of juvenile brown trout and variation in environmental factors (discharge, temperature, and food availability) were examined over 2.5 years (28 months) in an upstream and downstream reach of Silverstream, a lowland river in New Zealand. Population densities of juvenile brown trout varied between the upstream and downstream reach, with the latter showing considerably higher temporal variation in fish density than the upstream reach. Juvenile brown trout from the downstream reach were larger and had higher condition factors than those from the upstream reach over the study period. Discharge had a negative effect on fish densities in 1 year in the downstream reach, and high water temperatures were recorded in one summer, with possible negative effects on fish densities. However, the different effects of environmental variables on populations between the two reaches could not explain the higher temporal variation in fish densities in the downstream reach as spatial variation in population densities also occurred in years without high discharge events or high summer water temperatures. Variability in food availability over time was higher in the upstream reach, but differences between the two reaches were small. These results suggest that factors other than condition of the local environment are important in controlling brown trout population dynamics in Silverstream.

SummaryThe importance of terrestrial carbon in aquatic ecosystems is widely recognised, but patterns of terrestrial reliance can be variable. Fish often act as important links between terrestrial and aquatic ecosystems, but little is known about how resource seasonality and fish density influence fish reliance on terrestrial energy in lakes.We sampled a high‐latitude subarctic lake in Finnish Lapland during the open‐water season over three consecutive years to assess both patterns of terrestrial reliance and trophic niche structure of introduced brown trout (Salmo trutta), the only resident fish species. The small size of the study lake made it possible to sample the whole population by conducting a complete fish removal, allowing for a direct assessment of size structure and changes in brown trout density over time.We hypothesised that annual and seasonal shifts in the dietary niches of brown trout would directly track the availability of pulsed resources such as aquatic and terrestrial insects as well as rodents. We further expected that dietary niche shifts would be correlated with population density, leading to a smaller trophic niche size at lower densities. We therefore investigated the annual and seasonal patterns of resource use using measures of dietary niche and in particular of terrestrial reliance, derived from stomach content analysis and stable‐isotope analyses (SIA) of liver and muscle, along a temporal gradient of declining fish density.According to stomach content, terrestrial reliance in brown trout was the highest in each year at mid‐to‐late summer, evidently following the peak abundance of terrestrial invertebrates and rodents. Surprisingly, we could not detect annual or seasonal shifts in terrestrial reliance from estimates provided by isotope ratios in muscle or liver. Furthermore, fish density did not appear to influence either terrestrial reliance or trophic niche size. However, trophic position derived fromSIAof liver tissue decreased with decreasing densities, while fish condition increased.Large, consistent pulses of terrestrial invertebrates in mid‐summer (or rodents during their peak years) are likely important for brown trout in the long term and could explain the lack of density‐dependent correlation in terrestrial reliance. However, further studies are needed to link the abundance of pulsed resources to resource use by fish across wider gradients of lake size, productivity and fish density.

The Biotic–Abiotic Constraining Hypothesis (BACH) suggests that biotic interactions can have an overriding negative influence on a fish species, even when abiotic conditions are suitable. The abundance of a fish species is predicted to be largely regulated by abiotic habitat characteristics when densities of predators or competitors are low. However, when predator or competitor densities are high, the abundance of the fish species is suppressed by biotic interactions regardless of environmental conditions. We used data from the Salt River basin (Idaho–Wyoming) to investigate whether the BACH could be used to explain patterns in the density of cutthroat trout Oncorhynchus clarkii across a watershed. Using a combination of principal components and nonlinear multiple‐regression analyses, we determined that the BACH was useful for explaining patterns in cutthroat trout density. When brown trout Salmo trutta and brook trout Salvelinus fontinalis densities were low, cutthroat trout density was highly variable and was best explained by habitat characteristics. High cutthroat trout densities were found in high‐gradient reaches with little fine substrate and a low proportion of deep‐pool habitat (i.e., reaches with a diversity of pools, riffles, and runs). When brown trout and brook trout densities were high, cutthroat trout density was always low despite favorable habitat conditions. Brown trout density was highest in low‐elevation reaches with warm water temperatures and abundant instream cover, whereas brook trout density was highest in high‐elevation reaches with low water temperatures. This study suggests that the influence of brown trout and brook trout densities overrode the influence exerted by habitat conditions on cutthroat trout density in the Salt River basin and that the BACH is useful for explaining biotic and abiotic influences on lotic cutthroat trout populations.

We investigated electrofishing catchability (q) for brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss in the Colorado River, Grand Canyon National Park, Arizona, over a range of fish densities, water temperatures, turbidities, conductivities, shoreline types, and seasons. The covariance of q with rainbow trout density strongly resembled random distributions, thereby suggesting no relationship between q and rainbow trout density. The catchability of rainbow trout was greater in turbid water (≥480 nephelometric turbidity units (NTU)) than in clear water (≤10 NTU), although lower water temperature may have contributed to this effect. The catchability of rainbow trout was greatest over sand–silt shorelines. The catchability of brown trout increased sharply to levels above those predicted from random chance up to about 0.025 fish/m2 and then assumed an asymptotic or declining relationship with intraspecific fish density. In contrast to the situation with rainbow trout, the catchability of brown trout was higher over rocky shorelines (cobbles, boulders, and bedrock) than sand–silt shorelines, suggesting that the variability of q in relation to shoreline type is species specific. We hypothesize that the catchability of rainbow trout is influenced more by environmental variables than by density. We also hypothesize that brown trout catchability varies with density because a greater proportion of fish occur in shallow, nearshore areas (where electrofishing is most effective) when fish density is high. This effect is enhanced by high catchability over rocky substrates. Our findings emphasize the need to understand the biological and environmental factors affecting electrofishing catchability, especially in monitoring programs that rely on catch-per-unit-effort data to accurately represent fish population status and trends.

The purpose of this paper is to describe the annual variations in the density of an unexploited population of lotic brown troutSalmo truttathat has been censused annually for 19 years and to explore the importance of density‐independent and density‐dependent processes in regulating population size. Brown trout density and indices of stream discharge and water temperature were related to annual variations in natural mortality, recruitment, and growth. Annual mortality of age‐1 and older (age‐1+) brown trout ranged from 0.30 to 0.75 and was best explained by discharge during spring and by brown trout density. Recruitment to age 1 varied fivefold. Density of age‐1 brown trout was inversely related to spawner density and positively related to discharge during the fall spawning period. The median length of age‐1 brown trout was positively related to discharge during summer and fall. Relative weight was inversely related to the density of age‐2+ brown trout. The interactive effects of discharge and brown trout density accounted for most of the annual variation in mortality, recruitment, and growth during the first year of life. Annual trends in the abundance of age‐1+ brown trout were largely dictated by natural mortality.

Overcoming the dichotomy of implementing societal flood risk management while conserving instream fish habitat – A long-term study from a highly modified urban river