Limestone Treatment of Whetstone Brook, Massachusetts. II. Changes in the Brown Trout (Salmo trutta)and Brook Trout (Salvelinus fontinalis)Fishery

Density, age structure, and growth rates of wild brook trout (Salvelinus fontinalis)and brown trout (Salmo trutta)in Whetstone Brook in northcentral Massachusetts were monitored for 4 years before and 3 years during limestone treatment to mitigate acidic conditions. The population density of brook trout increased significantly during treatment. Liming did not have any significant effects on the growth rates of brook trout or brown trout. Actual survival rates of brook trout and brown trout were not calculated due to the low density of both species, but more older individuals of both species were captured during the treatment period. Fulton condition factors (an index of fish condition) increased significantly for both brook trout and brown trout during treatment. Seven-day in situ bioassays of brown trout and rainbow trout demonstrated that liming improved the chemical environment for fish in Whetstone Brook. During a pretreatment bioassay in 1987, 100% rainbow trout mortality was observed at both the control and treatment stations in Whetstone Brook. Brown trout mortality was 67% in the control station and 70% in the treatment station. The pH during the 1987 bioassay averaged 4.90 in the control station and 4.99 in the treated station. During a bioassay conducted in 1990 after treatment began, rainbow trout mortality was 100% in the control station and 0% in the treatment station. Brown trout mortality was 17% in the control station and 0% in the treatment station. The pH during the 1990 bioassay averaged 5.23 in the control station and 6.60 in the treatment station. Analysis of total aluminum in the gills of fish from the 1990 bioassay revealed higher levels in fish from the control station than in those from the treatment station.     

Differences in the sensitivity of brown trout, Salmo trutta L., and rainbow trout, Salmo gairdneri Richardson, to physiological doses of cortisol

Interspecific differences in the stress response of fish may be due, in part, to differences in the sensitivity of target tissues to cortisol. The relative response of brown and rainbow trout to a standardized dose of cortisol was assessed by monitoring condition (K factor), the number of circulating lymphocytes and mortality due to disease, following cortisol treatment. Cortisol implantation resulted in a significant decline in K factor and number of circulating lymphocytes in immature brown trout, but not in immature rainbow trout, despite plasma cortisol levels being similar in both cases. Cortisol implantation in mature brown and rainbow trout significantly increased the mortality rate due to bacterial and fungal infection compared with control fish. Furthermore, the mortality rate due to disease was significantly greater in brown trout than rainbow trout, despite both groups receiving similar doses of steroid.     

Morphological organ alterations and infectious diseases in brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss exposed to polluted river water

Poor water quality is discussed as a major factor causing a decline of brown trout populations in Swiss rivers. For our study we have chosen a river in the Swiss midlands, where the brown trout population has decreased dramatically during the last 10 yr and where feral fish have shown distinctive pathological alterations. The objective of our study was to investigate whether river water may be responsible for impaired fish health leading to an increased mortality in the river. In an active monitoring program, groups of brown and rainbow trout were exposed to polluted river water for 24 mo. Fish held in tap water served as a reference. Mortality, macroscopic and histopathologic changes, and infectious agents were investigated. Compared with the reference group, high mortality rates and severe pathological alterations of the inner organs were observed in fish held in river water. Especially gills, liver and kidney of these fish showed significantly higher changes than fish from tap water. These changes were dominated by degenerative and inflammatory reactions. Additionally, several infectious agents were diagnosed in fish exposed to river water. The most important findings were furunculosis and proliferative kidney disease. Brown trout seemed to be more sensitive than rainbow trout to environmental stress and infectious agents.     

Stocking hatchery‐reared brown trout in different densities into a wild population – a comparison of growth and movement

In spring 2001 and 2002 a small stream was stocked with tagged hatchery‐reared yearling brown trout ( Salmo trutta ), in order to study their influence on the resident brown trout population. The stream was separated into six sections: two sections without stocking, two sections where stocking doubled the trout population and two sections where the fish population was quadrupled. The working hypothesis was that due to food limitation (competition) growth of the wild fish will be negatively influenced by stocking, and wild fish will be displaced by the (possibly more aggressive) hatchery fish. Surprisingly, growth rate of wild and stocked fish of the same age was similar and independent of stocking density. Two main reasons may be responsible for this finding: only a low percentage of the stocked fish remained in the stream, and food was not limited during summer. Only 12–19% of the stocked fish were recaptured after six months, in contrats to 40–70% of one‐year old and up to 100% of older wild trout. The wild fish were not displaced by hatchery‐reared fish: During summer the wild fish remained more or less stationary, whereas most of the stocked trout had left their release site. The results indicate that in a natural stream stocking of hatchery reared brown trout does not influence negatively growth and movement of the wild fish independent of stocking density.     

The impact of an introduced predator on a threatened galaxiid fish is reduced by the availability of complex habitats

1. The availability of complex habitats such as macrophytes may be vital in determining the outcomes of interactions between introduced predators and native prey. Introduced brown trout (Salmo trutta) have impacted numerous small native freshwater fishes in the southern hemisphere, but the potential role of complex habitats in determining the direct outcomes of brown trout – native fish interactions has not been experimentally evaluated. 2. An in‐lake enclosure experiment was used to evaluate the importance of structurally complex habitats in affecting the direct impacts of brown trout on a threatened galaxiid fish. Five Galaxias auratus and a single brown trout were added to enclosures containing one of three different habitat types (artificial macrophytes, rocks and bare silt substrate). The experiment also had control enclosures without brown trout. Habitat‐dependence of predation risk was assessed by analysis of G. auratus losses to predation, and stomach contents of remaining fish were analysed to determine if brown trout directly affect the feeding of G. auratus and whether this is also habitat‐dependent. 3. Predation risk of G. auratus differed significantly between habitat types, with the highest mortality in enclosures with only bare silt substrate and the lowest in enclosures containing artificial macrophytes. This result highlights the importance of availability of complex habitats for trout – native fish interactions and suggests that increasing habitat degradation and loss in fresh waters may exacerbate the direct impacts of introduced predators. 4. Stomach contents analyses were restricted to fish in enclosures with artificial macrophytes and rocks, as most fish were consumed in enclosures with brown trout and only bare silt substrate. These analyses suggest that brown trout do not directly affect the feeding of G. auratus in complex habitats, but it is still unknown whether its feeding is reduced if complex habitats are unavailable.     

Habitat use and life history of brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) in some low acidity lakes in central Norway

Habitat utilization and the life history of browntrout Salmo trutta and Arctic charr Salvelinus alpinus were investigated in fivesympatric populations and five allopatric brown troutpopulations in Høylandet catchment, a atmosphaericlow deposition area in Mid Norway. There was asignificant inverse correlation in abundance ofepibenthic Arctic charr and brown trout in theselakes, indicating that the latter species is dominant.The largest numbers of sympatric brown trout andArctic charr were caught in epibenthic habitat. In twolakes, brown trout to some extent also occurredpelagically, while pelagic individuals of Arctic charrwere found in all five lakes. The main food items forboth epibenthic and pelagic brown trout wereterrestrial surface insects and chironomid pupae.Zooplankton was the primary food item for Arctic charrin both habitats. Although the age distribution wasvery different in the populations, neither speciesseem to suffer from recruitment failure. There was nosignificant difference in survival rates betweensympatric populations of brown trout and Arctic charr.We found a significant inverse correlation betweenepibenthic catches of brown trout and the mean weightof 4+ fish, the most abundant age group. However, ifusing weight data for three-year-old fish, no suchrelationship was found for Arctic charr. Brown troutand Arctic charr reached asymptotic lengths of197–364 mm and 259–321 mm, respectively. Both speciestypically reached sexual maturity at age 2–3, and nomaturation-induced mortality was evident. We concludethat fish populations in Høylandet lakes areregulated throughout their lifes by inter- andintraspecific competition.     

Redistribution of juvenile salmonid fishes after localized catastrophic depletion

Juvenile Atlantic salmon and brown trout were depleted at three sites ( c . 108–380 m²) of a natural stream during the summer months of 1991 and 1992. Local population changes and movements of fish marked in sections adjacent to each depleted area were monitored thereafter. There was very little movement of marked salmon parr into the central regions of the depleted areas following the immediate post-marking period. Upstream movement by young-of-the-year fish from high density sections in mid-late summer was noted for trout but not salmon. Unmarked 1-year-old salmon parr immigrated into depleted areas in June 1992, and the pattern of recolonization was consistent with migration upstream from the adjoining river. It is concluded that resident salmon were very strongly site-attached and resource tracking was of no functional significance as a compensatory mortality mechanism. The occurrence of a long distance migratory component in the population during early-mid summer indicates that this, rather than local resource tracking, constitutes a potential compensatory mechanism.     

Effect of mink, Mustela vison Schreber, predation on cohorts of juvenile Atlantic salmon, Salmo salar L., and brown trout, S. trutta L., in three small streams

Two cohorts of Atlantic salmon parr and one of brown trout were studied in periods with and without the presence of mink, Mustela vison . In all localities a marked increase in mortality rate was observed during periods when mink were present. Mink were observed catching salmon parr, and approximately 10% of the parr had bite marks, especially on the tail fins. In the smallest stream with brown trout, the mortality rate was 0.80 during a few days with mink present; remnants of trout were found along the stream. The present study suggests that mink predation may be a major cause of mortality of salmonids in small streams. The results indicate that predation efficiency may vary with characteristics of the habitat, especially stream width and discharge, and fish density.     

A 27-year study of brown trout population dynamics and exploitation in Lake Songsjøen, central Norway

From 1968–1984 (period I), a brown trout Salmo trutta , population in a 70-ha oligotrophic lake in central Norway was exploited using larger mesh gill-nets selectively removing the larger fish. From 1985–1994 (period II), intermediate sized fish were removed using smaller-mesh sizes gill-nets. Fishing mortality and CPUE were correlated positively with effort and numbers of fish >3 years old for period II. The gill-net catchability was correlated negatively with spawner biomass and number of trout >3 years old. The significant positive correlation between natural mortality and stock biomass and spawning stock biomass indicated density-dependent mortality. The significant correlation between spawning stock and recruitment described by the Ricker model, indicated density-dependent recruitment of 1-year-old trout. The fishing regimes in the two periods affected the population dynamics and density differently. Selective removal of smaller fish permitted the larger fish to survive, and was beneficial in reducing fish density and maintaining stocks at low levels, consequently, achieving the expected increase in fish growth rates.     

Comparative susceptibility of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta to Myxobolus cerebralis, the cause of salmonid whirling disease.

The susceptibility of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta to Myxobolus cerebralis, the cause of salmonid whirling disease, was assessed following dosed exposures to the infectious stages (triactinomyxons). Parallel groups of age-matched brown trout and rainbow trout were exposed to 10, 100, 1000 or 10,000 triactinomyxons per fish for 2 h and then placed in aquaria receiving single pass 15 degrees C well water. Severity of infection was evaluated by presence of clinical signs (whirling and/or black tail), prevalence of infection, severity of microscopic lesions, and spore counts 5 mo after exposure. Clinical signs of whirling disease, including a darkened caudal region (black tail) and radical tail chasing swimming (whirling), occurred first among rainbow trout at the highest dose at 6 to 7 wk post exposure. Black tail and whirling occurred among rainbow trout receiving 1000 and 100 triactinomyxons per fish at 8 to 9 wk post exposure. Only 1 of 20 fish had a black tail among rainbow trout receiving 10 triactinomyxons per fish, although 30% of the fish were infected at 5 mo post exposure. Black tails were observed in brown trout at 1000 and 10,000 triactinomyxons per fish beginning at 11 and 7 wk post exposure, respectively. There was no evidence of the tail chasing swimming (whirling) in any group of brown trout. The prevalence of infection, spore numbers, and severity of microscopic lesions due to M. cerebralis among brown trout were less at each exposure dose when compared to rainbow trout. Infections were found among rainbow trout at all doses of exposure but only among brown trout exposed to doses of 100 triactinomyxons per fish or greater. Risk of infection analyses showed that rainbow trout were more apt to be infected at each exposure dose than brown trout. Spore counts reached 1.7 x 10(6) per head among rainbow trout at the highest dose of exposure compared to 1.7 x 10(4) at the same exposure dose among brown trout. Spore numbers increased with dose of exposure in rainbow trout but not in brown trout. As microscopic lesion scores increased from mild to moderate, spore numbers increased in rainbow trout but not brown trout. The mechanisms by which brown trout resist infections with M. cerebralis were not determined. Cellular immune functions, including those of eosinophilic granular leukocytes that were more prominent in brown trout than rainbow trout, may be involved.     

Distribution of the flagellate Hexamita salmonis Moore, 1922 and the microsporidian Loma salmonae Putz, Hoffman and Dunbar, 1965 in brown trout, Salmo trutta L., and rainbow trout, Salmo gairdneri Richardson, in the River Itchen (U.K.) and three of its fish farms

The occurrence of Hexamita salmonis Moore, 1922 and Loma salmonae Putz, Hoffman and Dunbar, 1965 was investigated at 10 sites on the R. Itchen (five for brown trout only, three for rainbow trout only, and two for both brown trout and rainbow trout) and at three of its nine fish farms (two for rainbow trout, one for brown trout). Hexamita salmonis was recorded in brown trout from three river sites and the farm, and in rainbow trout from both farms and four river sites. Prevalence of Hexamita salmonis in farmed rainbow trout was higher than in farmed brown trout and was consistent with the former species being more susceptible to infection. H. salmonis was at significantly higher prevalence in rainbow trout from farm no. 5 than farm no. 2 for three size classes of fish. In wild brown trout and feral rainbow trout, the highest prevalences of H. salmonis were recorded at sites in the vicinity of farm no. 2. This distribution was consistent with an area of naturally high infection levels, and with infected fish unintentionally released from farm no. 2 serving as a source of infection, the infection subsequently becoming established in the river fish. Loma salmonae was recorded in wild brown trout and in rainbow trout from both farms. This appears to be the first recording of this parasite from British salmonids and also the first recording of the parasite from brown trout. The distribution of the parasite (particularly the prevalence being higher at farm no. 2 than farm no. 5) was consistent with it being introduced into the R. Itchen via rainbow trout from farm no. 2 (and probably no. 3) much of whose stock derived from imported Californian 'Shasta' rainbow trout.     

Density‐dependent growth in hatchery‐reared brown trout released into a natural stream

Hatchery‐reared brown trout Salmo trutta stocked in a natural stream in addition to resident wild brown trout grew more slowly than those stocked with an experimentally reduced density of brown wild trout. In both cases, hatchery‐reared brown trout grew more slowly than resident wild fish in control sections. Mortality and movements did not differ among the three categories of fish. The results showed that growth of stocked hatchery‐reared brown trout parr was density‐dependent, most likely as a consequence of increased competition. Thus, supplementary release of hatchery‐reared fish did not necessarily increase biomass.     

Predation and the prey community of a headwater stream

SUMMARY 1. Predatory, net-spinning larvae of the caddis Plectrocne-mia conspersa (Curtis) were abundant in the acid headwaters of some southern English streams where fish were absent, but were scarce or absent downstream where brown trout ( Salmo trutta L. ) occurred.
2. Field enclosure experiments showed that both underyearling and older brown trout reduced the density of P. conspersa . However, whereas small trout affected the overall density of caddis, older fish reduced that of large larvae only.
3. Although P. conspersa is itself an important invertebrate predator there was little evidence of an indirect effect of brown trout on the prey of P. conspersa . Perhaps the diets of brown trout and P. conspersa are so similar that fish simply replaced the caddis as top predator.     

Possible reasons for late summer brown trout (Salmo trutta Linnaeus 1758) mortality in Austrian prealpine river systems

For years, severe mortality of brown trout (Salmo trutta) in late summer has been reported in the prealpine river systems of Austria. For an initial understanding of the potential reasons for this mortality, brown trout were exposed to water under differing conditions from an affected river system. Blood parameters, histology of various organs, hepatic ethoxyresorufin‐O‐deethylase (EROD) activity, and occurrence of ectoparasites and fish diseases were investigated. In brown trout exposed to water from the affected river, concentrations of peripheral blood lymphocytes, granulocytes, plasma immunoglobulin and plasma lyszoyme activity decreased significantly, indicating a reduction in unspecific, innate and specific, adaptive immune response. Erythropoietic and thrombopoietic functions were also disturbed, with fish mortalities up to 100%. From the results of the present study it seems unlikely that ectoparasites, diseases, or effluent water from sewage plants were causative for the observed phenomenon. However, brown trout maintained in river water and in the dark survived longer than those under natural light conditions, and their immune systems were less drastically affected. As periods of high solar UV‐radiation occurred during the experiment and the Traun River water temperature was quite variable, these factors might have had an effect on immune systems. Laboratory studies were therefore conducted to ascertain whether water temperature variations and/or UV could indeed affect the immune system. The tested temperature variations (10°C/d) suppressed the brown trout immune system, whereas UV radiation (0.045 mW/m² for 6.5 h per day) had no effect. However, UV‐radiation did enhance the immune‐suppression effect of variable temperatures.     

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.     

Active biomonitoring with brown trout and rainbow trout in diluted sewage plant effluents

Brown trout Salmo trutta populations of numerous Swiss rivers are declining. Sewage plant effluents are discussed as a possible cause. To investigate the influence of sewage plant effluents, brown trout as well as rainbow trout Oncorhynchus mykiss were exposed to 10% diluted waste water over a period of 12 months. The effects were compared to those on trout kept in commercial tap water. The mortality rate was low and no pathogenic bacteria or viruses were recorded in exposed and tap-water animals. Parasitological examination revealed a mild infestation with Gryodactylus sp. in all groups. Macroscopically and histologically, only minor changes in gills, skin, and kidney of exposed animals were found when compared to fish kept in tap water. Degenerative and inflammatory reactions in the liver of exposed animals were the most prominent findings. Several brown trout caught in the River Langete showed marked proliferative, degenerative and inflammatory lesions of gills, liver, and kidney. The results do not suggest that waste-water effects would explain the decrease of fish populations. However, it is conceivable that the effluents in combination with other factors in the river enhance the development of changes.     

Transmission of Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea) to Fredericella sultana (Bryozoa: Phylactolaemata) by various fish species

Tetracapsuloides bryosalmonae is a myxozoan parasite of salmonids and freshwater bryozoans, which causes proliferative kidney disease (PKD) in the fish host. To test which fish species are able to transmit T. bryosalmonae to bryozoans, an infection experiment was conducted with 5 PKD-sensitive fish species from different genera. Rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta, brook trout Salvelinus fontinalis, grayling Thymallus thymallus and northern pike Esox lucius were cohabitated with T. bryosalmonae-infected Fredericella sultana colonies and then subsequently cohabitated with statoblast-reared parasite free Bryozoa. Statoblasts from infected colonies were tested by PCR to detect cryptic stages of T. bryosalmonae, which may indicate vertical transmission of the parasite. In this study, brown trout and brook trout were able to infect Bryozoa, while there was no evidence that rainbow trout and grayling were able to do so. Few interstitial kidney stages of the parasite were detected by immunohistochemistry in brown trout and brook trout, while rainbow trout and grayling showed marked proliferation of renal interstitial tissue and macrophages with numerous parasite cells. Intraluminal stages in the kidney tubules were only detected in brown trout and rainbow trout. In contrast to previous observations, pike was not susceptible to PKD in these trials according to the results of T. bryosalmonae-specific PCR. No DNA of T. bryosalmonae was detected in any statoblast.     

A Bioenergetic Analysis of Factors Limiting Brown Trout Growth in an Ozark Tailwater River

We examined prey utilization and energy consumption by brown trout, Salmo trutta, in a cold tailwater (Little Red River, Arkansas, USA; LRR) having low biodiversity and low availability of fish as prey. Stomach content analysis and age estimation were performed on thirty brown trout (10 each of three size classes for a total of 710 trout) collected monthly from an upstream and downstream site over a 1-year period. Diet diversity was low at both sites, as 80% and 70% of all prey consumed by upstream and downstream brown trout, respectively, were isopods. Piscivory (<0.5% of individuals sampled) and consumption of terrestrial invertebrates were rare. There was no relation between diet diversity and trout age, and a very small ontogenetic shift in brown trout diet. Second, we investigated brown trout growth rates relative to prey consumption and temperature. Temperatures and availability of prey were less than required for maximal trout growth. However, prey availability limited trout growth directly, but sub-optimal temperatures probably buffered the effect of this reduced energy consumption by reducing metabolic energy expenditures. Brown trout growth was 54.8–57.0% of the maximum predicted by a bioenergetics model. Instantaneous growth rates for age 1 and adult brown trout were slightly higher for those downstream (0.195) versus those upstream (0.152). Although isopods are abundant within this tailwater to serve as a forage base, the displacement of native fish fauna and subsequent lack of establishment of cold-tolerant forage fish species due to the thermal regime of hypolimnetic release from Greers Ferry Reservoir probably serves as a major barrier to brown trout growth.     

Continuous variation in the pattern of marine v. freshwater foraging in brown trout Salmo trutta L. from Loch Lomond, Scotland

Carbon stable-isotope analysis showed that individual brown trout Salmo trutta in Loch Lomond adopted strategies intermediate to that of freshwater residency or anadromy, suggesting either repeated movement between freshwater and marine environments, or estuarine residency. Carbon stable-isotope (δ¹³C) values from Loch Lomond brown trout muscle tissue ranged from those indicative of assimilation of purely freshwater-derived carbon to those reflecting significant utilization of marine-derived carbon. A single isotope, two-source mixing model indicated that, on average, marine C made a 33% contribution to the muscle tissue C of Loch Lomond brown trout. Nitrogen stable isotope, δ¹⁵N, but not δ¹³C was correlated with fork length suggesting that larger fish were feeding at a higher trophic level but that marine feeding was not indicated by larger body size. These results are discussed with reference to migration patterns in other species.     

The status of brown and rainbow trout, Salmo trutta and S. gairdneri as hosts of the acanthocephalan, Pomphorhynchus laevis

The status of brown and rainbow trout as hosts of Pomphorhynchus laevis was studied in the field and by means of laboratory investigations. Field data indicated that rainbow trout might belong to the group of preferred hosts of P. laevis , whereas brown trout belonged to the group in which the parasite achieved less than optimal growth and maturation. This was confirmed by laboratory infections. In rainbow trout P. laevis attained up to three times the growth rate in brown trout and maturation occurred whereas in brown trout establishment was lower, growth slower and no parasites matured. Changes in the behaviour of infected Gammarus pulex induced by the presence of P. laevis cystacanths were such as to render the shrimps more vulnerable to predation by trout and other surface and mid-water feeding fish, and selective predation upon infeged G. pulex by fish was demonstrated. nvestigations into the stimuli necessary for eversion of cystacanths of P. laevis revealed that the most important factor was a non-specific component of bile, and it was concluded that cystacanths were likely to evert in any species of fish. Recognition of the different status of brown and rainbow trout as hosts of P. laevis still fails to explain some peculiarities in the distribution of the parasite in the British Isles, where in Britain it occurs in trout in only one river but in Ireland in all rivers throughout the country. It is suggested that the Irish parasites may constitute a different strain of P. laevis , since they use a different species of intermediate host and are better able to survive in brown trout.