Assessing the status of British wild brown trout, Salmo trutta, stocks: a pilot study utilizing data from game fisheries

SUMMARY. 1. This paper describes an analysis of results from a questionnaire (Giles, 1987) which requested information on the status of game fish stocks (especially wild brown trout Salmo trutta L.) from those Game Conservancy members who own or manage fisheries. Also included are data arising from fishery log book records which are used to indicate trends in brown trout catch per unit effort (CPUE) over past decades.
2. The key finding from questionnaire returns was the widespread reporting of apparent declines of wild brown trout stocks in a total of twenty-seven sites throughout Britain. The affected waters are geographically widespread from the south-west of England through southern and south-east England, south Wales, northern England and Scotland including the Western Isles. Suspected stock declines of migratory (sea) trout, salmon Salmo salar L. and grayling Thymallus thymallus L. were also reported. Where brown trout occur alone, a significantly higher proportion of questionnaire returns recorded a stock decline compared with fisheries containing both brown and rainbow trout Salmo gairdneri Richardson.
3. Trout catch records from a small selection of differing fishery types are presented and the value of such data in fish stock assessment is discussed.     

Genetic differences among brown trout, Salmo trutta, stocks and their importance for the conservation and management of the species

SUMMARY. 1. Review of published studies on genetic variation, as shown by electrophoretic studies of protein variation, in natural brown trout ( Salmo trutta L.) populations from Britain and Ireland, Finland, France, Greece, Iceland, Norway, Sweden, U.S.A. and U.S.S.R., revealed abundant geographical variation in gene frequency with individual populations containing only a limited part of the gene diversity of the species.
2. Thirty-eight (54%) of the seventy gene loci examined have been found to be polymorphic in the species with an average population showing polymorphism at 16% of its loci (range 0-34.8%).
3. The brown trout is naturally subdivided into a large number of reproductively isolated and genetically distinct populations within, as well as among, drainages.
4. Two independent post-glacial colonizations, by genetically distinct races, followed by independent evolution in separate drainages over the past 13,000 years is seen as responsible for the genetic diversity of brown trout in north-western Europe.
5. Many genetically unique populations have been lost in the past 100 years and there is an urgent need to identify and conserve the remaining genetic diversity. Genetically unique populations are an irreplaceable resource for rational management in relation to angling and future aquaculture potential.     

Wild brown trout Salmo trutta: an important national and international resource

SUMMARY. 1. Brown trout were once given a variety of latin and common names, but are now regarded as belonging to only one polymorphic species, Salmo trutta L. A review of their geographical distribution shows that this species was originally native to Europe but has been successfully introduced in at least twenty-four countries outside Europe.
2. Brown trout provide valuable commercial and sports fisheries, e.g. commercial and rod catches of sea-trout in England and Wales averaged 110,547 fish per year from 1983 to 1986 and the minimum value of these fisheries is estimated to be £55M.
3. It is concluded from this brief review that the major objectives of scientific research on wild brown trout should be: (a) an assessment of the current status of stocks; (b) the maintenance of existing populations; (c) the development and improvement of mathematical models that can be used as tools for the conservation and management of this important national and international resource.     

Allozyme variability in brown trout Salmo trutta in Chile

1. Brown trout ova were imported during the last century from different locations in Europe to establish populations in Chilean rivers (South America). The rivers are currently occupied by naturalized populations that have adapted to very different environmental conditions, such as areas of semi-desert in the north, or rainy and cold areas in the south.
2. In this first study in this geographical area, electrophoretic variability of proteins encoded by twenty-five loci was screened in seven populations from northern to southern Chile.
3. The results show significant heterogeneity of allelic frequencies between populations in seven of eleven polymorphic loci detected. The estimated value of genetic diversity 0.1274 ( H _T) is higher than that observed in populations from areas of natural distribution of this species. However, only 12.64% of this genetic diversity was found between samples ( G _ST), indicating a low genetic divergence among Chilean populations. The observed associations among the Chilean and 'modern' group of European populations suggests the probable origin of the new populations.     

Molecular epidemiology of infectious hematopoietic necrosis virus reveals complex virus traffic and evolution within southern Idaho aquaculture

Infectious hematopoietic necrosis virus (IHNV) is a rhabdovirus which infects salmon and trout and may cause disease with up to 90% mortality. In the Hagerman Valley of Idaho, IHNV is endemic or epidemic among numerous fish farms and resource mitigation hatcheries. A previous study characterizing the genetic diversity among 84 IHNV isolates at 4 virus-endemic rainbow trout farms indicated that multiple lineages of relatively high diversity co-circulated at these facilities (Troyer et al. 2000 J Gen Virol. 81:2823-2832). We tested the hypothesis that high IHNV genetic diversity and co-circulating lineages are present in aquaculture facilities throughout this region. In this study, 73 virus isolates from 14 rainbow trout farms and 3 state hatcheries in the Hagerman Valley, isolated between 1978 and 1999, were genetically characterized by sequence analysis of a 303 nucleotide region of the glycoprotein gene. Phylogenetic and epidemiological analyses showed that multiple IHNV lineages co-circulate in a complex pattern throughout private trout farms and state hatcheries in the valley. IHNV maintained within the valley appears to have evolved significantly over the 22 yr study period.     

A model relating the results of low pH bioassay experiments to the fishery status of Norwegian lakes

SUMMARY. 1. Data from the literature concerning several Norwegian brown trout (Salmo trutta) populations are used to construct Leslie matrices in order to estimate their capacity to withstand egg and fry mortalities. These estimates are compared with the results of bioassay studies conducted at pH 4.5 and at various calcium concentrations in order to predict the percentage of trout populations which would be able to survive these conditions.
2. These predictions are in good agreement with the observed fishery status of Norwegian lakes at calcium concentrations <30 /μeq I⁻¹ but above this concentration the recorded status is worse than predicted.
3. Likely causes for this difference are considered to be effects acting on fish older than those used in the bioassay experiments (either mortality or reduction in fecundity possibly due to impaired growth) or the effects of other water quality factors, for example aluminium.
4. These effects need only be fairly slow acting as compared with the egg and fry mortalities so far studied in detail. Thus the discrepancy between predicted and observed results at pH 4.5 and 50μeq 1-⁻¹ calcium could be caused by an additional 80% mortality acting over the 4–8 year period before maturation.
5. An additional factor which could result in the observed fishery status being worse than predicted is temporal variation in water chemistry which is not assessed in the survey of Norwegian lakes. In particular, the adverse conditions occurring at time of snowmelt could be critically important.     

Environmentally induced spatio-temporal variations in the fecundity of brown trout Salmo trutta L.: trade-offs between egg size and number

1. Resident brown trout Salmo trutta in the Esva River basin (north Spain) live in a patchy environment with tracts of riparian forest or meadow along stream banks. This study assessed whether the reproductive traits of brown trout from four contrasting sites reflected site-specific factors.
2. Length at maturity (10.5–11 cm of 1 + individuals) was the same in the four sites examined but slowest growers in slow-growing sub-populations delayed maturity for 1 year relative to fast-growing fish. The analysis of monthly variations in egg size and number suggest that two 'decisions' in two consecutive years are required to complete spawning. The first concerns the number of eggs, determined when trout are still 0 +, and the second concerns egg size.
3. At three sites, egg size and number did not differ significantly between years but highly significant interannual variations were apparent at another site. Fish length was the major determinant of egg size and number at all sites but for any given length, brown trout at sites where the fish exhibited higher growth rates spawned more, but smaller, eggs than those at slow-growing sites. This spatial pattern was identical to the temporal pattern exhibited by trout at another site. The combination of temporal (year-to-year) and spatial (between rivers) variations in egg size and number showed a significant negative correlation, supporting the operation of a trade-off between these two traits.
4. The trade-off between egg size and number seems to be determined by site-specific factors, with slow-growing trout at sites which are fully covered by canopy spawning fewer, but larger, eggs than fast-growers in unshaded sites.     

Has the introduction of brown trout altered disease patterns in native New Zealand fish?

1. It is well recognised that non-indigenous species (NIS) can affect native communities via the 'spillover' of introduced parasites. However, two other potentially important processes, the 'spillback' of native parasites from a competent NIS host, where the latter acts as a reservoir leading to amplified infection in native hosts, and the 'dilution' of parasitism by a NIS host acting as a sink for native parasites, have either not been tested or largely overlooked.
2. We surveyed the helminth parasite fauna of native New Zealand fish in Otago streams that varied in the abundance of introduced brown trout Salmo trutta , to look for evidence of spillback and/or dilution. Spillover is not an issue in this system, with trout introduced as parasite-free eggs.
3. Seven native parasite species were present across 12 sites; significant inverse relationships with an index of trout abundance (i.e. dilution) were documented for three species infecting the native upland bully Gobiomorphus breviceps , and one species infecting the native roundhead galaxias Galaxias anomalus .
4. An inverse relationship between bully energy status and infection intensity of one parasite species suggests that parasite dilution could have positive effects on bully populations. Our failure to detect similar relationships for the other parasites does not preclude the possibility that dilution is beneficial to native fish, since parasites may have subtle or unmeasured impacts.
5. The parasite dilution patterns reported are compelling in that they occurred across several native host and parasite species; as such they have important implications for invasion ecology, providing an interesting contrast to the largely negative impacts reported for NIS. Mechanisms potentially responsible for the patterns observed are discussed.     

Plitvice Lakes National park harbors ancient,yet endangered diversity of trout (genus Salmo)

National park Plitvice Lakes is the oldest, the largest and the most visited national park in Croatia, well known for its stunning karstic watersheds that supply Plitvice Lakes and exceptionally valuable biodiversity. Lack of knowledge on the taxonomic status, population structure and viability of trout in the Plitvice Lakes National park, as well as the possibility of existence of cryptic diversity, motivated this study which aimed to determine: taxonomic status and phylogenetic relationship of Plitvice Lakes trout with other Salmo species, genetic structure and diversity of their populations, as well as future viability of populations. 150 samples of trout were obtained from 17 localities. Two mitochondrial gene markers, gene for cytochrome b and mitochondrial control region were used in phylogenetic reconstruction, as well as in population genetic investigation. Intraspecific and intrapopulation genetic diversity was described by calculating several measures of genetic polymorphism, while gene flow among populations was estimated as the migration rates and as the number of immigrants per generation. Phylogenetic trees revealed that Plitvice Lakes watershed is inhabited by two evolutionary independent units of the genus Salmo, representing two species, Salmo labrax Pallas, 1814 and Salmo trutta Linnaeus, 1758. Possibly ‘pure’ populations of both species were found alongside several admixed ones in which both species are present. Our data has shown low intraspecific and intrapopulational diversities for Plitvice Lakes trout and restricted gene flow. Small effective sizes of populations and weak interconnections between them, which are most likely a consequence of fragmentation and habitat degradation, accentuating the need for urgent conservation measures.     

Some impacts of human activities on trout, Salmo trutta, populations

SUMMARY. 1. The life cycle of salmonid fishes is described.
2. The performance and environmental requirements of the various life stages of the trout ( Salmo trutta L.) are reviewed, (a) The literature gives predictive relationships between water temperature and rate of embryonic development, food requirements and growth rate, (b) Water temperature, intragravel oxygen supply rate, water pH, the occurrence of mechanical shock, disturbance of spawning gravels, sedimentation and water chemistry can all influence the survival of the intragravel stages, (c) The survival and/or well-being of the free-swimming stages and the success of spawning are influenced by such factors as dissolved oxygen concentration, pH, water depth, water velocity and water chemistry.
3. Human activities such as impoundment, river transfer, drainage works, land improvement, afforestation and deforestation can all influence trout populations via changes in flow regime (and related effects such as sedimentation), temperature regime and water chemistry.
4. Man can also influence trout populations directly by cropping for food and/or sport and by artificial stocking.
5. Examples of practical application of present knowledge are given and some future research needs are listed.     

Growth, production and bioenergetics of brown trout in upland streams with contrasting riparian vegetation

1. A series of laboratory-based equations on trout growth and bioenergetics developed by J.M. Elliott were applied to data collected for brown trout ( Salmo trutta L.) under field conditions in Co. Mayo, Western Ireland. Fish were collected by electrofishing eight upland streams with contrasting riparian vegetation; grassland, open canopy and closed canopy deciduous.
2. Stream temperatures, one of the main influencing factors on fish growth and energetics, did not differ significantly between riparian types.
3. Observed growth rates were lower than the predicted maximum growth rates and were not influenced by riparian vegetation type. Growth ranged between 0.66% day⁻¹ for 0 + trout to 0.08% day⁻¹ for 2 + trout.
4. Production estimates showed no clear difference between riparian vegetation types over the growing season.
5. Fish densities and biomass tended to be greater in closed canopy streams particularly in summer.
6. Actual ration sizes calculated for trout were similar to the ration required for maintenance metabolism and were only 45–63% of the maximum potential rations. Although there was an ontogenetic increase in ration size with increasing fish age, the proportion of ration available for growth (i.e. the difference between actual and maintenance rations) did not differ between age classes but was greatest in summer. 1+ and 2+ trout show greatest ration available for growth in grassland streams.
7. Trout growth did not differ between riparian vegetation types but did vary seasonally with greatest attainment in summer. Growth was limited in the present study possibly due to combined effects of reduced prey available to fish and low stream temperatures reducing metabolic requirements. In such food limited systems, terrestrial invertebrate energy subsidies could have significant benefits to brown trout growth, production and bioenergetics.     

Changes in haematocrit values in blood samples treated with and without oxygen: a comparative study with four salmonid species

Changes in haematocrit values under lowered oxygen tension and aerobically treated blood samples of rainbow trout Salmo gairdneri (Richardson), landlocked Baltic salmon S. salar (L.), brown trout S. trutta lacustris (L.) and lake trout Salvelinus namaycush (Walbaum) have been studied in vitro . The mean haematocrit value increased during 2 h incubation under lowered oxygen tension by 32.7 ± 3.1% in rainbow trout, 28.2 ± 2.8% in landlocked Baltic salmon, 29.2 ± 5.6% in brown trout and 25.2 ± 2.8% in lake trout. During corresponding incubation with oxygen the mean haematocrit value decreased below the starting level by 18.1 ± 1.9% in rainbow trout, 18.3 ± 4.8% in landlocked Baltic salmon, 22.4 ± 0.7% in brown trout and 11.8 ± 1.7% in lake trout. Consequently, the changes in the haematocrit values were greater in the species belonging to the genus Salmo than in that of the genus Salvelinus . The increase in the haematocrit values seems to have resulted from swelling of the erythrocytes and their decrease is similarly attributable to shrinking of the cells. The reasons for this swelling, which may be complicated, and its apparent significance for haematocrit determinations are discussed.     

Introduction of the non-indigenous amphipod Gammarus pulex alters population dynamics and diet of juvenile trout Salmo trutta

Summary 1. The amphipod Gammarus pulex , introduced to Irish rivers with the aim of enhancing trout feeding, is displacing the native Gammarus duebeni celticus . These two species are generally associated with different environmental conditions and macroinvertebrate communities, confounding assessment of effects of the invader as compared with the native on fish populations. Here, we uncouple effects of the two Gammarus species from environmental gradients.
2. A weir dissects a lowland stretch of the River Lissan, slowing the upstream invasion by G. pulex and resulting in contiguous G. pulex , mixed species and G. d. celticus reaches. Total invertebrate abundance and biomass in the benthos were significantly higher in the G. pulex reach, driven by high invader abundance, with low abundance of other taxa. Gammarus pulex was particularly prominent in night-time drift.
3. Correspondingly, densities and biomass of 0+ trout were significantly higher in the G. pulex reach, while instantaneous loss rates were lower. Fish growth rates were similar among the three reaches.
4. In the G. pulex reach, this invader dominated the diet of 0+ trout, leading to ingestion of significantly higher invertebrate biomass than fish in the other reaches. Fish generally preyed on Gammarus in proportion to its abundance, but exhibited some positive selection for G. pulex in the invaded reach.
5. The negative effects of the invader on native invertebrates are contrasted with positive effects on juvenile trout. This indicates changes in energy flux after invasion, with differential resource use or assimilation by G. pulex probable underlying mechanisms. As the frequency of amphipod invasions increases globally, investigations of their role as strong interactors at multiple levels of ecological organisation is required if the consequences of deliberate and unintentional introductions are to be predicted, and ultimately, prevented.     

Preliminary empirical models of the historical and future impact of acidification on the ecology of Welsh streams

SUMMARY 1. We describe a preliminary approacb to modelling the impact of acidification on the ecology of two Welsb streams. Output from the hydrochemical Model of Acidification of Groundwaters in Catchments (MAGIC) was used to drive empirical models which predicted brown trout Salmo trulta (L.) survival, trout density and invertebrate assemblage type. The models were used for hindcasts between 1844 and 1984 under conifer forest and moorland conditions. Forecasts involved each of these land uses with sulphate deposition either continued at 1984 levels or reduced by 50%.
2. Trout survival times and trout densities in the models declined markedly between 1844 and 1984. The most severe decline occurred under simulated forest, where high aluminium concentration led to the virtual elimination of trout in both streams.
3. In forecasts, only in simulated moorland with sulphate deposition reduced by 50% of 1984 levels, was further decline in trout population retarded. There was no marked recovery in trout density under any of the conditions examined.
4. Invertebrate assemblages in streams during the nineteenth century may have differed from those now existing in nearby moorland streams which are presently circumneutral. Past chemical conditions were unusual (<3mg 1⁻¹ total hardness, but pH >5.7 and low aluminium) by present-day standards, and were outside the range of the invertebrate model until -1940.
5. Between the 1940s and 1984 there was no change in invertebrate fauna under the moorland scenario despite some acidification. However, simulated forest advanced the appearance of the most impoverished assemblage type, which did not recover in spite of reduced deposition.
6. We discuss several uncertainties with the models in their present form, but suggest some methods for their testing and validation.     

The impact of social status on the erythrocyte beta-adrenergic response in rainbow trout, Oncorhynchus mykiss

The objective of the present investigation was to determine whether chronic increases in circulating cortisol concentrations, resulting from the occupation of subordinate status in rainbow trout social hierarchies, resulted in an enhancement of the erythrocyte adrenergic response. Rainbow trout (Oncorhynchus mykiss) were confined in fork length matched pairs for 6 h, 18 h, 48 h or 5-7 days, and social status was assigned through observations of behaviour. Erythrocyte adrenergic responsiveness, determined in vitro as changes in water content following incubation with the beta-adrenoreceptor agonist isoproterenol, was significantly greater in subordinate than dominant fish at 48 h of social interactions but not after 5-7 days, nor when assessed as changes in extracellular pH (pHe). However, the activity of the Na+/H+ exchanger (beta-NHE), assessed in vitro as the pHe change following incubation with the permeable cyclic AMP analogue 8-bromo-cyclic AMP, was significantly lower in subordinate fish. The number of erythrocyte membrane-bound adrenergic receptors (Bmax) was significantly higher in subordinate than dominant fish at 48 h, but had decreased by 5-7 days to a value that was not significantly different from that for dominant fish. The apparent dissociation constant (KD) of these receptors was not significantly impacted by either social status or interaction time. Finally, the relative expressions of beta-3b adrenergic receptor (AR) and beta-NHE mRNA were determined using real-time PCR and were found to be minimally affected by social rank. Relative to a control group, beta-3b AR mRNA was significantly up-regulated in both dominant and subordinate trout at all time periods, whereas the expression of beta-NHE was in general significantly down-regulated. Unlike the situation in rainbow trout treated with exogenous cortisol, elevations in circulating cortisol resulting from low social status did not "pre-adapt" the erythrocyte adrenergic response, but rather may have served to offset the potentially adverse effects elicited by plasma catecholamines, which were elevated during social hierarchy formation.     

Rates of gastric evacuation in piscivorous brown trout, Salmo trutta

SUMMARY. 1. The dry weight of food remaining in the stomachs of piscivorous trout decreased exponentially with time. Gastric evacuation rates increased exponentially with increasing temperature but were unaffected by predator size, meal size or type of fish prey.
2. Mathematical models were developed to estimate both the rate and time for the gastric evacuation of different meal sizes (expressed as dry weight), and were applicable to piscivorous trout of different sizes (length range 10–32 cm) feeding on trout fry or sticklebacks at different temperatures (range 5–18°C).
3. The wet weight of food in the stomachs also decreased exponentially with time, but evacuation rates both increased with temperature and decreased with increasing meal size; the latter relationship occurred because relative rates of water loss from a meal also decreased with increasing meal size. Use of wet or dry weights can therefore lead to different conclusions about the effect of meal size on evacuation rates.
4. When piscivorous trout were fed three consecutive meals of varying size, the models predicted the total dry weight of food left in the stomach, but not the weight remaining for each individual meal. Interactions between meals led to an increase in evacuation rates for meals consumed early in the series and a decrease in evacuation rates for later meals.
5. Evacuation rates for piscivorous trout were compared with those for trout feeding on invertebrates in an earlier study, and were close to those for caddis larvae as prey, higher than those for mealworms and lower than those for a variety of invertebrate prey. Although a great deal is now known about the daily food intake and growth rates of trout feeding on invertebrates, there is little comparable information for piscivorous trout.     

The ecology of brown trout Salmo trutta in English chalk streams

SUMMARY. .1. Chalkstream trout are fast-growing, short-lived with a stable age structure that is a consequence of the lack of extremes in the abiotic environment, especially of water temperature and flow.
2. Levels of secondary (invertebrate) production arc high and there is no evidence that interspecific or intraspecific competition for food resources limits growth.
3. Dietary studies show a broad similarity between trout and other species, but indicate differences that reflect variations in the micro-habitat distributions between fish species.
4. Trout numbers appear to be limited initially by the availability of gravel spawning areas, and then by areas suitable for newly-emerged fry. Reduction in stream discharge in the spring, either naturally or by man, can lower the number of 0+ trout that survive.
5. Eels are not important predators on trout eggs or fry. but a reduction in pike numbers can lead to a decrease in the mean weight of pike. Small pike do not pose a serious threat to stocked yearling trout.     

The diversity of macro invertebrate and macrophyte communities in ponds

SUMMARY 1. The range of factors which may influence the wide variation in the structure of pond communities is reviewed.
2. Factors explaining the variation in numbers of macro invertebrate and macrophyte taxa are investigated, using multiple regression analysis, for a group of ponds in which biogeographical effects are likely to be minimal.
3. Numbers of macro invertebrate taxa in these ponds are highly correlated with pH and probably, therefore, with other associated aspects of water chemistry,
4. Many invertebrate taxa, from species to entire phyla, were not found below pH 5.5, but few characteristically'acidophilic'taxa were apparent.
5. Details of species composition of the macro invertebrate fauna differ widely even between ponds of similar pH.
6. The diversity and composition of macrophyte communities are not adequately predicted by any of the factors investigated.     

Density reductions by predatory trout increase adult size and fecundity of surviving caddisfly larvae in a detritus-based stream food web

1. In some situations, individuals surviving in environments where predation is intense can grow faster because the benefits of release from intraspecific competition outweigh costs associated with anti-predator responses. Whether these 'thinning' effects of predation occur in detritus-based food webs where resource renewal occurs independently of consumption by consumers was studied. We investigated how effects of predatory brown trout ( Salmo trutta ) on the larvae of the detritivorous stream caddisfly, Zelandopsyche ingens , influenced the size and fecundity of the caddisfly adults.
2. Trout substantially reduced the abundance of Z. ingens larvae, but adult male and female Z. ingens were significantly larger in trout streams compared to fishless streams. Females in trout streams had 33% more eggs than fishless stream females, and egg sizes were not significantly different. In mesocosms, Z. ingens larvae in low density treatments reflecting trout stream abundances grew significantly faster than larvae in high density treatments that were characteristic of fishless stream abundances. Non-lethal trout presence did not influence case building behaviour, feeding rates or growth or Z. ingens larvae, indicating non-lethal effects of predators were negligible.
3. Increased adult size and fecundity associated with trout stream individuals were probably a result of predator thinning of larval density indirectly releasing surviving Z. ingens from intraspecific competition. Thus, predator thinning did influence interactions between larvae in this detritus-based food web as larval growth was strongly density-dependent. However, extrapolating the total number of eggs potentially produced indicates the increased fecundity of females in trout streams would not compensate for losses of larvae to trout predation.     

Environmental stress and the survival of brown trout, Salmo trutta

SUMMARY. 1. The hypothalamic-pituitary-interrenal axis of the brown trout, Salmo trutta , is activated in response to most forms of environmental stress. This results in an elevation of blood cortisol levels.
2. Experimental elevation of blood cortisol levels in otherwise unstressed brown trout caused a dose-dependent increase in mortality rate due to disease. In our studies., Saprolegnia-infection , furunculosis and bacterial fin-rot were the principal diseases.
3. Chronic cortisol elevation also suppressed several of the endocrine processes controlling sexual maturation, resulting in a significant reduction in the size of the gonad in both male and female fish.
4. It is argued that many of the deleterious effects of sublethal pollution (including acidification) on natural trout populations can be attributed to chronically-elevated blood cortisol levels and that a knowledge of such physiological changes would allow an assessment of the impact of pollution events and act as an early warning of potential disease and recruitment problems.