The natural regulation of numbers and growth in contrasting populations of brown trout, Salmo trutta, in two Lake District streams

SUMMARY. 1. This short review summarizes a long-term investigation of brown trout in two populations that probably represent opposite extremes of life histories in this polymorphic species; Bhick Brows Beck serves as a nursery for the progeny of migratory trout (mixture of sea and estuarine trout) and Wilfin Beck is populated by resident trout. 2. Population density in Black Brows Beck was always much higher than that in Wilfin Beck, and was regulated by density-dependent survival in the early stages of the life cycle. There was no evidence for similar density-dependent regulation in Wilfin Beck; simple proportionate survival occurred with fairly constant loss-rates. Survival was reduced in both populations by summer droughts and also by spates in Wilfin Beck. 3. Black Brows trout were always larger than Wilfin Beck trout of similar age; fry size at the start of the growth period was chiefly responsible for these differences. Variations in water temperature were chiefly responsible for differences in growth rates between year-classes within each population. Food intake was not a limiting factor, except in the first winter of the life cycle and for adults over 3 years old in Wilfin Beck. Variation in individual size was inversely density-dependent in Black Brows Beck and decreased with age in Wilfin Beck, these changes being due to natural (stabilizing) selection. 4. There is strong evidence for genotypic differences between the populations. The implications of this are discussed, especially the need to conserve different populations that may contain unique genetic material, and the importance of restocking with fish reared from the indigenous population that should always contain the optimum genotypes for a particular habitat. Restocking with juveniles should be done with caution because it could lead to a decrease in both numbers and size variation when the population is regulated by density-dependent mechanisms. 5. One major objective of future work should be the development and improvement of mathematical models that can be used to predict the optimum density for trout in different populations, the maximum attainable growth rate in different habitats, and the effects on trout populations of environmental changes due to natural causes (e.g. droughts and spates) or human activities.     

The critical-period concept for juvenile survival and its relevance for population regulation in young sea trout, Salmo trutta

An example of density-dependent regulation is provided by a long-term investigation (1966-present) of a population of migratory trout (estuarine and sea trout), Salmo trutta L., in a Lake District stream. Evidence for the concept of a critical period for the survival of young fish is briefly reviewed and found to be rather equivocal. The concept is, however, relevant to the trout population. Loss rates were high before but low after a critical survival time ( t_c days after fry emergence) that varied between year-classes (range 33-70 days) and was inversely density-dependent on egg density. Survivor density and loss rates were strongly density-dependent on egg density before t _c, but proportionate survival with stable loss-rates occurred after t _c. Some trout established feeding territories soon after emergence and the number of fish without territories decreased from a high initial value to a negligible value at t _c. Fish size at t_c was not constant but increased as t _c increased. The range of t _c for the different year-classes was similar to that for survival times of unfed fry in the laboratory. A new stock-recruitment model, incorporating t _c, has been developed for the trout population and shown to be related to the model (Ricker curve) used in the long-term study. The critical time can also be regarded as the critical age for survival in young trout; this concept may be relevant to other fish species.     

Conservation and management of brown trout, Salmo trutta, in Scotland: an historical review and the future

SUMMARY. 1. Intensive research into the life history of brown trout started In 1948 when the Brown Trout Laboratory was opened in Pitlochry. Over the next 15 years significant contributions were made to the brown trout literature upon which the Laboratory based advice to landowners and anglers wanting to develop their fisheries. 2. Increasing pressure from the government for more work on Atlantic salmon tended to divert research funds and time away from brown trout investigations. The International Biological Programme's major study at Loch Leven from 1966 to 1972 ensured a continuing interest in trout in standing waters. Over this period little attention had been paid to trout in rivers. This changed when a number of investigations were started on the River Tweed by Edinburgh University. 3. A major constraint to brown trout conservation and management has been illegal fishing and lack of records on stocking activities and catches. The granting of Protection Orders under the Freshwater and Salmon Fisheries (Scotland) Act, 1976, has been a major incentive to increased interest in the improvement of trout fisheries. 4. Brown trout stocks have been reduced in certain areas due to the effects of afforestation, acidification, land drainage and farm wastes. Various remedial measures have been proposed and implemented. 5. To meet the increasing demands for trout fishing, many loch and reservoir fisheries are now stocked with rainbow trout in preference to brown trout. Attention should be paid to the interaction of these two species in both standing and in running waters., where fish farm escapees and inadvisable releases go unrecorded. 7. Research into the genetic effects on wild stocks from the liberation of large numbers of hatchery-reared brown trout has been lacking and probably many ‘pure’ indigenous stocks have been lost. More work in this field is essential. 8. Proposals are outlined for future brown trout research and recommendations are made for better management. Suggestions are also put forward for changes in the legislation to further protect Scottish brown trout stocks.     

The effects of population density and lake characteristics on growth and size structure of brook trout Salvelinus fontinalis (Mitchill, 1815) in boreal forest lakes in Canada

This paper examines the effect of lake characteristics on population density and how this variation affects growth, mortality and population size structure of brook trout, Salvelinus fontinalis. The study was conducted on 17 recreationally fished, reproductively isolated boreal forest lakes in Newfoundland, Canada from 1993 to 2000. A standardized sampling program, the Fyke Littoral Index Netting program (FLIN) was used to collected data that describes brook trout population parameters and life history attributes. Regression analyses showed significant relationships between fish density and biomass and characteristics of the lakes. Variation in fish density and biomass was explained by lake surface area and littoral habitat area. Significant relationships were found when growth, mortality and size structure were regressed against density. The proportional stock distribution and theoretical maximum size of brook trout were negatively related to density, and natural mortality was positively related to density. The largest maximum length and highest proportional stock densities occurred at brook trout densities of less than 30 fish/ha. In general, the higher the proportion of littoral habitat area the higher the densities of brook trout, which correspondingly had important effects on growth, natural mortality and size structure of the brook trout populations. This information is critical to the development of management strategies aimed at altering size distribution to produce specific fisheries management outcomes.     

The impact of trout on galaxiid fishes in New Zealand

Compared with the effect of invaders on the native terrestrial fauna of New Zealand, interactions between native fishes and introduced trout (sports fish in the genera Salmo, Oncorhynchus and Salvelinus) are less well known and there have been fewer efforts to remedy their effects. Trout have caused widespread reductions in the distribution and abundance of native galaxiid fishes, a family dominated by threatened species. The effects have been most severe on non-diadromous species (those lacking a marine migratory stage), which are commonly eliminated from streams by trout. Galaxiid populations in lakes, and those with migratory ?whitebait? stages, have also been affected, but the extent of the impacts are less understood. The mechanisms controlling negative interactions between trout and native fish, and how the environment modifies those interactions, will be important for future management. Experiments and field comparisons indicate size-specific predation by trout is the main driver of negative interactions. Large trout (>150 mm long) do the greatest damage and small galaxiids (those with adult sizes <150 mm long) are the most at risk. The fry stage of non-diadromous galaxiids is particularly vulnerable. Despite galaxiid fry production in some trout-invaded reaches, often no fry survive making them population ?sinks? that must be sustained by adult dispersal. Trout are also associated with changes in galaxiid behaviour and alterations to stream benthic communities. However, effects on galaxiid growth and fecundity have been little studied. Recent work also indicates that habitat conditions, especially floods, low flows and natural acidity, can mediate trout?galaxiid interactions. We argue that managers should be more proactive in their response to the plight of galaxiids, and we identify avenues of research that will benefit native fish conservation activities in the future.     

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.     

Early effects of the strategies of creating a genetic refuge and direct translocation for conserving and restoring populations of native brown trout

1. The conservation of salmonid inter‐ and intra‐specific diversity is a well‐known challenge, and general management guidelines and conservation processes are available. However, research demonstrating the outcomes of practical conservation actions is largely lacking. 2. We monitored the spatiotemporal genetic and demographic evolution of a native Mediterranean brown trout population in a river in the French Alps to assess the efficacy and early effects of genetic refuge (i.e. cessation of stocking) and wild trout translocation strategies. We also studied the use of angling as a tool to limit the introgression of the wild standing population. 3. We found that the rate of non‐native alleles in wild populations was age dependent, underpinning the importance of using age profiles in the design of genetic conservation studies. 4. Genetic refuge and direct translocation of wild trout resulted in a rapid and significant decrease in the percentages of non‐native alleles. Moreover, the genetic refuge strategy resulted in a significant reduction in the number of pure non‐native individuals, without changing trout densities, whilst direct translocations resulted in the establishment of dense, self‐sustaining native trout populations. Direct translocations changed the distribution of genotype categories and increased densities up to 55‐fold in 3 years. Our results also showed that angling resulted in a selective pressure on non‐native trout introduced at fry stage, whereas non‐native trout issued from natural recruitment were not affected. 5. Our study provides insights for improving the efficacy of practical conservation policies and can be used in other native freshwater fish conservation plans. Proactive measures such as direct translocation need to be implemented together with passive approaches such as genetic refuge policies. Before implementing such actions, accurate genetic and demographic studies at small geographical scales are essential to ensure that no self‐sustaining population of non‐native fish is present. To obtain rapid colonisation, we recommend introducing fish along whole river sections rather than concentrating on a few river stretches. Angling pressure can be used as an additional tool to improve restoration.     

Estimating the long-term effects of stocking domesticated trout into wild brown trout (Salmo trutta) populations: an approach using microsatellite DNA analysis of historical and contemporary samples

Indigenous salmonid fish gene pools are affected by domesticated conspecifics, derived from aquaculture escapes and deliberate releases. Variability was examined at nine microsatellite loci in order to assess the long-term impact of stocking domesticated trout in two brown trout populations. The study was based on analysis of two historical samples (1945-56), represented by old scale collections, and seven contemporary samples (1986-2000). In one population historical and contemporary samples were remarkably genetically similar despite more than a decade of intense stocking. Estimation of admixture proportions showed a small genetic contribution from domesticated trout (approximately 6%), and individual admixture analysis demonstrated a majority of nonadmixed individuals. The expected genetic contribution by domesticated trout was 64%, assessed from the number of stocked trout and assuming equal survival and reproductive performance of wild and domesticated trout. This demonstrates poor performance and low fitness of domesticated trout in the wild. In another population there was a strong genetic contribution from domesticated trout (between 57% and 88% in different samples), both in samples from a broodstock thought to represent the indigenous population and in a sample of wild spawners. Survival of domesticated trout and admixture with indigenous fish in the broodstock and subsequent stocking into the river, combined with a low population size of native trout relative to the number of stocked trout, could explain the observed introgression. Few nonadmixed individuals remained in the introgressed population, and I discuss how individual admixture analysis can be used to identify and conserve nonintrogressed remains of the population.     

Hydropower-related pulsed-flow impacts on stream fishes: a brief review,conceptual model,knowledge gaps,and research needs

The societal benefits of hydropower systems (e.g., relatively clean electrical power, water supply, flood control, and recreation) come with a cost to native stream fishes. We reviewed and synthesized the literature on hydropower-related pulsed flows to guide resource managers in addressing significant impacts while avoiding unnecessary curtailment of hydropower operations. Dams may release pulsed flows in response to needs for peaking power, recreational flows, reservoir storage adjustment for flood control, or to mimic natural peaks in the hydrograph. Depending on timing, frequency, duration, and magnitude, pulsed flows can have adverse or beneficial short and long-term effects on resident or migratory stream fishes. Adverse effects include direct impacts to fish populations due to (1) stranding of fishes along the changing channel margins, (2) downstream displacement of fishes, and (3) reduced spawning and rearing success due to redd/nest dewatering and untimely or obstructed migration. Beneficial effects include: (1) maintenance of habitat for spawning and rearing, and (2) biological cues to trigger spawning, hatching, and migration. We developed a basic conceptual model to predict the effects of different types of pulsed flow, identified gaps in knowledge, and identified research activities to address these gaps. There is a clear need for a quantitative framework incorporating mathematical representations of field and laboratory results on flow, temperature, habitat structure, fish life stages by season, fish population dynamics, and multiple fish species, which can be used to predict outcomes and design mitigation strategies in other regulated streams experiencing pulsed flows.     

Fall synchrony between leaf color change and brook trout spawning in the Laurentides Wildlife Reserve (Québec,Canada) as potential environmental integrators

Fall biological processes are driven by a combination of environmental factors, with cumulative effects over the length of the growing season, which are currently difficult to model. This study evaluated if leaf color change in fall (i.e., leaf yellowing) and brook trout spawning could be two biological processes that are synchronized at a regional scale and if leaf yellowing could be used to determine the peak of spawning activity of this species. To this end, we surveyed 551 brook trout redds and examined 193 digital images of forest trees in the Laurentides Wildlife Reserve (Québec, Canada) over the fall season. Results showed that leaf yellowing and brook trout spawning were synchronized, providing one of the first examples of temporal matching between freshwater and terrestrial biological processes at the regional scale. Considering the difficulty of monitoring the phenology of freshwater fish spawning at high spatial and temporal resolution, terrestrial integrators of environmental conditions, such as leaf color change, may prove to be promising predictors of spawning activity in the management of fish populations.     

Fish cognition: what can we learn, and what do they need to learn?

Fish provide a wonderful opportunity to explore processes that shape and select cognitive ability. In this presentation, I will illustrate three aspects of work that my colleagues and I have used to investigate fish learning and memory over the last decade. First, I will discuss how comparing different populations sampled from contrasting habitats allows differences in cognitive ability to be related to the evolutionary ecology of the fish. I will use examples that have investigated how differences in learning ability between populations of the same species can arise. Here, the examples will be taken from the ubiquitous three‐spined stickleback, and a Panamanian poecilid, Brachyraphis .
The second approach has used fish cognition as a tool to quantify behaviour to enable assessment of different aspects of fish welfare. For example, the recent work investigating pain perception in trout required the use of a learning task to quantify how fish behaviour was modified after noxious stimulation. Ways in which these, and similar, processes can be used in future studies of fish welfare will be discussed.
The final part of the presentation will consider recent work that addresses the problems of releasing hatchery‐reared fish for restocking purposes. Although a common practice, most of the hatchery‐reared fish die shortly after they are released. Much of the observed mortality apparently stems from the fishes' inexperience with a variable environment. Experiments with juvenile cod and brown trout suggest that both age, and the early rearing environment, have profound effects on fish learning and behaviour. I will discuss how simple modifications to current rearing practices may have large beneficial effects on the post‐release survival of hatchery‐reared fish.     

Optimum energy intake and gross efficiency of energy conversion for brown trout, Salmo trutta, feeding on invertebrates or fish

1. The chief objectives were to determine the daily optimum energy intake ( C OPT cal day−1) for growth and the gross efficiency ( K G%) for converting energy intake into growth for brown trout, Salmo trutta . Energy budgets for individual fish were obtained from experiments with 292 trout (initial live weight 1–318 g) bred from wild parents, and kept at five constant temperatures (5, 10, 13, 15, 18 °C) and 100% oxygen saturation. Most trout (252) were fed over a period of 42 days on a fixed ration of shrimps, Gammarus pulex , the ration levels varying between zero and maximum, but 40 of the larger trout were fed to satiation on freshly-killed sticklebacks ( Gasterosteus aculeatus ).
2. Energetics models developed in earlier studies on the same data were summarized briefly and were used to predict the relationship between the change in the total energy content of a trout ( C G cal day−1) and its energy intake ( C IN cal day−1), and hence to estimate C OPT. The models were also used to predict the relationship between K G and C IN. In both comparisons, there was good agreement between observed values from the experiments and expected mean values predicted from the models. For trout feeding on invertebrates, C OPT lay closer to the maximum, rather than the maintenance, energy intake. When the diet changed from invertebrates to fish, there was a marked increase in C IN, C G and K G.
3. For trout feeding on invertebrates, K G exceeded 30% within 7–11 °C, with a maximum K G of 31.8% at 8.9 °C. For piscivorous trout, K G exceeded 30% within 4–16 °C and 40% within 6.5–12 °C, with a maximum K G of 41.8% at 9.3 °C. These differences were discussed in relation to the results of previous workers, and the models used in the present study provided a method of exploring the limitations of the ' K -line' hypothesis for the relationship between K G and C IN.     

Evidence for selective angling of introduced trout and their hybrids in a stocked brown trout population

Comparisons of the genetic composition of brown trout Salmo trutta captured by anglers and by electrofishing based on three diagnostic microsatellite loci provided strong evidence that angling is selective in a stocked brown trout population. At two sites, anglers caught significantly younger trout and proportionally more introduced hatchery trout and hybrids than were observed in electrofishing surveys. Selective angling, in combination with a small legal catch size, may have considerably eliminated introduced trout and hybrids before spawning at the study sites, and thus may have reduced the introgression of alien genes into the local gene pool. Angling can be an important factor influencing the genetic structure of fish populations and should be taken into account in studies of introgressive hybridization in stocked fish populations and their management. In this study, demographic consequences of stocking were not assessed. Thus, even though the genetic consequences of stocking may be minimal or largely reversible through angling, resource competition between native and introduced trout, until they reach legal catch size, is expected to have a negative effect on the productivity of the indigenous trout population.     

Has stocking contributed to an increase in the rod catch of anadromous trout (Salmo trutta L.) in the Shetland Islands,UK?

The stocking of hatchery-origin fish into rivers and lakes has long been used in fisheries management to try to enhance catches, especially for trout and salmon species. Frequently, however, the long-term impacts of stocking programmes have not been evaluated. In this study, the authors investigate the contribution of a stocking programme undertaken to support the rod catch of sea trout in the Shetland Islands, UK. Once a highly productive recreational fishery, Shetland sea trout catches crashed in the mid-1990s. Around the time that stocking began, increases in rod catches were also reported, with advocates of the stocking highlighting the apparent success of the programme. Using a suite of genetic markers (microsatellites), this study explores the contribution of the stocking programme to the Shetland sea trout population. The authors found that the domesticated broodstock and wild spawned brown trout from seven streams were genetically distinct. Despite extensive stocking, wild spawned brown trout dominated, even in those streams with a long history of supplementation. The majority of sea trout caught and analysed were of wild origin – only a single individual was of pure stocked origin, with a small number of fish being of wild × stocked origins. This study suggests that stocking with a domesticated strain of brown trout has made only a very limited contribution to the Shetland Islands rod catch, and that the revival of sea trout numbers appears to be driven almost exclusively by recovery of trout spawned in the wild.     

Long-term captive breeding does not necessarily prevent reestablishment: lessons learned from Eagle Lake rainbow trout

Captive breeding of animals is often cited as an important tool in conservation, especially for fishes, but there are few reports of long-term (<50 years) success of captive breeding programs, even in salmonid fishes. Here we describe the captive breeding program for Eagle Lake rainbow trout, Oncorhynchus mykiss aquilarum, which is endemic to the Eagle Lake watershed of northeastern California. The population in Eagle Lake has been dependent on captive breeding for more than 60 years and supports a trophy fishery in the lake. Nevertheless, the basic life history, ecological, and genetic traits of the subspecies still seem to be mostly intact. Although management has apparently minimized negative effects of hatchery rearing, reestablishing a wild population would ensure maintenance of its distinctive life history and its value for future use as a hatchery fish. An important factor that makes reestablishment possible is that the habitat in Eagle Lake is still intact and that Pine Creek, its major spawning stream, is recovering as habitat. With the exception of an abundant alien brook trout (Salvelinus fontinalis) population in Pine Creek, the habitat factors that led to the presumed near-extinction of Eagle Lake rainbow trout in the early twentieth century have been ameliorated, although the final stages of reestablishment (eradication of brook trout, unequivocal demonstration of successful spawning migration) have still not been completed. The Eagle Lake rainbow trout story shows that long-term captive breeding of migratory salmonid fishes does not necessarily prevent reestablishment of wild populations, provided effort is made to counter the effects of hatchery selection and that natural habitats are restored for reintroduction. Long-term success, however, ultimately depends upon eliminating hatchery influences on wild-spawning populations. Extinction of Eagle Lake rainbow trout as a wild species becomes increasingly likely if we fail to act boldly to protect it and the Eagle Lake watershed.     

Variability of kokanee and rainbow trout food habits,distribution, and population dynamics,in an ultraoligotrophic lake with no manipulative management

Crater Lake is a unique environment to evaluate the ecology of introduced kokanee and rainbow trout because of its otherwise pristine state, low productivity, absence of manipulative management, and lack of lotic systems for fish spawning. Between 1986 and 2004, kokanee displayed a great deal of variation in population demographics with a pattern that reoccurred in about 10 years. We believe that the reoccurring pattern resulted from density dependent growth, and associated changes in reproduction and abundance, driven by prey resource limitation that resulted from low lake productivity exacerbated by prey consumption when kokanee were abundant. Kokanee fed primarily on small-bodied prey from the mid-water column; whereas rainbow trout fed on large-bodied prey from the benthos and lake surface. Cladoceran zooplankton abundance may be regulated by kokanee. And kokanee growth and reproductive success may be influenced by the availability of Daphnia pulicaria, which was absent in zooplankton samples collected annually from 1990 to 1995, and after 1999. Distribution and diel migration of kokanee varied over the duration of the study and appeared to be most closely associated with prey availability, maximization of bioenergetic efficiency, and fish density. Rainbow trout were less abundant than were kokanee and exhibited less variation in population demographics, distribution, and food habits. There is some evidence that the population dynamics of rainbow trout were in-part related to the availability of kokanee as prey.     

Community structure affects trophic ontogeny in a predatory fish

While most studies have focused on the timing and nature of ontogenetic niche shifts, information is scarce about the effects of community structure on trophic ontogeny of top predators. We investigated how community structure affects ontogenetic niche shifts (i.e., relationships between body length, trophic position, and individual dietary specialization) of a predatory fish, brown trout (Salmo trutta). We used stable isotope and stomach content analyses to test how functional characteristics of lake fish community compositions (competition and prey availability) modulate niche shifts in terms of (i) piscivorous behavior, (ii) trophic position, and (iii) individual dietary specialization. Northern Scandinavian freshwater fish communities were used as a study system, including nine subarctic lakes with contrasting fish community configurations: (i) trout‐only systems, (ii) two‐species systems (brown trout and Arctic charr [Salvelinus alpinus] coexisting), and (iii) three‐species systems (brown trout, Arctic charr, and three‐spined sticklebacks [Gasterosteus aculeatus] coexisting). We expected that the presence of profitable small prey (stickleback) and mixed competitor–prey fish species (charr) supports early piscivory and high individual dietary specialization among trout in multispecies communities, whereas minor ontogenetic shifts were expected in trout‐only systems. From logistic regression models, the presence of a suitable prey fish species (stickleback) emerged as the principal variable determining the size at ontogenetic niche shifts. Generalized additive mixed models indicated that fish community structure shaped ontogenetic niche shifts in trout, with the strongest positive relationships between body length, trophic position, and individual dietary specialization being observed in three‐species communities. Our findings revealed that the presence of a small‐sized prey fish species (stickleback) rather than a mixed competitor–prey fish species (charr) was an important factor affecting the ontogenetic niche‐shift processes of trout. The study demonstrates that community structure may modulate the ontogenetic diet trajectories of and individual niche specialization within a top predator.     

A new biological indicator to assess the ecological status of Mediterranean trout type streams

Fuelled by the generalized degradation of freshwater ecosystems, the development of tools to assess their ecological status has been the focus of intensive research in the last decades. Although fish are one of the key biological quality elements used to describe the ecological status of rivers, fish metrics that accurately respond to disturbances in Mediterranean trout type streams are still lacking. In these systems, multimetric indices are not optimal indicators because of their low species richness and abundances, thus alternative approaches are needed. Since carrying capacity defines the potential maximum abundance of fish that can be sustained by a river, its relationship with actual density (D/K ratio) could be an accurate indicator of population conservation status and consequently of the ecological status of the river. Based on this rationale, we modeled carrying capacity dynamics for 37 brown trout populations during a 12-year study period. We analyzed the response of the D/K ratio to a gradient of increasing environmental harshness and degradation in order to assess its suitability to accurately measure brown trout conservation status. Our results showed that the D/K ratio was highly sensitive to temporal and spatial variations in environmental conditions and the levels of human-induced environmental degradation. Variations in the environmental and human degradation factors included in the best explaining regression models developed for the whole population and by age classes accounted for between 58 and 81% of the variation in the D/K ratio. Likewise, the D/K ratio was sensitive to both general and life stage specific disturbance factors. Further analyses helped identify the factors limiting population abundance. Therefore, the D/K ratio could be an interesting indicator to consider when defining objective management plans and corrective actions in degraded rivers and streams subject to Mediterranean climatic conditions.     

Experimental sea ranching of brook trout, Salvelinus fontinalis Mitchill

An experiment to induce anadromy in a population of wild brook trout, Salvelinus fontinalis , was conducted near Sept-Iles, Quebec, in 1978–1979. Brook trout were captured from the Matamek River, tagged and transported to the Matamek River estuary during late spring and early summer, and allowed free movement between an impassable waterfall 0.7 km upstream and the sea. Fish were recaptured in autumn as they returned to fresh water. Over two years, 34.0% of the released fish were recaptured. Best returns were in the 2+ and 3+ age classes with 38.0 and 62.1% recaptured, respectively. Straying of transplanted fish appeared to be <1%. All age classes included sea run brook trout (sea trout) but the largest percentages of sea trout occurred in older fish. Growth was better in sea trout than in fish which did not develop anadromy, presumably a function of an increased food supply at sea. Severe tagging effects stunted growth and probably suppressed anadromy, especially among younger fish. Sexual characteristics of recaptured fish indicated suppressed maturation of gonads in sea trout compared to fish remaining in fresh water and there was a shift to a larger percentage of females in the sea trout. Comparisons between our results and data on other anadromous Salvelinus species underscore the potential for sea-ranching of trout and char as a moderate effort, high yield aquaculture technique.     

Higher growth variability and stronger responses to temperature changes in wild than hatchery‐reared sea trout (Salmo trutta L.)

Each year, millions of hatchery‐reared sea‐run brown trout Salmo trutta L. (the sea trout) juveniles are released into the natural environment in the Atlantic region. The aim of this work was to investigate the growth responses of sea trout to changing temperature conditions and to compare the growth plasticity between wild and hatchery‐reared fish. Scales were collected from sea trout in a selected river flowing into the southern Baltic Sea. We analyzed the scale increment widths as a proxy of somatic growth and investigated the interannual variabilities and differences in growth between fish groups (wild and hatchery‐reared). We used mixed‐effects Bayesian modeling and ascribed the variances in growth to different sources. Furthermore, we developed indices of interannual (2003–2015) growth variation in the marine and freshwater phases of the life cycle of the fish and analyzed the relationships between trout growth and temperature. Temperature positively affects fish growth, regardless of the origin of the fish. We observed stronger relationships between fish growth and temperature conditions in the marine phase than in the freshwater phase. Additionally, wild sea trout are characterized by stronger responses to temperature variability and higher phenotypic plasticity of growth than those of the hatchery‐reared individuals. Therefore, wild sea trout might be better suited to changing environmental conditions than hatchery‐reared sea trout. This knowledge identifies possible threats in management actions for sea trout with an emphasis on ongoing climate change.