Migration strategies of brown trout (Salmo trutta) in a subarctic river system as revealed by stable isotope analysis

We estimated the proportions of anadromous and freshwater‐resident brown trout (Salmo trutta) in different parts of the subarctic River Näätämöjoki/Neidenelva system (Finland and Norway) using carbon, nitrogen and hydrogen stable isotope analyses of archived scales as identifiers of migration strategy. Our results showed that carbon stable isotope values were the best predictor of migration strategy. Most individuals fell into two clearly distinct groups representing anadromous (47%) or freshwater‐resident (42%) individuals, but some fish had intermediate carbon values suggesting repeated movement between freshwater and the sea. The proportion of anadromous individuals decreased steadily with distance from the sea forming a spatial continuum in migration strategies which is probably maintained by the combination of several factors such as divergent availability of food resources, variable migration costs and genetic differences. These within‐catchment differences in migration strategies should be taken into account in fisheries management practices.     

River temperature and adult anadromous Atlantic salmon,Salmo salar,and brown trout,Salmo trutta

Abstract In terms of the spawning migration of adult salmon, Salmo salar L., water flow is often considered the primary factor controlling river entry and fluctuations in flow controlling when the fish subsequently migrate upstream. However, water temperature has also been suggested to modify the spawning migration of salmon, particularly their movements within estuaries and the timing of freshwater entry. Freshwater temperature is more likely to impact salmonid biology than flow, particularly in relation to temperature dependant metabolic costs, time of spawning and fecundity. Therefore, temperature may be more of a factor regulating salmonid populations in fresh water than flow itself. This study focuses on two aspects of the impact of temperature on salmonids in fresh water: first, how salmon may modify their behaviour to adapt to changes in temperature and second the potential relationship between temperature, environmental conditions (e.g. water quality) and physiology (e.g. maturation and olfaction) in regulating adult migration.     

Male‐biased sex ratio of nonanadromous Oncorhynchus mykiss in a partially migratory population in California

Abstract – Differential rates of anadromy between males and females are common in partially migratory salmonid populations, but this pattern is not fully clear for Oncorhynchus mykiss (rainbow trout/steelhead) from the limited but mixed data available. In particular, there are very few data on sex ratios of juvenile and nonanadromous (resident) fish to help assess sex composition of various life stages and life‐history types. We used a recently developed Y‐chromosome genetic marker to assess sex ratio of stream‐dwelling (i.e., juvenile and nonanadromous) O. mykiss in a small coastal basin in central California, USA. We analysed 384 samples collected from three contiguous study reaches over 3 years. Sex ratio was 1:1 among juvenile‐sized O. mykiss (<150 mm) but highly male‐skewed (83%) among nonanadromous‐sized individuals (≥150 mm), and this sex ratio × size pattern did not differ among years or study reaches. Our results suggest that the rate of anadromy differs between males and females in this basin. Our study also demonstrates the application of new genetic markers to determine sex composition of immature and nonanadromous salmonids, which will help assess sex‐specific life‐history behaviour in partially migratory populations of O. mykiss and other species.     

Water level influences migratory patterns of anadromous brown trout in small streams

Habitats modify the population ecology of species. Here, we show how low water level influences abundance and size of adult anadromous brown trout (Salmo trutta) entering a small, South Norwegian stream for spawning. After smolting, the fish appear chiefly to feed within 10 km of the home stream. In the autumn, South Norwegian streams typically flood because of heavy rainfall, when the anadromous brown trout entered from the sea. Mean annual duration of the upstream migration period was 34 days and ended when the flooding ended and the water temperature dropped to below 4°C. During most of the migration period, on average two trout ascended the river per day. The sexes entered the spawning area concurrently, and the male:female ratio of the anadromous trout was 1.27. No fish entered when the water depth just downstream of the spawning area was below 5 cm, and mean number of fish increased with increasing water depth to ca. 30 cm, but not at higher flows when the ascent gradually decreased. Mean and maximum size of the entering spawners increased with water depth between 5 and 16 cm. Among those that had been to sea, most were recaptured in the home stream, 4% in other streams, but only two of the strays were caught close to spawning time. The present results illustrate that population traits of anadromous brown trout from a small stream differ from those in larger rivers, probably because of selection associated with water flow.     

Marine migrations in anadromous brown trout (Salmo trutta). Fjord residency as a possible alternative in the continuum of migration to the open sea

Partial migration is a common phenomenon in many fish species. Trout (Salmo trutta) is a partially migratory species where some part of the population migrate to the marine environment, while another remains in freshwater. In the years 2008 and 2009, a total of 159 wild sea trout smolts were tagged with acoustic and PIT‐tags in the river Villestrup (Denmark) to study the initial postsmolt marine behaviour within a fjord system. We found that the strategies of the sea migrants vary: some stay in the fjord, while others migrate to the sea, suggesting that partial migration occurs even in the marine environments. Overall, a total of 53% of the tagged smolts migrated from the fjord to the sea, and 47% stayed (or potentially died) in the fjord. The ratios of fjord‐resident versus sea‐migrating postsmolts were consistent at the study times, and no differences between the early and late migration periods of the smolts were observed. The individual's size or body condition at the time of tagging did not affect survival or the migratory decisions in the fjord. High overall initial survival (74%) was found 30 days after the fjord entry. We suggest that within a continuum of migration to sea, there is a migratory decision point when sea trout postsmolts encounter a fjord system. At this point, postsmolts will assess the possibility of migration versus the alternative of fjord residency.     

Alternative routes to piscivory: Contrasting growth trajectories in brown trout (Salmo trutta) ecotypes exhibiting contrasting life history strategies

Large and long‐lived piscivorous brown trout, Salmo trutta, colloquially known as ferox trout, have been described from a number of oligotrophic lakes in Britain and Ireland. The “ferox” life history strategy is associated with accelerated growth following an ontogenetic switch to piscivory and extended longevity (up to 23 years in the UK). Thus, ferox trout often reach much larger sizes and older ages than sympatric lacustrine invertebrate‐feeding trout. Conventional models suggest that S. trutta adopting this life history strategy grow slowly before a size threshold is reached, after which, this gape‐limited predator undergoes a diet switch to a highly nutritional prey source (fish) resulting in a measurable growth acceleration. This conventional model of ferox trout growth was tested by comparing growth trajectories and age structures of ferox trout and sympatric invertebrate‐feeding trout in multiple lake systems in Scotland. In two of the three lakes examined, fish displaying alternative life history strategies, but living in sympatry, exhibited distinctly different growth trajectories. In the third lake, a similar pattern of growth was observed between trophic groups. Piscivorous trout were significantly older than sympatric invertebrate‐feeding trout at all sites, but ultimate body size was greater in only two of three sites. This study demonstrates that there are multiple ontogenetic growth pathways to achieving piscivory in S. trutta and that the adoption of a piscivorous diet may be a factor contributing to the extension of lifespan.     

Effects of size and morphology on swimming performance in juvenile brown trout (Salmo trutta L.)

Abstract –  Our study assesses swimming capacity (speed and stamina) and possible morphometric determinants of locomotor performance of juvenile brown trout ( Salmo trutta L.). We addressed these issues at the individual level to have an approach of the functional significance of intraspecific variation in morphological design. Both swimming speed and endurance time showed significant positive relationships with fish length. Size-corrected values of speed and endurance time were negatively correlated suggesting a phenotypic trade-off between burst and prolonged swimming. Size was also highly correlated with all the morphological variables measured. Therefore, we used the residuals of the regressions of those variables on fish length to remove the effect of body size. A principal components analysis (PCA) summarised the 12 morphological variables into two factors, which accounted for 44.3% of the variance. PC1 combined several measures of body depth and width, whereas PC2 represented mainly postanal length relative to abdomen length. Relationships between the scores of the two factors and size-corrected values of maximum swimming speed and endurance time were weak. PC2 showed a significant positive relationship with endurance time; that is, individuals with longer caudal regions were able to swim against water flow for longer periods of time. Stoutness (PC1) showed a marginally significant negative correlation with endurance time. The lack of stronger relationships could be because of the low morphometric variability among the test individuals, all proceeding from the same population, reared in a common environment, and measured at the same ontogenetic stage.     

Atlantic salmon Salmo salar L., brown trout Salmo trutta L. and Arctic charr Salvelinus alpinus (L.): a review of aspects of their life histories

Abstract – Among the species in the family Salmonidae, those represented by the genera Salmo, Salvelinus, and Oncorhynchus (subfamily Salmoninae) are the most studied. Here, various aspects of phenotypic and life‐history variation of Atlantic salmon Salmo salar L., brown trout Salmo trutta L., and Arctic charr Salvelinus alpinus (L.) are reviewed. While many strategies and tactics are commonly used by these species, there are also differences in their ecology and population dynamics that result in a variety of interesting and diverse topics that are challenging for future research. Atlantic salmon display considerable phenotypic plasticity and variability in life‐history characters ranging from fully freshwater resident forms, where females can mature at approximately 10 cm in length, to anadromous populations characterised by 3–5 sea‐winter (5SW) salmon. Even within simple 1SW populations, 20 or more spawning life‐history types can be identified. Juveniles in freshwater can use both fluvial and lacustrine habitats for rearing, and while most smolts migrate to sea during the spring, fall migrations occur in some populations. At sea, some salmon undertake extensive oceanic migrations while other populations stay within the geographical confines of areas such as the Baltic Sea. At the other extreme are those that reside in estuaries and return to freshwater to spawn after spending only a few months at sea. The review of information on the diversity of life‐history forms is related to conservation aspects associated with Atlantic salmon populations and current trends in abundance and survival. Brown trout is indigenous to Europe, North Africa and western Asia, but was introduced into at least 24 countries outside Europe and now has a world‐wide distribution. It exploits both fresh and salt waters for feeding and spawning (brackish), and populations are often partially migratory. One part of the population leaves and feeds elsewhere, while another part stays as residents. In large, complex systems, the species is polymorphic with different size morphs in the various parts of the habitat. Brown trout feed close to the surface and near shore, but large individuals may move far offshore. The species exhibits ontogenetic niche shifts partly related to size and partly to developmental rate. They switch when the amount of surplus energy available for growth becomes small with fast growers being younger and smaller fish than slow growers. Brown trout is an opportunistic carnivore, but individuals specialise at least temporarily on particular food items; insect larvae are important for the young in streams, while littoral epibenthos in lakes and fish are most important for large trout. The sexes differ in resource use and size. Females are more inclined than males to become migratory and feed in pelagic waters. Males exploit running water, near‐shore and surface waters more than females. Therefore, females feed more on zooplankton and exhibit a more uniform phenotype than males. The Arctic charr is the northernmost freshwater fish on earth, with a circumpolar distribution in the Holarctic that matches the last glaciation. Recent mtDNA studies indicate that there are five phylogeographic lineages (Atlantic, Arctic, Bering, Siberian and Acadian) that may be of Pleistocene origin. Phenotypic expression and ecology are more variable in charr than in most fish. Weights at maturation range from 3 g to 12 kg. Population differences in morphology and coloration are large and can have some genetic basis. Charr live in streams, at sea and in all habitats of oligotrophic lakes, including very deep areas. Ontogenetic habitat shifts between lacustrine habitats are common. The charr feed on all major prey types of streams, lakes and near‐shore marine habitats, but has high niche flexibility in competition. Cannibalism is expressed in several cases, and can be important for developing and maintaining bimodal size distributions. Anadromy is found in the northern part of its range and involves about 40, but sometimes more days in the sea. All charr overwinter in freshwater. Partial migration is common, but the degree of anadromy varies greatly among populations. The food at sea includes zooplankton and pelagic fish, but also epibenthos. Polymorphism and sympatric morphs are much studied. As a prominent fish of glaciated lakes, charr is an important species for studying ecological speciation by the combination of field studies and experiments, particularly in the fields of morphometric heterochrony and comparative behaviour.     

Genetic differentiation between anadromous and freshwater resident brown trout (Salmo trutta L.): insights obtained from stable isotope analysis

Abstract –  A genetic survey was carried out on recently emerging fry to investigate interbreeding between anadromous and freshwater resident brown trout in La Roche Brook, a major spawning tributary of the Oir River system (Normandy, France). Emerging fry were sampled at three different sites in spring 2002 and 2003. Stable isotope analysis (SIA) allowed the classification of each individual as the progeny of resident ( N  = 76) or anadromous ( N  = 58) female trout. Microsatellite DNA analysis showed that genetic diversity between fry samples can be explained by microgeographical (various sampling sites) and temporal (2 years of sampling) variations rather than maternal origin (anadromous vs. freshwater resident). We conclude that a high level of gene flow exists between the two morphs when anadromous adults have access to the spawning grounds of residents. We highlight the relevance of using SIA in conjunction with genetic analysis to study interactions between the two morphs.     

Habitat suitability analysis for lacustrine brown trout (Salmo trutta) in Lake Walchensee,Germany: implications for the conservation of an endangered flagship species

• 1. The lacustrine brown trout (Salmo trutta) is endangered and of high conservation importance. In the only spawning habitat of the population in the Bavarian Lake Walchensee, the River Obernach, a substantial decrease in spawning runs has been reported. In this study, the present ecological state of the spawning stream was analysed with the objective of identifying life‐stage specific limitations to successful recruitment attributable to deficiencies in (i) spawning migration, (ii) spawning habitat quality, and (iii) habitat quality for juveniles.
• 2. Structural stream analysis showed that discharge and several migration barriers — particularly near the river outlet into the lake — prevent successful spawning migrations at normal water levels. Migration barriers are probably the main limiting factor for reproduction of lacustrine brown trout, whereas structural variability of the Obernach meets the habitat requirements of both spawners and juveniles.
• 3. Spawning site quality was suitable for trout, as indicated by stream substratum texture and high exchange rates between free‐flowing water and the interstitial zone in physico‐chemical parameters (redox potential, dissolved oxygen, pH, temperature and conductivity).
• 4. Analyses of fish community structure revealed dominance of lithophilic species, in particular of riverine brown trout (Salmo trutta). Its density and intact demographic population structure suggest that spawning and juvenile habitat quality for salmonids is not limiting. Recapture of stocked lacustrine trout juveniles also indicates habitat suitability for the juvenile stage.
• 5. In conclusion, the results show that the methodology used in this study is suitable for the identification of life‐stage specific habitat deficiencies in lacustrine brown trout and other fish species. Availability of habitat data throughout the species' distribution range is a first crucial step for the development of an effective recovery plan. Copyright © 2009 John Wiley & Sons, Ltd.

     

Linking migratory patterns and diet to reproductive traits in female brown trout (Salmo trutta L.) by means of stable isotope analysis on ova

Abstract –  Many populations of vertebrates display alternative reproductive strategies involving delays in maturation processes and migrations to more productive habitats. Notably for females, migratory patterns and diet could influence reproductive traits such as ova size and number. To test this hypothesis, we targeted anadromous and freshwater-resident female brown trout of various sizes during their return to spawn in tributaries, and carried out stable isotope analyses (δ¹⁵N and δ¹³C) on ova to identify their feeding areas and diet. The study was performed in the Oir River (France), a small coastal system where trout migrations to sea and within the river basin have been monitored for the last 20 years. General trends were observed in reproductive traits, such as the positive correlation between female fork length, fecundity and ova size. Fecundity was size-dependent whatever the migratory origin of females, but major divergences were seen for ova size and isotopic ratios. Freshwater residents displayed a constant increase in ova size and a ¹⁵N-enrichment in ova, suggesting an investment in larger ova and a shift to piscivory as the fish become larger and older. Conversely, anadromous females had smaller ova compared with freshwater-resident females of similar body size and achieved higher fecundity as they grew bigger. Our results suggest the existence of a continuum of reproductive traits for freshwater-resident females whereas anadromous females clearly show a break with this continuum. This major dissimilarity could be explained by the difference in growing environments in terms of food availability and quality.     

The effects of ration size on migration by hatchery‐raised Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Abstract – The possibility to increase the proportion of migrating hatchery‐reared smolts by reducing their food ration was studied. Lake‐migrating, hatchery‐reared salmon (Salmo salar) and trout (Salmo trutta) smolts were either fed normal rations, based on recommendations from the fish‐farming industry, or reduced (15–20%) rations. They were released into the River Klarälven, western Sweden, and followed as they swam downstream to Lake Vänern, a distance of around 25 km. For both Atlantic salmon and brown trout, smolts fed a reduced ration migrated faster than fish fed a normal ration. Furthermore, a higher proportion of salmon smolts fed reduced rations migrated to the lake than fish fed normal rations in 2007 but not in 2006. This difference between years corresponded to greater treatment differences in size and smolt status in 2007 than in 2006. For trout, the proportion of migrating individuals and smolt development did not differ with ration size. Trout migrants fed a normal ration had a higher standard metabolic rate (SMR) than nonmigrants, whereas there was no difference in SMR between migrating and nonmigrating salmon. These results show that it is possible to use a reduced food ration to increase the migration speed of both Atlantic salmon and brown trout and to increase the proportion of migrating Atlantic salmon.     

Rehabilitation of brown trout, Salmo trutta, habitat damaged by riparian grazing in an English chalkstream

Abstract  Rehabilitation trials involving riparian fencing and limited pool excavation were conducted on the River Piddle and Devil's Brook, Dorset, England, which had been degraded by intensive riparian grazing. In one trial, based on two short (94 and 99 m) treated sections and two control sections, brown trout, Salmo trutta L., numbers were monitored from 1994 (pre-treatment) to 2000. In a second trial from 1996 (pre-treatment) until 2000, trout numbers were monitored in 900 m and 1400 m treated sections. After rehabilitation, juvenile trout numbers increased in the two short sections (Trial 1) but fell in one of the long sections. Adult numbers also increased markedly in the two short-treated sections relative to the controls and they increased markedly in one of the long sections despite a reduction in juveniles. Marking of trout in the short sections showed that they selected the rehabilitated habitat in preference to the control habitat and that immigration was the main source of adult trout, as it must also have been in the 1400 m section. While the results indicate that improvements can be made to adult trout habitat, more research is required on the impact on juvenile production before the impact of such work on the true population can be established.     

A test for interaction between brown trout (Salmo trutta) and inanga (Galaxias maculatus) in an artificial stream

Abstract –  The interaction between brown trout ( Salmo trutta ; fork length (FL) range 255–390 mm) and inanga ( Galaxias maculatus ; FL range 55–115 mm) was tested during summer through autumn in an artificial stream consisting of a single run-riffle-pool sequence with a natural food supply. Each experimental trial lasted for 15 days, and consisted of two brown trout and 50 inanga collected fresh from a nearby stream, with each species given prior residence in four replicate tests, totalling eight trials in all. In addition, two control trials (each 10 days), with 50 inanga in each, were run. Brown trout almost exclusively occupied the pool, whereas inanga occupied all habitat types, although in different proportions, when tested with and without brown trout. The proportion of inanga in the pool was appreciably lower in the experimental trials with brown trout than in the control trials with no brown trout; prior residence had no significant effect on inanga habitat use. Mortality of inanga attributable to predation by brown trout ranged from 0 to 40% with a mean of 14.5 ± 4.7%. The results suggest that habitat use and survival of inanga populations in small streams can be adversely affected by brown trout.     

The role of the environment in partial migration: food availability affects the adoption of a migratory tactic in brown trout Salmo trutta

Abstract –  Alternative reproductive tactics are commonly reported for salmonids (Pisces) and typically involve large migratory and small resident individuals. Variation in migratory tendency should reflect the different benefits and costs that the two different phenotypes face with regard to fitness. Therefore, the effect of food availability on the adoption of a migratory tactic in brown trout was investigated. Fifty trout were placed in each of 12 tanks and fed at three different levels. Growth-related variables were measured regularly, and at the end of the experiment, the proportion of migrants and residents was recorded. Low food availability led to increased numbers of migratory fish. The expected sex-bias was also present, with a lower percentage of resident females than resident males. As all fish originated from the same gene pool, the changing proportions of the migratory tactics can be classified as phenotypic plasticity. The study provides evidence that the different phenotypes reflect alternative tactics within a conditional strategy. Some differences in growth-related variables were present between the sexes, and a very pronounced difference in condition factor was found between resident and migratory males, but not in females. Thus, the results provide evidence that different selective forces may be acting on the sexes.     

Thyroid hormones in brown trout (Salmo trutta) reproduction and early development

Gravid brown trout (Salmo trutta) females were injected with various doses of a synthetic gonadotropin-releasing hormone analog (GnRHa), given with or without an injection of triiodothyronine (T₃), in order to investigate the potential of T₃ (a) to enhance the stimulatory effect of GnRHa on ovulation, and (b) to enhance the growth and survival of the produced progeny. From the time the hormonal treatments were initiated until ovulation was detected 5–38 days later, endogenous plasma T₃ levels increased from an average of 3.6 to 11.6 ng ml⁻¹. Injection with 20 mg T₃ kg⁻¹ body weight, further elevated plasma T₃ levels at ovulation (16.0 ng ml⁻¹. Mean time to ovulation was reduced significantly in fish injected with 10 μg kg⁻¹ of GnRHa, whereas treatment with lower doses was ineffective. Injection with T₃ did not enhance the ovulatory response of brown trout to GnRHa. Unfertilized eggs obtained from T₃-injected females had a higher T₃ content, suggesting a transfer of T₃ from the maternal circulation into the oocytes. Maternal T₃ injection had no effect on egg fertilization rates, embryo survival to eyeing and hatching, or the prevalence of abnormal larvae at the time of hatching. Length and weight gain of the progeny during yolk absorption was also not influenced by maternal T₃ treatment. At the completion of yolk-sac absorption, progeny from females injected with T₃ had a higher prevalence of skeletal abnormalities than controls. The results suggest that in teleosts like brown trout, which have high endogenous circulating T₃ levels, treatment of females with T₃ does not enhance responsiveness to GnRHa and it has the potential for deleterious effects on their offspring.     

Variation in growth and aggression of juvenile brown trout (Salmo trutta) from upstream and downstream reaches of the same river

Abstract –  To investigate differences in behaviour and growth between juvenile brown trout from spatially separated reaches, fish were collected from above and below a weir in Silverstream, New Zealand. Population level differences in mass-specific growth rates and aggression level were examined in tank experiments. Monitoring of mass-specific growth rates found that relative growth rates between fish of different sizes varied between fish from the upstream reach but not between fish from the downstream reach. Large individuals grew faster than smaller individuals from the upstream reach but no effect of relative fish size on growth was found among fish from the downstream population, indicating that social organisation in the two populations differed. When tested in pairs, juvenile brown trout from the downstream population were found to be more aggressive than those from the upstream population. Differences in the allometric relationship between growth rate and fish size and in the levels of aggressiveness appear to be related to the failure to form dominance hierarchies among fish from the downstream reach, a fixed behavioural trait most likely related to prior experience of crowding or different genetic backgrounds.     

Bigger is not always better: egg size influences survival throughout incubation in brown trout (Salmo trutta)

Classical optimality models for the evolution of egg size predict a single optimal investment for females inferior to the optimal investment for offspring because of the egg size–fecundity trade‐off and the assumption that ‘bigger is better’ for offspring fitness. Such models do not satisfactorily represent observed within‐population variation in egg size. We measured the influence of maternal investment in egg size on offspring survival in brown trout. Individual measures of egg size, metabolism and survival at different temperatures throughout ontogeny were carried out. We then developed a survival model with regard to egg size, incubation temperature and observed metabolic rate. Small eggs were found to survive at higher rates than large eggs, and the egg size–survival relationship was found to differ among females in accordance with average metabolic rate measured at hatching. These results provide insights for the understanding of the evolutionary significance of egg size variations within a population.     

Effect of supplementary stocking of juvenile brown trout,Salmo trutta,on yield in a Norwegian mountain reservoir

Abstract The effect of supplementary stocking of juvenile (age 0+), hatchery‐reared, brown trout, Salmo trutta L., on annual yields was assessed in a Norwegian mountain reservoir between 1979 and 2007. Fishing was mainly carried out by local fishermen with benthic gillnets. During the study period, annual stocking ranged from 0 to 52 500 fish (19.8 ha^?1). No stocking has been carried out since 1997. Annual yield varied from 1650 to 5653 kg, corresponding to 0.62–2.13 kg ha^?1. Exploitation rate in terms of number of gillnets and mean weight of 6+ fish (age when catchable size was reached) explained 64% of the variability in catches. Stocked fish contributed very little to the yield or catch‐per‐unit‐effort, exhibiting no positive correlation with stocking density. The lack of contribution from stocked fish was probably caused by a competitive bottleneck in the eroded epibenthic zone, causing high juvenile mortality. If stocking continues, it is recommended that fish with body lengths >15–20 cm are used.