Population regulation in adult, but not juvenile, resident trout (Salmo trutta) in a Lake District stream

1. Unlike a neighbouring sea-trout population that showed strong density-dependent survival, a resident trout population ( Salmo trutta L.) showed simple proportionate survival in the early life-stages. However, this persistent population fluctuated within narrow limits. Mature adults, especially during spawning, were the only possible life-stage left in which regulation might occur. 2. An October census, just prior to spawning, was made at five sites (total area 300 m ² ) from 1965 to 1983. Gravel nests (redds) associated with females of known size were excavated outside these sites to obtain a power-function relationship between egg density per redd and female length (range 181–280 mm, n = 26). This relationship and the census data for females (range 186–284 mm) were used to estimate egg densities in each year-class. 3. The census data for the early life-stages (0+, 1+, 2+ trout) confirmed proportionate survival with no evidence for density-dependent regulation. In contrast, the number of spawning females produced in each year-class was strongly density dependent on the initial number of females that laid eggs at the start of the year-class. Similarly, total egg production in each year-class was density dependent on initial egg density. 4 Both relationships were well described by the Ricker and Beverton-Holt stock-recruitment models (P < 0.001) and the goodness-of-fit was similar for both models. This study is probably the first to provide clear evidence for fish population regulation in the adult, rather than the juvenile, stage.     

The distances travelled by downstream-moving trout fry, Salmo trutta, in a Lake District stream

SUMMARY. 1. Field experiments were performed in the day and night at six modal water velocities (range 10–52cm s⁻¹), using: (i) newly- emerged fry without neutral buoyancy; (ii) older fry in poor condition (weight well below that expected for resident fry); (iii) older fry in good condition (weight similar to that of resident fry); (iv) dead fry.
2. An exponential model described the return rate of fry to the stream bottom; the mean distance travelled downstream varied considerably between the four fry categories, but always increased linearly with increasing water velocity.
3. Results were similar for dead fry and newly-emerged fry released at night; 50% of the fry returned to the bottom in 10–11 s and nearly all returned in c . 70s, the maximum distance travelled ranging from c . 7 m at 10 cm s⁻¹ to c . 37m at 52cm s⁻¹, Newly-emerged fry released in the day returned slightly faster (54s for 99% return to bottom).
4. Older fry in poor condition returned to the bottom slightly faster in the day than at night, but took about 2 min and travelled about twice the distance covered by dead fry. Older fry in good condition returned to the bottom at the fastest rate (3–6s for 50% and c . 30s for the rest), and travelled only about half (at night) or a third (in day) of the distance covered by dead fry.
5. The implications of this investigation are discussed and it is concluded that, apart from water velocity, the age and condition of the fry were the two most important factors affecting their downstream movement.     

Eubothrium crassum in migratory trout, Salmo trutta L., in the sea

The incidence and intensity of infection of various ages and conditions of migratory trout by Eubothrium crassum are described. A marine source of the worm seems likely and explanations for the characteristics of the parasite burden are given on this assumption. Within weeks of first going to sea the hosts accumulate large numbers of the parasite. One year suffices for the life cycle of Eubothrium but growth continues for at least two years and possibly for longer. The parasite burden decreases in older fish because of the stress imposed on the parasite by successive migrations of the host to and from fresh water and because older fish tend to feed earlier in the year.     

Population genetic structure of brown trout (Salmo trutta L.) within a northern boreal forest stream

Brown trout (Salmo trutta L.) population genetic structure and its temporal stability were studied within a small forest stream in central Sweden using five microsatellite loci. Both resident and migrating brown trout are present in the watershed. Tissue samples were collected from seven sections of the stream during two consecutive years. No differences were found in multilocus FST estimates between years within sections except in one case. Moreover, differences between age cohorts within sample sections were rare. The low interannual variation and the low heterogeny between cohorts is interpreted as indications of temporal stability. Pairwise multilocus FST estimates increased with increasing geographic distance, indicating isolation by distance. It is argued that the brown trout of the F?rs?n stream represent a population complex. The structure is probably maintained by precise natal homing and a limited, but important, amount of gene flow between closely situated sections within the stream supporting a stepping-stone model of gene flow.     

Validation and implications of a growth model for brown trout, Salmo trutta, using long-term data from a small stream in north-west England

Summary 1. The objectives were: (i) to check the validity of a new growth model; (ii) to examine the relationship between population density and both mean mass and mean growth rate and (iii) to discover if compensatory growth occurred. First (0+) and second (1+) year‐old juvenile sea‐trout were sampled by electrofishing at the beginning and end of the summer from 1967 to 2000. Additional samples were taken in some years in winter and in the critical period for survival when the fry first emerge from the gravel. The trout left the stream as pre‐smolts in May, soon after their second birthday. 2. A growth model ( Elliott, Hurley & Fryer, 1995 ) estimated the mean mass of the trout over the 2 years spent in fresh water. The date and mean mass at the start of the growth period were defined as the median date for fry emerging from the gravel and their mean mass at emergence, both being estimated from individual‐based models ( Elliott & Hurley, 1998a, b ). 3. The variation in mean mass among year‐classes was small for newly‐emerged fry (CV = 6.2%), maximum at the start of the first summer of the life cycle (CV = 38.1%), and then decreased gradually for successive life‐stages to a low value for pre‐smolts (CV = 10.8%). Mean mass was not related to population density and, therefore, mean growth rate was density‐independent. Growth in the first, but not the second, winter of the life cycle was lower than model prediction, but when it was assumed in the model that there was no first‐winter growth, there was good agreement in most year‐classes between model estimated values and observed mean mass. Exceptions were that mean masses and growth rates for 0+ trout after four summer droughts were lower than expected, but compensatory growth followed, so that observed and expected masses were similar for 1+ trout. 4. Pre‐smolt mean mass on 30 April measured total growth achieved in the freshwater phase of the life cycle. This was significantly related to mean mass at the end of the first and second summers of the life cycle, but not to the emergence date and mean mass of emerging fry. 5. These juvenile sea‐trout were growing at their maximum potential in most year‐classes but when this was not achieved, compensatory growth soon restored their mass to values expected from the model. This ensured a low variation in the mean mass of pre‐smolts just before they migrated to the sea. However, the latter mass was higher in more recent year‐classes (1987–98) than in previous ones (1967–86), demonstrating the effect of slightly higher stream temperature. This study has shown the importance of developing realistic growth models in order to detect departure from maximum potential growth, and the more subtle effects of temperature change, possibly due to the effects of climate change.     

Population dynamics,production and angling catch of brown trout,Salmo trutta,in a mature upland reservoir in mid-Wales

Synopsis The brown trout in Llyn Frongoch, a mature upland reservoir, and its nursery stream was sampled during 1983. The stream stock consisted largely of the 1983 and 1982 year classes, with fish reaching mean lengths of 7.0 and 11.6 cm at one and two years of age. The size and biomass of the stream stock at the beginning of 1983 and 1984 were estimated to be 120 and 125 (1.20 and 1.25 fish m^–2) and 1.41 and 0.69 kg (14.1 g m^–2 and 6.9 g m^–2) respectively. Annual stream production ranged from an estimated minimum of 2.49 kg (24.9 g m^–2) to an estimated maximum of 4.59 kg (45.9 g m^–2). Both downstream and upstream movements of 0+ juveniles were recorded. The adult spawning stock was estimated at 79 males and 32 females, a sex ratio of 2.5:1, with most spawners belonging to the 1980 yearclass. The average size of the lake stock over the year was estimated to be 1 650 (229 fish ha^–1) or 250.8 kg (34.8 kg ha^–1). The 1980 yearclass was predominant; there were few fish older than five years. Seasonal variations in netting catches suggested movements to and from the littoral region. Growth in the lake was moderately fast, with fish reaching mean lengths of 21.7 and 27.2 cm by three and four years of age. Fish entering the lake after one year appeared to grow faster than fish which remained in the stream for two years. Annual production in the lake was estimated at 136.7 kg (19.0 kg ha^–1). The total angling catch for the season was estimated to be 62.6 kg (8.7 kg ha^–1).     

Home range of juvenile Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, in a Norwegian stream

SUMMARY. 1. The sizes of home ranges of juvenile Atlantic salmon (age 1 +) and brown trout (age 2+ to 9+) in a Norwegian coastal stream were estimated by local movements of batch-marked fish from 12.5 and 25 m long sections. Only recoveries made in the release section and in up-and downstream neighbouring sections were considered.
2. There was no significant difference in the average percentage of recaptures of salmon and trout between 12.5 and 25 m sections; a stream area of about 40–50 m² defines the size of home range for stocks of both species.
3. The fraction of brown trout recaptured in release sections increased with increasing fish densities, indicating a smaller home range under these conditions.     

Growth, survival and production of juvenile salmon and trout in a Scottish stream, 1966–75

Investigations of the growth, survival and production of young salmon Salmo salar , brown trout and sea trout S. trutta in sections of a stream in Scotland were made during 1966–75. At the end of the growing season, in autumn, the size of the 0+ salmon ranged from a mean weight of 1.12 g in 1966 to 2.82 g in 1973, and the size of the 0+ trout ranged from a mean of 2.20 g in 1966 to 3.56 g (68.0 mm) in 1974. Growth rates of 0+ salmon between July to September were similar from year to year, as was the case with the 0+ trout. The greater size attained in their first year by trout, resulted from the longer feeding season, provided by earlier emergence of fry and ability to continue growing in colder weather in autumn. The lengths attained by 0+ salmon and 0+ trout in September were related to the population densities of 0+ salmon and the number of days above 0° C from 1 December. There was no discernible relationship between lengths of 0+ trout and the population densities of 0+ trout. Salmon and trout lost weight during the winter, which was made up by April. The densities of 0+ salmon in June varied between 2–12m ^–2. Rates of decrease of the population densities in their first year were related to their densities at the beginning of the season, and, more closely, to the densities of salmon and trout combined. At the end of the second year's growth there were between 0.06 and 0.25 salmon m ^–2. Size of the trout populations varied less from year to year than those of salmon. The life of a year class of salmon and trout could be divided into several stages characterized by different rates of decrease of the population.     

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.     

Movement and site fidelity in young brown trout Salmo trutta populations in a southern Irish stream

Populations of brown trout Salmo trutta were monitored at a number of sites within a single stream, using an individual marking technique and recapturing uniquely marked fish repeatedly over a period of 12 months. Individual 1 + and 2 + resident brown trout in the Glenfinish River were found to consist of stationary and mobile component populations. The latter population consisted of a number of individuals observed moving mostly in an upstream direction, within a range of 0.03–2.24 km. On a large spatial scale, individuals in the stationary component population exhibited some degree of home site fidelity within the stream, over a period of 3–4 months, after which the fish tended to move from the site. Within sites, fidelity to either riffle or pool habitats, mostly the latter, was apparent in a proportion of the population. On a smaller scale, fidelity to the exact position with respect to boulders in the stream was also evident in a number of individuals. Home range size was calculated amongst these individuals, with ranges of up to 20 m recorded.     

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.     

Synchrony in brown trout, Salmo trutta, population dynamics: a 'Moran effect' on early-life stages

Synchrony among populations (i.e. spatial covariation in temporal fluctuations of population size or growth rate) is a common feature to many animals. Both large-scale autocorrelated climatic factors (the 'Moran effect') and dispersal between populations are candidates to explain synchrony, although their relative influence is difficult to assess. Only a few investigations have reported patterns of synchrony among freshwater populations, and even fewer directly related these patterns to an environmental variable. In the present study, we analysed the spatio-temporal patterns of fluctuation of 57 brown trout populations widespread across France, each sampled continuously during 5 years. We compared the respective influence of connectivity and stream distance within basins (i.e. that potentially allow a basin-scale dispersal) and environmental factors (hydrological and air temperature variables, available for 37 sites) on the synchrony of brown trout cohort densities (0+, 1+ and adults). A series of Mantel tests revealed that the degree of synchrony was not related to connectivity or stream distance between sites, suggesting no effect of dispersal at the basin-scale. The degree of synchrony among sites for the 0+ fish was significantly related to the degree of hydrological synchrony (based on high flows during the emergence period). For all three age classes, the synchrony in the temperature patterns did not explain synchrony in trout dynamics. Our results allow us to discuss the respective influence of dispersal and climatic factors on the spatio-temporal patterns of trout dynamics at the basin scale.     

Environmental determinants of recruitment and their influence on the population dynamics of stream-living brown trout Salmo trutta

The relative importance of endogenous feedback mechanism vs environmental factors in the dynamics of animal populations is a long-standing, but not fully resolved yet, issue in ecology. We have addressed this subject by examining the dynamics of a stream-resident population of Salmo trutta in a northwestern Spain stream. Recruitment was the major determinant of population size and the abundance of recruits resulted from a combination of regional and local environmental factors. Stream discharge in March determined the amount of stream area suitable for newly emerged trout (r²=0.59–0.79%), that in turn determined the abundance of recruits at each site (r²=0.51–0.77%). Stream discharge determines the overall strength of annual recruitment. Discharge, however, combines with stream morphology at the site scale to result in a site-specific area suitable for juveniles and, hence, site-specific recruitment. Thus, our study exemplifies how an environmentally driven animal population may persist on time with little or no operation of endogenous regulatory mechanisms.     

High incidence of Atlantic salmon × brown trout hybrids in a Lake District stream

Hypervariable minisatellite DNA single-locus profiling and mitochondrial DNA analysis revealed that 18.48% of juvenile Atlantic salmon Salmo salar in Troutbeck, a stream in the R. Leven catchment of the English Lake District, were hybrids between Atlantic salmon and brown trout S. trutta , and that hybridization was bidirectional.     

Reproductive investment of female brown trout, Salmo trutta L., in a stream and reservoir system in northern England

Within the basin of Cow Green reservoir, upper Teesdale, the brown trout, Salmo trutta L., spend their first two seasons in running water. After this some females remain resident and others migrate to Cow Green reservoir. A cohort of 1000 females at age 2 that remains resident in tributary streams will, during the lifetime of the cohort, lay, on average, 3·0 times as many eggs as a cohort which resides in the reservoir. This conclusion is apparently not consistent with the observation that most females take up reservoir residence.
Reservoir females have a higher growth rate (Walford constant k=0·61, c.f. 0·86 for stream residents) and shorter life expectancy ( M year⁻¹= 1·19, c.f. 0·56) than the stream residents. However, they are of larger average size than the stream residents, lay larger eggs (mean 0·074 g, c.f. 0·066 g) and bury them more deeply (mean depth 11·0 cm, c.f. 8·5 cm).     

Recruitment as a driver of production dynamics in stream-resident brown trout (Salmo trutta)

1. The objective was to assess the role of recruitment as a determinant of the production dynamics of stream-resident brown trout ( Salmo trutta ) across replicate habitats of contrasting quality and population attributes. A total of 128-year-classes (YC) at 12 stream sites were examined along four tributaries of the Rio Esva drainage (northwestern Spain).
2. A meta-comparison revealed that growth, density, mortality and production were essentially site-specific. However, when all data were pooled, recruitment (as a delayed density-dependent process) affected both growth and mortality in a way such that individuals in YC with high recruitment grew less and had higher mortality.
3. The value of total YC production recorded covered the global range of variation in the production of stream salmonids reported in the literature. Linear regressions of log-transformed data revealed that 89.0%, 58.9% and 70.7% of the variation in YC density, biomass and production, respectively, were explained by variations in recruitment.
4. The inclusion of growth and mortality, together with recruitment, into a multiple regression increased the variance explained of the total YC production by 13.3%, from 70.7% to 84.0%.
5. The functional relationships between recruitment and the population attributes elucidated in this study appear to provide a useful tool for management applications, including forecasting population status.     

The population dynamics of young trout (Salmo trutta L.) in a Danish brook

The population size, dispersal of fry, growth of fry in relation to density, mortality and production of young trout ( Salmo trutta L.) were studied in a Danish brook during 1970–1972. In the first few months after emergence downstream emigration of fry took place both in 1971 and 1972, after which the trout became stationary. Growth was inversely density dependent and mortality was density dependent in the months just after emergence. Later mortality was constant and independent of density. Trout production varied between 10 g/m² and 18.6 g/m² in the 2 years.