The effects of intra- and inter-specific competition on the distribution of stocked juvenile Atlantic salmon, Salmo salar L., in relation to depth and gradient in an upland trout, Salmo trutta L., stream

The relative effects of inter- and intra-specific competition on the distribution of stocked salmon in relation to depth and gradient were investigated in an upland stream during two summer sampling periods. The stream was divided into two areas by an impassable fish barrier, and trout were removed from the upstream section prior to 2 years of salmon stocking. A small amount of trout re-immigration to the cleared area occurred. Under sympatric conditions in the downstream section the fry of both species were significantly more abundant in shallow water, whereas the yearling and older fish tended to inhabit the deeper, slow-flowing areas. Under near allopatric conditions in the cleared section salmon fry changed their distribution to include deeper areas, with shallow, fast-flowing water becoming the least preferred habitat. This trend for a wide distribution of salmon fry over all the available habitat in the upstream section was maintained in the second year when parr were present but trout remained at very low densities. It was therefore concluded that high inter-specific competition from trout was responsible for restricting the distribution of salmon fry to shallow habitat in the control area. Intra-specific competition from older salmon apparently only affected the growth and survival of salmon fry. The regulating mechanisms involved in these inter- and intra-specific effects are discussed in terms of competition for stream resources and predation. Recommendations based on the findings are made for stocking and habitat management of salmonid waters.     

Population changes after two years of salmon (Salmo salar L.) stocking in upland trout (Salmo tmtta L.) streams

The survival of salmon stocked in upland trout streams in the presence of salmon parr was found to be only about half the value recorded when trout alone made up the resident stock. Changes in the trout population were also recorded following the two years of salmon stocking, and these suggested that the presence of salmon parr may also influence trout fry survival. The findings are discussed in the context of habitat competition and total stream holding capacity.     

Growth and survival of Atlantic salmon (Salmo salar) in Llyn Dwythwch, North Wales

SUMMARY. Atlantic salmon fry have been annually stocked into Llyn Dwythwch, North Wales, since 1969, in an attempt to increase the natural stocks of the area. The growth and survival of 1- and 2-year-old salmon were investigated, and compared with that of other lake-reared populations and also with salmon in the natural stream environment. Lake-reared salmon follow the same patterns of slow and rapid growth as found for river fish, but the growth rate was superior in the former. The variation in length – weight relationship with age and sex was investigated. Survival rates in general compared favourably with the survival in rivers, with high mortality rates of salmon in Llyn Dwythwch resulting from predation at spring stocking by the resident brown trout. This was later avoided by stocking larger fish in the autumn.     

The food web positioning and trophic niche of the non-indigenous round goby: a comparison between two Baltic Sea populations

Native species may show invasiveness toward a recipient ecosystem through increases in abundance as a result of artificial stocking events. Salmonid species are typical examples of native invaders whose abundance is increased after stocking with hatchery fish. This study evaluated the effects of hatchery chum salmon fry on sympatric wild masu salmon fry, benthic invertebrate prey, and algae, after a single stocking event in Mamachi stream, Hokkaido, northern Japan. The results suggested that the stocked hatchery chum salmon fry decreased the foraging efficiency and growth of the wild masu salmon fry through interspecific competition, and depressed the abundance of Ephemerellidae and total grazer invertebrates (Glossosomatidae, Heptageniidae, and Baetidae) through predation. Also, the hatchery chum salmon fry may increase algal biomass through depression of grazer abundance by predation (top-down effect). These results suggested that a single release of hatchery chum salmon fry into a stream may influence the recipient stream ecosystem.     

Movements of brown trout, Salmo trutta, and juvenile Atlantic salmon, Salmo salar, in a coastal stream in northern Norway

Movements of resident brown trout (age 2+ to 9+ years) and young Atlantic salmon (age 1+), stocked as fry, were studied in July, August and September in a coastal stream in northern Norway. Between 85 and 89% of the brown trout were recaptured in the study area (346m, 1326m²) within 45m of their release point throughout the investigation period. Most specimens had moved less than 150m. Trout movements were related to local variation in density. Trout occupying those sections of stream with the lowest fish densities (5.3–10.9 fish 100m^?2) had a significantly lower movement rate than fish from sections with densities between 13.7 and 31.5 fish 100m^?2. Trout that moved from their marking section were significantly larger than specimens that did not leave their original site. There was a significant correlation between permanence of station each month and the mean water level that month. The majority of the trout (47%) were caught at undercut stream banks or at sites in the proximity of these. A decrease in water level during the study period resulted in a high loss (36%) of such habitat, probably forcing some individuals to move. The recapture data indicate that the trout population consists of one small (c. 15–20%) mobile, and one large sedentary component. Young salmon displayed high station permanence in July and August (93% and 96%). However, in the autumn they exhibited a significant downstream movement, and only 73% were recaptured within their release section. This movement was significantly higher for larger specimens, and is thought to occur because of a pre-winter change in habitat, initiated by a decline in stream temperature. In contrast to trout, salmon in sections containing the lowest densities (22.0–25.0 fish 100m^?2) did not show significantly lower movement rates when compared with salmon at higher densities (32.2–46.3 and 51.8–60.6 fish 100m^?2). The spatial distribution of young salmon indicated the formation of territorial mosaics over the stream bed, which are thought to reduce intraspecific competition.     

Prey availability and selective feeding of sea trout (Salmo trutta L., 1758) fry stocked in small forest streams

The food base and stomach contents of stocked sea trout (Salmo trutta) fry were determined in the first 4 months (April–August 2010) of life in two small lowland streams after resorption of the yolk sac. In each stream, 600 individual trout were released in a 200 m stocking‐section. The macrozoobenthos given as a food base for the fry were collected once a month from the streams using a bottom scraper. Three subsamples of macrozoobentos were considered as one sample from the stocking‐section. On the same day, 50 stocked fry were captured in each stream using electric fishing gear. Preferred food components were usually the taxa represented by number in a given stream in a particular month. Width size range of the prey in fish stomachs in May was from 0.10 to 1.47 mm, and from 0.11 to 3.78 mm in August. All food items found in both streams during the study months were also represented in intensity in the fish: Cyclopoida, and larvae of Baetidae and Chironomidae. Almost all Helodidae and Simuliidae larvae as well as the majority of Limoniidae and Nemouridae larvae were available as food each month.     

An electrophoretic method for distinguishing the young fry of salmon Salmo salar (L.) from those of trout Salmo trutta (L.)

Young fry of salmon were distinguished from those of trout using electrophoresis of whole fry extracts on cellulose acetate sheets. The separation patterns were revealed by general protein staining. The method was found helpful in resolving the mixed salmon and trout fry of an upland stream.     

An experiment on faster growth of salmon Salmo salar (L.) in a Scottish stream

The experiment was made in an attempt to modify the usual relationship in which young trout grow faster than young salmon in streams in which they occur together. A stretch of a trout stream was stocked with advanced salmon eggs, which produced fry earlier than the trout eggs laid naturally. The salmon grew faster than the trout and were longer than the trout at the end of the growing season. The mean length of 77.7 mm attained by the salmon is the largest known size reached by salmon in their first year when feeding on natural food supplies in streams in Scotland. Survival rate from egg planting to production of salmon of this mean length was high.     

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

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

Stocking experiments with 0 + and 1 + trout parr, Salmo trutta L., of wild and hatchery origin: 1. Post-stocking mortality and smolt yield

Wild and hatchery-reared 8–12-month-old (5–8 cm) trout, Salmo frurta L., were stocked in tributaries of the River Gudenb. Mortality was examined by means of electrofishing. Repeated electrofishing and handling caused a small increase in mortality. The daily instantaneous mortality rate Z was high during the first 2 months after stocking, ranging from 0.0070 for wild trout to 0.0326 for domestic trout at a stocking density of one trout per m² and from 0.0206 (wild trout) to 0.0888 (domestic trout) at a stocking density of two trout per m². Two months after stocking, Z decreased drastically ranging from 0.0007 (wild trout) to 0.0067 (domestic trout). When stocked, first-generation hatchery trout showed Z equal to domestic trout. Wild trout resident in the experimental stream were negatively affected by the introduction of domestic trout and wild trout from another stream. at a stocking density above the carrying capacity. It is concluded that the higher mortality of domestic trout was caused by changes in food, feeding and exercise, possibly combined with the lack of selection in the hatchery. Smolt yield at age 2+ was 3.2% (0+ trout stocked in the fall)-7.0% (1 + trout stocked in the spring) of the domestic trout stocked (approx. one-sixth to one-third of natural populations) and 65.2–68.7% of the domestic trout present before the smolt run. For first generation hatchery trout of wild origin the corresponding figures were 7.3% (age 0 +) and 93.4%, and for wild trout introduced to the experimental stream they were 11.1% (age0 +)and39.8%.     

The effect of stocking with 0+ year age‐class Atlantic salmon Salmo salar fry: a case study from the River Bush,Northern Ireland

An enhancement programme based on stocking 0+ year age‐class Atlantic salmon Salmo salar, conducted in the River Bush, Northern Ireland, U.K. over the period 1996–2005, was reviewed with reference to the performance and biological characteristics of wild fish. Wild ova to 0+ year fry (summer) survival was c. 8% with subsequent wild 0+ year fry‐to‐smolt survival c. 9%. Stocked unfed 0+ year juveniles gave c. 1% survival to smolt whilst fed 0+ year S. salar stocked in late summer exhibited survival at c. 5%. Stocking with unfed and fed fry contributed to increased smolt production and helped attain local management objectives between 2001 and 2005. Significant differences in biological characteristics were observed between wild and stocked‐origin fish. Wild‐smolt cohorts were dominated by 2+ year age‐class fish on the River Bush whilst smolts originating from fed fry mostly comprised younger 1+ year individuals. The mean mass of 1+ year smolts derived from stocked fed fry was significantly lower than that of wild 1+ year smolts, although these differences were not evident between older age classes. Differences in run timing between wild smolts and smolts derived from stocked fry were also apparent with the stocked‐origin fish tending to run earlier than wild fish. Although the stocking exercise was useful in terms of maximizing freshwater production, concerns over the quality of stocked‐origin recruits and the long term consequences for productivity are highlighted.     

The comparative feeding behaviour of brown trout, Salmo trutta L. and Atlantic salmon, Salmo salar L. in Llyn Dwythwch, Wales.

A detailed comparative study of the diets of natural brown trout and stocked Atlantic salmon in Llyn Dwythwch, North Wales, was carried out over a period of 13 months. The annual and seasonal composition of both diets was calculated by number, volume and occurrence methods. Food in relation to fish size was compared by Spearman rank correlation. Interspecific competition was only significant during the summer months when food was abundant. The bottom fauna was classified according to accessibility, and utilization of the fauna and electivity is discussed for each species. The similarity of diet between the salmon stocked into Llyn Dwythwch and those in Welsh rivers implies that the former is the result of inheritance rather than from interactive segregation with the lake trout.     

Effects of sea trout stocking on the population genetics of landlocked brown trout, Salmo trutta L., in the Conwy River system, North Wales, U.K.

The effect of the introduction of fry of anadromous sea trout, Salmo trutta L., on the genetic integrity of landlocked brown trout populations was evaluated. Samples were taken from six brown trout populations from streams above impassable waterfalls in the Conwy river system (North Wales, U.K.) in 1989 and 1990. Three of these streams had no known stocking history and three had been stocked with sea trout fry from the lower Conwy system over the last few years. Representatives of these sea trout were collected from two streams in the lower Conwy system and from a hatchery. Allele frequencies at 13 loci, six of which were polymorphic, were determined by starch gel electrophoresis.
The stocked populations were intermediate in their allele frequencies between unstocked brown trout and sea trout samples. A principal component analysis suggested significant numbers of hybrids in all of the stocked streams. This shows that some of the introduced sea trout did not migrate down the falls to the sea, but stayed in fresh water and hybridized with the local population. The significance of this finding for the conservation of the genetic resource of brown trout stocks is discussed.     

The maturation of anadromous female Atlantic salmon, Salmo salar L., stocked in a small pond in urban St John's, Newfoundland, Canada

Anadromous Atlantic salmon swim-up fry were stocked into a small pond (surface area 4.0 ha; maximum depth 3.6 m; mean depth 1.9m) in urban St John's, Newfoundland, Canada. The outlet from the pond was placed underground about 15 years ago. The growth rate of young salmon in this pond was two to five times as fast as in populations in insular Newfoundland. Females matured after a size at which they should have smolted and emigrated to sea. Growth, fecundity, and egg size of transplanted females are compared with those of the donor stock and with those reported for populations of landlocked salmon in insular Newfoundland. The evolutionary implications of the study are discussed.     

Mass-marking of brown trout (Salmo trutta L.) larvae by alizarin: method and evaluation of stocking

This study describes otolith marking of brown trout (Salmo trutta L.) larvae by immersion in different solutions of alizarin red S (ARS). The best results were obtained after marking with ARS at a concentration of 150 mg L^?1. To evaluate the efficiency of stocking with brown trout fry, 10 000 20‐day‐old larvae were marked in years 2002 and 2003 with ARS and released 2 weeks later into sections of a river with natural brown trout reproduction. Electro‐fishing surveys carried out 2 months after stocking in 2002 revealed that only 4.8% of all caught young‐of‐the‐year trout originated from stocking; in 2003 the percentage was 8.9%. Based on the substantial natural reproduction and the low ratio of stocked to wild trout, it was recommended to discontinue stocking.     

Interspecific relationships between emerging Atlantic salmon, Salmo salar, and coho salmon, Oncorhynchus kisutch, juveniles

Interspecific relationships between Atlantic salmon and coho salmon were studied at early life stages in laboratory and semi-natural stream channels. During emergence, the survival and dispersal patterns were similar for the two species in single or mixed populations. Survival of Atlantic salmon fry was reduced in the presence of older coho fry. However, no predation was observed. Microdistribution differed between the two species, with Atlantic salmon fry more numerous in riffles when coho were present.
Coho juveniles had a pelagic and gregarious distribution, in contrast to the benthic behaviour of the Atlantic salmon. In laboratory streams, Atlantic salmon fry moved out or adopted a subordinate cryptic behaviour which allowed them to escape predation while negatively affecting their growth.     

Survival and growth of salmon, Salmo salar (L.), planted in a Scottish stream

Salmon eggs and unfed fry were planted in reaches (total length 2.8 km, mean width 4 m) of a Scottish stream between 1971 and 1977 and their subsequent progress was studied by sampling 16 sections (areas 38–126 m²) of the stream. Brown trout are the only fish which spawn in the stream, waterfalls and a dam near its mouth preventing adult salmon and sea-trout passing upstream. There were no restraints on the downstream movement of fish except in 1977, when a fry trap was operated. In 1971 and 1974 boxes each containing 300 eggs were buried in groups of 3–6. In other years fry were evenly distributed at 3.6–29.3 m^?2. At the end of the first growing season, salmon occurred at decreasing population densities for a distance of 600 m below the plantings, but after two growing seasons there was little remaining indication of their pattern of dispersion when planted. Rates of survival between planting and the end of the growing season were 9.4–31%. Survival when eggs were planted (11.1–14.8%) was not affected by the numbers planted together at one point (900–1800) or the distance apart of groups of boxes (10–85 m). When fry were planted the instantaneous mortality rate (M) of the 0₊ salmon during their first growing season was related to the initial stocking density (D_p) by the formula M= 0.00637 + 0.00444 log₁₀D_p. Twenty-two to 88% of 0₊ salmon present at the end of the growing season were still surviving in the stream as 1₊ fish one year later. In 1973–1976 only a small number of 2₊ salmon occurred, the majority having migrated between the end of the second growing season and the following spring. There were more 2₊ salmon in 1977 and 1978 resulting from higher stocking densities in 1975 and 1976 and slower growth. Trout of several age classes were present but their population densities were never high (<0.6 m^?2). Salmon reached a greater size than trout by the end of the first growing season. Their mean weight (W^o, g) at this time was inversely related to their population density (D_o No. m^?2) and the biomass (B₁, g m^?2) of 1₊ salmon present, giving the relationship log₁₀w^o= 0.6584–0.0558 D₀-0.0352B₁. The mean weight of 1₊ salmon tended to be highest in sections where the 0₊ salmon had reached a relatively large size the previous year. When a reach of the stream was planted twice (11 and 30 May 1977) with salmon fry (total 13.9 m^?2) at the same stage of development, M during the first growing season was 0.0099 per day. This was less than that of fry in a control (M= 0.0107) where the stocking density was lower (6.8 m^?2) and also less than in previous years when single planting rates of approximately 14 m^?2 were used (M=0.0115). The double planting resulted in a wide range of lengths of 0₊ salmon in September and the highest biomass values encountered during all experiments.     

Production of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., within different sections of a small enriched Norwegian river

Growth, density and production of juvenile Atlantic salmon and brown trout were studied in three different sections of the Kvassheimsåna River in south-western Norway from 1979 to 1983. Section 1. in the upper part of the river, is located above a waterfall impassable for migratory salmonids and is surrounded by grazing land. Sections 2 and 3, in the middle and lower parts of the river, are influenced by agricultural activity. Total nitrogen concentration varied between 250 and 1000 μg l ^?1 in section 1 and 1500 and 2500 μg l^?1 in sections 2 and 3. Total phosphorus (Tot-P) concentrations also increased with decreasing altitude: 19–46 μg l^?1 in section I and 31–101 μg l ^?1 in sections 2 and 3. The number of 0 + salmon in sections 2 and 3 varied between 30.1 and 167.8 specimens 100 m ^?2, with means 90.2 and 95.2 specimens 100 m ^?2:, respectively; the density of 1 + salmon, with mean values of 16.3 and 51.0 specimens 100m^?2 was significantly correlated with the original fry density. The growth rate of 0+ salmon was not inversely related to cohort density, but was significantly so for 1 + salmon. Mean annual salmon production in section 2 was 1595 g 100 m^?2 year ¹, and in section 3 was 841 g 100m^?2 year ¹. A logarithmic function gave the best curve fit between salmon production and mean annual biomass. Thus, production levelled off for the highest values recorded in section 2, and perhaps approached the carrying capacity of the stream. A multiple regression analysis showed that yearly variation in 1 + salmon density was the single factor accounting for most of the total variability in production (60%). Variation in water temperature and nutrient content were not significantly related to variation in fish production. Densities of brown trout were low in all sections (<20 specimens 100m ^?2). Fry density was highest in section 3 and parr density in section 1. All age groups of sympatric brown trout grew significantly faster in sections 2 and 3 compared with allopatric brown trout in section 1.     

The survival, growth and diet of pike fry, Esox lucius L., stocked at different densities in experimental ponds

Yolk-sac pike fry were stocked at densities of 0.74 – 81.4 m⁻² in two ponds, each divided into eight sectors (mean area 155.8 m²). Growth and survival were recorded from May to December 1985. The growth rates were variable within each sector. The size-range of sampled fish increased throughout the year, but showed no significant correlation with density. Fry survival was initially density-independent but switched by late June/July to density-dependence, ranging from 0.5 to 43.6% of initial stocking numbers. The highest mean daily mortality rates occurred during May-July. The final survival in December ranged between sectors from 0.059 to 11.25% of the starting stock densities. The final biomass per unit area of pike surviving in December was not related to initial stocking density. In Pond 1 the mean biomass produced was 2.21 gm⁻² and in Pond 2 was 3.49gm⁻².
Pike fry < 30 mm fed only on invertebrates; those 30–100 mm took a wide range of invertebrates, cyprinids. sticklebacks and other pike. Cannibalism occurred at most densities between 5.45 and 81.4 fish m⁻².
Where attempts are made to increase pike production in managed populations by releasing small fry, an upper stock density of 5 fry m⁻² is advised if large, density-dependent mortalities are to be avoided.     

Impact of live food on survival and growth of hatchery‐reared sea trout (Salmo trutta trutta L.) parr in the wild

Survival rates and growth parameters of hatchery‐reared sea trout (Salmo trutta trutta L.) fry were determined after stocking in the wild. The larvae were hatchery‐reared for 12 weeks in two groups: fry were fed either on live zooplankton and live chironomidae larvae (LFG), or fed a pellet diet (PFG). The survival rate and specific growth rates were higher in the LFG than in the PFG group. Most effective for hatchery‐reared fish intended for stocking was the natural, live feed. The mean number of chironomid larvae found in the stomachs of fish that were initially captured in the wild was significantly higher in the LFG than in the PFG group. The live diet supplied in the rearing period had a positive impact on the foraging skills of the sea trout fry and their survival in the wild after their release on 24 April 2010.