Effects of stocked trout on native fish communities in boreal foothills lakes

Nasmith LE, Tonn WM, Paszkowski CA, Scrimgeour GJ. Effects of stocked trout on native fish communities in boreal foothills lakes.
Ecology of Freshwater Fish 2010: 19: 279–289. © 2010 John Wiley & Sons A/S Abstract – Ecological effects of stocking nonnative trout into lakes are receiving increased attention, especially in alpine environments. We assessed effects of stocked trout on native forage fishes in the boreal foothills of Alberta (Canada) by comparing fish density, population size structure and spatial and temporal activities in stocked and unstocked lakes over 3 years (2005–2007). The numerically dominant dace (primarily Phoxinus spp.) were larger in stocked lakes, consistent with size‐limited predation. Dace were also more crepuscular and concentrated on the lake‐bottom in stocked lakes, compared to more daytime activity in the water column in unstocked lakes. There were, however, no demonstrable effects of trout on the abundance of forage fish. The lack of major population‐level impacts of stocked trout suggests that current stocking practices, characteristics of boreal foothill lakes (e.g. thermal structure, abundant invertebrates, dense macrophytes) and/or behavioural adjustments of forage fish contribute to healthy native fish populations in our stocked lakes.     

Post-stocking Movements and Recapture of Hatchery-reared Trout Released into Flowing Waters - Effect of a Resident Wild Population

The post-stocking movements and survival of hatchery-reared brown trout (Salmo trutta L.) (length 18–19 cm) stocked into depopulated and control stretches of the Afon Clettwr, South Wales, were investigated. Data were obtained from electrofishing surveys and rewarded tag returns. Recapture rates ranged from 67–76% for both stretches. The resident population of wild brown trout had no significant effect on the dispersion of the stocked fish, the majority of which remained close to the point of stocking. No stocked fish were recovered from the experimental stretches in the year following their introduction. Within one year the depopulated stretch had been recolonized by wild trout. The implications upon restocking after a fish kill are discussed.     

Efficiency of Northern pike (Esox lucius) stocking in metropolitan France at large spatial and temporal scales

Stocking and electrofishing occurrence and abundance data for northern pike Esox lucius L. in >3800 km of French rivers across 7 years were compared to assess the effect of recreational fisheries stocking programmes on wild pike populations. A positive relationship was found between the additive effect of stocking and the size of the stocked pike. However, the stocking programmes implemented in France by recreational fishery managers from 2008 to 2013 increased the probability of pike occurring in the river network, without increasing abundance in well-established pike populations, because pike stocked in their early-life stages were used in most of the stocking programmes.     

Stocking of sea trout, Salmo trutta, in Lake Veere, south-west Netherlands

Lake Veere (1750–2050 ha), a brackish water lake in south‐west Netherlands, is a former branch of the Oosterschelde. The lake was closed off by the construction of two dams in 1961. Since the early 1970s Lake Veere has been regularly stocked for recreational and commercial fisheries with rainbow trout, Oncorhynchus mykiss (Walbaum), brown trout, Salmo trutta L., and glass eel, Anguilla anguilla (L.). Between May and September 1996 an experimental stocking of 18 054 trout of sea trout parentage (15–16 cm; 776 kg) was carried out to study their potential for recreational fisheries. The growth and mortality of the stocked trout were estimated from recaptures in eel fyke nets. The production and consumption of the stocked trout were estimated with a bioenergetics model. After 3.5 years (May 1996–November 1999) the stocked trout measured between 50 and 70 cm. The estimated annual total mortality was 84%. During the winter of 1996–1997, the biomass of the stocked trout reached a maximum of about 1800 kg. By November 1999 the biomass was estimated to be 100 kg. The maximum daily consumption by the trout population was 60 g ha^?1 in October 1996 and in June 1997. The total consumption of the stocked trout population over the 3.5‐year period was estimated as 54 244 kg. The analysis suggested that the stocked trout used about 0.2% of the average annual primary production of the lake system. Although the growth and initial production of the population are attractive from the perspective of a recreational fishery, the high mortality and infestation with the salmon louse, Lepeophtheirus salmonis (Krøyer), are serious drawbacks for a future stocking programme with trout in Lake Veere.     

The status of host fish populations and fish species richness in European freshwater pearl mussel (Margaritifera margaritifera) streams

• 1. The status of host fish populations and fish species richness was investigated at 36 sites of 20 extant freshwater pearl mussel populations, including the drainages of the Elbe, Danube, Rhine, Weser, Aulne, Kemijoki and Tuuloma in Germany, the Czech Republic, France and Finland, by carrying out comparative electrofishings.
• 2. Brown trout (Salmo trutta f. fario) were found to be the available host fish for pearl mussels in all except one of the streams investigated with mean densities of 2861 ha^?1 (range 0–8710 ha^?1) and a mean biomass of 119 kg ha^?1 (range 0–478 kg ha^?1). Streams that had been frequently stocked with brown trout had higher trout biomass and densities of host fish than natural populations, but trout stocking had no positive effect in two of the streams investigated.
• 3. Fish species richness ranged from 2 to 16 species per stream and showed a negative correlation with host fish biomass and host fish densities. Undisturbed oligotrophic pearl mussel headwater streams usually only yielded a low number of fish species. Habitat degradation can reduce competitiveness of specialized trout and result in an increased abundance of ubiquitous or atypical species.
• 4. A link between the lack of juvenile pearl mussels and a lack of suitable host fish was only rarely observed. Functional pearl mussel populations with relatively high numbers of juveniles had significantly lower densities and biomass of host fish than pearl mussel populations without recent recruitment.
• 5. This study suggests that 0+ host fish are not necessarily required to sustain functional pearl mussel populations. Low densities of host fish can be compensated by the higher glochidia carrying capacity of older host fish with limited previous contact with pearl mussel glochidia, by the long reproductive period of mussels, and by low mortality rates of juvenile mussels during their post‐parasitic phase.

Copyright © 2005 John Wiley & Sons, Ltd.     

Long-term variation in brown trout, Salmo trutta L., stocking success in a large lake: interplay between availability of suitable prey and size at release

Abstract –  Factors affecting long-term variation in brown trout, Salmo trutta L., stocking success were examined in a large lake, Lake Oulujärvi, in central Finland. Brown trout were stocked in spring (late May to early June) in 1974–1991 and in summer (late June to early July) in 1992–2001. The biomass of the vendace, Coregonus albula (L.), population (prey) at release time had the largest positive effect on stocking success within both periods: biomass of adult vendace in spring and both 0+ and adult vendace in summer. Increasing the size of stocked fish had a positive effect if the vendace available at release were only adults. The increasing trend of predator-catch-per-unit-effort (CPUE) [combined CPUE of northern pike Esox lucius L., burbot Lota lota (L.), and pike-perch Stizostedion lucioperca (L.)] through the study period and its negative effect on trout stocking success suggested an increasing effect of predation within the entire time series.     

Relationship between fish assemblage and angler catch in the Sulejów Reservoir,central Poland,in the context of a warming climate

The Pilica River has warmed at a rate of 0.06°C per year since 1969, threatening the ecological services provided by the Sulejów Reservoir. Gillnet sampling revealed that the reservoir is inhabited by a cyprinid‐dominated fish assemblage with roach, Rutilus rutilus (L.) (42% of the total biomass), the dominant species. However, the interest of the anglers was primarily focused on common bream, Abramis brama (L.) (almost 65% of the catch), as revealed by the catch statistics (2004–2014). No direct evidence was found that the temperature increase had an influence on the recreational fishery or fish assemblage structure. On the other hand, changes in the fish assemblage that were reflected in angler catch composition were observed. A decrease in the catch of common bream in gillnets was accompanied by a shift in angler choices towards roach and other fish species. The sustainability of recreational fisheries is more likely to be influenced by changes in the economic status of society, water quality (toxic cyanobacteria blooms) and water management practices in Poland. The question arises about which species will dominate in the impoundment in the future if the warming trend continues and common carp, Cyprinus carpio L., stocking is maintained.     

The balancing act of captive breeding programmes: salmon stocking and angler catch statistics

The debate over Atlantic salmon, Salmo salar L., stocking in Britain centres on the trade‐off between enhancing rod fisheries and harming wild populations. This article informs the debate by quantifying the relationship between stocking and angler catch statistics for 62 rivers over 15 years. After controlling for environmental factors affecting adult abundance, the 42 rivers with stocking had non‐significantly lower mean catch statistics than the 20 rivers without stocking. This difference increased with the age of stocked fish. Among stocked rivers, weak relationships between mean stocking effort and catch statistics also became more negative with the age of stocked fish. For stocked rivers, there was no evidence for a generally positive relationship between annual stocking efforts and catch statistics. Those rivers for which stocking appeared to improve annual rod catches tended to have lower than expected mean rod catches. The results suggest the damage inflicted on wild salmon populations by stocking is not balanced by detectable benefits to rod fisheries.     

Recaptures and resource use of native and non-native brown trout, Salmo trutta L., released in a Norwegian lake

Abstract Rate of recapture (gill netting), habitat use, and diet of three strains of stocked brown trout, Salmo trutta L., were compared with resident brown trout in a Norwegian lake. The strains originated from an alpine lake, from a boreal lake, and from the native brown trout population in the lake. Overall recapture rate was 5–8% for all strains. The low recapture rate could be due to the relatively small size at stocking; mean fish length varied between 13.1 and 14.5 cm with strain and stocking method. Two years after release, the frequency of the different strains decreased from about 12% in the first year to stabilize at about 1%. The alpine strain showed the highest overall recapture rate, whereas the native strain was recaptured at an intermediate rate. The overall recapture rate of scatter-planted brown trout was higher than that of spot-planted brown trout. Immediately after being stocked, introduced fish ate less and had a less-varied diet than resident trout; however, stocked fish adopted a natural diet within the first summer. The distribution of trout between the pelagic and the upper epibenthic habitat was similar for both the resident and the stocked brown trout. Results indicate that the habitat use of stocked brown trout is adaptive and becomes similar to that of indigenous fish.     

Resource use of native and stocked brown trout Salmo trutta L., in a subarctic lake

Habitat use, food composition and growth of stocked and native brown trout, Salmo trutta L., were studied in the subarctic Lake Muddusjärvi in northern Finland. Stocked brown trout and native brown trout preferred littoral and pelagic areas. Trout were stocked in October. In June stocked trout fed primarily on invertebrates while native fish were piscivorous. From July onwards the composition of the diet of both stocked and native trout was similar and consisted almost entirely of small‐sized whitefish. Brown trout were already piscivorous at a length of about 20 cm. The mean length of prey consumed was about 12 cm. Mean length‐at‐age was similar from the second year in the lake despite of the larger size of stocked fish during the first year in the lake.     

Effect of density on survival and growth of cyprinid fish fry

One of the bottlenecks in Israel's cold water ornamental fish industry is the large loss of fish during the post-larval stage. As a first step towards increasing survival rates, the optimal stocking density in earthen ponds for fry of each species should be determined. The results of five consecutive experiments with fry of goldfish (Carassius auratus), common carp and koi (respectively, edible and ornamental morphotypes of Cyprinus carpio) are presented here. The experiments were run in 18 cages of 1 m³ and 0.8 mm mesh size placed in a 0.02 ha earthen pond, stocked at 50,100 and 200 fry per cage. The variables analyzed were growth rate, harvesting weight, biomass and survival. Equations to estimate growth rates under different conditions are given. The three fish types present different growth rates and survival patterns in relation to stocking density and season, which lead to different management implications. Koi can be stocked up to 2 million fry per ha without showing negative density effects. Goldfish should be stocked at low density (500,000–1 million fry per ha) in spring and at higher rate (2 million fry per ha) in summer. Common carp stocking density should be adjusted to obtain the required fish size at harvest.     

Food consumption rates of piscivorous brown trout (Salmo trutta) foraging on contrasting coregonid prey

Knowledge of predator–prey dynamics is essential to understand ecosystem functioning. Quantification of such interactions is important for fisheries management, in particular in the case of stocking programmes. Here, food consumption rates (FCR) were quantified for wild and stocked piscivorous brown trout, Salmo trutta L., in three subarctic lakes with contrasting coregonid (Coregonus spp.) prey communities, using the Wisconsin and the Elliott–Hurley bioenergetic models. FCR was highest for stocked brown trout in lakes with the lowest predator densities, and lowest for wild brown trout. Although FCR estimate may vary somewhat depending on the specific model used, such tools are imperative for the proper impact assessment of brown trout stocking programmes and for the provision of advice on optimal stocking densities.     

Post-stocking Movements and Recapture of Hatchery-reared Trout Released into Flowing Waters: Effect of Time and Method of Stocking

Hatchery-reared brown trout Salmo trutta L. (length 23–26 cm) were stocked in the Afon Taf, South Wales. The effects of ‘spot-’ and ‘scatter-planting’ and of stocking at 1 and 4 weeks prior to the start of the angling season were investigated. Data on the percentage recapture and post-stocking movements of the trout were obtained from tag returns. Higher percentage recaptures were recorded for ‘spot-plantings’ (65% and 31%) than for ‘scatter-planting’ (16%). Stockings made 1 week before the start of the angling season yielded better returns (17.1%) than those made 3 weeks earlier (2.2%). Neither ‘spot-’ nor ‘scatter-planting’ resulted in stocked fish contributing to the catch for an appreciably longer period of time. The majority of trout stocked were caught in the area of stocking irrespective of the method or time of planting.     

Stocked rainbow trout (Oncorhynchus mykiss) affect space use by Warpaint Shiners (Luxilus coccogenis)

Due to widespread stocking, Rainbow Trout (Oncorhynchus mykiss Walbaum) are perhaps the most widely distributed invasive species in the world. Nonetheless, little is known about the effects of stocked Rainbow Trout on native non‐game species. We conducted experiments in an artificial stream to assess the effects of hatchery Rainbow Trout on home range and behaviour of Warpaint Shiners (Luxilus coccogenis Cope), a common minnow frequently found in stocked Southern Appalachian streams. We used the LoCoH algorithm to generate polygons describing the home ranges used by Warpaint Shiners. When a stocked trout was present Warpaint Shiners: (a) increased home range size by 57%, (b) were displaced into higher velocity mesohabitats, and (c) reduced mean overlap between the home ranges of individual warpaint shiners. Rainbow Trout did not significantly affect the edge/area ratio of Warpaint Shiner home ranges. Warpaint Shiner density (two and five fish treatments) did not significantly affect any response variable. Displacement from preferred microhabitats and increases in home range size likely result in increased energy expenditure and exposure to potential predators (i.e., decreased individual fitness) of Warpaint Shiners when stocked trout are present.     

Non‐indigenous fish in protected spaces: Trends in species distribution mediated by illegal stocking

1. Many freshwater non‐indigenous species (NISs) are stocked for recreational fishing, in some cases illegally in protected areas. In this study, fish communities were monitored using environmental DNA, electrofishing and anglers’ catches as the sources of samples in a mountainous Biosphere Reserve in Asturias (northern Spain), where stocking is forbidden.
2. Three NISs have been introduced illegally in the protected area and have shown increasing populations in the last two decades. Two species used as fishing bait, Squalius carolitertii (chub) and Phoxinus phoxinus (minnow), are expanding in running waters. Oncorhynchus mykiss (rainbow trout) was also detected and is likely to have been introduced for angling or from fish farm escapes.
3. The results suggest that sustained illegal stocking contributed to the increase of the three NISs. In contrast, Salmo trutta (brown trout) of northern European lineages, identified from *90 alleles at the LDH‐C1 locus, and formerly legally stocked for angling, is decreasing, most likely as a result of climate change. Climate change could also contribute to the expansion of the two non‐indigenous cyprinids to colder upstream areas.
4. Through the application of a social survey, it was found that unlike other population groups, anglers in the region significantly preferred stocking over environmental improvement for the management of fish populations. The results obtained suggest that raising the awareness of anglers about the importance of safeguarding native fish species could help to prevent the spread of NISs in protected areas.

     

Post-stocking behaviour of hatchery-reared European grayling, Thymallus thymallus (L.), and brown trout, Salmo trutta L., in a semi-natural stream

Hatchery-reared, juvenile European grayling, Thymallus thymallus (L.), and brown trout, Salmo trutta L., were each stocked six times into an area of a semi-natural stream. The order in which the two species were released was switched after every second experimental stocking. Temporal and spatial post-stocking dispersal, effects of previously stocked species, feeding behaviour and the influences of sex and size were studied. During each 48-h experimental release period, some fish were recaptured in a trap situated 200 m downstream from the stocking site, and fish remaining in the stream after each experimental release were caught by electric fishing. Significantly more grayling than trout moved downstream and left the semi-natural stream. Proportions of stocked grayling recaptured in the trap within 2 h and from 2 to 48 h post stocking in the stream were 36.4% and 10.0%, respectively. Corresponding recapture rates for brown trout were both 1.5%. Most of the grayling and brown trout that did not leave the stream early were recaptured in deep, slow-moving water at low velocities in the release area. The presence of grayling at the time that the brown trout were stocked resulted in significantly fewer brown trout staying in the upper part of the stream. Within 48 h of their release, 33.3% and 22.8% of the grayling and brown trout, respectively, had eaten natural food items. The mean length of brown trout recaptured in the upper part of the stocking area was significantly lower than that of fish recaptured in the lower part and in the trap. Among brown trout, males showed a significantly greater propensity to eat and to stay in the upper part of the stream near the stocking site compared with females . Brown trout with natural food items in their stomachs had significantly lower mean length than trout without such items. No sex- or size-related differences were found in the spatial distribution or feeding activity of grayling.     

Evaluation of Stocking Density during Second‐Year Growth of Largemouth Bass,Micropterus salmoides,Raised Indoors in a Recirculating Aquaculture System

Largemouth bass (LMB), Micropterus salmoides, are a highly desirable food fish especially among Asian populations in large cities throughout North America. The primary production method for food‐size LMB (>500 g) has been outdoor ponds that require two growing seasons (18 mo). Indoor, controlled‐environment production using recirculating aquaculture system (RAS) technologies could potentially reduce the growout period by maintaining ideal temperatures year‐round. Researchers conducted a 26‐wk study to evaluate optimal stocking densities for growout of second‐year LMB to food‐fish size in an indoor RAS. LMB fingerlings (112.0 ± 38.0 g) were randomly stocked into nine 900‐L tanks to achieve densities of 30, 60, or 120 fish/m³ with three replicate tanks per density. The RAS consisted of a 3000‐L sump, ¼ hp pump, bead filter for solids removal, mixed‐moving‐bed biofilter for nitrification, and a 400‐watt ultraviolet light for sterilization. Fish were fed a commercially available floating diet (45% protein and 16% lipid) once daily to apparent satiation. At harvest, all fish were counted, individually weighed, and measured. Total biomass densities significantly increased (P ≤ 0.05) with stocking rate achieving 6.2, 13.2, and 22.9 kg/m³ for fish stocked at 20, 60, and 120 fish/m³, respectively. The stocking densities evaluated had no significant impact (P > 0.05) on survival, average harvest weight, or feed conversion ratio which averaged 92.9 ± 5.8%, 294.5 ± 21.1 g, and 1.8 ± 0.3, respectively. After approximately 6 mo of culture, LMB did not attain target weights of >500 g. Observed competition among fish likely resulted in large size variability and overall poor growth compared to second‐year growth in ponds. Additional research is needed to better assess the suitability of LMB for culture in RAS.     

Emigration of hatchery‐reared Pallid Sturgeon,Scaphirhynchus albus (Forbes and Richardson), through a Missouri River dam

The middle Missouri River (MMR; Fort Randall Dam, SD to Gavins Point Dam, NE‐SD) is stocked with hatchery‐reared pallid sturgeon, Scaphirhynchus albus (Forbes and Richardson), from upper Missouri River broodstock to aid recovery of this federally endangered species. Emigration of these fish through Gavins Point Dam restores genetic connectivity that likely existed pre‐impoundment but could lead to outbreeding depression in the future. Recapture data of hatchery‐reared pallid sturgeon stocked in the MMR were evaluated to improve understanding of pallid sturgeon emigration. From 2004 to 2015, 219 emigrants were caught: 4 stocked at age ≥2 years and 215 stocked at age ≤1 year. Emigration of the 2001‐2007 year classes stocked at age 1 was a consistent phenomenon and appeared higher than emigration of year classes stocked at ages 2–3. Little evidence suggested emigration was associated with an unusually high‐water event in 2011. The annual emigration probability of individuals stocked at age 1 estimated from multi‐state mark–recapture models was 0.05 [95% confidence interval = 0.04–0.06] for fish ages ≥1 year. This study suggests that alterations to stocking practices (e.g. stocking age) may affect emigration rates and, therefore, connectivity among pallid sturgeon populations.     

Dispersion of stocked brown trout, Salmo trutta L., and landlocked salmon, Salmo salar L., from stocking sites in Lake Oulujärvi, Finland

Movement and recaptures of two hatchery-reared brown trout, Salmo trutta L., stocks and landlocked salmon, Salmo salar L., released at different sites in regulated Lake Oulujärvi, were studied in relation to release site. Five groups of fish from each stock were released in approximately equal numbers. Most of the fish released in June and July were recaptured within 3 months, whereas the majority of the fish released in early winter (October and November) were caught the following spring, about 7–9 months after stocking. The release site had a significant effect on recapture rate. The results showed that fishing restrictions targeted mainly at gill net fishing are needed to preserve the stocked fish from overfishing. Significantly fewer recaptures were observed from the landlocked salmon stocking compared with brown trout. The recaptures from the landlocked salmon stocking indicated more active movement and less clumping compared with the two brown trout stocks.     

Evaluation of Larval Growth and Survival in Mexican Mojarra,Cichlasoma urophthalmus,and Bay Snook,Petenia splendida,Under Different Initial Stocking Densities

Two experiments were conducted to evaluate the initial stocking density in larvae of Bay snook, Petenia splendida, and Mexican mojarra, Cichlasoma urophthalmus, using a recirculation system. Five initial stocking densities (0.5, 1, 5, 10, and 20 larvae/L) were evaluated by triplicate for 45 d. Weight and total length (TL) were measured every 15 d, and fish production was calculated for each density. The larvae stocked at the lowest densities (0.5 and 1 larvae/L) presented the highest growth for both species: C. urophthalmus (0.78 g and 45‐mm TL, and 0.76 g and 45‐mm TL, respectively) and P. splendida (0.80 g and 52‐mm TL, and 0.79 g and 49‐mm TL, respectively). However, lowest fish production was recorded (35 and 69 fish per tank, respectively, for C. urophthalmus and 34 and 70 fish per tank, respectively, for P. splendida) compared with those at densities of 5, 10, and 20 larvae/L (336, 584, and 604 fish per tank, respectively, for C. urophthalmus and 341, 679, and 912 fish per tank, respectively, for P. splendida). The polynomial model for biomass production related to the stocking density shows that the optimum stocking densities for C. urophthalmus and P. splendida are 12 and 14 larvae/L, respectively.