An assessment of salmon farms and wild salmonids as sources of Lepeophtheirus salmonis (Krøyer) copepodids in the water column in Loch Torridon, Scotland

Wild salmonids and farmed salmon can both be sources of Lepeophtheirus salmonis (Krøyer, 1838) larvae . Farmed salmon smolts free of L. salmonis infections are stocked in sea cages and may subsequently contract L. salmonis infections, probably from wild fish. The contribution of gravid L. salmonis at Atlantic salmon, Salmo salar L., farms to populations of L. salmonis larvae in the water column has in the past been based on estimated parameters, such as louse fecundity. This present study augments these calculations by combining empirical data on densities of infective L. salmonis copepodids in the field with estimates of the number of gravid L. salmonis on farmed and wild salmonids in Loch Torridon. Data collected between 2002 and 2007 show a significant correlation between mean densities of L. salmonis copepodids recovered in the water column and the numbers of gravid L. salmonis at the local salmon farms. Generally, the farms with greatest numbers of salmon were observed to have stronger correlations with densities of copepodids in the water than the farms with fewer fish. The study suggests that louse management approaches, e.g. treatment trigger levels, need to take account of individual farm biomass, or numbers of fish. This study highlights the importance of control of L. salmonis on salmon farms for the co-existence of both wild salmonid populations and the aquaculture industry.     

The epidemiology of the sea lice, Caligus elongatus Nordmann, in marine aquaculture of Atlantic salmon, Salmo salar L., in Scotland

Although Caligus elongatus is one of two major species of sea lice that parasitize farmed salmon, its epidemiology has not been extensively studied. In this communication, the abundances of the adult stage of C. elongatus in salmon populations from 33 farms in the West of Scotland between 1997 and 2000 have been analysed for evidence of seasonal and annual patterns. The findings indicate that the pattern of C. elongatus is remarkably consistent from year to year, and directly opposed to that reported for Lepeophtheirus salmonis, the other major caligid species. In particular, adult infestations rapidly increase from the start of July each year, and are more prevalent on salmon in the first year of production than the second year. Treatment is seen to have an effect on levels of infestation but it is not clear why this species should have significantly lower levels of abundance in the second year of production. There is also evidence that fallowing has no effect on abundance. Strategic management programmes for the control of sea lice on salmon farms, which are increasingly effective in controlling L. salmonis , may also have to give greater consideration to C. elongatus .     

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.     

Development of the use of wrasse to control sea lice on salmon farms in Ireland

Abstract. Developments in the use of wrasse as cleaner-fish in the control of sea lice ( Lepeophtheirus salmonis, Caligus elongatus ) in Ireland are reviewed. Corkwing, Symphodus melops , and goldsinny, Ctenolabrus rupestris , have controlled L. salmonis (<1 louse per salmon) on commercial farms. C. rupestris and rockcook, Centrolabrus exoletus , removed both adult and chalimus C. elongatus in aquarium trials. No egg-bearing lice were ever found on salmon in cages with wrasse, and numbers of mobile lice were maintained at <1 per salmon smolt.
Wrasse are fished using shrimp pots, optimally set for about 2 h. The fishery is seasonal and limited to >10cm wrasse occurring close to the farm, and distant from other farms. Localized reductions in the body length and numbers of wrasse have been detected in the second year of fishing in two areas respectively. Due to the fishery limitations and high fecundity of wrasse, impacts on wrasse populations should remain localized. All wrasse populations fished are screened for microbial diseases, and protozoan and metazoan parasites. Results are typical for wild marine fish but metazoan skin parasites are rare. No pathogens, including tests with an Aeromonas sp. found in wrasse, have been found to be transferable to salmon.
Wrasse are the only effective and continuous method of sea lice control legally available. Research efforts should be directed towards improving the availability, pathogen-free status, and husbandry of wrasse, to enable cleaner-fish technology to be more widely and efficiently utilized.     

Sea lice, Lepeophtheirus salmonis, transfer between wild sympatric adult and juvenile salmon on the north coast of British Columbia, Canada

We examine sea lice, Lepeophtheirus salmonis , on juvenile and adult salmon from the north coast of British Columbia between 2004 and 2006 in an area that does not at present contain salmon farms. There is a pronounced zonation in the abundance of L. salmonis on juvenile pink salmon, Oncorhynchus gorbuscha , in the Skeena and Nass estuaries. Abundances in the proximal and distal zones of these estuaries are 0.01 and 0.05 respectively. The outer zones serve as feeding and staging areas for the pink salmon smolts. Returning Chinook, Oncorhynchus tshawytscha , and coho salmon, Oncorhynchus kisutch , concentrate in these areas. We collected data in 2006 to examine whether L. salmonis on returning adult salmon are an important source of the sea lice that appear on juvenile pink salmon. Nearly all (99%) of the sea lice on returning Chinook and over 80% on coho salmon were L. salmonis. Most of the L. salmonis were motile stages including many ovigerous females. There was a sharp increase in the abundance of sea lice on juvenile pink salmon smolts between May and July 2006 near the sites of adult captures. As there are no salmon farms on the north coast, few sticklebacks, Gasterosteus aculeatus , and very few resident salmonids until later in the summer, it seems that the most important reservoir of L. salmonis under natural conditions is returning adult salmon. This natural source of sea lice results in levels of abundance that are one or two orders of magnitude lower than those observed on juvenile pink salmon in areas with salmon farms such as the Broughton Archipelago.     

Spatial and temporal distribution of Lepeophtheirus salmonis (Kr?yer) larvae in a sea loch containing Atlantic salmon, Salmo salar L., farms on the north-west coast of Scotland.

A plankton survey investigated spatial and temporal distributions of larval sea lice in a sea loch on the north-west coast of Scotland. Plankton tows were collected approximately weekly at two depths (0 and 5 m) at five sample stations over a 2-year period. The survey began in March 2002 when Atlantic salmon, Salmo salar L., farms in the study area stocked with fish and finished in March 2004, after farms completed harvesting. Over 90% of lice recovered were Lepeophtheirus salmonis (Kr?yer); therefore, the results of this paper refer largely to that species. The data were analysed using generalized additive models with explanatory variables for time, depth and station. Greatest densities of nauplii were recovered at stations adjacent to farms indicating that the local salmon farms were a likely source of larvae at times during the production cycle. There were significant temporal trends in larval densities and the stocking and harvesting of farms can possibly account for these increases and decreases, particularly at the start and at the end of the production cycle. The distribution of copepodids was more widespread than that of nauplii, indicating that the larvae can be transported several kilometres from the point of release.     

Differences in Lepeophtheirus salmonis abundance levels on Atlantic salmon farms in the Broughton Archipelago, British Columbia, Canada

Sea lice data collected from Atlantic salmon farms in the Broughton Archipelago between 2003 and 2005 were examined for inter-regional differences in mobile Lepeophtheirus salmonis (Krøyer) abundance using the generalized linear model procedure. Factors such as age of the salmon populations, location of farms and time of year had a significant effect on the abundance of the mobile stages of L. salmonis whereas water temperature and salinity did not. Separate evaluation of SLICE® treatment data found no significant difference in treatment frequency among the areas but did show that there were significantly lower numbers of farm treatments during the summer months when compared with other seasons. The role of environment and wild fish in influencing sea lice abundance on the farmed salmon is discussed. The findings suggest that effective management programmes for sea lice should not only be based on geographical location but should take into account other factors which could influence lice abundance levels.     

Recapture of pond- and hatchery-reared brown trout, Salmo trutta L., released in small streams

Abstract. Two groups of 0+ brown trout, Salmo trutta L., one pond-reared and one hatchery-reared (fish length 32-80mm), were released in eight small streams in three different areas of Norway between 1984 and 1987. Hatchery and pond fish were reared at different sites, but releases were made both near the rearing site and in the home area of the other group. A total of 2550 fish were recaptured by electrofishing the year after stocking. We found great variations in the recapture rates of pond and hatchery fish in one stream between years, and between streams in different areas the same year. These results indicate that the rearing method is not essential to the recapture rate. Both pond and hatchery fish generally had higher recapture rates in streams near their rearing site than in the distant areas. Thus factors associated with transportation seem to influence the survival of stocked fish. The results also indicate that size at stocking may be an important factor for the recapture rate.     

Survival of stocked hatchery-reared brown trout, Salmo trutta L., fry in relation to the carrying capacity of a trout nursery stream

Abstract. During the period June 1982-84 hatchery-reared brown trout, Salmo trutta L., fry were Stocked into stretches of the Owendoher. a trout nursery stream on the east coast of Ireland. These experiments were designed to examine the survival of stocked fry and to estimate the carrying capacity of the system. During the first year fry were stocked into sectors already supptorting wild fish at densities normal for the system. In the following year fry numbers were artificially reduced prior to stocking with the hatchery-reared fish. Mortality of the stocked fry was high after release with less than 33% of the fish surviving beyond the first 3 weeks. No stocked fish survived after October 1982. In the second year, however, 2-9% of the fish survived. The best survival rates were achieved where wild fry numbers were lowest. Regardless of the initial stocking density the various experiments yielded autumn fry densities (0.07-0.7 fish/m²) similar to those at unstocked sites (0.1-0.62 fish/m²).
Stocking did not increase recruitment to the 1+ group and again 1+ densities (0.15-0.35 fish/m²) similar to unstocked sites (0.07-0.39 fish/m²) were obtained at the end of each year. These results suggest that spawning and recruitment in the Owendoher yield population densities approaching the maximum carrying capacity of the stream. The system appears to support a maximum summer fry density in the region of 1 fish/m² and a maximum autumn density of 0.7 fish/m².     

Growth, Yield, Dressout, and Net Returns of Bighead Carp Hypophthalmichthys nobilis Stocked at Three Densities in Catfish Ictalurus punctatus Ponds

Abstract— Alternative fish species that can be cultured together with catfish Ictalurus punctatus provide an opportunity to diversify caffish farms. A 2-yr study was conducted in 0.10-ha earthen ponds to evaluate the effect of bighead carp (BHC) stocking density on growth, yield, dressout yield, and net returns. Initially, bighead carp (average weight of 22 g) were stocked at rates of 380, 750, or 1,130 fishha in ponds with catfish. Caffish were cultured under commercial conditions by stocking caffish at a density of 12,500/ha, aerating nightly and feeding at an average rate of 82 kgha per d. Stocking rates for 2-yr-old fish were reduced to 77, 260, and 435/ha in the second growing season. There were no significant differences among treatments ( P > 0.05) in summer growth of bighead carp in either year. Bighead carp stocked at 1,130 fishha had significantly higher yields than those stocked at 380/ha, but did not reach minimum market size of 2.2 kg during the first year ( P > 0.05). There were no significant differences ( P > 0.05) in caffish growth, yield, survival, or feed conversion ratios due to the bighead carp stocking densities. Partial budget analysis indicated that net benefits were positive for all three treatments over a range of prevailing prices of bighead carp. Bighead carp production in catfish ponds is economically feasible over a wide range of prices. Given the market risk of producing smaller fish at the higher density, the medium density is the preferred stocking density of fingerling bighead carp in catfish ponds.     

Temporal and spatial variations in lice numbers on salmon farms in the Hardanger fjord 2004–06

The long and narrow Hardanger fjord in western Norway has a high density of salmon farms and has had severe salmon lice, Lepeophtheirus salmonis, problems. In the years 2004–06, salmon lice numbers were recorded in selected salmon farms in the fjord as part of a larger research project. Most farm sites participated in a strategic control programme and were deloused between November and January in each year. The aim of the programme was to achieve a mean abundance of <0.3 adult female lice at this time and to minimize the infection pressure on wild smolts in the spring. Dedicated teams carried out detailed counting of lice on farmed fish in April–September each year. Temperature conditions were fairly similar throughout the fjord and amongst years, but wide variations in salinities were observed. The two innermost zones, B and C, had the lowest lice mean abundances, whereas the outermost zones, D and E, consistently had more lice. General linear model analyses showed that differences in adult female lice abundance between the zones were associated with differing levels of salinity and emamectin benzoate treatments strategically administered. Mean fish weight was significantly positively correlated with mean abundance of adult female lice.     

The effects of hydrogen peroxide treatment on different life-cycle stages of the salmon louse, Lepeophtheirus salmonis (Krøyer 1837)

The effects of hydrogen peroxide administered at a concentration of 1500 ppm for 20 min at 7.5 °C on different life-cycle stages of Lepeophtheirus salmonis were examined experimentally. The mobile adult and pre-adult stages of L. salmonis readily reattached to Atlantic salmon after hydrogen peroxide treatment. Adult female lice, but not adult males or pre-adults, reattached in significantly lower numbers than untreated controls. Survival of early chalimus stages (I and II) was not affected by hydrogen peroxide treatment, but their subsequent development to chalimus III and IV was delayed compared with untreated controls. Nauplii and copepodid larvae of L. salmonis were almost all dead by 1 hour post-treatment. A very few copepodids survived 24 h post-treatment. Egg strings of L. salmonis at an early stage of development failed to hatch after hydrogen peroxide treatment, but those with pigmented eggs did hatch, although in significantly reduced numbers. Treated egg strings did produce viable copepodids, although in significantly reduced numbers compared with controls.     

Environmental conditions at sea-cages, and ectoparasites on farmed European sea-bass, Dicentrarchus labrax (L.), and gilt-head sea-bream, Sparus aurata L., at two farms in Greece

Water quality in sea-cages, and metazoan ectoparasites on bass, Dicentrarchus labrax (L.), and bream, Sparus aurata L., were sampled at two fish farms in south-west Greece over 2 years. The seabed and marine life around the cages of one farm was surveyed using scuba. The most frequent and abundant parasites were the monogeans Sparicotyle chrysophrii and Furnestinia echeneis on the gills of bream, and Diplectanum aequans on the gills of bass. The copepod Caligus minimus occasionally occurred in the buccal cavity of bass, but only in low numbers. Prevalence and abundance of parasites increased with bass size but F echeneis levels decreased with bream size. There were no other clear relationships between water quality, parasite levels and fish size. Ammonia varied seasonally with sea temperature, and with oxygen levels below 6 mg 1^-1 on many occasions, it is possible that fish suffered stress from a combination of high ammonia and low oxygen conditions. There was no accumulation of waste food or faeces below the cages at the one farm surveyed, but there were thousands of wild fish, including mullet (Mugilidae), Spicara sp. and Diplodus spp., present around the cages. Water quality may be improved by changes in farm management, such as reducing stocking densities below the present 16 kg nr^-3. Oxygen should be monitored at the cages on site daily. How comparable environmental conditions and impacts, and parasitization are at other bass and bream sea cage farms is unknown.     

Experimental infection and horizontal transmission of Piscirickettsia salmonis in freshwater-raised Atlantic salmon, Salmo salar L.

Experimental infections with Piscirickettsia salmonis via intraperitoneal (IP), oral (PO) and gill (GS) routes were compared, and the importance of physical contact in the horizontal transmission of this organism was investigated. Atlantic salmon, Salmo salar L., under-yearling parr raised in fresh water were used in this study. Samples of liver, kidney, spleen, gill and brain were collected weekly for 5 weeks after challenge, and were examined using the indirect fluorescent-antibody technique (IFAT). The pathogen was transmitted horizontally to fish with and without physical contact. However, transmission of P. salmonis occurred significantly more rapidly among fish with physical contact. Mortalities occurred in 50% of fish experimentally challenged with P. salmonis and their cohabitants. The estimates of the relative risk of dying demonstrated that fish challenged by the IP and GS routes had a significantly higher probability of dying than fish challenged by the PO route ( P < 0.005). Contact cohabitants with infected fish had a higher probability of death than non-contact cohabitants ( P < 0.005). The sequential studying using IFAT indicated that a haematogenous pattern of infection occurred among fish infected by oral and gill routes, or by cohabitation. This was different from the capsular (serosal) infection pattern observed in intraperitoneally inoculated fish. Piscirickettsia salmonis was observed within the cytoplasm of leucocytes and renal tubules, the latter indicating that elimination of this pathogen through the urine may be possible. Aeromonas salmonicida was also detected (by IFAT) in some of the fish exposed to P. salmonis , suggesting that P. salmonis may cause immunosuppression, and thus, increase the susceptibility of the host to other pathogens.     

Stocking of brown trout, Salmo trutta L.: effects of acclimatization

One-year-old hatchery brown trout, Salmo trutta L., ( n  = 16 520) from a sea-run, local brood stock were marked and released (scatter-planted) into the River Laisälven in northern Sweden. Eight different groups were created using Alcian blue and Visible Implant Elastomer tags. Half the fish were kept in small enclosures in four stocking areas for 6 days before release. The other half were released just after transportation. To evaluate the effect of acclimatization on post-stocking performance, the areas were electric fished 2 months later. During the electric fishing survey, a higher number of the acclimatized hatchery fish were recaptured than those released immediately. The growth rate of stocked fish differed significantly between stocking areas and fish held in enclosures grew more than those released directly. The rate of recapture of hatchery fish varied between stocking areas (6.4–17.4%). Movements of juveniles within and between the stocking areas were low, and only 3.6% of the recaptured fish were found in an area not originally stocked. These results showed that acclimatization of fish before release increases the number and size at recapture within a stocking area.     

Impact of Climate Change on the Technical Efficiency of Striped Catfish,Pangasianodon hypophthalmus,Farming in the Mekong Delta,Vietnam

The technical efficiency of randomly sampled pangasius farms in the Mekong Delta of Vietnam was estimated using data envelopment analysis, and factors affecting technical and scale efficiency were examined with bootstrap truncated regression. The mean technical efficiency score assuming variable returns to scale was 0.84. The technical efficiency of downstream farmers was higher due to lower energy costs and stocking once a year. Most of the up‐ and midstream farms needed to pump water and stocked at least three times in 2 yr. Regression analysis showed a positive effect on technical efficiency of the farmers' education level and having experienced climate change impact through flooding or salinity intrusion in the past. Farms affected by salinity intrusion had a lower scale efficiency as they reduce stocking frequency and rate. In general, reducing fish mortality and the cost of inputs, increasing scale of operation, and being trained, using appropriate methods, in management strategies may improve technical efficiency.     

Survival, exploitation and movement of takeable size brown trout, Salmo trutta L., in a Norwegian river

Abstract. Takeable-sized (25-61 cm total length), hatchery-reared brown trout, Salmo trutta L., were released in early July from 1982 to 1984 in the sub-alpine River Sjoa in southern central Norway.
Of those recovered during the same season that they were stocked, 67-73% were caught within 10 days. Mean exploitation and survival rates in the same season of release ranged from 0 41 to 0-54 and 005 to 0.11, respectively. No tagged fish were caught 2 years after stocking.
The frequency of capture increased significantly with fish length. The length at stocking of those fish recovered after one winter in the river was greater than those caught in the same season as released. The migrant fish (n = 20) were significantly larger than stationary fish (n=434).
Between 87.5 and 95-6% of the fish with known capture sites caught during the first year (n=180) were recovered in the release area. The highest fraction of migrants was obtained in the year with the highest stocking density; the water discharge was also higher that year. The migrants (n=17) were caught 1.0 - 6.0km downstream, with the exception of one fish which moved 2.0 km upstream. The year after stocking, 63% (n=8) of the remainder were caught in the release area, while the migrants (n = 3) were caught 2.0-6.0km downstream.     

Effects of Stocking Density, Salinity, and Light Intensity on Growth and Survival of Southern Flounder Paralichthys lethostigma Larvae

Four separate studies were done on Southern flounder Paralichthys lethostigma larvae during first feeding and metamorphosis to determine the effects of stocking density, salinity, and light intensity on growth and survival. One study used stocking densities of 10, 20, 40, and 80 fish/L during first feeding; the second study compared the growth and survival of larvae stocked at 20 and 33 ppt; and a third experiment evaluated stocking densities of 1/L and 3/L under two different light intensities (1,600 lux vs 340 lux) during metamorphosis. The fourth experiment tested the effects of different salinities (0, 10, 20 and 30 ppt) on larval growth and survival during metamorphosis. Growth and survival (overall 6.9%) were not significantly different ( P > 0.05) for stocking rates up to 80/L. Larvae placed into 20 ppt salinity had survival through first feeding similar to that of larvae raised at 33 ppt. During metamorphosis, light intensity had no effect ( P > 0.05) on growth or survival, but fish stocked at 3/L had significantly lower ( P < 0.05) survival than fish at 1/L. Complete mortality of larvae occurred at 0 ppt. Growth and survival past metamorphosis were not significantly different ( P > 0.05) at 10, 20 and 30 ppt, but unmetamorphosed fish did not survive to day 60 at 10 ppt. Based on these results, practical larviculture of Southern flounder may require a two-step process with high stocking rates (80 fish/L) through first feeding and lower densities (1/L) through metamorphosis. Fingerling production in fertilized nursery ponds might he possible at salinity as low as 20 ppt.     

Quantitative estimates of freshwater fish stocking practices by recreational angling clubs in France

Although freshwater fish stocking is widely used by managers, quantitative assessments of stocking practices are lacking in many countries. The general objective of the present study was to determine the quantity and characteristics of fish stocking in metropolitan France. Using a survey-based approach, stocking practices for 2013 by recreational angling clubs in France were quantified, which represented the bulk of fish stocking undertaken in that year. Stocking was found to be practiced by 88.6% of angling clubs in France, representing, on average, 65% of their annual budget. Overall, 22 species were stocked, including 13 native and nine non-native species, with strong variations among species in terms of life stages and body sizes used for stocking. Using Bayesian modelling, a total biomass of 2.029 t, representing approximately 90 million fishes, was estimated to be stocked in France in 2013. In terms of biomass, the most widely stocked species were rainbow trout Oncorhynchus mykiss (Walbaum), brown trout Salmo trutta L., roach Rutilus rutilus (L.), common carp Cyprinus carpio L. and northern pike Esox lucius L. A stocking volume of approximately 60 fishes or 1.5 kg of fish biomass per angler per year seems commonplace in industrialised countries for which data are available.