Photoperiod in recirculation aquaculture systems and timing of seawater transfer affect seawater growth performance of Atlantic salmon (Salmo salar)

Production of Atlantic salmon smolts in recirculation aquaculture systems (RAS) is growing, and novel production protocols using continuous light in RAS are being implemented in the industry. In the present study, Atlantic Salmon parr were exposed to either a traditional protocol (short-day winter signal [12:12 L:D] for 6 weeks) or to continuous light. Both photoperiods were applied in freshwater (FW) and brackish water RAS. Salmon from all treatments were transferred to seawater pens at 200 and 600 g and grown until slaughter size. A control group was smoltified with a 6-week short-day winter signal and kept in FW until sea transfer at 100 g. Continuous light gave a higher growth rate in RAS but reduced feed intake and growth and increased feed conversion ratio during the first 8 weeks in seawater. However, at slaughter, fish exposed to continuous light was bigger than fish given a winter signal because of the higher growth rate in RAS. Slaughter weight was lowest in fish transferred to sea at 600 g, despite having the highest day-degree sum during their life span. The best performing group was the control group transferred at 100 g. All treatments handled transfer to seawater and survival and maturation were not affected by the treatments in RAS. The immune status was examined with a multigene expression assay on BioMark HD platform from parr stage to 5–7 months after seawater transfer. Overall, there was no significant effect of photoperiod or salinity on the expression of the selected immune genes. In sum, the results from this study indicate that using continuous light in RAS may have negative effects on performance shortly after transfer in fish transferred to sea at 200 g, whereas at 600 g, all treatments had reduced growth after transfer irrespective of treatment in RAS.     

Prediction of size-dependent risk of salmon smolt (Salmo salar) escape through fish farm nets

The escape of small smolt through farm cage netting is a major challenge faced by the Norwegian salmon farming industry. Escape can occur when the smolt placed in the cages are smaller than the size estimated by the farmers. Furthermore, one may assume wrong mesh-properties as the cage netting change shape and/or state from stiff (mesh bars with tension) to slack (mesh bars without tension) due to sea currents or waves and become more suitable for penetration. The latter represents an increased risk for cages placed in more exposed sea areas, which is a growing trend in the industry due to increased demand for farming sites. The potential influence of mesh shape and state on the risk of escape from salmon farm cages is predicted. The morphological characteristics of salmon smolt are assessed and used to determine the risk of potential escape through meshes of different sizes, shapes and states. The results showed that fish smaller than 47 g and 201 g have the potential to escape through 30 mm and 50 mm meshes, respectively. In general, the risk of smolt escape is highest when the meshes in the netting are slack. Semi-slack meshes with mesh openness of between 65 and 95 % also present a higher risk of smolt escape than square stiff meshes. The highest risk of escape was identified at approximately 80 % mesh openness. This study illustrates the importance of mesh states in fish farming cage nettings. In many cases the minimum smolt size needed to maintain an escape risk below 1 % was approximately twice as large for slack square meshes than for stiff square meshes of the same size.     

Growth and salt-water tolerance of juvenile Atlantic salmon, Salmo salar, reared under different combinations of dietary carbohydrate and photoperiod regime

The present study evaluated how different photoperiods, carbohydrate sources and inclusion levels influenced growth, plasma, organ and body compositions in Atlantic salmon (Salmo salar). Four different diets were used, containing 10 or 20% starch on a dry matter basis, using gelatinized corn (C10 and C20) or wheat (W10 and W20). Two different photoperiods were used, Continuous (Con) light throughout the study, Winter (Wi) two initial weeks of continuous light, thereafter 6 weeks of short day, ending with 6 weeks of continuous light. The winter regime resulted in reduced growth rate compared with continuous light. All Wi groups were smaller than Con groups when the experiment was terminated, except for fish given diet C10. Diet W10 resulted in better growth compared with all other groups when exposed to Con. regime. Atlantic salmon fed with diets containing 20% starch had lower growth rates than fish fed diets containing 10% starch, when the same starch source and light regime were compared. All fish belonging to the Con group exhibited similar and higher condition factors compared with fish from groups Wi. Condition factor was not influenced by dietary starch source. Higher whole body lipid concentrations were found when fish were given the corn compared with the wheat diets, without any responses to starch level or light regime. Whole body protein showed increased values in fish exposed to regime Wi compared with Con, without any influence from dietary manipulation. Liver glycogen values and hepatosomatic index reflected dietary starch levels and were significantly influenced by light regime, whilst muscle glycogen levels varied only as a response to light regime with lower values in fish from the Wi compared with the Con groups. Light regime influenced plasma glucose levels, except in groups fed with diet W10. All measured plasma nutrient concentrations were within normal ranges. At the end of the feeding period, all fish were evaluated for glucose and salt water tolerance to ascertain whether diet and/or light regime influenced glucose regulation capacity and/or smolt quality. Fish fed with 20% starch had reduced osmoregulatory ability measured as plasma chloride after a seawater challenge test. Fish from the Wi regime, but not from the Con regime developed characteristics associated with smoltification. Glucose tolerance in Atlantic salmon was substantially influenced by both photoperiod regime and diet. Fish reared under the Wi photoperiod showed reduced glucose regulation capacity compared with fish held under continuous light. Glucose regulation capacity was also reduced in fish adapted to the highest starch levels.     

Differences in sea migration between wild and reared Atlantic salmon (Salmo salar L.) in the Baltic Sea

The effect of origin, smolt size and year of release on the sea migration pattern of Atlantic salmon (Salmo salar L.) in the Baltic Sea was examined by tagging experiments conducted in 1991–1993 on wild and reared smolts of the Simojoki river salmon stock. The tag recovery data analysed by log-linear models revealed significant differences in both spatial and temporal sea migrations between the wild and reared salmon; the variation was attributed to the year of release and to the origin of the fish. Grilse accounted for the majority of reared returners (76%) but for a smaller proportion (46%) of the wild fish. The effect of smolt size could be studied only in the smolt groups tagged in 1991. Wild fish were more frequently (71%) caught in the Baltic Main Basin than were reared fish (51%) during their second sea year, and the size variation between wild and reared smolts did not explain the recovery site. No such differences in spatial distribution were found during the third sea year. The tagging place (hatchery/trap) of the reared fish did not affect their later sea migration. The differences in sea migration patterns suggest that the wild salmon are more vulnerable to the intensive salmon fishery in the Baltic Main Basin than are reared fish.     

A closer look at closed cages: Growth and mortality rates during production of post-smolt Atlantic salmon in marine closed confinement systems

The most controversial environmental problems in commercial salmon farming are the negative effects of sea lice (Lepeoptheirus salmonis, Caligus spp.), the genetic introgression of farmed salmon in wild populations, nutrient waste load and the emission of potentially toxic waste to coastal waters. Moving production from sea cages to land-based facilities, offshore farming or marine closed containment systems (CCS) are suggested as possible ways to solve these problems. However, there are few published studies on production capacity and fish welfare in such systems. The main aim of this study was to describe growth rates, mortality rates and mortality causes in the commercial-scale production of Atlantic salmon (Salmo salar) post-smolts in CCS from sea transfer until the size of 1000 g. From October 2014 to May 2017, we recorded growth rates, feed use, mortality and mortality causes during 23 CCS production cycles, including 18 CCS periods with off-season smolt (S0) and 5 CCS periods with one-year smolt (S1). The mean (SD) growth rate, thermal growth coefficient (TGC), for all 23 CCS was 3.03 (0.34), with no difference between cages with S1 (n = 5) and cages with S0 (n = 1 8). Cumulative mortality three months after sea transfer (CM_3mo) was 2.6 %, while cumulated mortality throughout the total trial period (CM_total, mean number of days = 159) was 3.6 %. Both CM_3mo and CM_total were higher in S1 groups than in S0 groups. Mean (SD) feed conversion ratio in CCS with S0 (n = 18) was 1.11 (0.07). The two main mortality causes were ‘Ulcers and fin rot’ (S1 and S0) and ‘Failed smolt’ (S1), accounting for 36.1 % and 19.3 % of the total mortality, respectively. Water flow, oxygen saturation and other water quality parameters were within safe limits for fish health and welfare.     

The effect of artificial light treatment and depth on the infestation of the sea louse Lepeophtheirus salmonis on Atlantic salmon (Salmo salar L.) culture

Two field studies were carried out with farmed Atlantic salmon (Salmo salarL.) in sea cages to examine various effects of artificial light (AL) and the vertical distribution of salmon on lice infestation.

The use of AL light caused an overall increase in lice infestation in both experiments. The first study showed that salmon held at 0–4 m depth in cages developed higher infestation than salmon held at greater depths (4–8 and 8–12 m) under both natural light (NL) and AL. In the second study, salmon maintained in 14-m deep sea cages that were exposed to AL with different light intensities. The AL treatments resulted directly in different diel and seasonal patterns of vertical distribution of the salmon and also different temporal patterns in lice infestations. So indirectly the infestation pattern appeared to be correlated with median day-time swimming depth of the salmon.     

Growth, osmoregulation and sexual maturation of underyearling Atlantic salmon smolt Salmo salar L. exposed to different intensities of continuous light in sea cages

Atlantic salmon smolts are usually transferred to sea water during early spring, but photoperiod treatment can be used to produce underyearling (0+) smolts for transfer to sea water in late autumn, 7–8 months earlier than usual. This study investigated the effect of exposure to additional continuous light (LL) of different intensities on 0+ smolts after transfer to sea water. 0+ smolts transferred to sea cages in mid-October were exposed to natural light (NL) or LL of one of three intensities (low, medium or high) until January, whereupon they experienced NL until harvest in December. Fish in all groups displayed a normal ability to hypoosmoregulate, assessed by monitoring plasma chloride concentrations after transfer to sea water. No consistent differences in length, weight or condition were found between LL groups. The LL groups had greater growth in length than the NL group during the time of LL exposure, followed by an increase in weight when exposed to NL. This resulted in fish of larger size than the NL group. By August, the fish in the NL group had caught up with the LL groups in respect of length, weight and condition factors. This suggests that the LL treatment led to increased winter growth and phase-shifted a seasonal pattern of growth. The incidence of sexual maturation was low (< 1%), with no differences between groups.     

Effects of starvation,subsequent feeding and photoperiod on flesh quality in farmed cod (Gadus morhua)

The study was designed to investigate the effect of four cycles of 5 weeks starvation followed by 10‐week refeeding compared with daily feeding under either natural photoperiod or continuous light (LL) regime on body composition and flesh quality in Atlantic cod in sea cages, northern Norway. The fish were sampled for body composition and flesh quality parameters at the start of the trial, twice at the end of a 10‐week feeding period and twice at the end of a 5‐week starvation period. There was effect of both feeding and light regime on growth, the two starving groups losing weight during starvation and regaining weight during refeeding, and the group under LL being heavier. But, the mean overall growth did not vary between groups. Starvation/refeeding regime resulted in higher slaughter yield, but no overall effect was seen on hepatosomatic index, water content, water holding capacity (WHC), muscle pH, hardness or flesh colour compared with control groups. Continuous light increased gutted weight and slaughter yield, lowered WHC and depressed maturation compared with fish under natural light regime. Increased growth rate resulted in softer fillets and lower muscle pH.     

Impact of smolt production strategy on vertebral growth and mineralisation during smoltification and the early seawater phase in Atlantic salmon (Salmo salar, L.)

This study investigates the effect of different smolt production strategies on vertebral morphology (radiology), composition (mineral content) and mechanical strength (load-deformation testing) in Atlantic salmon (Salmo salar). Rapid-growing underyearling (0+) smolt were compared with slower-growing yearling (1+) smolt and a reference group of wild smolt (w). The underyearling and yearling smolt were transferred to seawater in October 2002 and May 2003, respectively. The underyearling smolt were reared under continuous light and the yearling smolt under natural light during the first twelve weeks in seawater, at ambient temperatures. Thus, the underyearling smolt hit seawater at 13 °C and were reared at 10-13 °C during the early seawater phase, whereas the yearling smolt hit seawater at 7 °C and were reared at 7-10 °C during the early seawater phase. All groups displayed increased longitudinal growth (up to 9% increase in relative length) of the caudal vertebrae during parr-smolt transformation. However, at transfer to seawater, the underyearling smolt had significantly lower vertebral mineral content (0+ 44%, 1+ 47%, w 50%) and higher incidence of deformed vertebrae (0+ 1.5%, 1+ 0%, w 0%), and at twelve weeks after transfer to seawater significantly lower vertebral mineral content (0+ 36%, 1+ 41%, w 43%), yield-load (0+ 6492 g, 1+ 8797 g, w 9150 g) and stiffness (0+ 7578 g/mm, 1+ 15,161 g/mm, w 20,523 g/mm), and significantly higher incidence of deformed vertebrae (0+ 2.5%, 1+ 0.3%, w 0%). There was a significant correlation between the mineral content and mechanical properties of the vertebrae. The underyearling smolt had significantly elevated plasma concentrations of total Ca, and P and Ca²⁺ during the parr-smolt transformation and in the early seawater phase.The results show that underyearling smolt may have an increased risk of developing vertebral deformities. It is possible that this risk can be reduced by postponing the start of the short-day treatment. This will reduce the temperature during smoltification, the temperature and daylength during the early seawater phase, and increase the age at smoltification.     

Risk factors for furunculosis, infectious pancreatic necrosis and mortality in post-smolt of Atlantic salmon, Salmo solar L.

Abstract An epidemiological longitudinal study comprised information obtained from 124 randomly selected Norwegian sea-sites housing post-molts of Atlantic salmon, Salmo salar L. Information on crude cumulative mortality, fish characteristics and farm factors was gathered by the veterinary practitioners servicing the farms. The observation period lasted from the time of sea transfer of the smolts in the spring until 1 October 1991. On average, 116480 smolts were transferred to each sea-site. The mean crude cumulative mortality of post-smolts during the period concerned was 16.8%. The cumulative farm-level incidence was 54.0% for furunculosis, 39.5% for infectious pancreatic necrosis (IPN) and 10.5% for vibriosis. Occurrence of any of these specific infectious diseases in the post-smolt was significantly associated with mortality. The risk of furunculosis was significantly related to the location of the sea site. Mixing of smolt from many freshwater hatcheries in the sea farm increased the risk of IPN significantly. In addition, the risk of IPN was related to age of site and geographic location. The mean cumulative mortality in 1-year-old smolt groups was 17.5%, significantly greater than in two-year-old fish (7.9%). In addition, the method used to transport fish from the freshwater hatchery to the sea site was significantly associated with mortality. Neither the weight of smolts at sea transfer nor the date of transfer were significantly related to mortality.     

Survival of reared Atlantic salmon and sea trout in relation to marine conditions of smolt year in the Baltic Sea

The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) was examined in relation to marine conditions during post-smolt migration and in relation to stock traits. In 1970–2001, Carlin-tagged smolts were released in the Iijoki and Oulujoki rivers, the northern Baltic Sea. When both species were analysed together, the abundance of the three prey fish, herring (Clupea harengus), smelt (Osmerus eperlanus) and vendace (Coregonus albula) correlated positively with the survival of salmonids. In addition, the increase in smolt size appeared to improve the survival rate. Sea surface temperature (SST) may have affected indirectly through the abundance of prey fish during the post-smolt migration of salmon and sea trout. The smelt and vendace showed a statistical effect on survival only when the temperature effects were not included in the models. In sea trout, an increasing smolt length was not significantly correlated with the survival in good herring recruitment years, but in poor years survival increased very rapidly with increasing smolt size. The recapture rates of the salmonids tended to decrease between the years 1970 and 2001. During the same time period, the June SST slightly decreased. The positive correlation between the annual summer SST and recapture rate of salmon may partly explain the decreasing trend in recapture rates. An increase in smolt size did not compensate for the decline in the recapture rate of either species.     

Vertical distribution and sexual maturation in cage‐farming of Atlantic cod (Gadus morhua L.) exposed to natural or continuous light

The vertical distribution and seasonal timing of sexual maturation were investigated in Atlantic cod in four large commercial sea cages in Northern Norway during their second year of on‐growth. Replicate cages were exposed to either natural light (NL) or continuous light (LL) from May 2007 to January 2008. Hydro‐acoustic monitoring revealed that the cod in the NL cages were mainly distributed from 5 to 13 m depth at day while they were more dispersed and swam deeper at night. A clear ascent was observed prior to feeding events. The cod in the LL cages displayed a similar seasonal and daily pattern, but with a more dispersed vertical distribution range at both day and night. It is hypothesized that surface feeding motivation resulted in cod preferentially occupying the upper reaches of the sea cage, generally at depths < 13 m. This would also reduce the need for excessive alterations in swim bladder volume. Both the LL and the NL groups were exposed to LL the year before the study, resulting in first time sexual maturation (puberty) being delayed until the summer in both groups. However, while cod in the NL cages matured during the autumn 2007, the LL cod did not.     

Changes in condition factor and gonadosomatic index in maturing and non-maturing Atlantic salmon (Salmo salar L.) in Bay of Fundy sea cages, and the effectiveness of photoperiod manipulation in reducing early maturation

Changes in condition factor (cf) and gonadosomatic index (GSI) in maturing and non‐maturing Atlantic salmon (Salmo salar L.) cultured in Bay of Fundy sea cages was investigated under control (natural) and continuous light conditions. All experimental cages contained salmon stocked as smolts in April 2001. Six control cages received only natural illumination, three received continuous light from 3 November, 2001 until 31 May, 2002, and three received continuous illumination from 15 February, 2001 until 31 May, 2002. In mid‐July 2002, each cage was sampled, and sex, round weight, fork length (FL), mean muscle lipid content and gonad weight were recorded from each sampled fish. At harvest (August 2002 to February 2003), sex, maturation state, round weight and FL were measured from a sample of fish from each cage. The frequency distributions of GSIs from fish sampled in July indicated for both sexes a GSI>0.3 was indicative of early maturation. The relationship between cf and GSI suggested that salmon of both sexes had to have a cf greater than 1.3 in early summer for early maturation to develop. Continuous illumination from November greatly reduced the number of males with GSI>0.3 in mid‐July (0.8%, compared with 50% for control males); 17% of males from cages lit in February had GSIs exceeding 0.3. The percentage of control females exceeding 0.3 was 7%, compared with 0% and 4% for females from cages receiving continuous illumination from November and February respectively. For salmon with GSI of 0.3 or less, the percent muscle lipid increased linearly with cf. A multiple regression of cf and FL on GSI (GSI=?0.973+0.35 cf+0.013 FL) correctly identified 88% of maturing vs. non‐maturing salmon from control cages in July.     

Adiposity and growth of post-smolt Atlantic salmon Salmo salar L.

The long‐term adipose homeostasis seen in mammals gives rise to a ‘lipostatic’ model in which signals produced in proportion to fat stores serve to regulate energy intake. An extension of this predicts an impact of these signals on growth; downregulation of feeding in animals with increased adiposity should result in reduced growth. This was tested by monitoring fat deposition and growth in Atlantic salmon Salmo salar L. provided with feeds that differed in fat content. Salmon parr (mass c. 20 g) were fed either high‐ (H: 34%) or low‐fat (L: 22%) feeds, based on either fish (F) or vegetable (V) oils for 6 months to create groups of fish that differed in adiposity (10–12% and 5–7% body fat) at parr–smolt transformation (mass c. 130 g). Fish fed the high‐fat feeds deposited more body fat, and this was confirmed by measurement of fat concentrations in the fillet, viscera and remaining carcass. The fish were then grown‐on in sea water (c. 35 g L^?1, 8 °C, 24L:0D) for 14 weeks while being fed either high‐ or low‐fat feed formulated with fish oil to give the following treatments: HF→ H, HF→ L, LF→ L, LF→ H, HV→ H, HV→ L, LV→ L, LV→ H. Although fish exposed to the various feed treatments did not differ markedly in growth rate (SGR range 1–1.14% day^?1) over the 14 weeks of rearing in sea water, the results were in general agreement with predictions from the ‘lipostatic’ model, i.e. fish with the greatest fat reserves after the parr–smolt transformation grew more slowly than fish that were ‘leaner’ at this time. This suggests that adiposity, or ‘fatness’, may exert a negative feedback on feeding in salmon, thereby having an influence upon growth.     

Occurrence and prevalence of infectious pancreatic necrosis virus in rainbow trout (Oncorhynchus mykiss) cultured in cages in the Black Sea

Rainbow trout (Oncorhynchus mykiss) cultured in cage systems in the South Eastern Black Sea were surveyed for the type, occurrence and prevalence of infectious pancreatic necrosis virus (IPNV). Two nearby farms (designated as Farm A and Farm B) were visited monthly in 2007 and 2008. At each farm, 385 fish were selected randomly from five cages. Another farm with infected trout from a hatchery also was monitored for IPNV from the transfer to harvest. IPNV was found to be prevalent in both farms surveyed. In Farm A, IPNV was present throughout the growing period, from January to May, and all five randomly sampled cages tested positive for IPNV in March and April of 2007. In Farm B, IPNV was present only in February and March in 2007, and in 2008, IPNV was observed in January (two cages) and February (one cages) at low levels. Interestingly, IPNV was absent 2 weeks after transfer to the sea at 17.5°C. The same strain of IPNV, genotype III that was isolated from the same stock of fish at the hatchery, reoccurred when water temperatures dropped to 12°C in December in the Black Sea. Transferring fish to the sea at high water temperatures could lessen the negative impacts of IPNV on growth of rainbow trout in brackish water.     

Grow-out of juvenile seahorse Hippocampus kuda (Bleeker; Teleostei: Syngnathidae) in illuminated sea cages

This paper examines the feasibility of rearing 10–15-day- and 0.7–1.5-month-old seahorse Hippocampus kuda in illuminated sea cages to continue existing hatchery protocols to mass produce H. kuda for trade and enhance depleted wild stocks in their natural habitats. Thawed Acetes (a planktonic crustacean abundant in inshore seas) was fed to juvenile seahorses in lighted and unlighted sea cages while one group in lighted cages was not fed Acetes . After 10–12 weeks of rearing, both mean body weight and stretch height increased in all treatment groups, with lighted cage-reared seahorses fed Acetes being heavier (2 g) and longer (8 cm) than the other two treatment groups. Although instantaneous growth rates declined during the rearing period, these were generally higher among Acetes -fed seahorses in lighted cages (0.02–0.07) compared with those in the unlighted cages with Acetes and lighted cages without Acetes feeding. Mean survivorship in all groups ranged from 9% to 74% after the trials, but mean survivorship of juveniles in lighted cages with Acetes feeding (9–74%) was consistently lower than the two treatment groups as a likely result of crustacean and piscine predators being attracted by light and the odour of leftover Acetes in the lighted cages. These results demonstrate that light-attracted zooplankton prey supplemented by Acetes feeding may provide essential nutrients for the growth of H. kuda juveniles in illuminated sea cages. With further improvement in the grow-out protocol, it may provide a possible alternative livelihood to seahorse fishers and sufficient seed to re-populate depleted wild stocks of H. kuda .     

Early marine survival and movements of escaped Atlantic salmon Salmo salar L. juveniles from a land‐based smolt farm during autumn

The aim of this study was to examine early marine survival and movements of simulated escaped Atlantic salmon Salmo salar L. pre‐smolt and smolt from a commercial smolt farm during autumn. One‐third of the pre‐smolt most likely died in the immediate vicinity of the release location, whereas the corresponding mortality for smolts was lower (8.5%) during the 5‐week study period. The surviving pre‐smolt left the farm area after 2–3 days, predominantly along the shore. In contrast, most of the surviving smolts left the farm area during the first day and 54% seemed to move away from the shore and adopt a more pelagic movement pattern than pre‐smolt. The number of surviving fish recorded in the fjord decreased throughout the study period, possibly due to a combination of fish migrating out of the fjord or undetected mortality. Compared with existing knowledge on migration of released farmed smolts during spring, our results indicate less directional and slower movement rates during autumn. Only two of the tagged fish were detected upstream in the rivers following release. A rapid dispersion of escapees indicates that the potential for recapturing escapees is limited unless recapture efforts are initiated immediately after escape. Hence, there is a need for development of technology that detects and prevents escapees to enter the sea.     

Technology for cultivation of Atlantic salmon, Salmo salar L.: problems and possibilities

Abstract. The concept of farming Atlantic salmon, Salmo salar L., beyond smolt stage in sea water started in Norway and Scotland in 1967–1968, using natural enclosures or sea water ponds. Cage culture has since 1969 developed to be the dominating method. Atlantic salmon is cultivated in more than nine countries with a production of 235400t in 1990. Incubation of eggs, hatching, first feeding and smolt production have to a large degree been standardized. Various types of incubator systems and holding units are briefly presented and discussed. There has recently been a trend to move ongrowing cage farms from inshore and sheltered waters to more open and offshore locations. The various types of cages and supplemental equipment for the two options are compared and evaluated. The offshore trend is a more expensive and difficult technology than existing inshore farming methods. Sea enclosures and net enclosures are less attractive. There is new interest in pump ashore systems. Large land-based farms have been built in six countries with varying success. The amounts of water required are enormous and constructions costs are high, but running costs are comparable to cage culture. Great improvements have taken place for both smolt farms and pump ashore farms in treatment of water, use of heat exchangers, filtration of water, disinfection, aeration, oxygenation, photoperiod and light control, automatic feeding, internal transport, handling of fish and surveillance and control systems for fish and water. The problems and prospects of farming are discussed in relation to new and old technology.     

The prevalence of escaped farmed salmon, Salmo salar L., in the River Ewe, western Scotland, with notes on their ages, weights and spawning distribution

Abstract  The prevalence of escaped farmed Atlantic salmon, Salmo salar L., in the River Ewe, western Scotland, was assessed. After the establishment of smolt cages in the catchment and marine cages near the river mouth during 1986–1987, approximately 425 000 parr and smolts, and 122 000 growers have escaped. Between 1987 and 2001, farmed salmon occurred in the rod fishery in 13 of the 15 years, contributing at least 5.8% of the total catch, with a maximum annual frequency of 27.1%. It was estimated that <1% of fish escaping from the marine cages entered the river, but contributed at least 27% of potential anadromous spawners in 1997. Radiotagged, farmed fish in 2001 probably spawned in three subcatchments also used by tagged wild fish. Despite the likelihood of hybridisation there was no change in the median weight or marine age of wild fish, but smolt age decreased significantly ( P  < 0.02). The Ewe has a depleted wild salmon population (≤900 anadromous adults), and further genetic introgression by escapees should be prevented.     

The effect of spectral composition and light intensity on melatonin, stress and retinal damage in post-smolt Atlantic salmon, Salmo salar

Metal halide lights are currently used as standard in commercial Atlantic salmon sea cages as a means of enhancing productivity through grilse inhibition. However, such systems create bright point light sources that are neither environment specific nor species specific and could potentially compromise fish welfare. Light emitting diodes (LEDs) are a new form of lighting technology currently being developed for the fish farming industry that can be tuned to environment and species sensitivities through narrow bandwidth outputs. However, prior to implementing these new high energy alternatives, any potential adverse effects must be determined in fish. The objectives of this study were thus (1) to determine the effect of increasing intensities of blue LED light (0.199–2.7 W m^− 2, at 0.1 m from the light source) on light perception and stress response, and (2) to examine potential retinal damage under these conditions in post-smolt Atlantic salmon, Salmo salar. A white LED light was also tested, as well as a very high intensity metal halide positive control. Results demonstrated firstly that salmon perceived blue LED light (basal melatonin levels maintained) irrespective of intensity. Secondly, fish exposed to high intensity blue LED light showed an increase in plasma cortisol and glucose levels within 3 h, returning to a basal state 24 h post-light onset. This typical acute stress response was not observed in fish exposed to the white LED light and lower blue light intensities which could indicate differential sensitivities to spectral content of the light. No effects on the non-specific immune system (lysozyme activity) were observed. Finally, extensive histological examination of the retina from fish exposed to these various light treatments revealed no signs of damage. This demonstrates the efficiency of the adaptive mechanisms to light developed in fish.