Escaped farmed Atlantic salmon, Salmo salar L., feeding in Scottish coastal waters

Escaped reared salmon, Salmo salar L., were distinguished from wild salmon in the catch of a coastal salmon fishery on the west coast of Scotland. The stomach contents of 54 escaped fish were examined to determine their recent feeding history and 19 (35%) were found to contain food. The predominant prey were juvenile whiting, Merlangius merlangus (L.), unidentified Gadidae and sandeels (Ammodytidae), although other fish and invertebrates, mainly post-larval hermit crabs (Paguridae), were recorded. All these prey are pelagic or semi-pelagic. These observations demonstrate that escaped salmon feed on natural prey in coastal waters and extend our knowledge of the diet of salmon in their marine phase.     

Screamer disease in Atlantic salmon, Salmo salar L., in Chile

Investigation of cranial and spinal deformities in Atlantic salmon smolts from Chile immediately after sea transfer, over two successive crops, demonstrated ankylosis of the mandibular articulation, spinal foreshortening, fracture of vertebrae and rarefaction of osseous and cartilaginous tissues including the operculum. As a result the mouth was permanently agape and opercula could be folded back on themselves. All affected fish had been transferred to sea water at 20 °C plus and fed on particular commercial diets. Fish in cooler areas, or on diets high in vitamin C and phosphorus in the high temperature zone, were clinically normal. The condition has not recurred following dietary adjustment.     

Thermal habitat experienced by Atlantic salmon (Salmo salar L.) kelts in coastal Newfoundland waters

Thermal habitat was recorded by data storage tags (DSTs) applied to Atlantic salmon (Salmo salar L.) kelts during their seaward migration in the spring of 1998 at enumeration facilities in Highlands River, Humber River, Western Arm Brook, and Campbellton River, Newfoundland. In total, 139 DSTs were applied and data were downloaded from eleven of the recovered tags. The recovered tags had been applied at Highlands, Campbellton and Western Arm rivers and recovered in the coastal waters of Newfoundland and Québec and at the enumeration facilities at Highlands and Campbellton rivers. Water temperatures experienced by the fish were recorded for periods of 62–118 days at resolutions of 15–30 min. The data from the sea record on the DSTs were analysed for temperature patterns in relation to migration behaviour and diurnal movement of the fish. A variety of patterns were exhibited on the temperature records suggesting that the fish were behaving in various ways at different times. For Campbellton and Highlands fish over the course of some 24 h periods, night‐time temperatures changed little and were among the highest daily temperatures experienced by the fish, whereas daytime temperatures often showed dramatic and frequent shifts in temperature presumably as the fish rapidly and frequently changed depth. For the Western Arm Brook fish, rapid fluctuations in temperature occurred sometimes during the day and night without a consistent diurnal pattern. We also considered large‐scale aspects of the data by examining oceanographic conditions in relation to the temperatures recorded by the tags.     

Plasma protein changes in Atlantic salmon, Salmo salar L., with infectious salmon anaemia

Moderate to severe anaemia and hypoproteinaemia were reported in a Canadian outbreak of 'haemorrhagic kidney syndrome' in Atlantic salmon, later shown to be caused by a variant of infectious salmon anaemia virus (ISAV). The progressive anaemia associated with ISA has been previously reported, but hypoproteinaemia in salmon infected with European isolates of ISA virus has not been well documented. The present study showed a very significant positive correlation between decreasing haematocrit values and total plasma protein concentrations in Atlantic salmon infected with two Canadian and two Norwegian ISA viral isolates. However, variations in the concentration of individual plasma proteins, typical of acute phase responses in higher vertebrates, were not observed.     

Nocardiosis in tank-reared Atlantic salmon, Salmo salar L.

Nocardiosis is an infection caused by aerobic Gram-positive, branching, filamentous rods of the genus Nocardia . The organism is resident in both soil and plants ( Austin & Austin 1993 ; Frerichs 1993 ), and is closely related to Mycobacterium spp. Nocardiosis caused by Nocardia asteroides and N. seriolae (previously N. kampachi ) has been reported in several finfish species, both freshwater and marine. The first reported incidence was recorded by Valdez & Conroy (1963) in neon tetras, Hyphessobrycon innesi (Myers), and subsequently in other fish species, some of commercial significance including rainbow trout, Oncorhynchus mykiss (Walbaum) ( Snieszko, Bullock, Dunbar & Pettijohn 1964 ), brook trout, Salvelinus fontinalis (Mitchill), ( Campbell & MacKelvie 1968 ) and yellowtail, Seriola quinqueradiata (Temminck & Schlegel; Kubota, Kariya, Nakamura & Kira 1968 ). Despite two of these incidences occurring in salmonids, infection by Nocardia spp. in Atlantic salmon, Salmo salar L., has not previously been reported.     

Infectious pancreatic necrosis in Atlantic salmon, Salmo salar L

A clinical and histopathological review was carried out of 21 outbreaks of acute infectious pancreatic necrosis (IPN) in Scottish Atlantic salmon, Salmo salar L., farms (13 marine and eight fresh water) during 1991-2004. A distinctive syndrome was evident in both post-smolts in sea water and fry in fresh water, where liver lesions, which had not previously been associated with IPN, became a consistent finding in addition to the more typical pancreatic and intestinal changes. Initial cases were described in post-smolts in Shetland, but by the end of the period of investigation this type of pathology had extended down the West coast of Scotland and into Ireland. Limited viral strain analysis suggested that similar strains were involved in both fresh water and sea water and that these differed from earlier isolates from rainbow trout, Oncorhynchus mykiss (Walbaum). In fresh water, recovered fish frequently developed a greatly distended intestine associated with accumulation of undigested food. In sea water, after the initial, often significant (50% or more), losses, there were many fish which failed to grow and became chronically emaciated and prone to sea louse infection. Although use of transfer diets containing immune enhancers and the selection of IPN resistant broodstock has reduced losses the disease remains a serious cause of economic loss.     

Carbohydrate nutrition in Atlantic salmon, Salmo salar L.: growth and feed utilization

Five groups of Altantic salmon, Salmo salar L., (80 g postsmolt) were fed moist diets containing increasing levels of starch from 0% to 31% and concomitant decreasing levels of protein. The results showed that feeding a diet containing 22% lipid with no starch or a starch inclusion higher than 22% exerts negative effects on growth and feed utilization. A starch inclusion above 9% resulted in decreased starch digestibility, while protein digestibility was not influenced by the dietary starch content. Undigested starch is suggested to affect lipid digestibility in the same manner as dietary fibre. The reduced digestibility by high levels of dietary starch led to increased loss of particulate matter to the environment. Taking into account feed utilization and environmental aspects, the present experiment suggests that a diet containing approximately 9% starch is optimal.     

Target spawning requirements for Atlantic salmon, Salmo salar L., in Newfoundland rivers

Abstract In recent years, there has been increasing pressure from user groups in Newfoundland and Labrador, Canada, to manage Atlantic salmon, Salmo salar L., on a river-specific basis. The requirement to maximize benefits to users while ensuring that conservation goals are met has resulted in an initiative to develop river-specific target spawning requirements. In Newfoundland, anadromous juvenile Atlantic salmon make widespread use of lacustrine habitat for rearing. Depending on the river, lacustrine habitat can account for a substantial proportion of total production. Therefore, target spawning requirements for a number of rivers in several management areas were derived in terms of the contribution of both riverine and lacustrine habitats. The approach adopted, and derivations of associated parameter values, are reviewed, and limitations and research needs are discussed.     

Distribution of Atlantic salmon, Salmo salar L., in freshwater bodies of Europe

Data are presented on the current geographic distribution of Atlantic salmon, Salmo salar L., in freshwater bodies of Europe. They dwell in 14 hydrological systems, including basins of U lakes and three rivers. Nine of the lakes (Ladoga, Onega, Kuito and others) arc located in Russia, one in Finland (Saimaa) and one in Sweden (Vänern). All the rivers inhabited by freshwater salmon flow within Norwegian borders. Most of the lakes and rivers have good connection with the Baltic, White or Norwegian Seas. Over the past 20 years the freshwater salmon stock in Lake Imandra (the Kola Peninsula) has completely disappeared as a result of heavy human impact. The situation with such non-migratory salmon, which used to be important to fishing and trade, is also critical in all other water bodies. It is necessary, therefore, to elaborate effective measures for protecting and restoring freshwater salmon in the lakes and rivers of Europe.     

Incidence of hybridization between Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Ireland

The substantial growth of the farmed salmon industry in Europe since the 1970s has highlighted concerns regarding the genetic impact of escaped farmed salmon on wild salmonid stocks. High incidences of salmon × trout hybrids have been recorded in rivers situated near intensive salmon farming in Norway and Scotland, which may be indicative of a breakdown in reproductive isolation between salmon, Salmo salar L., and brown trout, Salmo trutta L. In the present study, salmonid fry and 0+ parr were collected from rivers in western Ireland. Allozyme and minisatellite DNA analysis were carried out on fry to determine the frequency of F1 hybrids from 10 rivers located within 38 km of salmon farms and three rivers at least 80 km from salmon farms. A total of 49 hybrids were recorded from 4135 salmonid fry (frequency = 1.2%). Mitochondrial DNA analysis showed that all hybrids arose from Atlantic salmon female × brown trout male crosses. Hybrid parr were recorded from one of the low-risk rivers (1.0%), but were present in seven out of the 10 catchments located within 38 km of salmon farms, with frequencies ranging from 0.7% to 3.1%. The results of the present survey, which represents the first extensive record of the levels of salmon-trout hybridization in Ireland, are discussed in relation to the continued growth of salmon farming in this country.     

Reproductive strategies in Atlantic salmon, Salmo salar L.

Abstract. Life-history strategies are means by which animals solve the problems of successful reproduction in varying environments. Their development patterns are consequences of responses to the opportunities the environment offers them. Understanding them requires an understanding of the way they evolved, their ontogenetic development, their physiological control, and their adaptive value. The present paper views the salmonids as marine fishes, which have radiated into fresh water through using river beds as protected spawning grounds. It also takes the view that the maturation process has priority over somatic growth in fish, and that it has already been initiated by the time of first feeding. Its completion is environmentally dependent, and can be arrested annually. Whether or not it will be arrested depends on the status of the energy stores of the individual at particular critical times of year. This mechanism has adaptive value both for immediate reproductive success — adequate energy to provision the next generation — and for later overwinter survival, ensuring that if energy stores are inadequate for reproduction they are spared. Atlantic salmon show variation in their reproductive patterns, and examples are given from laboratory and aquaculture experiments to demonstrate some environmental controls which result in these variations. A hypothetical model is presented to account for the operation of these controls.     

Predation of Atlantic salmon, Salmo salar L., smolts and parr by red-breasted mergansers, Mergus serrator L., on two Scottish rivers

Abstract Current models estimating the impact of red-breasted mergansers, Mergus serrator L., on salmon, Salmo salar L., fisheries in Scotland fail to take account of any annual variation in the proportion of the diet that is smolts. During the 1987–1990 smolt runs, the annual variation in the diet of mergansers was estimated from the stomach contents of birds shot on two Scottish rivers. The proportion of salmon in the diet was greatest early in the smolt run (76–91% by weight), and contained proportionately more smolts than later in the run, when coarse fish were more prominent. There was little annual variation in the proportion of the diet that was juvenile salmon. However, the proportion of these fish that were smolts, was twice as great in some years than in others and this appeared to be independent of estimated annual smolt production in the rivers.     

Pathogenicity of Saprolegnia spp. to Atlantic salmon, Salmo salar L., eggs

Live and dead Atlantic salmon eyed eggs were challenged with eight different Saprolegnia isolates, selected because of their varied origins, known morphological characteristics and growth/germination pattern. Some isolates were also tested for pathogenicity to Atlantic salmon parr. Challenge of eggs was performed by exposure to spores in suspension or by co-incubation of live eggs with infected dead eggs. The phenotypic characteristics of the isolates were evaluated in relation to their observed pathogenicity from the challenge experiment, to identify possible virulence factors leading to egg-infection by Saprolegnia. The results from the experiments confirm that live eggs are refractory to infection with Saprolegnia spores in suspension and that an infection of live eggs can only occur from an infection nucleus represented by dead eggs or debris. It was observed that strains pathogenic to salmon parr were not particularly infective towards eggs, and the isolates that gave the highest infection rates to eggs were species considered to be saprotrophs.     

Is there lipostatic regulation of feed intake in Atlantic salmon Salmo salar L.?

High‐energy feeds, with increased fat concentrations, are often used in salmon farming to improve feed:gain ratio. However, fish are thought to regulate ingestion to meet their energy and nutrient intake requirements. Further, feeds with excessive fat content will lead to increased adiposity, which is thought to exert a negative feedback on feed intake via lipostatic regulation mechanisms. A test of the lipostatic model of feed intake regulation was carried out on juvenile Atlantic salmon Salmo salar L. (c. 165 g) in which body fat content (5.6% and 9.4% body fat) had been manipulated by feeding feeds with different fat concentrations. Thereafter, the fish were offered the high‐ and low‐fat feeds (15.6% and 26.5% dietary fat content) simultaneously, and our hypothesis was that feed preference and intake would reflect the state of fat storage. Thus, we predicted that, when given a choice of feeds, the lean fish would eat more, and that the body fat status of the groups would converge over time. The results indicated a general preference for the leaner feed irrespective of adiposity level, but leaner fish consumed more feed, grew faster and deposited more body fat than their fatter counterparts. Over time, body compositions converged among treatments, and differences in feed intake ablated. These findings seem to provide supportive evidence for a lipostatic regulation of feed intake in fish.     

Feeding behaviour of wild Atlantic salmon, Salmo salar L., parr in mid- to late summer in a Scottish river

Abstract. From 25 July to 11 September 1984 Atlantic salmon, Satmo salar L., parr were observed underwater between 0800 and 1600 h, feeding actively from defended territories at two sites in the River Tilt, Perthshire, Scotland. Territories comprised one or more preferred stations on or just above the substrate, from which the fish intercepted drifting particles (75% of feeding), or foraged on the substrate (25%). Drift feeding occurred 36% at the surface, 35% in mid-water and the remainder at the feeding station. 35% of prey particles were attacked directly from the feeding station, and 65% indirectly after preliminary inspection, sometimes involving a drift downstream by the fish. 20% of captures were made upstream of the station and 80% downstream. 12–15-cm fish fed more frequently at the surface, while 10-cm fish fed in mid-water or at their stations. Surface feeding decreased proportionately during rainfall. Frequency of feeding increased with temperature. Feeding was depressed in the presence of large salmonids.     

Swimming height of Atlantic salmon, Salmo salar L., crossing a Crump weir

Abstract. A closed-circuit television system was used to determine the height in the water column at which salmon swam when ascending and descending over a Crump weir. The swimming depth distribution of fish ascending the weir was very skewed with 88.1% of the fish seen swimming within 10cm of the weir surface while the descending fish seen were scattered throughout the available water column. The distribution of water velocities over the downstream face of the weir showed that the velocity was lowest near the weir surface and highest at the water-air interface except at the weir crest where high velocity water was found at the weir surface, often causing fish to rise as they crossed the crest. This has important implications in choosing the position of electrode arrays in open-channel counters as the upstream electrode must not be too close to the weir crest or fish may be missed as a result of being too far from the electrode to produce a large enough signal.     

Growth, product quality and immune status of Atlantic salmon, Salmo salar L., fed wet feed with alginate

By-products, by-catch and offal from the fishing industry represent valuable resources of high-quality protein and energy. In order to utilize some of these resources, a method for preparing wet feed for Atlantic salmon, Salmo salar L., using a binder produced from seaweed, Ascophyllum nodosum L., was developed. The feed was formulated with offal from lean fish (haddock, Melanogrammus aegleflnus L., saithe, Pollachius virens L., pollack, Pollachius pollachius L., and cod, Gadus morhua L.) and fatty fish [mackerel, Scomber scombrus L., and argentine, Argentina silus (Ascanius)], fish meal extruded wheat, capelin oil, vitamin and mineral premix, astaxanthin, and seaweed binder. The feed was gellified in a solution of calcium chloride. Equipment for the production of such feed was developed and the feed tested on a large scale at a commercial salmon farm. As control, a high-fat, extruded, commercial dry feed was used. On a dry matter basis, the two feeds were comparable with respect to protein, fat and carbohydrate. Growth rates, appetite and feed conversion were similar for the two experimental groups. Fish fed dry feed seemed to be more active when handled before slaughter and had a higher incidence of skin damage than fish given wet feed. Chemical and sensory analyses of the fish showed no difference (P > 0.05) between the two groups. Fifty fish from each group were analysed for haemoglobin, protein, lysozyme, complement and haemolytic activity in the blood. There was a significantly higher level of lysozyme in the wet feed group than in the dry feed group, implying an immunostimulating effect of alginate. It was clearly shown that producers have an excellent opportunity to improve the utilization of the resources and decrease feed production costs in places where the natural conditions are favourable, i.e. locations where there is an integrated production of fish farming and processing of wild catch.