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.     

Atlantic salmon in the North Pacific

The first catches of Atlantic salmon, Salmo salar L., in British Columbia (BC) waters occurred in 1987. The first reported escape of Atlantic salmon (2000 individuals) occurred in 1988. From 1988 to 1995, 97 799 Atlantic salmon were reported escaped from net pens in BC but the true number was higher as not all escapes are reported. Since 1987 a total of 9096 Atlantic salmon was caught in the coastal marine waters of BC, Washington and Alaska, and 188 were caught in fresh water. Most catches occurred in the Johnstone Strait area, where the abundance of salmon farms is highest. The most distant recovery occurred in 1994 when an Atlantic salmon was caught near the western end of the Alaska Peninsula. There have been no reports of successful reproduction of Atlantic salmon in the wild and no feral juveniles have been found. Atlantic salmon caught in the ocean in BC have substantial amounts of adipose tissue and they are heavier at length than fish caught in Alaska. The proportion of fish with prey items in their stomachs is generally low but higher in Alaska (13.1%) than in BC (5.8%). Most fish caught in fresh water are either maturing or mature.     

Can smolting and maturation of hatchery-reared brown trout Salmo trutta L. be affected by food deprivation during the first and second years of rearing?

Smolting and maturation of 2+ brown trout Salmo trutta L. were evaluated after exposing the groups of trout to different feeding regimes during the summers at 0+ and 1+ ages. The hypothesis tested was based on the theory of smolting of a congeneric species, Atlantic salmon Salmo salar L., in which the physiological smolting decision is expected to be taken at the end of July or beginning of August. During the first summer, the growth of the trout was restricted in two groups out of four by low feeding frequency. During the second summer, food was totally withheld for 3 weeks in June–July (i.e. before the expected sensitive period), in August or not at all (control). The proportion of sexually mature males in November was 5.2% in the groups fasted during August, but somewhat lower in the groups fasted in June–July (average 2.3%) or in control fish (3%). The tendency for smolting was evaluated during the following spring in an artificial stream with the help of PIT-tag technology, which allowed monitoring of the movements of individually tagged trout. Seawater challenge tests were also carried out in April and June. Differences in osmoregulatory ability in seawater indicated that feeding treatments had a slight effect on the timing of smolting, but no differences were observed in movement behaviour between treatment groups. Mature and maturing males moved less at the peak migration time (mid-May) but more in October than immature fishes. These results suggest that the smolting decision in brown trout may be taken at a different time than in Atlantic salmon and that periodic poor growth conditions during the summer will not prevent smolting of trout during the following spring.     

A historical review of the key bacterial and viral pathogens of Scottish wild fish

Thousands of Scottish wild fish were screened for pathogens by Marine Scotland Science. A systematic review of published and unpublished data on six key pathogens (Renibacterium salmoninarum, Aeromonas salmonicida, IPNV, ISAV, SAV and VHSV) found in Scottish wild and farmed fish was undertaken. Despite many reported cases in farmed fish, there was a limited number of positive samples from Scottish wild fish, however, there was evidence for interactions between wild and farmed fish. A slightly elevated IPNV prevalence was reported in wild marine fish caught close to Atlantic salmon, Salmo salar L., farms that had undergone clinical IPN. Salmonid alphavirus was isolated from wild marine fish caught near Atlantic salmon farms with a SAV infection history. Isolations of VHSV were made from cleaner wrasse (Labridae) used on Scottish Atlantic salmon farms and VHSV was detected in local wild marine fish. However, these pathogens have been detected in wild marine fish caught remotely from aquaculture sites. These data suggest that despite the large number of samples taken, there is limited evidence for clinical disease in wild fish due to these pathogens (although BKD and furunculosis historically occurred) and they are likely to have had a minimal impact on Scottish wild fish.     

Prey selectivity and diel feeding chronology of juvenile chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon in the Columbia River plume

We studied salmon feeding selectivity and diel feeding chronology in the Columbia River plume. Juvenile chinook and coho salmon were caught by trawling at 2–3 h intervals throughout a diel period on three consecutive days (21–23 June 2000) at stations located 14.8 and 37 km offshore from the mouth of the Columbia River. A total of 170 chinook salmon were caught at the inshore and 79 chinook and 98 coho salmon were caught at the offshore station. After each trawl, potential prey were sampled at different depths with 2–3 different types of nets (1‐m diameter ring net, bongo net, neuston net). Despite the variability in zooplankton abundance, feeding selectivity was surprisingly constant. Both salmon species fed selectively on larger and pigmented prey such as hyperiid amphipods, larval and juvenile fish, various crab megalopae, and euphausiids. Hyperiid amphipods were abundant in the salmon diets and we hypothesize that aggregations of gelatinous zooplankton may facilitate the capture of commensal hyperiid amphipods. Small copepods and calyptopis and furcilia stages of euphausiids dominated the prey field by numbers, but were virtually absent from salmon diet. Juvenile chinook salmon, with increasing body size, consumed a larger proportion of fish. Stomach fullness peaked during morning hours and reached a minimum at night, suggesting a predominantly diurnal feeding pattern. In general, both chinook and coho salmon appear to be selective, diurnal predators, preying mostly on large and heavily pigmented prey items, in a manner consistent with visually oriented, size‐selective predation.     

Stomach Flushing of Salmonids: A Simple and Effective Technique for the Removal of the Stomach Contents

A stomach flushing technique is described which has been used for over a year, both in the laboratory and in the field. It is reliable, quick and relatively easy to perform and has been applied to juvenile Atlantic salmon (Salmo salar L.) and brown trout (S. trutta L.). The only limitation on its use was that fish <4 cm in length were considered too small to flush. Experiments carried out show that the technique removes 98.9% of the stomach contents from the fish and has very little effect on subsequent survival (99.3%) and condition. It is suggested that this technique is an improvement on previous designs.     

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.     

Quality of Alaska Salmon after Harvesting and Post-Tendering

Nearly 22,000 sockeye salmon (Oncorhynchus nerka) that were caught by gillnetters and delivered to tenders anchored on the fishing grounds were tagged in Alaska during the summers of 2010 and 2011. The fish were chilled in refrigerated seawater and transported, usually within 24 h, to one shore-based processing plant, where they were graded visually. Handling practices, weather conditions, and distance from the fishing grounds to the plant were documented. The closer to the shore plant the fish were caught, the better was the salmon quality. Time, distance, and foul weather had significantly negative impacts.     

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.     

Angling Success for Atlantic Salmon (Salmo salar) in the River Wye in Relation to Effort and River Flows

Salmon catch and effort data for 1977 from ten private fisheries on the River Wye are analysed and discussed. The 555 salmon caught represent 10.7% of the total Wye rod catch and the mean angling success rate of 0.045 ± 0.022 salmon per angler-hour, equivalent to about 22 h per salmon, compares favourably with that for other rivers in Britain and Canada. Over 75% of all salmon caught were 2 sea-winter fish, the age distribution of salmon caught in the sample being similar to those for the whole river. Angling success in the upper reaches during July and August may have been reduced by limited availability affected by water temperatures too high for migration, Flows most suitable for successful angling in each fishery are compared in relation to distance from the estuary and long-term average daily flows. Salmon appeared to be caught nearer the crest of a spate in the upper compared with the lower reaches of the river and this difference is discussed.     

Interannual and interdecadal variability in juvenile coho salmon (Oncorhynchus kisutch) diets in relation to environmental changes in the northern California Current

The feeding habits of juvenile coho salmon, Oncorhynchus kisutch, in the northern California Current were examined using samples from two different time periods (1980–85 and 1998–2003) of highly contrasting oceanographic conditions. The goal was to test the influence of interannual and interdecadal changes in taxonomic composition of prey, feeding intensity, and size spectra of teleost prey. Analyses were done for samples taken both early in the summer (June) shortly after the salmon enter the ocean, and also in late summer (September) following some ocean residency. Fish prey dominated coho salmon diets by weight during most years, but this trend was more pronounced during the 1980–85 sampling period. In terms of numerical composition, the diets were more variable on an interannual basis, but decapod larvae and euphausiids were important prey in most years. Pteropods and copepods were important prey during weak upwelling or El Niño years, whereas euphausiids were important during strong upwelling or otherwise highly productive years. Hyperiid amphipods comprised a substantial proportion of the diets only in 2000. Coho salmon showed highly significant differences in prey composition among years or between decades both in weight and numerical composition. The percentage of empty stomachs was highly variable by year in both June and September, but was significantly different only for September between decades. In contrast, an index of feeding intensity did not show many significant changes in either comparison. However, the relative size ratios for fish prey consumed were highly variable by year, and larger than average fish prey were consumed during 1998, leading to the highest feeding intensity observed.     

Effects of catch and release angling on Atlantic salmon, Salmo salar L., of the Conne River, Newfoundland

The effects of catch and release angling on survival of Atlantic salmon, Salmo salar L., at Conne River, Newfoundland, were investigated by retaining angled ( n =49; experimental group) and trap-caught ( n =20; control group) fish in holding cages for up to 40 days. Samples were obtained from 8 June to 4 July, 2000, and partitioned among four water temperature strata. Apart from not being angled, control fish were handled, tagged, and transferred to holding cages in a manner similar to angled salmon. Water temperatures and discharge were monitored throughout the duration of the study. Overall, 8.2% of salmon caught and released died, but 12% died among salmon angled in water temperatures ≥ 17.9 °C. No control fish died. There were no significant differences in time associated with angling, exposure to air, tagging, transfer to holding cages, nor total handling time between salmon that survived vs. those that died. Results of the study should encourage managers to continue to use catch and release as a viable tool in the management of Atlantic salmon stocks.     

The risk of individual fish being captured multiple times in a catch and release fishery

The proportion of angled Atlantic salmon Salmo salar L. being caught and released has increased. If individuals are repeatedly captured, this may have fish welfare consequences. Of 995 Atlantic salmon tagged during catch and release in eight Norwegian rivers, 10% were captured twice, while 3% were captured three times within the same fishing season. The probability that released salmon were captured again decreased with decreasing time left of the fishing season, decreased for larger‐sized fish and varied among rivers/years. Increased exploitation rates within the river, indicating an increased fishing pressure, strongly increased the probability that fish would be recaptured. However, the proportion of salmon caught a second time was much lower than the total exploitation rates in the same rivers (which was on average 46%). For fish tagged in the sea, the likelihood of being angled decreased with time since entering the river, which may explain why the recapture rates of caught and released fish were lower than the total exploitation rates.     

Increased ability to compete for food by growth hormone-transgenic coho salmon Oncorhynchus kisutch (Walbaum)

In salmonids, growth hormone (GH) stimulates growth, appetite and the ability to compete for food. This study tested the hypothesis that increased GH levels in GH-transgenic coho salmon Oncorhynchus kisutch (Walbaum) increase competitive ability through higher feeding motivation. The transgenic strain of salmon used contained a gene construct consisting of the sockeye metallothionein-B promoter fused to the type 1 growth gene coding region. The transgenic animals (mean size = 250 g) were F₁ individuals. In six consecutive feeding trials, the intake of contested food pellets by size-matched pairs consisting of one control (1 year older non-transgenic coho salmon) and one GH-transgenic coho salmon was compared. Pellets were provided sequentially until neither fish took three consecutive pellets; the identity of the fish taking each pellet was noted. Calculated on the three first pellets offered at each feeding trial, the transgenic coho salmon consumed 2.5 times more contested pellets than the controls, supporting the hypothesis that GH transgenesis increases the ability to compete for food. Overall, the transgenic fish consumed 2.9 times more pellets that the non-transgenic controls, indicating a high feeding motivation of the transgenic fish throughout the feeding trials. It appears that GH transgenesis and GH treatments can induce similar changes in the feeding behaviour of salmonids. Depending on how transgenic and wild individuals differ in other fitness-related characters, escaped GH transgenic fish may compete successfully with native fish in the wild.     

Growth, feed conversion and chemical composition of Atlantic salmon (Salmo salar L.) and Atlantic halibut (Hippoglossus hippoglossus L.) fed diets supplemented with krill or amphipods

The effects of partial replacement of fish meal (FM) with meal made from northern krill (Thysanoessa inermis), Antarctic krill (Euphausia superba) or Arctic amphipod (Themsto libellula) as protein source in the diets for Atlantic salmon (Salmo salar L.) and Atlantic halibut (Hippoglossus hippoglossus L.) on growth, feed conversion, macro‐nutrient utilization, muscle chemical composition and fish welfare were studied. Six experimental diets were prepared using a low‐temperature FM diet as control. The other diets included northern krill where 20, 40 or 60% of the dietary FM protein was replaced with protein from northern krill, and two diets where the FM protein was replaced with protein from Antarctic krill or Arctic amphipod at 40% protein replacement level. All diets were iso‐nitrogenous and iso‐caloric. Atlantic salmon grew from 410 g to approximately 1500 g during the 160 day experiment, and Atlantic halibut grew from 345 g to 500–600 g during the 150 day experiment. Inclusion of krill in the diets enhanced specific growth rate in salmon, especially during the first 100 days (P < 0.01), and in a dose–response manner in halibut for over the 150 day feeding period (P < 0.05). Feed conversion ratio did not differ between dietary treatments, and no difference was found in dry matter digestibility, protein digestibility and fish muscle composition. Good growth rates, blood parameters within normal ranges and low mortalities in all experimental treatments indicted that fish health was not affected either Atlantic salmon or Atlantic halibut fed the various zooplankton diets.     

The vertical distribution of juvenile salmon (Oncorhynchus spp.) and associated fishes in the Columbia River plume

Simultaneous trawling at surface and at depth at one location off the Columbia River, Oregon, in June 2000 identified the depth distribution of juvenile salmonids and associated fishes. Juvenile salmon off the Columbia River were distributed primarily near the surface, within the upper 12 m. Highest densities of subyearling chinook salmon (Oncorhynchus tshawytscha) off the Columbia River were associated with high surface currents and decreasing tidal levels, with time of day possibly a co‐factor. Densities of yearling chinook salmon increased with higher turbidity. Pacific herring (Clupea pallasi) was the most abundant and commonly caught forage fish, with density increasing at night, probably related to diel vertical migration. Catches of juvenile salmonids were not associated with catches of forage fishes. Daytime surface trawling appears to be an appropriate method for assessing the distribution and abundance of juvenile salmonids in marine habitats.     

Life‐history differences of juvenile Chinook salmon Oncorhynchus tshawytscha across rearing locations in the Shasta River,California

Salmon from different locations in a watershed can have different life histories. It is often unclear to what extent this variation is a response to the current environmental conditions an individual experiences as opposed to local‐scale genetic adaptation or the environment experienced early in development. We used a mark–recapture transplant experiment in the Shasta River, CA, to test whether life‐history traits of juvenile Chinook salmon Oncorhynchus tshawytscha varied among locations, and whether individuals could adopt a new life history upon encountering new habitat type. The Shasta River, a Klamath River tributary, has two Chinook salmon spawning and juvenile rearing areas, a lower basin canyon (river km 0–12) and upper basin spring complex (river km 40–56), characterised by dramatically different in‐stream habitats. In 2012 and 2013, we created three experimental groups: (i) fish caught, tagged and released in the upper basin; (ii) fish caught at the river mouth (confluence with the Klamath River, river km 0), tagged and released in the upper basin; and (iii) fish caught at the river mouth, tagged and released in the lower basin. Fish released in the upper basin outmigrated later and at a larger size than those released in the lower basin. The traits of fish transplanted to the upper basin were similar to fish originating in the upper basin. Chinook salmon juvenile life‐history traits reflected habitat conditions fish experienced rather than those where they originated, indicating that habitat modification or transportation to new habitats can rapidly alter the life‐history composition of populations.     

Aeromonas salmonicida infection levels in pre‐ and post‐stocked cleaner fish assessed by culture and an amended qPCR assay

Due to increasing resistance to chemical therapeutants, the use of ‘cleaner fish’ (primarily wrasse, Labridae, species) has become popular in European salmon farming for biocontrol of the salmon louse, Lepeophtheirus salmonis (Krøyer). While being efficient de‐licers, cleaner fish mortality levels in salmon cages are commonly high, and systemic bacterial infections constitute a major problem. Atypical furunculosis, caused by Aeromonas salmonicida A‐layer types V and VI, is among the most common diagnoses reached in clinical investigations. A previously described real‐time PCR (qPCR), targeting the A. salmonicida A‐layer gene (vapA), was modified and validated for specific and sensitive detection of all presently recognized A‐layer types of this bacterium. Before stocking and during episodes of increased mortality in salmon cages, cleaner fish (primarily wild‐caught wrasse) were sampled and screened for A. salmonicida by qPCR and culture. Culture indicated that systemic bacterial infections are mainly contracted after salmon farm stocking, and qPCR revealed A. salmonicida prevalences of approximately 4% and 68% in pre‐ and post‐stocked cleaner fish, respectively. This underpins A. salmonicida's relevance as a contributing factor to cleaner fish mortality and emphasizes the need for implementation of preventive measures (e.g. vaccination) if current levels of cleaner fish use are to be continued or expanded.     

Comparative effects of dietary Na+-lactate on Arctic char, Salvelinus alpinus L., and Atlantic salmon, Salmo salar L.

Arctic char, Salvelinus alpinus L., and Atlantic salmon, Salmo salar L., were fed a commercial diet with or without supplementation of 1.5% Na⁺-lactate. Dietary Na⁺-lactate enhanced growth of Arctic char, while that of salmon was unaffected. Dietary ¹⁴C-lactate was retained for significantly longer in the stomach of Arctic char than that of Atlantic salmon. Changes in intestinal cholytaurin hydrolase activity, a bacterially produced enzyme, may indicate that dietary lactate affects the intestinal microbiota of Arctic char but not that of Atlantic salmon. Analysis of bile acids of char showed that dietary Na⁺-Iactate influenced neither intestinal nor gallbladder bile acid composition. Although Arctic char possesses the classical entero-hepatic circulation pathway, no extra loss of bile acids from the fish was observed. It is concluded that the retention time of diet in the stomach of char is significantly longer than that of salmon. This may increase the antibacterial action of lactate in the former, favouring the colonization of lactic-acid-tolerant bacteria in the intestinal tract, some of which produce the enzyme cholytaurin hydrolase.     

The effect of dietary krill supplementation on epithelium-associated bacteria in the hindgut of Atlantic salmon (Salmo salar L.): a microbial and electron microscopical study

Atlantic salmon (Salmo salar L.) were fed fishmeal protein for 46 days, and 500 g kg⁻¹ of fishmeal protein substituted with meal from Northern krill (Meganyctiphanes norvegica). No differences were observed in weight gain, length gain, feed conversion or specific growth rate between the groups that could be attributed to dietary manipulation. The adherent microbiota in the hindgut of the two rearing groups were further investigated. By substituting fishmeal with krillmeal, the total viable counts of aerobic and facultative aerobic bacteria colonizing the hindgut of Atlantic salmon increased from 8.5 × 10⁴ to 2.2 × 10⁶. Furthermore, dietary krillmeal affected the adherent hindgut microbiota. The Gram‐positive bacteria Carnobacteria piscicola, Microbacterium oxydans, Microbacterium luteolum and Staphylococcus equorum spp. linens and the Gram‐negatives Psychrobacter spp. and Psychrobacter glacincola were not isolated from hindgut of fish fed the krill diet. On the other hand, Pseudomonas fulgida, Pseudomonas reactans and Stenotrophomonas maltophila were not isolated from the control group fed fishmeal. Acinetobacter lwoffi, which is not normally found in the fish gut, was isolated from both feeding groups. Transmission electron microscopy showed bacteria‐like profiles between the hindgut microvilli in both feeding groups indicating autochthonous microbiota. When fish were fed the krill diet, hindgut enterocytes were replete with numerous irregular vacuoles. These vacuoles were not observed in fish fed the fishmeal protein.