Supplementation of adult Atlantic salmon (Salmo salar) restores biofilm biomass and benthic metabolism

The present study was performed to assess to what degree supplemented dietary iodine (I) was retained in selected tissues, including the fillet of adult Atlantic salmon (Salmo salar) reared in sea water. Atlantic salmon weighing approximately 1.5 kg were randomly assigned to three net pens per treatment and fed moist pellets (based on minced saithe and herring) supplemented with 0, 40 or 80 mg iodine (as KI) kg−1 on dry weight basis for 150 days. The iodine concentrations in the experimental feeds were analysed to be 10, 54 and 86 mg kg−1 dry weight, respectively. Growth, mortality and blood haemoglobin concentration (Hb) were recorded. Iodine concentrations were measured in muscle, liver and kidney after 90 and 150 days of feeding by inductively coupled plasma-mass spectrometry. In addition, plasma thyroxine (T4) and triiodo-thyronine (T3) were determined. The weight gain during the period was approximately 1 kg for all treatments. There were no mortalities, and blood Hb levels were within normal ranges. The iodine concentration in muscle, liver and kidney were all affected by the dietary iodine level, despite wide intratreatment variation. After 150 days, fillets of fish fed 10, 54 and 86 mg I kg−1 showed mean concentrations of 0.4, 0.5 and 0.9 mg I kg−1 wet weight, respectively, whereas the iodine concentration in the liver and the kidney increased approximately three times in the dietary groups. Similarly, plasma T4 and T3 showed great variation within the treatments. No significant correlations were found between individual tissue iodine concentration and thyroid hormone concentration in any of the groups at any sampling time. This preliminary feeding experiment showed that fillet iodine in adult Atlantic salmon can be increased up to 1.4 mg I kg−1 wet weight by dietary iodine 80 times the minimum requirement for salmonids, without impacting health, performance or plasma thyroid hormone status.

Retention and readability of visible implant (VI) tags were evaluated in adult (2-yearold) Atlantic salmon (Salmo salar), adult (4-year-old) lake trout (Salvelinus namaycush), and yearling (10-month-old) fish of both species. Tags were implanted in adipose eyelid tissue posterior to the eye. After 10 months, retention in yearlings was 49% by Atlantic salmon and 41% by lake trout. In adults, retention was 84% by Atlantic salmon and 45% by lake trout. Tag retention in yearling Atlantic salmon increased with fish weight at tagging; tag retention, by weight-groups, was 0% for fish that weighed 20 g or less; 46% for 21–40-g fish; and 71% for 41–99-g fish. Tag retention in adults of Atlantic salmon and lake trout was not related to fish weight. After 10 months, the percentage of readable tags was 100% in yearling Atlantic salmon and 82% in adult Atlantic salmon, and 0% in yearling lake trout and 77% in adult lake trout. None of the tags in yearling lake trout were readable 294 d after implant, because of cloudiness of the adipose eyelid tissue. Tags recovered from fish with cloudy adipose tissue were readable. The proportion of total tag loss occurring in the first 70 d after tagging was 78% in yearling Atlantic salmon and 22% in adult Atlantic salmon, and 67% in yearling lake trout and 41% in adult lake trout. An illuminated magnifier lamp (1.7 × magnification) greatly increased the readability of the VI tags. The VI tags were effective for Atlantic salmon larger than 30 g, but they were not effective for lake trout.

Sodium balance in adult atlantic salmon ( Salmo salar L.) during migration into neutral and acid fresh water

Partial cloning of constitutive and inducible nitric oxide synthases and detailed neuronal expression of NOS mRNA in the cerebellum and optic tectum of adult Atlantic salmon ( Salmo salar)

Imaging sonars are used around the world for fish population monitoring. The accuracy of the length measurements has been reported in multiple studies for relatively short (<15 m) ranges and high image resolution. However, imaging sonars are often used at longer ranges (i.e., >15 m) where the images produced from sonar returns become less detailed. The accuracy of the length measurements from the Adaptive Resolution Imaging Sonar (ARIS) was tested by releasing n = 69 known-sized adult Atlantic salmon (Salmo salar) directly into the sonar field at ranges between 15 and 29 m, and measuring their echoes manually by four users and semi-automatically using a computer workflow in Echoview software. Overall, the length measurements were very variable: compared to true (fork) lengths, the mean of differences varied between -9.9 cm and 7.8 cm in the human-generated datasets, and between -42.8 cm and -20 cm in the computer-generated dataset. In addition, the length measurements in different datasets were only in poor or moderate agreement with each other (intraclass correlation <0.61). Contrary to our expectations, the distance from the transducer or the subjectively assessed echo quality did not have an effect on the measurement accuracy in most of the datasets and when it did, the effect was not systematic between the datasets. Therefore, a size class and length prediction model was implemented in a Bayesian framework to group salmon into two size categories: One-Sea-Winter (<63 cm) and Multi-Sea-Winter (≥63 cm) groups. The model correctly predicted the size category in 83% of the fish in the computer-generated dataset and ranged from 68% to 74% in the human-generated datasets. We conclude that fish length measurements derived from long-range imaging sonar data should be used with caution, but post-processing can improve the usefulness of the data for specific purposes, such as adult Atlantic salmon population monitoring.

Adult Atlantic salmon (Salmo salar) return to natal rivers several months before spawning and during summer can be subjected to temperatures that exceed their upper temperature tolerance limits. Salmon use thermal refuges to minimize exposure to high temperatures, but little information exists regarding behavioral thermoregulation by adult Atlantic salmon. We examined behavioral thermoregulation by Atlantic salmon during summer in-river residence in a Quebec river with a novel combination of thermal infrared remote sensing, river temperature monitoring, and acoustic telemetry. Adults engaged in behavioural thermoregulation at cooler ambient river temperatures (17–19 °C) than previously recorded for this species and maintained body temperature within a narrow range (16–20 °C) via use of cool and warm refuges. Adults used large, stable, stratified pools as refuges, allowing multiple individuals to thermoregulate simultaneously without leaving the pool. Low river discharge and high temperatures can be physical barriers to salmon migration, preventing them from accessing suitable refuges (e.g., pools). Identifying and maintaining connectivity to thermal refuges may be critical for persistence of Atlantic salmon populations as climate changes and rivers warm.

Hydroacoustic differentiation of adult Atlantic salmon and aquatic macrophytes in the River Wye, Wales

Leiomyosarcoma with associated retrovirus were found in North America for the first time in adult Atlantic salmon (Salmo salar) held in a quarantine facility at the North Attleboro National Fish Hatchery (NANFH), Massachusetts, USA. The fish had been collected as age 1-2 yr animals from the Pleasant River, Maine, and were to be used as brood stock in a population augmentation program for that river. Neoplastic disease was observed at NANFH initially in older (age 4 yr) fish, followed by age 3 yr fish. Disease was not observed in age 2 yr fish. The mortality pattern was chronic.

Serum thyroid hormone levels were determined in adult Atlantic salmon (Salmo salar) of both sexes caught in the ocean and at a sequence of locations on their return migration to spawn. Tri-iodothyronine (T3) levels were greatest in fish caught in coastal or estuarine waters or in a river near head-of-tide. T3 levels were lower in fish caught in rivers throughout the angling season and lowest in those captured entering a tributary near spawning. Thyroxine (T4) levels were lowest in immature fish captured in the ocean in winter but raised in fish captured in spring; many of the latter group showed endocrine evidence of their becoming sexually mature. T4 levels were greatest in fish captured in coastal waters and progressively lower in fish captured in an estuary, near head-of-tide and in rivers. T4 levels in fish captured at tributary entry near spawning exceeded those in fish caught in rivers earlier in the year. In general, these data support the hypothesis that motor activity level in migrant fish is a determinant of thyroid status.

Thyroxine and tri-iodothyronine levels were determined in groups of adult Atlantic salmon (Salmo salar L.) of both sexes at river entry in summer or as they entered a spawning tributary in late autumn. Hormone levels were considered in relation to river or stream discharge rate at capture. For tri-iodothyronine, there was a quadratic or linear relationship with discharge rate for each sex at both sites. For thyroxine, there was a quadratic relationship with discharge rate at tributary entry but no relationship with discharge rate at river entry. It is argued that raised hormone levels at higher discharge rates are a probable consequence of increased motor activity associated with greater water velocity. The quadratic nature of most of the models is attributed to physiological or behavioural limitation of the response.

Accelerated recovery of Atlantic salmon ( Salmo salar) from effects of crowding by swimming

Scales of adult Atlantic salmon (Salmo salar) were collected from 18 river systems in eastern North America from 55°33′N in Labrador to 44°33′N in Maine and to 44°23′N in Nova Scotia during 1968, 1973–75. Analysis of four scale characters, namely circuli in first annual river zone, circuli in second annual river zone, smolt age, and circuli in first annual marine zone, demonstrated that significant differences occurred in each of these characters between river systems and latitude. The numbers of circuli in each of the three growth zones increased from north to south, while the smolt ages decreased from north to south. A discriminant function applied to these four characters employing latitude as a "dummy" variable indicated that the discriminant function contributed significantly to the clinal trend and 73.7% of the total variation was explained by this discriminant function. Key words: adult Atlantic salmon; scale characters; salmon; latitudinal cline, scale characters; clinal discriminant function

The aim of the present study was to investigate the cataract preventive effect of dietary histidine regimes in adult Atlantic salmon (Salmo salar L.) in seawater, both through manipulating the dietary histidine level and feeding period. Mean body weight of individually tagged Atlantic salmon at the start of the experiment was 1662 (sd 333) g. Low prevalence of mild cataracts were recorded in the beginning of June. Three fishmeal and fish oil-based extruded diets (crude protein: 375 g/kg and fat: 342 g/kg), differing only in histidine content (low (L): 9.3, medium (M): 12.8 and high (H): 17.2 g histidine/kg diets), were fed to duplicate net pens in seawater. The experimental period was divided into three seasons (June-July; July-September; September-October), each starting and ending with individual cataract examination, assessment of somatic data, and sampling of lens and muscle tissues for analysis of histidine and histidine derivatives. In July and September, a part of the population fed L- and H-histidine feeds were transferred (crossed over) to respective series of replicate net pens fed L-, M- and H-histidine diets (i.e. eleven experimental feeding groups at trial conclusion). The fish doubled their body weight from June to October, with no systematic effects on weight gain of dietary histidine feeding regimes. Development of severe cataracts was observed between July and September. The cataract severity was directly related to the dietary histidine level fed during the first and second periods. Feeding histidine-supplemented diets (M or H) in the first period from June to July mitigated later cataract outbreaks. The status of selected free imidazoles in muscle and lens tissues reflected the dietary histidine feeding regimes, relative to both feed concentration and feeding duration. The study shows the risk for cataract development for adult Atlantic salmon, 1 year after the transfer of salmon smolts from freshwater to seawater, which to a major extent can be prevented by histidine supplementation just before and during the early phase of cataract development.

Regurgitation rates of tagged salmon in the wild are difficult to estimate as few fish are recovered and thus the fate of the majority of tags is difficult to assess unambiguously. During a study on the River Tweed between 1994 and 1996, 281 returning adult salmon were caught by net and coble, and radio tagged. Salmon recaptured by rod and line were used as a sample of the radio-tagged population from which the frequency of tag regurgitation was determined. A total of 27 tagged fish were recaptured by anglers over the 3 year study period, of which 4 (14.8% ± 1.2% standard error) did not have radio tags in their stomach. Regurgitation rates were consistent between years; 16.7% in 1994 (2 out of 12), 12.5% in 1995(1 out of 8)and 14.3% in 1996(1 out of 7).

Adult Atlantic salmon (Salmo salar) were exposed for 48 h to water from acidified (pH 5.2) Fossbekk River (Norway), with and without 94 µg aluminium (Al)/L added as AlCl3, and to water from circumneutral (pH 6.7) Ims River (Norway) (controls). Cardiac output, heart rate, and stroke volume were monitored throughout the exposure period with Doppler flow probes placed around the ventral aorta of the fish. Fish exposed to Fossbekk River water without added Al showed few physiological disturbances. When 94 µg Al/L was added to Fossbekk River water, most of the fish died before the end of the 48-h exposure period, and a large elevation in heart rate was observed together with a decrease in plasma chloride concentrations and an increase in haematocrit, plasma glucose and plasma cortisol levels. Cardiac output was maintained at basal levels during the first 24 h of exposure because the tachycardia was accompanied by a concomitant reduction of stroke volume. Signs of arrhythmia appeared after 32 h of exposure and were associated with a further decrease in stroke volume that caused cardiac output to decrease below basal levels. The incapacity of the tachycardia to elevate cardiac output and the subsequent death of the fish suggest that this response to low pH and Al is more of a maladaptation reaction than a compensatory or adaptative reaction.