Increasing Dip Net Mesh Size Results in More Fin Splits in Post-Smolt Atlantic Salmon (Salmo salar).

Dip nets are widely used to retrieve fish from the water but can cause injuries to the fish. The objective of this study was to document the potential effects of dip net mesh size with respect to external injuries, more specifically fin splits and scale losses, in Atlantic salmon (Salmo salar). The study included 273 post-smolt Atlantic salmon from two fish batches referred to as the Tank group (w- = 178 ± 36 g, n = 198) and the Cage group (w- = 1319 ± 334 g, n = 75). Four identical dip nets with mesh sizes of 5, 10, 15, and 20 mm, respectively, were used to net fish out of their enclosures before the external injuries were assessed by visual inspection. The results showed that the number and severity of fin splits increased with dip net mesh size, but no effect of mesh size on the percentage of scale losses was found. Dip-netting of five fish at a time instead of one with the 20 mm dip net, had no statistically significant effect on the examined injuries. It is concluded that the use of dip nets with smaller mesh sizes can be positive for fish welfare by reducing fin splitting.

Warm water treatment increased mortality risk in salmon.

Thermal treatment is a controversial method to control sea lice in the Atlantic salmon farming industry. This study aimed to complement the growing evidence base to document the impact of thermal treatments on salmon welfare, behaviour, physiology and health. Here, fish were treated two times (four weeks apart) for 30 s in either 27, 30, or 33 °C warm water, and parameters were compared to a procedural control (exposed to their holding temperature of 14 °C) or a negative control (where no treatments were applied). The fish had a clear behavioural response to the warm water, despite low difference between treatment and holding temperature (Δt = 13, 16 or 19 °C). Eye damages were more prevalent in the warm water treated groups than in the controls. Little difference was recorded between treatment groups in their growth and condition factor, blood plasma values, organ health, and long-term coping ability. There was, however, a significant increase in mortality as a function of temperature after the first treatment (14 °C: 6.5%, 27 °C: 5.3%, 30 °C: 12.4% and 33 °C: 18.9% mortality). The first treatment was performed only two weeks after the fish had been tagged and moved into the experimental holding tanks, while the fish had been allowed to recover for four weeks without any handling before the second treatment. The group of fish that were not subjected to any treatments (the negative control) had no mortality throughout the entire experimental period.

Minding the Gaps in Fish Welfare: The Untapped Potential of Fish Farm Workers.

The welfare of farmed fish is often regarded with less concern than the welfare of other husbandry animals, as fish are not universally classified as sentient beings. In Norway, farmed fish and other husbandry animals are legally protected under the same laws. Additionally, the legislature has defined a number of aquaculture-specific amendments, including mandatory welfare courses for fish farmers who have a key role in securing animal welfare, also with regards to noting welfare challenges in the production process. This article uses fish welfare courses as a site from which to inquire about the common-sense understanding of fish welfare in Norwegian fish farming. The focus is specifically on fish farm employees, their experiences of welfare-related issues and contradictions in their daily work, and the struggle to act responsibly in aquaculture settings. Through participant observation at welfare courses, as well as interviews and conversations with fish farm workers, the article details how challenges are experienced 'on the ground', and suggests how fish farm workers' own experiential knowledge might be mobilized to improve the general welfare of farmed fish.

The effect of fasting period on swimming performance, blood parameters and stress recovery in Atlantic salmon post smolts.

In this study, Atlantic salmon post smolts (~250 g, ~29 cm) were fasted for four weeks at 12 °C in full strength seawater. During this period, the critical swimming speed (Ucrit) was measured after 1, 2 and 4 weeks of fasting, as well as in a fed control group. Furthermore, blood samples were taken in each treatment group prior to the swim test, at fatigue, and following 3 h and 24 h of subsequent recovery. Four weeks of fasting gradually reduced the condition factor from 1.03 to 0.89. However, the Ucrit remained statistically unaffected at 3.5 body lengths s-1. Exhaustive exercise stress caused large increases in plasma osmolality, [Cl-], [Na+], [Ca2+], [lactate] and [cortisol], while haematocrit and [haemoglobin] also increased. Plasma ions and lactate had increased further after 3 h recovery, and osmolality, [Cl-] and [Na+] were still elevated above control levels after 24 h while other blood parameters were fully recovered. Osmotic disturbances may therefore be considered the most challenging stressor during strenuous exercise in seawater. Only minor effects of fasting period on blood parameters in response to exhaustive exercise were detected, which included slightly higher osmotic disturbances and a repressed response in red blood cell recruitment at fatigue in fasted fish. Furthermore, the 4-week fasting group had a reduced cortisol response following fatigue compared to the other treatment groups. In conclusion, these results show that Atlantic salmon maintain their full swimming capacity as well as their ability to respond and recover from acute stress during an extended period of food deprivation.

Skeletal deformities in wild and farmed cleaner fish species used in Atlantic salmon Salmo salar aquaculture.

As a first attempt to assess bone health in cleaner fish production, wild and cultured ballan wrasse Labrus bergylta and lumpfish Cyclopterus lumpus were examined by radiology. In C. lumpus, wild fish (57%) had more vertebra deformities (≥1 deformed vertebrae) than cultured fish (2-16%). One wild C. lumpus had lordosis and another was missing the tail fin. In L. bergylta, wild fish (11%) had fewer vertebra deformities than cultured individuals (78-91%). Among the cultured L. bergylta, 17-53% of the fish had severe vertebra deformities (≥6 deformed vertebrae) with two predominate sites of location, one between vertebra 4 and 10 (S1) in the trunk, and one between 19 and 26 (S2) in the tail. Fusions dominated S1, while compressions dominated S2. Although wild L. bergylta had a low vertebra deformity level, 83% had calluses and 14% had fractures in haemal/neural spines and/or ribs. The site-specific appearance and pathology of fracture and callus in wild L. bergylta suggests these are induced by chronic mechanical stress, and a possible pathogenesis for fish hyperostosis is presented based on this notion. In conclusion, good bone health was documented in cultured C. lumpus, but cultured L. bergylta suffered poor bone health. How this affects survival, growth, swimming abilities and welfare in cultured wrasse should be further investigated. SIGNIFICANCE STATEMENT: Skeletal deformities were studied in ballan wrasse and lumpfish of both wild and cultured origin for the first time to identify potential welfare issues when deploying them as cleaner fish in salmon sea cages. While cultured lumpfish showed good bone health, cultured wrasse had a high occurrence of vertebra deformities, which is expected to impact lice eating efficiency and animal welfare negatively. These deformities are most likely induced early in development.

Is it advantageous for Atlantic salmon to be triploid at lower temperatures?

Marine organisms living at low temperatures tend to have larger genomes and larger cells which suggest that these traits can be beneficial in colder environments. In fish, triploidy (three complete sets of chromosomes) can be induced experimentally following fertilization, which provides a model system to investigate the hypothesis that larger cells and genomes offers a physiological advantage at low temperatures. We tested this hypothesis by measuring metabolic rates and swimming performance of diploid and triploid Atlantic salmon (Salmo salar) post smolts acclimated to 3 or 10.5 °C. At 10.5 °C, triploids had significantly lower maximum metabolic rates which resulted in a lower aerobic scope compared to diploids. In addition, triploids initiated ram ventilation at lower swimming speeds, providing further evidence of a reduced capacity to meet oxygen demands during strenuous activity at 10.5 °C. However, at 3 °C, metabolic rates and critical swimming speeds were similar between both ploidies, and as expected substantially lower than at 10.5 °C. Therefore, triploidy in colder environments did not provide any advantage over diploidy in terms of metabolic rate traits or swimming performance in Atlantic salmon. We therefore conclude that traits, other than aerobic scope and swimming performance, contribute to the trend for increased cell and genome size in marine ectotherms living in cold environments.

Snorkel sea-cage technology decreases salmon louse infestation by 75% in a full-cycle commercial test.

Methods to prevent parasite infestations in farmed fish are becoming widespread, yet tests of their effectiveness often lack commercial relevance and statistical power, which may lead to technology misuse. Here, we examined salmon louse infestation on Atlantic salmon in triplicate commercial snorkel louse barrier and standard cages over a 12 month production cycle. Barrier cages reduced newly settling lice on Atlantic salmon by 75%, with variability in parasite reduction over time depending upon environmental variables. The commercial, triplicate, long-term study design serves as a template to validate performance and detect weaknesses in anti-parasite techniques in fish mariculture.

Sudden exposure to warm water causes instant behavioural responses indicative of nociception or pain in Atlantic salmon.

Thermal treatment has become the most used delousing method in salmonid aquaculture. However, concerns have been raised about it being painful for the fish. We studied the behavioural response of Atlantic salmon acclimated to 8 °C when transferred to temperatures in the range 0-38 °C. Exposure time was 5 min or until they reached the endpoint of losing equilibrium and laying on their side, a sign of imminent death. At temperatures below 28 °C, none of the fish reached endpoint within the 5-min maximum. At 28 °C four of five fish reached endpoint, and fish reached endpoint more rapidly as temperature increased further. Fish transferred to temperatures above 28 °C had higher swimming speed immediately after transfer and maintained a high swimming speed until just before loss of equilibrium. Their behaviour was from the start characterised by collisions into tank walls and head shaking. Just before loss of equilibrium they started breaking the surface of the water, swimming in a circle pattern and in some instances displayed a side-wise bending of their body. In other words, salmon transferred to temperatures above 28 °C showed instant behavioural responses indicative of nociception or pain.

Repeated sublethal freshwater exposures reduce the amoebic gill disease parasite, Neoparamoeba perurans, on Atlantic salmon.

Freshwater bathing is one of the main treatment options available against amoebic gill disease (AGD) affecting multiple fish hosts in mariculture systems. Prevailing freshwater treatments are designed to be long enough to kill Neoparamoeba perurans, the ectoparasite causing AGD, which may select for freshwater tolerance. Here, we tested whether using shorter, sublethal freshwater treatment durations are a viable alternative to lethal ones for N. perurans (2-4 hr). Under in vitro conditions, gill-isolated N. perurans attached to plastic substrate in sea water lifted off after ≥2 min in freshwater, but survival was not impacted until 60 min. In an in vivo experiment, AGD-affected Atlantic salmon Salmo salar subjected daily to 30 min (sublethal to N. perurans) and 120 min (lethal to N. perurans) freshwater treatments for 6 days consistently reduced N. perurans cell numbers on gills (based on qPCR analysis) compared to daily 3 min freshwater or seawater treatments for 6 days. Our results suggest that targeting cell detachment rather than cell death with repeated freshwater treatments of shorter duration than typical baths could be used in AGD management. However, the consequences of modifying the intensity of freshwater treatment regimes on freshwater tolerance evolution in N. perurans populations require careful consideration.

Predicting the effectiveness of depth-based technologies to prevent salmon lice infection using a dispersal model.

Salmon lice is one of the major parasitic problems affecting wild and farmed salmonid species. The planktonic larval stages of these marine parasites can survive for extended periods without a host and are transported long distances by water masses. Salmon lice larvae have limited swimming capacity, but can influence their horizontal transport by vertical positioning. Here, we adapted a coupled biological-physical model to calculate the distribution of farm-produced salmon lice (Lepeophtheirus salmonis) during winter in the southwest coast of Norway. We tested 4 model simulations to see which best represented empirical data from two sources: (1) observed lice infection levels reported by farms; and (2) experimental data from a vertical exposure experiment where fish were forced to swim at different depths with a lice-barrier technology. Model simulations tested were different development time to the infective stage (35 or 50°-days), with or without the presence of temperature-controlled vertical behaviour of lice early planktonic stages (naupliar stages). The best model fit occurred with a 35°-day development time to the infective stage, and temperature-controlled vertical behaviour. We applied this model to predict the effectiveness of depth-based preventive lice-barrier technologies. Both simulated and experimental data revealed that hindering fish from swimming close to the surface efficiently reduced lice infection. Moreover, while our model simulation predicted that this preventive technology is widely applicable, its effectiveness will depend on environmental conditions. Low salinity surface waters reduce the effectiveness of this technology because salmon lice avoid these conditions, and can encounter the fish as they sink deeper in the water column. Correctly parameterized and validated salmon lice dispersal models can predict the impact of preventive approaches to control this parasite and become an essential tool in lice management strategies.

The effect of stunning methods and season on muscle texture hardness in Atlantic salmon (Salmo salar L.).

UNLABELLED: Commercially collected records of Atlantic salmon (Salmo salar L.) muscle texture hardness were used to evaluate the effect of slaughter procedures and seasonality on texture quality. A database collected by Marine Harvest® contained flesh hardness records of Atlantic salmon slaughtered at processing plants in Norway from summer 2010 to summer 2011. The fish were slaughtered either by (1) percussion followed by automated bleeding ("Percussive") or (2) live chilling with exposure to carbon dioxide (CO2 ) followed by manual severing gill arches and bleeding ("CO2 ") or (3) live chilling with exposure to CO2 followed by percussive stunning and at the end automated bleeding ("CO2 ·percussive"). Hardness in salmon muscle cutlets was measured in Newtons (N) by Materials Testing Machine Zwick 500N. The hardness in salmon varied significantly over the study period (P < 0.05, mixed effect model) and showed the softest value of 21.2 (± 0.7) Newton (N) in summer 2011 and hardest 24.1 (± 0.2) N in autumn 2010. Slaughter procedures had a significant effect on salmon muscle hardness (P < 0.05, mixed effect model), where percussion followed by automated bleeding resulted in the hardest value (24.0 ± 0.4 N) as compared with CO2 stunning (21.8 ± 0.2 N) and combination of CO2 and percussive stunning (23.1 ± 0.15 N). CO2 is suspected as a causal factor in accelerated postmortem softening of the salmon muscle. PRACTICAL APPLICATION: Commercial use of CO2 in combination with live chilling results in accelerated postmortem softening of the muscle tissue in salmon and should be avoided.

The interaction between water currents and salmon swimming behaviour in sea cages.

Positioning of sea cages at sites with high water current velocities expose the fish to a largely unknown environmental challenge. In this study we observed the swimming behaviour of Atlantic salmon (Salmo salar L.) at a commercial farm with tidal currents altering between low, moderate and high velocities. At high current velocities the salmon switched from the traditional circular polarized group structure, seen at low and moderate current velocities, to a group structure where all fish kept stations at fixed positions swimming against the current. This type of group behaviour has not been described in sea cages previously. The structural changes could be explained by a preferred swimming speed of salmon spatially restricted in a cage in combination with a behavioural plasticity of the fish.

Effect of blood removal protocol and superchilling on quality parameters of prerigor filleted farmed Atlantic cod (Gadus morhua).

UNLABELLED: A total of 40 farmed Atlantic cod (Gadus morhua) were in 2 groups either fillet directly after stunning and spray washed or produced into fillets according to traditional slaughter methods including exsanguination for 30 min, gutting and washing. Both groups were either stored superchilled or traditionally on ice. After 7 d postmortem color (CIE L*, a*, b*) and fillet shrinkage was measured by computer imaging along with drip loss and texture hardness. Results show that superchilled fillets had significant lower core temperature than fillets stored on ice during the entire 7 d storage period. This resulted in reduced fillet shrinkage from 14.7% to 6.9% and less drip loss dropping from 9.45% to 3.99% in average. Processing the fish directly into fillets resulted in satisfactory blood drainage, where all groups were in particularly well exsanguinated with a* values below zero. No color difference was observed between filleting groups or chilling methods. Spray washing of the fillets resulted in water uptake and higher drip loss in interaction with chilling method. We conclude that filleting farmed fish in one step is feasible. PRACTICAL APPLICATION: Traditionally farmed fish are slaughtered and processed over several steps, which often include live chilling, stunning, exsanguination, chilling, gutting, rinsing, decapitation, filleting before the fillets are packed into polystyrene boxes and shipped with ice. These processes are often time, laboring, space, and energy consuming. A novel processing line for filleting of farmed fish is gutting and filleting the fish directly after decapitation and replacing exsanguination with spray washing the fillets. In addition, all the cooling steps are replaced by superchilling the fillets. This novel process line gives fillets with comparable if not superior quality compared to the traditional process.

Learning and anticipatory behaviour in a "sit-and-wait" predator: the Atlantic halibut.

We studied the learning capacities and anticipatory behaviour in a "sit-and-wait" predatory fish, the Atlantic halibut, Hippoglossus hippoglossus. In Experiment 1 two groups of halibut received series of light flashes (conditioned stimulus, CS) that started before delivery of food (unconditioned stimulus, US) and persisted until after food delivery, i.e. delay conditioning. Control groups received unpaired CS and US presentations. The anticipatory behaviour of delay conditioned halibut consisted mainly of take-offs towards the surface shortly after onset of the CS. In Experiment 2 six groups of halibut were trained in three trace conditioning procedures: Two groups with 20s, two groups with 60s and two groups with 120s trace interval. Learning was evident in the 20 and 60s trace groups and in one of the 120s trace groups. In contrast to delay conditioning the anticipatory behaviour of trace conditioned halibut was characterized by subtle movements near the tank floor with orientation towards the CS. The cautious responses of halibut after trace conditioning differed markedly from what is observed in other fish species and are suggested to reflect a "sit-and-wait" foraging strategy that requires the predator to remain undetected until the prey is within lunging range.