Swimming depth and thermal history of individual Atlantic salmon (Salmo salar L.) in production cages under different ambient temperature conditions

Salmon production cages at sites with a pronounced thermal stratification give individual fish an opportunity to choose their thermal environment. The behavioural responses of individual salmon to such stratification, however, are poorly documented. Information about spatial distributions and temperature experience of individual Atlantic salmon (initial weight 1.5 kg) was gathered over a period of 4 months (mid-August to early-December) using data storage tags. Fish were stocked at normal or high densities in triplicate 2000 m³ production cages at 5.6–14.5 (ND) or 15.7–32.1 (HD) kg m^− 3, and valid data were collected for 12 ND and 11 HD salmon. There were large inter- and intra-individual variations in swimming depth, with indications that the salmon performed behavioural thermoregulation in an attempt to maintain body temperature within the range of 8–20 °C. Stocking density influenced the average swimming depth and body temperature, indicating competition for preferred thermal space in periods of unfavourably high temperature (towards 20 °C) in large parts of the cage volume. Analysis of temporal behavioural patterns demonstrated a higher variability during day than night and that 60 to 70% of the individuals displayed cyclic diel patterns in either swimming depth or body temperature in at least one out of three sub-periods. The results are discussed in relation to bio-energetic and thermal stress theory and possible consequences for growth variation in salmon cages. Generally, this study suggests that individual swimming depth and body temperature is in part a response to available temperature interacting with stocking density and time of day, while some individual variation cannot be ascribed to the measured variables.     

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.     

Growth and feeding behaviour of caged Atlantic mackerel, Scomber scombrus L.

Wild Atlantic mackerel, Scomber scombrus L., aged 1 and 2 years, which overwintered in sea cages from October to June at temperatures above 5 °C had high rates of survival and adapted to feed on commercial 4-mm food pellets. Growth and food intake was related to temperature and the specific growth rate (w) approached zero at around 7 °C. In contrast to the natural cycle, there were only small reductions in weight and fat content from January to May. No consistent influence of a 24-h photoperiod was found on growth and maturation. Independent of the temporal food distribution, the mackerel exhibited high plasticity in growth and fat deposition in a starvation/re-feeding trial.     

From fright to anticipation: using aversive light stimuli to investigate reward conditioning in large groups of Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis>)

In this study, we demonstrate how an event that is initially frightening to Atlantic salmon is turned to a positive stimulus through habituation and associative learning. The study was carried out in four commercial sized tanks (5 m³) with near industry densities (>550 fish, 16 kg m⁻³), using a delay conditioning procedure with an aversive flashing light as the conditioned stimulus and food reward as the unconditioned stimulus. By using video image analysis of the distribution of the fish in the tanks, the changes in behaviour from trial to trial could be documented in great detail. The current study documents the change in behaviour across the individual conditioning trials, clearly showing the step-by-step nature of the transition. The salmon needed more than 26 trials to become fully habituated to the flashing light but showed clear anticipatory behaviour already after about 19 trials. This demonstrates that the learning process is a combination of habituation and associative learning.     

The effect of feed supply rate on growth of juvenile perch (Perca fluviatilis)

This study investigates the effect of the feed supply rate within a meal on growth of juvenile perch (Perca fluviatilis). Groups of PIT-tagged, feed-trained juvenile perch (12 g) were held in 100-L tanks at 18 °C, under a 24L:0D photoperiod and fed five meals per day in excess. Feed was applied at rates of 1.2 (Low), 4.2 (Medium) and 14.3 (High) pellets fish⁻¹ min⁻¹ during two periods, each of 50 days. Cumulative meal-time per day ranged from 10 to 225 min. Specific growth rates (SGR) in the two periods were positively correlated, indicating a consistent individual growth performance. There was growth dimorphism between the sexes. Females grew about 20% faster than males, resulting in final mean weights of ≈ 87 and 58 g respectively. Autopsy revealed differences in sex ratios between treatments and the effect of feeding rate was analysed according to sex. There was a tendency for fish fed at the lowest rate to have a higher SGR than fish in other treatments, both for females (L = 1.98 ± 0.0, M = 1.84 ± 0.08, H = 1.88 ± 0.19% bw day⁻¹) and males (L = 1.68 ± 0.01, M = 1.55 ± 0.16, H = 1.57 ± 0.02% bw day⁻¹). Fish fed at the lowest rate also tended to have the lowest feed conversion ratio (L = 0.88 ± 0.05, M = 1.02 ± 0.11, H = 1.04 ± 0.10) but the effects were not significant. These results suggest that juvenile perch can feed efficiently under a range of conditions.     

Voluntary responses and limits of tolerance to pressure reduction and swimbladder expansion in farmed Atlantic cod

Atlantic cod and other teleosts with closed swimbladders have slow gas resorption rates and are therefore vulnerable to sudden pressure reductions that lead to swimbladder expansion and possible loss of behavioural control. This problem is of particular relevance to fish farmers, who require safe protocols for lifting of cages containing cod that account for swimbladder expansion. To recommend a limit for pressure reduction, we experimentally determined the maximum pressure reduction, relative to neutral buoyancy pressure, free-swimming farmed cod would expose themselves to. Classical reward conditioning was used to motivate cod that were neutrally buoyant at 2?C3.3 ATM (10?C23 m depth) to respond to a sound signal by swimming rapidly upwards to a feeding station positioned at 1.5 ATM (5 m depth). Sixteen cod were implanted with data storage tags (DSTs) that registered their ambient pressure throughout the experiment. The DST data showed that cod voluntarily stopped rapid ascents before they reached the feeding station at an average maximum pressure reduction of about 41% relative to their neutral buoyancy pressure. This was equivalent to a 70% swimbladder expansion relative to neutral volume. During the experiment, cod seldom remained above their neutral buoyancy depths, except when feeding, indicating a behavioural aversion to positive buoyancy. To avoid the loss of buoyancy control in cod, we conclude that single lifting events of sea-cages, which forces swimbladder expansion, should never exceed a 70% expansion of the swimbladder above the pre-existing neutral buoyancy volume.     

Submerged light increases swimming depth and reduces fish density of Atlantic salmon Salmo salar L. in production cages

Artificial photoperiods that postpone sexual maturation and increase growth are now widely used in the Atlantic salmon Salmo salar L. farming industry. Few studies have been carried out to examine the effect of this treatment on fish behaviour and welfare in production cages. In this study, echo‐integration was used to observe the swimming depth and fish density of salmon in 20‐m‐deep production cages illuminated by lamps mounted above the water surface (SURF) or submerged in the cage (SUBS). From January to May, SUBS swam at a greater depth (5–11 m) than SURF (1–3 m) at night. SURF descended and SUBS ascended at dawn, but SUBS were still swimming at greater depth than SURF during the day from January to March. The difference in swimming depth resulted in SURF swimming at a median fish density about twice as high as SUBS at night and up to five times the calculated fish density. SURF increased the utilization of the cage volume as the biomass increased, but fish swimming at the highest density did so at up to 20 times the calculated fish density. The results suggest that salmon position themselves in relation to the artificial light gradient to maintain schooling behaviour and that the use of submersible lights may be a precaution to secure the welfare of caged salmon.