Anders Alanärä

The use of self-feeders in rainbow trout (Oncorhynchus mykiss) production

Abstract The aim of the present paper is to evaluate the use of self-feeders in rainbow trout production. The basic idea with self-feeders is that the fish themselves control the feeding level, which is thereby set by their feeding motivation. To determine the limitations of the technique, this work has focused on characterising: (1) how self-feeding activity is related to food demand; and (2) how the self-service food supply is related to growth and feed conversion ratios. For the feeding system to function correctly, rainbow trout must first learn how to operate it. Trout reared in tanks in groups of 100–300 individuals require about 25 days to reach a stable level of self-feeding. Under large-scale rearing conditions (e.g. in cages with group sizes of 1000–2000 ind.), however, learning seems to be of minor importance for the ability of fish to operate the system. When reared in small tanks in groups of up to about 300 ind., self-feeding activity in trout is strongly influenced by the development of dominance hierarchies. Under such situations, a small number of fish will dominate the actuation of the trigger and thereby have a disproportionately greater influence on the food supply of the group as a whole. Any change in temperature is quickly followed by a corresponding change in self-feeding activity up to about 15 °C. Above this temperature, trout lower their activity, indicating that self-feeding activity reaches a maximum at around 15 °C. The reward level (amount of food received in response to one trigger actuation) is the single most important factor requiring proper adjustment in order to optimise growth and feed conversion ratios when using self-feeding systems. The optimal reward is discussed in relation to differences in dietary energy content, number of fish, and water temperature. High-density conditions reduce the self-feeding activity of trout, and the recommended maximum rearing density is about 30 kg m −3 . An evaluation of growth and feed conversion data indicates that self-feeders have the potential to function well with rainbow trout under commercial rearing conditions.

Demand feeding as a self-regulating feeding system for rainbow trout (Oncorhynchus mykiss) in net-pens

Abstract Rainbow trout with an initial weight of 1.1 – 1.2 kg were fed an extruded high energy food using demand feeding and timer-controlled feeding techniques, during 4 summer months, in full-scale netpen farming conditions. Unrestricted and restricted demand feeding techniques were tested. The time-rcontrolled feeding was tested at a corresponding restricted level. There were no statistically significant differences in growth rate between trout fed restricted and unrestricted rations using the demand feeding technique. Trout fed using the timer-controlled technique exhibited a significantly lower growth rate compared with fish fed using demand feeders. Trout fed using the demand feeding technique and restricted rations had the best feed conversion, with a mean value of 1.08 during the experiment (feed conversion = kg food supplied/kg weight gain). The feeding of trout using the unrestricted demand feeding technique resulted in a feed conversion of 1.49, and using restricted timer-controlled feeding it was 1.36. The feed conversion in the unrestricted demand feeding groups was correlated with water temperature, indicating that the ability of the trout to regulate the feeders at a level where food losses were minimized, was temperature dependent. It is suggested that the feeding of fish, using a timer-controlled technique and restricted food rations, with short and regular feeding intervals induces stress and high competition for food. This consequently leads to increased swimming activity and metabolic energy losses.

The effect of time-restricted demand feeding on feeding activity growth and feed conversion in rainbow trout (Oncorhynchus mykiss)

Abstract The effect of restricted access to a demand feeding system was evaluated for cage-reared rainbow trout with an initial weight of 0.46 – 0.61 kg during 4 summer months. The purpose was to find the access time that produced optimal levels of food supply, as assessed by measurements of growth rate and feed conversion. Following a period during which fish had 24 h access to food per day, feeding regimes were reduced to 3, 5 or 7 h of access to the feeder per day for treatment groups. The time available for feeding within each restricted regime was divided into two feeding periods during each day. Fish held under the control feeding regime were given unrestricted access to the demand feeder (24 h/day). Restricted access time to the demand feeders resulted in a significantly reduced number of feeder actuations and thus lowered food supply compared with the unrestricted regime. Restricted access to the feeder also resulted in a decreased growth rate during the first 28 days after the transition from unrestricted feeding. This was particularly obvious for the trout allowed to feed for only 3 h per day. Following the initial period of low growth rate, specific growth rates increased in the restricted groups and became similar under all the feeding regimes, at about 1.1% per day. The feed conversion (FC) varied with food access time. The self demand food supply was used most efficiently by fish fed for 3 h per day (FC = 1.0) and least efficiently by fish fed on the unrestricted regime (FC = 1.6). Trout on the other two regimes, with 5 and 7 h feeding, showed intermediate conversion of 1.1 and 1.2, respectively. The results suggest that two feeding periods per day, each of about 2 h duration, are sufficient for optimal growth of large (0.5 – 2 kg) rainbow trout adapted to demand feeders. The number of bites per day was correlated with the water temperature. Biting activity peaked at 16 °C. After 2–3 h of feeding, trout on the time-restricted feeding regimes exhibited a significantly lower bite activity, probably due to the fish becoming satiated.

The effect of temperature, dietary energy content and reward level on the demand feeding activity of rainbow trout (Oncorhynchus mykiss)

Abstract The present study included three experiments, the objectives of which were to assess the effects of temperature (Expt. 1), dietary energy content (Expt. 2) and reward level (amount of food received in response to one trigger actuation) (Expt. 3) on the demand feeding activity of rainbow trout. Fish (initial size 30–120 g) were reared in 0.8-m 3 indoor tanks and fed using demand feeders. Each of the three experiments lasted about 60 days. The main parameters recorded were bite activity (number of trigger actuations), growth rate and feed conversion. Bite activity was significantly higher in trout reared at 15°C than in those reared at 5°C; consequently, food supply and growth rate were higher in the former. However, as evaluated by the growth and feed conversion rates, only fish raised at 5°C showed a bite activity that was close to the optimal, whereas the bite activity was too low at 15°C. Groups of fish fed diets with either a low (19.8 MJ/kg), medium (20.7 MJ/kg) or high (22.0 MJ/kg) gross energy content did not show any significant difference in bite activity or growth rate. This indicates that trout in aquaculture conditions are unable to adjust the bite activity in a way that matches the energy content of the food. The reward level significantly influenced the ability of the trout to meet their demand for food by regulating its supply. Among groups offered either a low (0.36 g), medium-sized (0.80 g) or high (1.65 g) reward, the high-reward groups exhibited the highest growth rates. The trout were, to some extent, able to compensate for a low reward by increasing their bite activity. However, at low or medium-sized rewards, fish were unable to compensate to the degree necessary to meet their food demand.

A test of the individual feeding activity and food size preference in rainbow trout using demand feeders

Feeding activity and food size preferences were investigated in three groups of rainbow trout with initial weights ranging from 200 to 1000 g. Fish were given the option of releasing food from either of two demand feeding devices that contained either 6 mm or 9 mm food pellets. Individual biting activity (to activate release of food) was continuously monitored using specially designed passive integrated transponders (PIT-tags). In each of the groups, biting activity was strongly influenced by three individuals, which accounted for 70–85% of the total. Thus, high bite activity by few individuals indicates the development of social dominance hierarchies. There was no statistically significant correlation between number of bites and growth rate, indicating that the fish that operated the demand feeders did not gain any growth advantage. Trout that dominated the actuation of the feeders were found among both medium- and large-sized fish. There was no evidence of active food size selection, and the fish that most frequently operated the feeders defended one area of the tank rather than preferring the trigger that would have delivered food of the optimal size.

Feeding behaviour of the Arctic charr in comparison with the rainbow trout

Abstract Appropriate feeding techniques are important in order to optimize feed intake and minimize feed losses in net-pen aquaculture operations. The ability of the Arctic charr to feed in the water column was studied using different techniques of feed pellet release. For comparison, the same study was carried out with rainbow trout. The pellets were all released simultaneously or with a 5-s interval between delivery and with or without dispersal of pellets over the water surface. When the pellets were all delivered simultaneously, the charr only caught 39% (not dispersed) or 65% (dispersed). The charr took more than 94% of the pellets in the water column if they were released at 5-s intervals, irrespective of whether or not the pellets were dispersed over the surface. The rainbow trout, on the other hand, caught nearly 100% of the pellets in the water irrespective of feeding technique. When reared in net-pens, Arctic charr should, therefore, be fed a few pellets at a time, and the pellets should be released at intervals and dispersed over the water surface.

GABAergic anxiolytic drug in water increases migration behaviour in salmon

Migration is an important life-history event in a wide range of taxa, yet many migrations are influenced by anthropogenic change. Although migration dynamics are extensively studied, the potential effects of environmental contaminants on migratory physiology are poorly understood. In this study we show that an anxiolytic drug in water can promote downward migratory behaviour of Atlantic salmon (Salmo salar) in both laboratory setting and in a natural river tributary. Exposing salmon smolt to a dilute concentration of a GABAA receptor agonist (oxazepam) increased migration intensity compared with untreated smolt. These results implicate that salmon migration may be affected by human-induced changes in water chemical properties, such as acidification and pharmaceutical residues in wastewater effluent, via alterations in the GABAA receptor function.

Do delivery rate and pellet size affect growth rate in Atlantic salmon (Salmo salar L.) raised under semi-commercial farming conditions?

Abstract How the daily feed ration is presented is an important aspect of feed management. In order to evaluate the effects of delivery rate and pellet size, Atlantic salmon ( Salmo salar L.) were raised in small (5×5×10 m), semi-commercial cages and were fed using 14 combinations of pellet sizes (0.03–2.68 g) and delivery rates (82.5–1320 g·min −1 (0.1–31.1 pellets·fish −1 ·min −1 )). Treatment groups were compared using the growth rate (Thermal Unit Growth Coefficient, TGC). No significant linear effect on growth due to pellet size or delivery rate could be detected. For pellet size, however, indications point to a non-linear relationship with growth rate. Salmon grew well (mean TGC (±S.D.)=2.68±0.49) irrespective of treatment indicating a high level of plasticity in the salmons ability to adjust to changes in pellet size and delivery rates within the ranges used and under the conditions tested here.

Post-release and pre-spawning behaviour of simulated escaped adult rainbow trout Oncorhynhus mykiss in Lake Övre Fryken, Sweden.

Using radio telemetry, the present study simulated the escape of 48 adult rainbow trout Oncorhynchus mykiss from a net-cage fish farm in the Lake Ovre Fryken, Sweden. The post-release dispersal of O. mykiss was fast, showed long-range dispersal behaviour, low winter survival and lacked the ability to find suitable spawning habitats. Thus, the present study suggested that reproducing for the first time may be an obstacle to the establishment of escaped farmed O. mykiss.

Significance of substrate and the timing of start-feeding in alevins of Arctic charr (Salvelinus alpinus)

Abstract In an attempt to find improved incubation and start-feeding techniques for Arctic charr, newly hatched alevins were subjected to one of three different treatments: reared on a flat-screened bottom and offered food at 9 days (94% yolk remained) after hatching; reared on a flat-screened bottom and offered food 34 days (55% yolk remained) after hatching; reared in a gravel-filled hatching cylinder and offered food at emergence. The time point at which 50% of the alevins had emerged from the cylinder was estimated to be 52 days after hatching. Gravel rearing enhanced the yolk conversion efficiency during the yolk-sac period, resulting in significantly larger alevins at emergence. During first-feeding this growth pattern was reversed, and alevins which were offered food 9 and 34 days after hatching exhibited significantly higher growth rates than those reared in gravel. Differences in growth rates between treatments is suggested to be an effect of differences in imprinting to the dry food, an early contact with food appearing to favour growth. Mortality was lowest in alevins offered food 34 days post-hatch, intermediate in alevins offered food 9 days post-hatch, and highest in gravel-incubated alevins.