Anders Fernö

The challenge of the herring in the Norwegian sea: Making optimal collective spatial decisions

Norwegian spring-spawning herring, Clupea harengus harengus L., are long-lived multiple spawners subject to strong variation in recruitment success. They tend to adopt low-risk, preferred-conservat...

Swimming behaviour as an indicator of low growth rate and impaired welfare in Atlantic halibut (Hippoglossus hippoglossus L.) reared at three stocking densities

Abstract The Atlantic halibut (Hippoglossus hippoglossus L.) is a new species in Norwegian aquaculture. However, in ongrowing trials, halibut shows variable and generally poor rates of growth. The halibut is a flatfish that spends most of its time resting on the bottom, and it was hypothesized that high stocking densities with frequent social interactions could lead to behaviour changes, reduced appetite and impaired welfare. Halibut were kept in six tanks at low, medium, and high densities (18%, 54% and 112% bottom coverage). All fish were individually tagged with Trovan™ passive implant transponder (PIT) tags, allowing an antenna to register fish swimming at the surface. Swimming and feeding behaviour was also recorded by underwater video cameras. Individual growth rates were highly variable, but food consumption and growth rates fell significantly with increasing stocking density, while individual swimming activity rose with increasing density. Frequent “surface swimmers” had a significantly lower growth rate than fish that were seldom recorded by the PIT antennae. Surface swimming may therefore be an indicator of suboptimal growth rates and impaired welfare in reared halibut.

Schooling dynamics of norwegian spring spawning herring (Clupea harengus L.) in a coastal spawning area

Abstract The behaviour of Norwegian spring spawning herring (Clupea harengus L.) was studied in a spawning ground off Kanney in southwestern Norway. A total of 47 schools were tracked from 5 to 60 minutes using multi-beam sonar and echosounder. Variables recorded included the horizontal area and vertical extent, depth, density and structure of schools and their swimming speed and direction. Herring schools were defined into five categories that differed in several respects: Immigrating schools had a large horizontal area and a more elongated shape than other schools. They swam deep and had a consistent swimming speed and direction. Searching schools were smaller and more dense and their swimming speed and direction varied. Spawning schools settled on the bottom; they had small vertical extent and low density. Emigrating schools were smaller in horizontal area and less dense than immigrating schools and they swam higher in the water column. Feeding schools were oflow density and were dynamic in their shape...

Diel activity pattern and food search behaviour in cod, Gadus morhua

Food search behaviour in cod, Gadus morhua, was studied by means of a stationary positioning system. Six fish in a fjord in northern Norway were tagged in situ by allowing them to shallow bait-wrapped acoustic tags, and their swimming behaviour was continuously recorded. The fish exhibited a diel activity rhythm, with higher swimming speeds and a larger range during the day than at night. Chemically mediated food search was studied in periods of both high and low activity by setting a string of baits in the morning and at night. During the period of high activity, more fish localized the baits (70 versus 45% of the observations) and the time to localization was 50% shorter, indicating that high swimming activity increased the probability of encountering the odour plume and the odour source. No diel variations in the response threshold to olfactory stimuli were found, as the proportion of fish in the odour plume that responded rheotactically, and their response distances were similar in the two periods (240 and 227 m respectively). Cod performed area-restricted searches on encountering the string of baits during the day. The probability of cod detecting prey by taste receptors, encountering the odour plume of prey or localizing a stationary food source after olfactory stimulation should be relatively independent of light, and this could explain why cod were active throughout the 24 h cycle. On the other hand, the visual detection distance and the ability to capture active prey are influenced by the light level, resulting in higher activity during the day.

Vertical distribution of Atlantic salmon (Salmo solar L.) in net pens: trade-off between surface light avoidance and food attraction

Abstract The vertical distribution of Atlantic salmon (Salmo salar L.) reared in marine net pens of 7 m depth was studied by means of an echo integration system in relation to temporal and environmental factors. Great variations in local densities were found. In summer up to 80% of the fish could be localized within a 1-m depth interval, whereas in winter the fish were more evenly spread out in the pen. High light levels resulted in deeper fish distribution. The fish usually descended at dawn and ascended at dusk and swam deeper in summer than winter, and there was a negative correlation between light level and fish density at the surface in summer. Feeding also influenced fish distribution most strongly in summer, with rapid upwards swimming when feeding started and a gradual descent during the course of feeding. The fish stayed generally deeper in a feeding regime with infrequent (twice daily) than frequent (every 10 min) feeding. The vertical distribution could largely be explained by a trade-off between light-induced surface avoidance and food attraction. Avoidance of the pen bottom and excessively high densities also seemed to play a role. The implications of the findings regarding choice of pen size, selection of feeding regimes and development of automatic feeding systems are discussed.

Surface activity of Atlantic salmon (Salmo salar L.) in net pens

Abstract The leaping and rolling activities of Atlantic salmon in net pens are behavioural features that may give important indications of the status of the fish. These surface activities were studied in fish in net pens in relation to environmental factors and operational procedures. The leap of salmon in net pens differs from the oriented leaps salmon make when ascending a river. The fish usually land on their side, and in about 6% of the leaps the fish hit the pen wall. Leaping activity was low in winter. Leaping increased with increasing louse infestation, and there was little leaping after de-lousing. However, during chemical de-lousing treatment, leaping activity was high. When rolling, the fish slowly breaks the water surface. Rolling activity varied between days, but was relatively constant throughout the year. Rolling activity increased after different types of disturbances, and may be related to compensation of swimbladder gas loss during a stress situation. High rolling activity is thus an indication of recent acute stress, whereas high leaping activity indicates heavy louse infestation or present acute stress.

Migration behaviour of norwegian spring spawning herring when entering the cold front in the Norwegian Sea

Abstract The distribution of Norwegian spring spawning herring when migrating to the feeding areas in the Norwegian Sea in spring was mapped by acoustic surveys in April 1995 and 1996. The schooling behaviour of the herring was recorded by a high-resolution sonar, and the swimming speed and swimming direction were quantified by tracking individual schools for up to one hour. In early April the herring migrated in the Norwegian Sea from the continental shelf off Norway, between 66° and 68°N, and westward to 2°W. When reaching the cold-water front, at about 0°, the herring turned southward along the front. During daytime the herring migrated in large schools at 300–400 m depth; at night they rose to surface and either dispersed or maintained schooling.

Overlap in distribution and diets of Atlantic mackerel (Scomber scombrus), Norwegian spring-spawning herring (Clupea harengus) and blue whiting (Micromesistius poutassou) in the Norwegian Sea during late summer

Abstract Based on two large-scale surveys in the Norwegian Sea in July 2004 and 2006 we investigated the potential overlap in distribution and diets and association with environmental variables for mackerel, herring and blue whiting. Mackerel and blue whiting had low overlap in both distribution and diets, and were associated with warm Atlantic and cold prey-rich Arctic waters, respectively. Also, herring and mackerel were negatively correlated, associated with different environmental variables and even had low diet overlap in Atlantic water where the highest overlap in distribution was observed, but these trends were much clearer in 2006 than in 2004. Prey was patchily distributed and data from single stations indicated that feeding was opportunistic, in particular for mackerel. However, mackerel diet width and feeding incidence were similar between the years, whereas herring was more selective in 2004, probably reaching the end of the feeding season. A delayed peak in Calanus availability was indicated in 2006 and may have prolonged the feeding season, partly explain the difference between the years. Our data did not indicate a prey shift due to encounters between mackerel and herring. Concentrated efforts in limited areas are needed to further elucidate this aspect of interaction.

Basic movement pattern and chemo-oriented search towards baited pots in edible crab (Cancer pagurus L.)

The basic (natural) movement pattern and gear-induced behaviour of female edible crab Cancer pagurus L. were studied by means of a stationary positioning system. Nine crabs were tagged with ultrasonic transmitters and their positions were monitored approximately every third minute for 9–24 days. Because the crabs sometimes hid, the signals were often received improperly, resulting in inaccurate position fixes, and a computer program to distinguish between movement and inactivity was developed.

Food-search strategy in ling (Molva molva L.): crepuscular activity and use of space

The activity rhythm and use of space in the food-searching behaviour of ling (Molva molva L.) were studied by means of a stationary positioning system. The system consisted of a fixed array of three hydrophones, which monitored the position of each individual being tracked once every three minutes. Five ling were tagged in situ by allowing them to ingest a transmitter wrapped in bait and were tracked continuously for 6 to 11 days. Ling showed a crepuscular activity rhythm with higher levels of swimming activity at dawn and dusk. The fish occupied a home range throughout the study, and during inactive periods (about 65% of the time) they remained within a small core area. Most of the time spent outside the core area was during the period of high activity at dawn. Food-search behaviour was studied by setting mackerel-baited fishing gear in the experimental area. When baits were present, ling moved more slowly and within a more limited area, indicating area-restricted searching in the presence of a food odour. Ling responded to and located baits in both their active and inactive periods during the day, but were not observed to react at night. Comparing these findings with those from similar tracking experiments on cod (Gadus morhua L.) suggested that ling are less active at night and show a more restricted use of space by occupying a core area. This may reflect different feeding strategies between these gadoid species with ling taking a higher proportion of mobile prey and being more dependent on visual stimuli.