Size-dependent social attraction and repulsion explains the decision of Atlantic cod Gadus morhua to enter baited pots.

The present study tested whether the presence of already retained fishes inside baited fish pots acted as a social attraction and affected the entrance probability of Atlantic cod Gadus morhua in a fjord in northern Norway. Video analysis revealed that the probability of an entrance initially increased with the presence of low numbers of fishes inside the pot, but subsequently decreased at a critical number of caught fishes. The critical number was dependent on the size of the G. morhua attempting to enter. This demonstrates that social attraction and repulsion play a role in G. morhua pot fishing and has important implications for the capture efficiency of fisheries executed with pots.

Behavioural responses of hatchery-reared and wild cod Gadus morhua to mechano-acoustic predator signals.

The behavioural responses of wild (predator-experienced) and hatchery-reared (predator-naive) cod Gadus morhua to standardized mechano-acoustic (MA) stimuli were compared in the laboratory. Wild fish responded mainly with freezing and fast-start escapes away from the stimulus, whereas hatchery-reared fish often ignored or approached the stimulus. Wild fish also had stronger responses, turning faster during escapes and reducing activity immediately after the stimulus. Both fish types were less active on a 'risky' bare substratum after the stimulus. The antipredator responses of wild fish were consistent to repeated stimuli, whereas hatchery-reared fish that had generally only encountered harmless stimuli showed more variable responses with lower repeatability. This suggests that experience plays a role in shaping the behavioural response of fishes to MA stimuli.

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.

The aggressive behaviour between territorial Pseudocrenilabrus multicolor (pisces, cichlidae) with unlimited or only visual contact.

The development of aggression between territorial fish has mainly been studied in laboratory experiments using only visual contact. In this study, comparisons of the aggressive behaviour were made between conditions of unlimited and only visual contact. Fighting between two territorial cichlids in direct contact decreased over time in a way similar to the aggressive activity between subjects permitted only visual contact. However, there were also important differences between the two conditions of contact. Low-intensity aggression with a distance between the subjects occurred frequently in free interaction but was generally not observed under conditions of visual contact. The presence of non-territorial fish increased the aggression between subjects with visual contact but conversely had no influence on fighting and decreased low-intensity aggression between freely interacting subjects. In addition, the low level of aggression after some time of unlimited contact was not transferred to a subsequent period of visual contact. The results indicate that the decrease of aggression under conditions of visual contact is caused by habituation, whereas in a free interaction a balance is also developed between attack and escape tendencies.

The aggressive behaviour between territorial Astatotilapia burtoni (pisces, cichlidae) with unlimited or only visual contact emphasizing low-intensity aggression during free interaction.

The relationship between territorial Astatotilapia burtoni was studied under conditions of both unlimited and only visual contact. The aggressive activity between territorial fish coming into visual contact remained at a high level during the seven days of observation. Territorial fish permitted unlimited contact showed, on the other hand, a marked decreased intensity of aggression during the first day of contact. Results from experiments using visual contact are therefore not necessarily relevant for a free contact situation. A detailed examination of low-intensity aggression indicated that a dynamic equilibrium between attack and escape tendencies, as regulated by distance to the territorial border, controlled aggression between freely interacting fish. Low-intensity aggression seldom occurred between fish separated by a glass partition and a similar equilibrium did not seem to develop in this situation. The difference between the findings on A. burtoni and results from similar experiments on another cichlid fish Pseudocrenilabrus multicolor could be explained by differences in the relative importance of two factors in the regulation of aggression; (1) a balance between attack and escape tendencies and (2) a decrease of the attack tendency (habituation) below a critical threshold value.

On the function of the 'eye-spots' in agonistic behaviour in the fire-mouth cichlid (Cichlasoma Meeki).

During frontal threatening, individuals of the fire-mouth cichlid (Cichlasoma meeki) dilate their opercula, thereby demonstrating two conspicous 'eye-spots'. In a hostile situation, a fish confronted with an opponent whose eye-spots have been experimentally removed is more likely to react with overt aggression towards the other. Fights also follow a more violent course than does a 'normal' fight between two intact males. A tentative explanation to these results is advanced which supposes that the eye-spots may exert an 'intimidating' effect upon the opponent during threat and fighting. This hypothesis is also supported by an experiment where males were confronted with a mirror. In this situation the distance between the mirror and the fish during frontal threat was significantly shorter in fishes lacking eye-spots, indicating that the intimidating effect emerging from the mirror image was diminished. In a third set of experiments when two males (one intact and one without eye-spots) competed for the only suitable territory site in an aquarium, the intact male eventually dominated the other in five of six aquariums.

Influence of previous experience on the mate selection of two colour morphs of the convict cichlid, Cichlasoma nigrofasciatum (Pisces, Cichlidae).

In a pilot test, individuals of two colour morphs of Cichlasoma nigrofasciatum showed colour preferences in their schooling behaviour according to previous experience. Two further experiments were undertaken to investigate if such experientially induced preferences could reflect on the choice of mate. In one experiment white males who had been reared differently with regard to the colour morph of parents and siblings were given females of the different colour morphs (white and normal) to choose from, the females being successively removed after pairing to induce further choices. In this test, however, the males predominantly chose normal females, probably because of dominance relationships among the females, which masked a possible colour preference. Instead, a free choice test was devised, where 74 white and 74 normal fishes were allowed to pair off freely in a large tank. Here, a statistically significant sexual preference for the previously experienced colour morph was found. These results, as well as the possible evolutionary consequences, are discussed.