Shoaling preferences of two common killifish (Fundulus heteroclitus and F. diaphanus) in the laboratory and in the field: a new analysis of heterospecific shoaling

Species-specific schooling behaviour of fish in the freshwater pelagic habitat: an observational study

Social living of animals is a broadly occurring phenomenon, although poorly studied in freshwater systems, fish schooling behaviour is an excellent example. The composition of fish schools, species-specific schooling tendencies and preferences of adult fish were studied in the pelagic habitat of the Římov Reservoir, Czech Republic. Video recordings captured over a total of 34 days (16 h per day) in the clear water period of three seasons were analysed. From four species identified as school-forming species - bream, bleak, roach and perch, 40% of the individuals observed formed schools of 3-36 individuals. Although conspecific schools prevailed, 20% of individuals formed heterospecific schools, except bleak that schooled strictly with conspecifics. Schools were composed of individuals of similar body size and life strategy. Heterospecific schools were significantly larger than conspecific schools and showed uneven proportion among species, that is, one species being more abundant when the school dimension increased. Probability of encounter in bleak was lowest and proved highest inclination for schooling. Gregarianism levels depended on species morphology and body size, with larger and morphologically advanced fish tending less to sociability. This indicates that the antipredator function of schooling behaviour is intensified with increasing vulnerability of the species.

Predation avoidance and foraging efficiency contribute to mixed-species shoaling by tropical and temperate fishes

The formation of mixed-species social groups, whereby heterospecifics form and maintain either transient or stable groups with each other, can confer substantial fitness benefits to individuals. Such benefits may arise via multiple mechanisms associated with both predation avoidance and foraging efficiency. In fishes, mixed-species shoaling reportedly occurs where displaced tropical species (known as "vagrants") interact with resident temperate species, although little is known about the nature and frequency of such interactions. To investigate this phenomenon, we used displaced tropical Indo-pacific Sergeant Abudefduf vaigiensis settling in temperate south-eastern Australia as a model system. Underwater visual surveys revealed shoal composition and size differed significantly between open-water and reef habitats, with shoals in open habitats being larger and more speciose. Shoals containing A. vaigiensis were mainly mixed-species, and larger and more speciose in open habitats than nearer to reef. Since both foraging efficiency (via access to plankton) and predation threat likely increase with increasing distance from reef habitat, we suggest that mixed-species shoaling mitigates predation risk whilst allowing increased foraging opportunities for A. vaigiensis in open areas. These findings provide support for the importance of mixed-species shoaling to the persistence of tropical reef fishes in temperate regions.

The shoaling behavior of two cyprinid species in conspecific and heterospecific groups

Mixed-species shoals of fish are frequently found in the field; however, little is known about individual-level interactions within these groups. We examined the collective motion of two cyprinid species (Chinese bream, Parabramis pekinensis, and qingbo, Spinibarbus sinensis) that occupy partially overlapping habitats but differ in social behavior (high vs low aggressiveness) and preferred flow regime (slow vs fast water velocity). We extracted measures of collective motion from video recordings of eight replicate groups of four individuals of either Chinese bream or qingbo (conspecific group) or two Chinese bream plus two qingbo (heterospecific group). Chinese bream in conspecific groups showed lower percent time moving and mean swimming speed but a similar speed while moving as compared to the qingbo conspecific groups. However, the difference in mean swimming speed and percent time moving vanished in the heterospecific group as Chinese bream elevated their swimming activity to coordinate with qingbo. This finding suggests that the two species may share similar interaction rules regarding shoaling behavior. The conspecific groups of qingbo exhibited a greater distance between group members than Chinese bream, suggesting a difference in cohesion. However, the inter-individual distances of all fish were similar in the heterospecific group. Qingbo in the heterospecific group swam more frequently at the front compared to Chinese bream, possibly due to their higher activity level. We also measured the startle response to an artificial stimulus and found that there was no significant difference among groups. In conclusion, the present study demonstrated that in the heterospecific groups, Chinese bream elevated their percent time moving while qingbo decreased their inter-individual distance to achieve consistent collective movement; thus, the two species showed similar behavior in the mixed-species group.

Impact of benzyl butyl phthalate on shoaling behavior in Fundulus heteroclitus (mummichog) populations

Fundulus heteroclitus preference for association with familiar conspecifics of similar body length was impacted by benzyl butyl phthalate (BBP); this was found to be a statically significant result with a p < 0.0001. When presented with equally sized shoals consisting of either large or small fish, the majority of unexposed (84%) and acetone exposed control (82%) fish selected the shoal of large fish. A small number of control fish chose either the shoal of small fish (6% and 10%) or the neutral zone (10% and 8%) where they were clear morphological outliers. Fish exposed to 0.1 mg/L BBP exposure daily for four weeks selected the shoal of small fish more often than unexposed or acetone controls (7.5- and 4.5-fold respectively). They also remained in the neutral zone and displayed agitation at levels more than twice that of control. Agitation and shoal choice disruption are quantifiable behavioral responses that support the use of F. heteroclitus as a model for detecting sub-lethal BBP exposure.