Less water renewal reduces effects on social aggression of the cichlid Pterophyllum scalare

Abstract

Changes in aquatic environments can affect fish behavior. Water renewal, for example, can dilute chemical signals, which are a social communication tool used by some fish species, and this dilution increases aggressive interactions in the group. Fierce and prolonged fights can affect fish welfare by increasing the probability of injuries, stress, and detrimental impacts on fish health. We tested whether the amount of water changed was associated with social aggression in the angelfish Pterophyllum scalare, a popular ornamental Amazonian cichlid. We designed an experiment with social groups (3 fish) that were assigned to one of three treatments (with 15 replicates of each treatment): 1) 50% water renewal; 2) 25% water renewal; or 3) 0% renewal (i.e., water removed and returned to the aquarium as a control). These treatments were referred to as T50%, T25%, and T0%, respectively. Fish behavior was video-recorded immediately before water renewal (baseline) and also 1 min, 1 h, 2 h, and 24 h after water renewal. The frequencies of attacks (overt fights) and displays (threats) were compared using the general linear model (GLM), with the treatment as the categorical factor, observation sessions as the repeated measures, and social rank as a continuous factor. Attacks increased after the water was changed, and they were higher in T50% than in T25% (P = 0.0001). In T25%, aggression returned to baseline levels after 1 h (P = 0.32), but remained increased after 24 h in T50% (P = 0.000001). Changing only a small volume of water at a time was therefore found to prevent exaggerated aggressive interactions among P. scalare specimens and to reduce the probability of injuries, stress, and detrimental impacts on fish welfare.

Social Behavior and Welfare in Nile Tilapia

“…In the juvenile angelfish cichlid, Pterophyllum scalare, for example, a change of 25% water stimulated aggressive interactions in the group for one hour, and after this period aggressive interactions returned to basal levels [12]. On the other hand, after changing 50% water, fish kept an increased aggressive interaction rate for 24 h [12]. Thus, it is possible that other cichlid species, such as Nile tilapia, also behave that way, although this hypothesis needs to be tested.…”

“…When the external environment is changed, social communication can be impaired and social interactions can be affected. For instance, for some fish species chemical information is important in social rank communication [10], however, water dilution can dilute such information, thus, disturbing social stability and increasing aggressive interactions [11,12].…”

“…This type of interaction is not only observed in adult males and females, but also in juveniles, which show a very similar aggressive behavior to that of adults. For example, the angelfish Pterophylum scalare and the Cichlasoma parananese show a similar social interaction, their social rank being clearly established when they are juveniles [12,18]. Aggressive behavior is, therefore, part of the competition for resources, for example, food, reproductive partners, spawning and brood care site, in which dominant individuals have priority over others to access such resources [19].…”

Aggressive communication in aquatic environments

“…This was most likely caused by the disruption of chemical communication due to changed water chemistry. Changing only a small volume of water at a time was in turn found to be a good solution to prevent exaggerated aggressive interactions in these fish, leading to a reduction of detrimental effects on fish welfare (Gauy, Boscolo, & Gonçalves‐de‐Freitas, ).…”

Unusual effect of chemical communication on social aggression in juvenile cichlid fish Cichlasoma paranaense (Cichliformes: Cichlidae)

“…Some differences in social signaling among them are expected, since the evolutionary mechanism favored differences in social behavior between these two large groups (Keenleyside, 1991;Barlow, 2000). In a recent study with juvenile Amazonian cichlid, Pterophyllum scalare (Schultze, 1823), Gauy et al (2018) found that chemical information is important to keep social stability within the group. For instance, the greater the renewal of aquarium water (and the more diluted chemical information), the greater the overt aggression in aggressive interactions, and longer time was needed to restore the baseline levels of aggression in the group.…”