Aquarium cichlid fish Tropheus moorii flexibly adjust hierarchy when tank fish species composition changes: A pilot study

An important aspect in the assessment of an individual's life quality is the impact of such changes on dominance hierarchy, which in turn affects physiology, food access and reproductive success. This study measured the effect of changes in tank fish composition on the hierarchy of a stock of 10 individuals of the cichlid fish (Tropheus moorii) hosted in the aquarium of the Natural History Museum of the University of Pisa. During data collection, to ensure animal welfare in the management process, 10 individuals of Neolamprologus multifasciatus were introduced and one individual of Simochromis babaulti removed (perturbance event). For the first time in a fish species, measures of linearity (corrected Landau's h' index based on binary dyadic dominance relationships), steepness (based on Normalised David's Scores), and triangle transitivity (based on the transitivity of dominance relationships within triads) were combined. A total of 932 agonistic encounters were collected across three observation periods: Periods 1, 2 and 3 (25 hours of observations/period). Hierarchical relationships were assessed using aggressor/aggressee socio-matrices. Aggression rates plummeted from Period 1 (prior to the perturbing event) to Period 2 (immediately following the event) probably due to novel-event related behavioural inhibition. Linearity and steepness levels decreased with an increase in unknown relationships, but the frequency of one-way relationships increased possibly because high ranking individuals targeted subordinates to avoid rank reversal. In Period 3, steepness, linearity and aggression levels increased to initial levels (Period 1). Only the alpha male remained unchanged across the three periods. Thus, dominance relationships remained linear, but the initial hierarchy was not fully restored following the perturbing event. In conclusion, to ensure welfare it is suggested that aquarium stocks of Tropheus moorii be monitored following tank composition changes, because although they may flexibly adapt to them this does not necessarily occur in the short term.