Behavioural Responses of Angelfish (Centropyge tibicen and C. nox) to Exogenous Food Around Peak Abundance (MaxN) Using Baited Remote Underwater Videos

Baited Remote Underwater Video systems (BRUVs) are widely used to quantify reef fish abundance, but the behaviour of fish during aggregation events is poorly understood. We examined how the behaviour of two detritivorous angelfish, Centropyge tibicen and C. nox, changed around peak abundance (MaxN) on the coral reefs of Pemba Island, Tanzania, using archived BRUV footage. Seven behaviours were scored from 11 deployments, selected from 79 screened, in the five minutes before and after MaxN. Binomial generalised linear models tested effects of time window, species and habitat structure (the first principal component (PC1) from a principal component analysis of benthic cover). Co-feeding tolerance, rapid escape and passing through increased after MaxN, including after adjustment for multiple comparisons. Feeding bites increased (p = 0.014) but not after adjustment (Holm-adjusted p = 0.056); substrate grazing, chasing and hovering did not change. There was no significant species effect, but with few videos per species this is not evidence that the species behave alike. Behaviour was recorded less often at coral-rich sites in keyhole angelfish (odds ratio: 0.98 per PC1 unit, p = 0.001) and in the pooled model, but not in midnight angelfish. Files from the same deployment are not independent and the counts were overdispersed, so p-values are likely to be too small. These exploratory results suggest that behavioural coding of archived BRUV footage could add information to abundance-based surveys, pending validation with larger, independently replicated samples.