Are mangroves important for reef fish on Mayotte Island (Indian Ocean)?
In order to understand the potential role of the mangroves of Mayotte Island for reef fishes, sampling was conducted monthly from April 2016 to April 2017 in three sites along a mangrove fringe: Bandrélé near fringing reef, Dzoumogné close to a river mouth and Malamani in a bay. Fishes were sampled using a fyke-net combined with visual census surveys (UVC) and eight environmental variables were recorded at each site to determine their relationship with the fish community structure. A total of 992 individuals reef fishes distributed among 44 species were recorded. Considering the sampling methods, 869 individuals comprising 35 species were observed by UVC and 123 individuals belonging to 25 species were captured with a fyke-net. Samples were mainly composed of juvenile fishes, whether 81% and 93.5% in relative abundance for UVC and catches, respectively. Whatever the sampling method, fish abundance and species richness were significantly higher in the Bandrélé mangrove. Overall, six species (Abudefduf sparoides, A. vaigiensis, Acanthurus blochii, Lutjanus fulviflamma, L. monostigma and Sphyraena barracuda) were dominant and accounted for 59% of total number of fish observed and captured. Species richness and fish abundance showed a seasonal variation for UVC and were significantly higher in wet season (November-April) with one important peak in January. Canonical RDA analysis revealed that the most significant factors explaining the distribution of fish community structure were temperature, reef proximity and fishing time for the catches, and temperature, water transparency and reef proximity for UVC. Temperature appears to be the most important factor explaining the seasonal variation, while spatial variation of reef fish abundance in mangrove may be related to reef proximity, water transparency and fishing time. First order carnivores, zooplanktivores and herbivores were mainly observed in Bandrélé, whereas piscivores were dominant in Malamani mangrove for both methods. Second order carnivores were well represented in Dzoumogné site. We propose that water transparency due to reef proximity with high salinity level and food availability were probably the most important factors structuring reef fish community in the mangrove. Moreover, the importance of Mayotte mangroves for reef fishes appears to relate to site, environmental variables and seasonal factor. Notwithstanding the important role of mangroves for several species, its role of nursery for reef fish should not be generalised.