Predation risks for juvenile fishes in a mangrove estuary: A comparison of vegetated and unvegetated microhabitats by tethering experiments

摘要

Predation rates for juveniles of three fish species, the resident cardinal fish Apogon amboinensis. vagile mojarra Genes erythrounis and benthic resident sand goby Favonigobius reichei, were compared in a creek between a mangrove-root area (fringing area with submerged portions of mangrove roots) and a bare sand area (unvegetated central area) in the Urauchi River mangrove estuary, Iriomote island, southern Japan, by daytime tethering experiments. A amboinensis, occurring restrictedly in the mangrove-root area, hovered inactively near mangrove roots during the day. The other two species, on the other hand, inhabited both mangrove-root and bare sand areas, G. erythrounis swimming actively and speedily in the water column and F. reichei resting immobile on the substratum, its body coloration similar to the later. The predation mortality rate of A amboinensis was significantly lower in the mangrove-root area than the bare sand area, whereas no differences in the rates for G. erythrounis and F. reichei were found between the two microhabitats. In addition, species and individual densities of piscivorous fishes, determined from a visual census, were significantly higher in the mangrove-root area, suggesting that the potential predation risk was not necessarily lower. The lower mortality rate of A amboinensis in such a predator-rich area may be due to their anti-predator tactic associated with mangrove structural complexity, sheltering behind mangrove roots when disturbed by predators. The similar mortality rates of the other two species between the microhabitats may have resulted from anti-predator tactics independent of mangrove vegetation structure, such as rapid flight and cryptic body coloration.