Anomalous Behavior of Free-Swimming Echeneis naucrates (Echeneidae) in the Aquaculture Areas of Nha Trang Bay (South China Sea, Southern Central Vietnam)

Abstract

The results of observations of the behavior of free-swimming sharksuckers Echeneis naucrates in a prosperous ecosystem (Maldives, Indian Ocean) and in the areas exposed to strong anthropogenic impact (Nha Trang Bay, South China Sea) are presented. In Nha Trang Bay, individuals of E. naucrates are not found on the reefs with a low density of forage objects but are shifted to the zone of aquaculture farms, where they form schools and feed on fodder of the aquaculture objects falling from the cages. It has been shown for the first time that negative buoyancy rises (there is no swim bladder in echeneids) when a significant amount of food is consumed near the cages. This leads to rapid fatigue of the fish at swimming in the pelagic zone, and they begin to settle to the bottom periodically. In different parts of the study area, seven such episodes were recorded on different days, of which two were timed: the school of E. naucrates stayed at the bottom for 2 min 38 s and 2 min 14 s; the pelagic period between two settles was 3 min 6 s. In the individuals in a settled state, suffocation was developed as manifested in an increased breathing rate. Fishes raised their heads to counteract the suction of the silt fraction from the bottom sediments. Abnormal behavior reflects the adaptation of E. naucrates in Nha Trang Bay to the consumption of artificially generated food resources and is associated with an uncharacteristic rhythm of nutrition for this species.