Modification of Schooling Behavior in Larval Atherinid Fish Atherina mochon by Heat Exposure of Eggs and Larvae

Abstract

We report the effects of short daily exposures to sublethal increases in water temperature during the egg and larval stages on the development of the schooling behavior of the little sand smelt Atherina mochon. The experiments were conducted to simulate the periodic exposure of different developmental stages to a transient thermal plume caused by the discharge of heated effluents into the spawning environment. Exposures were administered for eggs only, larvae only, and both eggs and larvae and consisted of nonlethal increases in water temperature from a 20°C acclimation temperature to 28.5°C over a period of 30 min, maintenance at 28.5°C for 15 min, and a gradual reduction to 20°C over 35 min. Heat treatments were administered to the egg-only and larvae-only groups for 10 consecutive days each; those administered at both the egg and larval stages were for 20 consecutive days. Behavioral testing of larval fish at 10, 15, 20, 25, 30, and 35 d posthatch showed significant modification or retardation of schooling behavior. There was an increase in number of approach–withdrawals (behavior that precedes parallel formation and involves the head-on approach of two fry and immediate veering away) as well as a decrease in parallel orientation (schooling), higher latency for the formation of the first schools, and shorter duration of the longest-persisting schools in most heat-exposed fish relative to the controls. Locomotor ability was diminished and schooling was unstable, with wide fish-to-fish distances and an absence of synchrony in the position and swimming speed of school members. These behaviors were indicative of a reduction in normal social interaction, in which fish swim close together and adjust to each other in a coordinated fashion. Additional research is required to determine whether subtle changes in schooling behavior are as important as immediate heat death to the survival of local fish populations.