Field and laboratory observations revealed tidal and diurnal cues for spawning in the atlantic silverside, Menidia menidia. In the field, spawning runs began approximately when daytime flood tide velocities ranged from 3 to 16 (x = 11) cm sec−1. Spawning runs ended at ebb tide velocities ranging from 5 to 22 (x = 17) cm sec−1.
In the laboratory, M. menidia were reared from embryos to sexual maturity in 10 months (April 1979 to January 1980). During this time, approximately 50 fish were held in each of two, 1 m diameter tanks. A pump was used to maintain a constant current velocity of 8 cm sec−1 in the holding tanks. Water temperature ranged from 16 to 25°C, the salinity was 30±2‰. Fish were fed TetraminRflake food and Artemia nauplii each day. During January and February 1980, the seawater circulation pump was turned off twice daily for one hour, from 1200 to 1300 and 2400 to 0100. Current velocities decreased from 8.0 cm sec−1 to 0.0 cm sec−1 during these periods. M. menidia held under a 24 h light:0 h dark (24 L:0 D) photoperiod spawned from 1200 to 1300 and 2400 to 0 100 in response to decreased current velocities. Modification of the photoperiod to 14 L:10 D (with the circulating pump turned off from 1200 to 1300 and 2400 to 0100) resulted in spawning between 0500 and 0600 in response to ‘lights-on’ and between 1200 and 1300 in response to decreased current velocities. No spawning occurred in response to the decreased current velocities during darkness.
The importance of timing of spawning runs to coincide with low current velocities was revealed in tests to determine the duration of viability of sperm and unfertilized eggs. Eggs remained highly viable for only 5 min, sperm for 10 min. Spawning when current velocities were low, reduced dispersion of milt, thus increasing the likelihood of egg fertilization.
Fish Tidal currents Diurnal cycle Intertidal zone Egg viability