Grunion have adapted to tidal cycles in a precise manner. Along the Pacific coast of North America the two daily high tides vary in height, and the higher of the two occurs at night during spring and summer months. Grunion spawn only on these higher tides, and after the tide has started to recede. Since waves tend to erode sand from the beach as the tide rises and deposit sand as the tide falls, it is obvious that if grunion spawn on a rising tide the succeeding waves would wash the eggs out. This danger is eliminated since spawning usually is confined to the falling tide. In addition, grunion nearly always spawn on a descending series of tides when succeeding tides are lower than tides of the previous night. The eggs would be washed out prematurely by succeeding tides if spawned during the ascending tidal series. The eggs mature and are ready to hatch in about 10 days or about the time of the next series of high tides. Thus, spawning must take place soon after the highest tide in a series if the eggs are to have adequate time to develop before the next series of high tides. Looking at the tidal cycle, it becomes apparent that there are only 3 to 4 nights following the highest tide that spawning conditions are right, and it is on these nights that grunion spawn. This is copy and pasted from wildlife.ca.gov/Fishing/Ocean...