Complex sentence: Estuaries can be highly variable and complex environments contributing to fluctuating biological interactions, with shifts in osomolarity, food sources, predators, etc. Since many adult "Alosa" species die after spawning, the young generally have to migrate to the sea from the spawning grounds.
Split into simpler sentences: Estuaries can be highly variable and complex environments contributing to fluctuating biological interactions. There are shifts in osomolarity, food sources, predators, etc. Since many adult "Alosa" species die after spawning, it is the young who generally have to migrate to the sea from the spawning grounds.