This results in snails getting quickly too big for crayfish to eat. When raised with fish, they grow slowly, resulting in sizes that are unappetizing to fish. The energy and resources that would have been invested in eggs can instead be invested in growth. Smart people would probably expect that snails grow slowly in the fish environment because they produce alot of eggs. The smart folks would be wrong, however.
Egg production is minimal in the presence of fish.
It would have been a tidy answer if they grew slowly because they produced alot of eggs, but I believe the real answer is wedded to the functional ecology of yet another trait. Behavior I said above that fish and crayfish attack and selectively eat different types of Physa . Here I should point out that the predators also locate prey by different means. Fish are visual predators that can locate snails anywhere in the water column. Crayfish chemotactically search the substrate for prey.