I am guessing the borrow pits and quarry have very deep areas meaning their is a lot of water volume that is harder to warm or cool. This fact means the temperature of these water bodies does not change as rapidly as a shallower pond would. This means RES which are more sensitive to this do better. Also, I think the waters have higher alkalinity levels supporting more diverse and larger numbers of molluscs like snails and clams and crustaceans like shrimp and crayfish which RES love to eat.
