Complex sentence: Commercial fishing together with dam building decimated the fish stock, which were unable to breed due to lower water temperatures in the reservoirs while dams blocked their movement into smaller channels (Nabhan 1988:553).
Split into simpler sentences: Commercial fishing together with dam building decimated the fish stock. The species were unable to breed due to lower water temperatures in the reservoirs while dams blocked their movement into smaller channels (Nabhan 1988:553).