I am sure shale oil would kill fish and earthworms But then again the fish wouldn't be populated in the first place if shale oil existed there for thousands of years.

So the presence of dead fish being found now is a indicator that the environment wasn't toxic before until the derailment happened.

Another example is if you find dead salamanders as they are very sensitive to minute toxic changes to their environment. Salamanders are basically the canaries in the coal mine for toxins entering a certain eco system.