The author hypothesizes that an overabundance of algae is harmful to smaller fish in this pond. This is based on the fact that over the past 15 years, the few times that the author has seen a lot of dead small fish has coincided with the times that he has noticed unusually large amounts of algae in the water.

The author assumes that the correlation between lots of dead small fish and lots of algae is explained by algae causing harm to the small fish. This overlooks alternate explanations for the correlation. Perhaps there’s a third factor that causes both the algae and the dead fish. The author also assumes that his own observations of the timing of dead small fish and lots of algae is representative of the general pattern of dead small fish and lots of algae in the pond.