This is not an assumption, because the guide’s belief about how long the water has been polluted doesn’t affect the relationship between the most populous species and being the best adapted.

Whether the catfish had to adapt very little doesn’t affect the possibility that it’s still the most adapted. A fish can be the most adapted even if it hasn’t adapted much. Also, the argument only concerns belief about what’s most adapted, not what is actually the most adapted.

The conclusion begins by saying “if” the studies are accurate. This conditions the rest of the conclusion on the hypothetical situation in which the studies are accurate. Whether they are actually accurate is irrelevant to this hypothetical situation.

If this possibility were true, then even if the catfish is the most populous species, the guide might not believe that the catfish is the most adapted. The guide might simply have a different species in mind, even if that species isn’t actually the most populous or most adapted.

The author doesn’t assume there’s only one species of catfish in the pond. The recent studies happen to identify the bullheat catfish as the most numerous species, but there can be other catfish species that are not as numerous.