From your reports it sounds like the darters are definitely faster, but we need to know more about them. If we can study one, perhaps we can figure out how to neutralize them more effectively. With your hunting skills, it should be possible to incapacitate one of the darters and bring it back here so that we can learn more about it. Track one down, wound it, rope it, and bring it back here to the cage we've set up for study.