OK, I get what you mean by the average BMI. IMO the estuary is too large to make an assumption that even a large percentage of the fish caught and measured spent at least some point in their life near the wiers. You have measured some of my fish in the past, but I can't remember if you asked where the fish were caught. Maybe this would help develop your hypothesis, that is, in future samplings ask the angler where the fish were caught. Assuming your hypothesis is correct, it may be useful to break down the data into groups, say fish caught at wiers, near the wiers on east bank, west cove, north end, etc... Then compare the data to see if indeed fish near the wiers have a lower body mass.
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