Scientist will grid off an area and assign number to different grid coordinates. Then a random number generator program is use to chose sample points on the grid. That way the sampling is truly random and unbiased. They will use lift nets to sample fish in those areas.
Fish can move great distances in a short time and they tend to school together by species and age. So a real good sampling grid would have to take that into consideration. Remember that 90% of the fish only occupy about 10% of the water at any on time.
But if they sample fish from areas that fishermen are most likely to catch fish then that would be a good way to find out if the fish we fishermen are likely to catch are contaminated or not. It's not an exact science but it the best that we can do giving the circumstances.
But if a single fish can accumulate enough selenium in it's body in it's life time then other fish could do the same thing. Remember they all school together a lot. So catching one fish out of the school would be a good indicator of what the other fish are doing in the same school. Schools of fish tend to feed on the same prey items and in the same areas.
I too have the same type of questions that pappy had in his taking the other side just for the sake of knowledge.
Heavy metals tend to accumulate in the bottom of the lake. If the selenium is coming out of the coal that's dumped near the water then it would be concentrated in that area for sure. But the biomass will tend to take up the selenium and then spread it though out the water column. And we know that it tend to accumulate in the fish or other organisms. Then they move around. Water flowing may not always move the fish in the direction of the water flow as fish are mobile. I'm not sure how strong the current is or this area so I'm talking in general.
They should try to catch and sample some of the trout to find out if they too are contaminated with the selenium. River's can tend to dilute the selenium and wash it down stream into other areas.
And there is no telling what was in those 55 gal drums before they were used to float docks. Scary stuff there. But at lease they were hopefully not full of what ever was in them at one time. Only the residue remained in the barrels hopefully. Still that could add to the problem of the contamination. The problem is that the testing people would have to know what was in the barrels before forming a testing plan to see if those contaminates are in the fish too.
But who's going to eat the fish if they have known high levels of selenium and smaller amounts of other toxin in the fish? The selenium alone would be enough to prevent me from fishing and eating fish in these waters.
You have to balance the cost of the sampling plan and the testing against the need to test the fish for human consumption. We can't afford to test for thousands of possible contaminates as it would be cost prohibitive.