I should have added the way cycles of success in breeding, or survival of fry may have depleted numbers of any species, I can remember roach going through very lean years, perch, chub too, in fact on the river Ivel at Biggleswade the river used to have a massive head of bream, in fact there was hardly a swim upstream of the mechano bridge where it wasn't possible to catch several but they slowly disappeared and I've only heard of one in about ten years, they were gone before the otters arrived, so can't imagine why....
I was thinking last night after my post, always a dangerous thing however, it occured to me the one thing that has changed in the last 10 years is the amount of flooding and water that goes through this river system. I have seen it at its highest a few times breaking previous records, there is a ea measuring station on one part, I think this maybe has something to do with the tench decline, the river stays pretty calm and sedate most of the year except when there is a lot of rain and the lock keepers open the sluices and let it out, this must have been happening a lot more in the last 10 years, when they open the river just rushes through and goes down in level a lot, maybe it has got too much for the tench; what do you think?
Another peculiar one is the same river, I started fishing it about 20 years ago with a mate, we always caught a few chub, always about 1-1.5lb. We thought maybe there had been a pollution event that got them and these fish were a recovering weight class, we caught them well for a few years, noticed they were getting a bit bigger but then they disappeared altogether, my mate has passed since but I have not had a chub on this stretch in 5+ years. I am sure they may have moved up river (or not, I don't know) but why no more all of a sudden. As I said funny old game river fishing, I don't worry now, just go with the flow but it is a shame sometimes especially with the tench, loved catching those, I think my biggest was about 6lb, 6lb of fighting river tench was a moment..