Thats an excellent post Howard, and as I said to Pete Reading recently, I have taken, what some would call is a somewhat radical view, are we really witnessing the decline of one type of Barbel,and that is the male fish, that in a normal world form perhaps 70% of a shoal of Barbel, thus giving the much larger females a degree of safety and protection in that shoal, now if that mix has been corrupted, it means that, successful spawning is threatened for obvious reasons but more importantly it makes those big females very vulnerable to Otters, because of the once safety of the shoal has been taken away (for what ever reason) they, the bigger females stick out like a sore thumb, yes, in the short term, we end with a huge spike in big Barbel being caught by Barbel angkers targeting those individual fish, but evidence that we describe as missing year classes, could be in effect and what we are acknowledging (by default) is that there are just no small males being caught because they aren't there anymore , so we have a river in the first instance of extra big barbel that dont and cant spawn anymore and then as time progress's as old age, angling pressure and Otters take their toll, we see a population collapse. Your point Howard on signal Crayfish, is worth considering, but if you take the collapse of the Temes population into account there are no signals present, yes the native white claws have always been there......but not in the epidemic proportions that we see of the Signal on some rivers...