It is possible that if they look really hard they may find there are more in Scottish rivers that was thought. Various wildlife trusts did this with dormice and were surprised at how many woods considered 'unsuitable' by the experts still contained them.
I realise A. Lamprey had very different problems and requirements to a dormouse, but with the improvement in cleanliness of rivers over the last few years, hopefully his kinsmen are just hiding from us mere humans, as I hope they are not all as big as the one seen in the picture.