Outliers?

It's nice to see reports that marine animals including Short-nosed seahorses and Little terns seem to be thriving in the seas in parts of the UK (https://www.theguardian.com/environment/2018/dec/31/conservation-push-yields-results-for-uk-sea-life-but-challenges-remain-plastic-pollution#img-1) but I am not wholly convinced that sporadic recording of such beasts (perhaps benefiting from conservation programmes) actually reveals a healthy marine environment. In some respects, seeing thriving populations of common species might well be a better indicator of 'health' than increases in the numbers of occasional animals often at the margins of their ranges.