The Snake Pipefish or Entelurus aequoreus used to be a relatively uncommon fish until about five years ago. Certainly they were never recorded in the north but one year a few appeared and then their population suddenly exploded. I saw over a hundred on one dive and crab fishermen, who had never before come across one, found their ropes and creels covered in them. They were common in rock pools and were often seen offshore up on the surface where sea birds caught them for food. However, due to having a somewhat crunchy external skeleton, they usually were regurgitated. Some species of birds, particularly Auks and Terns, resorted to trying to feed Snake Pipefish to their chicks as a substitute for their more normal diet of Sand-eels, which are in serious decline due to over-fishing and climate change. However, this proved disastrous as the Pipefish have limited nutritional value compared to the oily flesh of Sand-eels and their hard, long bodies caused many chicks to choke.
Nobody has been able to explain definitively why this sudden population increase came about but strangely after a couple of years, they vanished. I have never seen another one in the last four years or so.
North Sea - St. Abbs Marine Reserve