Mass occurrence of snake pipefish in the Northeast Atlantic: Result of a change in climate?

In 2004, a sudden mass occurrence of snake pipefish Entelurus aequoreus took place in the Northeastern Atlantic and the abundance has been increasing since. Before 2004, snake pipefish were mainly found in coastal areas and have been reported only occasionally in oceanic waters. The oceanic form lives free in the water column while the coastal form is found among sea weeds or in sea grass beds. Abundance indices (numbers per hour fishing) from inshore surveys have remained at the same level, while those from offshore surveys show a very strong increase since 2004. The length distributions differ significantly between surveys, with coastal snake pipefishes being larger. Although the outward appearance of the coastal pipefishes seems different from the pelagic specimens, no differences were found when comparing taxonomic features. Apart from appearance the habitat is different for the two types of snake pipefish.The oceanic specimens of snake pipefish are much leaner than the coastal specimens. Stomach contents of the oceanic snake pipefishes revealed remains of relatively small calanoids (mean length 2.4 mm). The calanoid population has recently changed and is nowadays dominated by the smaller Calanus helgolandicus. Here we put forward the hypothesis that the sudden appearance of the snake pipefish in the deeper waters is a result of the change in the average lengths of calanoids which in turn is caused by changes in the hydroclimatic environment.The mass occurrence of the snake pipefish is affecting the ecosystem. Seabirds are feeding their chicks with them and they are also found in stomachs of fish and sea mammals.