NATURE Sharks, dolphins and mackerel appear

The sun, the moon and the smarts


Marine Life
John paul Tiernan

May is quite the month in Mayo for marine life. The big favourites come back and some of the smaller ones too.
This year, however, the combination of some unseasonably sunny weather and a mid-April full moon meant that Easter came late, but some of our more remarkable and bigger animals turned up early.
The nothern side of Clew Bay usually gets the spoils at this time of year – basking sharks, the big animal in question, favour the deep water which is found just off Achill. However, last weekend it was in the relatively shallow waters just off Roonagh Pier where a Clare Island ferry operator had multiple sightings of two sharks over the space of a few days.
That wasn’t the only strange observation at Roonagh that evening. On the pier itself, another early arrival was surprising those who had bothered to drop a line into the rising evening tide: The easiest fish of all to catch, mackerel, were there in numbers. Mackerel will be caught offshore at this time of year but such runs of the fish in the inshore bays more often occur from July through to September.
What was causing these early arrivals? The answer had something to do with the sun, the moon and the tide. The same sunshine which drove grass growth in early April also caused an earlier than usual bloom in plankton (microscopic seaweed). This most likely caused an early surge in larger zooplankton (animal plankton), which mackerel and basking sharks seek out.
The full moon of two weekends ago meant a spring tide, which meant a huge flow of plankton-rich water into the bay; the hungry mackerel and sharks only too happy to hitch a lift on this teeming tide of indulgence.
Two days later, the basking shark’s only rival as the supreme animal of our inshore waters – the very different bottlenose dolphin – showed up en masse. If the basking shark is the quiet slow-moving giant of the sea, then the bottlenose is the brash and brilliant show-off. Their movements were at a minimum, however, on this particular morning as they moved steadily north to meet the now-dropping tide as it flowed out between the mainland and Clare Island.
Word must have got out that there were mackerel in the bay, as with impeccable timing, the dolphins arrived outside Roonagh. The spring tide, which two days earlier had been so good to those early-season mackerel, now carried them helplessly to meet the smiling jaws of a school of smug dolphins, who once again, had outsmarted everyone.

John Paul Tiernan, Louisburgh, runs www.irishmarinelife.com, a website dedicated to the creation of knowledge of our marine ecosystems. He is currently studying for an MSc in Marine Science.