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NATURE Dolphins sing in the summer

Outdoor Living
Spring blooms and summer beckons off Mayo coast


Marine life
John Paul Tiernan


Despite the surprising but very welcome warmth of last week, which confirmed suspicions of winter’s demise and reassured us of summer’s not-too-far away sunshine and growth, this is actually the coldest time of year in our coastal waters.
As we are reminded of what 17 degrees feels like again, the wildlife (and dedicated humans) who inhabit our marine territory are tolerating a baseline 8 degrees or so. If they are near an estuary this might even be a hostile 6 degrees or 7 degrees, as water from the much colder lakes and rivers mingles like an unwelcome guest with the warmer saltwater.
Why so cold then when our air temperatures are shooting up? The sea temperatures take a long time to catch up – trillions of gallons of water won’t heat overnight. In fact in June when a sweltering weekend drives people to the shore, the water might still be at a frigid 11 or 12 degrees. On the flip side, on an icy subzero day in December, the sea might offer a welcoming 11 or 12 degrees as it is equally slow to cool down from its high point in September.
Not to worry though – right now, signs of summer and renewed life are equally as evident offshore as they are on land. There, it is not so much temperature, but rather the amount of light that the longer days offer and the availability of food that are the critical factors.
Quiet indications of summer’s approach were found on the recent low spring tides – where turning over a rock at the very bottom of the shore, I disturbed a velvet swimming crab. Their aggressive personality (they will snap defensively at a too-close camera) is often the highlight of a summer snorkelling trip. This female however was spawning, which starts for them at this time of year, and which I left her to quietly do.
A far more emphatic sign of summer spotted was off the Mullet peninsula last week, where Dúlra Nature Tours from Belmullet (www.dulra.ie), were lucky enough to witness 1,000+ common dolphins in a single aggregation. They regularly see pods of 200 or more, but for four hours or so last week, they watched as endless waves of dolphins swam against the tide, switching from North to South as the tide turned to maintain position against the flow of water and fish.
The dolphins were most likely tracking offshore shoals of mackerel, which were responding to an early proliferation in their food stocks (copepods, small crustaceans), which in turn were responding to an earlier than usual spring plankton bloom.
The spring bloom, which occurs when increased sunlight allow the nutrients stirred up in winter to be utilised by microscopic algae at the sea’s surface (forming enormous blooms) is the definitive sign that winter is gone and a summer of long days, boat trips and spectacular encounters with dolphins is just around the corner.