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Coastal, General

Meán Fómhair; movements and migrations

Velvet Swimming Crab in September. Image © J P Tiernan 2010

After a fortnight of settled skies, seas and excellent visibility, a new regime slipped into place just before the halfway mark of this past month. For coastal populations, September means change; tourists and young return to work and further education, and paralleling them, a lot of our marine species have places to go too.
The first movement I noticed was an involuntary one. On last weekend’s spring tide, not only the height of the water was notable, but so too was it’s cloudy green tinge, quite a contrast to the almost transparent blue of the preceding two weeks. This was likely a mini bloom of dinoflagellates, which are microscopic single-celled algae. The huge volumes of water which are moved in the inshore region by an equinox spring tide in September can disrupt the thermocline, which is a layer of stratification where nutrients build up, determined by water temperature. This disruption allows nutrients to mix into the surface layer which is at a very pleasant 16°C or so, creating a short bloom of these algae and a very emerald-looking sea.
From October onwards, the sea will remain in a similar state as non-stop wave action means a constant mixing of sediments and nutrients into the water column. Another bloom won’t happen until spring, however, as temperatures dip below the critical point for these microscopic algae to thrive.
Crabs also start moving in September (along with lobsters) to deeper waters. From late September onwards, due to increasing wave action; it becomes too rough in the shallow rocky zones around our coast to eke out the unpredictable scavenger lifestyle of the crab. They move offshore and down, sometimes hundreds of metres, where the effect of the waves is lesser felt and where they can avoid the colder surface water in winter.
Meán Fómhair movements are not just up and down, but north and south also. The arctic tern goes almost completely unnoticed by most of us despite its presence on almost every shore in Mayo, mainly because it is such a tiny bird. Its migration, however, contradicts its diminutive size. The arctic tern likes to spend the summer in both hemispheres and some of the birds which will leave Mayo shores this September, after a summer of breeding, will beat a path to southern Argentina, South Africa or even Antarctica. The last of them should be gone by early October.

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Discussion

2 thoughts on “Meán Fómhair; movements and migrations

  1. lovely piece of writing…
    In Derrynane we still have Terns, Crabs and lots of Mackerel. A large red tide loomed only last week. Water temp at 14c surface and 12 beneath the thermocline.

    Posted by Vincent Hyland | September 29, 2010, 2:47 pm

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