Tuesday, 23 July 2013

Integrating the Impossible

The complexities of resource exploitation in a littoral ecosystem

Estuary of Afon Dyfi looking south-west.
(Picture E. Evans, used by permission)

It's not the Amazon rainforest, despite all the rain. It's certainly not the high Serengeti under Ol Doinyo Lengai.  And yet these estuaries around the British coast are some of the most biologically productive ecosystems anywhere on the planet.  Each cubic metre of glorious glutinous silt down there contains over 25000 kJ of calorific energy – about the same as a dozen (large) bars of chocolate.  Fish base their entire life cycles around these places.   Wading birds flock to them in winter because there's no food resource that compares.

The Dyfi estuary supports not just one food chain but hundreds.  At the bottom of each are microscopic organisms living in the mud and, near the top, one osprey nest.   It seems like a perfect location for ospreys to live and breed, so why only one nest so far?  That's an interesting question – but first, a shaggy dog story...

This is Polky, a good-natured little pooch who lives with one of my neighbours across the road. Polky and his almost-identical brother Cobblers (don't ask) go for walkies twice a day, and their favourite game is catching a thrown tennis ball on the grassy area between our respective apartments.  To be honest, this is the only time I can tell the two dogs apart because Cobblers is brilliant at catch ball. He makes it look easy and he never misses.  Polky, on the other hand, cannot catch ball for toffee. His eyesight is fine, his technique is sound and his timing appears to be good. He really tries but he just sucks at it.  If the ball hits him on the nose and rolls away, that's a good effort by Polky's standards. Both dogs have been playing this game since they were pups.  It's a game of skill and co-ordination; one that involves instinctive AND acquired abilities.

Some animals are better at learning how to do stuff than others.

In Britain, many osprey nests are close to reservoirs or freshwater lakes. Catching fish in such places is by no means easy, but at least the birds know where the fish are.  Given the innate hunting skills they were born with, any reasonably competent male osprey ought to be able to produce fish for his nest.   However, rivers and their estuaries are different.  The overall food resource there is greater and more varied, but accessing it can be much more complicated.   In a tidal estuary and river, the behaviour and distribution of fish changes on an hour-by-hour basis, AND varies over seasonal time scales as well.   Weather patterns far upstream can change the salinity of the seawater offshore, or affect its temperature and transparency.  At any given spot, there may be dozens of sizeable fish present on one day, and not a single one at the same time on the next.

In such a complex environment, instinctive hunting skills are not enough – not by a long chalk. There is no inherited information that says..

“Flounders can be caught along the sandy areas south of the estuary, for an hour either side of low tide.”

“Hunting migratory sea bass in mid-May is a waste of time because there aren't any.”

“If visibility is poor today, there's a well-stocked trout lake about 12 kilometers north-east from here."

“If it's been sunny for three days, shoaling mullet will be taking plankton near the surface in the mornings.”

All this, and much else, has to be learned.  The reason that Monty - and his neighbour at the Glaslyn nest to the north - are so successful is that they have achieved a complete mastery of all these factors.  They know and understand the dynamics of foraging on the Dyfi, or Tremadog Bay, and it has taken them years to amass all the detailed information needed to do this.

Other male ospreys may not be so proficient, because it's one thing to catch sufficient food to feed yourself – quite another to collect enough for a female and a brood of ravenous youngsters as well.

The nest-provisioning stats collected in Wales illustrate this problem.  A solo osprey only needs about 400g of fish per day to maintain condition.  That's two small trout or one very moderate mullet.  This graph shows the weekly take required when three chicks are being supported.   And it's not just about quantity: if conditions are unfavourable, the solo osprey might go hungry for a couple of days until fishing becomes easier.   This state of affairs won't do when there are young to be fed – the incumbent male has to be able to produce something every day, and only a bird that knows his home range like the back of his own talon can do this.

Unringed osprey "Dai Two-Dots" 26/8/12  ( (c) Dyfi Osprey Project)

Perhaps this is the real reason that “Dai Dot” and other satellite males have not attracted roving females and set up nests of their own in west Wales.   They can live along the estuaries in the summer quite comfortably, but they have never acquired the EXTRA skills and knowledge to be successful providers for a family.

Like Polky, they just don't have what it takes to catch the ball.

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