Date: 8/7/17 3:50 pm From: Nathaniel Wander <nw105...> Subject: [obol] Effects of last winter's abundant precipitation on pelagic species: terns, cormorants, murres
A friend at US Fish & Wildlife reports that cormorant and tern breeding in and around the Columbia Estuary is setting late records this summer. It's believed that the increased snow melt in spring and early summer resulted in a persistent fresh water lens that floated atop the marine water, suppressing coastal upwelling and disrupting the food chain. The birds paused or didn't begin breeding until the food fish abundance began returning to normal. This might also account for the dead murres washing ashore earlier in summer.
Birds that feed at or relatively near the water's surface can adapt behaviorally to the problematic functioning of terrestrial eyes in water. Cormorants, for example, are flush feeders: they don't need to see particularly sharply; they track the motion of their startled prey. Alcids feed deep in the water column, however, and have problems using vision to forage. Light is absolutely reduced; they can't remain under water nearly long enough for their pupils to completely dark adapt; typically rounded vertebrate corneas leave them far-sighted in water. (Rounded corneas help terrestrial vertebrates focus light as it transits from the less dense air outside the eye to the fluid within: these refractive benefits are negated when light transits between equally dense fluids.)
Some diving birds like penguins have adapted biologically, developing flattened corneas (they are congenitally near-sighted on land) and pupils that can contract nearly to pinpoints; they remain close to dark-adapted all the time. Common Murres don't have any obvious biological adaptations to these visual problems. It's believed they forage more or less blindly, swimming through shoals of small fish and picking them off as they make random contact--sort of like marine swifts! Success at this mode of feeding requires relatively high prey density. Suppressed upwelling could have disrupted the food chain sufficiently that prey were not densely enough available and murres' foraging efficiently plummeted. Nathaniel WanderPortland, OR
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