http://www.eurekalert.org/pub_releases/2006-11/plos-mea110706.php
Public release date: 13-Nov-2006
Mixing exploitation and conservation: A recipe for disaster
Commercial shellfish dredging in the Dutch Wadden Sea led to declines
in both the quality and amount of the red knot's food resources,
causing the population to crash.
Click here for more information.
Most governments around the world set conservation policy based on the
assumption that resource exploitation and species protection can
co-exist in the same place. These policies have led to Orwellian
"marine protected areas" that host commercial fishing operations,
leading one to wonder who's protecting whom. A new study reveals the
danger of this approach--showing that exploitation has led to a decline
of a seabird species by 80% in the Dutch Wadden Sea--and concludes that
it's time to let protection mean protection.
For decades, the Dutch government sanctioned mechanical cockle dredging
in three-fourths of the intertidal flats of the Wadden Sea, a natural
monument protected under two intergovernmental treaties. Before suction
dredging began in the 1960s, an estimated 2,000 tons of cockles were
hand-harvested from the reserve each year. In 1989, the high-pressure,
motor-driven water pumps used in suction dredging sucked up close to
80,000 tons of cockles. By 2004, the Dutch government decided the
environmental costs were too great and stopped the practice. Jan van
Gils and colleagues investigated the ecological impacts of commercial
cockle dredging on intertidal ecosystems by studying a long-distance
migrant shorebird that dines principally on cockles, the red knot
(Calidris canutus islandica). Up to 50% of the global red knot
population uses the Dutch Wadden Sea at some point during their annual
cycle.
Red knots are exquisitely adapted to their lifestyle. They have a
pressure-sensitive bill that senses hard objects buried in the sand and
a shell-crushing gizzard to accommodate the birds' penchant for
swallowing their catch whole. They even have a flexible digestive
system that minimizes the energy costs of flying up to 16,000
kilometers between their arctic breeding grounds and winter homes in
Europe and the tropics; their gizzard expands and contracts to balance
daily food intake and energy needs. To determine the effects of
dredging on the birds, the authors sampled prey quality and density
over 2,800 Wadden Sea sites during the late summer months (late July to
early September) for five years starting in 1998. Dredging occurred
each year from September to December, immediately after their sample
collections. In undredged areas, cockle densities increased by 2.6%
each year, and the quality remained stable. In dredged areas, cockle
densities remained stable, and their quality (flesh-to-shell ratio)
declined by 11.3% each year--paralleling the decline in the quality of
the birds' diet (as measured by droppings). This finding falls in line
with evidence that dredging disturbs the silt cockles like to settle
in, as well as their feeding conditions, which in turn reduces their
quality as a food resource.
Based on prey quality and densities, van Gils et al. predicted the
energy intake rate for knots with an average-size gizzard at each site
(all sites were pooled into 272 blocks, each with an area of 1 square
kilometer), then calculated the percentage of blocks that would not
yield sufficient intake rates for knots to avoid starvation. From 1998
to 2002, the percentage of blocks that couldn't sustain knots increased
from 66% to 87%--all attributable to dredging in previously suitable
sites. Reduced prey density caused some of this degradation, but most
stemmed from declines in both cockle density and quality.
The authors caught and color-banded the birds so they could estimate
survival rates the following year, and they measured gizzard mass with
ultrasonography. As expected, when prey quality declined, birds needed
larger gizzards to process the relatively higher proportion of shells
in their diet. Their chances of surviving conditions at the Wadden Sea
increased as a function of prey quality and gizzard flexibility. Birds
that did not return had much smaller gizzards than those that did.
Survival rate calculations based on gizzard size and prey quality
revealed that if birds could not expand their gizzard and prey quality
was low (0.15 grams of flesh per gram of shell), only 47% of arriving
birds would avoid starvation. A much greater proportion would survive
if their gizzard could expand by at least 1 gram (70% for 1 gram, 88%
for 2 grams).
These degraded food conditions, the authors conclude, explains why red
knot populations have declined by 80% in the Wadden Sea. And increased
mortality in the Wadden Sea, which the authors estimate at 58,000 birds
over five years, accounts for the 25% decline of red knots across their
entire northwest European wintering grounds. Dredging reduced the
quality of red knots' primary food source so drastically that even the
birds' extraordinarily adaptable digestive system could not save them.
The authors point out that dredging doesn't even provide significant
economic benefits--only 11 outfits manage 22 fishing boats--yet is
"directly responsible" for the widespread decline of a protected
shorebird. These findings put the lie to the notion that commercial
exploitation is consistent with conservation and underscore the risks
of disturbing critical habitat for threatened or endangered species.
###
Citation: van Gils JA, Piersma T, Dekinga A, Spaans B, Kraan C (2006)
Shellfish dredging pushes a flexible avian top predator out of a marine
protected area. PLoS Biol 4(12): e376. DOI:
10.1371/journal.pmed.0030376.
PLEASE MENTION THE OPEN-ACCESS JOURNAL PLoS BIOLOGY (
www.plosbiology.org ) AS THE SOURCE FOR THESE ARTICLES AND PROVIDE A
LINK TO THE FREELY-AVAILABLE TEXT. THANK YOU.
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