http://www.eurekalert.org/pub_releases/2005-12/uoc--omd121705.php
Public release date: 19-Dec-2005
Overfishing may drive endangered seabird to rely upon lower quality food
Berkeley -- The effects of overfishing may have driven marbled
murrelets, an endangered seabird found along the Pacific coast, to
increasingly rely upon less nutritious food sources, according to a new
study by biologists at the University of California, Berkeley.
The results, to be published online by early March 2006 in the journal
Conservation Biology, suggest that feeding further down the food web
may have played a role in low levels of reproduction observed in
contemporary murrelet populations, and has likely contributed to the
seabirds' listing as an endangered species, the researchers said.
"The dietary patterns of today's marbled murrelets might be artifacts
of the profound changes that coastal marine ecosystems world-wide have
undergone because of overfishing," said Steve Beissinger, professor of
conservation biology in UC Berkeley's Department of Environmental
Science, Policy, and Management and principal investigator of the
study. "What better place to investigate this process than in Monterey
Bay, where sardines once were king and Cannery Row formerly stood?"
The researchers decided to embark on this study when they noticed that
the population of 600 marbled murrelets they were studying in central
California had very low levels of reproductive success.
"You'd expect birds to nest every year, but we found that in some
years, as many as 90 percent of the murrelets we were studying weren't
nesting," said Beissinger. "Even in the good years, only 50 percent of
the seabirds were nesting, and for those that did, many nests failed
for a variety of reasons."
The researchers analyzed marbled murrelet feathers from 136 birds
collected in the Monterey Bay region between 1895 and 1911. The
historic feathers represent a time period prior to the onset of
intensive fishing, decades before the infamous sardine fishery collapse
of the late 1940s, and nearly a century before recent declines of
anchovies, squid and rockfish.
The researchers compared these historic samples with feathers they
collected between 1998 and 2002 from 201 murrelets in the same region.
The researchers had records of when and where all the feathers they
sampled were obtained.
Measures of stable carbon and nitrogen isotopes – derived from food
ingested at the time the birds create feathers and preserved into the
feathers – give clues to the birds' diet. More nutritious food items,
such as sardines, anchovies and squid, are distinguished in the
feathers by higher levels of nitrogen isotopes. Less valuable food
sources, including marine invertebrates such as krill, are signaled by
greater levels of carbon isotopes.
"It takes about 70 to 80 krill to equal the energy value of one
sardine," said Beissinger. "When murrelets have to rely upon less
nutritious foods, they need to spend more time diving underwater to
catch and eat them, and have less energy to make and lay eggs. The
availability of food commonly affects the reproductive success of
seabirds."
The researchers were able to determine the time of year that individual
feathers were grown. They pointed out that murrelets undergo two molts
of feathers each year, one right before the onset of the March through
April breeding season, and another after their nesting period ends
between August and September. Brown-tipped breast feathers are only
lost in the pre-breeding period, while all-white breast feathers and
wing feathers are lost in the post-breeding period.
The researchers were thus able to relate the diet of the murrelet to
the environmental and fishing conditions at the time the feather was
created. "In this paper, we're trying to tease apart climate signals
from human fishing impacts," said Ben Becker, the study's lead author.
Becker was a UC Berkeley Ph.D. student at the time of the study and is
now director and marine ecologist at the Pacific Coast Science and
Learning Center at the Point Reyes National Seashore.
The research team found that the proportion of the most nutritious food
sources was significantly higher for the murrelets of a century ago in
the Monterey Bay region compared with the birds' modern-day
counterparts. Compared with the murrelets' historic diets, typical
modern pre-breeding diets during cool ocean conditions are composed of
42 percent less high-nutrition prey.
In contrast, prior to breeding during cool ocean years, contemporary
murrelets rely more upon krill and mid-level prey, such as sand lance
and young rockfish, compared with their historic counterparts.
The researchers found that while ocean temperature was important, the
era when the birds lived had a far greater impact on pre-breeding diet.
That is, the murrelet diet in the era before the collapse of sardines
and declines in other fisheries consisted of a greater proportion of
more nutritious foods and fewer lower-value prey items.
Post-breeding diets experienced less change, regardless of ocean
temperature or era. The researchers explained that this might be due to
the fact that krill and young rockfish are less available in late
summer and early fall regardless of fishing pressure or oceanographic
conditions.
"If we look back 100 years ago in Monterey Bay, marbled murrelets were
almost always feeding on higher trophic level prey, regardless of ocean
conditions," said Becker. "Now, even in the same environmental
conditions, the murrelets are not able to access higher tropic level
prey items."
Becker said this study illustrates an additional factor related to the
decline of the marbled murrelet. "For this bird, the primary
conservation concerns have been loss of habitat, particularly the
coastal old-growth forests where the bird nests," he said. "But it's
possible that they're getting a double whammy. Not only are they losing
their nesting habitat, they're also experiencing a degraded prey
environment."
The researchers pointed out that the U.S. Fish and Wildlife Service has
recently decided to start the process to remove the marbled murrelet
from the Endangered Species list, denying it protection even though the
bird has not recovered throughout its range. The researchers argue that
such a move would be premature.
"The threats that have caused marbled murrelets to decline throughout
their range have not been remedied," said Beissinger. "Coastal old
growth forests will require 50 to100 years of restoration, and prey
resources haven't recovered from the sardine fisheries collapse in the
mid-20th century. Other prey items that the murrelets feed upon,
including rockfish, have had to have seasonal closures because of heavy
harvest for commercial markets. Similarly, the anchovy fishery in
central California has declined by 75 percent since 1974."
The study results support the need for fisheries to be managed to
address their broader impacts on the ecosystem, the researchers said.
"Traditionally, decisions on how much to harvest were made by
calculating the impact of fishing on a single species, or by
determining how much can be fished in order for the population to
rebound in subsequent years," said Becker. "Now, more people are taking
an ecosystem approach, considering how harvesting one species will
impact other species that rely upon it. However, so little is known
about the diet of the murrelet that a study like this is important in
informing fishery management decisions."
"Our study also highlights the need to consider locations that improve
the production of seabird prey in the ongoing process to determine
where to place marine protected areas along the California coast in
order to restore marine food webs and conserve seabirds," Beissinger
added.
The researchers are conducting additional studies using feathers from a
wider range of seabirds and from a greater selection of time periods to
get a clearer picture of impacts of oceanic conditions and fishing
pressure throughout the past century.
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