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 Ser
Source:University of California - Berkeley