Marine bird aggregations associated with the tidally-driven plume and plume fronts of the Columbia River
- Publication date
- Publisher
- 'Elsevier BV'
Abstract
Freshwater discharge from large rivers into the coastal ocean creates tidally-driven frontal systems
known to enhance mixing, primary production, and secondary production. Many authors suggest that
tidal plume fronts increase energy flow to fish-eating predators by attracting planktivorous fishes to feed
on plankton aggregated by the fronts. However, few studies of plume fronts directly examine piscivorous
predator response to plume fronts. Our work examined densities of piscivorous seabirds relative to the
plume region and plume fronts of the Columbia River, USA. Common murres (Uria aalge) and sooty
shearwaters (Puffinus griseus) composed 83% of all birds detected on mesoscale surveys of the
Washington and Oregon coasts (June 2003-2006), and 91.3% of all birds detected on fine scale surveys
of the plume region less than 40 km from the river mouth (May 2003 and 2006). Mesoscale comparisons
showed consistently more predators in the central plume area compared to the surrounding marine area
(murres: 10.1-21.5 vs. 3.4-8.2 birds km⁻²; shearwaters: 24.2-75.1 vs. 11.8-25.9 birds km⁻²). Fine scale
comparisons showed that murre density in 2003 and shearwater density in both 2003 and 2006 were
significantly elevated in the tidal plume region composed of the most recently discharged river water.
Murres tended to be more abundant on the north face of the plume. In May 2003, more murres and
shearwaters were found within 3 km of the front on any given transect, although maximum bird density
was not necessarily found in the same location as the front itself. Predator density on a given transect
was not correlated with frontal strength in either year. The high bird densities we observed associated
with the tidal plume demonstrate that the turbid Columbia River plume does not necessarily provide fish
with refuge from visual predators. Bird predation in the plume region may therefore impact early marine
survival of Pacific salmon (Oncorhynchus spp.), which must migrate through the tidal plume and plume
front to enter the ocean. Because murres and shearwaters eat primarily planktivorous fish such as the
northern anchovy (Engraulis mordax), aggregation of these birds in the plume supports the hypothesis
that it is the plume region as a whole, and not just the plume fronts, which enhances trophic transfer to
piscivorous predators via planktivorous fishes.Keywords: Marine birds, River plumes, USA, Tidal fronts, Columbia River, Predator prey interactions, Forage fish, Salmon, Washington, Orego