The Wilson Journal of Ornithology 124(l):66-72, 2012 
FORAGING OVER SARGASSUM BY WESTERN NORTH 
ATLANTIC SEABIRDS 
MARY L. MOSER'- 24 AND DAVID S. LEE' 
ABSTRACT.—Drifting reefs of Sargassum (a brown alga) are used by a variety of pelagic seabirds in the western 
Atlantic Ocean. We examined gut contents from 964 individuals of 39 seabird species collected 5 to 60 km off the coast of 
North Carolina for evidence of Sargassum use. Sargassum pieces or Sargassum- associated prey were found in nine of 10 
Procellariiformes species and less frequently among Charadriiformes (12 of 25 species). No Surgassum-associaied prey was 
found in Pclecaniformes examined, but observational data indicated that Atlantic tropiebirds ( Phocthon Upturns <mdF. 
uethereus) and Masked Boobies (Sulu clactylalm) commonly foraged ovet Sargassum. Four species were considered 
Sargassum specialists, having frequencies of occurrence >25% and high volumes of Sargassum -associated prey 
Audubon's Shearwater ( Puffinus Utemmieri). Royal Tent ( Thalttsseus muximus). Bridled Tern (Onychoprion anaedirtu. n 
and Red-necked Phalarope ( Pltalaropus lobatus). Seven species fed in Sargassum to a lesser extent, and nine species had 
ingested Sargassum pieces, but contained no Sargassum-ussoc’e.Hcd prey. It is likely that other seabird species foray; 
regularly over Sargassum. as our conclusions are based on relatively small sample sizes taken during random sampling in 
the open ocean, Our conservative analysis and extensive observational data indicate the Sargassum community is critical 
for leeding lor some western North Atlantic seabirds. Degradation of Sargassum habitats by oil development, harvest, and 
or ocean acidification would undoubtedly have negative effects on Illness ol these birds Received 22 March 2011 
Accepted 7 October 2011. 
Consolidated reefs of floating pelagic brown 
algae of the genus Sargassum are important and 
recurring features of tropical and sub-tropical 
marine environments. Holopelagic S. natans and 
S.fluitans support a diverse and abundant fish and 
invertebrate fauna in the western Atlantic Ocean 
(Fine 1970, Settle 1993, Casazza and Ross 2008). 
Recent remote sensing data indicates Sargassum 
reefs originate in the northwestern Gulf of Mexico 
in June each year and are advected into the 
western Atlantic in early summer by ihe Loop 
Current, moving northward with (lie Gulf Stream 
(Gower and King 2011). The floating weed moves 
to the south and west in fall and winter, becoming 
less buoyant with age. The circulation of Sargas¬ 
sum is consistent among years and is driven by 
predictable currents and trade winds (Gower and 
King 2011). 
Floating Sargassum can be extensive, yet 
ephemeral habitat for seabirds. Airborne imagery 
indicated that drift lines of the algae extend for 
continuous lengths ot at least 5 km and primarily 
consist ot 20-80 nr’ reefs of Sargassum (Marmor- 
ino et al. 2011). Satellite images indicate 
' Zoology Department. North Carolina State Universi 
Raleigh, NC 27695. USA. 
u , CUr ?w address; Nor thwcst Fisheries Science Cem 
National Marme Fisheries Service. 2725 Montlake Bou 
vard East. Seattle, WA 98112. USA, 
28336. e uSA° iSe R ° B ° X 7,,K2 ' White Lake. 
J Corresponding author; e-mail: mary.moser@noaa.gov 
Sargassum slicks can be even larger, ranging 
from 100 to 1,000 m in width and up to hundreds 
of kilometers in length (Gower et al. 2006). 
However, consolidated drift lines of Sargassum 
off the coast of Florida start to disintegrate as 
wind speeds exceed 5 m/sec (Marmorino et al 
2011). The amount of pelagic Sargassum in the 
North Atlantic was estimated at 0.54 metric ions; 
km’ in the Gulf Stream and 0.02 metric lons/knr 
over ihe Continental Shelf, for a combined 
standing crop of >50,000 metric tons off the 
Carolina*; (Howard and Menzies 1969). Gowei 
and King (2011) estimated the wet weight of 
Sargassum in the Atlantic has regularly exceeded 
1.8 million metric tons during the past decade and 
that even greater amounts can occur in the Gull ol 
Mexico. Thus. Sargassum reefs are important 
feeding stations and possible roosting sites lot 
pelagic seabirds (Haney 19S6). 
Studies to date have used observations of 
seabird behavior around Sargassum reefs to reach 
conclusions about why seabirds are attracted to 
this habitat (Haney 1985. 1986). We examined the 
gut contents of 39 species of pelagic seabirds for 
evidence of foraging over Sargassum. Percent 
frequency ol occurrence, numerical abundance 
and volume of Sargassum-assoclmed prey weft- 
used to ascertain the relative extent of Sargassum 
foraging exhibited by the species sampled. These 
data were supported by extensive visual observa- 
lion ol marine birds feeding in pelagic habitats off 
the coast of North Carolina. Our objectives were 
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