Moser and Lee • SARGASSUM REEFS AND SEABIRD FORAGING 
67 
lo: (1) identify seabird species that rely most 
heavily on pelagic Sargassum for feeding, and (2) 
document prey items most frequently targeted by 
these birds. 
METHODS 
Gut content analysis from 964 individual 
seabirds of 39 species collected 5 to 60 km off 
the North Carolina coast was conducted as 
described in Moser and Lee (1992). In addition, 
visual observations of seabird foraging were made 
from a vessel during 231 day trips (averaging 1.201 
seabird observations and 25.2 species/trip). Birds 
were collected during all seasons between 1975 
and 1989. although fewer sampling and observa¬ 
tion trips were made in winter. Observations and 
collections occurred over a wide geographic area in 
;tn attempt to census inshore coastal waters, the 
inner and outer Continental Shelf, and deeper 
waters over the Shelf's edge. Lee and Socii (1998) 
mapped the areas surveyed by month. 
Documentation of Sargassum use was not the 
original focus of seabird collections or observa¬ 
tions. The birds used in this study were collected 
opportunistically during their entire period of 
occurrence in North Carolina waters to obtain data 
on heavy metal accumulation, plastic ingestion, 
a £ e and sex ratios, body temperature, parasite 
load, moll sequence, behavior, and ecology (e.g., 
Moser and Lee 1992, Lee 1995, Lee and Haney 
! 'Wii. Foraging flocks quickly dispersed when 
approached by our survey boats and birds foraging 
over Sargassum were not targeted, nor were they 
particularly easier to collect. Sargassum is 
typically found in the vicinity of the outer 
Continental Shelf along the western wall of the 
rj "lf Stream and. to a lesser extent, in wind rows 
w 'thin the Stream. Only 40% of the surveys were 
near the Shelf edge where Sargassum typically 
" Ccu n., and the alga was frequently not in the 
'■"mediate vicinity of our survey sites. 
Contents from the stomach and gizzard ol each 
b|r d were combined, and birds with empty upper 
digestive tracts were excluded from the analysis. 
Percent frequency of occurrence of Sargassum 
''eaves or bladders) and Sargassum- associated 
Luna was calculated for each seabird species 
'"umber of birds with prey ‘A' divided by the 
,0 tol number of birds X 100). Sargassum 
associates were defined as those species (fish, 
c histaceans, and gastropods) that reside in Sar- 
Wmm during the life history phase when 
'"gested (following Dooley 1972, Settle 1993). 
Unidentifiable prey items were assumed not to be 
Sargassum associates, and the mean percent 
volume of Sargassum- associated prey in the 
digestive tract was used as a direct measure of 
the relative importance of Sargassum for foraging 
within species. Thus, our estimates of Sargassum 
use are conservative. 
RESULTS 
Gnt contents of birds from three Orders, five 
families, 16 genera, and 39 species were analyzed. 
Twenty-one species (53.8%) had ingested Sar¬ 
gassum pieces or Sargassum-ussociated prey 
(Table l). Birds were classified as Sargassum 
specialists (species that had >25% occurrence of 
Sargassum- associated prey). Sargassum users 
(those with up to 25% occurrence of Sargassum 
or associated prey), and Sargassum incidentals 
(species (hat contained only pieces of Sargassum 
and no associated prey ). We regarded the presence 
of alga in digestive tracts as evidence of foraging 
associated with pelagic Sargassum. Its presence 
among gut contents that lacked any identifiable 
Sa rgo.v.vHm-associated prey was probably a result 
of the bird’s inability to rapidly digest the alga. 
Evidence of Sargassum foraging was found in 
most Proccl I arii formes (9 of 10 species) and less 
frequently in Charadriiformcs (12 of 25 species). It 
is possible that Sooty Tern ( Onychoprlon fuscatus) 
could be added lo the species that use Sargassum , 
as four of 11 birds sampled contained Hying fish 
(Exocoetidae). There was equivocal evidence from 
digestive tract analysis for Sargassum use by two 
of the four Pelecaniformes. Relatively few indi¬ 
viduals of these species were collected. Our 
extensive visual observations of the pelagic 
members of this family indicated they forage over 
algal mats. Moreover, two of five White-tailed 
Tropicbirds ( Phaethan Icpturus) and two of three 
Red-billed Tropicbirds (P. aethereus) ingested 
flying fish. 
Species that had no Sargassum or identifiable 
Sargassum- associated prey in their digestive tracts 
included: Band-rumped Storm-Petrel (Oceano- 
droma castro , n = 12), White-tailed Tropicbird 
(n - 5). Red-billed Tropicbird (n = 3), Northern 
Gan net (Morns bassanus. n = 5), Double-crested 
Cormorant (Phalacrocorax auritus. n — I), 
Parasitic Jaeger (Stercorarius parasiticus, n = 4), 
Great Skua (S. skua, n = 1). Great Black-hacked 
Gull ( Lartis marinas, n = I). Herring Gull (L. 
argenlatus , n = 2), Ring-billed Gull (L dektwar- 
ensis, n = 2), Arctic Tern (Sterna paradisaea . 
