358 
Fishery Bulletin 106(4) 
Although a significant positive logarithmic relationship 
was observed between numbers of species and Sargas- 
sum wet weight (Fig. 3B), similar numbers of species 
were often collected regardless of Sargassum quantities. 
For example, the maximum number of species (n = 19) 
collected in one neuston tow coincided with a relatively 
low quantity of Sargassum habitat (6.8 kg) (Fig. 3B). 
Behavioral observations 
Underwater video recordings clarified the close asso- 
ciation of juvenile fishes to structure compared with 
open water. Many juvenile fishes rapidly explored and 
associated with any new substrata introduced near the 
Sargassum mats (e.g., snorklers, vessel). As in our other 
collections, the two most abundant families of fishes 
observed in the video recordings were Monacanthidae 
(mostly S. hispidus ) and Carangidae ( Caranx spp. and 
Seriola spp.). Fishes exhibited a size-related layering 
among and below the Sargassum (Fig. 5, A-C). Smaller 
juvenile fishes were usually very close to or within the 
Sargassum and were rarely observed more than one 
meter below the algae (Fig. 5A), whereas larger, more 
mobile juvenile fishes (e.g., carangids and kyphosids) 
were further below the Sargassum (Fig. 5B). Even deeper 
below the Sargassum (up to 3 m), larger predators (e.g., 
adult dolphinfish and jacks) were observed, usually in 
schools (Fig. 5C). When large predators swam below 
the Sargassum, smaller fishes moved upward into the 
algae (Fig. 5A). 
Other behaviors were also observed from the under- 
water video recordings. Individuals and groups (7-10 
individuals) of juvenile Aluterus monoceros (with a light 
to dark brown mottled pattern, Fig. 5D) were observed 
hovering just below the Sargassum with heads down at 
a 45° angle, tails near the surface (Fig. 5D). A school 
(about 65 individuals) of adult A. monoceros (silver body 
color) exhibited the same behavior under the hull of 
the vessel, which was adjacent to the weedline. On 15 
occasions, small groups (2-15 individuals) of juvenile 
S. hispidus were observed pursuing and nipping at 
lobate ctenophores, Mnemiopsis leidyi (Fig. 5E). These 
interactions took place about 1.5 m below the Sargas- 
sum, and S. hispidus was the only species observed 
displaying this behavior. A distinct boundary was ob- 
served between open-water and Sargassum habitats; 
the open water adjacent to the edge of the weedline 
was unpopulated, whereas a high density of juvenile 
fishes were observed underneath and within 
the Sargassum (Fig. 5F). 
Schools of adult dolphinfish (approximately 
10-50 individuals) were observed swimming 
under the weedline on seven occasions. Most 
of these appeared to be females based on head 
shape and estimated sizes, but some may have 
been immature males. A female (295 mm SL), 
ripe with eggs, was collected from the vessel 
by hook-and-line during video recording op- 
erations. On five of the seven occasions, adult 
Caranx bartholomaei and C. crysos were mixed 
with the school of dolphinfish or swam closely 
behind them. On one occasion, a single large 
juvenile dolphinfish (approximately 300 mm 
SL) swam rapidly upward into the Sargas- 
sum with its mouth open, turned away at the 
weedline, and swam away; however, no small 
fishes were observed under the Sargassum in 
the vicinity of the strike, and it was not pos- 
sible to determine the success of this apparent 
feeding attempt. 
Discussion 
y= 29.02x+ 11,64 
0.715, P<0.001, n=89 
— i 
90 
Figure 3 
Relationships between number of individuals (A) and number of 
species (B) and Sargassum wet weight (kg) for all fishes collected 
with neuston nets in Sargassum habitat during summer and fall of 
2000-2003 off North Carolina. Note differences in y-axis scale. 
Pelagic Sargassum habitat supports an abun- 
dant and diverse assemblage of juvenile fishes, 
providing structure and protection in relatively 
barren oceanic surface waters. Juvenile fishes 
dominate the Sargassum community, and the 
majority of fishes collected in this study from 
Sargassum habitat were comparable in size 
(<50 mm SL) to those reported from other 
Sargassum studies (Dooley, 1972; Wells and 
Rooker, 2004). As with seagrass ecosystems 
