FISHERY BULLETIN: VOL. 86, NO. 4 



(1976) and Moser and Ahlstrom (1970). Megalopae 

 were identified from references sited in Williams 

 (1984). Portunus spp. and Callinectes spp. mega- 

 lopae were separated using the characters suggested 

 by Smyth (1980). Penaeids were identified using 

 Cook (1966). No attempt was made to precisely iden- 

 tify the other groups of organisms counted. 



Statistical comparisons of the density of larvae 

 and flotsam in the different sampled habitats were 

 made using Wilcoxon's two-sample test (Sokal and 

 Rohlf 1969). Densities were considered to be sig- 

 nificantly different when P < 0.05. 



RESULTS 



During the summer of 1985 searches for internal- 

 wave-slicks were made on eight occasions. On three 

 days, despite favorable wind and sea conditions, no 

 internal-wave-slicks were observed. On 4 and 24 

 June, the observed sets of slicks were oriented 

 parallel to shore and the bottom topography while 

 on 14 and 20 July sets were oriented nearly perpen- 

 dicular to shore. On 21 August a set of slicks aligned 

 nearly perpendicular to shore was observed about 

 5 km offshore, and at about 9 km offshore a second 

 set of slicks was found oriented parallel to the bot- 

 tom topography. Both types of slicks, perpendicular 

 and parallel orientation, possessed all of the usual 

 characteristics of internal waves. Slicks were about 

 30 to 50 m wide, they were separated by one to 

 several hundred meters of rippled water, and the 

 sets moved (the parallel-to-shore slicks moved on- 

 shore while the perpendicular-to-shore slicks moved 

 north). 



The surface drifters released in front of sets of 

 slicks oriented parallel to shore on 4 June and 21 

 August could not be located following the plankton 

 tows. On 24 June, all of the drifters located at the 

 end of the plankton tows (48 of 50 released) were 

 found in the first two slicks of the set of internal 

 waves. The drifters, which had been released in a 

 250 m long line in front of the set of internal waves, 

 had been caught, concentrated, and carried about 

 4 km shoreward by the internal waves. Clearly this 

 set of internal waves was capable of carrying buoy- 

 ant flotsam shoreward. 



Sargassum floats were abundant in the plankton 

 tows. The floats are buoyant, do not extend above 

 the water (i.e., are not blown directly by the wind), 

 and, hence, the floats should act much like the re- 

 leased surface drifters. On all three dates when the 

 internal- wave-slicks were oriented parallel to shore, 

 the density of Sargassum floats was significantly 



higher (14- to 300-fold higher; Tables 1, 2) in the con- 

 vergence than divergence zones. These data indicate 

 that not only was the set of internal waves sampled 

 on 24 June capable of carrying flotsam shoreward, 

 but the sets oriented parallel to shore on 4 June and 

 21 August were also capable of carrying buoyant 

 flotsam shoreward. 



In contrast, on those days when the slicks were 

 oriented roughly perpendicular to shore (14 July, 20 

 July, and 21 August, Table 1) released surface 

 drifters were about equally distributed between slick 

 and rippled water and, despite the fact that the 

 sHcks moved northward during the observation 

 period, the drifters were not carried along with the 

 waves. The density of Sargassum floats was not 

 significantly higher in the slicks than the rippled 

 water between the slicks. These data suggest that 

 these internal waves were not transporting flotsam. 



Before larval densities can be interpreted, it is 

 first necessary to determine which larval types are 

 found exclusively in the neuston. An increase in the 

 neustonic density of organisms, which inhabit both 

 the neuston and the water column, could be due to 

 forces concentrating just the neustonic portion of 

 the population, or it could be due to animals from 

 the water column augmenting the population in the 

 neuston. Given the sampling regime of this study 

 it was impossible to differentiate between these two 

 possibilities. Because of this limitation a series of 

 tows were made in the neuston, the water column, 

 and bottom water to determine which organisms ex- 

 clusively inhabited the neuston. 



There was a distinct assemblage of megalopae and 

 larval fish that were caught in the oblique water 

 column and bottom plankton tows but were nearly 

 absent from the neuston tows (Table 3). Unfor- 

 tunately, organisms that were common in the 

 neuston tows made over or around internal waves 

 were abundant on only one of the three dates when 

 the vertical distribution samples were collected. On 

 this one date (19 June 1986, Table 3) there was a 

 group of larval fish and crabs that were only pres- 

 ent in the neuston tows. This latter group included 

 the megalopae of Portunus spp. and Callinectes spp., 

 juvenile Monacanthus hispidus, and juvenile and lar- 

 val Hyporhamphus unifasciatus and Sphoeroides 

 maculatus. Previous research also suggests that 

 these larvae and postlarvae as well as others are in- 

 habitants of the surface waters. Both the behavior 

 (Sulkin andVan Heukelem 1981) and the vertical 

 distribution (Smyth 1980; Johnson 1985a) of Calli- 

 nectes spp. megalopae suggest that they are usually 

 neustonic. The megalopae of Portunus spp. and 



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