FISHERY BULLETIN: VOL. 86. NO. 4 



and South Islands of New Zealand (Zeldis and Jillett 

 1982; Jillett and Zeldis 1985; Kingsford and Choat 

 1986). These areas are characterized by mesotides, 

 i.e., tidal range 2 to 4 m (Davis 1964). The first pur- 

 pose of the research reported in this paper was to 

 test if transport occurred in the Atlantic Ocean and 

 in an area of microtides (tidal range <2 m, Davis 

 1964), the South Atlantic Bight. 



To date researchers have compared the density 

 (no./m-) of larvae in the convergence zones (slicks) 

 over internal waves to the density present in the 

 divergence zone (ripples). Cases where the densities 

 of larvae were significantly higher in slicks than the 

 ripples were used as evidence for the proposed 

 hypothesis. Higher density in the convergence im- 

 plies that larvae tend to spend more time in the slick 

 than in the water between slicks. Because of the 

 speed at which internal waves are propagating, even 

 a brief residence in an internal-wave-slick could 

 cause appreciable shoreward transport. For exam- 

 ple, if a larvae spent an hour in a slick it could be 

 carried about 2 km (assuming an average internal 

 wave speed of 56 cm/s. Sawyer 1983). 



Higher larval densities in the convergence zone 

 is only one of several predictions which should be 

 true if internal waves are transporting larvae shore- 

 ward. The second purpose of this research was to 

 test several additional predictions. As an internal 

 wave moves shoreward, larvae from in front of the 

 wave will be swept by the currents associated with 

 the wave into the convergence zone. If these larvae 

 can remain at the surface in the convergence, they 

 will be carried shoreward. Predictions include 1) 

 larval density in the waters in front of the set of in- 

 ternal waves should be significantly higher than in 

 the waters behind the set; 2) because larvae will 

 accumulate in the convergence, the observed den- 

 sity of larvae over an internal wave will be signifi- 

 cantly higher than the density in the waters in front 

 of the set of internal waves; and 3) a rare larval 

 type may be carried into an area from a distant 

 source in which case these particular larvae may 

 only be present in the waters over the internal wave 

 and will be concentrated in the slicks. To test these 

 predictions, replicate neuston net samples were col- 

 lected in the convergence and divergence zones over 

 a set of internal waves, and in the waters imme- 

 diately in front and behind this set of internal waves. 



METHODS 



The study was confined to the ocean waters at the 

 northern end of Onslow Bay within about 20 km of 



Beaufort Inlet, NC, U.S.A. (long. 34°40'W, lat. 

 76°40'N; Fig. 1). Surface plankton tows from the 

 waters over internal waves were collected during 

 the summer of 1985 on 4 and 24 June, 14 and 20 

 July, and 21 August. On 24 June 1985 surface 

 plankton tows were also collected from immediate- 

 ly in front and behind a set of internal waves. On 

 14 June 1985 and 9 and 19 June 1986 samples were 

 collected at the surface and in the water column to 

 determine which taxa were exclusively neustonic in 

 distribution. 



Winds stronger than 10 to 15 knots obliterate the 

 slicks, which delineate the convergence zones over 

 internal waves. Searches for internal-wave-slicks 

 and sampling of the associated plankton were 

 limited to periods with winds less than a moderate 

 breeze (Beaufort scale 4). The procedure for locating 

 internal- wave-slicks was to proceed along shore until 

 the estuarine front associated with Beaufort Inlet 

 was crossed and thence perpendicular to shore un- 

 til we found a set consisting of at least three large 

 (at least 30 m wide by about 500 m long) linear slicks 

 separated by one to several hundred meters of 

 rippled water. Sets of large slicks separated by rip- 

 pled water are a unique surface signature of large, 

 usually tidally generated internal waves (Ewing 

 1950; LaFond 1959; Apel et al. 1975; Gargett 1976; 

 Fu and Holt 1982; Chereskin 1983; Sawyer 1983). 



To test if a set of internal waves was capable of 

 transporting flotsam, surface drifters (weighted 

 Styrofoam cups) were released in a line perpen- 

 dicular to and in front of the set of internal-wave- 

 slicks (Shanks 1983). Prior to the release of the sur- 

 face drifters and immediately after all sampling was 

 completed the position of the first slick in the set 

 was determined by either compass bearings on land- 

 marks or with loran. From these measurements we 

 were able to determine the distance that the set of 

 waves propagated during the period of observation. 



While the currents acted on these surface drifters, 

 3 or 4 replicate 5- to 10-min surface (<20 cm depth) 

 plankton tows were made in the slicks and rippled 

 water between slicks. These 1985 samples were col- 

 lected on 4 and 24 June, 14 and 20 July, and 21 

 August 1985. In addition, on 24 June 1985 replicate 

 surface plankton samples were also collected from 

 the water immediately in front and behind the set 

 of internal waves (within 200 m of the internal wave 

 set). Plankton samples were collected using a manta 

 net (Brown and Cheng 1981) with a mouth opening 

 of 0.95 X 0.26 m and a net mesh of 0.333 mm. A 

 flow meter mounted in the mouth of the net mea- 

 sured the volume of water filtered. Between tows 



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