SHANKS: SHOREWARD LARVAL TRANSPORT 



waves are capable of carrying fish larvae shoreward, 

 but shoreward transport seems to be confined to 

 postflexion stage larval or juvenile fish. 



In the samples from 24 June 1985 there are a 

 number of cases in which a fish species and/or stage 

 of development was common in the tows from the 

 convergence zones but was absent or very rare in 

 the divergence zones, in front, or behind the set of 

 waves (i.e., zero or one caught in the nine tows from 

 these three habitats. Table 4). Either these larval 

 fish are extremely rare in the waters surrounding 

 the internal waves in which case the convergence 

 zone must have accumulated larvae from a large 

 volume of water or the internal waves transported 

 the larvae into the study area from a distant source. 



On 24 June 1985, the density of total Brachyura 

 was significantly higher in the slicks over the inter- 

 nal waves than in any other area sampled (Table 2). 

 Portunus spp., a group which previous data had 

 demonstrated inhabited the neuston, made up the 

 bulk of the Brachyura caught. The densities of Por- 

 tuniis were significantly higher in the slicks than the 

 rippled water between slicks. The density of Por- 

 tunus spp. megalopae differed significantly from 

 0.3/100 m^ in front of the set of internal waves to 

 0/100 m- behind (Table 2). Portunus spp. first crabs 

 were absent from both the waters in front and 

 behind the set of internal waves; they were abun- 

 dant in the waters over the internal wave (Table 2). 

 The densities over the internal waves of Portunus 

 spp. megalopae and first crabs were significantly 

 higher than their densities in front of the set of 

 waves (Table 2). Callinectes spp. were uncommon 

 in the samples though their density was significantly 

 higher in the slick than in the rippled waters. In- 

 cluded under the category of miscellaneous mega- 

 lopae were the species Uca spp., Sesarma spp., and 

 majid crabs, all forms which were found to be more 

 abundant in the water column than in the neuston 

 (Table 2). There was not a significant difference 

 between their density in the slicks vs. the rippled 

 waters, suggesting that these megalopae were not 

 carried shoreward by the sampled set of internal 

 waves. The data indicate that definitely Portunus 

 spp. and possibly Callinectes spp. were carried on- 

 shore by the sampled set of internal waves. 



Other organisms counted in the samples were 

 adult amphipods, polychaete larvae, and Penaeus 

 spp. postlarvae. Densities of these types of or- 

 ganisms were significantly higher in the slicks than 

 the rippled waters (Table 2). While in each instance 

 densities were lower behind the set of internal waves 

 than in front, the differences were not statistically 



significant. The density of polychaete larvae and 

 Penaeus spp. postlarvae over the internal waves 

 were significantly higher than the density in front 

 of the set. These data suggest that these inverte- 

 brate larvae were also transported onshore by the 

 set of internal waves. 



DISCUSSION 



Tidally generated internal waves have been ob- 

 served in many areas of the world (Apel et al. 1975; 

 Fu and Holt 1982; Sawyer 1983). Larvae may util- 

 ize internal waves as a mechanism of onshore migra- 

 tion along many coastlines. Testing this hypothesis 

 would require the impossible task of making obser- 

 vations along all coastlines. An alternate technique 

 is to test for internal wave transport in areas with 

 different combinations of tidal range and shelf 

 width. Previous work has been done in areas of 

 mesotides (tidal range 2 to 4 m, Davis 1964) and 

 either narrow (<6 km. Shanks 1983) or moderate 

 (about 30 km, Kingsford and Choat 1986) shelf 

 widths. The Atlantic adjacent to the Beaufort Inlet 

 is characterized by microtides (tidal range <2 m, 

 Davis 1964) and a wide shelf (about 80 km). The first 

 purpose of this research was to test if onshore trans- 

 port of larvae could occur in an area with these 

 characteristics. On the three dates in which the 

 internal-wave-slicks were oriented roughly parallel 

 to shore the data suggest that internal waves were 

 transporting larvae and flotsam onshore. If the 

 waters around the Beaufort Inlet are representative 

 of localities with microtides and wide shelves, then 

 onshore migration of larvae via internal waves may 

 occur in other similar areas of the world. 



On three dates the internal-wave-slicks were 

 oriented nearly perpendicular to shore and were pro- 

 pagating roughly northward. Over the continental 

 shelf, internal waves oriented roughly perpendicular 

 to shore have been observed in satellite photo- 

 graphs. These internal waves are invariably associ- 

 ated with submarine canyons (Apel et al. 1976). 

 There are no submarine canyons in Onslow Bay. The 

 internal waves causing these slicks may have been 

 formed over Frying Pan Shoals (Fig. 1). These 

 shoals extend about 50 km across the shelf from the 

 end of Cape Fear and form the southern boundary 

 of Onslow Bay. Just north of the study area, Cape 

 Lookout and the Cape Lookout Shoals also extend 

 out across the shelf (Fig. 1). On a flight over Cape 

 Lookout numerous slicks oriented parallel to Cape 

 Lookout Shoals were observed propagating north- 

 ward into Rayleigh Bay (pers. obs.). The geography 



711 



