FURTHER SUPPORT FOR THE HYPOTHESIS THAT 



INTERNAL WAVES CAN CAUSE SHOREWARD TRANSPORT OF 



LARVAL INVERTEBRATES AND FISH 



Alan L. Shanks^ 



ABSTRACT 



In areas of mesotides (tidal range 2 to 4 m) and narrow continental shelves (<30 km) internal waves 

 can transport (i.e., convey from one place to another) the larvae of coastal organisms shoreward. Research 

 reported here was in an area of microtides (tidal range <2 m) and a wide continental shelf (>80 km), 

 the South Atlantic Bight. Half of the sampled sets of internal waves were aligned parallel to shore and 

 probably originated at the shelf break. The higher densities of larvae and flotsam in the slicks over these 

 internal waves (convergence zones) than in the rippled water between shcks (divergence zones) indicates 

 that these waves were transporting larvae and flotsam shoreward. All nontransporting internal waves 

 were aligned at a sharp angle to shore and may have formed over shoals oriented perpendicular to shore. 

 To further test the hypothesis that internal waves can transport larvae, surface plankton were col- 

 lected from the waters over, in front, and behind a set of internal waves. The density of Portunus spp. 

 megalopae was significantly higher in waters in front of the set than behind. The average densities of 

 a variety of larval fish and invertebrates were significantly higher over the internal waves than in front 

 of the set of waves. These data indicate that internal waves can cause shoreward transport of larvae 

 and flotsam. Precompetent larval fish were not carried shoreward by this set of waves while competent 

 stages (i.e., juvenile through postflexion) were transported shoreward. 



Recent papers have suggested that the planktonic 

 larvae of some coastal invertebrates and fish 

 (Shanks 1983, 1985, 1986; Jillett and Zeldis 1985; 

 Kingsford and Cheat 1986; Shanks and Wright 

 1987) as well as flotsam (e.g., an oil spill, Shanks 

 1987) can be transported (i.e., conveyed from one 

 place to another) by internal waves. As the tide ebbs 

 off the continental shelf or across some other sharp 

 change in the bottom relief (i.e., a reef or bank) a 

 lee wave is formed (Lee and Beardsley 1974; 

 Gargett 1976; Maxworthy 1979). When the tide 

 changes to flood this lee wave is "released" and 

 propagates away from its point of origin (see for ex- 

 ample Chereskin 1983). Most of the waves formed 

 at the continental shelf break propagate shoreward. 

 As the original internal wave moves onshore it 

 evolves from a solitary wave into a set of waves 

 (Osborne and Burch 1980). Stripes of glassy water, 

 slicks, are surface manifestations of currents over 

 the internal waves, and they delineate zones of con- 

 verging and downwelling currents situated between 

 the crest and the trough of an internal wave (Ewing 

 1950; LaFond 1959). The currents over the inter- 

 nal waves generate slicks by both perturbing small 



^University of North Carolina at Chapel Hill, Institute of Marine 

 Sciences, 3407 Arendell Street, Morehead City, NC 28557. 



surface waves and concentrating the organic sur- 

 face film (Ewing 1950; Gargett 1976). Buoyant flot- 

 sam will be carried into the convergence zone by the 

 surface currents, but because of the particle's buoy- 

 ancy they will not follow the water as it is down- 

 welled; the flotsam will be trapped at the surface 

 in the convergence and as the convergence zone 

 moves onshore so will the flotsam. The proposed 

 mechanism of larval transport suggests that any 

 organism which can remain at the surface in the con- 

 vergence zone either by swimming or other 

 behaviors (Shanks 1985) will, like flotsam, remain 

 in the slick and be transported onshore (Shanks 

 1983). 



The conditions necessary for the production of 

 tidally generated internal waves (tides, sharp bot- 

 tom relief, and some water column density stratifica- 

 tion) are present in the waters adjacent to nearly 

 all land masses, and the surface manifestations of 

 internal waves have been observed from both ships 

 and satellites in numerous locations (Apel et al. 

 1975; Fu and Holt 1982; Sawyer 1983). Evidence 

 that internal waves may be capable of transporting 

 planktonic larvae onshore has been collected in the 

 Pacific Ocean off Southern California (Shanks 1983, 

 1985, 1986), in the San Juan Archipelago (Shanks 

 and Wright 1987), and in the waters off the North 



Manuscript accepted July 1988. 



FISHERY BULLETIN: VOL. 86, NO. 4, 1988. 



703 



