FISHERY BULLETIN: VOL. 86. NO. 4 



and oceanography of the Cape Lookout and Frying 

 Pan Shoals are so similar that by analogy the inter- 

 nal waves oriented perpendicular to shore observed 

 during this study may have been formed over Fry- 

 ing Pan Shoals and propagated northward into 

 Onslow Bay. The mechanism for the formation of 

 these internal waves is unknown. 



As was observed in a previous study (Shanks 

 1983), only some sets of internal waves transported 

 larvae or flotsam. In this study only those slicks 

 aligned parallel to shore caused transport. It is not 

 clear why some sets of waves cause transport while 

 others do not. The physical characteristics of tidal- 

 ly generated internal waves are quite variable. The 

 amplitude (Cairns 1968) and decay distance of in- 

 ternal waves varies over the fortnightly tidal cycle 

 (Brink 1988). Further, the depth, wave length, and 

 shape of the internal waves is dependent on the 

 relative depths of the thermocline and the bottom 

 and wave amplitude (LaFond 1959; Lee 1961; Cairns 

 1967, 1968). What is needed is simultaneous mea- 

 surements of the physical characteristics of a set of 

 internal waves much like those made by LaFond 

 (1959) with measurements of the transport of flot- 

 sam or larvae. 



The second purpose of this research was to test 

 several new predictions derived from the hypothesis 

 that internal waves can transport larvae. If trans- 

 port was occurring, then one would predict that 

 1) due to the accumulation of larvae in the con- 

 vergence zones as internal waves propagate shore- 

 ward, the density of larval types transported by the 

 internal waves should be significantly lower behind 

 than in front of the set, 2) the observed density of 

 larvae over an internal wave should be significant- 

 ly higher than in the waters in front of the set of 

 waves, and 3) there may be types of larvae which 

 are only present in the slicks, suggesting that they 

 had been carried into the area from a distant source. 

 The appropriate samples to test these three predic- 

 tions were collected on 24 June 1985. The densities 

 of several larval types were significantly higher in 

 the waters in front of the internal wave set than 

 behind. There were many instances in which the 

 observed density over the internal waves of a type 

 of larvae was significantly higher than in the waters 

 in front of the set of internal waves. Lastly, there 

 were a number of organisms that were only caught 

 in the convergence zones over the internal waves. 

 In this set of observations the three predictions were 

 confirmed indicating that this set of internal waves 

 was carrying larvae and flotsam shoreward. 



The significant differences in larval densities 



observed on 24 June (i.e., in front vs. behind and 

 over the internal waves vs. in the front of the set 

 of waves) may have been due to fortuitous cross- 

 shelf patchiness in larval density. Because conditions 

 allowed only one opportunity to sample in front and 

 behind a set of internal waves, this alternate ex- 

 planation can not be rejected. Cross-shelf larval 

 patchiness is, however, probably not an adequate 

 explanation because of the very short distance over 

 which large differences in larval abundances were 

 observed. For example, the tows in front of the set 

 and the tows in the slick and rippled water over the 

 set were separated by at most 200 m, yet there were 

 many cases (14, Tables 2, 4) where larval abun- 

 dances were different by at least a factor of 10. Dif- 

 ferences in plankton abundance of this magnitude 

 and over this small a distance are almost always 

 associated with oceanographic features (e.g., fronts: 

 Boden 1952; Pingree et al. 1974; Owen 1981; Fogg 

 et al. 1985). The only apparent oceanographic 

 feature in the study area was the internal-wave- 

 slicks. The observed differences in larval density 

 were probably caused by the internal waves. 



The data in Figure 2, the density of fish by stage 

 of development over the internal waves vs. in front 

 of the set of waves, suggest that only the later 

 developmental stages of fish (juvenile through early 

 postflexion) were transported onshore by the set of 

 internal waves sampled on 24 June 1985. All five 

 of the abundant fish species (Table 4) inhabit near 

 shore or estuarine habitats as adults (Fritzsche 1978; 

 Hardy 1978; Johnson 1978; Martin and Drewry 

 1978). Flexion and preflexion larval fish are clearly 

 not competent to adopt the adult or nursery habitat, 

 and the data suggest that they were not carried on- 

 shore by the internal waves. There may be adaptive 

 advantages to planktonic larvae avoiding estuarine 

 waters during their development (Strathmann 

 1982). Juvenile fish and, perhaps, also postflexion 

 larvae, are competent to recruit into the adult or 

 nursery habitat. Transport onshore by internal 

 waves may, therefore, be adaptively advantageous 

 for those fish whose adult or nursery habitat is 

 coastal or estuarine. 



The larval development of the blue crab, Calli- 

 nectes sapidus, occurs at sea (Smyth 1980; 

 McConaugha et al. 1983; Johnson 1985b). The lar- 

 vae are present in the waters over the continental 

 shelf and out to the Gulf Stream (Smyth 1980). At 

 the end of the larval period the megalopae must 

 return to an estuarine habitat to continue its adult 

 existence. How the megalopae make this migration 

 is an open question (Johnson et al. 1984; Johnson 



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