sport and commercial fisheries of the State. Blue- 

 fish generally spend their first summer feeding 

 and growing in sounds and lagoons before moving 

 out into the ocean sometime in the fall. These 

 young bluefish reach about 1 lb in size before 

 they make this shift in habitat. The black seabass 

 [Centra pristis striata) is also an important con- 

 tributor to the sport fishing industry of North 

 Carolina. Although the Spanish mackerel {Scom- 

 beromorus maculatus) has not been included in 

 the list of species partially dependent upon food 

 species from intertidal flats, it sometimes leaves 

 the ocean to venture into sounds and estuaries. 

 When it does, it preys heavily on silversides and 

 other baitfishes which may have been feeding on 

 intertidal flats. 



Among the sharks listed in Table 3, perhaps 

 only the Atlantic sharpnose shark [Rhizopriono- 

 don terranovae) is often seen in shallow waters 

 in the sounds and estuaries of North Carolina. 

 Other sharks which are larger, more mobile, and 

 nocturnal in their visits to shallow water, may be 

 more involved with intertidal flats than is com- 

 monly recognized. Sharks represent one common 

 terminus of food chains originating in the inter- 

 tidal flat habitat. 



4.5 ESTUARIES AS NURSERY GROUNDS 

 FOR FISHES 



One of the most widely quoted functions of 

 estuaries and coastal lagoons is their role as nursery 

 areas for many marine fishes. Many fish species 

 which live or spawn at sea as adults utilize the 

 estuary as postlarval forms and as juveniles. 

 Presumably this split life history has developed in 

 response to (1) the very high productivity of estu- 

 arine systems which can satisfy the early nutri- 

 tional needs of the juvenile fishes and (2) the pro- 

 tection from predators provided by the shallow 

 waters. Those species which utilize the estuaries 

 as nurseries include most of the important com- 

 mercial and sport species as well as most bait- 

 fishes in the coastal marine systems of North 

 Carolina (Turner and Johnson 1973). The contri- 

 bution of estuarine systems in general to marine 

 fisheries is enormous, and the intertidal flat habi- 

 tat makes its contribution to this pattern. 



The dietary information [jresenled earlier 

 applies to juvenile and adult fishes. Most species 

 of bony fishes undergo radical changes in their 



trophic status as they grow and age. As larvae, 

 most marine fishes are considered part of the 

 plankton because of their small size. Even as post- 

 lar\al forms, most marine fishes are very small. 

 Both larval and early postlarval fishes feed upon 

 zooplankton. This is true of species which are 

 planktivorous as adults, as well as species which 

 are predatory when older (Kjelson et al. 1975). 

 Atlantic croaker, spot, striped mullet, and virtually 

 all other larval and postlarval fishes in marine sys- 

 tems spend much time in small size classes which 

 feed on the zooplankton. Because zooplankton 

 are supported by a diet of phytoplankton, inter- 

 tidal flats contribute indirectly to zooplankton 

 production through the mechanism of increasing 

 the area of the euphotic zone and supplying inor- 

 ganic nutrients, both of which stimulate phyto- 

 plankton production (see Chapter 2). Intertidal 

 flats also contribute directly to the success of 

 larval and postlarval fishes in another way. Be- 

 cause of their vulnerability to predators, these 

 postlarval fishes congregate in the shallowest 

 waters of the estuarine system for protection 

 against predatory fishes, which are ordinarily re- 

 luctant to risk the shallows. Many fishes which 

 are strictly marine as adults can thus be found 

 during their earliest life stages in the intertidal Hat 

 habitats. For instance, even some of the offshore 

 reef fishes, various snappers and groupers, spend 

 their larval and early postlarval days as plankti- 

 vores in the shallows of estuaries and lagoons in 

 North Carolina (.\dams 1976a), although it is un- 

 clear how important these juveniles are to the 

 total reproductive effort of the reef species. 



Not only do marine fishes raised in estuaries 

 demonstrate predictable changes in trophic status 

 as they grow, but they also show unpredictable 

 changes in feeding habits in response to varying 

 abundances of preferred foods. Predatory fishes 

 in estuaries commonly supplement their diets 

 with detritus when their usual prey are scarce 

 (Darnell 1964). Although growth rates on pure 

 diets of detritus arc quite low, this plastic feeding 

 behavior at least permits the predators to survive 

 periods of food scarcity. Detritus always is avail- 

 able as a dependable alternate source of energy in 

 any estuarine system. 



4.6 SEASONAL CHANGES IN THE FISH 

 FAUNA OF INTERTIDAL FLATS 



Seasonal variation in ihc fisii fauna iound 

 over intertidal flats at higher tides is (juite substan- 



46 



