Occasional nigrants, as the naine im- 

 plies, are only present infrequently and 

 unpredictably. Representatives include 

 large carnivores of offshore or oceanic 

 origin such as carangids and scrombrids. 

 Organisns of this type represent only a 

 snail proportion of the biomass present, 

 but tnay bo important in determining fish 

 community structure. 



This system (Kikuchi 1961, 1962, 

 1966) aids in classifying the fish fauna, 

 but is not exact. For example, the king 

 mackeral could possibly be found over the 

 back reef grass beds much of the year, but 

 during winter large schools move through 

 the region. Thus this fish could be 

 classified as a seasonal resident and as 

 an occasional migrant. 



Stru c ture and Function 



Because fishes that occupy grass beds 

 are important to commercial fishermen and 

 because the seagrass habitat is apparently 

 important in the life histories of those 

 fishes, it is surprising that relatively 

 little is known concerning the distribu- 

 tion of fishes within the grass bed 

 itself. 



Densities of fishes are typically 

 greater in grass bed habitat within south 

 Florida's estuaries and coastal lagoons 

 than in adjacent habitats (Reid 195^; Tabb 

 et al. 1962; Roessler 1%S; Yokel 1975a, 

 1975b; Weinstein et al. 1977). Yokel 

 (1975a, 1975b), using a trawl, reported 

 greatest densities of fishes in seagrass 

 meadows as opposed to bare sand and shell 

 bottoms in the Ten Thousand Island region 

 of south Florida. In the Rookery Bay Sanc- 

 tuary, 3.5 times as many fishes were cap- 

 tured in crass as in other habitats 

 (Yokel 1975a). Similar results have been 

 reported in Biscayne Bay (Roessler 1965; 

 Roessler et al . 1974; Thorhaug and Roes- 

 sler 1977). As is true for invertebrates, 

 often highest densities and greatest spe- 

 cies richness of fishes are associated 

 with the red algal complex (Roessler 

 et al. 1974; Thorhaug and Roessler 1977), 

 although this is not necessarily an exten- 

 sive habitat. Clark (1970) in Whitewater 

 Bay observed high densities of fishes as- 

 sociated with patchy shoal grass and the 

 calcareous green alga, Udotea congluti- 

 nata. 



Although it is v/el 1 documented that 

 fishes are abundant over grass within 

 south Florida's estuaries and coastal 

 lagoons (Figure 19), knowledge of vithin- 

 habitat distributional patterns relative 

 to grass bed characteristics (i.e., struc- 

 tural complexity, prey densities) is poor 

 at best. It would seem more often than 

 not that patterns attributable to inverte- 

 brates are assumed in principle to also 

 apply to fishes. Fishes are generally 

 larger and more mobile than invertebrates 

 and the extrapolation may not be valid. 

 In Tague Bay, St. Croix, U.S. Virgin 

 Islands, abundance of coral reef fishes 

 feeding over grass at night exhibited a 

 distributional pattern strongly correlated 

 with habitat complexity as measured by 

 plant biomass and bottom topography 

 (Robblee, in prep.). Fish predators may 

 be responding to grass bed characteristics 

 other than just the grass carpet. 



Some fish commonly utilize inverte- 

 brate fauna found among seagrass (Carr and 

 Adams 1973; Brook 1975, 1977; Adams 1576b; 

 Robertson and Howard 1978). The results 

 of experimental manipulations of predation 

 by exclosure caging have attempted to 

 evaluate the effect predation has in 

 structuring invertebrate populations in 

 seaqrass beds. Exclusion of fish preda- 

 tors usually causes increases in species 

 abundance and densitv (Orth 1977b; Young 

 et al. 1976; Young and Young 1977). If 

 expected increases fail to appear, the 

 abundance of decapod predators probably 

 increased sufficiently to reduce the abun- 

 dance and composition of the other inver- 

 tebrates (Young and Young 1977). 



Plant biomass and invertebrate abun- 

 dance relationships observed in Panamanian 

 grass beds are governed largely by preda- 

 tion mediated by the structural complexity 

 of the grasses (Heck and Wetstone 1977). 

 Numbers of macrobenthic animals increased 

 noticeably in the fall with emigration of 

 fishes from grass beds in Apalachee Bay 

 (Stoner 1980b). Amphipods consumed most 

 frequently by the pinfish were epifaunal 

 (Stoner 1979). In studies by Nelson 

 (1979a) infaunal amphipods were 1.3 times 

 more abundant than epifaunal tube-dwelling 

 amphipods and 4 times more abundant than 

 free-living epifaunal amphipods during 

 the seasonal influx of pinfish. These 

 results reiterate the role predators play 



51 



