Figure 19. Small grouper (Serranidae) foraging in seagrass bed. 



in controlling abundances and species com- 

 position within sea grass beds (Nelson 

 1979a; Stoner 1979). 



Little is known about how fishes 

 respond to the structural complexity of 

 the grass canopy. Noting the size distri- 

 bution of fishes typically inhabiting sea- 

 grass beds, Ogden and Zieman (1977) specu- 

 lated that large predators, such as bar- 

 racudas, jacks, and mackerels, may be 

 responsible for restricting permanent 

 residents to those small enough to hide 

 within the grass carpet. For fishes larger 

 than about 20 cm (8 inches) the grass bed 

 can be thought of as a two-dimensional 

 environment; these fishes are too large to 

 find shelter within the grass carpet. 

 Mid-sized fishes (20 to 40 cm or 8 to 16 

 inches) are probably excluded from the 

 grass bed by occasional large predators. 

 Mid-size fishes are apparently restricted 

 to sheltered areas by day and may move 



into the beds at night when predation is 

 less intense (Ogden and Zieman 1977; Ogden 

 1980). The size of the individuals in 

 these groups is a function of the length 

 and density of the grass beds. In Flor- 

 ida, where the seagrasses are typically 

 larger and denser, the grass beds offer 

 shelter for much larger fish than in St. 

 Croix, where the study of Ogden and Zieman 

 (1977) was done. 



Heck and Orth (1980a) hypothesized 

 that abundance and diversity of fishes 

 should increase with increasing structural 

 complexity until the feeding efficiency of 

 the fishes is reduced because of interfer- 

 ence with the grass blades or because 

 conditions within the grass canopy become 

 unfavorable (i.e., anoxic conditions at 

 night). At this point densities should 

 drop off. Evidence indicates that feeding 

 efficiency does decline with increasing 

 structural complexity. 



52 



