heavily fished areas, showed the traditional opposite patterns. Hixon added 

 that Hay's general observation that urchins are often rare where large urchin- 

 eating fishes are abundant is evident on Hawaiian reefs. Where fishing is 

 prohibited on marine reserves, such as Hanauma Bay and Coconut Island on Oahu, 

 urchins appear to be relatively rare. 



Hay stressed that his major point was that many patterns documented on 

 human-impacted reefs may be recent, having prevailed only for the past few hundred 

 years. Because herbivory by fishes may select for different evolutionary responses 

 in algae than herbivory by urchins, attempts to extract evolutionary implications 

 from ecological data gathered on heavily fished reefs is not justified. 



Les Kaufman (New England Aquarium) objected to a number of Hay's assertions, 

 particularly the idea that urchins on heavily fished reefs necessarily are 

 freed from predation by fishes. He took issue with Hay's data which indicated, 

 on one hand, a decrease in grazing intensity with increased depth at Salt River 

 on St. Croix but, on the other hand, an increase in grazing intensity with 

 increased depth at nearby Teague Bay on the same island. Based on his diving 

 experience at these sites, Kaufman felt that both areas supported fishes capable 

 of eating urchins. Thus, other factors besides the overall abundance of fishes 

 may explain the patterns that Hay attributed to fishing pressure. Further, 

 Kaufman felt that Hay failed to consider the importance of juvenile mortality 

 in urchin populations. He suggested that predation by wrasses on juvenile 

 urchins may be considerable on heavily fished reefs, such as Teague Bay (St. 

 Croix) and Discovery Bay (Jamaica), so that fishing pressure on larger fish 

 species may not ultimately affect urchin densities. 



Substantial discussion centered on the adequacy of Thalassia as a bioassay 

 of grazing intensity. Robert Carpenter (University of Georgia) suggested that 

 different measures of grazing could produce different depth profiles of grazing 

 intensity (see also Steneck, 1983). Carpenter felt that the Thalassia technique 

 measured mostly parrotfish grazing and that a more general measure of grazing 

 is provided by counting the number of herbivore bites per unit reef area. Hixon 

 agreed, noting that bite marks by fishes from different families (e.g., parrotfishes 

 vs. surgeonf ishes) can be individually identified and counted on flat substrates 

 (e.g., Hixon and Brostoff, 1983). Morrison also concurred and suggested that, 

 instead of Thalassia , a food readily consumed by all herbivores should be used 

 to measure overall grazing intensity. Based on his own and other's work, 

 Morrison indicated that filamentous algae seem to be preferred by urchins and 

 nearly all herbivorous fishes (especially parrotfishes and damsel fishes). 



Sara Lewis (Duke University) noted that some algal species (e.g., Lobophora 

 variegata ) are differentially susceptible to grazing on different reefs. She 

 suggested that these within-speci es differences may represent geographical 

 variation in the development of plant defensive compounds. Hay agreed, noting 

 that herbivory on a local level possibly could induce the production of chemical 

 defenses. He went on to stress the problem of interpreting data from herbivore 

 food-preference observations. Data on food preferences provide information on 

 the responses of herbivores to different plants, but not necessarily on the 

 selective pressure that herbivores impose upon the plants. Clearly, more 

 information will be required to elucidate the reciprocal interactions between 

 reef herbivores and algae. 



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