Theses and Dissertations

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Defense Date

1999

Document Type

Dissertation - NSU Access Only

Degree Name

Ph.D. Oceanography/Marine Biology

Department

Oceanographic Center

First Advisor

Richard E. Spieler

Second Advisor

James Bohnsack

Third Advisor

Curtis M. Burney

Fourth Advisor

Richard E. Dodge

Fifth Advisor

Peter Sale

Abstract

I conducted a multifactorial field study examining the relative importance of and interactions between reef-based and recruitment processes on small model reefs off the southeast coast of South Florida. Four manipulative experiments were completed using 40, 1m3 model reefs. Reefs were deployed at a depth of seven meters on sandy substrate in a grid pattern 30 meters from each other and a minimum of 30 meters from any natural reef. Ten reefs were permanently assigned within the grid as count-only (CO) reefs and were not manipulated during the study. The remaining 30 reefs were evenly assigned and rotated monthly between three additional treatments. Fishes on each of the these reef treatments were removed monthly with an ichthyocide. Reefs assigned to the clean (CL) treatment were manipulated only by removing all fishes each month. The remaining 20 reefs were assigned to treatments that were designed to add prey refugia and exclude predators. Partially caged (PC) treatment reefs had cage material placed on two sides of the reef while the fully caged (CG) reefs had cage material placed on all four sides of the reef totally excluding predators. Divers using SCUBA recorded fish abundance, size distribution, species richness, and species composition for each reef monthly from April 1995 through October 1996.

Experiment I examined the importance of competition for limited reef-based resources on reefs with identical structural refugia (CO and CL reefs). The removal of resident fishes from the CL reefs did not provide evidence for reef-based resource limitation. Recruit abundance (fishes less than five cm total length (TL)) was not affected by the additional resources available on the CL reefs. Recruit species richness and composition were also not affected by the removal of resident fishes. Experiment 2 examined the importance of post-settlement predation and refuge availability. When prey refugia were added to the reefs excluding predators (CG and PC reefs), the abundance of recruits was significantly increased. Recruit abundance was greatest on the CG reefs followed by the PC reefs, both of which supported significantly greater recruit abundance than the non-caged reefs (CO and CL). Predation did not appear to affect recruit species composition. The eight most abundant recruit species were common on all treatments.

Temporal variations in recruit species abundance and composition was identified. This variable recruitment affected the importance of post-settlement predation and refuge limitation. The relationship between predation and refuge availability was shown to be density-dependant. During times of high recruitment, higher peaks in recruit abundance were seen on the CG and PC reefs than on the CO and CL reefs. When recruitment was low, differences in mean recruit abundance between the treatments were reduced. Recruitment variability appears to be very important in determining species composition on the reefs. Recruit species richness nearly always paralleled recruit abundance.

This study illustrates the importance of the interactions among multiple processes in structuring reef fish assemblages. Recruitment variability and the coupling of postsettlement predation and refuge limitation influenced the structure of fish assemblages on small model reefs off South Florida. In addition to providing specific information on the processes important in this area, this study provides clear evidence that reef fish assemblages are not structured by any single process but by interactions between reef-based and recruitment processes.

Comments

Funded by a grant provided by the Florida Department of Environmental Protection (DEP), Division of Marine Resources, Office of Fisheries Management. Additional funding provided by South Florida Sport Fishing Classic Inc., NSU OC and the Bud Huch Memorial Scholarship.

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