Grant Number C-6729 Final Project Report: Fundamental Design Parameters for Artificial Reefs: Interaction of Patch Reef Spacing and Size

Lindberg, William

Artificial reefs are popular among Florida fishermen and designated in Florida Statutes (F.S. Ch . 370.0608) as a fishery enhancement. Florida has the most active artificial reef program in the nation, with nearly 300 built over the past decade and more than half of all reefs Sites in the United States. Reef construction expenditures in Florida since 1987 have exceeded 5 million dollars. However, serious questions were raised by fishery scientists more than a decade ago about the true benefits derived from artificial reefs. At issue was whether artificial reefs actually increase fish production or simply concentrate fish, making them easier to catch. In other words, should artificial reefs be considered a tool in fishery conservation or simply another form of fishing gear? This issue became known as the Attraction-Production question, and ultimately motivated the research project reported here. Researchers are in agreement that this issue cannot be resolved by a direct scientific test at the level of a fish population or fishery stock. Instead, we must come to the truth by a process of elimination, by attempting to disprove the various mechanisms which might explain the abundance of fish at artificial reefs. The Suwannee Regional Reef System (SRRS) was designed and built to test one of the leading hypotheses for how artificial reefs might benefit fish. Our previous work had given rise to a general three-step hypothesis: (I) We assumed that reefs give fish refuge from predation and perhaps from currents. (2) Reef-dwelling fishes that feed on sand-dwelling prey were expected to deplete their prey around reefs, resulting in a negative feedback to the predators' energy budgets, residency times, and local abundances. (3) The patchiness of reef habitat was expected to affect the degree of prey depletion and the intensity of negative feedback, such that smaller, more widely spaced patch reefs may be more favorable to such reef residents than are larger, more closely spaced patch reefs. The 5-year project reported here experimentally tested the effects of patch reef size and spacing on distribution patterns of potential prey and on the abundances or local standing stocks of reef-associated fishes. As such, this project was an initial test of the general hypothesis, and the first phase of a more comprehensive research program. The results reported here support our original hypothesis, but also require some modification to it. Reef habitat patchiness significantly affected the densities of benthic invertebrates living in sand around reefs. Patch reef size and spacing significantly affected the abundance of most fishes that were expected to respond to these reef treatments. Effects of patch reef size on fish abundance had previously been tested by other investigators, but ours was the first experiment to also test patch reef spacing. Reef spacing was a critical factor for testing the hypothesis because of its predicted effects on fish abundance. Of the reef-dwelling fishes that could be tested statistically, six can feed over sand, and of these, four (black sea bass, white grunt, gray triggerfish, and cubbyu) were most abundant on the widely spaced patch reefs, as expected. The other two species (tomtate and sheepshead) were unaffected by reef spacing, which we attribute to unique characteristics of their life histories. Patch reef size significantly affected all of these species except black sea bass and cubbyu. However, a four-fold difference in the amount of reef material generally yielded only a 2-3 fold difference in the number of fish, consistent with what others have found. In addition to reef treatment effects, seasonal differences and trends over time were significant for many species. The need to modify our original hypothesis comes mostly from results for gag grouper, the top resident predator on these reefs, and the primary target for reef fishermen of the region.