INTRODUCTION:

In recent years, global deployment of artificial reefs has occurred mainly with the intention of enhancing fisheries and recreational activities. The large majority of deployments have been performed without regard to how the space isolating artificial reefs from one another affects abundance and species richness of the associated fish assemblages. This study addressed two questions: 1) given a certain limited amount of reef material and deployment area, what spatial configuration of reef material, in terms of isolation distance, yields the greatest fish abundance and species richness and 2) is abundance and species richness correlated with the amount of reef material available in a single reef?

MATERIALS AND METHODS:

This study, performed offshore Fort Lauderdale, Florida, USA (26?07N, 80?05W) in approximately 8m of water, used concrete reef modules (~ 1m³) (Photo 1) placed at the apices of four differently sized equilateral triangular configurations. These treatments had sides of 25m, 15m, 5m, and 0.33m, each with two replicates (Figure 1).

In addition to these four triangular treatments, two configurations represented by a solitary module (Single) and two modules separated by 0.33m (Double), also with two replicates, were used to examine the effects of increasing reef material on fish assemblage structure. Upon completion of module placement, all fishes were removed using an ichthyocide. Twenty-two monthly data collections (in a 24 month period) were performed using a visual sampling technique in which a SCUBA diver counted and recorded all fishes present within one meter of the modules. Each fish was placed into one of five size classes: < 2cm, > 2-5cm, > 5-10cm, > 10-20cm, and > 20cm total length (TL). Analysis (ANOVA) of abundance and species richness values were executed using the PROC GLM procedure and Student-Newman-Keuls comparison of means (SAS Institute Inc., Cary, NC, USA).

RESULTS:

In the twenty-two monthly censuses performed, a total of 37939 fishes from over 80 species was recorded. Statistically significant differences (p<0.05) among triangular treatments were present for abundance and species richness (Figure 2). The 25m and 0.33m treatments exhibited significantly higher abundance and species richness values than both the 5m and 15m treatments. Though no significant differences occurred between the 25m and 0.33m treatments for abundance or richness, the 25m treatment exhibited the highest mean abundance value while the 0.33m treatment recorded the highest mean richness value. Comparison among individual modules from triangular treatments with the Single module treatment (representing the highest amount of isolation) indicates significantly higher abundance values occur when isolation space increased (Figure 3). Likewise, species richness also increased as isolation space increased. Examination of Figure 4 shows that by increasing the amount of reef material both abundance and richness increased significantly.

CONCLUSIONS:

The results of this study highlight the importance of small-scale (<25m) spatial dynamics on the structure of reef fish assemblages. Spacing, at the scale of this study, has the potential to affect recruitment and early post-recruitment processes, benthic forging, prey refugia, microhabitat availability, and other reef-based ecological processes. The results indicate, with small artificial reefs, increasing spacing between reefs may offer the optimal configuration for acquiring the most abundant and likely the most diverse fish assemblages within a given area. Note that abundance and richness increased with increasing spacing (the Single module was >35m from any other module) (Figure 3). Interestingly, no significant differences in abundance and richness were found between the 0.33m and 25m configurations (Figure 2). This suggests that reefs spaced further apart may exhibit characteristics of several solitary reefs while the 0.33m treatment acted as a single, albeit larger patch reef. Furthermore, comparisons between the Single, Double, and 0.33m treatments revealed, on this particular scale, the amount of reef material significantly altered fish abundance and species richness but lacked a direct linear relationship. That is, doubling and tripling the amount of reef material did not double and triple the abundance or richness of the associated fish assemblages.

ACKNOWLEDGEMENTS:

We thank personnel from the Broward County Department of Planning and Environmental Protection, as well as, all the volunteer graduate students from NSUOC for technical assistance. This research was supported by Grant C-8400 from the Florida Department of Environmental Protection and Broward County Department of Planning and Environmental Protection.

Back to NCRI Projects Page