The role of artificial reefs and natural banks in the distribution and abundance of economically important fishes

In the northern Gulf of Mexico, artificial reefs have become an important resource enhancement tool. These structures, along with natural banks, act as fish aggregation sites on a relatively barren continental shelf region that contains little to no vertical structure for fishes to seek refuge. Generally, a lack of fishery-independent studies comparing artificial reefs to their natural counterparts has led to uncertainty surrounding the role of artificial reefs. Furthermore, with increasing legislative regulations on the types of materials permitted for artificial reefs, it is important to examine which materials provide the most benefit. Thus, the goal of this dissertation was to present stakeholders and managers with the tools to understand how artificial reef design, placement, and proximity to other reefs, which are difficult to ascertain, affect population dynamics to guide future reefing projects. In Chapter 2, I used a combination of remotely operated vehicle and hydroacoustic surveys to characterize fish communities at artificial reefs and natural banks and provide absolute abundance estimates of four economically important fish species: Red Snapper, Greater Amberjack, Gray Triggerfish, and Vermilion Snapper. My results show high abundance of reef fish, particularly Red Snapper, Vermilion Snapper, and Greater Amberjack. Additionally, age 2+ Greater Amberjack extrapolated estimates were 221% of the most recent stock assessment for this species and identify a need for reevaluation of the habitat use and abundance estimates for this species. In Chapter 3, I conducted surveys using stereo-remote underwater video surveys to address the effect of prefabricated pyramid reefing location and orientation on fish community structure. Although reefing location nor orientation had a significant effect on species richness, diversity, and evenness, I identified differing communities between both variables (reefing location and orientation) with Red Snapper, Greater Amberjack and Gray Triggerfish among the main contributing species to these differences. Fish measurements identified primarily juvenile individuals which emphasizes the importance of lower relief reefs as habitat for younger cohorts. Absolute abundance estimates were derived from combining the video surveys and hydroacoustic surveys and were used to calculate two types of cost-benefit ratios based on overall fish density and the densities of economically important species such as Red Snapper, Greater Amberjack, Gray Triggerfish, Vermilion Snapper, and Almaco Jack. Port O’Connor nearshore reef was considered the least cost-effective reefing location in terms of overall fish density, but not for economically important species density. Port O’Connor was configured with science in mind and produced the most consistent results compared to the other reefing locations, which highlights the importance of developing a reef with science in mind to evaluate reef benefits more competently. In Chapter 4, I used the same methods from Chapter 2 to address the effect of artificial reefing material (prefabricated pyramids and railroad ties) on fish community structure and cost benefit. Species richness was found to increase with increasing tonnage of railroad ties and is likely due to differences in structural complexity. In contrast, evaluation of the two different types of cost-benefit ratios identified a massive 4000-ton railroad tie pile as the least cost effective in terms of overall fish density and economically important species density. As a result, I do not recommend considerably large piles of reefing material if the goal of the reef is to provide habitat for economically important species, but rather to spread this material in smaller and widely dispersed groupings. Importantly, this study identifies the need for specifying clear objectives when designing artificial reefs to maximize efficiency. In Chapter 5, I developed an individual-based model to evaluate Red Snapper, Greater Amberjack, and four competing species, population responses to changing artificial reefs in more complex, but realistic, habitat scenarios like those observed on the Texas continental shelf. My results identify the importance of artificial reefs in the growth and overall biomass of Red Snapper and Greater Amberjack, particularly in areas, like the Gulf of Mexico, where structured habitats are limited. This model also validated age- or size-based transitions in habitat for Red Snapper that have been observed in other studies, suggesting a transition from habitat-centric to forage-centric movement for this species. This model presents resource managers and stakeholders with a tool to guide future reefing projects to discern the impact of placement and density of artificial reefs on population dynamics of economically important fish species. Collectively, my findings suggest that artificial reefs in the western Gulf of Mexico play an important role in shaping fish communities and in the population dynamics of economically important species. Furthermore, I developed a unique approach to quantifying the efficacy of artificial reefs and revealed two types of reefs that were significantly less cost-effective than the others explored in this study. Future artificial reefing projects should avoid large, highly dense reefs and should consider proximity to other artificial or natural habitats. I also present a tool for resource managers and stakeholders to guide future reefing projects to discern the impact of differing artificial reef designs on populations of economically important species. This dissertation improves the scientific understanding of the role of natural and artificial habitat in fish communities and provides fisheries managers with key abundance estimates needed for stock assessment models for several economically important species on the Texas coast.