Small-scale habitat interactions for Gulf of Mexico reef fishes over natural and artificial reefs
Thompson-Grim, Jill Catherine
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Red Snapper (Lutjanus campechanus) is the most economically important fish in the Gulf of Mexico (GOM). The species is a prominent long-lived, reef-associated finfish and commonly inhabits natural banks and artificial reefs. However, reef habitat in the northwestern GOM is limited considering less than 3% of the continental shelf is composed of substrate with vertical relief greater than 1 m. Therefore, reef habitat when present, generally aggregates fishes and may serve as hotspots of abundance and diversity. The goal of this thesis was to evaluate previously unknown small-scale habitat driven effects on fish abundance and community structure on high relief areas in the northwestern GOM. Chapter 1 examined the relationship between fish density and geospatially identified relief anomalies (i.e., biotope and slope) at the South Texas Banks (STB). Densities were greatest at the crest in regions where 5° to 20° changes in vertical relief were observed. Fish community composition was similar across biotopes but varied depending on the magnitude of the nepheloid layer (i.e., layer of resuspended sediment). ROV surveys revealed high relative abundances of federally managed species (45.5%) like Red Snapper (10.8%) in highly turbid environments and highlighted previously unknown dynamics and importance of the STB. In Chapter 2, I evaluated the optimal reef site design of decommissioned oil and gas platforms in the northwestern GOM. Data indicated a linear relationship between fish abundances of nearly all federally managed species and quantity of platform structures within a reef site. Benefit-cost analysis suggested reef sites with the highest density of adjacent platform structures had the highest fish abundances and long-term benefit. vi For Chapter 3, remotely operated vehicle (ROV) and hydroacoustic (echo sounder) surveys were run concurrently at artificial and natural reefs in the Texas region of the northwestern GOM to determine if species-specific density estimates depended on the survey method used. I also assessed the value of adding a rear-facing camera to an ROV to evaluate relative abundances of federally managed species. Data suggested that gear efficiency (i.e., survey method and camera placement) was specific to species and habitat of interest. Red Snapper density at both reef types was best evaluated using hydroacoustic surveys with species composition derived from paired forward and rear-facing cameras. I estimated the absolute abundance of Red Snapper with this method and found 1,058,051 (CV = 29%) Red Snapper at artificial reefs and 6,118,605 (CV = 38%) Red Snapper at natural banks within the region. Studies presented in this thesis assess the variation of fish density and community structure within a reef (i.e., natural bank relief anomalies and artificial reef site configuration). Collectively, findings indicate abundances of Red Snapper and other federally managed species can be enhanced through creation of artificial reef sites containing the maximum number platform structures and through protection of natural banks within the STB characterized by high amounts of crest area with 5° to 20° changes in vertical relief. I provide suggestions to habitat and fisheries managers about methods to optimize surveying Red Snapper as well as enhancing populations at reef habitats in the northwestern GOM.