Growth and feeding ecology of groupers on artificial and natural reefs in the northern Gulf of Mexico Charlotte H. Rivas, Lynne Wetmore, Jay Rooker (Undergraduate) Department of Marine Biology, Texas A&M University at Galveston. ♦ Abstract

The main objective of this study was to understand the habitat value of three different reef types (coral reefs, clay banks, oil platforms) for demersal fishes in the Gulf of Mexico. Two ecological components of habitat quality, growth and diet, were assessed for two species of grouper: graysby (Cephalopholis cruentata) and rock hind (Epinephelus adscensionis). Otolith microstructure analysis was used to determine age, and age-length relationships were developed to estimate growth rates. Diet was determined by conventional gut content analysis. Growth rates obtained for graysby were similar between clay banks and oil platforms, but significantly lower for individuals from coral reefs. Growth rates for rock hind were equivalent between clay banks and oil platforms; however, this species was rarely observed at the coral reef sites; only three specimens were collected. Percent gut fullness was lowest for graysby and rock hind collected from coral reefs. Diets of graysby and rock hind from oil platforms and clay banks contained mainly small fishes, while prey species of grouper on coral reefs were primarily invertebrates. Results from this study indicate that the diverse reef types may serve different functional roles for demersal fishes in the northern Gulf of Mexico.

♦ Introduction ♦ Results ♦ Gut Content Analysis

400 Fig 4. Dietary analysis Hard bottom reef habitats play a significant role in the preservation of demersal fish Southeastern USA Fig 2. Age-length relationship for Graysby for graysby. populations, but they constitute less than 3.0% of the total area in the northwestern Gulf of Graysby (Potts & Manooch 1999) graysby. The percentage of fish Mexico (GOM) (Parker et al. 1983); hence, understanding the ecological value of these Growth rates were similar between clay 350 with full guts (≥1 prey item banks and oil platforms, but significantly habitats is important to maintain integrity of fish communities. Diet and growth are two in stomach) (Fig 4A) was slower for graysby at coral reefs ecological components that are often used as proxies to evaluate habitat quality for marine Clay Bank Curacao lowest at coral reefs (ANCOVA, P < 0.05). Thus, fishes could 300 Coral Reef (Nagelkerken 1979) (25%) but higher at clay fishes. Thus, the main objective of this study was to characterize the relative habitat value of not be pooled across habitats in growth banks (41%) and oil two natural reef types (coral reefs, clay banks) and one artificial reef type (oil platforms) in the Platform models. Gulf of Mexico by comparing the growth and dietary composition of two small grouper platforms (67%). species: graysby (Cephalopholis cruentata) and rock hind (Epinephelus adscensionisa). 250 The von Bertalanffy model was used to create an age-at length curve for graysby There were also marked Results were also compared to previously published growth models from other geographic at coral reefs, and growth at this reef type differences in prey item locations in order to assess regional variability in growth for these two grouper species. 200 in the GOM was substantially slower than composition among Northern Gulf of Mexico predicted by the two previously published habitats (Fig 4B). Diet was similar between clay (GOM) models for graysby in other regions. banks and oil platforms, Reef habitats available to demersal fishes in the GOM 150 (Coral reef only) Because of limited samples, separate with 100% of graysby at

Total length (mm) Total -0.17(t + 1.00) Lt = 248(1 - e ) growth curves were not created for clay these reef types Coral Reef Coral zone Clay Bank Coral Oil Platform banks and oil platforms. However, in the containing only fish in Zone 100 size range available, growth of graysby at their stomachs. However, these two reef types in the GOM at coral reefs, the majority appeared similar to graysby in other of graysby (60%) Coral zone 50 regions. contained only invertebrates in their Results indicate that growth of graysby stomachs, while a may differ significantly among habitats in 0 minority contained only the GOM, and fishes at coral reefs may Clay/Siltstone 0 2 4 6 8 10 12 14 fish (20%) or a grow substantially slower than graysby in combination of fish and Age (years) other regions. invertebrates (20%). • Large reef crest area • Smaller reef crest • Small area of coral/ sponge • Uniform, high (50-60%) • Limited coral /sponge cover cover on crossbeams coral cover (mainly clay flats) • Located throughout • Located shelf-edge • Located mid-shelf continental shelf 450 Fig 3. Age- length relationship for rock Fig 5. Dietary analysis Rock Hind Southeastern USA hind. Rock for rock hind.

400 (Potts & Manooch 1995) The percentage of fish No significant differences in growth were Hind with full guts (Figure A) Clay Bank detected among habitats (ANCOVA, P > was 0% at coral reefs, but 0.05), and fishes from all three reef types 350 Coral Reef substantially higher (≈ ♦ Materials & Methods were pooled to create a single age-at- Platform 42%) at both clay banks length curve using the von Bertalanffy and oil platforms. 300 growth model. Field collections: In 2009-2010, graysby (n = 65) and rock hind (n = 142) Prey item composition were collected from: Growth of rock hind at all three reef types was similar between clay 250 in the GOM appeared to be substantially banks and oil platforms. • 2 coral reefs (East & West Flower Garden Banks) slower than predicted by the only The majority (55-56%) of previously published growth model rock hinds collected at • 2 clay banks (Sonnier Bank, Stetson Bank) 200 both of these reef types Northern Gulf of Mexico • 4 oil platforms (HI-389, VR-315A, VR-331A, VR-340A) Results indicate that growth of rock hinds, contained only fish in their (GOM) although similar among habitats, may vary stomachs, while a minority Total length (mm) Total 150 (All habitats pooled) substantially from region to region. contained invertebrates -0.46(t + 0.66) (21-22%) or fish and Lt = 212(1 - e ) invertebrates (22-24%). Figure 1. Whole otolith with 100 Otolith microstructure analysis: SUMMARY yellow lines showing sectional SUMMARY cuts (A) and transverse otolith •Graysby: growth at coral reefs was • A single otolith was chosen randomly from each fish, 50 •Percent gut fullness was section (B) and detail (C) from a significantly slower than at other reef types embedded in epoxy resin and sectioned using a lowest at coral reefs 240 mm graysby (age =11 years) in the GOM, and also slower than graysby Beuhler Isomet saw collected from East Flower 0 in other regions •The primary prey item at Gardens Bank. Arrows denote coral reefs was inverts, but • Otolith sections were mounted on glass slides, and 0 1 2 3 4 5 6 7 8 •Rock hind: growth at all three reef types annual growth increments the primary prey at clay annual growth increments (annuli) were enumerated in the GOM was substantially slower than (annuli). Age (years) banks and oil platforms was in other regions under a compound light microscope using the software fish Image Pro Plus A B

Gut content analysis: ♦ Discussion & Conclusions

• Stomachs were dissected under a microscope, and gut Results from this study indicate that different reef types may fill different ecological roles for groupers in the GOM, possibly as a function of food availability. Growth rates for graysby were similar at clay banks and oil C Age 1 fullness was recorded for each fish: Empty (0 prey Age 2 platforms but significantly slower at coral reefs, and it appears that this observed variability in growth may be driven by dietary differences among habitats. Gut fullness was lowest at coral reefs, potentially indicating lower items) or Full (≥ 1 prey item) Age 3 foraging efficiency, and while the diets of graysby at both clay banks and oil platforms were composed entirely of fish, the primary prey item at coral reefs was invertebrates, which typically have a lower nutritional value. Age 4 Like graysby, growth rates of rock hind were similar between clay banks and oil platforms; however, this species was extremely rare at coral reefs, where only three specimens were collected, and all three of these fish had • All prey items were identified to the lowest possible empty stomachs. Although much of the conservation and management efforts for demersal fishes in the GOM have focused on coral reefs, the increased growth rates and dietary differences observed at clay banks and oil taxon, and each fish was assigned to a dietary Age 11 platforms emphasize the potential importance of these two reef types as essential habitat for groupers in the region, particularly for rock hind, which was only found almost exclusively at these two habitats. category: “Fish”, “Fish/Invert”, or “Invert,” based on prey item composition The marked differences in growth for both graysby and rock hind in the northern GOM compared to previously published models from other geographical locations (i.e. southeastern USA, Curacao) indicate that regional variability in growth for these two grouper species may be substantial. If this is the case, effective management of these species will require region-specific life history information in order to develop accurate fishery models, and the growth models presented here may represent valuable baseline data for graysby and rock hind in the northern GOM. ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦