ABSTRACT
Fishery productivity and trophodynamics of platform artificial reefs in the northwestern Gulf of Mexico
Carl R. Beaver, B.S., Corpus Christi State University;
M.S., Texas A&M University-Corpus Christi


Nearly 5000 oil and gas structures in offshore waters of the northwestern Gulf of Mexico compose the largest complex of artificial reef structures in the world. Platforms have long been known to increase the biological productivity of sessile attached invertebrates. A portion of this productivity has been assumed to move up the food chain to reef fishes; however, no detailed investigation into the productivity and energy transfer within these systems has been conducted.

Two platforms in the northwestern Gulf of Mexico were studied to: characterize the composition and biomass productivity of their fouling communities and to estimate energy flow between fouling-community elements and selected reef fishes.
Analysis of photographic transect data indicated significant differences in fouling community diversity with depth. East Breaks 165A exhibited five depth zones from the surface to a depth of 53m. High Island 389A displayed three depth zones to 53m.
Only four sessile fouling community organisms and five species of amphipods were found to comprise more than 1% of the diets of any of the reef fish examined. Standing-stock energy values were calculated for all nine fouling community species; values ranged from 0.13 kcal/m2 for Corophium biglovia, to 24,000 kcal/m2 for the bivalve Isognomon bicolor. Annual production for the five amphipods was also calculated. Values ranged from 5.4 gDW/m2 for Corophium biglovia to 43.8 to gDW/m2
for Corophium louisiana. Total annual production for all five species of amphipods was estimated to be 98.1 gDW/m2 or 384.4 kcal/m2.
Three species of reef fish, Paranthias furcifer, Epinephelus adscensionis and Balistes capriscus, were selected for stomach content analysis. Additionally, population levels and biomass were determined for each of these species. Paranthias furcifer was the most common planktivore at either structure. Populations and biomass varied seasonally and between platforms, ranging from a low of 318 (mean weight 0.48 kg) during the winter at HI389A to a summer high of 636 (mean weight0.43 kg) at EB165A.
Populations of Epinephelus adscensionis and Balistes capriscus varied between platforms. Mean population levels of E. adscensionis populations were 301 and 214 while B. capriscus populations were 86 and 167 at EB165A and HI389A respectively.
Stomach contents were weighed and categorized by habitat of origin. Daily and seasonal differences in consumption rates were examined for each species of fish. Electivity indices were calculated for each prey species. Consumption rates were then used to determine daily levels of caloric intake. Daily caloric intake for P. furcifer varied between summer and winter months with a mean daily intake of 42.4 during the summer and 24.9 during winter months (465 g mean fish weight). Epinephelus adscensionis consumed 58.5 kcal/day (496 g mean fish weight) while Balistes capriscus consumed 78.7 kcal/day (646 g mean fish weight).
Using standard bioenergetics conventions, energy flow models were developed for each species. On an average summer day, P. furcifer utilized 17.6 kcal or 42% of consumed energy for metabolism and 16.3 kcal or 38% of consumed energy for growth. During winter P. furcifer utilized 59% of consumed energy for metabolism (14.8 kcal) with 21% (5.1 kcal) being available for growth.
Epinephelus adscensionis used 35% (20.7 kcal) of consumed energy for metabolism and 45% (26.1kcal) for growth while Balistes capriscus dedicated 35% (27.3 kcal) to metabolism leaving 45% (35.6 kcal) for growth. Fish production rate from fouling community diet elements ranged from <0.1 g/(stock·m2·day) for P. furcifer during the winter to 0.7 g/(stock·m2·day) for B. capriscus.
While E. adscensionis and B. capriscus fed almost exclusively upon fouling community and associated organisms, P. furcifer fed primarily upon planktonic organisms throughout the year. Paranthias furcifer diets did show a significant increase in the number of fouling community organisms consumed during winter and spring sampling periods.
Data from examination of fouling and fish communities were used to construct food webs and energy flow diagrams for the artificial reef ecosystem. Results indicate that platform artificial reefs are allochthanous systems obtaining the majority of their energy from the plankton community. Consequently the highest levels of secondary productivity within the fouling community occurred within 30 m of the surface where plankton concentrations were greatest.
Food chains were generally short, containing 4 to 5 nodes and 3 to 4 links with the shortest chains being those containing planktivorous fishes.
This study indicates that the productive potential of platform artificial reefs lies in their ability to redirect energy flow. The filter-feeding organisms of the fouling community function to trap energy from the primary and secondary producers of the plankton community and redirect the flow of this energy into a reef community where it then becomes available to predatory reef and pelagic fishes.