Texas Great Barrier Reef

The Relative Importance of Recruitment, Attraction and Production of Reef Fishes on Natural and Modular Artificial Reefs
Researchers: Lead researcher: Michael McGowan. Others: Patrick J. Walsh, James A. Bohnsack and William J. Richards.

Project Number and Duration: R/LR B-22, 1987 - 1989

Question: This project sought to address several questions surrounding artificial reefs. These included 1) Does the size of the reef make a difference in how many fish settle on the reef versus how many visit and then move on? 2) How do the fish assemblages on similar-sized and natural reefs differ? 3) How many and how diverse are the fish larvae that occur near the reef, and how many larvae settle on the reef?

Project: Researchers built 50 identical concrete artificial reef blocks -- essentially, large (2950-kg. or 5700 lb.) square blocks with two holes in each wall -- and sunk them in varying small- to large groups about 6.4 kilometers (four miles) east of Key Biscayne in Dade County. They also monitored a much larger, natural live-bottom reef nearby. The researchers' methods varied depending on their goals:

1) To probe the impact of reef size on visiting and resident fishes, divers counted, identified and measured fishes using standard visual methods, including counting large fish as they approached the reef and using small mirrors to peek into the reefs' openings and count smaller fish. They classified resident fishes as fishes seen on more than one census or fishes known to be reef residents and visiting fishes all fishes spotted only during one census (or that appeared to be using the reef only for temporary shelter or feeding.) Fish classified as transients were fish that moved away during sampling and appeared not to react to the reef.

2) To determine the role of the artificial reef in attracting larvae, the researchers towed a specially made 1-meter net behind a boat. Fastened to a "sled" made of PVC pipe, the drag of the net caused it to rise slowly to the surface, so that the water column was sampled as uniformly as possible. The researchers made multiple tows, then identified captured larvae in the laboratory.

Results: Researchers found that the largest proportion of fishes (65 percent) that appeared on the reef were residents, while 20 percent were visitors and 16 percent were transient. Generally speaking, the larger the reef, the more fish arrived to colonize it. These included several species valued commercially or recreationally, including yellowtail snapper, Ocyurus chrysurus, and snowy grouper, Epinephelus niveatus, although these fish tended to disappear from the reef after a period of time. Other commercially important species also colonized the reef, including hogfish, Lachnolaimus maximus, lane snapper, Lutjanus synagris, and mutton snapper, Lutjanus analis. Overall, the researchers found that the number and weight of fish on artificial reefs was far higher than on similar-sized natural reefs.

In 16 one-day cruises and a total of 96 samples, researchers also collected 41,821 fish larvae from the area within the artificial reef. Perhaps most interestingly, they found that the amount of larvae varied in direct proportion to the amount of zooplankton in the water during the previous month - in other words, spawning and survival of fish larvae directly followed the availability of plankton for forage. The larvae of herrings, the family Clupeidae, jacks, the family Carangidae, clingfishes, the family Gobiesocidae, and gobies, the family Gobiidae, tended to dominate, while larvae of coral reef species, were few and far between.

Impact: This study provided quantitative data on the impacts of artificial reefs that will prove useful for reef planners and managers.

Publications: Articles about this research appeared in The Bulletin of Marine Science and a book, Habitat Structure: The Physical Arrangement of Objects in Space.