December 23, 2013 | 4
Since July 2012, I’ve been posting about a study of artificial reefs along the Texas coast. Scientists at the Harte Research Institute for Gulf of Mexico Studies in Corpus Christi conducted the research, funded by the Texas Parks & Wildlife Department, to determine whether these structures increase fish populations, and whether their location, type and size matter.
For the most part, the Gulf of Mexico lacks complex habitat; clay, sand or silt with little structure dominate its floor (especially near the coasts). This leaves fishermen and recreational divers dependent on artificial reefs to find much marine life. Texas has one of the largest rigs-to-reef programs in the US, with 140 oil and gas platforms reefed since 1990, but few assessments of these structures (and their economic and ecological importance) had been done previously.
HRI researchers reported preliminary results of the study at their 66th annual meeting of the Gulf and Caribbean Fisheries Institute in Corpus Christi between Nov. 4 and 8.
In one presentation, “Relative abundance and size structure of Red Snapper, Jutjanus Campechanus, across habitat types in the northwestern Gulf of Mexico,” Matthew Streich, Mattew Ajemian, Jennifer Wetz and Greg Stunz reported on the relative abundance and size of red snapper across three different types of habitats: standing oil and gas platforms, artificial reefs and natural banks. Researchers had captured 396 red snapper ranging in length from 282 to 735 mm on vertical longlines between Oct. 2012 and July 2013. Larger hooks caught larger fish, and fish caught on natural and artificial reefs were longer than those hooked on standing platforms (see graph).
Wetz, Ajemian and Stunz also presented “Fish community structure and abundance on Texas artificial reefs: A preliminary assessment.” As previously reported, HRI scientists have been monitoring 15 sites using dive surveys, ROV surveys, and vertical longlines. Snapper species accounted for 26 percent of the total fish abundance at sites surveyed in 2012, with a total of 52 fish species from 18 families identified. Wetz noted that the majority of the monitored reefs are made from decommissioned platform jackets (the part of the structure below the water), but the sites are varied and complex, ranging from two to 12 structures.
One of the study questions is whether and how different structure types affect the fish communities observed. Preliminary analysis of 2013 data suggests that reef type—ships, toppled or cutoff platform—influences the fish community structure and abundance. Toppled platforms had greatest abundance of red snapper, Wetz said, while larger specimens were found around cutoffs. (Anglers will be happy to note that red snapper show up on all types of artificial reefs.) Ninety percent of catches on longlines—which represent fish from deeper water—are red snapper, which could reflect a cutoff’s higher relief. Continued monitoring will help confirm that suspicion. Researchers will also be analyzing ROV data from various sites, comparing fish communities by a reef’s depth, the number and age of structures, proximity to other structures (natural and artificial) and the distance from shore. Ajemnian says they’ll also be looking at how far from an artificial reef site its effects are carried. But anecdotally, he says, “when we’re using longlines, we know we’re not close to the structure anymore when we aren’t catching fish.”
This year the study added three sites for repetitive scuba surveys, which saw more species variety than last year’s ROV surveys. The study has been extended through 2015 and Wetz says these sites will be re-surveyed to see whether that difference holds. In general, though, different survey methods yield different results, so multiple methods at each site are best. (In a previous post, I discussed the challenges of various survey methods.)
Artificial reef programs arose from the unexpected development of marine life on offshore oil and gas platforms, notes HRI director Larry McKinney. The first offshore drilling platform was installed off the coast of Louisiana in 1937. In 2001, there were 4,043 platforms in the northern Gulf. Removal of non-producing platforms began in 1973, and the rate of removal accelerated in 2010; about 200 platforms come out each year and 3,085 were left as of March 5, 2013.
“Around these platforms are the most desirable fish in the Gulf,” McKinney says, referring to red snapper. People started fishing around platforms because that’s where the fish were and the structures were easy to find, especially in pre-GPS days. In 2001, the peak year for platforms, saltwater anglers contributed $621 million in retail sales. In 2006, that number was $981 million, and the full economic impact in Texas alone was $1.7 billion.
Meanwhile, oil and gas production moved farther offshore and from fixed to floating structures. Artificial reefs were seen as a way to fill the void, and data from this study have shown the value of this habitat.
“Artificial reefs do contribute to ecosystem productivity,” McKinney says. “We need to address to what degree—in other words, are artificial reefs fish concentrators or producers.”
Either way, the structures do reduce fishing pressure on natural structures. “There is no place in the Gulf of Mexico that cannot be found or reached with our sophisticated navigation tools and powerful boats,” McKinney says. “To me, the question is not whether artificial reefs are an effective [fisheries management] tool. They’re a critical tool. Recreational fishermen are the largest contributor to conservation funding, through taxes, to the tune of $7.3 billion since 1952. On the conservation side, artificial reefs supplement natural productivity and, more importantly, they reduce pressure on natural reefs. Fishermen are going to go somewhere.”
NOAA researchers are less convinced of the value of artificial reefs for production or achieving habitat functions. GCFI presenter Kristopher Benson, with NOAA’s restoration center in Galveston, agreed that artificial reefs can divert fishing pressure, but said the agency doesn’t consider the issue of production versus concentration to be resolved. “The literature shows reefs function on a spectrum between the two. Whether the services of artificial reefs outweigh those of existing habitat where the reefs will be placed remains unanswered.” Further study is needed, he added, as is research on optimal siting of artificial reefs, appropriate design for specific ecosystem management goals, and the functional development of these reefs over time.
Fortunately, HRI scientists are working to address those very questions. It’s been an interesting couple of years out on the Gulf of Mexico. Thanks for coming along.