ACCESS NUMBER: 30660-19926 

STUDY TITLE: Across-Shelf Larval, Postlarval, and Juvenile Fish Collected at Offshore 

Oil and Gas Platforms and a Coastal Rock Jetty West of the Mississippi River Delta 

REPORT TITLE: Across-Shelf Larval, Postlarval, and Juvenile Fish Collected at 

Offshore Oil and Gas Platforms and a Coastal Rock Jetty West of the Mississippi River 

Delta 

CONTRACT NUMBER: 14-35-0001-30660-19926 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Central Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1995-1997 

COMPLETION DATE OF REPORT: March 2002 

COSTS; FY 1995 $126,681; FY 1996 $119,034; FY 1997 $196,198 CUMULATIVE 

PROJECT COST $441,193 

PROJECT MANAGERS: Richard F. Shaw 

AFFILIATION: Coastal Fisheries Institute, Louisiana State University 

ADDRESS: Coastal Fisheries Institute, Louisiana State University, Baton Rouge, LA 

70803 

PRINCIPAL INVESTIGATORS: F.J. Hernandez, Richard F. Shaw, Joseph S. Cope, 

James G. Ditty, Mark C. Benfield, and Talat Farooqi 

KEY WORDS: Northern Gulf, juvenile fish assemblages, artificial reefs, rock jetties 

BACKGROUND: Oil and gas platforms can enhance fisheries by providing attachment 

substrate for habitat-limited sessile invertebrates, thereby creating food and habitat for 

reef-dependent species that are trophically-dependent on sessile and motile 

invertebrates associated with reefs. Since reef fish assemblages are among the most 

diverse and taxonomically rich in the aquatic biosphere, platform communities may 

significantly enhance biodiveristy. In addition, oil and gas structures may offer refugia 

for species that are trophically-independent of the biofouling community, but are 

ecologically-important resident, seasonal, or transient members of the hard substrate 

fish community. The extensive range of this artificial substrate may also serve as 

migratory routes for tropical and subtropical species. 

OBJECTIVES: (1) To provie information on the role that oil and gas platforms may play 

as nursery/recruitment grounds and/or refugia for postlarval and juvenile fish, which 

could contribute to fish production. (2) To respond to specific requests for more basic 
ACCESS NUMBER: 30660-19926 

biological information on reef fish, e.g., larval, postlarval, and juvenile taxonomy, 

seasonality, lunar periodicity, distribution, and relative abundance. (3) To evaluate the 

ecological significance that this artificial habitat building, which has occurred on an 

unprecedented scale in the north central Gulf, may have had on the early life history 

stages of fish. 

DESCRIPTION: Ichthyoplankton and juvenile fish assemblages were sampled at three 

petroleum platforms and a coastal rock jetty, which served as a low salinity, artificial 

habitat end-member. Mobil’s Green Canyon (GC) 18 was sampled monthly during new 

moon phases over a 2-3 night period during July 1995-June 1996. Mobil’s Grand Isle 

(GI) 94B was sampled twice monthly during new and full moon phases over a threenight period during April-August 1996. Exxon’s South Trimbalier (ST) 54G was sampled 

twice monthly during new and full moon periods during April-September 1997. The 

stone rubble jetties at the terminus of Belle Pass near Fourchon, Louisiana were also 

sampled over a two night period in 1997 simultaneously with the sampling of ST 54. At 

the platforms, fish larvae and juveniles were collected within the platform structure using 

passive plankton nets and light traps fished at depth (approximately 20 meters) and 

near-surface, and about 20 meters downstream of the platforms with a light-trap floated 

downstream on the surface. At Belle Pass, fish larvae and juveniles were collected with 

light-traps deployed at the surface within two meters of the rock walls and with a bowmounted plankton pushnet fished along the length of the jetty. 

SIGNIFICANT CONCLUSIONS: This study represents the first comprehensive look at 

the ichthyoplankton and juvenile fish assemblages collected within oil and gas platforms 

in the northern Gulf of Mexico. It is also a first attempt at comparing such assemblages 

across different depth zones and geographical regions. It is apparent that a diverse 

larval and juvenile fish community is supplied to, and/or inhabits the waters near the 

platforms and that these structures may be important to reef fish feeding or population 

dynamics. From a management perspective, fish early life history data from a crossshelf study of petroleum platforms could provide useful information in deciding the future 

placement of artificial structures and in determining whether or not the platforms serve 

as nursery areas/refugia for reef species. Based on our results, two obvious 

conclusions stand out, the peak in taxonomic richness and diversity at the mid-shelf 

platform (GI 94) and the relatively low abundance of reef-associated and reefdependent postlarvae and juveniles present at the platform. 

STUDY RESULTS: In general, while reef-associated and reef-dependent taxa were 

collected at all platform sites, taxonomic richness and diversity was highest at GI 94 

(mid-shelf). Due to the pelagic nature of most reef-dependent eggs and larvae, 

dispersal in the oceanic environment plays a large role in the eventual settlement and 

recruitment of postlarvae and juveniles to adult environments. While some studies 

have determined mechanisms of larval retention in reef environments, it is widely 

believed that recruitment is variable and dependent, in part, on the supply from nearby 

reefs. Off the coast of Louisiana most oil and gas platforms are concentrated along the 

inner and mid-shelf within the 200-meter isobath. At GI 94, the intermediate location, 

depth, and proximity to a high density surrounding platforms may create generally 
ACCESS NUMBER: 30660-19926 

favorable conditions for the recruitment of reef taxa. The presence and proximity of 

upstream reefs and spawning habitats, therefore, may play an important role in the 

eventual makeup of the pre-adult assemblages. 

The fact that we collected relatively few individuals of reef-dependent and reefassociated taxa, particularly lutjanid and serranid specimens, is not surprising for 

several reasons. First of all, due to the high mortality rates experienced by pelagic 

larvae prior to settlement, reef-dependent juveniles are relatively rare in general. This, 

coupled with potentially high predation rates at the settlement site itself, may result in a 

very low abundance of juveniles available for capture. Secondly, recruitment events for 

these taxa can be extremely episodic, with most reef fish replenishing over the course 

of 1-3 nights. Although we targeted peak times of settlement and recruitment (new and 

full moon periods), it is still very possible that we missed settlement peaks through the 

course of the study. 

The abundance of postlarval and juvenile synodontids and scombrids near the platform 

suggests that predation pressure is high. This may also contribute to the relatively low 

numbers of reef-dependent juveniles collected, since most syonitids and scombrids are 

piscivorous as early as the postlarval stage. The high numbers of piscivorous juveniles 

collected in our study, primarily with light-traps, indicate that predation is important in 

determining local reef assemblages. 

While oil and gas platforms may be very suitable habitat for adult fishes, the physical 

structure of these artificial reefs may not be ideal for settling postlarvae and juveniles. 

Previous studies have shown that smaller reefs tend to hold a greater cumulative 

number of total and resident species, higher fish densities, and more settlers. The 

higher carrying capacity and settlement success of smaller reefs is probably a function 

of their: 1) greater edge effect (higher ratio of perimeter to reef area); 2) lower vertical 

relief which often favors juvenile over adult reef fish; and 3) greater availability of small 

shelter holes, or porosity, which has been repeatedly shown to be important for postsettlement survival. Petroleum platforms, in contrast, are large reefs and are generally 

characterized as having a higher profile (high vertical relief), less complexity, and lower 

porosity than natural reefs. 

STUDY PRODUCTS(S): Hernandez, F.J., Jr., R.F. Shaw, J.S. Cope, J.G. Ditty, M.C. 

Benfield, and T. Farooqi. 2001. Across-shelf Larval, Postlarval, and Juvenile Fish 

Communities Collected at Offshore Oil and Gas Platforms and a Coastal Rock Jetty 

West of the Mississippi River Delta. OCS Study MMS 2001-077. Prepared by the 

Coastal Fisheries Institute, Louisiana State University. U.S. Department of the Interior, 

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 168 pp.