ACCESS NUMBER: 30951-17802 

STUDY TITLE: Importance of Zooplankton in the Diets of Blue Runner (Caranx crysos) 

Near Offshore Petroleum Platforms in the Northern Gulf of Mexico 

REPORT TITLE: Importance of Zooplankton in the Diets of Blue Runner (Caranx 

crysos) Near Offshore Petroleum Platforms in the Northern Gulf of Mexico. 

CONTRACT NUMBER: 30951-17802 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Central Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 2000; 2001; 2002 

COMPLETION DATE OF REPORT: February 2003 

COST(S): 

PROJECT MANAGER(S): M.C. Benfield, S.F. Keenan 

AFFILIATION: Louisiana State University, Coastal Fisheries Institute 

ADDRESS: Wetland Resources Building, Baton Rouge, LA 70803 

PRINCIPAL INVESTIGATOR(S): M.C. Benfield, S.F. Keenan 

KEY WORDS: Zooplankton; diet; blue runner; Caranx crysos; offshore petroleum 

platforms; northern Gulf; acoustic techniques; acoustic Doppler current profiler; biology; 

artificial reef; stomach contents. 

BACKGROUND: There are over 4000 offshore petroleum platforms in the northern 

Gulf of Mexico. Large numbers of reef-associated and pelagic fish species are 

associated with these structures, but little is known about the relationships between 

platforms and fish communities. A preliminary study by our group indicated that 

zooplankton were important in the diets of a pelagic fish (Caranx crysos) commonly 

found around Gulf platforms in the summer months. Related research has indicated 

that zooplankton densities may be elevated near platforms, which may provide a trophic 

subsidy to these fish populations. This project was initiated to examine the role of 

zooplankton in the diets of blue runner and evaluate zooplankton spatial and temporal 

distribution near platforms through traditional (net collections) and acoustic techniques. 

OBJECTIVES: (1) To quantify the diets of platform-associated and open water adult 

blue runner; (2) To evaluate the diel feeding periodicity, diel shifts in prey selection, and 

prey size selection of platform-associated blue runner; (3) To evaluate zooplankton 

composition and abundance near two petroleum platforms; and (4) To demonstrate the 
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utility of high-frequency acoustics to visualize the spatial patterns of sound scattering 

particles near platforms. 

DESCRIPTION: The study was conducted on or near mid-shelf platforms located in the 

north-central Gulf of Mexico. The project consisted of platform-based trips (summer 

1996 and 1999) and research cruises (summer 2000) to evaluate the feeding habits of 

blue runner and distribution of zooplankton in waters near platforms. During 1996 and 

1999, sampling was conducted from the Grand Isle 94B (GI94B) platform and the Main 

Pass 259A (MP259A) platform, respectively. Platform-based sampling consisted of 

collecting blue runner by hook and line angling using artificial lures. Stomach contents 

were harvested from each fish and the contents were identified in our Louisiana State 

University laboratory. Number and wet mass of the contents were used to calculate 

dietary indices. A limited number of plankton samples were collected to examine prey 

size selectivity. 

In the summer of 2000, research cruises were made aboard a charter fishing vessel to 

the Viosca Knoll 203A (VK203A) and Main Pass 140A (MP140A) platforms. This vessel 

was equipped with a 1 m2

 plankton net and an acoustic Doppler current profiler (ADCP). 

The use of this vessel provided mobility for sampling in close proximity to the platform 

as well as far field collections of fish and zooplankton. A total of seven trips were made, 

which are summarized in the following table: 

Date Destinatio Objective 

30May–1 VK203A Preliminary cruise, test equipment, identify suitable platforms

5–8 June VK203A Plankton net tows (1 day set, 1 night set), blue runner 

12–16 June MP140A Plankton net tows (2 day, 2 night sets), blue runner 

26–29 June MP140A Plankton net tows (2 day, 2 night sets), blue runner 

17–20 July VK203A Plankton net tows (2 day, 2 night sets), blue runner 

1–3 August VK203A Diel feeding study (n=117), open water blue runner 

6–9 August Open Open water blue runner trip (n=13) 

Blue runner were collected for dietary analysis using identical methods as previous 

sampling and one trip (1–3 August) was dedicated to evaluating the feeding periodicity 

of these fish near platforms. Plankton samples were collected during the day and night 

to examine the distribution of zooplankton in relation to the platforms and examine the 

relative abundance of zooplankton prey available to blue runner. Acoustic surveys were 

conducted around each platform following plankton collections. The ADCP provided a 

qualitative view of the distribution of sound scattering particles (ie. zooplankton) near 

the platforms. 

SIGNIFICANT CONCLUSIONS: Adult blue runner in proximity to offshore petroleum 

platforms feed primarily on meso- and macro-zooplankton during the day and macrozooplankton and fish at night. Blue runner in open waters appeared to forage on similar 

organisms as platform-associated fish, however the sample size (n=14) was limited. 

Feeding periodicity results showed blue runner around platforms fed after dark, which 

may be due to the light field produced from the platform. This nocturnal feeding 
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represents a potential energy subsidy available to fishes near platforms. Plankton 

sampling did not indicate elevated (or reduced) densities of zooplankton near the 

structure, however sampling was conducted at distances over 100m from the structure, 

which may not be close enough to perceive a platform effect. Acoustic surveys 

indicated sound scattering was greater at night than during the day and scattering was 

elevated near the platform (<100m) during some surveys. 

STUDY RESULTS: A total of 323 platform-associated and 14 open-water adult and 

sub-adult blue runner were collected over the course of this study, which is the most 

comprehensive examination of the diets of this species. Adult blue runner were 

previously considered to be piscivorous, however our findings indicate that zooplankton 

(e.g. larval and adult decapods, larval stomatopods, hyperiid amphipods, and larval fish) 

comprise a large proportion of their diet. Feeding periodicity results found the blue 

runner were capable of feeding throughout the night at the same intensity as the day. 

Size selectivity results indicated that blue runner consumed larger prey such as fish 

during the night and smaller decapod crustaceans were consumed during the day. 

A total of 168 plankton net tows were conducted; 84 during the day and 84 at night. All 

plankton tows were conducted between 100–1000m of the respective platform. 

Although certain zooplankton taxa exhibited increased densities near the platform 

during some cruises, plankton net sampling did not demonstrate consistently elevated 

densities of zooplankton in proximity to petroleum platforms. Zooplankton density was 

generally greater during the night samples, however a significant platform enhancement 

effect was not observed. 

Within each cruise, positive relationships existed between volume scattering strength 

and zooplankton density and wet mass for many taxa indicating that zooplankton was 

the primary source of sound scattering recorded by the ADCP. Results of four acoustic 

surveys were completed for this report. These surveys demonstrated the utility of an 

ADCP to examine small-scale patterns in sound scattering layers in 

surface waters around platforms, however preliminary analysis did not establish that 

elevated (or reduced) levels of scattering existed near platforms. 

STUDY PRODUCT(S): Keenan, S.F. 2002. The Importance of Zooplankton in the 

Diets of Blue Runner (Caranx crysos) Near Offshore Petroleum Platforms in the 

Northern Gulf of Mexico. Master’s Thesis. Louisiana State University. Baton Rouge. 

Keenan, S.F. and M.C. Benfield, 2002 Zooplanktivory by Blue Runner: An Energetic 

Subsidy to Gulf of Mexico Fish Populations at Petroleum Platforms. Pages 499-513 in

M. McKay, J. Nides and D. Vigil editors, Proceedings: Gulf of Mexico Fish and 

Fisheries: Bringing Together New and Recent Research, October 2001 U.S. Dept. of 

the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, 

La. 

Keenan, S.F., M.C. Benfield, R.F. Shaw and H.L. Haas, 2000. Importance of 

Zooplankton to Platform Productivity. Pages 33-41 in M. McKay and J. Nides editors, 
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Proceedings: eighteenth annual Gulf of Mexico information transfer meeting, December 

1998. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS 

Region, New Orleans, La. OCS Study MMS 2000-030. 

Keenan, S.F., M.C. Benfield, and R.F. Shaw. In press. Zooplanktivory by blue runner: 

an energetic subsidy to Gulf of Mexico fish populations at petroleum platforms. 

Transactions of the American Fisheries Society special publication. 
ACCESS NUMBER: 30951-17802