HYPOXIA IN LOUISIANA COASTAL WATERS DURING 1983: 



IMPLICATIONS FOR FISHERIES 



Maurice L. Renaud 1 



ABSTRACT 



Hypoxic bottom water (<2.0 ppm dissolved oxygen) was present in shallow (9-15 m) waters south of cen- 

 tral Louisiana in June and July 1983. It was patchy in distribution from south of Barataria Pass to south 

 and west of Marsh Island. Data suggested that bottom water hypoxia did affect the abundance and distribu- 

 tion of shrimp and bottomfish. Offshore bottom water dissolved oxygen was significantly correlated with 

 1) combined catches of brown and white shrimp (r = 0.56, P < 0.002), 2) fish biomass (r = 0.56, P < 

 0.001), and 3) vertical density gradient (r = -0.73, P < 0.001). Several hypoxic stations were in regions 

 designated as potentially hypoxic through a posteriori analysis of satellite data. Micrapogonius undulatus 

 was the dominant fish species nearshore and offshore Penaeus aztecus and P. setiferus were sparsely 

 distributed throughout the study area. 



The presence of bottom water hypoxia (<2.0 ppm 

 dissolved oxygen) in the nearshore Gulf of Mexico 

 is a common, recurring, and mostly seasonal (June- 

 August) event. It is generally thought to be associ- 

 ated with temperature and salinity stratification ini- 

 tiated by freshwater runoff and with phytoplankton 

 blooms during hot, calm weather (Fotheringham and 

 Weissberg 1979; Bedinger et al. 1981; Comiskey and 

 Farmer 1981; Turner and Allen 1982a, b; Boesch 

 1983; Leming and Stuntz 1984). Phytoplankton 

 respiration and decomposition of sinking organic 

 matter are major oxygen consuming processes. High 

 oxygen demand of the organic load in freshwater 

 runoff (Gallaway 1981) and lack of a direct oxygen 

 replenishing mechanism (strong winds) in the pres- 

 ence of vertical stratification contribute to hypoxia 

 formation (Harris et al. 1976; Ragan et al. 1978; 

 Swanson and Sindermann 1979; Harper et al. 1981). 

 Christmas (1973) and Boesch (1983) discussed possi- 

 ble nitrate pollution in rivers and coastal hypoxia. 

 Boesch (1983) presented a brief history of hypoxia 

 in the Gulf of Mexico and evaluated its causes and 

 consequences. The extent to which any factor is in- 

 volved with hypoxia formation is unknown. 



Hypoxia in the Gulf of Mexico has been most 

 noticeable in shallow (<20 m) Louisiana waters. It 

 has been reported infrequently on the Texas shelf 

 (Harper et al. 1981; Gallaway and Reitsema 1981). 

 Low oxygen levels have also been measured east of 

 the Mississippi River Delta inshore of barrier islands 



'Southeast Fisheries Center Galveston Laboratory, National 

 Marine Fisheries Service, NOAA, 4700 Avenue U, Galveston, TX 

 77550. 



and in inland bays (May 1973; Christmas 1973) and 

 offshore of Mobile Bay, AL (Turner and Allen 1982b). 

 Abnormally high concentrations of moribund fish 

 and crustaceans near the shoreline ("jubilees") in 

 Alabama have also been linked to hypoxia (May 

 1973). 



Considerable interest in hypoxia has been renewed 

 by a less than average shrimp harvest in 1982 (Klima 

 et al. 1983) and 1983 2 . In this paper I report the loca- 

 tions and extent of Louisiana coastal hypoxia in 1983 

 and discuss the interrelationships of fish and shrimp 

 abundance and distribution with environmental 

 parameters. 



METHODS 



Nearshore data were collected in a 7.3 m Aqua- 

 Sport at a total of 56 stations from nine transects 

 west of the Mississippi River Delta (long. 89°33'W 

 to 90°14'W) from 1 to 16 June 1983 (Fig. 1). The 

 transects, perpendicular to shore, ranged from 5 to 

 8 km in length and 1 to 16 m in depth. The six east- 

 ernmost transects were sampled twice, with a sam- 

 pling interval of 14 d. Shrimp and bottomfish were 

 collected at 23 of 56 stations in 15-min tows with 

 a 3.0 m box trawl. Towing speed was about 3 kn. 

 Before each tow, water temperature, salinity, and 

 dissolved oxygen concentration were recorded at 1 

 m depth intervals with a Hydrolab 8000. Hydro- 

 graphic profiles were made at the remaining 33 

 stations. 



An offshore study area extending from long. 



2 1983 Gulf Coast Shrimp Data, NOAA, NMFS. 



Manuscript accepted January 1985. 



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