RENAUD: HYPOXIA IN LOUISIANA WATERS 



and its implications regarding shrimp management. 



The effect of hypoxia on shrimp is not completely 

 understood. It is possible that an extensive area of 

 hypoxic bottom water can act as a physical barrier 

 to juvenile shrimp migration offshore and to post- 

 larval migration into nursery grounds. Limited in- 

 direct evidence supports this hypothesis. Gazey et 

 al. (1982) described a shrimp mark-release study in 

 Louisiana. Extensive longshore and offshore move- 

 ment occurred before the recapture of the shrimp 

 during 1979, when hypoxia was not reported off 

 Louisiana (Fig. 5). In 1978, when hypoxia was wide- 

 spread along the Louisiana coastline (Fig. 6), shrimp 

 did not move comparable distances. It was possible 

 that hypoxia reduced shrimp movement into offshore 

 waters. 



The most extensive occurrence of hypoxic bottom 

 water recorded in Louisiana coastal waters occurred 

 from May 1973 to May 1974 (Flowers et al. 1975; 

 Ragan et al. 1978). It was widespread between 

 Barataria and Timbalier Passes and extended up to 

 30 km offshore in some regions. Ragan et al. (1978) 

 reported several areas to be anoxic The duration and 

 severity of this hypoxic condition may have had an 

 impact on the offshore brown shrimp fishery in 1973. 



Total brown shrimp catch and CPUE (catch per unit 

 effort) in 1973 were significantly lower (paired £-test, 

 P < 0.05) than in 1972 (fn. 2). Catch declined 36% 

 (2.8 million kg) and the mean CPUE was reduced 

 by 120 kg/vessel per d. Movement of juvenile brown 

 shrimp to the offshore fishery occurs from May to 

 August (Cook and Lindner 1970). Monthly catch and 

 CPUE of brown shrimp from January through April 

 1973 did not differ from the same time period in 

 1972; however, catch and CPUE from May through 

 December were significantly lower (paired £-test, P 

 < 0.01) in 1973. Postlarval recruitment of brown 

 shrimp occurs from January to May (Baxter and 

 Renfro 1966). An interaction between hypoxia and 

 postlarval recruitment in 1974 might have been 

 responsible for the continued poor harvest of brown 

 shrimp that year. Catch and CPUE were still sig- 

 nificantly lower than in 1972 (paired i-test, P < 0.05). 

 It was not until 1976 that brown shrimp catch sur- 

 passed the 1972 levels (Table 1). A decline in total 

 shrimp catch of Louisiana in 1982 may have been 

 related to a large region of hypoxic bottom water 

 reported by Stuntz et al. (1982). 



Although hypoxia has not been directly linked to 

 declines in annual catch, its presence during critical 



LOUISIANA 



C~-30 ^ f 1 



Figure 5— Movement of tagged juvenile brown shrimp from Caillou Lake and Barataria Bay expressed as days at 

 large before recapture (from Gazey et al. 1982). Shrimp were released in July 1979. Hypoxia was not documented 

 off this coastal area in 1979. 



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