FISHERY BULLETIN: VOL. 84, NO. 1 



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BOTTOM WATER DISSOLVED OXYGEN 

 CONCENTRATION (PPM) 



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Figure 3.— Offshore shrimp abundance in relation to bottom water 

 dissolved oxygen concentration. 



water trawls above previously identified hypoxic 

 areas. Therefore, it was concluded that they were 

 captured from the upper water column as the trawl 

 passed through it. Four brown shrimp, three lesser 

 blue crabs, Callinectes similus, and one mantis 

 shrimp were the only crustaceans captured in five 

 midwater trawls. The relationship between shrimp 

 and bottomfish abundance and distribution indicates 

 that they do not pass through or over hypoxic water 

 masses. Actual avoidance behavior in the field has 

 not been documented. 

 Nearshore, a total of 20 fish and 5 invertebrate 



species were collected. Atlantic croaker was the 

 dominant species. Brown shrimp were present in low 

 numbers at most stations. White shrimp; blue crabs, 

 Callinectes sapidus; lesser blue crabs; and sea bobs, 

 Xiphopenaeus sp., were the only other crustaceans 

 collected. A high variability in fish and shrimp abun- 

 dance was probably due to the low fishing efficiency 

 of the small net at the deeper nearshore stations. 

 As a result, no significant correlation was present 

 at nearshore stations between bottom water dis- 

 solved oxygen concentration and fish or shrimp 

 abundance 



Vertical density stratification was present at both 

 nearshore and offshore stations. Dissolved oxygen 

 concentration and vertical density gradient were 

 negatively correlated (r = -0.73, P < 0.001) (Fig. 

 4). This agrees with Leming and Stuntz (1984) who 

 found a high correlation between bottom dissolved 

 oxygen content and surface to bottom density gra- 

 dients off Louisiana in 1982 (r = -0.74, P < 0.001). 

 Offshore, the mean difference between surface and 

 bottom dissolved oxygen was 6.4 ppm (standard er- 

 ror = 0.40) in hypoxic areas and 1.6 ppm (standard 

 error = 0.08) in nonhypoxic areas. Temperature 

 generally did not vary more than 2°C between the 

 surface and bottom regardless of the area. 



During the first week of July, 92% of the hypoxic 

 stations were in areas predicted as potentially hypox- 

 ic through a posteriori analyses of remote sensing 

 data. Hypoxic areas were characterized by surface 

 water temperatures near 30 °C, which agrees with 

 Leming and Stuntz (1984). They discussed satellite 

 data acquisition, its value in identifying and 

 forecasting hypoxic regions in the Gulf of Mexico, 



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Figure 4.— Bottom water dissolved oxygen concentration in 

 relation to vertical density gradient of the water column. Den- 

 sity gradient is expressed as (bottom sigma-t minus surface 

 sigma-t)/depth. 



0.5 1.0 1.5 



DENSITY GRADIENT 



22 



