DAGG ET AL.: DISTRIBUTION OF COPEPOD NAUPLII 



to clearly identify and segregate these regions. 

 The overall region did not always appear signifi- 

 cantly better than the other regions because only 

 parts of it are better. 



On the continental shelf in the Gulf of Mexico, 

 as in other shelf regions of the United States, the 

 typical pattern is for the water to be vertically 

 well-mixed during the wintertime (Parker 1968). 

 Autumn cooling breaks down the thermal strati- 

 fication that has existed throughout the summer 

 and allows the isothermal water column to be 

 easily mixed. The major exception to this general 

 pattern in the Gulf of Mexico is the shelf region 

 influenced by the large volume of freshwater 

 runoff from the Mississippi River. During this pe- 

 riod, the freshwater influx is of sufficiently large 

 volume (average flow for December to March in 

 1975 through 1979 was 17,290 m^/second (U.S. 

 Army Engineer District, New Orleans Corps of 

 Engineers, 1980)) to physically stratify the shelf 

 waters hundreds of km downstream from the 

 delta. We suggest that this salinity-induced strat- 

 ification is a vital component of the recruitment 

 success of Gulf menhaden because it provides an 

 environment in which prey aggregations can 

 occur. It has been suggested that vertical stratifi- 

 cation by other small-scale physical phenomena 

 (e.g., Langmuir circulation) allows significant 

 patchiness of prey items to exist (Lasker 1975). 

 This patchiness provides small regions of compar- 

 atively high food concentrations for fish larvae, 

 and results in improved feeding success. In this 

 study, copepod nauplii were aggregated vertically 

 at stations with physical stratification and were 

 nearly homogenously distributed at stations lack- 

 ing physical stratification. Maximum naupliar 

 concentrations (no. per m^) at stratified stations 

 were typically 2-10 times greater than at non- 

 stratified stations. 



In conclusion, we believe that the large fresh- 

 water inflow of the Mississippi River during the 

 wintertime spawning period of the Gulf men- 

 haden contributes to the feeding success and 

 survival of larval fish by providing physical strat- 

 ification which in turn results in a vertical strat- 

 ification of phytoplankton and microzooplankton 

 in layers or patches of relatively high concentra- 

 tions. 



ACKNOWLEDGMENTS 



Part of this work was done in cooperation with 

 personnel from the National Marine Fisheries 

 Service at Beaufort, NC and at Pascagoula, MS. 



In particular, we would like to thank D. Hoss for 

 providing time and space in his field programs, 

 and J. Govoni for discussion of several portions 

 of the manuscript. S. Cummings, L. Hill, 

 E. Walser, J. White, J. Turner, P. Morgan, 

 C. Neill, and E. Turner assisted in collecting and 

 analyzing samples. Part of this work was done 

 in cooperation with D. Checkley and A. Amos at 

 the University of Texas; their contributions are 

 greatly appreciated. Thanks also to the captains 

 and crews of the F.R.V. Oregon II, the RV 

 Longhorn, and the R. J. Russell. This work was 

 supported by the Louisiana Universities Marine 

 Consortium, by NSF Grant OCE-81 19848, and by 

 the Ocean Assessment Division, National Ocean 

 Services, NOAA. 



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