386 



Fishery Bulletin 102(2) 



small juvenile brown shrimp (Farfantepenaeus aztecus) 

 are more transient in suboptimal habitat (open bays and 

 deeper channels) and exhibit less movement upon reach- 

 ing optimal habitat (ponds and shallow channels). 



The only study with which we can directly compare our 

 findings on residence time and movements was conducted 

 by Knudsen et al. (1996) near Calcasieu Lake, Louisiana, 

 where tidal flux was considerably lower (mean tide range 

 <0.6 m) and the system (marsh impoundments) was less 

 open than that in the present study. Knudsen et al. (1996) 

 marked batches of juvenile white shrimp (45-69 mm TL) 

 by injection of colored pigments and released them into a 

 pair of 35-ha. impoundments, each connected to the open 

 estuary through a narrow channel that was fitted with 

 screen deflectors and traps designed to collect all emigrat- 

 ing nekton. The mean time from release to emigration of 

 juvenile white shrimp ranged from 30.2 to 59.9 days. Our 

 estimates of tidal creek residence time for juvenile shrimp 

 in Georgia tidal creeks was about half that reported for 

 impoundments in Louisiana and may be explained by the 

 differences in tidal flux and openness between the two 

 systems. However, the values we observed were likely 

 underestimates of the actual residence period of survivors 

 within the creeks because they included losses due to mor- 

 tality as well as emigration. 



It seems clear from the studies conducted thus far that 

 juvenile penaeids, once having entered the estuarine 

 nursery, tend to remain within a limited spatial range 

 where they are exposed to local conditions for several 

 weeks. Our findings also provide evidence of spatial 

 variation for both residence time and growth rate of ju- 

 venile white shrimp that is possibly attributable to struc- 

 tural differences in tidal creek subsystems. We suggest 

 there may be an optimal value for the ratio of subtidal to 

 intertidal drainage area within marsh creek systems that 

 can achieve a favorable balance between suitable habitat 

 (space) at low tide, which tends to enhance residence time 

 and density of juvenile shrimp, while providing sufficient 

 intertidal foraging habitat and predator refugia at high 

 tide to promote high rates of juvenile shrimp growth and 

 survival. Spatially explicit information on growth rates 

 and the extent to which individual shrimp move within 

 their estuarine nurseries are necessary initial steps to- 

 ward meeting the challenge of maintaining quality nurs- 

 ery habitat for a sustainable shrimp fishery and satisfying 

 other demands associated with human development in 

 and around estuarine watersheds. 



Acknowledgments 



Several individuals provided field and laboratory assis- 

 tance for this project, but we especially thank K. Feeley, 

 J. Kneib, and J. Spicer for helping on a regular basis. 

 The primary source of funding was a National Estuarine 

 Research Reserve System Graduate Research Fellowship 

 to S. Webb (NA870R0284) (Estuarine Reserves Division, 

 Office of Ocean and Coastal Resource Management, NOS, 

 NOAA), and matching funds provided by the University 

 of Georgia Marine Institute. The Georgia Sea Grant Col- 



lege Program contributed funds for the purchase of an 

 additional tag injector unit, which substantially improved 

 the effectiveness of the mark-recapture program. The con- 

 ceptual basis for this project was derived from research 

 conducted under a grant from the National Science Foun- 

 dation ( DEB-9629621 ), which also contributed supplemen- 

 tal student support. 



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