224 



Fishery Bulletin 98(1) 



of logjQ-lagged river discharge and number of trawl- 

 ers (/•=0.80, P<0.001). CPUE was not significantly 

 (P>0.05 ) correlated with river flow or number of trawl- 

 ers ( Table 1 ), nor with total landings ( r=-0.26, P=0.3 1 ). 

 The equation of best fit (0.64) for predicting shrimp 

 landings took the form 



where 



X,= 

 Y = 

 M = 



Y=a +m{X,X2), 



log,Q-lagged river discharge (Mm'^/yr); 



number of trawlers; 



shrimp landings (t/yr); 



the slope of the equation (1.67); and 



the y-intercept (232 t/yr). 



Discussion 



Our analyses represent a first attempt to identify re- 

 lationships between variability in shrimp landings in 

 the upper Gulf of California and factors influencing 

 these landings. Total shrimp landings and the size 

 of the shrimping fleet at San Felipe have declined 

 over the past 15 years. Social and economic changes 

 have affected shrimping. In the late 70s and early 

 80s shrimping was reserved for social units (cooper- 

 atives), with the result that privately owned shrimp 

 trawlers were banned from the fishery. In addition, 

 the government subsidized building of additional 

 vessels and many new unskilled fishermen entered 

 the industry. Then policies were reversed in the 

 late 1980s, private boats returned, interest rates 

 increased, and many of the shrimp trawlers were 

 removed from the fleet. 



We found a significant relationship (P<0.001) be- 

 tween total catch and the rate of freshwater discharge 

 of Colorado River water into the marine ecosystem, 

 although the mechanisms through which river dis- 



charge might affect the shrimp fishery are unknown. 

 Lower salinity may improve the survival of early 

 life stages by providing "enlarged nursery" protected 

 habitat (Garcia, 1991), even though L. stylirostris 

 and P. californiensis are generally considered eury- 

 haline species ( Hernan, 1997 ), having large numbers 

 of postlarvae and juveniles in hypersaline habitats 

 (Brusca, 1980; Page'^). Salinity and nutrient gradi- 

 ents in the estuary and upper Gulf during river flows 

 have not been reported to our knowledge. 



Future plans for the Colorado River will likely 

 decrease freshwater discharge into the estuary as 

 more water is diverted upstream for farms and 

 domestic use (Morrison et al., 1996). Our analyses 

 suggest that decreases in river discharge to the delta 

 and estuary may adversely affect shrimp landings. 

 The United States and Mexican governments should 

 initiate a research program on the effects of river 

 flow on ecologically and commercially important spe- 

 cies in the upper Gulf of California and incorporate 

 these findings into a comprehensive management 

 plan for the Biosphere Resei-ve as well as the Colo- 

 rado River Basin at large. 



Literature cited 



Alvarez-Borrego, S., B. P. Flores-Baez, and 

 L. A. Galindo-Bect. 



1975. Hidrologia del Alto Golfo de California II. Condiciones 

 durante invierno, primavera y verano. Ciencias Marinas 

 '2(1 ):2 1-36. 

 Brusca, R. C. 



1980. Common intertidal invertebrates of the Gulf of Cali- 

 fornia. Univ. Arizona Press, Tucson, AZ, 513 p. 



' Page, M. 1999. Marine Science Institute, University of Cali- 

 fornia, Santa Barbara, CA 93106. Unpubl. data. 



