GULF OF MEXICO COMMERCIAL SHRIMP POPULATIONS 



383 



Figure 28C suggests tluit maximum rate of 

 growth in the northwestern Gulf of Mexico white 

 siirimp stock, idtliough varying in magnitude 

 from generation to generation, occurs, on tlie 

 average, somewhere witiiin that size range having 

 hmits eciuivalcnt to the wcigiits at which 25 

 to 30 individual whole shrimp weigh 1 pound 

 (41-50 count, heads off). This compares with 

 evidence as to size at which growth in weight 

 reaches a maximum in the northeastern stock, 

 as well as in the Gulf's major stocks of brown 

 and piidv shrimp. But because information 

 concerning natural mortality rates is lacking, 

 answers to the question : "Where on the population 

 growth curve do weight losses due to natural 

 mortality begin to offset weight gains due to 

 growth?" cannot be given. If natural mortality 

 from the juvenile stage upward proves negligilile, 

 harvesting should be restricted to shrimp whose 

 growth rate is approaching or has readied a 

 maximum. Should natural mortalit}- prove appre- 

 ciable, utilization at a smaller average size may 

 be indicated. 



As in the case of northern Gulf of Mexico 

 brown shrimp populations, semiannual periods 

 of intensified spawniing activity in coexisting white 

 shrimp populations defy explanation. The me- 

 chanics of physiological adaptation to a highly 

 variable environment are not understood, but 

 temperature is believed to be a major if not the 

 primary factor governing spawning activitA" in 

 httoral Penaeidae. This relationship has already 

 been considered in discussing the reproductive 

 cycle of the browm shrimp. It was concluded 

 that heightened spawning in brown shrimp popu- 

 lations oS the northern Gulf coast was related to 

 seasonal temperature reversals and not to some 

 fixed "optimum" temperature. 



A similar conclusion nuiy apparently be drawn 

 for the white shrimp except that increased 

 spawning activity seems more closely related to 

 reduction in rate of temperature change as seasonal 

 minimum and maximum temperatures are ap- 

 proaclied. This can be construed to be in agree- 

 ment with Lindner and Anderson (1956), who 

 make the very general statement: "Spawning in 

 Louisiaiui appears to be more closely associated 

 with rising and falling temperatures than with 

 absolute temperature." 



Inshore population phases — Over the period 

 1956-59, white shrimp comprised about 45 



* COMMCROAL riELO |ir>lll.« 

 • •LOUtSl&NA C0A9T 

 "-TEXAS COAST 



I p«H"S*. "•aM'S") 



A 





A 



COAST coAsr 



S YIELD pen DAY'S TRAWLINO 



vj" 





\Ji 



^1 



Vs 



CkOStO It'SONt 



rm — rrrr 



- II IAS C««fT 



Figure 29. — Total and average yield of white shrimp 

 populations in inshore waters along the northwestern 

 Gulf coast, 1956-59. 



percent of all shrimp taken atuiually from inshore 

 waters in the Louisiana and Texas Coast areas. 

 Inshore catches usually accounted for about 

 one-half of all white shrimp taken in the former 

 area, and two-thirds of that harvested each year 

 from the latter area. Yearly white shrimp land- 

 ings from Louisiana's inshore waters fluctuated 

 between a high of 11.4 million pounds in 1956 

 and a low of 2.8 million pounds the following 

 year. Inshore landings from the Texas Coast 

 area ranged from a low of 1.3 milHon pounds in 

 1956 to highs of about 3.6 million pounds in 

 1958 and 1959 (fig. 29A). Prospects for the 

 Texas inshore fishery are encouraging. On the 

 basis of incomplete data for 1960, the upward 

 production trend established duriiig 1956-59 is 

 being maintained. 



A negative production trend signified by the 

 decrease in Louisiana's 1957 inshore landings was 

 countered with legislative action in the form of 

 closed seasons more stringently enforced tluin 

 those previously in effect. Accordingly, laws 

 closing inshore waters of the State to all shrimping 

 from the beginning of July to mid-August ■* 

 and again from mid-December to the end of the 

 following April, went into effect in 1958. Pro- 

 duction in the offshore white shrimp fishery has 

 since recuperated nicely, but the trend in the 

 inshore fishorv is still far from that desired. 



• The "spring" season was extended to mid-July In 1%0. 



