GULF OF MEXICO COMMERCIAL SHRIMP POPULATJONS 



367 



corresponding; to those forthcoming in hite suninier 

 and fall in eastern waters are generated slightly 

 later, nsually during fall and winter. 



The coalescence of fall and spring broods 

 described for eastern portions of the upper Gulf's 

 brown shrimp stock was simulated in stock seg- 

 ments lying off Louisiana. However, the relative 

 importance oi each brood to the offshore fishery 

 displayed a reversal. Whereas spring broods 

 (light arrows) sustained the commercial fishery to 

 the east, fall or winter broods (shaded arrows) 

 sustained it in the western Gulf, the Louisiaiui 

 coastal area seemingly being the transition point. 



Spring broods in Louisiana waters usually did 

 not contribute measurably to the offshore fishable 

 biomass until late the same year or early the year 

 following. Offshore j-ields at such times were 

 small but, on the average, composed of mature 

 individuals surviving from the previous spring. 

 Their presence was reflected on the bionuiss curve 

 by minor, early-year modes (cf. figs. 14A, 14B, and 

 14C). Dominant modes, on the other liand, 

 invariably indicated the presence, during mid- 

 summer, of the stronger fall or winter broods. 

 These contributed the greater portions of annual 

 yields though mean shrimp size during peak har- 

 vest (July-August) was somewhat less than that 

 characteristic of spring broods fished during the 

 winter months immediately following their 

 appearance. 



Late fall or winter broods also dominated the 

 year-round fishery off Texas and eastern Me.xico. 

 Not as well defined in offshore waters as their 

 winter counterparts, spring broods in this area 

 played obscure roles. Their contributions to the 

 offshore fishery were negligible during brood 3'ears 

 but probably significant in succeeding years (figs. 

 14C and 15C). The extent to which spring broods 

 supplement each year's spawning populations 

 remains problematical but there is no question 

 that they sustain important inshore bait fisheries. 

 Lack of appropriate data precludes further 

 discussion. 



Factors operating to produce two "generations" 

 of brown shrimp annually are as intangible as 

 their mechanics are complex. But worthy of 

 consideration as an indicator of what underlies 

 this phenomenon is an easily measured environ- 

 mental parameter, namely, temperature, if tiic 

 species spawns within an optimum temperature' 

 range Iving somewhere between animal minimum 



and maximum temperatures on the floor of the 

 continental shelf, two spawnings per year would 

 be expected; one at some point on the ascending 

 limb of the annual temperature curve, the other 

 at a corresponding point on the descending limb. 

 Taking into account the species capacity for rapid 

 growth, it is reasonable to conjecture further that 

 at least portions of a brood forthcoming at one 

 intercept of the spawning isotherm and seasonal 

 temperature curve would complement the spawn- 

 ing population giving rise to the brood appearing 

 at the subsequent intercept. Until a brood be- 

 becomes extinct, its residuals would be expected 

 to attain maturity and breed at successive 

 intercepts. 



Alignment of periods of maximum spawning in- 

 tensity with anrmal sea temperature curves sug- 

 gests, however, that spawning is associated with 

 seasonal temperature reversals rather than with 

 some optimum temperature. Periods of peak 

 spawning were determined: (1) by extrapolation 

 from brood curves projected backward in time, 

 inferring some knowledge of early growth in 

 penaeid populations (Hudinaga, 1942; Pearson, 

 1939); and (2) through cursory but systematic 

 observations of ovarian development in spawning 

 populations off the upper Texas coast, such ob- 

 servations being made in the course of research 

 conducted by the Bureau of Commercial Fisheries 

 Biological Laboratory, Galveston Tex. Roughly 

 speaking, heightened spawning activity in the 

 northwestern Gulf's brown shrimp stock seems 

 to occur, on the average, during the period March- 

 April, and again during the period September- 

 October. 



Continuous sea temperatiu-e data for the 

 northern Gulf shelf are scanty with most of those 

 available representing surface measurements taken 

 at selected shore stations. Lindner and Anderson 

 (1956, p. 621) present comparative bottom tem- 

 perature curves (inshore; offshore at 3 fm.; and 

 offshore at 10 fm.) constructed from measurements 

 taken during 1931-34 in the Barataria Bay, 

 JjOuisiana, area. Annual sea temperature curves 

 derived from measurements obtained over 

 extended periods at various other locations are 

 given in figure 16. The more extensive of these, 

 however, reflect conditions in areas some distance 

 removed from what are believed to be principal 

 brown shrimp spawning areas. How well they 

 match seasonal temperature patterns at, say, 



