366 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



parison of curves also revealed the same westward 

 lag in niaximuni seasonal production noted earlier 

 in the eastern Gulf. Peaks usually occurred 

 during July-August off Louisiana, August-Octo- 

 ber off Texas, and September-November off 

 eastern Mexico. 



Fishable biomass. — Biomass curves derived for 

 western portions of the northern Gulf's brown 

 shrimp stock paralleled those describing eastern 

 portions. Dominant modes of the curve deter- 

 mined from data of commercial operations off 

 Louisiana occurred slightly ahead of or at the 

 same time as tliose of the Texas curve (fig. 14 A). 

 Modes of the curve for eastern Mexico were 

 generally displaced still later (fig. 15A). As also 

 indicated by corresponding yield curves, this sug- 

 gests a pattern of gross westerly drift for major 

 portions of vernal broods, probably associated 

 with their seaward migration from upper Gulf 

 nursery grounds. 



A gradual westward increase in brown shrimp 

 abundance is demonstrated by biomass curves for 

 successive coastal areas being displaced on their 

 ordinates at increasingly higher levels. Four-year 

 abundance trends, on the other hand, exhibited a 

 decline from east to west. Louisiana's available 

 biomass was slightly upward, that off Texas expe- 

 rienced a mild decline, while that off eastern 

 Mexico fell off moderately (cf. figs. 14B and 15B). 

 A pattern thus emerged for the northern Gulf's 

 brown shrimp stock — an upward 4-year trend for 

 eastern segments, a nearly static situation for 

 those centrally located, and a perceptibly down- 

 ward trend for westernmost segments. 



Whether this pattern developed purely by 

 chance or resulted from factors operating differen- 

 tially yet systematically on adjacent biomass units 

 is problematical. The cumulative effect of a com- 

 pounded fishing mortality associated with the 

 seasonal westward movement of newly recruited 

 broods is a distinct possibility. Of significance 

 here is the fact that suitable inshore nursery 

 grounds diminish rapidly in extent as one proceeds 

 westward from the Delta. Heavj- fishing on 

 broods produced to the east coupled with light 

 recruitment from areas to the west could con- 

 ceivably result in a systematic population decline 

 from east to west. 



It should be noted that biomass curves depicting 

 western portions of the brown shrimp stock may 

 not always be comparable to those describing 



eastern portions. Commercial fishing off Texas 

 and Mexico, for example, is t^-pically more selec- 

 tive than that off the remaining upper coast. 

 Texas markets are such tliat small shrimp are 

 ordinarily unacceptable and fishermen conse- 

 quently avoid landing them, often through dis- 

 carding, but usually by seeking out aggregations 

 of larger specimens. Moreover, legislation enacted 

 in 1959 encourages fishermen in the Texas Coast 

 area to defer from trawling on predominantly 

 small brown shrimp during June and July bj' 

 closing coastal waters up to a distance of 10 miles 

 offshore. 



Obviously, these factors act to minimize the 

 amount ot information yielded by biomass indices 

 computed from conmiercial statistics. Further- 

 more, attempts to explain differences in inter area 

 population trends are defeated if fishing practices 

 in adjacent areas are such that resulting statistics 

 lead to incomparable fishable biomass indices. It 

 is quite apparent (fig. 14C) that the definition of 

 fishable biomass as given earlier in terms of 

 "standard" Gulf trawling gear does not hold for 

 all areas. 



Population characteristics. — Despite the com- 

 mercial fishery's tendency toward increased selec- 

 tivity of larger shrimp on the Gulf's western 

 grounds, catches during 1956-59 periodically con- 

 sisted of brown shrimp covering a size range 

 sufficient to describe (although somewhat sketchily) 

 the structure of the biomass whose components 

 would ordinarily be vulnerable to the gear cm- 

 ploj^ed under conditions of nonselective fishing 

 (figs. 14C and 15C). More specifically, catch 

 composition data included in statistics of fishing 

 activities off Louisiana appeared relatively free of 

 the selectivity bias, whereas those included in 

 statistics of operations off Texas and Mexico did 

 not. The effects of "biased sampling" frequently 

 complicated the picture of population size struc- 

 ture in the latter areas. 



Distribution of modes of weight-frequency 

 curves derived for monthly landings taken off 

 Louisiana and Texas provided additional evidence 

 for semiannual brown shrimp broods in upper 

 Gulf waters (fig. 14C). In the western Gulf, how- 

 ever, the timing of each brood's appearance seemed 

 advanced somewhat beyond that of its counterpart 

 in the eastern Gulf, indicating correspondingly 

 later spawiiing peaks in western areas. This was 

 especially apparent in Texas waters where broods 



