362 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Biologically speaking, the brown shrimp seems 

 to differ little from other commercial species. 

 Taxonomic differences, for example, arc quite 

 subtle; and except for minor differences in chronol- 

 ogy, events in its morphological development 

 parallel those characterizing life histories of other 

 shallow-water Penaeidae. Ecological factors 

 would therefore be expected to play the dominant 

 if not the more apparent role in separating thjs 

 species from its relatives. For instance, although 

 the greater proportion of each commercial variety's 

 developing young may temporarily utilize (at 

 different times) the same estuarine habitat, the 

 parents show marked differences in their bathy- 

 metric distribution. Thus brown shrimp adults 

 commonly are found on the outer reaches of the 

 continental shelf, suggesting a greater transport 

 distance for newly hatched larvae, and indicating 

 a correspondingly longer migration for juveniles 

 from estuary to offshore spawning ground. In 

 addition, cursor}' observations tend to support the 

 consensus that a substrate appreciably softer than 

 those ordinarily occupied by its commercial rela- 

 tives typifies the brown shrimp's habitat (Hilde- 

 brand, 1955; Williams, 1958). If the substrate 

 is a major limiting factor, such a recjuirement may 

 well explain the dominance of this species on the 

 continental shelf from Mississippi Sound westward. 



In examining the recent status of the brown 

 shrimp in Gulf of Mexico waters, I view its bio- 

 mass as a continuous "stock" or population over 

 the range of primary exploitation. However, the 

 possibility of this stock being composed of units 

 or subpopulations enjoying varying degrees of 

 "discreteness" should not be discounted. Though 

 our knowledge in this area is nil, the likelihood of 

 any unit being isolated physicallj' or genetically 

 would seem quite remote. Recent mark-recapture 

 studies in Texas and Louisiana waters indicated, 

 for example, an east to west movement of at least 

 portions of successive generations, each member of 

 which can trace its lineage to a spawning aggre- 

 gation maintained to the east, presumably in the 

 western part of the Delta area (U.S. Fish and 

 Wildlife Service, 1960; Louisiana Wild Life and 

 Fisheries Commission, 1960). 



For convenience in presenting and interpreting 

 data, the brown shrimp stock is divided into two 

 units between which interchange is believed mini- 

 mized by a natural barrier, namely, the Mississippi 

 River Delta and impinging edge of the continental 



shelf. Subdivision of the areas lying east and 

 west of the Delta is strictly arbitrary. And 

 although the term "populations" is used freely 

 to identify biomass units within these coastal areas 

 (and subareas), it is used in an artificial sense and 

 in no way implies that actual subpopulations (or 

 "races"), if these exist, have been defined. 



Eastern Gulf Populations 



Data of fishing effort and yield as related to 

 commercial utilization of brown shrimp popula- 

 tions along the northeastern Gulf coast are given, 

 respectively, in appendix tables 1 and 2. These 

 have been condensed because of space considera- 

 tions from more extensive tables giving catch-by- 

 depth data on a subarea basis, the subareas in- 

 volved here being those numbered from 8 through 

 12 (fig. 1). The two general areas upon which 

 consolidation was based are identified as the 

 Apalachicola and Pensacola-Mississippi River 

 areas. Note that indices used to assess popula- 

 tions or portions of a stock occupying any coastal 

 area were computed from uncondensed data, i.e., 

 data tabulated on a statistical subaroa-depth zone 

 basis. Inshore catches are given by specific 

 water body in appendix table 5. 



Commercial yield. — During the period 1956-59, 

 brown shrimp production off northwestern Florida 

 (Apalachicola area) was greatly overshadowed by 

 that off Alabama, Mississippi, and eastern Louisi- 

 ana combined (Pensacola to Mississippi River 

 area) (fig. 12A). Population yields as reflected 

 by commercial landings showed a steady increase 

 from 0.1 to 0.7 million pounds annually in the 

 Apalachicola area, and marked Huctuation be- 

 tween 5.3 and 12.6 million pounds annually in the 

 Pensacola-Mississippi River area. Production in 

 botli areas experienced typical midwinter lows 

 and midsummer highs. Although of a much lower 

 oi'der of magnitude, production peaks in the 

 former area occurred slightly in advance of those 

 in the latter. 



Fishahle Momasft. — Comparison of biomass in- 

 dices revealed upward trends in annual overall 

 brown shrimp abundance from the Mississippi 

 River eastward (fig. 12B). Marked deviations 

 from what might be considered normal fluctuations 

 in seasonal abundance were not apparent. As 

 suggested by the corresponding yield data, a 

 greater biomass, on the average, occupied the 

 more westerly portion of the northeastern Gulf 



