364 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERMCE 



often weak due to the magnitude and success of 

 interim spawning being sustained at compara- 

 tively high levels. The exact position of one peak 

 relative to the other is therefore difficult to resolve. 



The evidence for two broods per 3'ear substan- 

 tiates earlier opinion as to the seasonal reproduc- 

 tive activity in brown shrimp off the upper Gulf 

 coast (Gunter, 1950). Additional evidence in a 

 later section will permit further discussion of this 

 feature of brown shrimp life history. 



Inshore population phases. — Up to this point 

 discussion has been limited to the dj-namics of 

 offshore phases in the eastern Gulf's brown shrimp 

 population. If attention is turned to inshore 

 phases, parallels to events in offshore population 

 phases may be readily noted. 



In both the Apalachicola and Pensacola-Missis- 

 sippi River areas (but more noticeably in the 

 latter), distributions of monthh commercial 

 yields from offshore and inshore waters corre- 

 spond very precisely during the period 1956-59 

 (cf. figs. 12A and 13A). Annual yields fluctuated 

 in like manner although the offshore catches 

 were about twice those of inshore catches. 



Closer comparison of curves describing seasonal 

 patterns for offshore and inshore yields from the 

 Pensacola-Mississippi River area reveals that, 

 on the average, peaks in offshore yields lagged 

 slightly behind those in inshore yields (figs. 12A 

 and 13A). This lag reflects the expected pattern 

 for juveniles (spring brood) migrating from in- 

 shore "nursery grounds" to parental habitat 

 offshore, pointing up the relationship between 

 inshore and adjacent offshore fisheries and stini- 



A COMMERCJAL riELD (m.ll.ons of pounds modi.e 

 • APALACHICOLA 

 PENSACOLA-MISSISSIPPI RIVER 



\ 



\ 



/A 



e VlELO PEP OAVS TRAWLING 



• APALACHICOLA 



<• PENSACOLA-MISSISSIPPI RIVER 



A 





xlJ%/. 



^d/V 





Is 



J M " J S NJMMJSNJMMJSNJIUMJSN 

 1956 1957 I95B 1959 



TTI 1111 



1956 - 59 



Figure 13. — Total and average yiekl of brown shrimp 

 populations in inshore waters along the northeastern 

 Gulf coast, 1956-59. 



ulating speculation as to the effect harvesting 

 a brood's prerecruits (in terms of the offshore 

 fishery) could have on attaining the maximum 

 yield from that brood. 



Taking into account tlie species concurrent 

 migratory and growth patterns, it is presumed 

 tliat such attainment would be realized shortly 

 following egress from inshore waters, or at a point 

 in space and time where shrimp size is equivalent 

 to that at which individuals average about 30 

 (heads-on) to the pound. The reasoning here 

 derives from an examination of curves in sections 

 B and C of figure 12. Those for the Pensacola- 

 Mississippi River area provide the best points 

 of departure. In 1956 and 1957, maximum 

 population biomass, mainly due to the contri- 

 butions of late winter-spring broods (light arrows, 

 fig. 12C), was attained during August -Sept ember 

 (fig. 12B). Observe now that the brood curves, 

 reflecting average growth of shrimp comprising 

 each brood, exhibit inflections which occurred 

 during the same period. Thus, with growth rates 

 having reached a maximum and the broods them- 

 selves attaining maximum weight, biomass from 

 this point on was largely governed by mortality 

 factors. The average size of shrimp at the time 

 of greatest brood mass is shown to have been, 

 as stated above, roughly that at which 30 whole 

 (50 headless) uniform-size individuals weigh 1 

 pound. In the present example, the greater 

 proportion of shrimp taken by the offshore fleet 

 was, on the average, composed of individuals not 

 much larger than those taken by the inshore fleet. 

 Corroborating the case for two population 

 broods annually, curves fitted to average monthly 

 yields from inshore waters likewise indicated the 

 occurrence of two broods, one in late summer or 

 fall, the other in late winter or early spring, re- 

 spectively, the "reproductive" and "commercial" 

 broods referred to earlier (fig. 13B). Moreover, 

 sustained trawling provided crude year-round 

 indices of brown shrimp abundance in tlie eastern 

 Gulf's inshore waters. These indices suggested, 

 despite lessened reliability of midwinter values, a 

 continuous influx of larvae and tended to confirm 

 the hypothesis of protracted spawaiing activity. 

 Four-year trends in average commercial yields 

 from inshore population phases closely approxi- 

 mated those derived for offshore phases. 



Summary of J^-year status. — During 1956-59, 

 eastern segments of the northern Gulf of Mexico's 



