shore water throughout the year, but that peaks 

 of abundance occur, which vary from one year 

 to another. In Florida Bay a large peak was i-e- 

 ported from April to June by Tabb, Dubrow, and 

 Jones (1962) and from July through October by 

 Jones et al. (1964). A secondaiy peak of abun- 

 dance is known to take place late in the fall or 

 during the winter. In Tampa Bay an extremely 

 large peak was recorded in July by Eldred et al. 

 (1965). In Mississii^pi, Christmas et al. (1966) 

 fomid pink shrimp postlarvae from May through 

 December, but only a few before July. In the 

 Aransas Pass, Tex., inlet, Copeland and Ti'uitt 

 (1966) observed a maximum peak of postlarval 

 P. d. duorarum in August and September. 



Small pink shrimp grow rapidly in the nursery 

 areas and move toward the se^ as they approach 

 maturity. They leave mostly on the ebb tides since 

 sample catches are usually much larger then than 

 during flood tides, and largest when the tidal 

 current is strongest. Costello and Allen (1966) 

 estimated that shrimp remain in the estuaries of 

 southwest Florida for a period ranging from about 

 2 to 6 months. 



Hughes (in press) has suggested that the dis- 

 placement of the postlarvae inshore on flood 

 tides and the emigration of the "juveniles" off- 

 shore on ebb tides appear to be effected by the re- 

 spective responses of the two stages to changes in 

 salinity. Juveniles usually exhibit a positive 

 rheotaxis; however, when the salinity of the water 

 decreases downstream swimming ensues. This en- 

 sures that in nature juveniles will swim against the 

 current and, thus, resist displacement in an in- 

 shore direction by the flood tide but will swim 

 offshore with the ebb tide. Postlarval shrimp re- 

 spond to a decrease in salinity by dropj^ing to the 

 substrate; when the salinity increases they become 

 active in the water column. Consequently, post- 

 larvae are mo\'ed inshore on the flood tide and 

 evade displacement seaward on the ebb tide. 



A number of studies have been carried out in 

 different localities to follow the migi-ation of pink 

 shrimp as they leave the estuaries. Tabb, Dubrow, 

 and Jones (1962) and Idyll et al. (1966) investi- 

 gated the emigration from the estuarias to Florida 

 Baj'. The former authors observed that many 

 shrimp move to the Bay at about 18 to 20 mm. c.l. 

 (about 82 to 90 nun. t.l.) and practically all be- 

 fore attaining a carapace gi'eater than 25 mm. 

 (about 105 mm. t.l.). Idyll et al. (1966) estimated 



that the montlily mean size of the migrating 

 shrimp ranged from 9.9 mm. c.l. (about 46 mm. 

 t.l.) to 18 mm. c.l., and averaged 14 mm. c.l. (about 

 65 mm. t.l.). The relative abundance of shrimp 

 moving to Florida Bay fluctuates seasonally. Num- 

 bers are greatest in late summer and early fall, 

 and abundance reaches a peak in September. A 

 second peak occurs from January to April. Eldred 

 et al. (1961) in Tampa Bay found that shrimp 

 began their seaward movement at 85 mm. t.l. in 

 Ai^ril and continued to leave through July. Mass 

 migration to offshore waters did not seem to occur 

 during the fall and winter. Copeland (1965) stated 

 that most shrimp leave through Aransas Pass, 

 Tex., inlet, at 70 to 90 mm. t.l., from April through 

 October. Joyce (1965) gathered evidence that in 

 northeast Florida the young leave the nursery 

 grounds at a length of 85 to 90 mm. 



Williams (1955a) in North Carolina observed 

 that pink shrimp that arrive at the nursery 

 grounds in spring grow rapidly and after having 

 reached the subadult stage move back to the ocean, 

 where as young adults they are harvested in late 

 simimer and fall. Individuals that overwinter in 

 the estuaries migi'ate to the sea in May and June 

 and become the object of a spring fishery. 



Several investigations have been made to estab- 

 lish the movements of pink slirimp leaving the 

 nursery areas of southwest Florida for the spawn- 

 ing grounds. Costello and Allen (1966) concluded 

 that: (a) some pink shrimp from shallow coastal 

 waters of southwest Florida follow broad migra- 

 tory routes in moving to offshore grounds; (b) 

 shrimp from each nursery area seem to assume "a 

 distinct pattern of distribution on the offshore 

 grounds,'" that is, those found in the Tortugas 

 come from shallow waters ranging from north- 

 east to east of the grounds, whereas those taken on 

 the Sanibel grounds are immigrants from the ad- 

 jacent coastal waters; (c) ranges of the stocks 

 from which the Sanibel and Tortugas populations 

 are derived overlap along the southwest coast of 

 Florida and in the offshore water between the 

 two trawling grounds; (d) shrimp enter the Tor- 

 tugas grounds from the north, east, and southeast. 

 Furthennore, it seems that the Tortugas shrimp 

 do not migrate to the Sanibel fishing areas, and 

 south Sanibel shrimp seldom migrate to the Tor- 

 tugas gi-ounds or to the northwest Sanibel grounds. 

 The above conclusions wei'e based on various mark- 

 recapture exi^eriments by Costello and ,Allen ( 1960, 



518 



U.S. FISH AND WILDLIFE SERVICE 



