Sheridan: Forecasting the fishery for Penaeus duorarum 



745 



1969). Rapid salinity changes, as might be experi- 

 enced during rainy season floods, may also force 

 shrimp out of nearshore habitats (Hughes, 1969). 

 Emigration of juvenile pink shrimp from nurseries 

 in summer and fall has been postulated to form the 

 fall and winter landings of new recruits by the fish- 

 ery (Higman et al., 1972). Juvenile and subadult pink 

 shrimp marked and released in southwest Florida 

 coastal habitats were primarily recaptured on the 

 Tortugas fishing grounds (Costello and Allen, 1966; 

 Gitschlag, 1986). Apparent pink shrimp movement 

 speeds were 1-2 km/day (Costello and Allen, 1966); 

 thus pink shrimp could reach the fishing grounds 

 100 km southwest of Cape Sable (Fig. 1) in 50-100 

 days, as postulated by Higman et al. (1972). 



Environmental determinants of pink shrimp 

 growth and survival have not been examined exten- 

 sively. Most available information consists of pink 

 shrimp abundance and size by season or habitat, with 

 coincident measurements of temperature and salin- 

 ity. Pink shrimp have wide tolerances for salinity and 

 temperature (0-65%c and 11-40°C; Costello and 

 Allen, 1970; Costello et al., 1986). Maximum growth 

 of postlarval pink shrimp (7.8-10.1 mm total length 

 [TL]) was found at 30— 35°C under constant salinity 

 (28-32%c, Teinsongrusmee, 1965). The only experi- 

 mental analyses of pink shrimp survival versus com- 

 bined temperature and salinity variations was con- 

 ducted by Williams ( 1960). Survival of juveniles (35- 

 100 mm TL) was highest (77-100%) at 15-30%c and 

 8.8-28.4°C but was significantly lower (62-67%) at 

 10%o and 8.8-28.4°C due to impaired os- 

 moregulation. Higman et al. (1972) con- 

 ducted enclosure experiments to deter- 

 mine pink shrimp growth in the field but 

 felt that poor water quality conditions 

 confounded their results. Neither Will- 

 iams (1960) nor Teinsongrusmee (1965) 

 addressed the hypersaline conditions of- 

 ten experienced in Florida Bay (up to 

 70%e; Mclvor et al., 1994). 



The Tortugas fishery began in 1949, 

 and since 1956 monthly catch and effort 

 data have been collected by NMFS per- 

 sonnel using standard methods (Nance 

 and Patella, 1989). The fishery has 

 landed an average of 4,350 metric tons 

 (t) annually during 1960-85 (NMFS 2 ) 

 and was relatively stable (coefficient of 

 variation=17%; Nance and Patella, 1989). 

 Since that time, however, the fishery has 

 shown a singular, and as yet unexplained, 

 decline and apparent recovery (Fig. 2). 

 In conjunction, the bimodal trend in 

 monthly catch per unit of effort (CPUE) 



of the smallest pink shrimp size class (&68 tails to 

 the pound, or "68-count") has changed (Fig. 3). The 

 fall peak in recruitment of 68-count pink shrimp that 

 dominated in early years (1960—69) has shifted in 

 favor of a spring peak. This shift may have been the 

 result of management measures enacted during 

 1961-81 to restrict the catch of small shrimp 

 (Caillouet and Koi, 1981; Gulf of Mexico Fishery 

 Management Council, 1981). 



Materials and methods 



Fishery yield forecasts often depend upon indices of 

 larval or juvenile abundance as indicators of recruit- 

 ment into a fishery. Forecasting brown shrimp yield 

 off Texas depends upon landings of juveniles by the 

 Galveston Bay bait shrimp industry immediately 

 prior to their emigration from the bay (Berry and 

 Baxter, 1969; Baxter and Sullivan, 1986). Forecast- 

 ing western rock lobster, Panulirus cygnus, yield is 

 based on seasonal settlement of the final planktonic 

 stage (puerulus) on collectors at a single site, Seven 

 Mile Beach, Western Australia (Phillips, 1986). In 

 Florida, however, there are no long-term fishery-in- 

 dependent data sets describing larval or juvenile pink 

 shrimp abundance in coastal habitats. Thus, my 

 models are based on fishery-dependent catch statis- 

 tics and on environmental variables that could af- 

 fect the survival, growth, and recruitment of pink 

 shrimp, even though causal mechanisms may be 



S 3000 



Annual 

 2 in the 



960 1970 1980 1990 



Fishing year (November-October) 



Figure 2 



landings by the Tortugas pink shrimp fishery (NMFS, Footnote 

 text). 



