Murphy and Chittenden: Reproduction, age and growth, and movements of Pepnlus bum 



13 



simple in species with a complex life history like 

 P. burti as it is in species that spawn during one major 

 period a year. 



The apparent cessation of somatic growth as P. bur- 

 ti approach spawning in the northwestern Gulf con- 

 trasts with large annual increments in the northcentral 

 Gulf. This difference, combined with their disap- 

 pearance from the northwestern Gulf at 13-16 months 

 of age and their smaller maximum sizes and younger 

 maximum ages there (see below), indicate fish from the 

 northwestern Gulf (1) spawn and die at age I, or more 

 probably (2) in some presently unclear combination 

 emigrate offshore and to the northcentral Gulf prior 

 to, or immediately after, spawning. 



Few P. burti apparently exist larger than 190-200 

 mm in length. The largest we captured (180 mm, Texas; 

 198mm, northcentral Gulf) are similar to maxima in 

 other studies' sampling to at least 80-100m depths 

 (173mm, Hildebrand 1954; 184mm, Franks etal. 1972; 

 169 mm, Chittenden and McEachran 1976; 193 mm, 

 Allen et al. 1986). Maxima are even smaller from 

 estuaries or the shallow Gulf (154mm, Gunter 1945; 

 131mm, Miller 1965; 122 mm, Perret et al. 1971; 

 133 mm, Christmas and Waller 1973), which agrees 

 with our findings that P. burti disperse to deep water 

 as they mature. The largest records were from the 

 northcentral Gulf (Allen et al. 1986; our study). 



Our estimate that t L = 1-1.5 years in the north- 

 western Gulf agrees with Chittenden and McEachran 

 (1976) who suggested a 1-2 year maximum age. A 

 higher t L (2-2.5 years) in the northcentral Gulf ap- 

 pears realistic, because the largest records there are 

 not much larger than the mean size at age II. Based 

 on these maximum ages, theoretical estimates (Royce 

 1972, Hoenig 1983) of total annual mortality rate 

 (1 - S) are nearly 100% in the northwestern Gulf and 

 82-90% in the northcentral Gulf. Murphy (1981) cal- 

 culated similar values of 1 - S for the northwestern 

 Gulf (99%) from observed time-specific abundance data 

 for consecutive Winter or Fall cohorts. If mature fish 

 from the northwestern Gulf emigrate offshore and to 

 the northcentral Gulf, as we suggest, our values for 

 maximum age and total mortality are under- and over- 

 estimates, respectively, for P. burti in the north- 

 western Gulf. 



The presence of the largest P. burti in the north- 

 central Gulf follows a pattern in other marine and 

 estuarine, demersal and pelagic species (Cynoscion 

 nothus, DeVries and Chittenden 1982; Stenotomus 

 caprinus, Geoghegan and Chittenden 1982; Micro- 

 pogonias undulatus, Rivas and Rothmayr 1970, 

 Gutherz et al. 1975, White and Chittenden 1977; Bre- 

 voortia patronus, Nicholson 1978; Larimus fasciatus, 

 Standard and Chittenden 1984; and probably C. are-' 

 narius, Shlossman and Chittenden 1981). Small differ- 



ences between areas also exist in other population at- 

 tributes of P. burti, as in C. nothus and L. fasciatus : 

 younger age compositions and maximum ages, smaller 

 maximum sizes, and higher total annual mortality rates 

 occur in the northwestern Gulf. At least three explana- 

 tions could account for this: (1) There may be no basic 

 differences between areas, just much greater biomass 

 (Moore et al. 1970) at age in the northcentral Gulf; (2) 

 differences may be real, not related to biomass, imply- 

 ing slight, but fundamental, population dynamics dif- 

 ferences between areas; and (3) differences reflect 

 some presently unclear combination of an offshore and 

 spawning or postspawning movement of older, larger 

 fish from the northwestern to the northcentral Gulf. 

 The first implies Chittenden and McEachran (1976) and 

 Chittenden (1977) are correct that shrimp communities 

 on the Gulf continental shelf have a common popula- 

 tion dynamics pattern. The other explanations imply 

 that their arguments need modification for slightly 

 longer life spans and lower mortality rates in the north- 

 central Gulf, and that shrimp communities are a little 

 more sensitive to fishing than Chittenden's (1977) 

 simulations indicate. 



Movements, recruitment, and 

 spawning areas 



Peprilus burti probably spawn offshore. We found fish 

 congregate in 36-100 m depths as they mature and size 

 gradients that indicate an offshore dispersal. In agree- 

 ment, Allen et al. (1986) found a positive correlation 

 between mean size and depth at 200-290 m. Finucane 

 et al. (1979) collected larvae in water 22-182 m deep 

 on the continental shelf off Texas. The young make 

 their way inshore to 5-27 m depths off Freeport, 

 Texas— the white shrimp community (Hildebrand 1954, 

 Chittenden and McEachran 1976)— where they recruit 

 to the bottom. At least two mechanisms could explain 

 their arrival inshore. For one, Ekman surface trans- 

 port, associated with prevailing downcoast alongshore 

 currents in the spawning season, could, in theory, bring 

 the young inshore. However, Dentzau and Chittenden 

 (1990) rejected this idea. It implies fishes of the brown 

 shrimp community would also recruit inshore, but, in 

 actuality, there is a clear separation between the two 

 communities (Chittenden and McEachran 1976). A 

 more likely alternative, suggested for Polydactylus 

 octonemus (Dentzau and Chittenden 1990), is based on 

 the hydrography and cyclonic gyre of the northwestern 

 Gulf (Kelly et al. 1981, 1983, 1984): The eastward 

 counterflow of the gyre is diverted inshore at the 

 Mississippi River delta and ultimately extends down- 

 coast as an alongshore current. Members of the white 

 shrimp community could spawn anywhere in this flow, 

 in the offshore northeastward flowing arc or in the 



