892 



Fishery Bulletin 92(4), 1994 



lected from Mosquito Island and was estimated to 

 have been spawned on 1 November 1989. 



Growth of larval fish (and other vertebrates) typi- 

 cally proceeds through a series of consecutive inter- 

 vals (thresholds) which characterize ontogeny. Peri- 

 ods of rapid growth are generally followed by peri- 

 ods of slower development during which complex 

 structures prepare for the next series of changes 

 (Balon, 1984). The Laird-Gompertz equation has been 

 used to describe larval fish growth when the length- 

 age plots are nonlinear and upper asymptotes are 

 apparent (Zweifel and Lasker, 1976; Methot and 

 Kramer, 1979; Laroche et al., 1982; Warlen and 

 Chester, 1985; Warlen, 1988). This model was used 

 to estimate the age-specific growth rate and the ex- 

 ponential decline in the age-specific growth rate as 

 larval gulf menhaden approach metamorphosis to 

 juveniles. 



Growth rates were estimated from larvae between 

 5.8 and 16 mm SL collected offshore and larvae be- 

 tween 17 and 24 mm SL collected within Fourleague 

 Bay. The average daily growth of larvae collected off- 

 shore (0.44 mm/day) was greater than within 

 Fourleague Bay (0.12 mm/day). The average daily 

 growth rate of larval gulf menhaden from the com- 

 bined data was 0.25 mm/day. 



Postlarval gulf menhaden are estimated to be 15- 

 25 mm SL (Shaw et al., 1988) upon entering the es- 

 tuary and begin transformation to the juvenile stage 

 around 20 mm SL. Transformation is complete at 

 about 30 mm (Suttkus, 1956; Hettler, 1984). Between 

 20 and 30 mm SL, however, growth characteristics 

 change (Fig. 2): mouth parts and gill rakers are modi- 

 fied and the body begins to thicken and take on the 



deep-bodied characteristics of juveniles and adults 

 (Suttkus, 1956). During this threshold, postlarval 

 (prejuvenile) gulf menhaden growth in weight is dis- 

 proportionately greater than growth in length 

 (Deegan and Thompson, 1987). 



The period of slowed growth in length just before 

 and during juvenile transformation is followed by a 

 dramatic increase in growth rate (Springer and 

 Woodburn, 1960; Deegan and Thompson, 1987). Av- 

 erage daily growth of gulf menhaden between 18 and 

 82 mm SL reportedly ranges from 0.20 to 0.48 mm/ 

 day within Fourleague Bay (Deegan and Thompson, 

 1987). Our estimate of average daily growth rate for 

 postlarvae (17-24 mm SL) within Fourleague Bay 

 was expectedly lower (0.12 mm/day), since our lar- 

 vae were approaching or were in the process of trans- 

 formation. 



Our estimate of average daily growth from offshore 

 is similar to Warlen's (1988) growth estimates for 

 larval gulf menhaden collected off Southwest Pass, 

 Louisiana (0.28-0.42 mm/day). Some marine larvae 

 have been shown to grow faster at higher tempera- 

 tures (Laurence et al., 1981); however, this has not 

 been demonstrated for gulf menhaden (Warlen, 

 1988). During the winter, surface water temperatures 

 are generally warmer offshore than within 

 Fourleague Bay; however, this difference is gener- 

 ally minimal (Raynie and Shaw, in press). Our tem- 

 perature data are insufficient (and may be atypical 

 of the average conditions) to evaluate the relation- 

 ship between growth and temperature. The differ- 

 ence in growth rates between environments, how- 

 ever, is most likely the result of ontogeny. A 71% de- 

 crease in growth rate between larval and juvenile 



30 40 50 60 



Age (days posthatch) 



Figure 2 



Laird-Gompertz growth models for larval gulf menhaden collected from 

 offshore (n=36) and Fourleague Bay (n=75), Louisiana, between 23 and 

 25 January 1990. n=number of larvae from each environment. 



