NOTE Raynie and Shaw: Comparison of larval and postlarval growth rates of Brevoortia patronus 



891 



Samples were initially preserved with 95% etha- 

 nol, stored in ice, and later preserved with a 70% 

 ethanol solution in the lab. Temperature and salin- 

 ity were measured with a Beckman Portable Elec- 

 tronic Salinometer (Model No. RS5-3). 



Laboratory analysis 



Notochord lengths (preflexion, NL) or standard 

 lengths (SL) were measured to the nearest 0.1 mm 

 with an ocular micrometer under a dissecting micro- 

 scope. Sagittal otoliths were then removed from a 

 random subsample (^=111) of larvae under a dissect- 

 ing microscope with polarized light. Otoliths were 

 air-dried and mounted on a glass microscope slide 

 with S/P Accu-mount 60. Otoliths were sufficiently 

 thin and rings sufficiently spaced to allow for optical 

 sectioning (focusing to the plane of maximum clar- 

 ity) under a compound microscope (400x or l,000x) 

 to make total increment counts and otolith diameter 

 measurements. Increments were independently 

 counted by each author and averaged. 



Spawning dates were back-calculated for each 

 larva by subtracting the estimated age from date of 

 capture (i.e. capture date -[ring count + 5 days for 

 egg incubation and yolk-sac absorption]) (Warlen, 

 1988). It was assumed that there were no differences 

 in the age at first increment deposition (5 days) 

 among larvae. 



Statistical analysis 



Age and growth data from each 

 environment (offshore and es- 

 tuarine) and the combined data 

 were fit to the Laird version of 

 the Gompertz growth equation 

 (Laird et al., 1965) by means of 

 nonlinear least squares regres- 

 sion techniques (SAS Institute, 

 Inc., 1985): 



L t =L e \ I, 



where L t = standard length of 

 larvae at day f ; L = initial 

 length; K = A /a;A = age-spe- 

 cific growth rate at L ; and a = 

 the exponential decline in the 

 age-specific growth rate. Be- 

 cause five days were added to 

 otolith counts to attain age es- 

 timates (2 days incubation + 3 



i c , 



10 



K 5 



days between hatching and exogenous feeding and 

 first increment formation), two days were subtracted 

 from the age estimates, so that the Y-intercept would 

 approximate the hatching length. The length at hatch- 

 ing has been observed from laboratory data (2.6-3.0 

 mm NL; Hettler, 1984) and estimated from field data 

 (2.4 mm NL; Warlen, 1988). With these data, we fixed 

 the hatching length at 3.0 mm NL in our models. 

 Average daily growth rate was estimated by 



Average daily growth = 



( standard length - 3.0 mm ) 

 days posthatcb 



(after Deegan and Thompson, 1987). 



Results and discussion 



The mean surface water temperature offshore at the 

 time of capture was 17.8°C (range 17.5-18.0°C) and 

 the mean salinity was 31.0 ppt (range 29.0-33.0 ppt). 

 Within Fourleague Bay, the mean temperature was 

 19.0°C (range 17.5-20. 1°C) and salinities ranged from 

 2.7 to 7.3 ppt, with the exception of our 24 January 

 1990 Lower Bay collection when the salinity was 

 23.9 ppt. 



According to age estimates, most of the larvae col- 

 lected offshore were spawned within one week be- 

 tween 27 December 1989 and 3 January 1990. Vir- 

 tually all larvae collected within Fourleague Bay 

 were spawned between mid-November and 24 De- 

 cember 1989, with a peak between 17 and 24 De- 

 cember 1989 (Fig. 1). One 85-day-old larva was col- 



□ Offshore, n =36 



 Fourleague Bay, n=75 



IjiLlIijjiiII 



,n , ri n 



Estimated Spawning Date 



Figure 1 



Percent frequency of birth (spawning) dates estimated from otolith increment 

 data for gulf menhaden collected offshore and within Fourleague Bay, Louisi- 

 ana, between 23 and 25 January 1990. rc=number of larvae collected from 

 each environment. 



