Flores-Coto and Warlen: Spawning time, growth, and recruitment of larval Leiostomus xanthurus 



shore. If cohort survival is similar, the birthdate dis- 

 tributions would not be expected to differ. Age of lar- 

 vae recruited to the estuary is a useful measure of the 

 transport period from offshore spawning to recruit- 

 ment into the estuary. Transport time may be expected 

 to vary seasonally as a function of transport rate and 

 spawning distance from shore (transport distance). 



The objectives of this study were to (1) determine 

 age and size of spot larvae immigrating into the New- 

 port River estuary through Beaufort Inlet, (2) esti- 

 mate the spawning season (back-calculated from ages 

 of recruited larvae) and the relative contribution of 

 birthweek cohorts to the total larval spot recruitment, 

 (3) relate the density, age and size distributions, and 

 back-calculated birthweeks of spot larvae collected off 

 the North Carolina coast to the same cohorts collected 

 later just inside the mouth of the estuary, and (4) esti- 

 mate the growth rate of larvae. 



Methods 



Spot larvae used in this study were collected in both 

 marine and estuarine areas. A 1x2m neuston net 

 frame with a 945 |xm mesh net (Hettler 1979) was used 

 to collect larvae (some specimens were early juveniles) 

 at a station adjacent to Pivers Island in the lower 

 Newport River estuary about 2 km inside Beaufort In- 

 let (Fig. 1). The fixed net was fished, with the top of 

 the frame just under the water surface, from a bridge 

 platform in the center of the channel. A flowmeter 

 (General Oceanics model 2030) was attached to the 

 mouth of the net to estimate the amount of water 

 sampled. Four consecutive sets were made during 

 nighttime hours at mid-flood tide, when current was 

 the strongest. Because of the expected seasonal varia- 

 tion in larval fish densities, sets were 2-8 min long 

 (x = 5 min) and sampled 58. 4-443. Om 3 of water. 

 Samples were collected weekly from 10 November 1987 

 to 4 May 1988. The mean of data from all sets on a 

 given night was used as the estimate of density of spot 

 larvae. 



Sampling for spot larvae was also conducted off the 

 North Carolina coast from Cape Fear to Oregon Inlet 

 (Fig.l) during two phases ( 12-13 January and 2-5 Feb- 

 ruary) of cruise 172 of NOAA ship Oregon II in 1988. 

 Sampling stations were arranged along a transect from 

 Beaufort Inlet approximately south-southeast to the 

 400 m isobath, then along the Gulf Stream to about 

 36° N latitude, and finally inshore along a transect to- 

 ward Oregon Inlet (Fig. 1). Additional stations along 

 bands parallel to the coast were added in February in 

 Onslow Bay. Samples at a station were obtained from 

 either a day or night oblique plankton tow using a 

 60 cm Bongo frame fitted with 333 and 505 jxm mesh 



nets and rigged with flowmeters. The amount of water 

 filtered on a tow was depth-dependent and varied be- 

 tween 72.4 and 476.8 m 3 . Ichthyoplankton samples from 

 both estuarine and marine areas were preserved in 

 95% ethanol. Spot larvae from each sample were 

 counted and density estimated as larvae/100 m 3 . 



Larval spot were randomly sampled from each estua- 

 rine collection for aging. If the total number of larvae in 

 a collection was <20, then all fish up to a maximum of 

 10 were used. If there were more than 20 specimens in 

 a collection, then 10-40 larvae were taken for the sample 

 depending upon the coefficient of variation (CV) of the 

 standard length (SL) of 20 fish. If CV was 0.10-0.12, a 

 subsample of 20 fish was taken. If CV was >0.12, the 

 subsample was 30 fish, except when more than 1000 

 fish were caught, at which time 40 fish were used. Lar- 

 vae were measured to the nearest O.lmmSL. Age was 

 determined according to the method of Warlen & Chester 

 (1985). The estimated age of a larva was the observed 

 number of sagittal growth increments from one reading 

 plus the estimated number of days from hatching to 

 first increment formation (5 d). The precision in dupli- 

 cate readings of otoliths from 25 larvae (range 32-86 d) 

 was estimated from the differences in paired readings. 

 The mean (±SD) difference was 1.52±1.26 growth incre- 

 ments, and the range was 0—4 increments. The birthdate 

 (= spawning date) of each larva was back-calculated by 

 subtracting its estimated age from the date of capture. 

 Larvae spawned in a given calendar week were consid- 

 ered in the same calendar birthweek cohort. The spawn- 

 ing period of spot was estimated from the back-calcu- 

 lated birthdates of larvae recruited to the estuary over 

 all seasons. For each weekly collection, the percentage 

 of larvae from each birthweek cohort was determined. 

 Each percentage was multiplied by the corresponding 

 weekly density total (larvae/100 m 3 ) to give the density 

 of larvae from each birthweek cohort. The densities for 

 each birthweek cohort were summed over all collections 

 and their percentage contribution to the total density 

 (1540 larvae/100 m ! ) for all birthweek cohorts was cal- 

 culated. The Laird version (Laird et al. 1965) of the 

 Gompertz growth equation was used to describe growth 

 of spot larvae from the combined marine and estuarine 

 collections. To stabilize the variance of length over the 

 observed age interval, we used the log-transformed ver- 

 sion of the Gompertz growth equation. 



Results 



Estuarine abundance and age/size 

 distribution of larvae 



Abundance of larvae A total of 9760 spot larvae was 

 collected at Pivers Island between 2 December 1987 



