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



19 



- 0.024 



ESTIMATED AGE (days) 



Figure 9 



Growth of larval spot Leiostomus xanthurus collected from the estuary and oce- 

 anic waters off North Carolina, November 1987 to May 1988. A Laird-Gompertz 

 model was fit to the age/size data for 663 fish. Estimates of the parameters were 

 obtained by fitting the log-transformed version of the model to the data. L„=length 

 at hatching, A,i=specific growth rate at hatching, and a = exponential decay of the 

 specific growth rate. 



Stream (Fig. 6) suggest that larvae are being trans- 

 ported to areas north of Onslow Bay. The origin of 

 these larvae is probably south of Cape Hatteras and 

 most likely Onslow Bay or southward. These data sup- 

 port the hypothesis of Norcross and Bodolus (1991) 

 that spot spawned south of Cape Hatteras on the outer 

 continental shelf contribute to recruitment in Chesa- 

 peake Bay. Spring (March-May) spawned bluefish 

 Pomatomus saltatrix are also thought to be transported 

 to the Middle Atlantic Bight from spawning areas near 

 the edge of a northerly flowing warm-water mass (Gulf 

 Stream) in the South Atlantic Bight (McBride & 

 Conover 1991). 



The extended recruitment period of 5 months ( Fig. 2 ) 

 is a reflection of the length of spawning period (Fig. 5), 

 although the time from spawning to recruitment var- 

 ies throughout the season. The beginning of the maxi- 

 mum recruitment period coincides with increasing es- 

 tuarine water temperature. Warlen & Burke (1990) 

 found that peak immigration into North Carolina es- 

 tuaries of fall-winter spawned ichthyoplankton 

 matched the period of rising water temperature. This 

 idea agrees with the fact that spot abundances are low 

 during cold periods. Low water temperatures (<10°C) 

 can cause cold stress by increasing larval respiration 

 rate and can kill spot larvae (Hoss et al. 1988). The 

 maximum estuarine recruitment period in North Caro- 



lina probably varys slightly from year 

 to year, but is basically midwinter to 

 early spring. Our estimate of the maxi- 

 mum recruitment period (mid-Febru- 

 ary to mid-April) is similar to that 

 (February-March) recorded by Hettler 

 & Chester (1990) and that (mid^Janu- 

 ary to mid-March ) found by Warlen & 

 Burke ( 1990). Apparently once spot lar- 

 vae are in the estuary they move to- 

 ward fresher water and utilize upper 

 reaches of estuaries as nursery areas 

 (Weinstein et al. 1980, Allen & Barker 

 1990). Peak recruitment of spot to the 

 marshes of the Cape Fear River estu- 

 ary in North Carolina occurred during 

 March and April (Weinstein 1979). 

 Interannual variations may be ex- 

 pected as a consequence of the sea- 

 sonal changes that trigger emigration 

 of the adults from estuaries to oceanic 

 spawning areas and the subsequent 

 transport rates of larvae back to the 

 estuary. 



The sum of the weekly mean larval 

 densities over all collections ( 1540 lar- 

 vae/100 m 3 ) was almost double that of 

 1985-86 (estimated from Fig. 2 of Warlen & Burke 

 1990), but less than that of 1989-90 and about equal 

 to 1986-87 and 1988-89 (S.M. Warlen, unpubl. data). 

 Allen & Barker (1990) also recorded variable patterns 

 of larval spot abundance in South Carolina estuaries 

 during 1981-84. The four highest peaks of recruitment 

 density (10 and 24 February, 23 March, 13 April), that 

 contributed about 74% of all spot larvae, could be con- 

 sequences of concentration mechanisms of larvae out- 

 side the inlet and the subsequent facilitation of the 

 influx of pooled larvae to the estuary. Lyczkowski- 

 Shultz et al. (1990) suggested that larvae of spot, as 

 well as other offshore spawners, accumulate in 

 nearshore areas to develop and grow prior to recruit- 

 ment. Tide may be an important mechanism which 

 forces the larval gathering process outside and inside 

 inlets (Pietrafesa & Janowitz 1988). The higher abun- 

 dance of larvae just inside the inlet compared with the 

 abundance at inshore stations seems to be a common 

 feature for many estuarine-dependent species (Warlen 

 1982, Lewis & Judy 1983, Warlen & Chester 1985). 



Because any birthweek cohort can be widely dis- 

 persed in the ocean, their larvae may reach the estu- 

 ary over a period of 2-10 weeks. However, in general, 

 >50% of the larvae of any birthweek cohort are re- 

 cruited to the estuary in one week (Table 1). Birthweek 

 cohorts of 25 October to 15 November had bimodal 



