Warlen: Spawning time and recruitment of Brevoortia tyrannus 



421 



recruitment is also a measure of the 

 total time from offshore spawning to 

 recruitment to the estuary. Valuable 

 new information on the early life his- 

 tory of larvae can be obtained when 

 estimates of recruitment densities 

 are combined with estimates of sea- 

 sonal age composition of the catches. 

 Back-calculated birthdate distribu- 

 tions of larvae in each sample can be 

 multiplied by the catch density to de- 

 termine the relative contribution of 

 birthweek cohorts to the number of 

 immigrants. 



The purposes of this seven-year 

 study were to document the duration 

 and relative abundance of larval At- 

 lantic menhaden recruitment to the 

 estuary near Beaufort, North Caro- 

 lina; to measure size and estimate 

 ages of recruited larvae; to back-cal- 

 culate birthdate distributions of lar- 

 vae; and to determine the contribu- 

 tion of birthweek cohorts to the num- 

 ber of immigrants for an estimation 

 of temporal spawning within and 

 among sampling years. 



Methods 



Larval collection 



CAPE LOOKOUT 



Larval Atlantic menhaden were col- 

 lected at a station adjacent to Pivers 

 Island ( Fig. 1 ) as they recruited to the 

 Newport River estuary from the At- 

 lantic Ocean. Sampling, designed to cover the ex- 

 pected recruitment period of mid-November to April 

 (Lewis and Mann, 1971), ran for seven consecutive 

 years beginning with the 1985-86 sampling year. 

 Larvae were sampled from 13 November 1985 to 23 

 April 1986 with a 60-cm bongo frame with paired 

 505-|im mesh nets and flow meters pulled by a 

 6.7-m boat. Tows were made weekly during night- 

 time hours just after mid-flood tide, when the cur- 

 rent was strongest, to reduce potential net avoidance 

 by larvae. Data from the catch of both nets from two 

 consecutive tows, one surface-bottom-surface (double 

 oblique) in 3-5 m of water and one just under the 

 surface (subsurface), were averaged to estimate lar- 

 val density (number larvae/100 m 3 water fished) and 

 to provide fish for ageing. The lack of a significant 

 difference (paired comparison r-test, P>0.05) in catch 

 density from subsurface and double oblique tows al- 



Figure 1 



Location of the sampling site for larval Atlantic menhaden, Brevoortia 

 tyrannus, in the lower Newport River estuary near Pivers Island, North 

 Carolina. 



lowed the paired data to be combined. In the other 

 six sampling years (19 November 1986-30 April 1987, 

 10 November 1987-4 May 1988, 16 November 1988- 

 3 May 1989, 15 November 1989-2 May 1990, 14 No- 

 vember 1990-24 April 1991, and 13 November 1991- 

 6 May 1992) larvae were collected, with a 1 x 2 m 

 neuston net frame fitted with a 945-um mesh net 

 and flow meter fished just under the water surface 

 from a bridge platform over the center of the chan- 

 nel adjacent to Pivers Island. This location was only 

 meters from the site sampled with the bongo nets in 

 1985-86. As in 1985-86, all samples were collected 

 weekly during nighttime hours at mid-flood tide. 

 Three consecutive sets were made each night in 

 1986-87 and four each night in subsequent years. 

 Because of the expected seasonal variation in men- 

 haden abundance, sets were between 2 and 16 min- 

 utes long; most were 5-7 minutes long. Volume of 



