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Back-Calculated Spawning Date 



Figure 6 



Birthdate frequency distributions calculated from the relative abundances of larval Atlantic menhaden, Brevoortia 

 tyrannus, collected in the lower estuary at Pivers Island, North Carolina, in each of seven recruitment years. In 

 each distribution, the vertical line is the median value and the box represents the 2nd and 3rd quartile intervals 

 (central 50%). Lines beyond the boxes represent the range of data. 



close to the new moon phase of the lunar cycle, which 

 would also reduce available light during spawning. 

 Spawning of Atlantic menhaden off the southeast- 

 ern Atlantic states is apparently timed to ensure 

 transport of larvae across the continental shelf and 

 arrival of most of the larvae (about 85% on average) 

 during a time of optimal survival conditions in the 

 estuary. Atlantic menhaden larvae that recruit to the 

 estuary during the peak period (February-April) 

 arrive when the water temperature is rising, prey 

 abundance is high, and predator abundance and es- 

 tuarine resident larval fishes and invertebrate ( cteno- 

 phore) competitor abundances are low (Warlen and 

 Burke, 1990). Some Atlantic menhaden larvae (about 

 15%) recruited to the estuary early in the season, 

 November— December, before the period of coldest 

 water temperatures in January (Warlen and Burke, 

 1990). These early recruited larvae may experience 

 cold-related mortality if estuarine water tempera- 

 tures drop to <4°C (Lewis, 1966; Wilkens and Lewis, 

 1971). However, larval Atlantic menhaden may tol- 



erate lower temperatures than larvae of other spe- 

 cies (e.g. spot, Leiostomus xanthurus) that recruit 

 over the same period in the Beaufort area (Hoss et 

 al., 1988). In milder winters, earlier recruited lar- 

 vae may survive overwinter and, in the following 

 spring and summer, may be larger than the more 

 abundant Atlantic menhaden larval groups recruited 

 several months later. Ahrenholz et al. ( 1989) observed 

 a multi-modal distribution in lengths of juvenile 

 menhaden collected in the summer in North Caro- 

 lina which may reflect several seasonal abundance 

 groups of immigrating larvae (early, middle, late) 

 from a single spawning season. 



Variation in egg production, mortality of eggs and 

 larvae, and losses of larvae by advection to other ar- 

 eas of the coast probably account for the observed 

 differences in relative recruitment abundances at the 

 Pivers Island collection site. There was more than 

 an order of magnitude difference between the rela- 

 tive abundances of the most abundant and the least 

 abundant years. Lewis and Mann (1971) also ob- 



