NELSON ETAL.: LARVAL TRANSPORT OFBREVOORT1A TYRANNUS 



coefficient (Table 4). When combined with the 

 other factors considered above, Chesapeake Bay 

 discharge accounts for an additional 6% of the 

 residual variance in density-independent year- 

 class strength. A fairer test of the effects of dis- 

 charge on larval transport would require that we 

 isolate that portion of the total larval production 

 that would enter Chesapeake Bay under varying 

 conditions. Our knowledge of Atlantic menhaden 

 spawning activities is not sufficient to do this with 

 reasonable precision. 



An absence or reduction in the number of larvae 

 in estuaries during periods of extreme cold has 

 been noted by June and Chamberlin (1959) and 

 Reintjes and Pacheco ( 1966). Kendall and Reintjes 

 (1975) hypothesized that severe winters, par- 

 ticularly in the northern segment of the spawning 

 range, result in heavy kills of overwintering lar- 

 vae in the estuaries. In addition, laboratory ac- 

 climation studies have shown high mortality rates 

 when menhaden larvae were held for several days 

 at temperatures below 3°C (Lewis 1965). A time 

 series of minimum mean monthly sea surface 

 temperatures was located for the mouth of Dela- 

 ware Bay from National Ocean Survey Tide Sta- 

 tion Observer Records (U.S. Department of 

 Commerce 1973). These data were considered 

 representative of mid-to-northern coastal areas in 

 the Middle Atlantic Bight. Correlation of the 

 survival index for the entire population and the 

 minimum temperature yielded a low correlation 

 coefficient (Table 4). The correlation is somewhat 

 of an artifact, however, and probably is biased by 

 the positive correlation between Ekman transport 

 and year-class strength. Westward Ekman 

 transport is generated by winds from the north. 

 Years of high westward transport in winter 

 months are years of sustained north winds, which 

 are associated with cold air masses. Under such 

 conditions, we would expect cooler sea-surface 

 temperatures in those years, particularly in or 

 near shallow estuarine areas. There may be a posi- 

 tive correlation between temperature and survi- 

 val, but the relationship probably is masked by the 

 overriding effects of wind-generated Ekman 

 transport (Table 5). The low correlation coefficient 

 could also indicate that only a small portion of the 

 population would overwinter in northern waters 

 where temperature stress might be a significant 

 factor. 



If low temperature reduces survival, a transport 

 mechanism to carry fall-spawned larvae south- 

 ward along the Middle Atlantic Bight into the 



vicinity of estuaries that have milder winter 

 temperatures would be a positive survival factor. 

 Therefore, the meridional (north-south) compo- 

 nent of Ekman transport in the Middle Atlantic 

 Bight at lat. 39°N, long. 72°W near the edge of the 

 shelf off Delaware Bay was considered. A corre- 

 lation between the survival index and the 

 southward transport for the October-December 

 spawning period resulted in a coefficient of 0.336, 

 which accounts for about 10% of the total variance 

 in density-independent recruitment. However, 

 the contribution to reduction in residual variance 

 was minimal, because all of the variation due to 

 southward transport was accounted for by linearly 

 related east-west zonal Ekman components al- 

 ready considered. A relatively steady state 

 southward transport mechanism exists in the 

 Middle Atlantic Bight in the form of a southward 

 flowing current over the shelf (Bumpus 1973). 

 Because this current is quasi-permanent, vari- 

 ations in southward Ekman transport may be of 

 little significance and may only create minor 

 fluctuations in strength of an existing transport 

 mechanism. 



RECRUIT-ENVIRONMENTAL MODEL 



The logic used in the selection of environmental 

 parameters for inclusion in a model of en- 

 vironmental effects is depicted schematically in 

 Figure 5. The heavy line represents an intuitive 

 weight of density-dependent and density- 

 independent factors in the survival of menhaden 

 larvae from the time of spawning through their 

 oceanic phase. In the upper Middle Atlantic Bight, 

 for example, spawning takes place close to shore or 

 in major bays and sounds, reducing or eliminating 

 the time spent by larvae in the open ocean. This 

 would reduce dependence on favorable currents 

 for transport. Under such conditions, environ- 

 mental factors influencing mortality may be rela- 

 tively stable, with variation in the number offish 

 spawning in the area being the probable cause of 

 most of the variation in the number of recruits 

 produced. In the South Atlantic Bight, however, 

 spawning takes place offshore, and dependence on 

 favorable ocean currents would seem to have 

 greater weight than spawning stock size on 

 survival. Large annual variations in transport 

 would produce large variations in survival in the 

 South Atlantic Bight at a given stock size. The 

 lower Middle Atlantic Bight seems to be an in- 

 tergrade between the two extremes, with sig- 



33 



