NELSON KT AL.: LARVAL TRANSPORT OFBREVOORT1A TYRANNUS 



physiological requirements for estuarine de- 

 pendence, but metamorphosing larvae are rarely 

 taken in the ocean, indicating that apparent 

 requirements (food, shelter, etc.) provided by 

 estuaries are essential in the life cycle of 

 menhaden. Transport to the vicinity of estuaries 

 should increase the opportunity for entering 

 nursery grounds, resulting in good year classes 

 from years of strong onshore transport. Weak 

 onshore transport or water movement offshore 

 would increase the distance that must be actively 

 traversed, reduce chances of survival, and result 

 in a poor year class. If variation in survival is due 

 to variation in the efficiency of transport of larval 

 menhaden from offshore areas to estuaries, then 

 knowledge of the transport mechanisms would be 

 useful for understanding and predicting variation 

 in year-class strength. 



Menhaden larvae have been found to be more 

 abundant in the upper 15 m of the water column 

 than in the underlying 18-33 m in extensive 

 surveys of our Atlantic shelf waters (Kendall and 

 Reintjes 1975; Chapoton see footnote 5). It is 

 assumed, therefore, that they remain in the upper 

 mixed layer and are transported along with it. 

 Horizontal transport in the surface layer is 

 principally the result of extensive quasi-steady- 

 state currents and local, variable currents, which 

 are strongly influenced by wind and run-off. 

 Steady state currents, by definition, cannot be 

 responsible for year-to-year variation in larval 

 transport and recruitment, so attention was first 

 turned to the local, variable currents which are 

 superimposed on the quasi-steady-state circula- 

 tion of the surface layer. 



In the search for a westward transport 

 mechanism which varies seasonally and from 

 year-to-year, wind drift data computed from mean 

 monthly atmospheric pressure distributions for 

 the period 1946 to the present were considered 

 first. In particular, plots of zonal (eastward or 

 westward) wind-driven (Ekman) transport 

 produced by the Pacific Environmental Group, 

 NMFS, NOAA were studied (for method see 

 Bakun 1973). A grid point (lat. 35°N, long. 75°W) 

 located about 56 km southeast of Cape Hatteras 

 was selected as being representative of the wind 

 field in the area of interest. The seasonal variation 

 of Ekman transport at lat. 35°N, long. 75°W 

 generally includes relatively strong WSW-SW- 

 SSW transport during the first quarter of each 

 year. Because of the SW-NE trend of the coastline 

 south of Cape Hatteras, Ekman transports sig- 



nificantly west of southwestward (those with a 

 stronger westward component) would be most 

 effective in transporting eggs and larvae toward 

 estuarine nursery areas. Plots of the monthly 

 zonal transport at this point revealed conditions of 

 eastward or weak westward transport during most 

 of the year, shifting to moderate or strong west- 

 ward transport during January-March; a 

 periodicity which matched that of spawning of 

 menhaden south of Cape Hatteras (Figure 1). 



In coastal waters of the Middle Atlantic Bight 

 between Virginia and Long Island, N.Y., com- 

 putations of monthly zonal Ekman transport 

 exhibited a pattern similar to that found south of 

 Cape Hatteras. Monthly zonal Ekman transport 

 values computed for this area show that stronger 

 westward transport generally occurs in the 

 November-February period of menhaden spawn- 

 ing activities, possibly providing a mechanism for 

 transporting menhaden larvae into the vicinity of 

 estuarine environments. 



A model of the circulation of the shelf waters off 

 the Chesapeake Bight was developed and cited for 

 its application to menhaden year-class strength by 

 Harrison et al. (1967). The model was used in an 

 attempt to explain the difference in "production of 

 young menhaden" in Chesapeake Bay from the 

 1958 year class, an unusually productive one, and 

 the 1964 year class, which was well below average. 

 The model yielded inappropriate surface current 

 regimes to explain strong shoreward larval 

 transport in 1957-58, and Harrison et al. chose 

 near-bottom currents, which appeared more 

 favorable, as an explanation. As cited earlier, data 

 collected in comparative net tows indicate that 

 menhaden larvae are more abundant in the upper 

 layer than the near-bottom layer, a condition 

 which weakens the premise on which the argu- 

 ment is based. 



Application of the Ekman drift data to the 

 problem of explaining the large difference in 

 menhaden production in Chesapeake Bay in 1958 

 and 1964 leads to a more satisfactory biological 

 conclusion than the bottom-layer-transport model 

 used by Harrison et al. (1967). The average 

 monthly westward Ekman transports for the 

 November-March period at two points in the 

 Middle Atlantic Bight for 1957-58 (Table 1) were 

 about twice as large as those for 1963-64, qual- 

 itatively implying that variation in wind-driven 

 surface layer transport of larvae may be at least 

 partly responsible for the amount of variation in 

 menhaden year-class strength. 



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