Smith and Morse Larval distribution patterns of Oupea harengus on the Georges Bank 



341 



<2 weeks 



2 to 5 weeks 



5 to 8 weeks 



"""I ?o 



Figure 3 



Composite representation of the distribution of Atlantic herring Clupea harengus larvae by age in the Georges Bank area, 1971-75. 



in the western North Atlantic for at least 15yr (Tibbo 

 et al. 1958, Boyar et al. 1973b, Anthony & Waring 

 1980, Lough et al. 1985, Grosslein 1987). The magni- 

 tude of spawning activity, as measured by the abun- 

 dance of larvae, was low in 1971 and 1972 but in- 

 creased sharply in 1973 and continued upward in 1974, 

 the initial spawning years of the strong 1970 year- 

 class. In 1975, larval abundance on Georges Bank de- 

 clined dramatically and spawning activity on Nantucket 

 Shoals equaled that on the bank. By 1976, when sur- 

 veys showed only negligible spawning activity east of 

 the 69° meridian, Georges Bank had lost its 

 longstanding status as the principal spawning ground 

 for herring in the western North Atlantic (Fig. 2). 



ICNAF surveys of 1971-75 identified two geographi- 

 cally-separate spawning areas: one centered along the 

 eastern edge of Nantucket Shoals, the other on north- 

 eastern Georges Bank (Fig. 3). Within 2-5 wk of hatch- 

 ing, larvae began to disperse and the two spawning 

 centers were no longer discrete. At 5-8 wk after hatch- 

 ing, some larvae originating on the Northeast Peak 

 were transported westward across the southern half of 

 Georges Bank by anticyclonic currents. Together with 



larvae from Nantucket Shoals, their distribution ex- 

 tended over shelf waters from Cape Cod eastward and 

 continued into adjacent slope waters. Although distri- 

 butions of both Nantucket Shoals and Georges Bank 

 larvae exhibited the influence of known circulation pat- 

 terns within 5 wk of hatching (Fig. Id), centers of abun- 

 dance remained near their points of origin for at least 

 8wk. Thereafter, a single center of abundance for lar- 

 vae >8 wk old emerged over the shallow central part of 

 Georges Bank (Fig. 3). 



Herring larvae in all four age-groups occurred largely 

 in coastal waters immediately adjacent to Cape Cod 

 during the 1976-84 time-interval, a distribution pat- 

 tern that differed significantly from those observed dur- 

 ing the first 5yr of the time-series (Fig. 4). We found 

 no larvae <2wk old on Georges Bank in 1977. In 1978, 

 only a few larvae in the youngest grouping were caught 

 over the once-productive Northeast Peak. For the next 

 decade, we caught no recently-hatched larvae anywhere 

 on Georges Bank. Larvae <2wk old were essentially 

 absent on Nantucket Shoals during the 1976-84 time- 

 interval as well. The center of larval abundance in 

 each of the four groupings occurred in Massachusetts 



