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



345 



mixing of early-stage larvae reported here, we see no 

 compelling argument for the separation of herring 

 stocks through larval retention in the western Gulf of 

 Maine and Georges Bank areas. 



We consistently found sufficient larval drift and mix- 

 ing to question the stock structure of Sinclair & lies 

 ( 1985) in the Georges Bank region. However, Figs. 3-6 

 lend some support to the retention hypothesis by show- 

 ing that a significant portion of the larvae up to 8wk 

 old remained near their points of origin. Campana et 

 al. (1989a, 1989b) reported similar results for haddock 

 Melanogrammus aeglefinus and Atlantic cod Gadus 

 morhua larvae, respectively, on Browns Bank. Some 

 larvae of the two species were transported shoreward 

 while others remained near the spawning grounds 

 longer than known water residence time. Both of the 

 1989 studies concluded that the instability of the 

 Browns Bank gyre accounted for the dichotomous dis- 

 tribution patterns. They interpreted the onshore drift 

 as an all-or-none situation for individual larvae. Some 

 readily exited the influence of a weakly established 

 gyre, while most were retained during periods of pro- 

 nounced gyral circulation. In our view, their theory 

 provides a more plausible explanation for the shifting 

 distribution patterns of herring larvae around Georges 

 Bank, an area also influenced by gyral circulation, than 

 the stock hypothesis proposed by lies & Sinclair ( 1982). 



Stephenson & Kornfield (1990) attributed the in- 

 creasing biomass of herring on Georges Bank during 

 the late 1980s to the resurgence of residual fish rather 

 than to recolonization by fish from surrounding areas. 

 Based largely on a 4yr dataset that excluded Nan- 

 tucket Shoals and Massachusetts Bay, they concluded 

 that (1) adult herring caught on Georges Bank dif- 

 fered in age composition from herring on Jeffreys Ledge 

 and southwestern Nova Scotia, (2) some of the Georges 

 Bank herring exhibited different isoenzyme character- 

 istics than neighboring herring, and (3) the return of 

 herring to Georges Bank took longer than they ex- 

 pected through recolonization. 



In our view, the change in spawning sites on Georges 

 Bank during the 20 yr time-series, coupled with the 

 recession and subsequent expansion of the spawning 

 range over time, makes a convincing case for recoloniz- 

 ation rather than resurgence. Larval distribution pat- 

 terns on Georges Bank during the closing years of the 

 time-series differed markedly from those observed at 

 the outset of the program. Whereas recently-hatched 

 larvae were concentrated on Northeast Peak during 

 the initial survey years, their distribution on the bank 

 progressed eastward only as far as Georges Shoals 

 during the final years of survey activity. If, as pro- 

 posed by Stephenson & Kornfield (1990), resurgence of 

 residual fish accounted for the recovery of spawning 



herring on the bank, we would expect little or no change 

 in the location of spawning centers over time. Instead, 

 our 20 yr time-series shows dramatic change. 



When the herring population in the study area de- 

 clined in the early 1970s, its spawning range receded 

 from Georges Bank to Massachusetts Bay. As the popu- 

 lation began to grow in the 1980s, the spawning range 

 expanded, initially from Massachusetts Bay to Nan- 

 tucket Shoals, then from the shoals to Georges Bank. 

 Whether the recovery resulted from an expanding adult 

 population, from fortuitous transport and survival of 

 larvae, or from a combination of the two, is not clear. 

 However, it is worth noting that the reoccupation of 

 spawning beds on both Nantucket Shoals and Georges 

 Bank occurred 3-4 yr after we first observed evidence 

 of larval drift towards these two subareas. This time 

 period, perhaps coincidently, represents the age-at- 

 maturity for Atlantic herring. 



Acknowledgments 



Although too numerous to mention individually, we 

 herewith acknowledge everyone involved in the collec- 

 tion of the 8590 plankton samples that form the basis 

 of this document; our colleagues at the Polish Sorting 

 Center in Szczecin for their diligence and persever- 

 ance in sorting the samples; the technicians and biolo- 

 gists responsible for implementing quality control, data 

 entry, and database management procedures; and those 

 who reviewed earlier drafts of the manuscript and pro- 

 vided constructive comment. 



Citations 



Anthony V., & G. Waring 



1980 The assessment and management of the Georges 

 Bank herring fishery. Rapp. P.-V. Reun. Cons. int. 

 Explor. Mer 177:72-ill. 

 Bartsch, J., K. Brander, M. Heath, P. Munk, K. 

 Richardson, & E. Svendsen 



1989 Modelling the advection of herring larvae in the 

 North Sea. Nature ( Lond. ) 340:632-636. 

 Boyar, H., R. Cooper, & R. Clifford 



1973a A study of the spawning and early life history of 

 herring iClupea harengus harengus L.) on Jeffreys 

 Ledge in 1972. Int. Comm. Northwest Atl. Fish. Res 

 Doc. 73/96, 27p. 

 Boyar, H., R. Marak, F. Perkins, & R. Clifford 



1973b Seasonal distribution and growth of larval her- 

 ring iClupea harengus L.) in the Georges Bank-Gulf 

 of Maine area from 1962-1970. J. Cons. Cons. Int. 

 Explor Mer 35:36-51. 

 Butman, B., & R. Beardsley 



1987 Physical oceanography. In Backus, R., & D. Borne 

 (eds.), Georges Bank, p.88-98. MIT Press, Cambridge. 



