Fl.lKKI.ANDWKOHl.KWSKI; WARM Ci iKK CCLK S'I'KKAM kINCS 



They suggested that deviations from normal trans- 

 port conditions may be a cause of the very large 

 recruitment variations observed in the fisheries for 

 sardine and anchovy. Whether warm core rings 

 represent anomalous conditions or whether the shelf 

 fishes of the Northwest Atlantic have adopted 

 spawning patterns which minimize the losses by en- 

 trainment is yet to be discovered. 



G. Laurence-* has suggested that in some instances 

 vertical migratory behavior may prevent significant 

 numbers of larvae from being advected offshore. If 

 the entrainment feature is shallow and the fish lar- 

 vae avoid the surface layer, then the offshore trans- 

 port could be much less than predicted by our model. 

 The National Marine Fisheries Service is currently 

 surveying entrainment features associated with 

 warm core rings to assess the losses occurring off the 

 shelf. Recently, Wroblewski and Cheney (1984) 

 reported finding significant numbers of the white 

 hake, Urophycis tenuis, larvae 140 km seaward of 

 the Scotian Shelf break in Slope Water which had 

 been entrained by a warm core ring. Urophycis 

 tenuis spawm on the Scotian Shelf and upper con- 

 tinental slope. Wroblewski and Cheney concluded 

 that the ring altered the usual larval drift pattern 

 along the shelf edge. Curiously, no larvae of cod or 

 haddock, which also spawn on the shelf during spring 

 and summer, were found in the ichthyoplankton net 

 tows. It remains to be demonstrated whether suffi- 

 cient numbers of shelf-spawned larvae are trans- 

 ported offshore by rings to affect recruitment of 

 shelf stocks. 



The advective losses predicted by our model may 

 be overestimates if only larvae present near the shelf 

 edge are susceptible to entrainment and if their den- 

 sity is much lower than that further inshore. The 

 influence of the ring may not reach the shallower 

 regions where many larvae are found. Also, biologi- 

 cal losses may be larger than assumed in the model, 

 so that the relative importance of ring-induced losses 

 may be less. 



Warm core ring entrainments are not the sole 

 mechanism by which fish larvae can be transported 

 off the shelf. In 1955, Chase found a relationship be- 

 tween haddock recruitment and the strength of the 

 wind-driven current normal to the southern side of 

 Georges Bank. We also recognize that there could 

 even be beneficial effects to the Georges Bank eco- 

 system if the ring-induced cross-shelf flows push 

 nutrient-rich Slope Water onto the Bank and fertilize 



the system (G. A. Riley^). Rather than exploring all 

 mechanisms, we have chosen to assess one particular 

 source of variability which may contribute to fluctua- 

 tions in year-class strength for the fish stocks spawn- 

 ing on Georges Bank. 



We have also assumed that the fish larvae are well 

 mixed across the shelf, although we know that they 

 are generally distributed in patches. Thus our model 

 solutions may be comparable with field data only if 

 one integrates the field data over x and z as we have 

 done in our simplified model. Fisheries recruitment 

 data naturally reflects this integration over large 

 spatial scales and we are encouraged by the apparent 

 relationships in Table 2. However, loss during the 

 larval period is only one factor affecting recruitment. 

 Events during the postlarval stages are also signifi- 

 cant; Sissenwine et al. (1983) showed that predation 

 in these later stages is an important factor in year 

 class success. 



Our model has given quantitative but crude esti- 

 mates of the importance of ring events for larval sur- 

 vival and suggested that the impact of the ring 

 depends strongly upon its motion; investigations 

 with more highly resolved and more complex models 

 and further survey work for assessing both ring and 

 mesoscale eddy influences on larval fish distributions 

 and subsequent recruitment are the next steps. 



ACKNOWLEDGMENTS 



This research was supported by U.S. National 

 Science Foundation grant number OCE-8019260 

 to G. R. Flierl and J. S. Wroblewski and by Na- 

 tional Sciences and Engineering Research Council of 

 Canada grant number A 0593 to J. S. Wroblew- 

 ski. 



LITERATURE CITED 



BiSAGNI, J. J. 



1976. Passage of anticyclonic Gulf Stream eddies through 

 deepwater dumpsite 106 during 1974 and 1975. NOAA 

 Dumpsite Eval. Rep. 76-1, 39 p. 

 BoLZ, G. R., AND R. G. Lough. 



1981. Ichthyoplankton abundance, diversity and spatial pat- 

 tern in the Georges Bank - Nantucket Shoals area, autumn 

 and winter season 1971-1977. NAFO SCR Doc. 81/IX/136, 

 23 p. 

 CELONE, p. J., AND J. L. Chamberlin. 



1980. Anticyclonic warm-core Gulf Stream eddies off the 

 northeastern United States in 1978. Ann. Biol. (Copenh.) 

 35:50-55. 



^G. Laurence, Northwest Fisheries Center Narragansett Labora- 

 tory, National Marine Fisheries Service, NOAA, R.R. 7 A, Box 

 522 A, Narragansett, RI 02882, pers. commun. June 1983. 



^G. Riley, Department of Oceanography, Dalhousie University, 

 Halifax, Nova Scotia, Canada B3H 4J1, pers. commun. January 

 1983. 



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