FISHERY BULLETIN: VOL. 83, NO. :i 



(1981). The number of rings present in the Slope 

 Water is less important than the trajectories of the 

 rings. If they pass too far offshore or appear too 

 early or too late in the spawning season, they may 

 have little impact. It is difficult to measure the 

 magnitude of the onshore-offshore flows from the 

 remote sensing data, but we have tried to consider 

 the size of the shelf edge affected and the longshore 

 speed of the eddy when judging the strength of a par- 

 ticular interaction. Unfortunately, we have no infor- 

 mation for the years considered on the timing or 

 location of spawning which undoubtably is important 

 in determining the impact of individual rings. 



The spring of 1975 was a time of low warm core 

 ring activity (Table 2), and both cod and haddock pro- 

 duced good year classes, essentially the best since 

 the 1960's (Smith et al. 1979). This high recruitment 

 was not due to large numbers of larvae being spawn- 

 ed; indeed, the abundance of cod and haddock larvae 

 was at a 5-yr low (Smith et al. 1979). Apparently, the 

 few larvae present on Georges Bank experienced 

 exceptionally high survival. There was only one ring 

 (designated eddy 6 by Bisagni 1976) which by our 

 model predictions could have influenced recruitment. 

 Eddy 6 was in the vicinity of Georges Bank from 

 February- through April but did not seem to be en- 

 training shelf water for this entire period. 



During the spring of 1976 there were three, 

 possibly four, warm core rings interacting with 

 Georges Bank between January and May. Mizenko 

 and Chamberlin (1979a) presented the track lines for 

 eddies 751 (which later possibly became 76B), 76A, 

 and 76C. Rings with these trajectories had the poten- 

 tial to affect recruitment. The 1976 year class 

 strength for both cod and haddock stocks was weak 

 (Smith et al. 1979). 



The spring of 1977 was a year of unusually high 

 ring activity. Figure 1 shows five rings (77A, 77C, 

 77E, 77G, and 77H) simultaneously present in the 

 Slope Water region on 11 May 1977. Mizenko and 

 Chamberlin (1979b) presented trajectories for all the 

 eddies observed in 1977. Of these rings, 76F, 76G, 

 77A, and 77E appear to be most detrimental. The 

 1977 year-class strength for both cod and haddock 

 stocks was again weak (Serchuk and Wood 1981; 

 Clark et al. 1982). 



The relationship of low ring activity and high 

 recruitment observed in 1975 holds true again for 

 1978. This was a good year for cod and a very good 

 year for haddock recruitment (Serchuk and Wood 

 1981; Clark et al. 1982). Examining the trajectories 

 of warm core rings present during 1978 (Celone and 

 Chamberlin 1980), we find no rings interacting with 

 Georges Bank until late May or June. During the 



60-d period from mid-February to mid-April, no anti- 

 cyclonic eddies were apparent off the northeast 

 coast. In late April and May, there were two rings 

 (78A and 78B), but they occurred too far south or too 

 far offshore to affect the Bank. 



Finally in 1979, the most recent year for which 

 there are both fisheries recruitment data and trajec- 

 tory records, we find three potentially dangerous 

 rings: 781, impinging upon Georges Bank from 

 March to July (Fitzgerald and Chamberlin 1981) and 

 two eddies, 79A and 79B, lingering southwest of the 

 Bank from February through May. The 1979 year 

 class was weak for cod and only average for haddock 

 (Serchuk and Wood 1981; Clark et al. 1982). 



CRITIQUE 



According to our calculations, warm core rings 

 may have considerable influence on the distribution 

 and ultimate survival of cod and haddock larvae 

 spawned on Georges Bank. Our simple mathematical 

 model demonstrates the possible effects of cross- 

 shelf flows induced by a ring upon larval fish distri- 

 butions and abundance. The ring's size, strength, and 

 translation speed are critical in determining its 

 potential impact. A stationary ring may cause a 20 to 

 50% loss of larvae by inducing advective transport 

 off the shelf. If a ring is moving, the impact can be 

 even greater, especially for an eddy travelling long- 

 shore at a slightly slower speed than the shelf water. 

 In this most catastrophic case, a group of larvae can 

 catch up to the back side of the eddy and be entirely 

 swept off the shelf. 



In briefly examining the published literature on 

 ring trajectories and fisheries recruitment statistics, 

 we have found what appears to be a relationship be- 

 tween years of frequent ring interaction with 

 Georges Bank during the late winter and early 

 spring spawning season and weak year classes of cod 

 and haddock. Our analysis can be criticized as cur- 

 sory and incomplete since we had no detailed infor- 

 mation on entrainment features or on many impor- 

 tant biological factors such as the timing and location 

 of spawning. Future studies may show this relation- 

 ship was fortuitous; however, our results certainly in- 

 dicate that future study is warranted. 



An implicit assumption in our analysis of ring 

 events and fisheries recruitment data is that advec- 

 tive losses can affect year-class success. This has not 

 yet been demonstrated for the Northwest Atlantic; 

 however, Parrish et al. (1981) presented convincing 

 arguments that the dominant exploitable fishes off 

 the west coast have adopted spawning behaviors 

 which minimize losses due to offshore transport. 



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