THE POSSIBLE INFLUENCE OF WARM CORE GULF STREAM RINGS 

 UPON SHELF WATER LARVAL FISH DISTRIBUTION 



G. R. Flierl' and J. S. Wkoblewski=* 



ABSTRACT 



We propose a simple one-dimensional mixlel for examining the impact of warm core riiij^s upon the larval 

 fish distribution and abundance over the continental shelf off the northeastern United States. The model in- 

 cludes (in a cross-shelf averaged sense) the loss of larvae due to biological causes of mortalit^t', the advective 

 transport of larvae due to the mean down-shelf currents, and the changes in larval density pnxiuced by the 

 on-shelf or off-shelf flow s t)ccurring when a ring approaches the shelf-slope front. The results of this highly 

 idealized mt)del indicate that the decreases in larval abundance caused by cross-shelf flows may be as large 

 as those caused by biological factors and, furthermore, the effects are strongly dependent upon the rate of 

 motion of the ring. A stationary ring may cause a 20 to 50% drop in abundance, depending on the strength 

 and size of the ring and on the longshore velocity in the shelf water. When the ring is slowly moving, the im- 

 pact can be even greater: a patch of larvae being advected downshelf by the longshore current could, when 

 catching up to the back side of an eddy, essentially be swept off the shelf, decimating the patch. 



Model predictions are compared with historical MARMAP data of larval cod and haddock density in the 

 Georges Bank area. There does appear to be a relationship between the frequency of ring interactic)n with 

 Georges Bank and the subsequent year-class strength of cod and haddock stocks. Thus we suggest that fur- 

 ther investigation of the impacts of rings is warranted, both from the observational and the theoretical view- 

 points. These studies should include detailed measurements in entrainment features, further analysis of 

 ring-Bank interactions factoring in the closeness of the ring, the strength of its currents and its translation 

 rate, and more detailed modelling of entrainment events and larval fish ecology. 



Warm core rings form in the Slope Water region be- 

 tween the North American continental shelf and the 

 Gulf Stream. These rings are eddies 100 to 200 km in 

 diameter which result when a Gulf Stream meander 

 separates from the main current. The potential im- 

 pact of warm core rings upon the continental shelf 

 ecosystem has become more apparent with routine 

 satellite infrared images of the sea surface. Charts of 

 sea surface temperature prepared from these im- 

 ages, e.g., Figure 1, (Halliwell and Mooers 1979; 

 Chamberlin 1981) frequently show rings entraining 

 cold water from the continental shelf. This interpre- 

 tation is supported by ship observations and current 

 meter records of water transport onto and off the 

 continental shelf induced by warm core rings 

 (Morgan and Bishop 1977; Smith 1978; Smith and 

 Petrie 1982). Physical and biological oceanographers 

 have long been aware of occasional intrusions of 

 anomalously warm water onto Georges Bank (for a 

 review, see Bolz and Lough 1981), which are now 

 likely to be attributable to warm core ring activity. 

 In 1961 Colton and Temple hypothesized that large 



'Department of Earth, Atmospheric and Planetary Sciences, 

 Massachusetts Institute of Technology, Cambridge, MA 02139. 



^Department of Oceanography, Dalhousie University, Halifax, 

 Nova Scotia, B3H 4J1, Canada; present address: Bigelow 

 Laboratory for Ocean Sciences, West Boothbay Harbor, ME 04575. 



numbers of larval fish of shelf species can be drawn 

 off Georges Bank into warmer Slope Water where 

 they succumb to unfavorable environmental condi- 

 tions (c.f. Laurence and Rogers 1976). 



The purpose of this paper is to examine theoreti- 

 cally the possible influence of warm core rings on the 

 abundance and distribution of larval fish in continen- 

 tal shelf waters off the northeastern United States, 

 in particular the shelf region associated with cod and 

 haddock spawning on Georges Bank. Our study 

 makes estimates of the advective losses of larvae 

 because of the entrainment of shelf water by an eddy 

 and predicts changes in larval density (the observ- 

 able quantity in ichthyoplankton surveys) because of 

 the onshore and offshore flows induced by a ring. We 

 also consider the possible biological causes of mortali- 

 ty (e.g., predation, physiological death). In other 

 words we wish to estimate the relative importance of 

 the physical and biological losses via a mathematical 

 model. While our model does not describe either in 

 great detail, we feel that it does indicate the impor- 

 tance of ring-induced entrainment and the 

 dependence of this effect upon the speed of transla- 

 tion of the ring, the width over which it interacts 

 with the shelf, and the strength of its currents at the 

 shelf break. We compare our model predictions of 

 the spatial and temporal distributions of larval fish 



Manuscript accepted October 1984. 

 FISHERY BULLETIN: VOL. 83. NO. 3, 1985. 



313' Jd 



