FISHERY BULLETIN: VOL. 83. NO. 3 



and Wroblewski 1984), but it is important to 

 remember that these also include the advective 

 losses. We have therefore chosen a smaller value of ^u 

 to reflect only biological processes; alternate values 



N 



of /i will be considered shortly. The graph shows the 

 extreme situation where the inflow or outflow is 

 uniform over the whole downstream distance. The 

 decay rate with distance travelled is always increas- 

 ed for offshore flow. For onshore flow, the decay rate 

 can be reduced below the "no-ring" case but only 

 very far downstream (x > 500 km) where the flow 

 rate down the shelf is huge {U = 30 cm/s). Since on- 

 shore flows of 20 cm/s over a 500 km stretch of shelf 

 are not likely to occur, we can conclude that the 

 spatial decay rate will be enhanced in both the 

 regions of onshore flow and the areas with offshore 

 flow. 



The net result, when a stream of larvae moving 

 down the shelf and declining in density due to biolog- 

 ical losses encounters a stationary eddy, can be calcu- 

 lated by combining the result for offshore flow in the 

 region < x < Z> with the one for onshore flow in x > 

 D where the ring is centered at the point x = D.ln 

 Figure 5 we compare the solution without the eddy 



N 



exp 



-fXX 



(16) 



to that with the eddy 



exp(-MT) 

 UiD) 



forO<x<D 



Uix) 



exp {-^AT)forD < x<W 



, T = 



s 



' dx' 

 U{x') 



(17) 



where W is the length of the shelf domain, taken to 

 be 500 km. Most of the decrease in population den- 

 sity occurs in the onshore flow regime; the final den- 

 sity is only half of that which would occur in the 

 absence of the eddy. In assessing the causes of the 

 population decrease, it is clear that the physics and 

 the biology play comparable roles: the decrease in 

 density when there is no eddy is a factor of two dur- 

 ing the transit down the shelf (Fig. 5). The changes 

 in density which occur predominantly in the region 

 of onshore flow when the currents are present, give 

 another factor of 2.5 decrease. Note that recruit- 

 ment cannot be inferred directly from these density 

 patterns; we will address the implications for recruit- 

 ment in the section on Moving Eddies below. 



We summarize the impact of stationary rings for 

 various values of the onshore-offshore flow rates and 

 the scale length along the shelf impacted in Figure 6. 

 Here we plot contours of the ratio of the population 

 density far downstream of the eddy (N ) to that 



N_(Vo • 0) 

 No 



Figure 5. - Same as Figure 3 except biological lo.sses {^/ = 10 

 s" ) are included. 



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320 



