Sensitivity to percent loss of a year class 



The sensitivity of the two selected species to percent loss of a year class 

 has been investigated by projecting the age-specific density vector of each 

 equilibrium population for 50 years after simulating 10 percent incremental 

 reductions in the probability of survival of the first cohort through year-0. 

 The results of these simulations for the herring and cod fisheries are shown in 

 tables 5 and 6, respectively. 



The interest of these simulations comes from the fact that within the 

 limitations of the fishery model itself they cover the full range of impacts 

 resulting from any oil spill event that might occur on Georges Bank regardless 

 of size, type, timing, and duration, as long as two or more successive cohorts 

 of the same species are not impacted. It is interesting to note that even the 

 elimination of an entire year class (i.e., 100% cohort reduction) results in 

 rather moderate losses to the fishery in the case of cod. On the other hand, 

 the loss of an entire year class in the herring population results in ultimate 

 losses to the fishery equivalent to more than a one-year equilibrium catch 

 (table 5). 



These differences in sensitivities of the cod and herring are the result 

 of the different population structures and fecundities of each species, together 

 with the different compensatory formulations used to model them. The strongest 

 compensation occurs in the cod, which typically follows a dome-shaped (Ricker 

 1958) form of stock and recruitment relationship. The herring compensatory 

 ratio is about 25 percent less than for the cod. In addition, the herring com- 

 pensatory mortality follows the Beverton and Holt (1954) form of stock recruit 

 relationship, which weakens even further the ability of this species to compen- 

 sate for year class reductions. The net result is that the herring stock 

 demonstrates less dynamic response to large cohort reductions than the cod stock. 



Finally, the largest one-year catch loss (model output variable A), for the 

 two species can be understood by examining the age structure of the total catch. 

 It is clear that the value of A cannot be larger than the largest percent 

 contribution of a single age-class to the total catch. For instance, A is 17.4 

 percent for the cod for the loss of an entire year class, which is the contribu- 

 tion of the four year olds to the total catch. Thus, a fishery based on a long- 

 lived species such as cod, in which the catch is typically composed of many 

 year classes, is much less susceptible to large catch losses from single event, 

 acute oil spills. A fishery based on the current herring stocks on the other 

 hand, will tend to rely upon only one or two age groups to supply most of the 

 catch, so that recruitment fluctuations caused by oil spills (or variations in 

 natural mortality) will be more strongly reflected in the catch on a percentage 

 bas is . 



Effect of Georges Bank gyre on impact predictions 



An issue which has consistently been raised at the public hearings on the 

 lease sales of tracts off the New England coast concerns the possible formation 

 of an anticyclonic gyre on Georges Bank. There is a variety of oceanographic 

 data supportive of such a gyre during the spring and early summer (Bigelow 1927; 

 Bumpus and Lauzier 1965; EG&G 1979). The existence of such a feature makes 

 sense biologically in that ichthyoplankton would be retained in the Bank's 



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