Abstract. - Annual assessments 

 of the Northwest Atlantic mackerel 

 stock have occurred every year since 

 1973, providing useful advice to fish- 

 ery managers involved in the deci- 

 sion making process for this impor- 

 tant pelagic resource. Since 1985, 

 assessment advice based on an F A 

 management strategy has indicated 

 that catches in the 300,000 mt range 

 are feasible because stock biomass 

 has increased greatly after the col- 

 lapse of the fishery in the mid-1970s. 

 However, indications from previous 

 research are that compensatory pro- 

 cesses are very important, so a sto- 

 chastic simulation model with den- 

 sity-dependent growth, maturity, and 

 natural mortality was constructed to 

 study how these mechanisms might 

 affect our ability to provide short- 

 and long-term advice for this impor- 

 tant stock. Model results suggest 

 that our present assessments may be 

 too optimistic relative to yield pro- 

 jections and that minimum spawn- 

 ing-stock biomass levels may be dif- 

 ficult to maintain even with an F 0] 

 fishing strategy. Model results also 

 reveal that natural mortality rates 

 are probably much higher than pre- 

 viously thought and are important in 

 determining trends in abundance in 

 this stock. 



Impact of Compensatory 

 Responses on Assessment Advice 

 for the Northwest Atlantic 

 Mackerel Stock 



William J. Overholtz 

 Steven A. Murawski 

 William L. Michaels 



Woods Hole Laboratory, Northeast Fisheries Science Center 



National Marine Fisheries Service. NOAA. Woods Hole, Massachusetts 02543 



Manuscript accepted 19 September 1990. 

 Fishery Bulletin, U.S. 89:117-128 (1991). 



The Northwest Atlantic stock of At- 

 lantic mackerel Scomber scombrus 

 has historically been important to the 

 U.S. domestic fishery; records from 

 the early 1800s to the 1980s suggest 

 that cumulative landings have been 

 7-8 million mt over that time-period 

 (Sette and Needier 1934, Hoy and 

 Clark 1967, Anderson 1985). A thriv- 

 ing domestic industry utilized mack- 

 erel well into the 1940s until landings 

 dropped because of declines in abun- 

 dance, availability, and increased pro- 

 duction of fresh and frozen white fish 

 products (e.g., haddock, cod) (Hoy 

 and Clark 1967, Jenson 1967). A re- 

 surgence of the fishery occurred in 

 the 1970s when distant water fleets 

 from eastern Europe and the Soviet 

 Union landed an average of 310,000 

 mt annually from 1970 to 1976 (Fig. 

 1A). Since many of the important 

 groundfish species in the region have 

 declined recently, the U.S. industry 

 has become more interested in mack- 

 erel as a volume (high-catch, low- 

 price) fishery. 



The fishery has been managed under 

 the auspices of the Mid- Atlantic Fish- 

 ery Management Council since 1977. 

 Current management objectives for 

 this stock include maintenance of a 

 minimum spawning stock (600,000 

 mt), annual quotas based on an F .i 

 catch strategy and a recognition of 

 the necessity for keeping the total 

 stock at some reasonably high level 



to insure that the recreational fishery 

 remains viable. 



Recent assessments suggest that 

 the stock has increased since collaps- 

 ing in the late 1970s. A succession of 

 moderate to good year-classes from 

 1981 to 1985 promoted rapid recov- 

 ery of the stock to levels observed in 

 the early 1970s (Fig. IB, C). Assess- 

 ment advice during the last several 

 years based on an F .i management 

 strategy has indicated that annual 

 catches in the 300,000 mt range are 

 feasible in the short term. Allocations 

 to joint ventures have increased over 

 the last several years, amounting to 

 about 75,000 mt in 1987, but recent 

 landings have remained well below 

 the 300,000 mt level and fishing mor- 

 tality has averaged only about 0.07 

 since 1980 (Overholtz and Parry 

 1985). 



Pelagic fishes such as mackerel are 

 important in the trophic dynamics of 

 fishery ecosystems, supporting popu- 

 lations of predatory fish, birds, and 

 marine mammals. Additionally, these 

 species may also increase to densities 

 that inhibit their own population pro- 

 cesses (e.g., growth, reproduction) 

 and those of competitors. Evidence 

 exists that Atlantic mackerel exhibit 

 density-dependent growth (McKay 

 1979, Lett 1980, Overholtz 1989). 

 Other factors such as maturation 

 rates, fecundity-at-age, and preda- 

 tion mortality rates may also vary 



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