126 



Fishery Bulletin 89(1), 1991 



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Fishing Mortality 



Figure 1 1 



(A) Mean catch, (B) coefficient of variation of catch, and (C) 

 proportion of the time spawning-stock biomass (SSB) falls 

 below 600,000 mt for year 10 of a set of 10-year simulations 

 of the standard assessment (STD) (darkened bar) and density- 

 dependent simulation (DDM) (cross-hatched bar) models, with 

 fishing mortalities of 0.1-0.6 in 0.1 increments. 



progressively more variable as fishing mortality in- 

 creases (Fig. 11B). Both models have relatively high 

 CV's at all levels of fishing mortality, suggesting a 

 large range in possible catches. 



The minimum spawning-stock criteria of 600,000 mt 

 is a threshold biomass defined in the present fishery 

 management plan. This value was chosen by managers 

 because a relatively clear demarcation point between 

 low and high recruitment is evident in the 1962-85 

 stock-recruit data series (Anderson 1985). When the 

 simulation results are expressed relative to the pro- 

 portion of times the SSB drops below the 600,000 mt 

 level for each model, the STD model appears to be 

 much less prone to risk and, therefore, is much more 

 optimistic than the DDM model results. The propor- 

 tion ranges from 0.0 at F = 0.1 to about 0.52 at F = 0.6 

 for the STD model, while P ranges from 0.01 to 0.94 

 for the DDM model (Fig. 11C). The STD model results 

 suggest that there is little risk of the SSB dropping 

 below the threshold, even at fishing mortality rates 



of 0.4-0.5 (Fig. 11C). The DDM model results are 

 much less optimistic, suggesting the need for some 

 concern at fishing mortality levels of 0.3 and greater 

 (Fig. 11C). 



Discussion 



Biological interactions may play an important role in 

 regulating marine ecosystems (Sherman et al. 1981, 

 Walters et al. 1986, Overholtz and Tyler 1986, Over- 

 holtz et al. In press). Species interactions are becom- 

 ing an important fishery management issue, and as- 

 sessment advice is increasingly contingent on these 

 mechanisms (Anderson and Ursin 1977; Pope 1976, 

 1979; Shephard 1984; ICES 1987,1988). This study in- 

 dicates that the stock dynamics of Atlantic mackerel 

 are not only influenced by fishing, but that predation 

 and intraspecific compensatory mechanisms including 

 density-dependent growth are strong influences that 

 probably effect yield forecasts and management advice 

 in the short and long term. 



Large differences in mackerel growth suggest that 

 year-class size partially influences the initial pattern 

 of growth during a cohort's first several years. Adult 

 stock size probably plays an important role in regu- 

 lating growth after a year-class recruits to the adult 

 portion of the population (Overholtz 1989). Declines in 

 growth are probably significant as stock biomass in- 

 creases (Overholtz 1989). Model results, although not 

 presented in this paper, suggest that mackerel would 

 reach a larger size if the mackerel stock were fished 

 more heavily. Larger catches of Atlantic mackerel 

 would probably cause growth to stabilize at higher 

 rates, and more of the annual production would be 

 available for harvest. 



Predation mortality rate has usually not been ac- 

 counted for in the past in most assessment work, but 

 recent studies have shown the importance of including 

 this mechanism to enhance stock assessments (Ander- 

 son and Ursin 1977; Shephard 1984; ICES 1987,1988; 

 Overholtz et al. 1990). Our analysis suggests that 

 predation probably has a major influence on the 

 dynamics of Northwest Atlantic mackerel. Predation 

 mortality is probably the largest component of natural 

 mortality on this stock, since other sources such as 

 general diseases, parasitism, and epizootics are not 

 thought to be important sources of mortality on most 

 fish stocks on an annual basis (Anderson 1979). Strong 

 year-classes of mackerel may attract elevated levels of 

 predation, in contrast with the usual assumption of con- 

 stant natural mortality. Other studies have suggested 

 that predation mortality rates should continually 

 decline or remain constant as abundance increases 

 (Sparre 1984). Our model results indicate that preda- 



