Fishery Bulletin 89(1), 199) 



1967 1970 1973 1976 1979 1982 1985 



Year 



Figure 1 



Descriptive data for the Northwest Atlantic mackerel stock, 

 1967-85. (A) Yield (ooo's mt); (B) Recruitment at age 1 

 (billions of fish); (C) Spawning-stock biomass (ooo's mt); (D) 

 Mean weight at ages 1, 2, and 3 from commercial samples 

 (g); (E) Percent maturity for ages 2 and 3. 



with stock size. An analysis of the food habits of 

 mackerel predators suggested that natural mortality 

 rates (M2) for this stock were higher during 1973-75 

 when relatively large numbers of juvenile mackerel 

 were available, and declined during 1976-80 when 

 there were few small fish (NEFC 1987). 



This study examines the impact that compensatory 

 changes in growth, sexual maturity, and natural mor- 

 tality rates may have on the Northwest Atlantic mack- 

 erel stock. Implications of responses in these factors 

 on catch and spawning-stock biomass are evaluated 

 using a simulation model. The model was designed so 

 that changes in these compensatory factors as well as 

 the influence of fishing mortality patterns and strate- 

 gies could be assessed. 



Model background 



Data on potential density-dependent population reg- 

 ulatory mechanisms were obtained from research ves- 

 sel survey cruises and commercial fishing operations 

 conducted on the U.S. eastern coast. Information from 

 spring groundfish surveys conducted by the Northeast 

 Fisheries Center (NEFC) during 1973-85 and a com- 

 mercial fishery conducted by Poland during 1981-86 

 were examined to quantify compensatory relationships. 

 Analyses were performed to study changes in growth, 

 maturation rates, and natural mortality (Overholtz 

 et al. 1988, Overholtz 1989). 



Significant negative relationships between mean 

 weight-at-age and stock size were confirmed for both 

 research and commercial data sources (Overholtz et al. 

 1988, Overholtz 1989). Mean size-at-age for recent 

 year-classes was also found to be significantly different; 

 large year-classes grew more slowly (Overholtz 1989). 

 In addition mean weight-at-age was also negatively 

 related to year-class size, indicating that large cohorts 

 may depress growth rates of individuals from that par- 

 ticular year (Overholtz 1989) (Fig. ID). These analyses 

 helped us to quantify the relationship between growth 

 and density for this stock in our modeling exercises. 



Maturity data from 1981-86 were evaluated to ascer- 

 tain if percent maturity at age 2 and 3 changed over 

 the time period. No fish were mature at age 1, and all 

 fish were mature at age 4 + (Overholtz et al. 1988). Per- 

 cent maturity at age 2 ranged from 17% in 1981 to 53% 

 in 1983 (Fig. IE). Percent mature at age 3 ranged from 

 67% in 1986 to 98% in 1983 and 1984 (Overholtz et al. 

 1988) (Fig. IE). 



The maturity data were collected in conjunction with 

 age sampling and were not a priority item. During the 

 critical time of gonadal development in mid- to late 

 April, maturity samples were often sparse because age 

 sampling requirements had been fulfilled; no additional 

 maturity samples were collected because of this. A cur- 

 sory examination of the data revealed that there was 

 an apparent negative relationship with increased stock 

 size at age 2 and no consistent pattern at age 3 (Over- 

 holtz et al. 1988) (Fig. 1). The null hypothesis of no im- 

 pact of density on maturity rates could not be accepted 

 or rejected with the data at hand; we therefore included 

 this potential mechanism in our modeling studies to 

 ascertain its overall importance. 



During 1967-85 the northwest Atlantic mackerel 

 stock underwent profound changes in recruitment with 

 a subsequent decline in biomass (Fig. IB, C). We con- 

 sidered this a good time period to study changes in 

 predation on mackerel and associated possible changes 

 in natural mortality rates (M). For the purposes of 

 discussion in this paper we define natural mortality rate 

 (M = Ml + M2), where Ml = sources of natural mortal- 



