Overholtz et al.: Assessment advice for Northwest Atlantic mackerel stock 



127 



tion mortality rates on Atlantic mackerel are probably 

 much higher than previously thought. 



The results of the model projections show that un- 

 less the impacts of compensatory mechanisms are ac- 

 counted for, evaluations of current stock status using 

 the current standard assessment methodology may, in 

 fact, be optimistic and risky if catches are increased 

 to high levels in the future (Fig. 11). The differences 

 in results between the two models are, of course, con- 

 tingent on the parameterizations of the growth and 

 predation mortality submodels and how recruitment is 

 scaled in the density-dependent model. Two advances 

 in research on mackerel would help to improve our 

 ability to assess the stock: An MSVPA to provide cor- 

 rectly scaled estimates of recruitment, and a general 

 predation mortality model that would provide useful 

 estimates of M2's for forecasting purposes. Although 

 recent assessment advice indicates that catches can be 

 increased on the mackerel stock (Overholtz and Parry 

 1985), it perhaps needs to be modified to accommodate 

 the results of this study. 



The current management regime relies on catch and 

 stock size projections based on an F .i strategy. The 

 use of a reference point such as F .i is probably not 

 very useful for mackerel since growth, sexual matur- 

 ity, and natural mortality rates appear to fluctuate con- 

 siderably. This concept is best applied in situations 

 where these important variables are stable in the long 

 term. A more appropriate approach might be to remove 

 a moderately large sustainable catch annually or apply 

 an appropriate constant effort level over several years, 

 preserve a reasonable amount of spawning-stock bio- 

 mass, and monitor the results. This method would be 

 keyed to some of the uncertainties in stock dynamics 

 that we have investigated in this study and would pro- 

 vide information on stock responses with a low prob- 

 ability of stock collapse (i.e., F = 0.2-0.3; Fig. 11C). 



Additional analyses are necessary to confirm the 

 population processes that were modeled in this study. 

 Weights of individual fish should be monitored closely 

 to assess future changes. Sexual maturities of ages 2-3 

 fish should also be followed annually. Collection of these 

 data would also allow better parameterization of the 

 growth and maturity models. Sufficient samples must 

 be collected at the correct times to assess whether 

 these two variables, particularly percent maturity, are 

 continuing to change with stock density. Additional 

 food habits sampling at critical times and places would 

 help confirm and quantify the relationships found in 

 this analysis. Obtaining some information on predation 

 mortality on age-0 mackerel would be valuable. 



Preliminary data suggest that predator preference 

 may play an important role in determining the levels 

 of predation on available prey species. Recent declines 

 in sand lance Ammodytes dubius populations may in- 



crease predation mortality on mackerel and Atlantic 

 herring Clupea harengus. This points to the need for 

 a multispecies VPA where simultaneous impacts of 

 predation may be investigated. Improved predation 

 models that account for predator preference and prey 

 abundance would allow for more accurate predictions 

 of the impacts of these important factors, and better 

 management advice could be provided (Livingston 

 1986). Larger mackerel are preyed upon by marine 

 mammals, large pelagic fishes, and sea birds (Stillwell 

 and Kohler 1982, 1985; Payne and Selzer 1983; Payne 

 et al. 1984; Overholtz et al. 1990). The impact of these 

 predators is no doubt important, but was not evaluated 

 in this study. 



Acknowledgments 



We thank the personnel from the Northeast Fisheries 

 Center and other institutions who have collected data 

 on research surveys over the last 25 years. We are 

 grateful to the foreign fishery observers and scientists 

 who collected data from the Polish commercial fishery. 

 Special thanks to Louise Dery who provided the age 

 data for the study. We would also like to thank Brian 

 Rothschild for his guidance and insight in the initial 

 phases of this research. 



Citations 



Anderson, E.D. 



1985 Status of the Northwest Atlantic mackeral stock— 1984. 

 Ref. Doc. 85-03, Woods Hole Lab., Northeast Fish. Sci. Cent., 

 Natl. Mar. Fish. Serv.. NOAA, Woods Hole, MA, 46 p. 

 Anderson, K.P., and E. Ursin 



1977 A multispecies extension to the Beverton and Holt theory 

 of fishing, with accounts of phosphorous circulation and 

 primary production. Medd. Dan. Fisk. Havunders. 7:319-435. 

 Anderson, R.M. 



1979 The influence of parasitic infection on the dynamics of 

 host population growth. In Anderson, R.M., B.D. Turner, and 

 L.R. Taylor (eds.), Population dynamics, p. 245-281. Blackwell 

 Sci. Publ., Oxford. 

 Bowman, R.E., and W.L. Michaels 



1984 Food of seventeen species of northwest Atlantic fish. 

 NOAA Tech. Memo. NMFS-F/NEC-28, Northeast Fish. Sci. 

 Cent., Natl. Mar. Fish. Serv., NOAA, Woods Hole, MA, 

 183 p. 

 Bowman, R.E., R. Eppi, and M.D. Grosslein 



1984 Diet and consumption of spiny dogfish in the northwest 

 Atlantic. ICES Demersal Fish Comm., ICES CM 1984/G:27, 

 16 p. 

 Holling, C.S. 



1965 The functional response of predators to prey density and 

 its role in mimicry and population regulation. Mem. Entomol. 

 Soc. Can. 91:293-320. 

 Hoy, D.L., and G.M. Clark 



1967 Atlantic mackerel fishery, 1804-1965. Fish. Leaf]. 603, 

 U.S. Dep. Inter., Wash. DC, 9 p. 



