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



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Figure 2 



(A) Percent by age group of Atlantic mackerel sampled as prey 

 in Atlantic cod, silver hake, and spiny dogfish stomachs in 1982 

 (n = 8), 1983 (n = 12), 1984 (w = 32). (B) Length frequency of 

 mackerel sampled in Atlantic cod, silver hake, and spiny 

 dogfish stomachs, 1982-84. (C) Cumulative numbers of ages 

 1 and 2, and the total stock of mackerel from VPA stock size 

 estimates for 1970-85. 



ity other than predation, and M2 = predation mortal- 

 ity (ICES 1987). Since there was no multispecies vir- 

 tual population analysis (MSVPA) available to examine 

 annual trends in predation mortality rates (M2), we 

 decided to use another method to examine possible 

 changes in M2. We wanted to investigate possible 

 predation mortality models for mackerel for this period 

 of time. 



Summaries of NEFC food habits data indicate that 

 spiny dogfish Squalus acanthias, Atlantic cod Gadus 

 moruha, and silver hake Merluccius bilinearis are the 

 most important fish predators on mackerel (Langton 

 and Bowman 1980, Bowman and Michaels 1984, Bow- 

 man et al. 1984). Food habits data were collected from 

 1973 to 1980, but did not include individual lengths of 

 prey items from these predators. However, maximum, 

 minimum, and average lengths of fish prey were re- 



corded. Almost all the mackerel consumed in these 

 years were less than 30 cm (Overholtz et al. 1988). 



To study the problem in more detail, food habits data 

 from 1982-84 were examined to determine the size and 

 age distribution of mackerel as prey items in the three 

 fish predators. These data were chosen since detailed 

 records of predator and prey length were available. 

 Mackerel up to 35 cm were taken as prey by the three 

 species, but fish 30 cm or less composed the bulk of 

 the prey. These fish were predominantly ages 1 and 

 2 from the 1981-83 year-classes (Fig. 2A, B). Mackerel 

 appeared to be consumed roughly in proportion to their 

 abundance in the sea during 1982-84 (Fig. 2A, C). 



Our analysis thus centered on predation by these 

 three predators on age-1 and -2 mackerel. Total food 

 consumed (all species) by each predator was calculated 

 and the average percentage of mackerel by weight com- 

 prising the diet of each predator was estimated separ- 

 ately for the periods 1973-76 and 1977-80 (Table 1). 

 A period average was used because there was not 

 enough information available for annual estimates. 

 These two time-periods were chosen because the design 

 of the food-habits sampling regime was different in 

 each period, and because the abundance of small 

 mackerel was much different in each of the periods 

 (Fig. 2C). 



The method used to calculate consumption was based 

 on residence times of the predator and prey, percent 

 by weight of mackerel in the predator diet, daily ra- 

 tion estimates for the predator by season, if available, 

 abundance of 1- and 2-year-old mackerel in the sea, and 

 biomass of predators of the correct size distribution 

 (Bowman et al. 1984, Rexstad and Pikitch 1986). This 

 method assumed that the estimate of predator biomass 

 was an appropriate measure of the average standing 

 stock present during the year, that mackerel consumed 

 were only age-1 and -2 fish, and that predators con- 

 sumed mackerel relative to their abundance in the sea. 

 The estimates were made for 1973-80 for each pred- 

 ator species and the total number of age 1- and -2 fish 

 consumed annually was estimated (Table 2). 



