NOTE Yang: The trophic role of Pleurogrammus monopteiygius 



1049 



at the population level was calculated by using the 

 following equation: 



^C, =DR, xDxB, xP,, 



where C, 



DR. 



D = 



B. 



the consumption (by weight) of Atka 

 mackerel by size group / of a predator 

 species; 



the daily ration (as a proportion of 

 body weight per day) of predator size 

 group / (Table 1); 



the number of days when Atka mack- 

 erel were vulnerable to predation ( from 

 1 May to 30 September in this study); 

 the biomass of the predator size group 

 i (Table 2); and 



the proportion by weight of prey Atka 

 mackerel in the diet of predator size 

 group / (Table 3). 



Daily rations were calculated by using annual 

 growth increments and conversion efficiencies (Brett 

 and Groves, 1979). Predator biomass data were ob- 

 tained from the RACE Division database at the 

 AFSC. The percent by weight of prey Atka mackerel 

 in the diet of predators (P^) was calculated for 

 arrowtooth flounder, Pacific cod, and Pacific halibut. 

 Because of the high similarity (>60'^) between the 

 diets of Kamchatka flounder and arrowtooth floun- 

 der (Yang and Livingston, 19861, the percent by 

 weight of Atka mackerel that arrowtooth flounder 

 consumed was also used for Kamchatka fiounder. 



Results 



Atka mackerel as prey of marine fishes 



Eleven commercially important groundfish species 

 were collected: Pacific cod, Gadus macrocephalufi: 

 walleye pollock, Theragra chalcogramma: Pacific 

 halibut, Hippoglossus stenolepis; arrowtooth fioun- 

 der, Atheresthes stomias; Greenland turbot, Rein- 

 hardtius hippoglossoides; Atka mackerel, Pleuro- 

 grammus monopterygius; Pacific ocean perch, 

 Sebastes alufus; northern rockfish, S. aleutianus; 

 shortraker rockfish, S. borealis; and shoi'tspine 

 thornyhead, Sebastolobus alascauus. According to 

 stomach content data, Atka mackerel (mostly juve- 

 niles) were found in the stomachs of Pacific cod. Pa- 

 cific halibut, and arrowtooth flounder. The detailed 

 food habits of these species were previously reported 

 (Yang, 1996). The mean percent weights of Atka 

 mackerel consumed by the predators are given in 

 Table 3 by predator size and subarea. Arrowtooth 



flounder >40 cm FL consumed large amounts (36.6% 

 by weight) of Atka mackerel in the western Aleutian 

 area, whereas Pacific cod >60 cm FL and Pacific hali- 

 but (especially fish >80 cm FL) both consumed great 

 amounts of Atka mackerel in the central Aleutian 

 area (42'^/i and 72.9% by weight, respectively). 



Atka mackerel as predator 



A total of 238 Atka mackerel stomachs were analyzed, 

 231 (97%) of which contained food. Atka mackerel 

 ranged from 22 to 44 cm FL, with a mean and SD of 

 33.5 cm and ±4.5 cm, respectively. Table 4 lists the 

 percent by weight of the prey found in Atka mack- 

 erel stomachs. More than 90% of the total weight of 

 stomach contents was invertebrates and less than 

 5% was fish. Euphausiids (mainly Thysanoessa 

 inei'jnis) were the most important prey, represent- 

 ing more than 28''^ of the total stomach contents. 

 Calanoid copepods were another important prey of 

 Atka mackerel, representing approximately 23% of 

 the total stomach content weight. Larvaceans and 

 Themisto sp., a hyperiid amphipod, occurred fre- 

 quently but each represented less than 10% of the 

 total weight of stomach contents. Squid were another 

 invertebrate prey of Atka mackerel; they represented 

 5% of the total weight of stomach contents. 



Several fish species were preyed on by Atka mack- 

 erel. Walleye pollock represented 2.4% by weight of 

 the total stomach contents. Myctophids, bathylagids, 

 zoarcids, stichaeids, and pleuronectids were also 

 found. However, each of them i-epresented less than 

 V'f of the total weight of stomach contents. Canni- 

 balism of eggs was also observed in Atka mackerel 

 stomachs, and eggs represented 5.5% of the total 

 weight of stomach contents. 



Figure 2 shows the percentage by weight of the 

 main prey items for different Atka mackerel size 



