104 7 



The trophic role of Atka mackerel, 

 Pleurogrammus monopterygius, 

 In the Aleutian islands area 



Mei-Sun Yang 



Alaska Fisheries Science Center 

 7600 Sand Point Way NE 

 Seattle, Washington 98115-0070 

 E-mail address Mei Sun Yangia noaa gov 



Atka mackerel (Pleurogrammus 

 monopterygius) is presently the 

 most abundant marine fish species 

 in the Aleutian Islands. Its exploit- 

 able biomass in 1998 was estimated 

 to be 535,500 metric tons (t) (Lowe 

 and FritzM. The commercial catch 

 of Atka mackerel increased from 

 26,740 1 in 1991 to 104,000 1 in 1996 

 (Lowe and FritzM. Atka mackerel 

 has been identified as food for other 

 marine fishes, i.e. Pacific cod (Ga- 

 dus maci-ocephalus) (Andriyashev, 

 1937), coho salmon (Oncorhynchus 

 kisutch ) (Wing, 1985; Volkov et al., 

 1995; Davis-), arrowtooth fiounder 

 (Atheresthes stomias) (Mito, 1974); 

 sea birds such as thick-billed 

 murres (Uria lomvia) (Ogi, 1980); 

 and marine mammals such as Steller 

 sea lions (Eumetopias jubatus) 

 (Merrick et al., 1997). The western 

 stock of Steller sea lions has recently 

 been listed as endangered. A connec- 

 tion between the observed decline 

 in Steller sea lions and the sea lion's 

 dependence on commercially ex- 

 ploited fish species such as Atka 

 mackerel as prey has been sug- 

 gested (Merrick et al., 1997). Be- 

 cause of the great abundance of 

 Atka mackerel and its important 

 role as food for marine fish and 

 marine mammals, it is essential to 

 understand the trophic role of Atka 

 mackerel in the Aleutian Islands 

 area. The objective of this study 

 was to describe the role of Atka 

 mackerel as prey and as predator 

 in the Aleutian Islands ecosystem. 



Methods 



Study area 



From July to September 1991, the 

 Resource Assessment and Conser- 

 vation Engineering (RACE) Divi- 

 sion at the Alaska Fisheries Science 

 Center (AFSC), Seattle, Washing- 

 ton, conducted a groundfish re- 

 sources trawl survey in the Aleu- 

 tian Islands region. This area (Fig. 

 1 ) was divided into four major geo- 

 graphical units: southeastern 

 Bering Sea area (from long. 165°W 

 to 170°W, 42,604 km2); eastern 

 Aleutian area (long. 170"W to 

 177°W, 338,818 km^); central Aleu- 

 tian area (long. 177°W to 177°E, 

 322,234 km-); and western Aleu- 

 tian area (long. 177°E to 170°E, an 

 area of 343,083 km2). Two char- 

 tered fishing vessels. Ocean Hope 

 and Green Hope, were used. Both 

 vessels used standard RACE 

 Division Poly-Nor'eastern, hard 

 bottom, high-opening bottom 

 trawls constructed of 12.7-cm (5- 

 inch) stretched-mesh polyethylene 

 web, with a 3.2-cm (1-1/4 inch) 

 stretched-mesh nylon liner in the 

 codend to retain smaller specimens 

 (Harrison, 1993). Trawls were 

 towed at 3 knots for 30 minutes. 

 Scientists from the Resource Ecol- 

 ogy and Fisheries Management 

 (REFM) Division, Resource Ecology 

 and Ecosystem Modeling Task, col- 

 lected fish stomach samples during 

 this survey. 



Sample collection 



Fish stomach samples were col- 

 lected during trawling operations 

 by scientists aboard the charter 

 vessel Green Hope. Both Atka 

 mackerel and their potential fish 

 predators were sampled and mea- 

 sured, and stomach samples were 

 collected. Because of the previously 

 demonstrated high similarity ( >QQ'^/c ) 

 in diet between arrowtooth flounder 

 and Kamchatka flounder (Yang and 

 Livingston, 1986), no stomach 

 samples of Kamchatka flounder 

 were collected. Before excising a 

 stomach, fish were examined for 

 evidence of regurgitation or net 

 feeding. If a fish had food in its 

 mouth or around the gills, or if its 

 stomach was everted or flaccid, the 

 fish was categorized as having re- 

 gurgitated food and the specimen 

 was discarded. If a predator had 

 fresh food (usually fish) sticking out 

 of its mouth or throat, it was catego- 

 rized as a net-feeding fish and was 

 also discarded. When a sampled 

 stomach was retained, it was put into 

 a cloth stomach bag. A field tag with 

 the species name, fork length (FL), 

 and haul data (vessel, cruise, haul 

 number, and specimen number) 

 was also put into the bag. All of the 

 samples collected were then pre- 

 served in buckets containing a 10% 

 buffered formalin solution. When 



' Lowe. S. A., and L.W. Fritz. 1997. Atka 

 mackerel. In Stock assessment and fish- 

 ery evaluation report for the groundfish 

 resources of the Bering Sea/Aleutian Is- 

 lands regions as projected for November 

 1997, compiled by the plan team for the 

 groundfish fisheries of the Bering Sea and 

 Aleutian Islands. North Pacific Fishery 

 Management Council, P.O. Box 103136, 

 Anchorage, AK 99.510. 



- Davis, N. D. 1990. U.S. -Japan coopera- 

 tive high seas salmonid research in 1990: 

 summary of research aboard the Japanese 

 research vessel Hokuho marii, 4 June to 

 19 July. (INPFC Doc. 3.508) FRI-UW- 

 9010. Fish. Res. Inst.. Univ. Washington, 

 Seattle, WA 98195, 24 p. 



Manuscript accepted 11 December 1998. 

 Fish. Bull. 97(41:1047-1057 11999). 



