platyceros, Brandt. Can. Tech. Rep. Fish. Aquat. Sci. 1 129, 

 29 p. 



Glen S. Jamieson 



Department of Fisheries and Oceans 



Fisheries Research Branch 



Pacific Biological Station 



Nanaimo, British Columbia V9R 5K6, Canada 



Ellen K. Pikitch 



Department of Fisheries and Wildlife 



Oregon State University 



M. 0. Hatfield Marine Science Center 



Newport, OR 97365 



Present address: 



School of Fisheries WH-10 



University of Washington 



Seattle, WA 98195, USA 



MORPHOLOGICAL DIFFERENCES BETWEEN 



TWO CONGENERIC SPECIES OF 



PLEURONECTID FLATFISHES: ARROWTOOTH 



FLOUNDER, ATHERESTHES STOMIAS, AND 



KAMCHATKA FLOUNDER, A. EVERMANNI 



The two flatfishes of the genus Atheresthes (family 

 Pleuronectidae) are commonly caught in the eastern 

 Bering Sea commercial trawl fishery. From 1977 to 

 1983, they comprised an estimated 10.03% of the 

 total flatfish catch by the foreign trawl vessels in 

 the Bering Sea/ Aleutian Islands region (data com- 

 piled from U.S. Foreign Fisheries Observer Pro- 

 gram, Northwest and Alaska Fisheries Center). 

 However, these two species, the arrowtooth 

 flounder, A. stomias, and Kamchatka flounder, 

 A. evermanni, are morphologically similar and 

 hence difficult to distinguish. As a result, fisheries 

 workers in the field often lump the two species or 

 misidentify them. Because the two species may have 

 biological differences not presently known, it is im- 

 portant for management considerations and stock 

 assessments to distinguish the species in fisheries 

 surveys. The objective of this paper is to describe 

 gross morphological differences between the two 

 species more explicitly, so that the two can be ac- 

 curately identified in the field. 



Norman (1934) thought that these two species of 

 Atheresthes were so similar that eventually they 

 were shown to be identical. However, based on elec- 

 trophoretic evidence, Ranck et al. (1986) concluded 

 that A. stomias and A. evermanni are valid species. 

 Wilimovsky et al. (1967) previously had reached this 

 same conclusion by using a special morphological 



character index to separate the two species. This 

 index is a function of caudal vertebrae number, gill 

 raker number, distance from anterior eye margin 

 to dorsal origin, and eye diameter. Unfortunately, 

 the index is too complex to use in the field because 

 it is based partly on characters that cannot easily 

 be evaluated by gross external examination. This 

 study describes a simpler method for differentiating 

 the two species based on previously described ex- 

 ternal morphological characteristics and two new 

 morphological characters. 



Methods 



Collections were made in the eastern Bering Sea 

 in an area between lat. 54° and 59°N, long. 163° 

 and 174°W (Fig. 1) aboard the National Marine 

 Fisheries Service RV Chapman in summer 1984. 



Specimens were unselectively sampled in the field 

 from trawl catches containing A. stomias and A. 

 evermanni. The fork length and sex of the fish along 

 with location of sample were recorded, and each 

 specimen was preserved in 3.7% seawater/formal- 

 dehyde solution. 



In the laboratory, the following characteristics 

 were examined: 



1) Upper eye position: Specimens were first 

 classified according to the position of the upper eye, 

 following Norman (1934). If the orbit of the upper 

 eye interrupted the profile of the head (Fig. 2A), the 

 specimen was classified as A. stomias. If the upper 

 eye did not interrupt the profile of the head (Fig. 

 2B) and was completely on the right side of the 

 head, the specimen was classified as A. evermanni 

 (Norman 1934; Wilimovsky 1967). 



2) Gill raker counts: After initial separation of 

 the specimens on the basis of the upper eye posi- 

 tion, the four gill arches of the eyed side were 

 removed and the gill raker count of each of the four 

 arches recorded. Counts of the upper and lower 

 limbs were recorded separately and the two counts 

 were separated by a plus sign (for example, 4 + 12 

 means 4 rakers on the upper limb and 12 on the 

 lower limb). If a gill raker straddled the angle of the 

 arch, it was included in the count of the lower limb. 

 In this study, only the lath-shaped structures were 

 counted as gill rakers; the rudiments were not 

 counted. 



Results 



A total of 251 fish was examined. Based on the 

 upper eye position, 170 specimens were classi- 



608 



FISHERY BULLETIN: VOL. 86, NO. 3, 1988. 



