Zimmermann and Goddard Biology and distribution of Atheresthes stomias and A evermanni 



359 



species during resource assessment surveys con- 

 ducted by the AFSC. 



The purpose of the present work was to document 

 the geographical overlap of the two flounder species 

 in Alaskan waters and to describe the ecological dif- 

 ferences, such as length at age, sex-ratios, and depth 

 and temperature preferences that allow such exter- 

 nally similar species to coexist. Incorporation of this 

 information into future AFSC surveys may prove es- 

 sential for the management of the two species. 



Materials and methods 



Data were obtained from bottom trawl surveys con- 

 ducted from research vessels over the continental 

 shelf (<200 m) and slope (200-800 m) of the eastern 

 Bering Sea and off the Aleutian Islands (<500 m). 

 The surveys were conducted from June through Sep- 

 tember 1991. Sample design, fishing gear and meth- 

 ods, catch sampling procedures, and data analyses 

 are described by Goddard and Zimmermann 3 for the 

 eastern Bering Sea shelf and slope and by Harrison 

 (1993) for the Aleutian Islands. The bottom trawl 

 used in the Bering Sea shelf survey was different 

 from that used in the Bering Sea slope and Aleutian 

 Islands region. The bottom trawl used on the Bering 

 Sea shelf had smaller meshes in the net wings and 

 body and was used without roller gear; therefore it 

 fished heavily on the bottom. The bottom trawl used 

 on the Bering Sea slope and in Aleutian Islands re- 

 gion was used with bobbin roller gear and had larger 

 meshes in the net wings and body. Additional data 

 were incorporated from the western Gulf of Alaska 

 bottom trawl survey (<500 m) conducted in 1993 but 

 were used only for defining geographic ranges. Nets 

 were towed at approximately 5.5 km/hr (3 knots) for 

 30 minutes, covering a distance of 2.8 km (1.5 nmi). 

 Mean net width, as measured with SCANMAR net 

 mensuration gear, was multiplied by the distance 

 traveled to calculate the area swept by the net. Catch 

 per unit of effort (CPUE) was calculated for each 

 species for each tow in kilograms per hectare 

 (kg/ha). Either micro-bathythermograph probes fixed 

 to the net or expendable bathythermograph probes 

 dropped at the haul sites recorded bottom tempera- 

 tures (±0.1°C). 



Arrowtooth and Kamchatka flounders were distin- 

 guished by the position of their upper eyes. The up- 



per eye of arrowtooth flounder interrupts the profile 

 of the head and can be seen from the blind side. The 

 upper eye of the Kamchatka flounder does not inter- 

 rupt the head profile. Fish whose upper eye was not 

 located with any certainty or which had a damaged 

 upper eye were distinguished by number of gill rakers 

 (Yang, 1988). After sorting, each species was weighed 

 separately. Fish size was recorded as fork length, and 

 the sex of each fish was determined by making an inci- 

 sion posterior to the abdomen on the blind side and by 

 visually inspecting the gonadal tissue. 



Comparisons were made between the abundance 

 of the two species in CPUE (kg/ha) by depth interval 

 and water temperature interval. When there were a 

 sufficient number of hauls, depth was divided into 

 25-m intervals and temperature into 0.1°C intervals. 

 When there were few hauls, catches were grouped 

 into 100-m depth intervals and 1.0°C intervals. For 

 trawl hauls in which both species were caught, a lo- 

 gistic model was developed to show the relationship 

 between the proportion of arrowtooth flounder in the 

 total Atheresthes catch and depth; 



(\ni p/\- p) = a + px), 



where p is the proportion of arrowtooth flounder in 

 the total Atheresthes catch rate, 1 - p is the propor- 

 tion of Kamchatka flounder in the total Atheresthes 

 catch rate, a and ji are constants, and x is the depth 

 in meters. 



The geographic distributions of both species were 

 described from our thorough but depth-limited bot- 

 tom trawl survey data, supplemented by data sup- 

 plied by the Fishery Observer Program of the AFSC, 

 which also supplied species composition data on 

 catches from deep waters in the Gulf of Alaska. Non- 

 linear regressions related mean length of arrowtooth 

 and Kamchatka flounder by haul to depth; 



L = L -A 



JiM 



1 Goddard, P.. and M. Zimmermann. 1993. Distribution, abun- 

 dance, and biological characteristics of groundfish in the east- 

 ern Bering Sea based on results of the U.S. bottom trawl sur- 

 vey during June-September 1991. AFSC Processed Rep. 93- 

 15, 342 p. Alaska Fisheries Science Center, Natl. Mar Fish 

 Sen-.. NOAA, 7600 Sand Point Way NE, Seattle. WA98115-0070 



where L is estimated length, L is theoretical maxi- 

 mum length, M is depth in meters, and A and B are 

 constants. 



Initial observations indicated an unusually high 

 percentage of females in the arrowtooth flounder 

 population. Juvenile fish from which the sex could 

 not be determined, generally less than 20 cm in 

 length, were not included in the analysis. The per- 

 centage of females in the arrowtooth flounder popu- 

 lation was compared against the percentage of fe- 

 males in the Kamchatka flounder population for both 

 the Aleutian Islands and the eastern Bering Sea. 

 Comparisons were also made against the other ma- 

 jor flatfish species in the eastern Bering Sea to de- 



