636 



Fishery Bulletin 103(4) 



which little information is available, it is important to 

 determine specific reproductive traits by using the most 

 accurate methods and to compare the results with those 

 of simpler methods. 



Rikuzen sole (Dexistes rikuzenius) (also known as 

 Rikuzen flounder, FAO) is a coastal flatfish that lives at 

 depths of 100 to 360 m in the waters off the south coast 

 of southern Hokkaido, Japan, and the southern Korean 

 Peninsula (Sakamoto, 1984). It inhabits sandy bottoms 

 and preys mainly on benthic invertebrates (Fujita et al., 

 1995). It is relatively abundant in the North Pacific off 

 the coast of Japan and is an important fishery resource 

 for bottom trawlers (Ishito, 1964; Ogasawara and Ka- 

 wasaki, 1980). The commercial catch of flatfish such as 

 the Rikuzen sole has fluctuated widely in this area over 

 the past few decades (Anonymous, 2002), and therefore 

 fisheries management is needed to maintain stable and 

 appropriate fish-density levels. 



In addition to fisheries, various internal and external 

 conditions may affect the fluctuations in abundance of 

 fish populations. Understanding reproductive traits, 

 or survival in the early life stages, is a step toward 

 revealing population dynamics. Although both sexes 

 have indeterminate growth trajectories, conspicuous 

 sexual dimorphism occurs during the growth and life 

 span of Rikuzen sole. Females are larger at any given 

 time after age 1+ and live longer than males (Ishito, 

 1964). The spawning period of the Sendai Bay popula- 

 tion occurs from late October to late January and peaks 

 from November to December (Ogasawara and Kawasaki, 

 1980). Using measurements of oocyte diameter and the 

 appearance of the whole ovary, Ogasawara and Kawa- 

 saki (1980) revealed that females spawn several batches 

 of eggs during one spawning season. However, because 

 histological observations of the gonads have not been 

 conducted, details of the reproductive biology, such as 

 annual cycle of oocyte development, and body size and 

 age at maturity, have not been determined. In addition, 

 no information about fecundity has been reported. 



We examined the oogenesis of Rikuzen sole caught 

 in the North Pacific Ocean off the coast of Japan over 

 a period of one year. The aim was to determine the 

 mode of maturation, annual reproductive cycle, and age 

 at first maturity based on histological examinations, 

 age determinations from otolith growth increments, 

 and gonadosomatic indices (GSIs). Using these results, 

 we were able to estimate body size and age-related 

 potential fecundity and were able to develop a simpler 

 method for determining potential fecundity. 



Materials and methods 



From May 2000 to April 2001, except for July and 

 August when commercial bottom trawl fishing was pro- 

 hibited, Rikuzen sole samples were collected once or 

 twice a month from the fisheries market in Hachinohe, 

 Aomori Prefecture, Japan. All samples were caught by 

 bottom trawl nets in the coastal waters off Shitsukari 

 (41°22\ 141°33'E) and Hachinohe (40°43'N, 144°44'E), 



44 



40° 



36° 



32 



128' 132 : 136° 140° _ I 144" 



41- - 



40° 



39° 



38 L 



37° 



Sendai Bay 



140° 



14V 



142° 143° 



Longitude (E) 



1 44° 



145° 



Figure 1 



Catch area for Rikuzen sole (Dexistes rikuzenius) 

 in the Northern Pacific Ocean off the northeast 

 coast of Japan, 2000-2001. 



from depths of 70-300 m (Fig. 1). During July and 

 August, samples were collected with bottom long lines 

 off the coast at Onezaki (39°12'N, 141°56'E) from a depth 

 of 85-109 m. 



A total of 1031 females were collected and their 

 standard lengths (SL) to the nearest mm, total body 

 weights, eviscerated body weights, and ovary weights 

 to the nearest 0.1 g were measured. The GSI and body 

 condition (BC) of each specimen were calculated with 

 the following formulas: GSI = (gonad weight/eviscerated 

 body weight)xl00, and BC = (eviscerated body weight/ 

 SL 3 )x 100. Ovaries and sagittal otoliths were removed 



