Okiyama Reproductive style of two species of sandfishies 



547 



Figure 4 



Comparison between terms and diu'ation of early developmental stages of Arctoscopus japonicus and Tnchodon trichodon. For sources, see text. 



These ecological differences in spawning are com- 

 pared with seasonal variations of the surface water 

 temperatures among several stations covering most of 

 the distribution areas of the two species (Fig. 5). By 

 inspection, the location of two groups of stations may 

 be recognized as corresponding to the zoogeography 

 of the two species. Temperature ranges for A. japoni- 

 cus are uniformly broader than those for T. trichodon: 

 -2°-10°C to 18°-25°C versus 2°-7°C to 9°-14°C. 

 Furthermore, temperature differences where the two 

 species occur are particularly gi-eat during the warmer 

 seasons. In the Sea of Japan, A. japonicus apparently 

 copes with these extreme environmental conditions by 

 precisely timing their early ontogeny to cold seasons 

 and by gradually shifting the habitat toward deeper 

 levels (up to 200-300 m) as growth proceeds (Nishimura 

 1969, Minami and Tanaka 1985). 



Alternatively, the lowest temperature ranges of the 

 eastern Pacific stations lie intermediate to those of the 

 western Pacific stations. This stands in sharp contrast 

 with the highest temperatures, which are not over- 

 lapping. Considering the year-round occurrence of the 

 eggs of T. trichodon in the rocky intertidal area, it is 

 quite natural to suppose that this species demands a 

 particular set of physical environmental conditions for 

 the specific spawning site, such as less variable water 

 temperatures and above-freezing temperatures. These 

 specific thermal conditions may also be responsible for 

 the indistinct ontogenetic migration of T. trichodon. 



Although there is no reliable information on the 

 abundance of T. trichodon, except catch data of Allen 

 and Smith (1988), A. japoyiicus surely has greater 

 stocks than the former species. Total landings oi A. 

 japonicus in the Sea of Japan by Japanese vessels 

 peaked at 3.3x10^ tons in 1966. Of these, 2.1x10^ 

 tons were caught in the restricted area along the 

 northwestern district of the Japanese mainland, 

 namely, Akita Prefecture, where one of the biggest 

 spawning grounds occurs regularly (Okiyama 1970, 

 Kato 1980). 



Figure 5 



Seasonal patterns of surface water temperatures from ten stations 

 within the ranges oi Arctoscopus japonicus (A-E, solid lines) and 

 Trichodon trwhtidon (F-J, dotted lines) (adapted from Wadachi 1960). 

 For locations, see Figure 1. 



Figure 6 shows the long-term fluctuations of the 

 catches of A. japonicus in this district. Coastal catches, 

 comprised exclusively of fully matured specimens, are 

 separated here from the total catches. Drastic changes 

 in catches occurred during a 30-year period with peaks 

 occurring around 1965-68 and 1974-75. The decrease 

 between these peaks is slight as compared with the 

 events in other periods such as around 1955 and after 

 1978. In 1984, the coastal catch dropped to a low of 



