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Fishery Bulletin 103(2) 



of adequate feeding compared to fish that had been fed 

 from the time of first feeding. These observations sug- 

 gest that ichthyoplankton prey availability can strongly 

 influence growth and survival of S. niphonius larvae. 



Scomberomorus niphonius is distributed in the coastal 

 waters of Japan and supports important commercial 

 fisheries in the Seto Inland Sea. The total catch ex- 

 ceeded 6000 metric tons (t) in the middle 1980s but 

 decreased to less than 1000 t in the late 1990s in the 

 Seto Inland Sea. Spawning migration of S. niphonius 

 into the Seto Inland Sea occurs in May (Kishida and 

 Aida, 1989) and the larvae are distributed in May and 

 June in the Sea of Hiuchi, the central Seto Inland Sea 

 (Kishida, 1988). In order to ensure that catches remain 

 at stable levels and to establish more efficient fisheries 

 management, it is necessary to accumulate biological 

 information to elucidate the recruitment process of the 

 species. 



The objective of the present study is 1) to investigate 

 spatial and temporal distribution of S. niphonius larvae 

 and their prey and 2) to compare feeding conditions and 

 growth of S. niphonius larvae for two consecutive years 

 with contrasting levels of recruitment. 1995 and 1996, 

 in the Seto Inland Sea, Japan. The catch-per-unit-of- 

 fishing-effort (CPUE: no. offish/boat/day) of 1-year-old 

 S. niphonius (Fig. 1) fished by drift gill net in May, the 

 major fishing season for the species, at the Kawarazu 

 Fisherman's Association (Fig. 2) has been used as a 

 recruitment index in the Sea of Hiuchi (Kishida, 1991). 

 The CPUE fluctuated tenfold in the 1990s (Ehime Pre- 

 fecture Chuyo Fisheries Experimental Station Toyo 

 Branch 1 ) and indicated recruitment in 1995 was more 

 successful. Egg, larval, and larval prey distributions, 

 larval feeding incidence and growth, and young-of-the- 

 year (YOY) fish abundance were investigated in 1995 

 and 1996 in the Sea of Hiuchi. 



Materials and methods 



Ichthyoplankton sampling 



Three research cruises were carried out in 1995 (11-16 

 April, 24-28 May, and 20-23 June) and in 1996 (10-13 

 May, 27-30 May, and 18-21 June) in the Sea of Hiuchi 

 (Fig. 2). Ichthyoplankton sampling and hydrographic 

 survey were conducted from the RV Shirafuji (138 t) of 

 the National Research Institute of Fisheries and Envi- 

 ronment of Inland Sea (NRIFEIS). Double oblique tows 

 from the surface to 5 m above the bottom were made 

 by using a bongo net (0.7-m diameter, 0.315-mm mesh) 

 at 80 stations during the cruises in 1995 and at 50 

 stations in 1996. Average depth of the Sea of Hiuchi is 

 approximately 17.8 m (Montani, 1996). Scomberomorus 



1990 



1992 1994 1996 

 Year class 



Figure 1 



Catch per unit of fishing effort 

 (CPUE: no. offish/boat/day) of 1- 

 year-old Scomberomorus niphonius 

 in the 1990-99 year classes at the 

 Kawarazu Fisherman's Association 

 in the central Seto Inland Sea. Data 

 were obtained from drift gill-net 

 catches in May. the major fishing 

 season for S. niphonius. 



1 Ehime Prefecture Chuyo Fisheries Experimental Station Toyo 

 Branch. 2000. Unpubl. data. Kawarazu, Toyo, Ehime 

 799-1303, Japan. 



niphonius larvae were quickly sorted from the samples 

 and were preserved in 95% ethanol. Other ichthyoplank- 

 ton were fixed in 10% formalin seawater for sorting in 

 the laboratory. Flow meters were mounted in the mouth 

 of the net to determine the filtered volume. Each tow 

 followed a salinity-temperature-depth sensor cast to 

 measure the water temperature and salinity profiles at 

 each station. 



YOY fish abundance 



YOY S. niphonius have been reported to occur in the 

 southern part of the Sea of Hiuchi from late June to 

 early July (Watanabe, 1994). To detect a potential dif- 

 ference in S. niphonius recruitment abundance between 

 1995 and 1996, YOY fish abundance was assessed in the 

 southern part of the Sea of Hiuchi. YOY S. niphonius 

 were collected from catches by a seine fishery in the 

 southern part of the Sea of Hiuchi (Fig. 2). The seine 

 fishery primarily targets young and adult Japanese 

 anchovy iEngraulis japonicus). The codend of the net 

 has a 2-mm mesh aperture and was towed by two boats 

 for about 1 hour at a ship velocity of 3 to 4 knots. Two 

 to 10 kg of the catch by the seine fishery was sampled 

 weekly (five times each year) from mid June to late July 

 in 1995 and 1996. YOY abundance was expressed as the 

 number of S. niphonius per 10 kg of the catch. 



Laboratory procedures 



Larval SL was measured to the nearest 0.1 mm, and 

 stomach contents were identified under a dissecting 

 microscope. After removal of S. niphonius larvae, the 

 bongo-net samples were processed to estimate concentra- 

 tions (no./lOO m 2 ) of S. niphonius eggs. Larvae of two 



