Sho|i and Tanaka: Feeding and growth of Scomberomorus niphomus 



373 



Figure 2 



Map of the Sea of Hiuchi, central Seto Inland Sea, showing the 

 sampling stations where ichthyoplankton were collected with a bongo 

 net during the three cruises in 1995 Iclosed small circles) and in 

 1996 (large open circles). Catch data for 1-year-old S. niphonius were 

 obtained at Kawarazu Fisherman's Association (asterisk). Young- 

 of-the-year Japanese Spanish mackerel were collected by the seine 

 fishery in the southern waters indicated by the shaded area. 



clupeid species, gizzard shad (Konosirus punctatus) and 

 Japanese sardine iSardinops melanostictus), that were 

 the major prey organisms of the post-first-feeding S. 

 niphonius larvae (see "Results" section) were counted 

 to estimate prey concentrations. 



Scomberomorus niphonius larvae were aged by count- 

 ing daily increments on otoliths. Right-side sagittal oto- 

 liths were removed under a dissecting microscope and 

 the number of increments on the otolith were counted 

 using an image-analysis system (ARP, version 4.21, 

 Ratoc System Engineering Co., Ltd., Tokyo, Japan) con- 

 nected to a compound light microscope at 400 to 1000 x 

 magnification. Daily increments begin to be deposited 

 on the sagittal otoliths of S. niphonius larvae at first 

 feeding (Shoji and Tanaka, 2004). Scomberomorus ni- 

 phonius larvae initiate feeding on day 5 under 19.0°C 

 (Shoji et al., 2001). Larval age was therefore estimated 

 by adding five to the increment count because the water 

 temperature in the southern part of the Sea of Hiuchi 

 where S. niphonius larvae were abundant ranged be- 

 tween 18° and 20°C (see "Results" section) in late May. 

 Data from cruises in late May only (the second cruise 

 in both years) were included in the feeding and growth 

 analyses because no S. niphonius larvae were collected 

 during the first cruise and too few were collected during 

 the third cruise in both years. 



Results 



Physical environment 



The surface water temperature was higher in the south- 

 eastern area and lower in the northwestern area in all 

 cruises. Mean surface temperatures (±SD) were 12.3° 

 (±0.4), 18.6° (±1.2), and 20.5° (±0.6)°C in 11-16 April, 

 24-28 May, and 20-23 June, 1995, and were 14.3° (±0.6), 

 19.0° (±1.3), and 19.4° (±1.0)°C in 10-13 May, 27-30 

 May, and 18-21 June 1996, respectively (Fig. 3). Salinity 

 ranged between 32.5 and 34.3 ppt and was lower in the 

 southeastern area in all cruises. In late May, during the 

 seasonal peak in abundance of S. niphonius larvae, the 

 mean surface temperature was slightly higher in 1996 

 although there was no significant difference between the 

 two years (ANOVA: F=3.14, P=0.08). 



Scomberomorus niphonius eggs and larvae 



A total of 1018 eggs and 272 larvae of S. niphonius were 

 collected during the cruises. No eggs and larvae of S. 

 niphonius were collected during the first cruise in both 

 years. The egg and larval abundance peaked in late May 

 and decreased thereafter in both years (Fig. 4, A and B). 

 The eggs were abundant in the northwestern waters in 



