FISHERIES SCIENCE 2001; 67: 238–245 Original Article Daily ration of Japanese Spanish mackerel Scomberomorus niphonius larvae J SHOJI,1,* T MAEHARA,2,a M AOYAMA,1 H FUJIMOTO,3 A IWAMOTO3 AND M TANAKA1 1Laboratory of Marine Stock-enhancement Biology, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto 606-8502, 2Toyo Branch, Ehime Prefecture Chuyo Fisheries Experimental Station, Toyo, Ehime 799-1303, and 3Yashima Station, Japan Sea-farming Association, Takamatsu, Kagawa 761-0111, Japan SUMMARY: Diel successive samplings of Japanese Spanish mackerel Scomberomorus niphonius larvae were conducted throughout 24 h both in the sea and in captivity in order to estimate their daily ration. Using the Elliott and Persson model, the instantaneous gastric evacuation rate was estimated from the depletion of stomach contents (% dry bodyweight) with time during the night for wild fish (3.0–11.5 mm standard length) and from starvation experiments for reared fish (8, 10, and 15 days after hatching (DAH)). Japanese Spanish mackerel is a daylight feeder and exhibited piscivorous habits from first feeding both in the sea and in captivity. Feeding activity peaked at dusk. The esti- mated daily ration for wild larvae were 111.1 and 127.2% in 1996 and 1997, respectively; and those for reared larvae ranged from 90.6 to 111.7% of dry bodyweight. Based on the estimated value of daily rations for reared fish, the total number of newly hatched red sea bream Pagrus major larvae preyed by a Japanese Spanish mackerel from first feeding (5 DAH) to beginning of juvenile stage (20 DAH) in captivity was calculated to be 1139–1404. KEY WORDS: consumption, daily ration, feeding, gastric evacuation rate, Japanese Spanish mackerel, larvae, Scomberomorus niphonius, Seto Inland Sea. INTRODUCTION However, for a long time, most studies had focused primarily on the biology of adult fish.3–6 Recently, Japanese Spanish mackerel Scomberomorus information on early life history has been niphonius is commonly distributed in south- accumulated and several peculiar ecological western Japan and is one of the important fisheries features have been clarified: precocious develop- resources particularly in the Seto Inland Sea. Total ment in the digestive system of the larvae,7 catch in the sea exceeded 6000 t in the mid 1980s, piscivorous habits from the first feeding stage,8 but has recently decreased to less than 500 t mainly brief larval occurrence synchronized with peak because of overfishing.1,2 It is hoped to recover abundance of their prey fish, rapid growth in early the decreased stock by fishing regulations and life stages,9 and diel changes in vertical distribu- releasing seedlings. tion and feeding rhythm.10 Because of the unique In order to stabilize the catch and to establish feeding habits of larvae (i.e. a consistent piscivory more effective fisheries management, it is neces- from first feeding) a sudden increase in Japanese sary to accumulate biological information and to Spanish mackerel biomass could have larger pre- elucidate the recruitment process of the species. dation impacts on coastal pelagic fish commu- nities than those of many other piscivorous fish *Corresponding author: Tel: 81-75-753-6225. Fax: 81-75-753- species whose feeding habits shift from plank- 6229. Email: [email protected] tivory to piscivory at a more advanced stage of life. aPresent address: Agricultural Administration and Fisheries Division, Uwajima Regional Office, Ehime Prefecture, Uwajima, Assessment of predation impacts on other fish is Ehime 798-0036, Japan. needed for future stock enhancement of this Received 16 May 2000. Accepted 23 October 2000. species. Daily ration of Japanese Spanish mackerel larvae 239 Although Japanese Spanish mackerel has also fecture Chuyo Fisheries Experimental Station) in been expected as a target species for sea-farming the Hiuchi-nada Sea, the central waters of the Seto because of its high growth potential [reaching Inland Sea (Fig. 1). A total of nine and 10 sets of 10 mm total length (TL) in a month and 600 mm TL larva-net tows were conducted with an interval of in the first growing season: May to November], 2–3 h from 11:37 h on 13 to 11:42 on 14 June 1996 sibling cannibalism in early life has constituted a and from 10:33 on 3 to 09:57 on 4 June 1997, major difficulty in mass production.11,12 By quanti- respectively. Details of the sampling method are fying their food consumption, an appropriate described in Shoji et al.10 Japanese Spanish mack- feeding schedule is indispensable for mass pro- erel larvae were sorted in the laboratory and stan- duction of this fierce fish in captivity. dard lengths (SL) were measured to the nearest This study is part of a larger project designed to 0.1 mm using a dissecting microscope. Stomach estimate the predatory impact that young Japanese contents of 143 (3.9–11.5 mm SL) and 209 (3.0– Spanish mackerel have on their piscine prey 10.3 mm SL) Japanese Spanish mackerel larvae populations. We estimated the daily ration of were examined for 1996 and 1997, respectively. Japanese Spanish mackerel larvae using the speci- Then the larval bodies and stomach contents were mens obtained from successive 24 h samplings in dried for 24 h at 60°C and the stomach contents the central waters of the Seto Inland Sea. Estimates weight index (I) was calculated as follows. of daily ration were compared with those under laboratory conditions. dry stomach contents weight I È ˘ 100 (1) = Í ˙ ¥ Î dry bodyweight ()W ˚ In order to detect growth in the early life stages MATERIALS AND METHODS of Japanese Spanish mackerel in the sea, we exam- ined sagittal otoliths of sea-caught fish:9 early Field sampling larvae collected by periodical sampling using the larva-net in the surveyed area from May to July Two sets of a 24h survey were carried out during 1997 and late larvae and early juveniles collected in cruises of the R/V Hiuchi (Toyo Branch, Ehime Pre- June and July by the ‘Patchi-ami’, a boat seine Fig. 1 Central area of the Seto Inland Sea showing sampling stations of Japanese Spanish mackerel larvae over a 24 h period on 13–14 June 1996 (᭝) and 3–4 June 1997 (᭡). Rearing experiments were conducted in Japan Sea-farming Association (JASFA) Yashima Station (ଙ) during May to June 1999. 240 FISHERIES SCIENCE J Shoji et al. fishery which is operated primarily for Japanese where C is the consumption of food during the anchovy Engraulis japonica larvae and juveniles time interval from 0 to t, I0 and It are average I at in the Hiuchi-nada Sea. These fish collected in time 0 and t, and R is the instantaneous gastric 1997 for growth analysis were preserved in evacuation rate. 90% ethanol. In the laboratory, according to the The field study suggests that Japanese Spanish otolith increments validation, right-side sagittal mackerel larvae are daylight feeders.10 Assuming otoliths of 70 fish were removed under a dissecting there is no feeding from sunset to sunrise, R for the microscope and increments were counted for age sea-caught larvae can be estimated from the deple- determination.9 tion of I during the night. The evacuation rate is therefore given by -Rt¢ IIesr= ss Sampling in captivity where R is calculated from the average Is of Artificial fertilization was carried out with a pair the samples collected at sunset (Iss), at sunrise of adult Japanese Spanish mackerel captured in (Isr), and the time interval between sunset and a drift gill net in the Harima-nada Sea, the eastern sunrise (t¢). Seto Inland Sea in May 1999. Eggs were trans- The R for fish in captivity was estimated from ported to Yashima Station, Japan Sea-farming the depletion of I during the starvation experi- Association (JASFA; Fig. 1), Takamatsu, Kagawa, ments using the following equation: and maintained in 0.5 m3 tanks under natural light conditions. Water temperature ranged from RtII= ()(1 ln 0 t ) (3) 18.2 to 19.5°C during the experiments. The initial To calculate the daily amount of food consump- density of fish at the beginning of the experiments tion, estimated C for each time interval during was about 15/L. Newly hatched larvae of red 24 h under both wild and rearing condition were sea bream Pagrus major, which are usually used summed. Using daily ration estimates (%W) of the as prey in culturing Japanese Spanish mackerel, predator (S. niphonius) and dry bodyweight of prey were fed sufficiently from the first feeding (mean of 100 newly hatched red sea bream larvae: (5 days after hatching (DAH)).11,12 0.029 mg), the total number of red sea bream con- In order to estimate the gastric evacuation rate sumed by a Japanese Spanish mackerel from first of Japanese Spanish mackerel larvae, starvation feeding (5 DAH) to early juvenile stage (20 DAH) experiments were conducted at the age of 8, 10, was calculated. and 15 DAH with the mean fish size of 6.8, 8.7, and 12.1 mm SL, respectively. Because Japanese Spanish mackerel larvae are piscivorous and begin sibling cannibalism without piscine prey, 100 fish RESULTS were isolated from one another using 2 L plastic cups at the beginning of each starvation experi- Diet and stomach fullness in the sea ment. Then 10–15 fish were sampled at once with the interval of 0.5 or 1.0 h and Is were calculated. During both of the sampling periods in 1996 Evacuation rate was estimated using the depletion and 1997, the diets of Japanese Spanish mackerel of I up to 10 h from the onset of the starvation larvae consisted exclusively of fish larvae of other experiment.