Title Studies on Early Development and Spawning Ecology in Japanese Sardine Sardinops melanostictus
Authers Masanobu MATSUOKA
Keywords Japanese sardine, Sardinops melanostictus, early development, spawning ecology, egg abundance
Citation Bull. Fish. Res. Agen. No. 22, 87-183, 2008
In the most resource period and decrease one of Japanese sardine Sardinops melanostictus, the early development and spawning ecology of Western Kyushu population were investigated, from biological aspect.
 Artificial fertilization was undertaken, using adults caught by fishing on the research vessel. Successfully fertilized eggs started cleavage after one hour and five minutes of insemination. In three hours, eggs reached the morula stage and attained to the blastula stage in four hours. Hatching occurred within 30 to 36 hours. Unfortunately, all larvae died within 1.5 days after hatching , probably because of the deterioration of the rearing sea water and high temperature.
 Wild eggs collected from adjoining areas of Nagasaki Harbor were incubated from Ab-Ba stage at 7.3-26.7℃. At 7.3 and 10.8℃, total and viable hatching rates, survival rate at first-feeding stage and rate of first-feeding success were lower than at 13.8-21.0℃. At 23.5℃, these rates were lower than at 13.8-21.0℃, and no normal larvae and no feeding ones were observed at 26.7℃. Eggs were also incubated at salinities of 17.4-53.8. At salinities of 26.1-39.6, total and viable hatching rates were high, mostly over 90%. Viable hatching rate at salinity of 17.4 was considerably low. At salinities of 44.3 and 53.8, total and viable hatching rates were relatively low. Survival rate at first-feeding stage and rate of first-feeding success at salinities of 17.4, 44.3 and 53.8 were lower than at salinities of 26.1-39.6.
 Incubation and rearing experiments at temperature of 18℃ were undertaken, using wild eggs. Total length of just hatched larvae was 3.44 mm. Fourth day-old larvae after hatching (about 5.7 mm TL) began to eat S type rotifers. 17th day-old larvae reached 9.9 mm TL on the average (range 8.50-11.95 mm), and the rudiments of the dorsal and caudal fin rays and fin-supports were partly formed. 29th day-old larvae reached 15.37 mm TL(maximum, 20.0 mm). In groups deprived of food from first-feeding stage, eighth day-old, 12th day-old and 17th day-old after hatching, most larvae could not take rotifers which were given on after three or four days.
 Developmental process of all cartilages and bones was described, using artificially raised and wild specimens. Changes in feeding and swimming functions were clarified from osteological side, and a developmental step forming four periods and eight phases was prescribed.
 Differential sequence of red muscle, pink muscle, white muscle and tonic-like fibers in the lateral muscle as an important locomotor organ was investigated. A layer of red muscle fibers was functionally developed at first-feeding stage. Beyond about 20 mm SL, stratification of red fibers, differentiation of tonic-like fibers and mosaic appearance of white fibers occurred. By 30 mm SL, pink fibers and two types of tonic-like fibers were differentiated. The structure of lateral muscle in a 37 mm SL juvenile was complete.
 Formative process of eye, olfactory organ, taste buds, lateral line system and inner ear was examined. The visual cell layer consisted of only single cones at first-feeding stage. In a 20.9 mm SL larva, rod-like cells and twin cones appeared. Both ciliated and microvillous receptor cells in olfactory organ were found shortly after hatching. Formation of olfactory nostrils and lamellae began at about 20 mm SL. Taste buds first appeared at 11.2 mm NL. Newly hatched larvae were equipped with 12 pairs of neuromasts on the head and trunk. The formation of head lateral line canal commenced at about 20 mm SL and four canals had ossified by 32.5 mm SL. Three semicircular canals formed by first-feeding stage. The structure of inner ear was entirely formed by 32 mm SL.
 The most spawning period was in March and the most GSI rate of females was over 20. From the ratio of specimens with hydrated eggs or postovulatory follicles at the regressing stage 0 (spawning day), presumptive example of the average spawning interval was calculated to be from 4 to 6 days.
 Spawning time and duration of egg development of this species and Japanese anchovy Engraulis japonicus were investigated by 46 time vertical net samplings in the waters off southern Kyushu. Spawning of sardine occurred mainly at about 20:00 on 12 March and during 19:00-20:00 on 13 March, 1991. On the other hand, spawning of anchovy seemed to occur mainly during 00:00-01:00. Spawning times of both species were not overlapped at all.
 The vertical distribution of sardine eggs was investigated to clarify their spawning depth. Horizontal net tows were simultaneously conducted at five layers of approximately 1 m, 20 m, 40 m, 60 m and 80 m. AA stage eggs (from fertilization to beginning of enlargement of perivitelline space) were mostly collected at 40 m and 60 m depths. This means that the spawning depth was approximately 40-60 m. The spawning depth of Japanese anchovy was considered to be 0-20 m. The spawning depths of both species were not overlapped, also the spawning time.
 Morphological changes of unfertilized and fertilized sardine eggs were observed continuously. Unfertilized eggs could be divided into three types on the bases of their morphology: a distorted type, a narrow perivitelline-space type and a normal perivitelline-space type. The first type of eggs disintegrated and sank to the bottom of the containers in a short time. The latter two types of eggs formed not only a perivitelline space but also a blastodisc. They disintegrated gradually within 12 hours after stripping. Common to eggs of these three types, the egg membrane finally broke and the eggs contents were mainly lost. The examination of the field-collected eggs shows that some wild eggs were very similar in their morphological characteristics to unfertilized and disintegrated sardine eggs mentioned above. These facts suggest that unfertilized sardine eggs may be commonly present in natural spawning, especially in the area of the south from Kyushu.
 Short term (from 1 hour to 1 week) variations at the 9 stations in the number of collected sardine eggs were investigated in the adjoining area of Nagasaki Harbor. The maximum variations within 0.5 day were 2.3 or 6.6 times. The maximum variations between days were 2.0 or 5.1 times. The maximum variation at 9 stations was 33.8 times. This result indicates that the number of sardine eggs collected by plankton net changes considerably during short time periods and between adjacent stations, and that the total egg production calculated from spawning survey data may include wide variations.
 Egg abundance and distributional changes of sardine were studied in the waters around Kyushu from 1979 to 1995. A total 13,138 tows was undertaken for collecting eggs by six prefectural experimental stations and the Seikai National Fisheries Research Institute. The egg abundance was calculated for each 30' ×30' square and grouped into three large areas from north to south (AreasⅠ(north of 34゜N), Ⅱ(34゜-31゜30' N), Ⅲ(south of 31゜30' N)). The total egg number considerably fluctuated from 47×1012 in 1995 to 2,873×1012 in 1987. The fluctuation mainly occurred in AreaⅢ.The egg abundance of this area drastically decreased from 1991, and was only 0.3×1012 in 1995. The spawning month showed secular change. In AreaⅠ, spawning occurred mainly in March in 1979 and 1989, April in 1981-1983, and May in 1984-1987. Spawning after 1988 has occurred in March and April. The main spawning grounds were located in AreaⅠ and AreaⅡ in 1979 and 1980. After 1981, the main spawning grounds gradually shifted to the south, and in 1987 most of the spawning occurred in AreaⅢ. The spawning temperature changed with the shift of the spawning area. In 1979, eggs were spawned in 13-19℃, although spawning occurred at the 21℃ level in 1987. After 1991, the spawning temperature returned to 14-19℃.
 Although the sardine resource is very low in present, from past events on records, I may presume that it will increase to the maximum size after present to scores of years.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull22/matsuoka.pdf