Biology of the Threadfin Bream, Nemipterus Japonicus (Block)
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BIOLOGY OF THE THREADFIN BREAM, NEMIPTERUS JAPONICUS (BLOCK) B. KRISHNAMOORTHI Central Mafiae Fisheries Research Institute; Sub-station, Visakhapatnam-3 ABSTRACT Nemipterus Japonicus attains an average length of 150 mm in the I year; 210 mm at the end of II year; and 240 mm at the end of III year. The fishery along the Andhra- Orissa coast is supported by fish which have completed one year. November marks the commencement of spawning and N. japonicus may spawn for the first time at 160-170 mm and for a second time, perhaps, at 220 mm. N. japonicus is undoubtedly a carnivore, feeds substantially on crustaceans, molluscs, annelids and echinoderms in that order of abundance and in one of the 1,726 stomachs examined over a three year period, vegetable matter was encountered. It is actively predaceous and possibly a sight-feeder. There was a certain degree of regurgitation, particularly during the spawning period from November to March. The feeding intensity did not increase with an increase in size, but the nature of food com ponents was size-dependent. There was a significant increase in the feeding intensity as A''. japonicus advanced to maturity. The peak catches in the fishery coincided or followed peaks in feeding intensity. INTRODUCTION Simultaneous with studies on the distribution and magnitude of abundance of Nemipterus Japonicus off the Andhra-Orissa coast (Krishnamoorthi, 1968), an investigation, extending over a three-year period from August 1964 to July 1967, was carried out to study the growth, reproduction and food and feeding habits in N. japonicus and the results are dealt with in the present account. MATERIAL AND METHODS The material Was largely drawn, as in an earlier study (Krishnamoorthi, 1968), from the catches of the Govt, of India trawlers M.T./IjAofc, MN. Champa and M.V. Sea Horse. Though M.T. Asholc operated a 15 m trawl and M.V. Champa and M.V. Sea Horse operated a 14 m trawl, the mesh size of the cod end of all the trawls was similar. Hence, biological data of N. japonicus collected in a month from all the vessels were pooled together. Samples were obtained at Weekly intervals. Though large numbers (100 specimens on an average) of N. japonicus were measured every week for length, fewer numbers (as indicated in the sections concerned) Were used for collection of other biological data. All measurements were, made on fresh specimens. A scale of only six stages of maturity, as followed 2 B. KRISHNAMOORTHI by Prabhu (1955) for Trich'mms haumela, was adopted in the present studies. While Prabhu (1955) did not come across stage VI in T. haumela, I could, on many occasions, observe N. japonicus in an oozing stage which, along with fish in a stage slightly more advanced than stage V, were treated as belonging to stage VI. AGE AND GROWTH Length frequency A total of 6,865 specimens of both males and females of N. japonicus was measured during the three-year period of this investigation (1964-65: 3,147; 1965-66: 1,288 and 1966-67: 2,430 specimens). The frequency polygons at size intervals of 10 mm for the three years are presented in Figs. 1 to 3. 1964-65. There were, during this year, principally 6 modes a, b, c, d, e and /(Fig. 1), whose progression could be followed. Modes a and c could be traced only from August to October. While the former progressed from 160 mm to 180 mm the latter progressed from 150 mm to 160 mm. In other words, mode a moved by 20 mm in 3 months at the rate of 6.66 mm per month, and mode c pro gressed by 10 mm in 4 months at the rate of 2.5 mm per month. The mode b progressed by 60 mm in 10 months, modes d and e by 60 mm in 7 months and mode /by 50 mm in 4 months at the respective rates of 6, 8.6 and 12.5 mm per month. Thus, the size group at mode / had the greatest rate of growth (12.5 mm per month) and the size group at mode c the least rate (2.5 mm per month). Both the size groups at modes d and e had a similar rate of growth (8.6 mm per month) and the size groups at modes a and b had more or less comparable rates of growth (6.66 and 6 mm per month)'. Since it is known that with increase in age the rate of growth tends to fall in fishes, it could be assumed based on the rates of growth presently obtained for the various modes, that the size group at mode / may re present the I year class, at modes d and e the II year class, and at modes a and b the III year class. In other words, N. japonicus attained a size of 150 mm at the end of the I year, 230 mm at the end of the II year and 2b0 mm at the end of III-IV year in 1964-65. 1965-66. This year there were principally only 4 modal groups, namely, a {a and oi), b (b, bi and bz), c (c and Cj) and d (Fig. 2). While the mode at d (220 mm) in November could not be traced in the succeeding months, the pro gression in the rest of the modes could be followed through the months from August to July. Thus, the modal group a (180 mm) of August progressed to 220 mm in May, i.e., a growth of 40 mm in 10 months or 4 mm per month. The group b which appeared in September at 150 mm progressed to 200 mm in July i.e., a growth of 50 mm in 11 months or 4.5 mm per month. The group c at 100 mm in September progressed to 170 mm in July and had a growth of 70 mm in 11 months or 6.4 mm per month. Extending the same arguments as for the modes of the previous year, the group c may be considered as the I year class because it had the highest u o o •n X > o z ea w > S 40 SO 60 70 60 90 lOO liO 120 llo 140 150 160 170 ISO I90 ZOO ZIO 220 230 240 250 260 270 280 Z90 300 310 320 LENGTH GROUPS 1N mm FIG. 1 Length frequency distribution in N. japonicus during the various months of the year 1964-65. 9^ 2 •zX > o o H X 40 50 60 70 ao 90 100 110 I20 130 l40 150 WO 170 ISO 190 ZOO 210 220 230 240 250 260 270 280 290 300 510 320 53(7 LENGTH GROUPS IN mm FIG. 2 Length frequency distribution in N. japonicus during the various months of the year 1965-66. BIOLOGY OF THREADFIN BREAM 5 (6.4 mm per month) growth rate, the group b as the II year class which had a growth rate of 4.5 mm per month, and the group a as the III year class since it had the least rate of growth ( 4 mm per month). Compared to the previous year's, the rates of growth obtained in the present year Were less. 1966-67. This year also there were principally 4 modal groups, viz., a (a and Oi), b (b and bi) c and d (Fig. 3). While the progression of modal group a and c could be followed from August to May, the group b could be traced from August to June only. The mode rf at 120 mm-fresh recruits to the fishery perhaps-appeared in February and progressed to 140 mm in May, i.e., a growth of 20 mm in 4 months at a rate of 5 mm per month. The growth rates of the other modal groups a, b and c respectively were 5, 3.3 and 3 mm per month; and based on these growth rates they respectively represent the I, II and III year classes in the fisheries for A''. japonicus. The sizes reached at the end of I, II and III years, being 140, 220 and 250 mm, were comparable with those obtained during 1964-65. Reviewing these studies, it may be said that N. japonicus attains an average size of 150 mm in its I year (140-170 mm), 210 mm (200-230 mm) at the end of its II year, and 240 mm (220-260 mm) at the end of its III year. Ben-Tuvia (1968) has reported a size of 130 mm for a fish of'O' year class and 170 mm for one-year-old N. japonicus obtained in the trawl catches in the South Red Sea along the Ethiopian Coast. Sizes of 110-170 mm (mean: 140 mm) for 1 year old, 170-250 mm (mean: 230 mm) for 2 year old, and 290-300 mm for 3 year old males of N. virgatus from the South China Sea have been reported (Eggleston, 1970). Eggleston (1971) reports also the mean sizes of the males of N. bathybus as follows: 1 year -120 mm., 2 year - 150 mm, and 3 year - 190 mm. The present results obtained for 1 and 2 year olds are more or less comparable with those reported for N. virgatus, though the average size presently obtained for the 3 year olds (240 mm) is much less than the range (290-300 mm) given by Eggleston (1970). From the studies of Ben-Tuvia (1968), it is not known which year class contributes most to the N.