Some Aspects Of The Biology And Fishery Of The Grey Mullet Liza Subviridis (Valenciennes, 1836)

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Date
1976-03
Authors
Chan, Eng Heng
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Abstract
Studies on some aspects of the biology of the greenback grey mullet, Liza subviridis, including distribution, food types, feeding habits, reproduction and growth were conducted in Penang to provide baseline information for future development and management of the fishery. The existing condition of the mullet fishery in West Malaysia was also analysed. Distribution studies show that ~ subviridis is widely distrib. lted in the coastal habitats and seems to predominate in the lower reaches of streams, estuarine areas, mangrove swamps and IIl.Jd-flats, Its tolerance to a wide range of salinity, dissolved ox~en content and temperature shows that it is a hard y species and its distribution is therefore net limited by these physical parameters. The fish is a bottom feeder and ingests benthic diatoms, filamentous algae and an y organic detri tus that is presS'lt in the habitat in which it feeds. The diet changes fran a plcsnktonic to a bottom feeding type as the fish grows from the post-larva to the adult. Heterosexuality is exhibited and fertilization is external. Sexual differentiation is based on examination of the gonads. Six maturity stages have been identified for the male and seven for the female. The stages are differentiated by macroscopic as well as microscopic changes in the development of the gonads, Sexual maturity is attained at a relatively small size. Male and female fish attain first sexual maturity in the size ranges of 9.0 - 10.5 and 11.0 - 11.5 em respectively, and in the corresponding ages of six and eight months. Cecundity ranges from 40,000 - 140,000 eggs in fish measuring 10.3 13.9 em. The relationship between fecundity, F, and length, L ca~ be expressed as F = 1.9044 L4.2998 Spawning is restricted to a short and definite period and all ripe ova are released within a single act. No major spawning season can be detected and spawning occurs all the year round. The presence of post-larval stages of ~ subviridis as well as adults with sexually mature gonads in shallow coastal waters indicate that the fish probably spawns in inshore waters. Post larval development can be divided into the post-larva, early juvenile, late juvenile and adult stages. Ctenoid scales become developed at 30 - 35 mm total length, pigmentation patterns change from the distinct spot concentrations to diffused dots outlining the scales at about 15 mm total length, the pre-orbital bone becomes serrated from 15mm total length and the adipose eyelid starts to form at 30 mm. total length. Growth can be represented by the von Sertalanffy equation which was calculated to be It = 20.7429 (1 - exp (-0.0035)(t - 14.7059)) The calculated maximum size attainable is 20.74 em. The lengthweight relationship may be expressed by a single equation, log W = 2.8184 log L - 1.4751 Catch statistics shows that the mullet fishery has declined drastically over the last 5 years. The main gears used in the fishery are the seine nets and gill nets. A seasonality is exhibited from October to December. The chief mullet producing states are Perak, Penang, Johore and Belangor. The various biological studies conducted on -Liza _s_u" b;;';v';; ir_i; ,;d,;,,;;i0's;;;' point to the possibility of the destruction of coastal habitats by pollution and land reclamation programmes as the primary cause of the decline of the mullet fishery. To combat the decline in the fishery, it is suggested that mullet be introduced as a culture species in brackish water as well as fresh water pond systems. The potential of mullet culture and further work on induced spawning and culture aspects are also discussed with reference to the findings embodied in this thesis.
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Baseline information for future development , and management of the fishery
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