IJA Volume 53, Issue 2, 2001
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ItemProtandrous Hermaphroditism In Australian Silver Perch, Bidyanus Bidyanus (Mitchell, 1836)(Israeli Journal of Aquaculture - BAMIGDEH, 2001)Gonadal development in two progenies of the Australian silver perch, Bidyanus bidyanus, was studied from the larval stage to 18.5 months. For approximately one month after hatching, it was impossible to visually or histologically determine the sex of the fish. Two to three months after hatching, anatomical and cytological sex differentiation occurred. At 4-5 months, the testis in most of the fish longer than 7-8 cm exhibited all stages of spermatogenesis including spermatozoa. Very few females were found among the 4 and 6.5 month fish. Up to 18.5 months, 7.1-23.5% were female (except in one batch). There were only 25 females, all at early stages of oogenesis, amongst 204 fish with differentiated sex gonads. In 10 of 17 histologically studied females, there were degenerating male cells (possibly spermatocytes) among a majority of developing oocytes. The predominance of phenotypic males, and the occurrence of females with ovaries and degen- erating male cells, indicate that during the first years of life, this species is a protandrous her- maphrodite. The testis develops during ontogenesis in a direct manner, whereas the female gonad develops indirectly, passing through an intermediate masculine stage.
ItemThe Israeli Market For Cultured Gilthead Sea Bream (Sparus Aurata)(Israeli Journal of Aquaculture - BAMIGDEH, 2001)With average annual growth at 38%, sea bream production is the main growth factor in Israel's fish- eries industry. It is estimated that cultured sea bream constituted over 10% of the volume and 20% of the value of domestic fisheries in 2000. Per capita consumption of sea bream grew from 0.11 kg in 1995 to an estimated 0.39 kg in 2000. Rapid industry development lowered market prices but, in general, farm-gate prices in Israel are higher than in major Mediterranean producers. Fluctuations in sea bream prices reflect fluctuating demand, but the percentage that sales have increased is greater than the percentage that prices have decreased. Cultured sea bream in Israel lacks competition from wild or imported sources. Increasing supplies of sea bream will come from the local mariculture industry, but growth is slowing and several constraints must be overcome. Intensive recirculating aquaculture systems may enhance inland marine farm production. The sea bream market in Israel has not reached saturation level, as indicated by consumption which has increased without signifi- cant marketing efforts. Demand could increase to 4,500 tons (0.7 kg per capita) by 2005. Whether this forecast is overly optimistic or not, it reveals potential for mariculture development.
ItemThe Influence Of Daily Feeding Frequency On Growth And Feed Consumption Of Rainbow Trout Fingerlings (Oncorhynchus Mykiss) Reared At 18.5-22.5°C(Israeli Journal of Aquaculture - BAMIGDEH, 2001)Feed consumption, growth and feed conversion ratios were studied in rainbow trout (Oncorhynchus mykiss) juveniles (mean weight 9.0±3.1 g) fed two, three or four times daily and reared at 18.5-22.5°C. The fish reached final mean weights of 26.8-31.6 g over the 40-day trial and growth rates (SGR) ranged 2.57-3.01% per day. Fish fed four times a day had a significantly high- er SGR and final mean weight than those fed two or three times each day. Similarly, the condition factors of the groups fed three or four times a day were higher than that fed twice (p<0.01). Daily feed consumption rates (3.1-3.7% of the body weight) increased significantly with increasing feed- ing frequency (p<0.05), while differences in feed conversion ratios (1.06-1.16) were significant only between the groups fed two and three times (p<0.01). At water temperatures of 18.5-22.5°C, rain- bow trout juveniles fed four times a day grow faster than those fed two or three times.