The tropical anguillid eel, Anguilla bicolor McClelland, experiences significant growth up to the adult size in both freshwater and estuarine ecosystems, encountering salinity gradients that may impact their reproduction. Therefore, this study aimed to investigate the reproductive aspects of A. bicolor collected from Serayu River (freshwater) and Segara Anakan (estuary), Cilacap, Central Java, during July 2020 field surveys. The examined parameters included eye index (EI), fin index (FI), gonadosomatic index (GSI), hepatosomatic index (HSI), and sex ratio of eel specimens in freshwater and estuary. Furthermore, the total length (TL) and body weight (BW) ranged from 260 - 630 mm and 18 - 419 g in freshwater, and 260 - 630 mm and 18 - 447 g in estuary. Significant differences (P<0.01) were observed in the reproductive characteristics between the two habitats (P<0.01), with higher eye index, fin index, gonadosomatic index, and hepatosomatic index in freshwater-caught eels than the values obtained in estuary inhabitants. The mean of these four parameters were 6.99 ± 2.98%, 4.08 ± 0.71%, 1.70 ± 1.13%, and 1.92% ± 1.07 in freshwater, while 3.48 ± 1.60%, 3.45 ± 0.479%, 1.12 ± 0.23%, and 0.28 ± 0.19% were found in estuary. The sex ratio was significantly affected by habitat, with female eels dominating in freshwater (62.5%) compared to estuary (32%). These results suggested that eels in freshwater habitats might exhibit early gonadal maturation compared to those in estuary.

Previous results demonstrated the stimulating effect of soybean phosphatidylcholine (PC) on the utilization of dietary neutral lipid in larval and postlarval fish. The present study further investigated the effect of the degree of saturation of dietary PC on the enhancement of dietary fatty acid incorporation in lipids of turbot. Newly-weaned turbot were fed for 20 days on four isolipidic diets containing the same amount of highly unsaturated fatty acids (HUFA), presented either as neutral lipid, i.e. fish oil ethyl esters, or as polar lipid. Diet FO was a phospholipid-free control diet. Diets HPC, SPC and FPC were supplemented with 3% hydrogenated soybean PC, 3% native soybean PC and 3% marine fish roe PC, respectively. The three PC-supplemented diets resulted in better growth and higher muscle triacylglycerol levels than the PC-free diet FO. The fish fatty acids were determined in 3 lipid classes (neutral lipid, PC, phosphatidylethanolamine) of 3 organs or tissues (eye, brain and muscle). Despite the identical amounts of n-6 and n-3 fatty acids provided by the soybean oil and by the HUFA ethyl esters, the substitution of 3% hydrogenated coconut oil in diet FO by 3% hydrogenated PC in diet HPC caused, averaged over the various tissues and lipid classes, a 7 to 12% higher incorporation of 18:2n-6, 20:4n-6, 20:5n-3 and a 32% higher 22:6n-3 level in turbot lipid. Diet HPC appeared as efficient as diet SPC for enhancing the incorporation of the n-3 HUFA from the ethyl esters. Feeding diet FPC, in which the n-3 HUFA were provided through the marine PC source, resulted in slightly higher levels of these fatty acids in the fish than feeding the ethyl ester HUFA diets, even if supplemented with PC. Present results confirm the positive effect of PC, either hydrogenated or native, on the utilization of fatty acids provided in the diet as neutral lipid. The slightly higher incorporation of HUFA, when esterified on dietary PC instead of neutral lipid, raises the question regarding the form of intestinal absorption of PL in fish.

There is a growing interest in the aquaculture of tropical anguillid eels as an export commodity. However, studies on feeding strategies, and the present demand to reduce feed costs need to be addressed to ensure the economic viability of eel farming. In this study, the effects of daily (DF) and alternate day (ADF) feeding on growth, feed utilization, body composition, blood chemistry, liver and intestinal morphology, and economic viability in the Pacific shortfin eel Anguilla bicolor pacifica were examined. Each feeding group of 30 elvers (166.25 ± 16.23 g mean initial wt) were randomly stocked in triplicate 4 m³ outdoor concrete tanks. These were fed for 155 d with formulated eel powder diet (49.77% crude protein; 10.21% crude lipid) made into a paste. Growth and survival were not significantly different between the two feeding groups. However, feed efficiency was improved in the ADF group, with significantly higher protein efficiency ratio (0.63) than in the DF group (0.39). Feed conversion ratio (FCR) was lower in ADF (3.85) than in the DF group (6.44), resulting in a 40% reduction in total feed consumption. Biometric indices, body proximate composition, and blood chemistry were not significantly affected. Likewise, liver and intestinal morphology showed no apparent alterations between groups. Partial costs-and-returns analysis showed that ADF yielded higher net profit and profit index. These results suggest that feeding on alternate days promote compensatory growth, better feed utilization, and normal physiological condition of A. bicolor pacifica, and reduce feed cost in the rearing process. Therefore, alternate day feeding should be promoted as a sound feed management strategy in the tank culture of tropical anguillid eels.