In happened to hatchery-raised juvenile salmonids when released

In the present study, Japanese eels in
each treatment stop feeding after they were acclimatized to seawater and kept
inside the floating fibre-glass tank resulted BW showed a decreasing trend
probably from water loss. The seawater adaptation of Japanese eels such as
drinking salt water as reported in European eels, revealed that salinity
evoking enteric neuronal plasticity caused a reduction of circular muscle layer
(CML) in the eel gastrointestinal tract purposely for compensation of water
loss during migration (Sorteni et al.,
2014). Moreover, the intestine weight reduced due to less developed CML will be
more functional for osmoregulation rather than active nutrient absorption (Aida
et al., 2012). Apart weight reduced from
water loss, the degeneration of digestive tract with no stomach or intestine
content examined from wild anguillids captured by trawl near the spawning
ground (Tsukamoto et al., 2011) could
also contributing to weight loss of Japanese eels in the present study.

In the present study, Japanese eels in
each treatment probably experienced anorexia as TL showed decreasing trend, resulted
in shrinkage of size. Anorexia in fish is the decrease or cessation of feeding
when fish is under unfavourable environmental condition (Islam & Woo,
1991). Anorexia can be happened to Japanese eels, as reported in feeding trial
with formulated feed containing low concentration of ascorbic acid which caused
poor growth (Ren et al., 2005).  Anorexia happened to hatchery-raised juvenile
salmonids when released to natural environment in autumn which resulted in body
shrinkage in term of body weight and total length due to lower preying ability
compared to wild salmonids (Huusko et al.,
2011).

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In the present study, Japanese eels in
each treatment showed decreasing trend in BG. This probably due to the starvation
as utilisation of energy in the form of fat reserved in the eel body for its
metabolic activities, and protein mobilisation was slightly faster than fat in
starving silver eels (Rankin & Jensen, 2012). BG increases faster instead
of TL during growth to sexual maturation as fat reserves in the body for
spawning migration purpose whereas BW decreased more rapidly than TL during
starvation (Knights, 1982).  

In the present study, Japanese eels in
each treatment showed decreasing trend in CF. Previous study reported on good CF
of European eel, Anguilla anguilla is
an ideal corpulence in relation to TL with CF value of l.08 and values greater
than that of 1.10-1.38 was considered in poor condition (Knights, 1982). The CF value of European eels may not
suitable as a guideline for ideal corpulence of Japanese eel since CF value in present
study all were greater than 1.08. However, higher value of condition factor was
reported in 68 wild male Japanese eels captured in Yangtze River with CF value of
1.33-2.16 and average CF value of 1.63±0.18 (Guo et al., 2011). In the present study, 1D (50%, two eels), 3D (50%, two
eels), 5D (50%, two eels) and 7D (25%, one eel) with CF values fall in the
range of 1.33-2.16 while the rest was between 1.12-1.29.

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