The stress of handling or confinement led to an
increase of ion branchial efflux and high Na+ loss in
rainbow trout (McDonald et al., 1991).Therefore, the
use of adequate amounts of salts (as NaCl) in the water
for transport could reduce the plasma–water gradient
and consequently the loss of ions from fishes into the
environment (Wurts, 1995). However, in the present
study pirarucu transported in water without addition of
salt did not show any significant loss of ions (except for
K+ after 3h), because ion net fluxes were close to zero.
If the fish would loose ions through transport, there
would be a significant net ion efflux. Consequently, the
use of salt in the water to avoid Na+ or Cl− loss is not
necessary. Moreover, there was a significant increase of
Na+, Cl− and Ca2+ influxes in fish transported in the
water with higher amounts of salt, and these influxes
increased up to 2h of transport. This was expected, since
all osmoregulatory effort of freshwater fishes consists of
obtaining these ions from the water (or avoids ion loss)
(Baldisserotto, 2003), and when transferred to water
with higher salt content freshwater fish must adapt to the
new conditions, i.e., reduce ion influx.