One of the ways and common practices in freshwater fish facilities
that mitigate transport stress is the addition of salt to thewater.Whereas
this practice has shown variable success depending on the fish
species of study, it is not yet known why salt can sometimes help freshwater
fish species to better respond to stress responses. It has been proposed
that salt aidswith osmoregulatory disturbances that occur during
transport (Crosby et al., 2006; Oyoo-Okoth et al., 2011) but othermechanisms
of action may also play a significant role. We hypothesize that
the addition of salt to transport water aids with returning to homeostasis
thanks to the effects of salt on skin mucosal immunity and the skin
microbiota.
In order to test our hypothesis, the present work specifically evaluates
the changes that trout skinmorphology, epithelial barrier integrity,
innate immune gene expression and bacterial communities undergo
following a 5-hour transport event in the presence or absence of salt
in the transportwater. Our results showthat despite a clear commensal
overgrowth in stressed individuals, bacterial translocation from skin to
systemic organs did not occur. Rainbow trout skin maintained homeostasis
in a number of ways such as increasing tight junction and mucin
gene expression, decreasing anti-microbial peptide expression and
inducing an anti-inflammatory phenotype via the TGF-β pathway.
Adding salt to transport tanks ameliorated stress parameters and
reduced the overgrowth of skin bacteria suggesting that previously beneficial
effects of salt addition may be explained by the effects of salt on
the fish microbial communities. Our findings have significant implication
to the field of aquaculture and underscore the importance of skin
mucosal health during transport operations