As mentioned above, the large plasma volumes required for the hormone
radioimmunoassays allowed only a limited number of assays for each group.
The only significant difference in the growth hormone and prolactin data was in
plasma tPRL188 levels, which were lower in fish in sea water and 1·6# sea water
compared to freshwater fish (Table I). Plasma tPRL177 levels were detectable
only in fresh water (8·22&2·83 ng ml"1, n=4). This result is consistent with
other data on this tilapia species being transferred from fresh water to sea water
(Nicoll et al., 1981; Ayson et al., 1993; Yada et al., 1994). As this ‘ freshwateradapting
’ hormone retains sodium, this response is appropriate for ionic
regulation in sea water.