A two generation selection experiment in Nile tilapia (O. niloticus, Lake Manzala, Egypt) was carried out to test whether the
proportion of males after temperature treatment could be sustainably influenced by selection. Starting with 45 families as a base
population the families were tested for their thermal sensitivity in the following way. After a 9-day incubation period at 28 °C, fry
of each family were subdivided into a control (28 °C) and a treatment group, each consisting of 110 fish. The treatment group was
kept at a temperature of 36 °C for 10 days. Thereafter treated fry were gradually adapted to 28 °C again. Each treatment and
corresponding control group was raised separately until sexing (by microscopic inspection of gonad squashes at a minimum age of
90 days). A random sample of ten males and females from control groups of each family was kept for later selection decisions. The
percentage of males in treatment groups was the selection criterion and the families showing more than 80% males or less than 60%
after temperature treatment were selected. Over two generations (G0 and G1) around 14% of the tested families were selected to
produce two divergent lines (high and low line) which showed a high or low response to temperature treatment regarding their sex
ratios. In the first and second generation of selection 66 and 26 families were tested in the high line and 72 and 13 families in the
low line, respectively. After two generations of selection the temperature treated groups in the high line showed a male percentage
of 90%, whereas the weakly sensitive low line showed an average male proportion of 54%. The realized heritability estimated from
the cumulated realized selection response and -differential in successive generations was 0.69 in the high line and 0.86 in the low
line. This study provides the first evidence that a surplus of males in temperature treated groups can be selected for as a quantitative
trait. Thus, temperature sensitive lines could be produced as a consumer- and environment-friendly approach to significantly
increase the proportion of males in Nile tilapia, if large numbers of broodstock could be tested.