Assuming that y is the number of fish and x is the number of months. The equation can be solve using the properties of logarithms.
log y = 0.9013.logx+ 0.6935
since x = 12 months, and substitute it to the equation
log y = 0.9013.log(12)+ 0.6935
log y = 1.6662
y = 10^1.6662
y = 46 fish in 12 months