  We implement a simple hydrodynamical model to study behavioural swimming tilt
angle of open swimmbladder fish. For this purpose we study the stability of
forces acting on a fish swimming horizontally with constant velocity.
Additionally, the open swimbladder compression with the depth is modelled by
Boyle's law. With these, our model gives an analytical solution relating the
depth with the body tilt angle and the velocity. An interesting result for
steady horizontal swimming is that the body tilt decreases with velocity almost
like $v^{-1}$. Moreover, we give an expression for the maximum tilt angle.
Then, by introducing the assumption of constant swimming power we relate the
swimming velocity with the tilting. Furthermore, we show that the
hydrodynamical influence of a temperature gradient produced by a thermocline
seems to be negligible for the fish tilting. These results are considerably
helpful for more realistic modelling of the \emph{acoustic target strength} of
fish. Finally, we tested our results by comparing the hydrodynamics solutions
with others obtained from acoustic observations and simulations of target
strength for Argentine anchovy.
