Answer
If the fish descends, then the diameter of the circle increases.
Work Step by Step
We can use Equation (33-45) to find the critical angle $\theta_c$ at the interface between water and air:
$\theta_c = sin^{-1}~\frac{n_2}{n_1}$
$\theta_c = sin^{-1}~\frac{1.00}{1.33}$
$\theta_c = 48.75^{\circ}$
The maximum refraction angle of the light as it enters the water is $48.75^{\circ}$
Let $D$ be the depth of the fish in the water.
We can find an expression for the radius of the circle:
$\frac{r}{D} = tan~48.75^{\circ}$
$r = D~tan~48.75^{\circ}$
We can see that as the depth increases, then the radius also increases.
Since the diameter is twice the radius, the diameter of the circle also increases.
If the fish descends, then the diameter of the circle increases.
