This is the Best answer for What is the mass of the fish? You can ignore the mass of the spring.?
The formula for the oscillation of a mass on a spring is T=2π√(m/k)
The relationship between frequency and period is T=1/f so the period is 1/2.60=0.3846 s.
From the information given, a force of 200N will stretch the spring by 0.125m.
That enables us to find the spring constant since k=F/x
Rearrange the formula