The following differential equation is given to model the population of fish which is affected by a fishery:
The first half of this equation describes the logistic growth of the population and is pretty easy to understand. The second half though seems way less obvious. No further information is given on what $\alpha$ or $H_2$ are supposed to be. $U$ is the concentration of fish present and $K$ is the capacity.
The term seems to always give us a value between $0$ and $1$, which sounds like a ratio. But we subtract that value, rather than multiplying, which seems to show that it is not a ratio. Furthermore, I am not even sure what $H_2$ is supposed to represent. If it is a representation of the effort the fishery puts into fishing, then it is placed somewhat counterproductively. It seems like the higher $H_2$ is, the less fish is actually fished.
How is the second part supposed to be interpreted so that this is a sensible model? What are $H_2$ and $\alpha$?