Why does overfishing lead to the decrease of fishery resources and the smaller and smaller fish caught?

Fish begin to lay eggs and reproduce when they grow to normal size under natural conditions. Overfishing will force fish to lay eggs before reaching the normal breeding age. In order to cope with this phenomenon, fishermen use fishing nets with smaller meshes, which in turn makes fish caught at a younger age, forcing them to lay eggs further in advance, leading to a vicious circle. The yellow croaker in the East China Sea fishing ground is getting smaller and smaller.