Why captured European fleets would strain the Kriegsmarine

What if Germany absorbed the ships of the navies it overran in Europe?

The ships could add useful capabilities, but the transfer's scope, trained personnel, and support burden prevent paper totals from establishing usable strength.

The counterfactual gives Germany the prewar navies of countries it occupied by August 1940. Polish and French destroyers would be attractive additions, but the availability of trained personnel is immediately questionable. Partial occupation creates disagreement over whether Germany receives an entire fleet or only the ships and infrastructure in occupied territory. The Channel Islands expose how far the broader interpretation could be stretched to include the Royal Navy. Even on that reading, the diversity and scale of the acquired fleet are expected to overwhelm German support capacity. Receiving hulls leaves the problems of crewing and sustaining them unresolved.