As a guide: Brown rats can swim better than black rats which can swim better than house mice. Mice, however, are high-risk stowaway invaders.
At 50m all rodents can easily swim to an island, and will do so frequently.
At 500m black rat will invade but the frequency of incursions may be low.
At 500m brown rat could, in many circumstances, be expected to reach the island every year
Use this handy guide to learn how to identify rodents and their signs. Entrance holes, droppings, nest material and gnaw marks can all help alert you to a rodent on your boat.
Do not land on a seabird island. Never push stowaways overboard dead or alive! Do not assume only one individual stowaway is aboard. Return to the mainland.