This reserve, in the far northwest of Rio Muni, spans 335sq km and is a Ramsar Wetland of International Importance. Turtles, hippos and goliath frogs abound. It is managed by INDEFOR-AP in Bata, from whom you can obtain permits to visit.
There are some cabins within the reserve to rent for turtle-watching, though they can be unavailable at times. Check with reserve management before making your way here.