This is completely normal for the bluegill, which is found along the Swedish west coast. What is unusual is that it happens in front of the public and can be documented, the Øresund Aquarium says in a press release. Otherwise, it usually happens far down on the seabed.
Female bluegill are called redbills, as they are often red in color. When the females change sex, a process that can take up to eight months, they look almost like tropical fish with blue-orange coloration. This makes them easier for predatory fish to spot, and therefore it is an advantage that this happens only later in life, when they have grown large enough to defend a territory.
