I think it's important for you to have ethics committee permission. But it may depend on the purpose of the dissection. For example, if the purpose is to dissect the fish for a didactic activity or experiment, authorization from the ethics committee will ensure that you have taken all necessary precautions. Some people may not feel well following a dissection, even if it is dissecting fish.

If possible, you can consult well-established ethics committees in your region for more information, mainly in relation to legislation.

Ethical consideration is not only needed as in human's consent to be part of a research project, the project itself need to be ethically approved to justify the experimentation and research.

In your example, the second one can be obtained and is considered an ethical approval.

Any biological research utilizing living beings especially animals should be supported though legal ethical permission from the concerned authority.