There are three levels of Spanish shorefishing laws: European level, state level, and autonomous community levels. Each Spanish region is allowed to issue their licences and specific rules that you must follow while fishing there. The state law allows people who already have a licence from one region, to fish in another, as long as they follow the rules of the area where they are fishing. If all you want to do is catch and release, there isn’t much to worry about banned species nor minimum sizes.
Spanish Shorefishing Licence Law
One of the most important fishing laws is in effect since the 26th of February of 1999. Here is the link (in Spanish).
The part that talks about reciprocal agreements is in Article 13:
In accordance with the Royal Decrees of transfers in fishing in inland waters, shellfish and aquaculture, each Autonomous Community will recognize maritime recreational fishing licenses issued by the other Autonomous Communities.
Here it is in Spanish:
Conforme a los Reales Decretos de transferencias en materia de pesca en aguas interiores, marisqueo y acuicultura, cada Comunidad Autónoma reconocerá las licencias de pesca marítima de recreo expedidas por las demás Comunidades Autónomas.