There are three levels of Spanish shore fishing laws: European level, state level, and autonomous community levels. Each Spanish region can issue their licences and specific rules you must follow while fishing there. The state law allows people with a licence from one region, to fish in another. You must follow the rules of the area where you are fishing. If all you want to do is catch and release, there isn’t much to worry about banned species or minimum sizes.
Spanish Shore Fishing Laws: Reciprocity Agreement that Lets You Fish Nationwide
One of the most important fishing laws has been in effect since the 26th of February of 1999. Here is the link (in Spanish).
https://www.boe.es/buscar/doc.php?id=BOE-A-1999-5160
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 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.
Spanish Shore Fishing Laws: Canary Islands
The Canary Islands government officially recognises licences issued by other Spanish governments, on their fishing department website. Click here to read more
When you show up at any fishing office to apply for a licence, no one will probably ask whether you already have a licence from another area. It is in their interest to collect fees for their region. It is your responsibility to know the laws, not theirs. So, if you plan to fish in different areas of Spain, save money and time by being smart. You just need one licence.