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. 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 nor minimum sizes.
Spanish Shorefishing Laws: Reciprocity Agreement that Lets You Fish Nationwide
One of the most important Spanish shorefishing 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 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.
When you show up at a 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 really your responsability to know the Spanish shorefishing 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.