Yes it's compulsory.
Hefty fines,  if you are found out to have ANY assets, other than here in Spain, worth over 50.000 euros. and havn't filled in the form.
So it means, bank accounts, shares, property etc. over the value of 50.000. This is per item.
This is not to do with paying taxes, in either country.
I believe you can find the form on on line now, but not in English!
If you wish to speak to someone about this, Carlos is a nice guy who speaks English. He is based in Tortosa. Tel: 977449022