This is caused by the marine phenomenon currently known as “Red Tide”.

It’s not always red and it’s not always tidal.  It’s a certain type of algae which exists in normal quantities, harmlessly, but when they become in greater concentrations  can kill marine life by paralyzing/poisoning or oxygen starvation.   This happens at various places around the world: us obviously, Florida and Texas also have them  a lot.  During a red tide it is advised to not eat oysters or shellfish – in the USA they have ways of banning such sales, which is of course detrimental to that industry.  Still not known what causes this extra concentration – sometimes associated with change in water temperatures, but the general feeling is leaning towards pollution by mankind.