There are not many speicies of poisonous fish that are commonly found in the waters around Majorca, however there are a few that you should be aware of before venturing into the sea or going fishing.
Weever fish (Araña)
Live in sandy beds beneath the surface, they are easily captured, but be very careful when handling. The thorns on the first dorsal and areas around the gills are poisonous! They can cause severe prolonged pain, fever, vomiting, slowing of heart rate, difficulty breathing and in some rare cases even death. Return to the sea without touching it.
While underwater diving / snorkelling, if you sea Weever fish, swimming in a wavy motion and with the dorsal open, you should swim away immediately. It is preparing to attack.
Rays (Pastinacas), Eagle Rays and Others
Found in sandy beds, beneath the surface. They are more easily caught at night or when underwater fishing (i.e. spear gun fishing). They have 1, 2 or even 3 poisonous thorn type stings. Their poison is deactivated by heat. Always return to the sea, as they are endangered in the area.
Scorpion Fish (Escorpora) / Red scorpionfish (Cabrachos)
Frequent stone or posidonia sea grass beds. The thorns on the head and dorsal are poisonous causing extreme pain. They are also difficult to capture.
Dogfish and Spiny Dogfish
A small shark usually captured by lure fishing. Has two poisonous thorns, one in front of each dorsal. The poison produces very painful wounds.
If fishing, you should always carry forceps / tweezers and protective gloves to remove the hook or cut the line without touching the fish. Once dead or even frozen, the poison can still be active and for this reason alone, they should always be returned to the sea.
If you are stung by a poisonous fish go straight to a doctor. They will apply the appropriate medication for type of sting, so it helps to know what has stung you. Applying 50C+ of heat to the sting might help, but be careful not to burn yourself. This is normally achieved by placing the stung limb in hot water.