well a friend of mine asked the RSPCA about feeder fish using piranha as an example. still doesnt really answer it as it seems no one is sure what the law actually is and theres a lot of grey area.
Thank you for your e-mail about Feeder fish. We have spoken to our Wildlife Department about this matter and they have said that as far as they are aware, there are no legal constraints on feeding live fish to other fish. There is also no legislation that specifically allows the use of certain fish to feed certain other fish, such as suggested by the pet shop staff. However, there is clear evidence that fish are 'sentient' (meaning that it has been proven that they feel pain). Such action of feeding live fish to other live fish could potentially be an offence under the Protection of Animals Act 1911, if a vet agreed that unneccesary suffering had been caused to the fish fed to the other fish. The difficulty in proving there has been an offence committed, would be in trying to prove that the fish fed to the other fish were intentionally and knowingly caused pain by the pet shop staff because of their actions. Piranhas (depending upon the species) will scavenge fish scales off live fish, take dead fish or meat sinking through the water, or if hungry enough, could attack other live fish. However, piranhas do not require the offering of live fish in captivity to trigger feeding, as feeding can also be triggered by dead meat/fish being dropped into their water tank. Another issue that could have an inpact on the welfare of the piranhas and the live fish being added to their tank as their food, is that the taking of fish from one water environment and then releasing them straight into water of a different temperature, salinity etc., can instantly shock the a fish's system and could prove fatal. If after receiving this information you feel that the pet shop staff are practicing a method of feeding which is against the welfare of the fish concerned, you can contact our 24 hour National Advice and Cruelty Helpline on 0870 55 55 999 and report the pet shop concerned, in complete confidence. One of our local RSPCA Inspectors can then visit the shop and discuss these practices with the staff and make recommendations on how best the piranhas can be fed in a humane way that does not involve feeding them live fish. Thank you for contacting us with your concerns. Kind regards RSPCA Enquiries Service