In the past three weeks all the red-billed gulls brought to Kaikōura Wildlife Rescue with toxic algal bloom poisoning have been euthanised.

Kaikōura Wildlife Rescue has been inundated with sick birds, with what appears to be a form of toxic algal bloom poisoning.

In the past three weeks the centre has treated more than 20 sick adults and juveniles red-billed gulls.

Manager and zoologist Sabrina Luecht said the first sick red-billed gull was reported on December 29 and there was no sign of the sick birds stopping. She was no longer counting the number of red-billed gulls she had taken in.

PIPPA BROWN/STUFF This young red-billed gull, which has a black beak as a juvenile, was euthanised yesterday by Sabrina Leucht manager of Kaikōura Wildlife Rescue from suspected toxic algal bloom poisoning.

The birds had been found all over Kaikōura, and a few have been run over as they were left floundering on roads.

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All the birds displayed symptoms of neurotoxin poisoning.

Leucht said the primary symptoms were paralysis of the legs, loss of balance and motor skills, and vomiting.

"The legs are often unable to move, are lifeless and have no reflex or feeling.

"Due to the low likelihood of full recovery or potential length of recovery, the decision is generally to euthanise."

"Recovery is limited and the toxicity often too progressive, meaning we are euthanising to put these birds out of their misery," she said.

Leucht said the birds were unlikely to recover and it was the only humane way to treat them.

Periodic algal blooms occur in marine environments throughout the world.

"Although many algal blooms are merely a nuisance, some are harmful and produce biotoxins that kill can fish, shellfish, mammals and birds, said Leucht.

"It takes 12- 24 hours after eating the shellfish for the symptoms to show up in the bird, which makes it harder to detect where the bloom might be."

Not all algae blooms can be seen, but may be visible as coloured patches in the water, usually as a brown, green or red bloom. Only a small percentage are harmful when they produce biotoxins or phytotoxins.

"It could be a long way off shore or it could be an algae bloom you don't really see but it has affected the shellfish which is the gull's main diet.

"​Shell fish are filter feeders and absorb a lot of plankton, which includes algae so it is quite concentrated in the animal itself and a gull only needs to eat a few and it is overloaded with toxins very quickly."

The far end of the Kaikōura peninsula has one of the largest colony of red-billed gulls in New Zealand. They appear on the threatened species lists, alongside the kiwi and the kakapo.

Leucht will send the next dead gull away to Massey University for necropsy to determine the cause.

NIWA marine biologist Hoe Chang, who has been working with toxic algae blooms for 30 years said he wasn't aware of any reports of a bloom near Kaikōura. He suspected the mortality was caused by paralytic shellfish poisoning (PSP) but was surprised that only one seagull species had been affected.

It would be necessary to send a water sample and a sample of the bird's gut contents to be analysed to accurately determine the cause.

There was a chance the mortality was caused by something else, even a viral infection, he said.

Chang said there was some risk to humans and pets if people ate contaminated shellfish and it was confirmed to be a neural toxin caused by NSP or PSP-producing algae. NSP was more common in the North Island.

"If a human feeds on shellfish poisoned by PSP producing organisms the first symptoms would be tingling fingers and numbness of lips."

"Paua could be affected, but crayfish is less likely to be because shellfish are filter feeders and have a concentration of toxic algae, whereas crayfish are scavengers so it is less likely," Chang said.

Juvenile red-billed gulls can be mistaken for black-billed adult gulls. They may look the same with a dark bill and darker feathers but an adult black-billed gull sports a black bill and has a red eye ring, whereas a juvenile red-billed gull has a black eye ring.