Dr McSweeney said the oldest of the eels were between 50 and 80 years old, which made them the oldest living inhabitants of the Moeraki River valley.
Fishermen were supposed to leave behind any eels bigger than 4kg, but Dr McSweeney said nobody was policing the take.
Eighty tonnes a year could be taken on the West Coast, including 16 tonnes for cultural purposes and 1.6 tonnes for recreational fishermen.
He said the law appeared to stipulate the rest had to be for human consumption, but there was speculation that some ended up in pet food.
Dr McSweeney said there was currently no way of protecting the eels.
They could be legally fished in Kahurangi National Park, where it was considered a traditional activity, but were protected in the Arthur's Pass and Westland national parks.
However, that meant they were open game in most West Coast rivers.
"They are the biggest eels in the world, you could almost call them river monsters."
Most eels in New Zealand are short-finned, with the rarer long-finned eels making up only about 20 per cent of the total catch.
Radio NZ reported this morning that researchers for the Manaaki Tuna group have been looking into who is buying the eels.
Addiction Foods, of Te Puke, exports a range of gourmet pet foods, including eel, which it markets in the United States and elsewhere as 'earth-friendly and hypo-allergenic'.
The Ministry for Primary Industries said in a statement today that catch limits were reviewed periodically, and in 2007 the total commercial catch limit for North Island long-fin eel stocks was cut from 193 tonnes to 81 tonnes.
Several research projects were under way to monitor eels stocks.
As commercial fishing in national parks and reserves was generally prohibited, about 33 per cent of New Zealand was within protected areas, the ministry said.
Eels had faced major changes to their environment including hydro-electric dams and turbines, changes in water quality, continuing drain clearance, and flood plain drainage. But freshwater eels were a commercially fished species, and there were no restrictions on their commercial catch.
As a premium, high-value species it would be uneconomic for whole eels to be used as a key ingredient for uses such as pet food, the ministry said.
Dr McSweeney said if long-finned eels were protected, commercial eel fishermen could still catch the more widespread short-finned eels.
- The Greymouth Star
