The UN's Food and Agriculture Organisation (FAO) said donor agencies have so far delivered or promised over 10,000 small canoes to tsunami-hit fishing villages, well over the nearly 7,000 small boats the government estimates were lost.
"Relief agencies are right in concentrating on the livelihoods of the poorest fishermen but this has to be done in a responsible manner," Pierre Gence, FAO's representative in Sri Lanka, said in a statement.
"An over-supply of modern fibreglass versions of the traditional "oru" one-man canoes will further deplete near-shore fish stocks," the FAO statement added.
Most of Sri Lanka's trawler fleet was also destroyed by December's tsunami, which killed around 40,000 people along Sri Lanka's southern, eastern and northern coastline.
"If you have too many boats in these waters, then already marginal incomes will go down even more," Sri Lankan Fisheries Biologist Lesley Joseph told the FAO. "Increased competition from an over-supply of canoes means that things could get even worse."