An environmental group on Friday urged the government to provide a timetable for taking back private wetlands and fish ponds under its Northern Metropolis development plan, warning that uncertainty over the process has far-reaching implications on the fish farming industry and ecology there.

The outgoing chief executive, Carrie Lam, announced in her policy address last year that authorities would resume hundreds of hectares of private wetlands and fish ponds to create three wetland conservation parks under an ambitious plan to develop the northwest New Territories.

The Hong Kong Bird Watching Society said it interviewed 63 operators in Deep Bay between February and March, with the majority of them being fish pond tenants.

While more than half of the respondents backed the government's blueprint, a number of fish pond operators renting private or ancestral land have complained about significant rent hikes, or had been refused by landowners to renew their tenancy.

"When the government announced the Northern Metropolis [plan], there are a lot of expectations on the land issue...Some of the landowners would like to take back their own land, or even increase the rent so that they can get more income, or they might expect the land near the Deep Bay area to be developed or to be resumed at a high price," said deputy director Woo Ming-chuan.

Woo urged the government to come up with a clear roadmap as soon as possible, saying the ecological value of the area would deteriorate if fish ponds were left idle for a prolonged period of time.

"We [want to] ask the government if there's any other measure that can be implemented, so that the fish ponds’ ecological value and the conservation value can be maintained from now to the moment when the land is resumed," she said.

"We hope that there can be ecological elements, [which] can be one of the considerations during the land resumption, so that the [compensation] can be higher. If some ecologically-friendly devices or some ecologically-friendly structures were present in the fish ponds, then they could be resumed in a higher rate."

Woo said installing regular drainage to create foraging grounds for birds and managing ponds without birds traps, for example, can help preserve the ecological value in the area during the transition period.