A conservation charity said it's concerned by what it calls a "growing trend" of wild fish killed by the salmon farming industry on British Columbia's coast.

Stan Proboszcz, science advisor with Watershed Watch Salmon Society, said nine times as many wild fish were reported inside open-net pen farms in 2017 compared with 2011.

"We saw quite a staggering increase in incidental catch over the years," he said. "Something's wrong here if we're seeing this large increase in wild fish being killed inside salmon farms."

The society crunched the number of "incidental catches" self-reported by industry to government during harvests, fish transfers and farm relocations.

But Proboszcz said the available data is incomplete and there needs to be more transparency about the apparent increase.

The farms raise Atlantic salmon in netted areas of the Pacific Ocean that allow a fresh flow of water and other sea life to enter.

Shawn Hall, a spokesman for the B.C. Salmon Farmers Association, said the industry is working to reduce its incidental catch or "bycatch" numbers, but they remain relatively low compared with the bycatch of commercial wild fisheries.

A 2017 report by Oceana Canada found on average, only about half of what is caught by commercial wild fisheries certified by the Marine Stewardship Council is the target species. The figures in the Watershed Watch report show incidental catches on salmon farms represented about 0.5 per cent of the total fish killed in 2017.

"Typically speaking our bycatch is less than one per cent of the total," Hall said. "That isn't to say there isn't more to do — there is. We should continue to keep that number as low as possible and keep driving it down through new processes, technologies, innovation, research into how to take that ever lower."

The growth in wild fish caught on salmon farms could have to do with the growth of Pacific herring populations, he said.

The report found herring to be the most common wild fish found on the farms, representing 70 per cent in 2017, followed by sablefish and cod.

The Department of Fisheries and Oceans did not respond to questions Tuesday about whether and how it keeps track of wild fish in salmon farms, or if it audits the numbers submitted by industry.

But it said in a statement that it is working to improve how it regulates the industry to meet global demand for seafood products while protecting wild fish populations.

The department said it has a study underway on alternative technologies for aquaculture, including closed containment pens that would separate farmed stocks from wild ones. It's also developing a framework for aquaculture risk management based on the precautionary approach and creating a single comprehensive set of regulations to bring clarity to industry, stakeholders and the public, the department said.

Proboszcz said the numbers reported still don't tell the whole story because the reports are only required at specific times — during a harvest, fish transfer or when a farm is moved. The society estimates that about 13.2 million wild fish may be held in B.C.'s 65 salmon farms at any given time, and an additional 653 tonnes of wild fish may be hanging around outside the farms because they're attracted by things like food and lights.

The presence of wild fish on salmon farms is concerning for several reasons, beyond the fact that they may be killed, which could affect their populations, he said.

"The farms are known to be amplifiers of pathogens, parasites and viruses. Are these things being spread to wild fish?"