Traders betting against pot stocks made almost $1 billion in 2019. Here are the 8 stocks most responsible for those gains.

• Short-sellers betting against the cannabis sector have made $993 million in mark-to-market gains year-to-date, according to a Wednesday note from financial-analytics provider S3 Partners.
• Those gains come even after solid sector performance erased $132 million from short-seller profit in December on the 20 stocks that were most shorted, according to the report.
• This year, traders have increased short positions on the most popular cannabis companies as stocks have struggled to perform, S3 said.
• While there is some evidence of short covering, or traders exiting short positions, the sector has "surprisingly been immune to short squeezes," wrote Ihor Dusaniwsky of S3 Partners.
• Read more on Business Insider.

Betting against pot stocks has been a lucrative bet for short-sellers this year.

Traders shorting the stocks of cannabis companies have mark-to-market gains of $993 million year-to-date, according to data from financial-analytics firm S3 Partners.

Those gains come even after solid cannabis sector performance in December brought short-sellers in the 20 most-shorted stocks down $132 million on a monthly basis, according to the report.

Traders have increased their short positions in cannabis companies this year as the sector has struggled to perform. So far this year, short exposure to the sector is up $843 million, or 35%, according to Ihor Dusaniwsky, the managing director of predictive analytics at S3.

"Short selling is fairly concentrated to a handful of names, with only six stocks topping $100 million of short interest," Dusaniwsky wrote.

Still, gains across the sector in the last few days have forced some short-sellers out of their positions, as shorting cannabis companies is "an expensive proposition," according to Dusaniwsky. The high cost of financing discourages some short-sellers, as does the lack of "borrowable supply," he said.

That's meant that as many as 11.6 million shares of the most-shorted shorted cannabis stocks were covered in the last 30 days, Dusaniwsky wrote. The companies that saw the most short covering were Aphria, Canopy Growth, Aurora Cannabis, and Tilray, according to S3 data.

But short covering wasn't unanimous across the industry, Dusaniwsky wrote. In the last 30 days, traders poured $25.9 million into shorting Cronos Group, and $20.2 million into shorting GW Pharma.

Given the cost to bet against cannabis, the sector has "surprisingly been immune to short squeezes as shorts have been profitable in most of the heavily shorted names," said Dusaniwsky.

Here are the top eight stocks that contributed to short-seller gains in cannabis year-to-date, ranked from lowest to highest mark-to-market profit.