This aerial photo of the Southwest River taken June 19 shows a distinct green tinge. (Submitted by Katie McInnis)

Rivers stinking with rotting vegetation are not unusual on P.E.I., but provincial officials are getting worried that the problems are starting earlier in the year.

Provincial biologist Cindy Crane says a longer season and longer anoxic events will be an issue for aquatic life. (CBC)

The problem goes beyond a bad smell. It starts with excessive nutrients in the water, usually from agricultural fertilizers. This causes a bloom of aquatic vegetation, which then dies. As it rots, it not only smells, but it also sucks oxygen from the water, a condition known as anoxia.

The lack of oxygen can be deadly to fish and shellfish.

The 11 reports of anoxic rivers this year are not unusual, but what is worrying is the timing of the first event. The Southwest River in New London went anoxic on June 18, almost a month earlier than usual, and the event lasted longer.

"As the anoxic issues tend to worsen in the province, we're tending to see them happening earlier and earlier, so this could be part of that trend," provincial biologist Cindy Crane told CBC News Tuesday.

Crane doesn't know why the Southwest River became anoxic so early, but she wonders if heavy rains washed nitrate-laden silt into the river, encouraging more algae to grow. She worries this longer season could mean some rivers may have more than one anoxic event a year.

"If we're having longer periods, and for longer times, it's certainly going to be an issue with aquatic life," she said.

Some dead fish have been found in the Mill River, where the entire system went anoxic this summer.

Crane said that's the only system where dead fish have been found, likely because in other rivers, fish have been able to swim to places where there still is oxygen.

But she noted that's something shellfish can't do, which has been a serious issue for shellfish farmers in the past.