Many streams in Pennsylvania have fewer fish species swimming in them than they did 20 years ago, and it may be due to acid rain.

Robin Heard (graduate-environmental pollution control) studied the diversity of fish species in 50 Pennsylvania streams and compared her results to a historical survey taken of the same streams between 1961 and 1971.

As in the historical survey, Heard measured levels of pH, alkalinity and dissolved ions -- or specific conductants -- in the water. In addition, she measured the total dissolved aluminum in the water. This was added to the study because recent research indicates that as pH levels decrease, or the acid level of the water increases, the amount of aluminum present also increases.

During certain stages of life, fish are sensitive to these conditions, she said, and depending on how long the conditions last, the fish may die.

Through a process called electrofishing, Heard and her adviser used a backpack generator to momentarily stun the fish in a section of each stream. Once stunned, the fish floated belly-up at the top of the stream, where Heard picked them up with a net and placed them into a bucket. After she recorded the species and size of each fish, she returned it to the stream.

Heard found that 38 of the 50 streams lost fish species since the survey was taken. These 38 streams also had a significant decline in pH, alkalinity and specific conductant levels, she said.

"We believe that acid rain is what is causing the decline in fish diversity," Heard said.

In order to better correlate fish losses with acidification, Heard plans to retest 30 of the streams during a period of higher water flow. After a storm, more acid rain is present in the water, causing a lower pH and an increase in aluminum, she said.