If you want to do your best to protect our salmon or other marine life while driving pile, why not choose a vibratory hammer to drive that next underwater pile? With such an emphasis on the environment these days, a recent definitive study commissioned last year by Caltrans on the issue of the effects of pile driving on marine life lists vibrating pile as the best pile driving method to protect Sea Life. In section 2.5 titled: Common Underwater Sound Reduction Measures, vibration of pile is listed as a preferred method to reduce sound in underwater pile driving compared to other pile driving methods. Evidence suggests it is the safest method for wildlife and the only method not to reach the dangerous 183db level. Attached is the entire report which is a must read for all you contractors driving underwater pile. There is also a ton of material about how to best handle underwater noise issues when driving any kind of pile with most types of hammers.
Link to the Hydroacoustic Effects of Pile Driving on Fish: http://www.apevibro.com/pdfs/Guidance_Manual_Sound_and_Underwater_Piledriving.pdf