I've been getting this question a lot lately so I thought it might be helpful to post the info here.
The below list can be found at http://swr.nmfs.noaa.gov/deter/Dets_Fisher.htm
Potential Deterrence Methods for Pacific Harbor Seals and California Sea Lions
The following list of "potential methods" and "deterrents to avoid" is not an exhaustive list of non-lethal methods or techniques. If you have questions about protecting your property and/or fishing gear and catch from nuisance Pacific harbor seals and California sea lions, please contact Marine Mammal Specialists (562-980-3232) or NOAA's Office for Law Enforcement (1-800-853-1964).
Potential methods for use by fishers to deter Pacific harbor seals and California sea lions from damaging gear or catch (anglers must be actively fishing with gear deployed).
Visual Repellents/Noise Makers:
boat hazing, circling pounding on hull
pyrotechnics (e.g., bird screamers, bangers, underwater firecrackers, cracker shells)
starter pistols*
horns, bells, whistles
Physical Contact:
sling shots
paint ball guns* (paint balls must be non-toxic and water soluble)
non-lethal ammunition (e.g., rubber bullets, sabot rounds, game stingers)
*The equipment used in these methods must be easily identifiable, at a distance, as a non-lethal weapon. Use of automatic discharge when using these methods is NOT recommended, due to increased risk of injury to the public and marine mammals.
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Methods to Avoid-the following methods and techniques have an increased likelihood to cause injury or mortality and should be avoided.
No Firearms with "live" lethal ammunition
No Devices with Injurious Projectiles (e.g., archery gear, crossbows, spear guns, bangsticks)
No Sharp/Pointed Objects (harpoons, spears, gaffs, nail studded bats/poles/clubs)
No Entangling Devices (e.g., loose webbing, snares, concertina wire)
No Aggressive Tactile Methods (e.g., striking animals with bats, hammers, etc., impact with vehicles or boats)
No Tainted Baits or Poisons
My personal favorites: Slingshots and Roman candles. I shop Tulalip's "Boom City" for the largest candles, "whistler" bottle rockets and I always have 'em handy!