The hope for a tax write-off is that write-offs are taken from your profits. I've gone through this for thirty years as a journalist. Others are correct that you must maintain detailed records and file a separate form each year. I send in quarterly payments on estimated supplimental earnings (as do others).

A large issue on guiding as a hobby is that it tends to dilute the customer-pool for guides who are full-time professionals at a time in the sport when each guide's customer base is increasingly critical to he or she successfully scrapping out a living.

It appears to me that we are living in a time when fishing guides would be well served to consider forming a Washington State Guide Association with requirements that must be met in order to obtain a guide license (First-Aid, CPR, boating expertise, proof of insurance, etc.). At this time anyone can send in $190 (or so) and become a licensed fishing guide in the state of Washington. Then if they work full or part-time the customer will know that he/she is getting a quality ride.
Finally, becoming a part-time guide is a poor way to pay for your new drift boat -- because you don't get to fish, which is the reason you rifled your kid's college fund to buy the thing in the first place.

Les Johnson