I have plenty of experience of gambling losses - not much in the way of winnings!! But this is one source of income which is exempt (99% certain anyway), part of the reason is that losses would obviously have to be deductible (I think this is the case in the states, where losses are deductible from any income, ie not ring fenced to gambling income). The other reason would be that historically there was a 10% tax on betting, this could either be paid in advance or paid on your winnings, so in effect additional tax would have been a double taxation. Currently this rate has been reduced to 3% and the majority of bookies pick this up, basically if you still pay this 3% you really are'nt trying!
Anyway - best of luck and keep on fishing!