Personally I work with my dad doing construction - not exactly possible for most people.
The work you'd be doing to make any money off of selling fish (if you're just going for money) could be much better spent, profit-wise
Ask your neighbors and family members if they have any odd-jobs they need done. If you have an elderly neighbor they would probably have regular tasks that you could do, especially in the winter and fall.
In the states, I know that the towns (at least near me) offer summer jobs for 15 yr olds and younger, so if you'd like a steady summer job, inquire at your town hall or ask at your school if they know anything about a program like that.
Wash cars, walk dogs, rake lawns, feed pets while they go away, carry in groceries.... there are a whole lot of little jobs that people are willing to pay for.
You could even ask around for soda cans and cash them in.
Just a warning, though, don't get your hopes up - some people are real cheapskates
