I think your teacher is going to expect an experiment. The paintball idea has some potential. As far as fishing related ... some ideas....
If you can get your hands on a seine net, compare the amount of bait in two different areas (ex. a beach vs a marsh, or the outlet of an estuary or marsh compared to the marsh itself). Or in a particular spot at different tides.
Compare the populations of worms in the sediment in a sandy beach vs a mud flat. Or clams... There are lots of cool creatures in the sediment and you could learn alot while still keeping the project relatively simple.
Compare the populations of crabs in a rocky area to a sandy area.
You get the idea... if you need anything in particular, feel free to pm me. 8th grade science fair is basically what I do for a living
