See if you can find out what the biologist intends to cover so you're not overlapping the topics too much.

Keep at the forefront of your mind that your audience is very young, around 9 i think, so talking a lot will bore the bujeezus out of them quickly.

Have a large poster display of the stages a fish goes through and why these fish can get so big. Point out the differences in sea running fish and fish that spend their life in fresh water only.

Kids might get a kick out of the idea that the steelhead may be a world traveler. You could have props associated with other places so the kids could get a perspective on how far a fish can travel in their life.

Ask for 5 voluteers - put them up at the front of the class and have them pretend that they are steelhead. You be the angler and have a rod with a dummy lure. Toss the dummy lure and have one of the kids run off with it - not far away though , then reel them back in. Keep the fish. Do it again, keep the fish. Do it again, keep the fish. Then show them how many of them are left. Talk to them about catch and release and the affect this practice could have on their future fishing opportunities.

Pass out small bags of m&m's to each child, and have the children open the bag, and look at all the m&m's they have. Then ask them to take 10 m&m's from the bag - explain that out of all the eggs a fish lays not all of them will hatch. Then ask them to put 5 m&m's back in the bag - explain to them that out of the eggs that hatched 5 of them died from being eaten by other fish, etc. Tell them that the five fish they have now go to sea and grow to be large adults. Then have them put 3 back in the bag and explain to them that other mammals and commercial fishing have removed them from the sea. Now explain that out of that whole bag of m&m eggs that they have in their hand only two of them came back to the river to river to start the process over again. You don't have to be accurate as to the percentages but this type of illustration will have more impact than simply explaining it to them.

However you approach this keep in mind that a childs mind is like a sponge. If they are excited about the material you are presenting they will learn and retain the experience. Children can be one of the best teaching tools to adults too. What a child learns from your instruction may have a benefit from this point on if the child takes what he learns and passes it on to his mom or dad when they're out fishing.