I've used several methods including stringer, cooler in the back hatch, fish bag in the front hatch. The stringers are easy but create drag when paddling if you don't pull it back into the kayak. Also, depending on where you are fishing, turtles and snakes (alligators too) will come try to snatch your catch. I've seen some videos of people using a clothes basket with pool noodles attached so it floats. You can then velcro or zip tie some mesh over the top to keep them from jumping out.