Reference article
Fishing Net (Duck Life 9: The Flock)
A handy net for catching Runts
Obtaining
Talk to 
Benson the Bear at 
Fishing Beach. He gives you the net when introducing fishing. Reach the beach through the cave route cleared after beating 
Igor.
Using it
Approach a bubbling fishing spot near the shore and press the action control as a fish jumps out to catch a 
Runt. Trade a Runt to Benson for a 
Fishing Rod to fish farther from the shore.