Both are species of shad... skipjacks are technically known as river herring. They can be anywhere from a few inches up to 2 or 3 lbs. They fight like demons, jump sky high... fished live they make great rockfish bait. Threadfins are just that... threadfin shad, also known as yellowtails. Grow to 7 or 8 inches... usually smaller. Prolific breeders and are THE primary baitfish in local waters. They're ganged up below area dams, and certain spots on the lakes, spawning right now. Very soon will start dispersing back into open water where they stay most of the time. Threadfins cannot tolerate cold water.... anything below 48 degrees and they start stressing out... below 44 they started dying. That's why you'll have huge numbers (like we have now) following mild winters. If we have a real cold winter it will really knock 'em back. However some say that spring fishing is better following a cold winter because the threadfin die-off. Therefore game fish are more desperate and susceptible to anglers. Just a theory, but I know several folks who subscribe to it.