Tennessee Lake Management: the Truth about F1 Largemouth bass

There was a time when I stocked F-1s regularly. Most other pond management companies working in this state still stock F-1s almost exclusively. F-1s are raised by far more fish farms in the southeast than are pure Floridas, which makes them much easier to procure consistently; and my competitors are all about doing things the easy way. There’s a genetic phenomenon called outbreeding depression, whereby the offspring of hybrids of two species often display inferior traits compared to either parent species. One of the first weakened traits that typically manifests is poor growth potential. This is why many state agencies now caution pond owners against stocking hybrid bluegill, or warn that a pond stocked with them will have to be totally renovated, i.e. poisoned and re-stocked, every four to six years. If you’ve ever fished a pond that had had hybrid bluegill for several years and you caught bluegill that averaged three inches long, this is why. Some biologists claim that Florida- and northern-strain largemouth are only two separate subspecies of the same species; but there are also those who claim they are two entirely separate species. Regardless of which is the case, there’s enough difference between the two genetically that anytime they cross, there is the potential for outbreeding depression in their offspring, just as with hybrid bluegill, hybrid crappie, or any other hybrid. One of the saddest large lakes I have ever worked on is a sixty-acre lake near Chattanooga. According to a resident on the lake, large bass were common the first few years after the lake was stocked. Neither that resident nor any of the other residents who fish could figure out why the bass size had dropped so precipitously; as I kept asking questions, I found the probable answer, before we ever saw the lake in person. It was stocked with F-1s, exclusively, from the start. When we electrofished it in October 2017, the largest bass we captured weighed 1.21 pounds. The bluegill-to-bass ratio was 4:1, which is not great, but better than the 2.18:1 ratio we found in a 150-acre lake we shocked that same month that had only pure-strain northern largemouth and yielded five bass over 18” in our survey including one that weighed just under five pounds. The difference? Those F-1 bass in that sixty-acre lake have been breeding with one another since the lake was stocked in the early 2000’s. We electrofished a two-acre pond in west Tennessee in June 2018; the pond had been stocked several years prior with F1 largemouth. The largest bass we found in our survey? Ten inches. I am not a genetic specialist; I couldn’t tell you all the ins and outs of outbreeding. I know from the fish genetics class I took that hybrids (whether fish or any other animal) are subject to a wide array of problems at the gene level, including aneuploidy which is when the fish has too few or too many chromosomes. We regularly stock pure Floridas into lakes that only have pure northern-strain largemouth, and those Floridas are almost guaranteed to interbreed with the northerns. The difference is that the pure Floridas can still choose to breed with other pure Floridas, and when F-1s happen, some of those fish are going to breed with pure Floridas or pure northerns, which should minimize the potential for outbreeding. The best example of how well stocking pure Floridas into a population of pure northerns can work, even after many years, is Chickamauga Lake. TWRA first began stocking pure Florida largemouth into the lake on a yearly basis in 2001; in March 2015, the state record of fourteen pounds, eight ounces that had stood since 1954 was bested by a fifteen-pound, three-ounce bass from Chickamauga. Ten-pound largemouth have become common on the lake, a size that was a true rarity when it only had northern-strain largemouth. But TWRA isn’t stocking F-1s into the lake - they’re stocking pure Floridas. And they stock hundreds of thousands of new pure Floridas into the lake every year. Outbreeding depression in F-1 largemouth is by no means my discovery: I’m just passing on to my clients the latest in scientific knowledge on largemouth bass. Floridas are much harder to source, which is why we’re now raising our own stock. If the company you’re talking with is pushing you to stock F1 largemouth, ask them about the negatives that come with these fish. If they tell you there are no negatives, you’re dealing with a company that cares more about their wallet than your lake.