Pond Boss Forums STOCKING A NEW POND Types of fish to choose F1 LMB and Outbreeding Depression

Dr. Neal,

I am glad to have seen your presentation at the PB conference. As a recognized producer of the now infamous F1 largemouth I am somewhat concerned about the implications of outbreeding depression.

I have a few questions for you. If you have the time to respond that would be very much appreciated.

About how many years or generations does it take for significant depression to occur with F1s?

How does the mixing of pure strains (50F:50N or 70F:30N or 33F:33N:33F1s) as lmb are stocked initially influence genetic depression down the road?

How do you recommend that growers in my position deal with ponds/lakes that have already been stocked with F1s only or mixed pure strains in the future?

Pure northern lmb are available almost year-round due to the fact that they are easily feed-trained. This makes larger fingerlings available for good survival rates. However, as you probably know, pure florida bass are difficult to feed-train and so they are only available in May-July time frame at a small size. Given this practical consideration, what is your recommendation for stocking existing lmb ponds with more bass fingerlings for genetic enhancement/inbreeding relief?

Thanks again for your presentation. For me this information may have more impact on the industry than any other presentation at the conference.

Todd:

I have modified the following description of outbreeding depression from the Wikipedia description:

�Outbreeding depression refers to cases when offspring from crosses between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population. This phenomenon can occur in two ways. First, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. Gene flow between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population.

A second way outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different populations. Within local, isolated populations, alleles are selected for their positive, overall effects on the local genetic background. Due to non-additive gene action, the same genes may have rather different average effects in different genetic backgrounds--hence, the potential evolution of locally co-adapted gene complexes. In other words, individuals from Population A will tend to have genes selected for the quality of combining well with gene combinations common in Population A. However, genes found in Population A will not have been selected for the quality of crossing well with genes common in Population B.

However, it is critical to understand that reduced inbreeding depression in first generation hybrids can, in some circumstances, be strong enough to more than make up for outbreeding depression. Because of this and because of the uniformity and predictable outcome of a first generation hybrid (F1 hybrid) farmers keep purebred strains for the purpose of outcrossing. Crossing the hybrids will give unpredictable outcomes and outbreeding depression will remain or worsen so that is not common practice.

As a general rule of thumb, hybrid vigor (another way of saying a reduction of inbreeding depression) is strongest in first generation hybrids and gets weaker over time. In contrast, outbreeding depression can be relatively weak in the first generation. But outside the context of ruthless selective pressure, outbreeding depression will increase in power through the further generations as co-adapted gene complexes are broken apart without the forging of new co-adapted gene complexes to take their place.

It is important to keep in mind that these two mechanisms of outbreeding depression can be operating at the same time. However, determining which mechanism is more important in a particular population is very difficult.�

So to answer your question � no one really knows! I relate the bass situation to the hybrid bream. The first generation is great, but the F2, F3, and subsequent generations are much less desirable. I don�t think hybrid bass will experience this deterioration to the same extremes or as rapidly, because the parent fish are much more closely related than bluegill and green sunfish. In fact, many would argue that we have precious few pure strain populations left, and many suspect that the world record Georgia bass was an intergrade between the two bass strains.

I tell people that if they already have hybrid bass, don�t worry about it. Manage for food and age and you�ll be fine. However, will selecting a fish to stock, I think the safest bet is to stock a pure strain.

It is very difficult to make predictions about different stocking ratios of pure fish, since bass don�t necessarily spawn randomly. However, if you assume random spawning, a gross approximation of genetic composition (percent of population) following stocking at a 50:50 ratio would be:

Gen 1: 25%F, 25%N, 50%F1
Gen 2: 6.25%F, 6.25%N, 12.5%F1, 75%Fx
Gen 3: .39%F, .39%N, .78%F1, 98%Fx
Gen 4: 0%F, 0%N, 0%F1, 100%Fx

Hence, by the third generation the reproductive output is primarily Fx hybrids. The stocked fish and the first two generations would provide superior growth for probably 10-12 years or so, but after that you should see the effects of outbreeding depression.