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Posts: 236
| I'd first like to thank you Dave for taking the time to answer our questions. I just have some questions about your quote.
"Dr. Sloss, Dr. Moyer (the other instructor from Oregon State University) and every other fish geneticist in the country strongly advocate that the first priority of selecting wild broodstock should be to maximize genetic diversity. That means taking good numbers of many sizes of fish from many lakes over a period of time that spans the spawning period. (This is what the Wisconsin DNR is gearing up to do next spring, at Dr. Sloss' recommendation.) Why the focus on genetic diversity rather than simply size? If we use only the biggest fish available, it's possible that those individuals could lack an allele (one form of a gene) that might allow some individuals in the population to adapt to future changes in environmental conditions such as climate, habitat, or newly introduced disease organisms. There was a large die-off (disease mechanism unknown) of muskellunge last year in the Thousand Islands Region of the Upper St. Lawrence River. Things like this happen. We want our musky populations to have the greatest possible chance to adapt and thrive under changing conditions, so genetic diversity must take priority over domestication (managing for one or more traits like size that we find to be immediately gratifying). "
1. Why are you assuming that a gene(or genes) for large growth will make an individual more likely to receive a certain disease? Is thier any evidence in other fisheries of only big fish getting a disease?
I know how certain human races have higher rates of diseases, but i've never seen where individuals within that race (Such as larger Men) are more likley to recieve the disease.
2. Have you seen how southern fisheries biologist only use large females bass (15lbs or bigger) for getting thier eggs? And if you have seen it, what is your opinion on this? and do you think they are risking those fish having poor fitness?
3. Do you feel larger females have higher rates of fecundity? The malo fish was reported of having 850,000 eggs, I've seen data suggesting the average female only carries 250,00. Have you seen any empiracle evidence suggesting that larger females carry more eggs? If you are looking for the highest rates of fecundity would it make sense to want large females keeping all other variable equal?
Once again Thank you for your time and knowledge. I give you very much respect for even entering this war zone.
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Location: Rhinelander, Wisconsin | Wouldn't the answer to the first question be somewhat the same if asked about medium sized fish? I interpret the statement quoted to mean overall diversity, all size fish, if you will. Selecting for a certain trait (in this case size) creates the pitfalls mentioned. Am I on the right track here? | |
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Posts: 178
| MuskieBum, I am pleased to try to answer thoughtful questions such as yours. Here are your questions and my responses:
MB: 1. Why are you assuming that a gene(or genes) for large growth will make an individual more likely to receive a certain disease? Is thier any evidence in other fisheries of only big fish getting a disease? I know how certain human races have higher rates of diseases, but i've never seen where individuals within that race (Such as larger Men) are more likley to recieve the disease.
DAVE: Actually MB, I am not making that assumption. I need to elaborate a bit on this concept. There is no current evidence for a linkage between fast growth rate (or large ultimate size) and disease susceptibility. At present, we don't even suspect one. If anything, one might assume individual fish that have attained a large size have immune systems that are at least as good, if not better, than many of their cohorts (other members of their year class). But that is beside the point. In any given fish population (all individuals of a species residing within a particular lake or river system), there is genetic variability. Alleles (forms of a particular gene) can be prevalent, common, or rare in that population. The frequency with which a particular allele appears among members of that fish population is a function of both random processes (pure coincidence) and selective pressures (conditions that favor the survival and reproduction of individual fish possessing the allele). It is those RARE ALLELES that interest geneticists (and should interest all of us), because they may code for the production of currently uncommon proteins or enzymes that will one day increase the ability of the population to adapt to changes in their overall environment, including changes in climate, spawning habitat quality, fish community composition, and the development of new or variant forms of diseases associated with bacteria, viruses, fungi and parasites. Geneticists know that such rare alleles are not to be found ONLY among the biggest or fastest-growing members of the fish population. In fact, they know that restricting the hatchery egg harvest to JUST those big fish (or fish with any other specific, desirable trait) significantly reduces the probability that a potentially useful rare allele will get passed along to future generations for use when needed. THAT is the reason why conscientious fishery managers nationwide are beginning to incorporate the principles of genetic diversity into the selection of broodstock for hatchery propagation purposes. This really is an oversimplification of the issue, but I am not an expert in genetics, and so you're getting only the basic concept here. If any real geneticists read this, please feel free to correct me or elaborate further if this is not clear.
MB: 2. Have you seen how southern fisheries biologist only use large females bass (15lbs or bigger) for getting thier eggs? And if you have seen it, what is your opinion on this? and do you think they are risking those fish having poor fitness?
DAVE: I am vaguely familiar with the way Florida-strain largemouth bass are being propagated in places like Texas and California. I really should not comment on this program because I have not been involved in it. (Once we learned that Florida-strain largemouth bass were maladapted for reproductive survival in Missouri, we stopped experimenting with them there.) Do keep in mind that much of the work being done with these fish is outside their native range. In Texas and California, Florida-strain largemouth bass are an exotic strain of fish, but they have been introduced into artificial ecosystems (man-made impoundments) in which there were few or no largemouth bass native to the river system prior to impoundment. With muskellunge in northern Wisconsin, we are talking about an endemic species in their native range. The two situations are not very comparable. It would be interesting to hear the thoughts of real experts like Dr. David Phillip (Illinois) on this southern bass program.
MB: 3. Do you feel larger females have higher rates of fecundity? The malo fish was reported of having 850,000 eggs, I've seen data suggesting the average female only carries 250,00. Have you seen any empiracle evidence suggesting that larger females carry more eggs? If you are looking for the highest rates of fecundity would it make sense to want large females keeping all other variable equal?
DAVE: That depends. If your question is about "absolute individual fecundity" the answer is YES. It is well-documented in the scientific literature that the size of female fish of most species is highly correlated to the number of mature eggs carried in the ovaries. However, if your question is about "relative individual fecundity" the answer is NOT NECESSARILY. One measure of relative individual fecundity is the ratio of the weight of eggs prior to spawning to the total weight of the fish, often called the gonadal somatic index (GSI). That particular measure of individual fecundity usually is correlated with the AGE of fish in the population, not their size. There seems to be an optimal age for producing healthy, viable eggs for many species. Beyond that age, absolute individual fecundity may continue to increase as body size increases, but GSI and egg quality (freedom from genetic defects that may affect hatching viability or offspring fitness) declines. The bottom line is this: The best eggs may not come from the oldest (and often the largest) fish. They are more likely to come from healthy fish in their prime. Because we cannot determine the age of adult muskellunge accurately without killing them, it is best to collect eggs from a full range of sizes in order to ensure that some fish in their "prime" have been selected for hatchery propagation purposes. (Many of those fish will have the genetic capacity to achieve trophy size if they have not done so already.) There are MANY other reasons to collect eggs from a wide size range of females, but this is one of them.
I hope this answers your questions, MB. Happy New Year to one and all.
Dave Neuswanger
Fisheries Team Leader, Upper Chippewa Basin
Wisconsin DNR, Hayward
Edited by Dave N 1/8/2006 10:42 AM
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Posts: 236
| Thank you Dave, Happy New Year. | |
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