Hypothetical Stock Mixing Impact Analysis

Category: article

 Aug 4th, 2005 by OutdoorsFIRST 

Modified Aug 4th, 2005 at 12:00 AM

Hypothetical Stock Mixing Impact AnalysisDave NeuswangerFisheries Team Leader, Upper Chippewa BasinWisconsin DNRJune 2, 2005 Presentation to Hayward Lakes Chapter, Muskies, Inc.Assumptions:1,000-acre seepage lake in northwestern WisconsinJust for fun, let’s call it Mutt Lake.Stocked with 500 muskies (0.5/acre) every other yearDensity and Survival of Stocked Fish:If every stocked fish lived exactly 20 years, we would have 5,000 muskies in Mutt Lake, or 5 fish per acre. Such high density is rarely if ever observed.Typical adult density is 0.25 per acre (250 fish in 1,000-acre Mutt Lake). Therefore, at any given time, only a small proportion (<5%) of fish stocked in the last 20 years are alive as adults.When and to what degree does mortality occur?What happens if we inadvertently introduce fish with questionable genetic characteristics (i.e., "bad fish") during a particular stocking event? Stock Mixing Example:Let's say we inadvertently stock 500, 10-12" musky fingerlings with presumably undesirable genetic characteristics in one year. What happens to those fish, and what are the risks associated with using Mutt Lake as a source of hatchery broodstock in the future?Mutt Lake Mortality TableFor Year-Class of Presumed Undesirable Fish500- Year 1195 -Year 2137 -Year 396 -Year 467 -Year 547 -Year 633 -Year 723 -Year 8% Total Mortality61 (A) Y130 (B) Y230 Y330 Y430 Y530 Y630 Y730 Y8Total # Remaining195 Y1137 Y296 Y367 Y447 Y533 Y623 Y716 Y8% of Mature Adult Muskieswith "Bad" Genes0 Y10 Y20 Y30 Y40 Y518.8 Y613.2 Y79.2 Y8AHansen, D. A. 1986. Population characteristics and angler use of muskellunge in eight northern Wisconsin lakes. American Fisheries Society Special Publication 15:238-248.BLyons, J., and T. Margenau. 1986. Population dynamics of stocked adult muskellunge (Esox masquinongy) in Lac Courte Oreilles, Wisconsin, 1961-1977. Wisconsin Department of Natural Resources, Technical Bulletin 160.BChristianson, J. 1991. An evaluation of the Spirit Lake muskellunge population with emphasis on survival and actual growth. Iowa Department of Natural Resources Annual Completion Report 1991:53-62.CA rough guess, for illustrative purposes, of average age at sexual maturation for males and females combined. Some males will mature earlier, and some females will mature later. Implications of Model Scenario:In Year 6 a DNR hatchery manager may have a 1-in-5 chance of gathering milt or eggs from a "bad" fish. By Year 8, that probability is reduced to 1-in-10.Even if a "bad" fish is used, there is no assurance that its undesirable genotype will prevail and be expressed phenotypically in the hatchery-produced offspring. Conclusion:It would be best not to engage in stock mixing of this nature (when one stock has known performance problems), but the damage is likely to be minor and temporary provided there is not a continual influx of "bad genes" into the system.This is especially true if the "bad genes" in question are among those that code for proteins used to govern growth rate (probably 10-20), exposing fish with the "slow-growth genes" to a relatively higher risk of size-biased predation by adult muskellunge and other predators. But what if some natural reproduction occurs in Mutt Lake? Wouldn't the hypothetical introduction of "bad fish" screw things up?Population geneticists have determined that fish populations have an "effective population size" (Ne) that is the number of individuals in a population reproducing and contributing to the alleles present in the next generation.In most fish populations, the ratio of Ne to Nc (censused population size) is less than 20%. For Escanaba Lake walleyes one year, it was only 3%.In our hypothetical example for Mutt Lake, we had a censused population size (Nc) of 250 adult muskies. Assuming even a high Ne/Nc ratio of 20% (50 fish),we would expect the following results:Years after Stockingin "Mutt Lake" Frequency of Fishwith "Bad" Genes Number of "Bad" Fish Contributing toFuture Genome6 .188 x 50 = 9.47 .132 x 50 = 6.68 .092 x 50 = 4.6Would 9, 7, or 5 "bad fish" in any given year contribute enough bad genetic material to the existing stock in 1,000-acre Mutt Lake to cause outbreeding depression and permanently ruin the fishery? The geneticists and the math tell us this is highly unlikely, but it's also a risk we should avoid if possible. It's also something we should check as we begin to inventory and characterize existing genetic stocks.

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