Research: Everyone’s Favorite Topic — Stocking!
By Jordan Weeks, Research Editor
Recently while scrolling through my social media newsfeed I saw a very nice promotional video from Muskies Inc. which focused on stocking. While at “bear camp” in September, my friends at Flambeau Forest Outfitters and I talked about musky stocking more than bear hunting. Two weeks ago, I drove six hours roundtrip to a meeting to talk about — you guessed it — STOCKING!
The current boom in musky fishing popularity and success is due to many factors including habitat enhancement, regulations, fisheries research and stocking. Fishing is better than it has ever been, largely because of scientific advancements, of which stocking efficiency is one. Unfortunately, too often I confront ideas surrounding this topic that are just plain wrong. In my experience, most anglers have an ultra-simplistic view of stocking that goes something like this: obtain the progeny from the hatchery, source, or strain that has the reputation for producing the largest fish and stock as many as you can.
If it were that easy and effective, we would all be looking east to the St. Lawrence River for our hatchery products.
But it’s not that easy. Even if there were fish available, would they survive the trip? I mean, the St. Lawrence is like a 24-hour drive from Wisconsin. Okay, let’s say we could overcome that hurdle effectively. Would they survive outside their native geographical area? My guess is probably not.
The reason we have different stocks/strains is due to adaptation to local environments or reproductive isolation.
Sara Andree, Joseph Parkos, Matthew Diana, Curtis Wagner, and Dr. David Wahl, researchers from the Illinois Natural History Survey, sought to determine if survival and growth of two distinct stocks varied under the same environmental conditions. This article will outline what they found and discuss some of the challenges to stocking muskies in wide geographic areas in their paper, “Evidence for Cogradient Growth and Survival Between Two Geographically Distinct Stocks of Muskellunge.”
Many studies have observed differences in survival, growth, and other life history traits of muskellunge stocks. Some studies speculate that differences in these variables are due to thermal preferences, genetics, phenotypic plasticity (changes in an organism’s behavior, morphology and physiology in response to a unique environment) and habitat. Cogradient variation is when a stock has a “positive association between genotype (genes) and environments that maximize growth, increasing phenotypic (outward appearance) divergence among thermal environments along a latitudinal gradient,” which means demonstrating the most efficient growth under native thermal conditions. Counter-gradient variation exists where “geneotype (genes) acts in opposition to environmental effects on growth, where the maximum growth rate is fastest for stocks at the highest latitudes of a species’ range where growing seasons are shorter, and winters are colder than they are for stocks at lower latitudes.”
Got it? Good.
Muskellunge are native to the Great Lakes, upper Mississippi River, Ohio and St. Lawrence rivers in the U.S. Stocks or strains have developed in these areas over time. Stocking has influenced some of these changes. Despite the propensity for interbreeding between different stocks, “genetic studies have found low contributions from stocked muskellunge, a pattern that may result in part from the poor growth and survival of translocated strains” (Miller et al. 2017).
The objectives of this study were to: compare the over-winter and 12-month growth and survival of a muskellunge stock within its native range (Ohio River Stock), to those of a stock transplanted from outside its native range (Upper Mississippi Stock). Researchers hypothesized that the native stock would show higher growth and survival than the non-native stock.
Muskellunge eggs were obtained from each respective native range. Ohio stock came from Cave Run Lake and the Mississippi stock originated in Leech Lake. Broodfish from each population were captured during the normal spring spawn. Young muskellunge were raised initially in hatcheries closest to each source population. In fall, fish were transferred to Sam Parr Biological Station, Illinois. At this time average length differed slightly (~10mm) between stocks but average weight did not differ significantly.
Three one-acre ponds were used for the experiment. All muskellunge were PIT (Passive Integrated Transponder) tagged. Individuals were measured and weighed prior to release in the ponds. Equal numbers from each stock were placed into each pond.
Survival and growth were evaluated overwinter (October to April) and summer (April to October). All ponds were inundated with fathead minnow forage. “Forage was added at higher density in summer in an attempt to create a forage-rich environment during the warmer months, when energy demands of muskellunge should be higher.” Hourly water temperatures were measured throughout the experiment.
Based on latitude, environmental conditions were more similar to the natural habitat condition for the Ohio stock. Maximum surface water temperature was 89.6 degrees for 18 days, but bottom temperature never exceeded 84 degrees, thus offering test fish some thermal refuge. Dissolved oxygen (DO) was not measured during the study. However, “literature values indicate that DO was likely no lower than 6.8 mg/L at the highest recorded temperature (Kalff 2001), so lethal effects of hypoxia were unlikely during the experiment,” researchers said.
During the first portion of the experiment (overwinter) survival was similar between stocks (77 percent). “However, pond draining the following fall reveled differences in survival over the summer months. Survival of the Ohio muskellunge averaged 73 percent and was significantly higher than that of Mississippi fish during the second half of the experiment (20 percent).” In addition, “the Mississippi stock had lower growth than the Ohio stock throughout the experiment.”
Based on these findings it appears genetic adaptation to native environments should be of the utmost importance when choosing which strain to stock in a waterbody. These researchers found that “differences in survival between the stocks of muskellunge in this experiment are consistent with a cogredient pattern of adaptation. The Mississippi stock experienced lower survival over the summer than the Ohio stock, suggesting that the Ohio fish were better adapted to the conditions at the latitude of the experiment.” This research also suggests that stock may be more important than size at stocking in some situations.
The next time your musky club is looking to stock, be sure to consider the latitude of origin as it relates to the receiving water of the strain you intend to stock. Maximize the bang for your buck!
Andree, S. R. et al. 2018. Evidence of Cogradient Growth and Survival Between Two Geographically Distinct Stocks of Muskellunge. NAJFM 38:922-929
Kalff, J. 2001. Limnology. Prentice-Hall, Upper Saddle River, New Jersey
Miller, L. M. et al. 2017. A Review of Muskellunge Population Genetics: Implications for Management and Future Research. Pages 385-414 in Muskellunge Management: Fifty years of cooperation among anglers, scientists, and fisheries biologists. American Fisheries Society, Symposium 85, Bethesda Maryland.
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