This article originally appeared in Musky Hunter’s August/September 2017 issue.

By Jordan Weeks, Research Editor

Stocking and genetics are two subjects that never get old, and it seems musky anglers are always looking for more information about these issues. Scientists are, too! This article outlines research performed by scientists at the Illinois Natural History Survey.

Matthew J. Diana, Curtis P. Wagner, and David H. Wahl explored the premise that stocked muskellunge survival is a function of genetic variation and adaptation, which is likely due to spatial and temporal variation in habitats. Or, in other words, fish survive best in their native range and survive less so when stocked in non-native habitats and geographical regions.

Specifically, the objectives of the study were to determine whether: (1) relative abundance, survival, and growth differed among three stocks of muskellunge after stocking events in three Illinois impoundments; and (2) post-stocking survival and growth varied in relation to stocking date, temperature at stocking, and winter and summer severity.

These researchers used three stocks of muskellunge in this study: Upper Mississippi River drainage stock (hereafter, MRS), Ohio River drainage stock (ORS), and a mixed source that was bred from a cross between the MRS and ORS (Illinois stock, hereafter ILX) and has been propagated in Illinois hatcheries since 1993. Sources of these stocks were as follows: MRS — Leech Lake, Minnesota; ORS — Cave Run Lake, Kentucky, Clear Fork Lake, Ohio, and Lake Chautauqua, New York; ILX — North Spring Lake, Illinois.

To account for variation in latitude and climate, and to better understand spatial and temporal differences, average annual temperature, cooling degree days, and heating degree days were calculated for each stock. The definitions of these terms are not terribly important for the purposes of this article. However, understand that because MRS fingerlings originated farthest north, they had the lowest average annual temperature, fewest cooling degree days and the most heating degree days. The most southern stock (ORS) had the highest average annual temperature, most cooling degree days and the fewest heating degree days.

Since the study was to be performed in Illinois, researchers aimed to see if stocks from the region outperformed stocks from outside the region when raised and released under similar conditions. In each study year (2003, 2004, 2005, and 2007 into Mingo and Pierce Lakes, and 2005, 2007, and 2008 into Sam Dale Lake), all three stocks were introduced into the study lakes at the same time. These three lakes were stratified spatially in the state of Illinois. Pierce is near the Illinois/Wisconsin border, Mingo is located in east central Illinois, and Sam Dale Lake exists in southern Illinois.

Attempts were made to introduce each stock at similar densities and size across all lakes. When this was not possible samples were standardized. Sub-samples of each group were held in predator-free cages for 48 hours to assess post stocking mortality. Lake water temperatures were recorded hourly in all study waters, and fish were recaptured at night via boat electrofishing. Sampling occurred in March-April and October-November of each year. All recaptured fish were measured, weighed and identified according to the correct stock.

Results

Stocking date, size, and stocking density varied among stocks (contrary to the study design). Date was similar for the ORS and ILX (September 18) but due to lack of available fish, the MRS were introduced a month later (November 8). Despite this difference, average stocking size was similar (ILX 10.7 inches, ORS 9.8 inches, MRS 10.4 inches). Stocking densities also varied by population due to availability of fish (ILX 12/acre, ORS 9.6/acre, MRS 7.6/acre). Post-stocking mortality differed among stocks but not among years. ORS experienced the highest 48-hour mortality (28 percent) followed by the ILX and MRS (8 percent and 1.6 percent, respectively).

Researchers found that water temperature at stocking was significantly related to stocking mortality! All high mortality events (>7 percent) occurred before September 24, with water temperatures over 69.8 degrees Fahrenheit. Catch rates for all stocks were generally low and did not vary among stocks when corrections were made to adjust for differences in stocking rate.

Survival among stocks varied. Average overwinter survival the first winter for all three stocks was 40.7 percent. Survival differed slightly the following summer (S1), with the ILX surviving best, followed by the ORS and the MRS. Survival over the second winter (W2) did not differ among stocks. Total length (TL) varied among stocks. After the first winter ILX were similar in total length to ORS but were significantly larger than MRS (but the difference between MRS and ORS was not significant).

The average total length of fish in the first fall was not correlated to catch per unit effort (CPUE) but was related to overwinter survival. By the second fall, average total length of the three stocks did not differ. However, by the second spring, ILX and ORS were both significantly larger than MRS. Summer temperature severity did not vary among lakes despite latitudinal differences, but daily average temperature did vary north to south (coolest north to warmest south).

Summertime survival was lowest for MRS but was not related to daily average temperature or summer temperature severity. ORS summer survival did not vary and was also not related to daily average temperature or summer temperature severity. ILX experienced the highest and most variable summertime survival but was also not affected by daily average temperature or summer temperature severity. Researchers found that “stock” was the only significant variable related to survival.

Discussion

Overall, researchers concluded there were significant differences in stock survival one-year post stocking. In this study, the MRS stock exhibited poor survival over the summer compared to the ORS and ILX. This survival was reflected in catch rates during that time period. They also suggest that differences between stocks can also result in divergent growth. Greatest mortality across all stocks occurred in the summer and the first year of life for all stocks. However the mechanism is unclear.

Researchers found that stocking muskellunge later in the fall, at the largest size possible, and at cooler water temperatures increased survival. I have written about this in the past. It is something that should be considered whenever muskellunge are stocked.

Muskellunge stocks in this study experienced significant growth differences. “In general, the MRS fish were consistently smaller than the ORS and ILX (except during the second fall post stocking); however, these growth differences were only significant during the second spring post stocking when catch rates of MRS fish were low,” researchers wrote. They go on to say “the small size of the few MRS fish we observed provided evidence that they were not performing well in Illinois waters. Other than the first spring post stocking, ILX and ORS muskellunge were similar in size throughout the study.”

Diana, Wagner and Wahl also noted that growth rate differences vary greatly on a latitudinal gradient and may be a component of local adaptation. This may explain why the MRS stock general exhibited poorer growth and survival compared to more naturalized stocks. These effects may not be observed in fish stocked in a more northerly climate. Overall, these researchers provide evidence that fish which are locally adapted to similar climates may outperform those from different latitudes regardless of the survival and growth potential in their native range.

Reference: Matthew J. Diana, Curtis P. Wagner & David H. Wahl (2017) Differences in Stocking Success among Geographically Distinct Stocks of Juvenile Muskellunge in Illinois Lakes, North American Journal of Fisheries Management, 37:3, 633-643