Agencies commonly stock Muskellunge Esox masquinongy to maintain or augment populations throughout much of North America. Muskellunge poststocking survival is variable, and adult populations are vulnerable to periodic lapses in recruitment. Radiotelemetry evaluated whether changes in stocking techniques caused recruitment declines of spring-stocked yearling Muskellunge in Spirit Lake, Iowa, or whether total length, condition, or 10-d dispersal influenced poststocking survival. The workers fitted hatchery-reared Muskellunge (38–40 fish; n = 78 fish in total) with radio transmitters and stocked in May 2016–2017 by using one of two techniques: 1) fish stocked immediately following 6-h transport in a hatchery distribution truck with Edwards treatment–laden water or 2) fish provided 36-h recovery period after a hauling event and then stocked. Radiotelemetry revealed that stocking techniques did not influence Muskellunge survival or dispersal, but survival significantly improved for fish stocked at larger sizes. Using this information, production techniques in 2018–2019 included an additional 33–40-d rearing period for Muskellunge less than 330 mm when measured for total length in mid-May. We radiotagged a subset of these fish (14–16 fish/y; n = 30 fish total), and survival improved to more than 73.4%. Subsequent application of telemetry was via a side-by-side comparison in 2020 for greater than or equal to 330-mm Muskellunge fitted with radio transmitters and stocked in May (n = 17 fish) and those reared using the grow-out procedure and stocked in June (n = 21 fish). Both cohorts experienced poor survival (39.2–56.1%), resulting from both fish and great blue heron Ardea herodias predation, despite large differences in dispersal. Regardless of stocking technique, this study identified that most mortality occurred within the first 25 d poststocking. These results indicate that Muskellunge populations in natural lakes would benefit from a better understanding of poststocking survival and stocking practices that minimize predator threats during this critical period.

Nearly half of North American Muskellunge Esox masquinongy populations are a result of agency introductions, and many require hatchery supplementation (Kerr 2011). Consequently, Muskellunge stocking programs have evolved substantially as culture methods improved and information regarding stocked fish survival and their contribution to the fishery became available to managers. In general, these improvements led to agencies stocking large fingerlings and yearling fish reared primarily on live forage to maximize the contribution of stocked Muskellunge (Margenau 1992; Szendrey and Wahl 1996; Larsheid et al. 1999; McKeown et al. 1999). In addition, numerous Muskellunge stocking evaluations have provided insight on potential system-specific ecological interactions that may be influencing poststocking survival (Wahl and Stein 1989; Hanson and Margenau 1992; Wahl 1999; Warren 2013; Wagner et al. 2015; Weber and Weber 2020). Yet, inconsistencies in Muskellunge poststocking survival remain one of the major concerns of those managing Muskellunge populations today (Owensby et al. 2017; Weber and Weber 2020). Thus, continued research on the mechanisms regulating cohort success of stocked Muskellunge is the focus of some agencies, especially in systems where adult Muskellunge are susceptible to reduced annual survival rates resulting from traditional angling-related mortality and less conventional sources of loss such as emigration (Meerbeek and Weber 2020) or brood-stock collection (Meerbeek and Weber 2019).

Telemetry is a valuable tool for managers interested in obtaining immediate results that can help guide management decisions to improve poststocking survival rates (Brownscombe et al. 2019). This strategy is especially important for long-lived fishes such as Muskellunge that might require several years to recruit to the adult fishery. Surprisingly, poststocking survival and dispersal of Muskellunge in natural lakes is the focus of few studies. One such study in two northern Wisconsin natural lakes found that most fall-stocked radiotagged Muskellunge fingerlings (307–333 mm in total length [TL]) dispersed from the stocking location within 14 d and had variable survival over 34 d (43–85%; Hanson and Margenau 1992). Other radiotelemetry evaluations examining age-0 or age-1 Muskellunge poststocking movement and survival are from southern U.S. rivers and midwestern reservoirs, but these studies documented poor survival despite high poststocking dispersal (Warren 2013; Owensby et al. 2017; Weber and Weber 2020). Thus, the initial behavior of stocked Muskellunge and factors that may be influencing survival remain largely unstudied in natural lakes, including those managed for Muskellunge in Iowa. Prior studies cited herein evaluated the contribution of stocked Muskellunge to age-4 (via mark recapture) and found that spring-stocked yearling Muskellunge (hereafter Muskellunge) were superior to their fall-stocked counterparts and led to reducing stocking rates by more than one-half (Larscheid et al. 1999; Larscheid 2008). Although this strategy worked well in several of Iowa’s managed Muskellunge fisheries, adult Muskellunge population densities in Spirit Lake, Iowa, declined from 2011 to 2016, as age-4 survival rates decreased to less than 11% (J.R.M., unpublished data).

Naturally, there was interest by managers in the factors associated with the large decrease in Muskellunge survival in Spirit Lake. Notably, in 2007, the discovery of zebra mussel Dreissena polymorpha veligers in hatchery source waters lead to changes in stocking policy beginning in 2008, with water used to transport fish from the hatchery to the stocking destination undergoing treatment with a potassium chloride and formalin solution (i.e., Edwards treatment; Edwards et al. 2002). Managers hypothesized that these additional transportation stressors influenced Muskellunge poststocking dispersal and vulnerability to predators. There was concern among managers that differences in stocked fish size or condition among hatchery production years may contribute to poor poststocking survival of Muskellunge. Lastly, top-level predator densities in Spirit Lake during this period of low Muskellunge poststocking survival were presumably high. It was largely unknown whether one or a combination of these variables, or a variable not identified, was responsible for the decline in Muskellunge survival in Spirit Lake. Thus, the objectives of this study were to 1) evaluate differences in poststocking dispersal and survival of Muskellunge stocked immediately after Edwards treatment and of Muskellunge provided a period to recover from the treatment; 2) determine whether there was a relationship between fish size or condition at stocking and short-term poststocking mortality; 3) identify sources of fish mortality; and 4) develop a stocking protocol for Spirit Lake that has high potential to meet management objectives for adult Muskellunge (see Meerbeek [2014]). To meet study objectives, we used radiotelemetry to track a subset of Muskellunge stocked in Spirit Lake for at least 80 d poststocking (DPS) between 2016 and 2020 that included fish stocked or reared by various techniques. Consequently, this telemetry evaluation differs from many traditional telemetry studies because we used the information obtained during each year’s stocking evaluation in a dynamic approach to review and adjust subsequent production and stocking techniques to maximize poststocking survival of Muskellunge in Spirit Lake.

Spirit Lake (2,300 ha) is a eutrophic natural lake located in northwestern Iowa (Figure 1) with a watershed size of 15,349 ha that consists primarily of agricultural land. The basin of Spirit Lake is bowl shaped, with a mean depth of 4.8 m, a maximum depth of 7.4 m, and water conductivity of 357 µS/cm (SE = 9.9). Several small rock reefs line the periphery of the lake, with emergent vegetation mostly limited to the southeastern and northern portions of the northeast bay. Submersed aquatic vegetation is abundant during the open-water season in Spirit Lake and includes several native species, such as wild celery Vallisneria americana, bushy pondweed Najas flexilis, clasping-leaf pondweed Potamogeton richardsonii, and coontail Ceratophyllum demersum, and one nonnative species, curlyleaf pondweed Potamogeton crispus. The Spirit Lake fishery is diverse, and the piscivore assemblage includes Muskellunge, Northern Pike Esox lucius, Walleye Sander vitreus, Largemouth Bass Micropterus salmoides, and Smallmouth Bass Micropterus dolomieu.

Figure 1.

Location of Spirit Lake (Dickinson County, Iowa) and individual yearling Muskellunge Esox masquinongy locations (black dots) determined via radiotelemetry, 2016–2020. The yellow star indicates the Muskellunge stocking location (i.e., Hales Slough boat ramp).

Figure 1.

Location of Spirit Lake (Dickinson County, Iowa) and individual yearling Muskellunge Esox masquinongy locations (black dots) determined via radiotelemetry, 2016–2020. The yellow star indicates the Muskellunge stocking location (i.e., Hales Slough boat ramp).

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Muskellunge rearing and stocking

We captured broodstock Muskellunge annually between 2015 and 2019 from Spirit Lake and East Okoboji Lake in early April by using gill nets (1.8 m × 97.5 m × 6.35-cm bar mesh) and transported them to the Spirit Lake Fish Hatchery where they were held in covered indoor concrete raceways until fish culturists spawned and fertilized Muskellunge eggs. We cultured Muskellunge fry in indoor concrete raceways (4.8 m × 1.3 m × 1.0 m) where automatic feeders dispensed pellet feed. At approximately 100 mm in TL, the fish diet changed to Fathead Minnow Pimephales promelas. During fall of each year, culturists transported fish from the Spirit Lake Fish Hatchery to an outdoor concrete tank with a volume of 125 m3 at a 0.6-m depth and a flow rate of 946 L/min located at the Rathbun Fish Hatchery. Muskellunge continued to be offered a Fathead Minnow diet until early May, at which time we again loaded Muskellunge into hatchery distribution trucks containing Edwards treatment–laden water and transported them 459 km back to Spirit Lake Fish Hatchery for final distribution and stocking.

In 2016 and 2017, workers separated Muskellunge that arrived at the Spirit Lake Hatchery into two cohorts. They transported one cohort, referred to as the direct stock cohort, an additional 6.1 km to the Hales Slough boat ramp (Figure 1) and stocked in Spirit Lake by using a distribution tube. They transported the other cohort, referred to as the hatchery holdover cohort, to the concrete raceways and held them for an additional 36 h to recover from transport stress while being offered a Fathead Minnow diet. After the recovery period, we loaded fish onto a hatchery distribution truck (absent the Edwards treatment) and stocked them using a distribution tube.

In 2018–2020, we used the same rearing and transportation system, which included the Edwards treatment, to transport fish from the Rathbun Fish Hatchery to the Spirit Lake Hatchery. However, at the Spirit Lake Hatchery, we transferred all fish from the distribution truck to the indoor concrete raceways. Within the first week, we sorted the fish by size into two cohorts. We loaded fish greater than 330 mm or 200 g, referred to as the direct-large cohort, into a hatchery distribution truck (absent the Edwards treatment) and stocked into Spirit Lake. We placed fish less than 330 mm or 200 g, referred to as the grow-out cohort, in separate concrete raceways and fed them a diet of Fathead Minnows for 33–40 d to improve size and condition before stocking. At the end of the grow-out period, we measured the fish (TL, in millimeters) and weighed (in grams) them, loaded them into hatchery distribution trucks (absent the Edwards treatment), and stocked them at the Hales Slough boat ramp.

We radiotagged some fish from each stocking event that occurred in 2016–2017, those provided a grow-out period between 2018 and 2020, and the direct-large cohort stocked in 2020 as described below to evaluate dispersal and survival. We loaded them into hatchery distribution trucks, along with untagged members of their cohorts. We stocked the fish into Spirit Lake less than 1 h from being transferred to the hatchery distribution tanks.

Fish tagging

We tagged fish following procedures outlined in Ball and Weber (2015). We first anesthetized fish and when immobilized we measured for TL (in millimeters) and weight (in grams). Next, we placed the fish ventral side up on homemade surgery cradle and used a scalpel to make an incision anterior to the pelvic girdle that was approximately 25 mm in length. We then created an exit hole for the transmitter whip antenna by inserting a 16-gauge hypodermic needle into the incision and exiting anterior to the anus. We then threaded the whip antenna of the radio tag (model F1440, 2.1 g in water; 158-d battery life, 0.4–0.9% body weight; Advanced Telemetry Systems, Isanti, MN) through the needle and carefully inserted the tag into the fish’s body cavity. We closed incisions by using two to three sutures that were either nylon monofilament or multifilament. We placed all surgical tools and the transmitter in a 2% chlorhexidine gluconate solution, and we sprayed the incision with iodine to prevent bacterial infection before placing the fish back in the raceway to recover.

In 2016, we tagged 20 direct and 18 hatchery holdover Muskellunge at Rathbun Fish Hatchery 18 d before loading onto hatchery distribution tanks on May 10, 2016 (Data S1, Supplemental Material). In 2017, we tagged 20 direct and 20 hatchery holdover fish at Rathbun Fish Hatchery 11 d before loading onto hatchery distribution tanks on May 10, 2017 (Data S1, Supplemental Material). In 2018, 2019, and 2020, we transported all Muskellunge from Rathbun Hatchery to the Spirit Lake Hatchery before fitting them with radio transmitters. We used the same tagging procedures for all Muskellunge selected for the additional grow-out period (n = 16 fish in 2018, n = 14 fish in 2019, and n = 21 fish in 2020; grow-out cohort; Data S2, Supplemental Material); however, recovery time before stocking typically ranged from 13 to 33 d. In 2018, three fish either died or expelled their tags during the grow-out period. In these instances, we disinfected the telemetry tags and reinserted them into new Muskellunge that had a recovery period ranging from 2 to 4 d before stocking. On May 18, 2020, we also inserted radio tags into 17 large (Data S2, Supplemental Material) Muskellunge. We held these fish for a recovery period of 4 d before stocking into Spirit Lake.

Radiotelemetry and survival

Muskellunge tracking consisted of using an Advanced Telemetry Systems model R4000 receiver connected to a three-element folding antenna (model 13860 Yagi; Advanced Telemetry Systems). Each year, we began telemetry the day after stocking and continued daily for up to 10 d, with the goal of locating each tagged fish within 10 m of its location. Wind and weather conditions did not always allow for meeting these objectives. After the first 10 DPS, we conducted telemetry a minimum of once each week and attempted to locate all tagged fish. However, we were often unable to locate all tagged fish on a weekly basis. Because its size made it impractical to methodically search the entire lake, we limited weekly tracking to the northeast bay and to water depths less than 4.0 m. However, we periodically searched unconventional areas of the lake to locate missing fish.

In 2016, we located fish either by holding the Yagi in-hand or by affixing it to a 1.8-m steel rod protruding vertically from the stern of the boat and traversing the lake. To improve Muskellunge detectability in 2017–2020, we first located fish by using a Yagi affixed to a 4.3-m aluminum rod mounted starboard on the vessel and capable of being rotated by a worker 360°. Tracking first consisted of slowly (<16 kph) navigating the perimeter of the lake approximately 100 m from shore with the Yagi pointed toward shore. If we did not detect a fish, we completed 100–150-m offshore transects, measured using a Lowrance™ Gen 2 HDS-7 unit, until we located all of the fish or the workday ended. We scanned each tag frequency for 2 s with the receiver set to either maximum volume and gain or at a lower level to avoid interference. After detection, we determined the direction of the tagged fish by rotating the Yagi until we achieved a maximum signal. As workers approached the tagged fish, they slowly reduced the gain and volume until the fish signal was faint. Next, we used the handheld Yagi to pinpoint the location of the fish by reducing the receiver gain to the lowest achievable setting, while maintaining strong signal strength. We recorded fish locations with a handheld Garmin GPSMAP 78SC (Data S3, Supplemental Material), and we transferred waypoints by using DNRGPS 6.0 software.

We calculated Muskellunge minimum movement (in meters) between two consecutive daily or weekly locations by using distance tools provided in Google Earth Pro 7.3. We frequently reviewed movement distances and locations, and we identified fish that displayed no or little movement over two or more tracking events. During the subsequent tracking event, attempts were made to force movement of these fish by using a push pole fitted with a magnetic bar. We considered the fish a mortality if no movement occurred or we recovered the tag. We reviewed the data from recovered tags to determine the fish’s last live location (Wagner and Wahl 2011). In instances where fish movement ceased in water depths greater than 3.0 m, we classified the fish as a mortality after at least three consecutive tracking events with no movement (Bettoli and Osborne 1998; Pine et al. 2012). We reviewed mortalities of this nature and classified them as either resulting from a fish predator or from an unknown mortality source. Specifically, we determined the occurrence of fish mortality when 1) we detected the tagged fish alive and moving in the lake for greater than or equal to 12 DPS, indicating a low probability of tag expulsion; and 2) we observed the predatory fish during a tracking event. In addition to those tags found deposited within the lake, we recovered some telemetry tags at a great blue heron Ardea herodias (GBHE) rookery and once on top of a muskrat Ondatra zebethicus lodge where GBHEs frequented. We considered all of these instances to be mortalities resulting from GBHE predation. Besides fish classified as dead, fish could remain alive in the fishery or could have an unknown fate. We considered alive those fish with regular movements from previous detections. We gave an unknown designation (i.e., censored individual) to fish that had gone undetected at some point and remained so until the conclusion of that year’s tracking study (Data S1-S3, Supplemental Material).

Data analysis

I compared mean TL at time of stocking among the direct and hatchery holdover Muskellunge in 2016 and 2017 via a paired t-test. I analyzed biotelemetry data from Muskellunge encounters by using a time-to-event model, where both the outcome of interest (death or censorship) and the event time (DPS) are known (Hosmer et al. 2008). Assumptions of the model include 1) status of Muskellunge is known at time of encounter and 2) survival probabilities for censored and uncensored individuals are equal (White and Garrot 1990; Miller 1998). To meet assumptions for survival analyses, I assumed that the tag did not affect individuals’ survival or behavior and that individuals of a cohort have the same detection probabilities. Often, the time-to-death for tagged Muskellunge occurred between tracking events that spanned several days. In these instances, I inferred mortality as a function of each fish’s last known live detection; I calculated time-to-death as the last known DPS alive plus half the average days between tag detections (Koeberle et al. 2023). Evaluation of the effect of the Edwards treatment on Muskellunge survival was determined by pooling telemetry data collected in 2016–2017 and examining survival outcomes of direct and hatchery holdover cohorts by using Kaplan–Meier curves (Kaplan and Meier 1958). I used a nonparametric test of equality (PROC lifetest in SAS, SAS Institute, Cary, NC) to compare the survival functions of the direct and hatchery holdover cohorts. I then used Cox proportional hazard regression models (Hosmer et al. 2008) to evaluate whether subject-level covariates such as 10-d mean dispersal, TL, or condition influenced fish survival. I calculated the covariable “condition” by using the relative weight formula as described by Wege and Anderson (1978) by using standard weight values reported by Neumann and Willis (1994). I calculated dispersal by using the optimal path tool in ArcMap 10.1 that determined the minimum in-water distance (in meters) from the stocking location to the location of a live-tagged Muskellunge (Data S3, Supplemental Material). I pooled the values for each Muskellunge between 1 and 10 DPS and reported them as them as mean 10-d dispersal. If Cox models indicated a significant interaction for a covariable, I further explored covariate values by using survivor functions to visualize covariable effects.

I estimated survival from biotelemetry data collected from each grow-out cohort stocked between 2018 and 2020 and the direct-large cohort stocked in 2020 by using time-to-event models. Similarly, I examined survival outcomes by using Kaplan–Meier curves (Kaplan and Meier 1958). I compared survival among grow-out and direct-large Muskellunge stocked in 2020 by using a nonparametric examination of the survival function (PROC lifetest in SAS). I also calculated mean dispersal in 10-d increments up to 80 DPS for all Muskellunge cohorts to examine differences in dispersal patterns among cohorts. I conducted all analyses within SAS software and maintained α at 0.05 for all statistical procedures.

Table 1 summarizes the mean TLs and ranges for all direct and hatchery holdover Muskellunge cohorts. I detected no differences in mean TL between direct and hatchery holdover cohorts stocked in 2016 (t = 0.50; pooled df = 36; P = 0.6216) or 2017 (t = −0.89; pooled df = 38; P = 0.3769). In total, we collected 963 live detections up to 108 DPS for both the direct (n = 528 detections) and hatchery holdover (n = 435 detections) cohorts. Of the 40 direct stocked tagged fish, 20 survived at least 80 DPS, 10 fish were of unknown fate, and the remaining 10 fish we declared as dead (Table 1). Of those that died, eight fish died from fish predators and two died from GBHE predation. Of the 38 hatchery holdover tagged fish, 17 survived at least 82 DPS. Of the remaining 21 fish, we determined that 12 died from fish predation (n = 8), GBHE predation (n = 2), or unknown (n = 2) causes, whereas 9 fish had unknown fates (Table 1). Estimated greater than or equal to 106 DPS survival was 70.7 ± 7.9% (95% confidence interval [CI]) for the direct cohort and 63.8 ± 8.6% (95% CI) for the hatchery holdover cohort, and I detected no difference in survival functions between cohorts (log-rank test for the model: χ2 = 0.1051; df = 1; P = 0.75; Figure 2). Both direct and hatchery holdover Muskellunge exhibited low mean dispersal (<274 m; SE ≤ 67.3) within the first 10 DPS (Table 2), and I detected no difference in regression models relating either dispersal (P = 0.16) or condition (P = 0.18) to survival. However, I observed a significant interaction between size at stocking and survival (P = 0.046), with Muskellunge greater than or equal to 325 mm surviving at higher rates (Figure 2). Mean dispersal for both stocked Muskellunge cohorts increased between 11 and 40 DPS and were generally the highest between 41 and 80 DPS. Overall, mean dispersal for direct and hatchery holdover Muskellunge was low (mean = 621 m; SE = 56.7 and mean = 736 m, SE = 76.6, respectively) and only 7 of the 58 known fate fish dispersed more than 1.3 km from the stocking location.

Figure 2.

Kaplan–Meier survival curves (a–c) for radiotagged yearling Muskellunge Esox masquinongy with estimated survival probabilities (solid lines) and 95% confidence interval (dashed lines) for each stocked cohort in Spirit Lake, Iowa, from 2016 to 2020. (a) Pooled 2016–2017 data from fish stocked immediately following Edwards treatment (direct cohort; n = 40) and fish provided a treatment recovery period of 36 h before stocked (holdover cohort, n = 38). (b) Fish less than 330 mm in May 2018, 2019, and 2020 reared an additional 33–40 d before stocked in June (grow-out cohort; n = 14–21 fish/y). (c) Fish greater than or equal to 330 mm stocked in May 2020 (direct-large cohort, n = 17). (d) Estimated 50-d survivorship curves from pooled 2016–2017 yearling Muskellunge biotelemetry data by using Cox proportional hazard models for fish stocked at 25-mm increments between 275 and 375 mm. TL = total length.

Figure 2.

Kaplan–Meier survival curves (a–c) for radiotagged yearling Muskellunge Esox masquinongy with estimated survival probabilities (solid lines) and 95% confidence interval (dashed lines) for each stocked cohort in Spirit Lake, Iowa, from 2016 to 2020. (a) Pooled 2016–2017 data from fish stocked immediately following Edwards treatment (direct cohort; n = 40) and fish provided a treatment recovery period of 36 h before stocked (holdover cohort, n = 38). (b) Fish less than 330 mm in May 2018, 2019, and 2020 reared an additional 33–40 d before stocked in June (grow-out cohort; n = 14–21 fish/y). (c) Fish greater than or equal to 330 mm stocked in May 2020 (direct-large cohort, n = 17). (d) Estimated 50-d survivorship curves from pooled 2016–2017 yearling Muskellunge biotelemetry data by using Cox proportional hazard models for fish stocked at 25-mm increments between 275 and 375 mm. TL = total length.

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Table 1.

Number (n) of radiotagged yearling Muskellunge Esox masquinongy for eight cohorts stocked in Spirit Lake, Iowa, from 2016 to 2020 and their fate (alive, dead, or unknown) up to at least 80 d poststocking. Also provided are date stocked, mean total length (TL), and mean relative weight (Wr). Direct = fish stocked immediately following Edwards treatment; Holdover = fish provided a treatment recovery period of 36 h before being stocked; Grow-out = fish less than 330 mm in May reared an additional 33–40 d before stocked in June; Direct-large = fish greater than or equal to 330 mm stocked in May.

Number (n) of radiotagged yearling Muskellunge Esox masquinongy for eight cohorts stocked in Spirit Lake, Iowa, from 2016 to 2020 and their fate (alive, dead, or unknown) up to at least 80 d poststocking. Also provided are date stocked, mean total length (TL), and mean relative weight (Wr). Direct = fish stocked immediately following Edwards treatment; Holdover = fish provided a treatment recovery period of 36 h before being stocked; Grow-out = fish less than 330 mm in May reared an additional 33–40 d before stocked in June; Direct-large = fish greater than or equal to 330 mm stocked in May.
Number (n) of radiotagged yearling Muskellunge Esox masquinongy for eight cohorts stocked in Spirit Lake, Iowa, from 2016 to 2020 and their fate (alive, dead, or unknown) up to at least 80 d poststocking. Also provided are date stocked, mean total length (TL), and mean relative weight (Wr). Direct = fish stocked immediately following Edwards treatment; Holdover = fish provided a treatment recovery period of 36 h before being stocked; Grow-out = fish less than 330 mm in May reared an additional 33–40 d before stocked in June; Direct-large = fish greater than or equal to 330 mm stocked in May.
Table 2.

Mean 10-d poststocking (DPS) dispersal rates (meters from stocking location) for eight cohorts of yearling Muskellunge Esox masquinongy implanted with radio tags and tracked up to 80 DPS from 2016 to 2020 in Spirit Lake, Iowa. The 95% confidence intervals are provided in parentheses. Direct = fish stocked immediately following Edwards treatment; Holdover = fish provided a treatment recovery period of 36 h before stocked; Grow-out = fish less than 330 mm in May reared an additional 33–40 d before stocked in June; Direct-large = fish greater than or equal to 330 mm stocked in May.

Mean 10-d poststocking (DPS) dispersal rates (meters from stocking location) for eight cohorts of yearling Muskellunge Esox masquinongy implanted with radio tags and tracked up to 80 DPS from 2016 to 2020 in Spirit Lake, Iowa. The 95% confidence intervals are provided in parentheses. Direct = fish stocked immediately following Edwards treatment; Holdover = fish provided a treatment recovery period of 36 h before stocked; Grow-out = fish less than 330 mm in May reared an additional 33–40 d before stocked in June; Direct-large = fish greater than or equal to 330 mm stocked in May.
Mean 10-d poststocking (DPS) dispersal rates (meters from stocking location) for eight cohorts of yearling Muskellunge Esox masquinongy implanted with radio tags and tracked up to 80 DPS from 2016 to 2020 in Spirit Lake, Iowa. The 95% confidence intervals are provided in parentheses. Direct = fish stocked immediately following Edwards treatment; Holdover = fish provided a treatment recovery period of 36 h before stocked; Grow-out = fish less than 330 mm in May reared an additional 33–40 d before stocked in June; Direct-large = fish greater than or equal to 330 mm stocked in May.

Grow-out Muskellunge (n = 16) in 2018 averaged 310 mm (SE = 2.5) on May 14, 2018. After a 39-d grow-out period, fish grew on average 36 mm (SE = 2.0) and 73 g (SE = 5.6). Table 1 summarizes TLs at time of stocking in June. We detected fish on 364 occasions up to 108 DPS. Only one fish died from fish predation within the first 15 DPS. Two more died from fish predation and one died from GBHE predation greater than or equal to 39 DPS. Survival for the 2018 grow-out cohort was 73.9 ± 22.2% (95% CI; Figure 2). Mean dispersal ranged from 543 m (0–10 DPS; SE = 124.4) to 1,129 m (71–80 DPS; SE = 196.8) and averaged 858 m (SE = 63.0; Table 2). Nine of the 16 fish never moved more than 1.2 km from the stocking location, and the maximum dispersal observed was 2.3 km.

Grow-out Muskellunge (n = 14) in 2019 averaged 310 mm (SE = 3.6) on May 16, 2019. After a 40-d grow-out period, fish grew on average 31 mm (SE = 1.9) and 70 g (SE = 4.8). Table 1 reports the TLs at time of stocking. We tracked Muskellunge for up to 114 DPS, with a total of 290 live detections. Of the 14 tagged fish, we determined 1 fish died from fish predation within the first 27 DPS, 2 fish went undetected after 24 and 58 DPS, and 11 fish survived at least 83 DPS. Survival for the 2019 grow-out cohort was 92.3 ± 14.5% (95% CI; Figure 2). Mean dispersal ranged from 338 m (0–10 DPS; SE = 79.4) to 774 m (71–80 DPS; SE = 153.9) and averaged 650 m (SE = 52.6; Table 2). Half of the fish never moved more than 860 m from the stocking location; the remaining six fish had maximum dispersal between 1.3 and 1.8 km.

Grow-out Muskellunge (n = 21) in 2020 averaged 312 mm (SE = 2.4) on May 21, 2020. After a 33-d grow-out period, fish grew on average 24 mm (SE = 1.8) and 51 g (SE = 4.6). Table 1 reports TLs at time of stocking. In total, we recorded 346 live detections of fish up to 127 DPS. Both fish and GBHE predation were substantially higher for grow-out Muskellunge stocked in 2020 than those observed in 2018 or 2019; 5 (23.8% of stocked fish) were prey for fish predators and 4 (19.0% of stocked fish) were prey for GBHE and most (8 of 9; 88.9%) of this mortality occurred within the first 26 DPS. We detected one fish on 10 occasions within the first 10 DPS, but this fish went undetected the remainder of the study and we classified it as an unknown fate. All of the remaining fish (n = 11) survived at least 120 DPS and estimated survival was 56.1 ± 21.6% (95% CI; Figure 2). Mean dispersal during the first 10 DPS for grow-out Muskellunge stocked in 2020 was 2–16 times greater (mean = 1,242 m, SE = 241; Table 2) than any other stocked cohort and averaged more than 1,500 m (SE ≥ 317.9) for each period beyond 10 DPS. Five of the 19 fish were never detected more than 870 m from the stocking area; the remaining 14 fish had maximum dispersal between 1.1 and 4.3 km.

Muskellunge in the direct-large cohort stocked in May 2020 experienced fates similar to the grow-out cohort stocked in June 2020: 10 of the 17 fish perished from either fish (n = 2) or GBHE (n = 8) predation within the first 17 DPS. Likewise, we detected no difference in survival functions between cohorts (log-rank test for the model: χ2 = 0.1925; df = 1; P = 0.19; Figure 2). One fish went missing after 6 DPS, and we considered this fish of unknown fate. In total, we reported 149 live detections for the fish that remained alive (n = 6) throughout the study duration, with known survival extending beyond 137 DPS. Survival for the 2020 direct-large cohort was 39.2 ± 23.9% (95% CI; Figure 2). Mean dispersal for direct-large Muskellunge was similar to that observed during other May stocking events (i.e., direct and holdover fish in 2016 and 2017), exhibiting low mean dispersal during the first 10 DPS and increasing thereafter (Table 2). Mean dispersal for the remaining six fish that survived at least 80 DPS was more than 1 km (SE = 126.0), with a maximum dispersal of 1.7 km.

The effects of the Edwards treatment (Edwards et al. 2002) on fish physiology and behavior are poorly understood, and several studies have been conducted recently to monitor and evaluate transport and poststocking stress and survival of Walleye exposed to this treatment in Iowa (Ball et al. 2019; Weber et al. 2020; Grausgruber and Weber 2021). However, less is known regarding the impacts of this treatment on other important fish species managed for sportfish populations. In this study, Muskellunge stocked after being transported 459 km in distribution tanks with the Edwards treatment for 6 h had similar short-term dispersal and survival rates as fish provided a 36-h recovery period before being stocked. Ball et al. (2019) reported that fingerling Walleye exposed to the same treatment appeared lethargic and demonstrated a lack of fight-or-flight response before being stocked, but concluded that exposure time or transportation distance did not influence short-term (48-h) survival rates. Furthermore, studies of Walleye fingerlings in Iowa that used an Edwards treatment to transport fish to natural lakes and impoundments documented high (76%) poststocking survival (Weber et al. 2020). In those cases where there was a reduction in Walleye survival, predation was the primary factor contributing to poststocking mortality (Grausgruber and Weber 2021). These studies combined provide little support that the Edwards treatment or an accumulation of stressors involved in fish transportation durations exceeding 3 h significantly influence short-term survival of stocked fish in Iowa.

Despite my inability to document reduced mortality from changes in hauling or stocking practices or Muskellunge dispersal patterns, I was able to quantify the effect of stocking size on poststocking survival of Muskellunge in Spirit Lake. These findings, although intuitive to many staff who perform stocking evaluations, surprised managers. In Iowa, stocking of Muskellunge is often at average TLs greater than or equal to 305 mm (Iowa Department of Natural Resources 2013), sizes that are considerably larger than those of previous studies that documented positive TL and survival correlations (Wahl and Stein 1993; Szendrey and Wahl 1996; McKeown et al. 1999; Wagner et al. 2017). Furthermore, the results reported herein estimated a greater than or equal to 20% increase in short-term poststocking survival if fish reached TLs greater than or equal to 325 mm compared with those traditionally reared (∼300 mm). Increased survival rates of this magnitude could result in more consistent Muskellunge recruitment patterns and more stable adult populations. However, multiple variables could influence the relationship of size at stocking to survival observed herein, such as the size of predominant predators (Scharf et al. 2000; Dörner and Wagner 2003) at the time of stocking and their foraging behaviors (Cooper 1995), or the timing of the stocking event (Larscheid et al. 1999). Nevertheless, this was the first documentation of a strong positive relationship between TL at time of stocking and survival and was the first observation that both fish and GBHE consume stocked Muskellunge in Spirit Lake. Other variables, such as fish condition and dispersal rates, showed no relationship to Muskellunge survival, further supporting the concept of stocking larger Muskellunge to maintain the fishery.

The observed effect of Muskellunge size on stocking success during the 2016 and 2017 telemetry investigations led to immediate changes in fish production and stocking techniques for Spirit Lake beginning in 2018. In lieu of stocking small (<330-mm) Muskellunge that had a lower estimated probability of short-term survival, I used a dynamic approach to stocking that included 1) stocking large (>330-mm) fish in May and 2) rearing small fish for an additional 33–40-d grow-out period. Because my primary objective was to have an immediate effect on the adult Muskellunge population in Spirit Lake, a side-by-side comparison of direct-large and grow-out Muskellunge was not originally of interest. Thus, in 2018 and 2019, I conducted telemetry only for grow-out Muskellunge. Of the 30 grow-out fish fitted with radio tags, 26 grew to sizes greater than or equal to 330 mm before being stocked in June and short-term survival improved to greater than or equal to 73.9%.

Besides being larger, telemetry investigations in 2018 and 2019 found substantially reduced predation from GBHE by stocking fish in June (only one of five known-fate mortalities and the mortality event occurred after 76 DPS). In comparison, 4 of the 13 known mortalities in 2017 (May stocked) were a result of GBHE predation and these mortalities all occurred within the first 25 DPS. I initially hypothesized that the reductions in water clarity and timing of the stocking in relation to GBHE nesting and foraging behavior would decrease GBHE predation on June-stocked grow-out Muskellunge. In a side-by-side evaluation of direct-large (May stocked) and grow-out (June stocked) Muskellunge stocked into Spirit Lake in 2020, GBHE predation based on known-fate observations showed a reduction from 50% (8 of 16) to 20% (4 of 20) by stocking fish in June when fish dispersed more rapidly. Unfortunately, it is not uncommon for piscivorous birds to account for substantial predation of sportfish stockings (Hodgens et al. 2004; Warren 2013; Owensby et al. 2017; Weber and Weber 2020). Stocked Muskellunge appear particularly vulnerable to avian piscivory, presumably due to their poststocking behavior and the clear, well-vegetated lakes where they are typically managed. Herein, I documented that most of this predation occurs shortly after stocking and within close proximity to the stocking location; however, I also documented mortality via GBHEs for fish that dispersed more than 2.7 km within the first 24 h poststocking. In addition, Meerbeek (2021) found avian predation on Muskellunge stocked as large as 454 mm; thus, there is no practical size to which Muskellunge can be reared that will avoid avian predation. Combined, these results provide evidence that for many managed Muskellunge populations in Iowa, stocking practices that reduce the risk of avian predation are necessary for maintaining adult Muskellunge populations.

I identified several key concepts and rearing practices to improve Muskellunge poststocking survival in a dynamic approach to telemetry experiments conducted between 2016 and 2020 in Spirit Lake. Of those, it is important to note the effect of DPS on Muskellunge survival. In this study, a total of 46 of the stocked Muskellunge were known-fate mortalities and 36 (78.3%) of these mortalities occurred within the first 25 DPS, regardless of rearing or stocking technique. These findings were consistent with those reported previously for stocked Muskellunge (Warren 2013; Owensby et al. 2017), including Iowa’s impoundments (Weber and Weber 2020), and for Walleye fingerling stocked in natural lakes and impoundments (Weber et al. 2020; Grausgruber and Weber 2021). Muskellunge cohort survival beyond 25 DPS was generally very high (88%; 76 of 86), but habitat characteristics of the recipient waterbody also influence survival (Weber and Weber 2020). Thus, it is important for managers to consider stocking approaches that account for spatial and temporal habitat variability in waterbodies wherein Muskellunge population management occurs by using supplemental stockings.

Lessons learned from stocking evaluations conducted in Spirit Lake during this study and reported by others led to further experimentation with stocking techniques for Muskellunge in an attempt to reduce nearshore avian predation during the first 25 DPS. For example, in 2021, I conducted a short-term (37 DPS) telemetry experiment in Spirit Lake for grow-out Muskellunge that included transferring fish from the hatchery distribution tank to a boat containing a large holding tank and then stocking fish offshore haphazardly around the periphery (∼100 m) of the northeast bay. Of 13 fish fitted with radio tags, only 3 fish died (77% survival), all as a result of fish predation (J.R.M., unpublished data). Furthermore, a study using these same stocking methods at Spirit Lake for large (mean TL = 418 mm) Muskellunge stocked in May documented 86.7% survival over 112 DPS (Meerbeek 2024). Collectively, these findings are encouraging for the future of Muskellunge management in Iowa, particularly in lakes where avian predation is a known significant source of poststocking mortality.

This study used radiotelemetry in an unconventional manner to make inferences on Muskellunge poststocking success in Spirit Lake that ultimately improved adult Muskellunge populations and provided cost-effective approaches to culture and management of an important sport fishery. During the course of this study, adult populations in Spirit Lake more than doubled between 2016 and 2023 as a direct result of improved poststocking survival (J.R.M., unpublished data). Other fisheries managed for Muskellunge that replicate the stocking methodology presented herein may not realize this same magnitude of survival, but this case study provides a unique example of how telemetry experiments can help document key factors that may be limiting stocked Muskellunge survival in lakes where the fishery is dependent on high cohort stocking success. Unfortunately, many jurisdictions do not have the opportunity to evaluate Muskellunge poststocking survival to this extent or must make stocking decisions based on limited knowledge of stocked Muskellunge behavior and predator dynamics of the recipient waterbody. In these cases, it is especially important that managers attempt to understand which interactions may be influencing poststocking survival, such as those reported herein and by Wahl (1999), to develop lake-specific stocking criteria that have high potential for success.

Please note: The Journal of Fish and Wildlife Management is not responsible for the content or functionality of any supplemental material. Queries should be directed to the corresponding author for the article.

Data S1. Radio tag frequency (in megahertz), total length (TL; in millimeters), weight (Wt; in grams), and relative weight (Wr) of yearling Muskellunge Esox masquinongy stocked using two methods into Spirit Lake, Iowa, in May 2016 and 2017. Also provided is summary information of radio tag total detections, number (n) of days known to survive, number of days to event, fate at end of study, and mortality source (great blue heron Ardea herodias [GBHE]). Direct = fish stocked immediately following Edwards treatment; Holdover = fish provided a treatment recovery period of 36 h before stocking.

Available: https://doi.org/10.3996/JFWM-24-006.S1 (15.9 KB XLSX)

Data S2. Radio tag frequency (in megahertz), total length (TL; in millimeters), weight (Wt; in grams), and relative weight (Wr) of yearling Muskellunge Esox masquinongy stocked in Spirit Lake, Iowa, by using either the grow-out rearing technique (2018–2020) or the direct-large technique. Also provided is summary information of radio tag total detections, number (n) of days known to survive, number of days to event, fate at end of study, and mortality source (great blue heron Ardea Herodias [GBHE]). Grow-out = fish less than 330 mm in May reared an additional 33–40 d before stocking in June; Direct-large = fish greater than or equal to 330 mm stocked in May.

Available: https://doi.org/10.3996/JFWM-24-006.S2 (15.4 KB XLSX)

Data S3. Individual fish locations (latitude and longitude) for 146 radiotagged yearling Muskellunge Esox masquinongy reared using four techniques and stocked in Spirit Lake, Iowa, between 2016 and 2020. Also provided is date of fish stocking, date of tracking event, days poststocking, distance (in meters) to stocking location, and Muskellunge fate (0 = dead, 1 = alive, 3 = unknown) at the end of each year’s tracking study. Direct = fish stocked immediately following Edwards treatment; Holdover = fish provided a treatment recovery period of 36 h before stocking; Grow-out = fish less than 330 mm in May reared an additional 33–40 d before stocking in June; Direct-large = fish greater than or equal to 330 mm stocked in May.

Available: https://doi.org/10.3996/JFWM-24-006.S3 (144 KB XLSX)

Reference S1. Ball EE, Weber MJ. 2015. Protocols for radio and acoustically tagging freshwater fishes. Department of Natural Resource Ecology and Management, Iowa State University, Ames, Iowa.

Available: https://doi.org/10.3996/JFWM-24-006.S4 (1.09 MB DOCX)

Reference S2.Iowa Department of Natural Resources. 2013. Iowa DNR Fisheries Bureau Fish Stocking Policy. Des Moines, Iowa.

Available: https://doi.org/10.3996/JFWM-24-006.S5 (151 KB DOCX)

Reference S3. Kerr SJ. 2011. Distribution and management of muskellunge in North America: an overview. Peterborough: Ontario Ministry of Natural Resources. Report MNR 62714.

Available: https://doi.org/10.3996/JFWM-24-006.S6 (545 KB DOCX)

Reference S4.Larscheid J. 2008. Contribution and survival of stocked muskellunge and population dynamics of adult muskellunge in Spirit, East Okoboji, West Okoboji, and Clear lakes. Des Moines, Iowa: Iowa Department of Natural Resources. Federal Aid to Fish Restoration, Annual Performance Report, Project No. F-160-R.

Available: https://doi.org/10.3996/JFWM-24-006.S7 (2.38 MB DOCX)

Reference S5. Meerbeek JR. 2014. Iowa’s muskellunge management plan. Des Moines, Iowa: Department of Natural Resources.

Available: https://doi.org/10.3996/JFWM-24-006.S8 (1.48 MB DOCX)

Reference S6. Meerbeek JR. 2021. Short-term stocking survival of yearling muskellunge raised in a recirculating aquaculture system. Des Moines, Iowa: Iowa Department of Natural Resources. Federal Aid in Sport Fish Restoration, Study 7060 Completion Report.

Available: https://doi.org/10.3996/JFWM-24-006.S9 (6.93 MB DOCX)

Reference S7. Warren LH. 2013. Spawning and nursery habitat of wild muskellunge and fate of stocked muskellunge in middle Tennessee River. Master’s thesis. Cookeville, Tennessee: Technological University.

Available: https://doi.org/10.3996/JFWM-24-006.S10 (963 KB DOCX)

There is no conflict of interest declared in this article. Funding for this study was from Iowa fishing license sales and the Federal Aid in Sport Fish Restoration Program. Donations of additional telemetry equipment was by the Upper Great Plains (Chapter 29) and Heartland Chapter (Chapter 10) Muskies, Inc., with grant support provided by the Hugh C. Becker and Gil Hamm foundations. I thank Iowa Department of Natural Resources staff at the Rathbun Fish Hatchery and Spirit Lake Fish Hatchery for rearing and hauling Muskellunge. DJ Vogeler, Emily Grausgruber, and Andrea Sylvia assisted with radio tag implantation. DJ Vogeler, Steve Pecinovsky, and Dominic Foley assisted with Muskellunge tracking. Erienne Davis provided assistance for ArcMap analyses. Comments on earlier version of this manuscript by George Scholten, three anonymous reviewers, and the Associate Editor improved the readability.

Any use of trade, product, website, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Author notes

The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service.

Supplemental Material