https://orcid.org/0000-0001-9533-1352
Abstract
Track cohort survival, individual growth, and PIT tag retention, as well as relative growth rate, from fall to the following spring for age‐0 Muskellunge Esox masquinongy held in an extensive culture pond.
A total of 806 age‐0 Muskellunge were provided a 12‐mm PIT tag in October 2021 and placed into a 0.1‐ha pond. Individual total length (mm) was recorded for each fish. When the pond was drained in March 2022, an assessment of survival and PIT tag retention was calculated for the cohort, as well as assessment of mean individual growth and relative growth rate.
Muskellunge experienced high survival (91.9%) but low individual mean growth (21.7 ± 0.2 mm [mean ± SE]). The mean length of surviving Muskellunge (267.8 ± 0.9 mm) was significantly greater than nonsurviving Muskellunge (251.4 ± 3.1 mm). However, smaller surviving Muskellunge did exhibit slightly improved growth compared to individuals that were initially longer.
The results provide an expectation for overwinter survival and growth in an extensive culture pond. This can assist managers in developing expectations on the numbers and length of Muskellunge that will be available to stock the following spring, which has largely been unreported previously.
The results from this study provide fishery culture and management staff with an expected survival and growth related to overwintering age‐0 Muskellunge in an outside pond. Knowing that fish will have high survival but minimal growth can help managers determine how many they would like to put in the pond to meet stocking goals as well as anticipate the potential survival of these fish in waters with different predators.
INTRODUCTION
In response to growing interest from avid anglers and the potential to use a large esocid predator to assist with top‐down biological control (Søndergaard et al. 1997; Kerr 2011; Perrion and Koupal 2017), the distribution of Muskellunge Esox masquinongy has become such that nearly half of North American Muskellunge waters are not considered native (Kerr 2011). Artificial production has failed to meet population objectives in many waters (Kerr 2011), as natural recruitment of Muskellunge in native and introduced waters has historically been limited (Eslinger et al. 2010). Lack of natural reproduction is one reason why fishery managers have sought out new waters for introduction. Failure to produce year‐classes naturally has been associated with lack of quality spawning habitat (Zorn et al. 1998; Farrell et al. 2007; Kapuscinski et al. 2007), insufficient broodstock (Eslinger et al. 2010), dynamic abiotic conditions (Eslinger et al. 2010), human development of shorelines (Jennings et al. 1999; Rust et al. 2002), and excessive direct or indirect competition for food resources (Inskip and Magnuson 1986; Eslinger et al. 2010). Thus, state agencies often rely on stocking to sustain populations or achieve specific management goals. Recruitment of stocked individuals to adult populations is frequently sporadic (Owensby et al. 2017), but a common finding in stocking evaluations is that larger stocked products are more likely to recruit to the adult stage (Szendrey and Wahl 1996) and exhibit 40% greater survival than smaller cohorts (Johnson and Margenau 1993). The desire by managers to stock larger Muskellunge has been accompanied by a chronological shift from fry to fingerlings, to fall fingerlings, to spring yearlings, to fall yearlings (Margenau 1999; Kerr 2011), and with each request to produce larger fish comes added expense and mortality.
The development of larger Muskellunge for stocking has extended the time and number of life stages through which hatchery staff must hold these fish. Specific techniques to improve growth and poststocking survival have looked at diet (Mooradian 1986; Larscheid et al. 1999), predation exposure (Szendrey and Wahl 1995), and the use of intensive culture and recirculating aquaculture systems to improve water quality filtration and provide warmer overwinter water temperatures (Ebeling and Timmons 2012; Meerbeek 2021). While most reported studies have evaluated contribution to year‐class strength from various stocked Muskellunge cohorts captured after stocking, very few studies have documented succession of different stages of development within the rearing process itself (Younk et al. 2010; Andree et al. 2018). Studies specific to overwinter mortality have shown variable survival and growth (Wagner et al. 2007; Weber and Flammang 2017; Andree et al. 2018); thus, more case study examples can improve managers expectations for overwinter performance. As part of an on‐going management effort to evaluate stocked Muskellunge and determine growth and mortality rates, the Nebraska Game and Parks Commission was placing passive integrated transponder (PIT) tags in extensively cultured Muskellunge during the fall. Because these fish were being placed back into a pond for overwinter rearing, we had a unique opportunity to track individual growth and PIT tag retention, as well as relative growth rate and survival rate, from fall to the following spring in one hatchery.
METHODS
Adult Muskellunge were collected for broodstock by boat electrofishing in April 2021 at Cottonwood‐Steverson Lake located in north‐central Nebraska. Eggs from each ripe female were extracted and fertilized with milt from three unique males. Fertilized eggs were transported to the North Platte Hatchery, where they were hatched in 12.8°C water. Prior to the swim‐up stage, the fish were transferred to a 0.91‐m‐diameter recirculating system that was supplied by directional flow piping with a 19–23‐L/min flow that was directed into the tank wall to dampen the spin rate to a 11–15‐L/min flow rate. At this time, the fish were provided an Otahime weaning diet, with feed size ranging from 360‐ to 650‐μm pellet granules and ending with 2.3‐mm extruded pellets. The temperature in the recirculating aquaculture system was slowly raised to 22.8°C during the first week, and the fish remained in this system with water from the Nebraska public power district canal until they reached 834 individuals/kg. The fingerling Muskellunge were transferred to a 0.4‐ha earthen‐bottom pond on June 21, 2021, that had maximum temperatures of 25.0°C. The pond was stocked with approximately 227 L of adult and juvenile Fathead Minnow Pimephales promelas. On October 6, 2021, the pond was drained through an external raceway, and a total of 806 surviving fingerling Muskellunge (mean total length [TL] = 267.8 mm; standard error = 0.9) were recovered.
Collected Muskellunge were held in the external raceway and transferred 15 fish at a time into a sedation tub filled with 56.8 L of pond water with 5 mg of AQUI‐S 20E. Fish were typically removed from sedation prior to complete loss of equilibrium, and guidelines for anesthetic exposure and recovery were targeted for exposure and recovery times (Marking and Meyer 1985). Each Muskellunge was removed from the sedation tub and measured (TL; mm). A 12‐mm APT12 PIT tag was inserted with a 12‐gauge needle in an anterior direction into the abdominal cavity with an insertion point dorsally located to the pelvic girdle following guidelines reported by Prentice et al. (1990). After PIT tag insertion, a PIT tag reader was used to detect the tag, and the individual tag number was recorded. Fish were placed into a recovery tub filled with pond water and allowed to recover until equilibrium was regained (≤20 min) before being placed into a stocking truck filled with the same water source. All fish were then transported to a 0.1‐ha earthen pond for overwintering. Water temperatures within the pond were not collected, but historic thermograph readings from a 0.4‐ha earthen pond on the North Platte hatchery indicated that water temperatures were generally <15°C at this time of year and <10°C when the pond was drained. Ice cover typically initiates in middle December and lasts through early March. The overwinter pond was also stocked with Fathead Minnows collected from the drained pond along with an additional 151 L of adult Fathead Minnows to ensure ad libitum feeding. On March 23, 2022, the overwinter pond was drained through an external raceway and 744 yearling Muskellunge were collected. Each surviving Muskellunge was checked for PIT tag retention and was measured (TL; mm) to calculate overwinter (October–March) growth. A single carcass was recovered when the pond lost ice cover that did have the PIT tag present and had grown 16 mm overwinter. Thus, a total of 745 fish were used for PIT tag retention assessment. This single fish was included in tag retention and mortality calculations but not growth analysis.
Overwinter survival was calculated by dividing the number of surviving Muskellunge collected in March by the number stocked into the overwinter pond in October. Tag retention was determined by summing the number of surviving and deceased recovered Muskellunge with tags and dividing by the total number of Muskellunge (both living and deceased) collected in the spring after ice out. Growth of individual tagged Muskellunge was calculated as the difference between March and October lengths, and mean growth was calculated over all surviving Muskellunge that had retained the PIT tag. Relative daily growth rate was calculated with the equation that was reported by Wagner et al. (2007). A two‐sample t‐test assuming unequal variance was used to compare October lengths between those individuals that survived and those that were deceased. Muskellunge that did not retain a PIT tag were not included in the deceased category of fall length since it is not known which fall measurement they should be assigned to from the database. A post hoc two‐sample t‐test assuming unequal variance found no difference between the spring lengths of surviving Muskellunge with and without a PIT tag. A simple linear regression was used to determine if overwinter growth was related to the starting October length. Significance level was set at α = 0.05 for all tests.
RESULTS
From the 806 tagged Muskellunge, estimated overwinter survival was 92.3% (n = 744) in the extensive culture pond. During the pond‐draining process, three Muskellunge received physical damage within the external kettle from either the crowding screen or being stepped on, which slightly reduced overwinter survival to 91.9%. The PIT tags were retained in 98.9% (n = 737) of the surviving and spring‐collected‐carcass Muskellunge.
Total length appeared to be related to both overwinter survival and growth. The fall mean (±SE) length of surviving Muskellunge (267.8 ± 0.9 mm) was significantly greater (t = 5.12; p ≤ 0.001) than the fall mean length of overwinter mortalities (251.4 ± 3.1 mm). The mean overwinter growth for surviving Muskellunge was 21.7 ± 0.2 mm, with an individual growth ranging from 0 to 85 mm. The relative daily growth rate was 0.0005 mm·mm−1·day−1. Fish that were smaller when they were tagged in October appeared to grow more than those that were larger (Figure 1; r2 = 0.07; p ≤ 0.001).
Plot of observed overwinter growth (total length; mm) of Muskellunge collected from an extensive culture pond in a Nebraska hatchery in March 2022 compared with the original total length when those individual Muskellunge were stocked into this pond in October 2021.
DISCUSSION
An important objective of hatchery production is to provide improved survival through different life stages and potential recruitment bottlenecks. The survival exhibited in this case study exceeded the 77% overwinter hatchery pond survival estimates observed for two Muskellunge stocks in Illinois (Andree et al. 2018). The cause of the mortality that was observed in these trials was likely either cannibalism or perhaps bird predation as was suggested in other overwinter extensive pond culture trials (Koupal 1999). Since age‐0 Muskellunge cannot be cultured to exceed sizes that preclude bird predation, inclusion of cover (natural or artificial) or increased turbidity within culture ponds may potentially protect against bird predation. Overall, considering that survival of stocked age‐0 fall fingerlings is typically reported to be <30% (Carline et al. 1986; Margenau 1992; Warren 2013; Owensby et al. 2017), holding Muskellunge overwinter may protect these hatchery fish through a potential overwinter recruitment bottleneck and may improve recruitment to adult stages and better help achieve management objectives.
With an increasing interest in stocking larger Muskellunge by fisheries managers, the assessment of growth at different life stages can be useful. The mean relative growth rate observed in this study was higher than that reported from trials that used a shorter winter time frame (Wagner et al. 2007) but lower growth than trials that were longer (Wagner et al. 2007; Andree et al. 2018). Raceway experiments that maintained 4–7°C for 112 days found only 1 mm of total growth (Weber and Flammang 2017), so the growth reported in the other trials was most likely experienced within the first or last few weeks of the trials, when water temperatures were elevated. The pond used in this study likely had 2 months of temperatures between 5°C and 15°C. Our results also included an unexpected juxtaposition, with the smaller Muskellunge growing more than larger individuals, resulting in less length variability within the hatchery products. To our knowledge, the observed reverse length‐selective growth relationship has not been reported previously in esocid populations. Smaller‐sized cohorts could survive better than larger ones due to food availability at the time of stocking being more fitting for those individuals instead of those that have switched to different prey (Kaemingk et al. 2014). Further, birds, some predatory fish, and anglers may harvest larger individuals (Sogard 1997; Conover et al. 2009). However, these factors are not likely to have affected growth in this study, considering that the hatchery conditions provided a similar environment and protected them from all predators such that larger tagged individuals appeared to survive at higher rates. The observed reverse length‐selective growth was more likely attributed to smaller fish gaining a bioenergetic advantage through more effective movement patterns (Busch and Mehner 2012) or improved agility and prey capture (Weatherley 1990). Despite these results, small differences in length have been linked to improved short‐term survival in some fish (McCormick and Hoey 2004), and larger‐sized stocked Muskellunge have recruited better poststocking (Szendrey and Wahl 1996). We suggest exploring different options to improve growth, such as the use of recirculating aquaculture systems. However, some of these options may have their own drawbacks, such as higher rates of bacterial infection and poor fin conditions (Meerbeek 2021) or higher costs of rearing as compared with traditional systems.
Retention of the abdominally inserted PIT tags was better than (Weber and Flammang 2017) or comparable to (Wagner et al. 2007) previous reported studies for fall age‐0 Muskellunge (approximately 225 mm TL). Besides confirming that PIT tags were suitable for use in this life stage of Muskellunge, this study also expanded the use of PIT tags to understand size‐selective growth and mortality within culture production. The retention of these PIT tags poststocking will also provide long‐term monitoring of population dynamics and provide known‐aged individuals (Rude et al. 2017; Sheffer et al. 2022). An additional concern was whether the placement of the PIT tag may disrupt growth and survival of individual fish. Considering that our reported survival was similar, but growth was reduced, it is possible that placement of the PIT tag in the abdominal cavity may have negatively influenced growth during the winter. However, use of similar 12‐mm tags in previous research indicated similar growth in fall age‐0 Muskellunge (Weber and Flammang 2017; Walton‐Rabideau et al. 2019). Thus, reduced growth observed in these trials is likely related to thermal conditions rather than tag insertion or location.
The demand for more and larger age‐1 Muskellunge is growing (Kerr 2011) and will come with the cost of added expense and loss of fish within the production process. To meet this growing demand, fish culture personnel will need to examine specific steps related to the development of these hatchery products. Adding a case study example that reports overwinter survival and growth from an extensive culture pond can improve the understanding of variability surrounding these characteristics. Additionally, alternative culture techniques can be explored and compared, such as the use of cover or predator bird protection within these extensive culture ponds (Koupal 1999) and recirculating aquaculture systems (Meerbeek 2021). Ultimately, consideration of cost‐benefit values should be incorporated that consider the input costs of staff, lost opportunity for use of the pond or recirculating aquaculture system, and recruitment of stocked Muskellunge to the respective fisheries.
ACKNOWLEDGMENTS
We would like to recognize that this research was funded in part by the Hugh C. Becker Foundation Grant. Additionally, we appreciate the field support provided by North Platte and Valentine Nebraska Games and Parks Commission fisheries staff, Jenna Ruoss, William Schreiner, Logan Dietrich, and Drayden Bellamy.
CONFLICT OF INTEREST STATEMENT
The authors have no conflict of interest.
DATA AVAILABILITY STATEMENT
Data collected as part of this research study are available through a public data request submitted to the Nebraska Game and Parks Commission Fisheries Division or by contacting the corresponding author.
ETHICS STATEMENT
Protocol and procedures established by the Nebraska Game and Parks Commission fisheries aquaculture division were followed to ensure ethical care and treatment for all Muskellunge handled in this study.
