Water wolves and tigers: Testing for directional hybridization and introgression between northern pike and muskellunge

Tiger muskellunge (muskellunge Esox masquinongy x northern pike Esox lucius) are commonly stocked in the United States but not in Canada. Much research has been conducted on the culture and stocking of these fish: however, there has been little research on natural hybrids or the genetics of tiger muskellunge. Hybridization and introgression occur within and among different taxa and can have both positive and negative impacts. Hybridization can lead to speciation events and can also be used for genetic rescue but introgression has the potential to cause extinction of populations when hybrids repeatedly backcross with parental species. Studies on hybridization utilize species-specific markers because unique differences between species allows for more reliable and accurate detection of hybrids. In this study, the cytochrome b region of mitochondrial DNA (mtDNA) was sequenced from 6- tiger muskellunge from various lakes in Ontario as well as hatcheries and lakes in New York, Wisconsin and Minnesota in order to determine whether tiger muskellunge had muskellunge or northern pike mtDNA. Since mtDNA is maternally inherited the data allo9wed the directionality of hybridization to be determined. Tiger muskellunge were genotyped using 20 nuclear microsatellite loci to confirm hybridization and to test for introgression. Muskellunge and northern pike have historically faced population declines and information on hybridization between these two species may help to understand these declines particularly if introgression is occurring. Anthropogenic effects such as habitat fragmentation are know to increase the rate of hybridization in species and if there is evidence of introgression here would be a need for management action in order to protect genetically distinct populations of both muskellunge and northern pike.

Artificial hybrids between Amur pike (Esox reicherti) and North American esocids

Artificial hybrids between Esox reicherti, the only species in the family Esocidae that does not occur naturally in North America, and North American esocids were developed. Five of a possible 10 crosses are described in detail. Three crosses failed (those involving the males of E. niger and E. americanus) for reasons other than methodology, the cross involving females of E. americanus americanus was not made, and a fifth cross involving the male of E. masquinongy was successful but no data are included. Interspecies fertility was suprisingly high, and an inverse relationship existed between survival of crosses and the difference in potential maximum size of parent species. At least one cross was fertile, and an F2 generation and backcrosses were developed

Identification of muskellunge, northern pike and their hybrids

Discrimination among live and even dead esocids can sometimes be very difficult. In areas where muskellunge and northern pike cohabit, there is inadvertent (sometimes illegal) harvest of the rare and more valuable muskellunge because many anglers can’t readily distinguish between them. Muskellunge x northern pike hybrids are recognized only by the most avid muskellunge angler and fisheries specialist. This study reviews criteria for identifying and distinguishing among esocids and researches new techniques that can be applied specifically for muskellunge, northern pike and their hybrid, the tiger muskellunge.

Body color pattern and external appearance of young, juveniles and adults provide discriminating characteristics as do the submandibular pores and cheek and gill cover scalation. External morphology such as body proportions can be used to distinguish between northern pike and muskellunge however their hybrids generally show intermediate meristic and morphometric characteristics that somewhat overlap muskellunge. Differences in the shape of dentary teeth and shape and dentition of the vomer and palatine tooth patches provide distinct diagnostic features. Scale pattern and shape and zonation of both scales and cleithra can nbe used to distinguish among these species and their hybrids. Scales and cleithra in large hybrids are larger relative to body size than in the parent species The internal anatomy also has distinguishing characteristics (e.g., location of the dorsal aorta, vertebral column and centra).

The fish were compared electrophoretically for 18 enzyme systems encoded by 35 lo ci. Muskellunge and northern pike are electrophoretically distinct for 54% (19) of their loci. The hybrid has intermediate isozyme patterns for all but two of these loci.

The results describe not only better and more easily recognizable characteristics for the angler but also more sophisticated, analytical laboratory techniques which can be used as forensic tools for discrimination among muskellunge, northern pike and their hybrids.

Reviewing the esocid hybrids

A review of the hybrids of the Esocidae is presented. Artificial crosses of the smaller pickerels, chain pickerel, grass pickerel and redfin pickerel and their reciprocals produced fertile progeny. However the crosses of the larger pike, muskellunge, northern pike and Amur pike although successful produced only one with fertile progeny, the Amur pike x northern pike. Crosses of the larger pike with smaller pickerel were generally unsuccessful or produced sterile young. Natural hybridization has been prevented by difference in distribution, habitat, size of mature fish, behaviour, spawning time, spawning sites and immunological barriers. Crosses of northern pike with muskellunge and Amur pike produced hybrids which appear to have greater potential as a port fish. These hybrids are relatively easy to rear, utlize artificial foods, grow fast and eventually produce a sport fish which is prized to anglers.

A comparison of muskellunge and hybrid muskellunge in a southern Wisconsin lake

A comparison of age and growth, return to the creel and harvest size characteristics was made for muskellunge and hybrid muskellunge in Little Green Lake, Wisconsin. Most hybrids reached 30 inches by their fourth summer while most muskies attained that length by their fifth summer. Since 1963, in a voluntary musky registration program, an average of 108 muskellunge and 125 hybrid muskellunge have been recorded annually. Anglers have registered 11.6% of the musky stocked and 14.6% of the hybrid musky stocked; their average size was 33.1 and 31.6 inches respectively. Total harvest was 2.03 lb/acre for musky and 1.98 lb/acre for hybrids. Exploitation rates were 26.6% for musky and 33.3% for hybrid musky. Live bait was used to catch 55.1% of the hybrids while artificial baits accounted for 87.0% of the musky caught. Since 1972, 147 muskellunge and 128 hybrids have been caught and released. Due to the hybrid’s greater vulnerability to angling muskellunge may be the better choice for stocking intensively fished waters.

Hybridization in the family Esocidae

Of 11 possible hybrids of species in the family Esocidae, six were known from nature and they are reviewed. The occurrence of a seventh, Esox lucius × Esox americanus americanus, is recorded. Of 22 possible reciprocal combinations of these species, five were previously known, five are still unknown, and 12 are newly described here. These 12 are based on artificial hybrids of known gametic constitution. Data are given on: description (young and oldest material available); growth; fertility; vitality and meristics are compared with parent populations. The low level of interspecific sterility was surprising. Some interspecific sterility exists between the two largest (Esox masquinongy Mitchill, Esox lucius Linnaeus) and the two smallest (Esox americanus Gmelin) forms. Total sterility does not exist as artificially one or other of the reciprocals was successful in each cross, including Esox masquinongy × Esox americanus. The hybrids exhibited the blending and intermediate nature usual in fish hybrids but colour pattern seemed tied to a particular parent. In nature only certain species hybridize, but it would appear that incompatibility of gametes has proceeded only to a limited extent. Other factors such as distribution, habitat, size, and behavior may be preventing hybridization.

Comparative population characteristics of muskellunge, northern pike and their hybrid

We compared growth, survival, diet, and angler catch of muskellunge (Esox masquinongy), northern pike (E. lucius), and tiger muskellunge (E. masquinongy x E. lucius) through 5 yr after their introduction into three Ohio reservoirs. Muskellunge grew slower than northern pike and tiger muskellunge through the first year but faster than northern pike in subsequent years. Large stocked esocids (180-205 mm) survived better than small ones (145 mm). Survival patterns established through the first fall were maintained through age 5; northern pike survived best, followed by muskellunge and tiger muskellunge. Angler catch reflected differences in survival as well as catchability among taxa. Northern pike were caught at smaller sizes and younger ages than other taxa. Gizzard shad (Dorosoma cepedianum) dominated esocid diets for all taxa and age classes, followed by centrarchids and cyprinids. Prey length consumed increased linearly with esocid length; northern pike selected larger gizzard shad than either muskellunge or tiger muskellunge. These differences in population characteristics among esocids should influence management and stocking programs. Whereas northern pike maximize angling opportunities, muskellunge probably will provide trophy fisheries. Although tiger muskellunge can be reared inexpensively, they appear to provide little recreational fishing in return.