Document Category: Interactions with Other Species

We documented differential vulnerability of fathead minnows (Pimephales promelas), gizzard shad (Dorosoma cepedianum) , and bluegills (Lepomis macrochirus) to predation by muskellunge (Esox masquinongy), northern pike (E. lucius) , and tiger muskellunge (E. masquinongy × E. lucius) in a 700‐L tank. Individual esocids (150–225 mm in total length) were combined with singlespecies groups (N = 15) of optimal‐sized prey (25–30% of predator length for bluegills, 37–43% for fathead minnows, and 30–36% for gizzard shad). Capture ability did not differ among esocids; however, mean captures per strike were higher for fathead minnow (0.67) and gizzard shad (0.78) than for bluegill (0A4). Morphology and antipredatory behavior, unique to each prey species, contributed to this differential vulnerability. In the field, we introduced equal numbers and similar sizes of these esocids into two systems, one with centrarchid prey and one with both centrarchid and gizzard shad prey. As predicted from laboratory work, esocids ate fewer prey and grew more slowly when centrarchids were the only prey than they did when gizzard were available, In a third reservoir, containing bluegills and gizzard shad, esocids strongly preferred gizzard shad over bluegills. To maximize growth and survival, esocids should be stocked in systems with soft‐rayed or fusiform prey, such as cyprinids or shad, rather than in centrarchid‐dominated systems.

The feasibility of reintroducing anadromous salmonids into reservoirs above high‐head dams is affected by the suitability of the reservoir habitat for rearing and the interactions of the resident fish with introduced fish. We evaluated the predation risk to anadromous salmonids considered for reintroduction in Merwin Reservoir on the North Fork Lewis River in Washington State for two reservoir use‐scenarios: year‐round rearing and smolt migration. We characterized the role of the primary predators, northern pikeminnow (Ptychocheilus oregonensis) and tiger muskellunge (northern pike Esox lucius × muskellunge E. masquinongy ), by using stable isotopes and stomach content analysis, quantified seasonal, per capita predation using bioenergetics modeling, and evaluated the size and age structures of the populations. We then combined these inputs to estimate predation rates of size‐structured population units. Northern pikeminnow of FL ≥ 300 mm were highly cannibalistic and exhibited modest, seasonal, per capita predation on salmonids, but they were disproportionately much less abundant than smaller, less piscivorous, conspecifics. The annual predation on kokanee (Oncorhynchus nerka) (in biomass) by a size‐structured unit of 1,000 northern pikeminnow having a FL ≥ 300 mm was analogous to 16,000–40,000 age‐0 spring chinook salmon (O. tshawytscha) rearing year‐round, or 400–1,000 age‐1 smolts migrating April–June. The per capita consumption of salmonids by Northern Pikeminnow having a FL ≥ 200 mm was relatively low, due in large part to spatial segregation during the summer and the skewed size distribution of the predator population. Tiger muskellunge fed heavily on northern pikeminnow, other nonsalmonids, and minimally on salmonids. In addition to cannibalism within the northern pikeminnow population, predation by tiger muskellunge likely contributed to the low recruitment of larger (more piscivorous) northern pikeminnow, thereby decreasing the risk of predation to salmonids. This study highlights the importance of evaluating trophic interactions within reservoirs slated for reintroduction with anadromous salmonids, as they can be functional migration corridors and may offer profitable juvenile‐rearing habitats despite hosting abundant predator populations.

In western North America, nonnative Brook Trout Salvelinus fontinalis in alpine lakes threaten the persistence of native trout and often offer limited sport fishing opportunity as they are prone to stunting. Stocking tiger muskellunge (Northern Pike Esox lucius × Muskellunge E. masquinongy ), which are reproductively sterile, may be an option to eradicate Brook Trout in some alpine lakes. We used floating gill nets to survey 17 alpine lake Brook Trout populations, then stocked 13 lakes with tiger muskellunge, with four additional lakes serving as controls. Tiger muskellunge were stocked at a mean TL of 317 mm and a density of 40 fish/ha. Brook Trout were resampled for 4 or 5 years after stocking to evaluate changes in Brook Trout TL and CPUE (fish/net‐night). Declines in CPUE were substantial for both treatment and control lakes but were significantly greater in treatment lakes. Mean Brook Trout CPUE in treatment lakes declined from 23.1 fish/net‐night to 2.3 fish/net‐night 5 years after stocking tiger muskellunge, whereas in control lakes, CPUE declined from 25.5 fish/net‐night to 7.8 fish/net‐night 5 years later. Complete eradication appeared to occur in two lakes within 2 years, and in two more lakes by year 5. In lakes where tiger muskellunge were stocked, the proportion of Brook Trout ≥250 mm TL in the catch increased significantly in years 1, 2, and 4 after stocking (compared with prestocking data), whereas no increase occurred in control lakes. Tiger muskellunge were most successful in reducing Brook Trout CPUE in lakes with no inlets or outlets, while elevation and lake area may also have played a role. Our results suggest tiger muskellunge can improve the size structure and potentially eradicate Brook Trout populations from some alpine lakes. However, we recommend combining any tiger muskellunge stocking with other conventional removal methods to increase the likelihood of successful eradication.

Inverse trends in relative abundance of muskellunge and northern pike have been reported for numerous lakes and one large river. In each case, muskellunge appeared to decrease in abundance while northern pike appeared to increase. Most instances involved colonization of native muskellunge lakes by northern pike but shifts in relative abundance of populations with a history of co-occurrence also have been noted. These trends have been interpreted as evidence of a negative interaction between the two species. Predation, competition and hybridization are possible mechanisms of interference. Earlier spawning in the spring, shorter generation time, a more aggressive nature and greater food conversion efficiency have been suggested as possible advantages for northern pike.

Predation by young-of-the-year (YOY) northern pike on YOY muskellunge was proposed as a likely mechanism of interactions nearly 35 years ago and it remains the most attractive hypothesis, more on the basis of its intuitive appeal and explanatory power than on actual field evidence. Under this hypothesis the apparent compatibility of muskellunge and northern pike in some waters can be explained by the availability of sufficient spawjning habitat to permit spatial separation of the species, either because of differences in preferred spawning habitat or because eggs are spread over a large enough area that encounters between YOY muskellunge and northern pike and minimized . The existence of one mechanism of interaction does not preclude the existence of others.

Circumstantial evidence of a negative interactions between the two species is persuasive and not to be discounted. However, negative associations in relative abundance might arise even in the absence of any direct species interactions. For example, changing environmental conditions could affect the two species quite differently if they have different tolerances or optima for certain environmental variables. Base on zoogeographic evidence it appears that northern pike might be favoured by cooler temperatures and more lentic conditions Cultural development more often results in the conversion of lotic habitat to lentic habitat than vice versa. Sequelae of human settlement such as increased turbidity, siltation and accumulation of organic sediments might be expected to have a more severe impact on a species adapted for life in flowing water than on one which evolved for life in still water habitats. Differences in the environmental requirements of the two species have been postulated but nor proven.

Effective muskellunge management in waters containing northern pike will require deeper understanding of how the two species interact, what factors modify the intensity and outcome of the interaction and how environmental conditions and fishing pressure can otherwise favour one species over the other.

With the exception of a few one-year-old fish, no muskellunge were found in tributaries of the upper Niagara River during 1975-1977. In contrast, nearly all (97%) northern pike were collected in tributaries despite extensive river sampling. The species are spatially segregated throughout much of their life cycles. Differential adaptation tot river current may be the most important factor permitting coexistence of the two species. The geologic history of the Niagara River suggests that the muskellunge population may have been established rather recently (since 5,500 years before present). However the present distribution of northern pike indicates that this population was probably established much earlier (12,300-10,400 BP).

Tiger muskellunge (muskellunge Esox masquinongy x northern pike Esox lucius) growth, condition and diet as well as the effect of stocking on northern pikeminnow (Ptychocheilus oregonensis) were studies at Curlew Lake, Washington, from 2001 to 2006. Curelw Lake (373 ha) was stocked with tiger muskellunge beginning in 1998 to reduce an overabundant northern pikeminnow population and to create a unique trophy fishery. Historically, Curlew Lake had provided good fishing opportunities for stocked rainbow trout (Oncorhynchus mykiss) as well as naturally reproducing largemouth bass (Micropterus salmoides) and smallmouth bass (M. dolomieu). The quality of trout fishing, however, had declined throughout the 1990s commensurate with anecedotal observations of increased numbers of northern pikeminnow in the sport catch. To monitor changes in species relative abundanc3e, the lake was sampled annually in the fall with standardized boat electrofishing surveys. Additionally the lake was sampled by boat electrofishing monthly from spring through fall to collect tiger muskellunge diet samples by gastric lavage. Rainbow trout and northern pikeminnow were the most important prey species for tiger muskellunge in Curlew Lake while largemouth bass were a distant third. Diet varied seasonally with rainbow trout being the most important prey during spring while northern pikeminnow was most important in summer. Both rainbow trout and northern pikeminnow were important in the fall. The relative abundance of northern pikeminnow in Curlew Lake significantly declined over the duration of the study. The high proportion of northern pikeminnow observed in the tiger muskellunge diet analysis indicates that the reduction can be attributed to the added presence of tiger muskellunge to the community. Therefore, the goal of northern pikeminnow population reduction through tiger muskellunge introduction (biological control via predation) has been successful. Continues biannual monitoring of the fish community to assess northern pikeminnow abundance should provide the necessary data to refine future tiger muskellunge stocking rates in Curlew Lake.

Changes in spawning habitat of northern pike (Esox lucius) may affect their segregation from and coexistence with the closely related muskellunge (E. masquinongy). We estimated the areal coverage of robust and shallow emergent vegetation in three shared-spawning bays in the Upper St. Lawrence River from aerial photographs taken from 1948 to 2003. Robust emergent vegetation (e.g., cattail) increased in coverage by 155–241% while shallow emergents (sedges) decreased by 46–96%. The loss of sedges, an important northern pike-spawning habitat, may facilitate greater spawning overlap in offshore-submersed aquatic vegetation within bay habitats used by muskellunge. Development rates and characteristics of northern pike and muskellunge eggs and larvae were compared to better understand the implications of greater spawning overlap. Northern pike eggs developed faster than muskellunge eggs at temperatures of 4.7–19°C, and adhesive eggs and the presence of adhesive papillae were present in both species. Equations were used to predict degree-day requirements for hatching and swim-up in three habitats (shallow emergents, bay, and offshore shoal) along a temperature gradient. Northern pike required more estimated degree days to reach hatching in bay and offshore shoal habitat relative to shallow emergent habitat due to cooler temperatures. Significant spawning overlap is known to occur within bay habitats, but poor success of northern pike in deep bay habitats and overall reductions in abundance are hypothesized to currently buffer muskellunge from potential negative interactions between these species.

To better understand why stocked esocids survive poorly, we estimated mortality rates of tiger muskellunge (F1 hybrid of female muskellunge Esox masquinongy x male northern pike E. lucius) that were placed into two Ohio reservoirs (mean fish total lengths, 171 and 179 mm; 62 fish per hectare). Because pond experiments showed that hybrids stocked at night experienced mortality rates as high as those released during the day, we stocked tiger muskellunge into lakes during the day. Mortality of stocked hybrids (estimated by catch per effort of electrofishing) exceeded 95% within 40 days in both lakes. Population estimates of largemouth bass Micropterus salmoides coupled with stomach-content data revealed that these predators accounted for 26% and 45% of the numbers stocked in the two lakes. In addition, some hybrids died from thermal stress. Improved survival of tiger muskellunge should result if they are stocked at lengths greater than 250 mm to reduce predation losses, and late in fall when thermal stress is reduced.