Muskellunge scale interpretation: The questions of aging accuracy

Age and growth data extracted from scales are often used when managing fish populations. However, accurate interpretation of scale annuli in certain species, such as muskellunge Esox masquinongy , has been suspect. The accuracy of age determination was tested with three experienced esocid scale readers who interpreted positive photo images made from acetate slide impressions projected on a microfiche reader. Images were made from scales of 25 known‐age muskellunge (3–10 years old) from five Wisconsin lakes. Each scale image was interpreted twice; once with fish length, sex, and date of capture given and once without any information. Of 149 scale interpretations, 32% were assessed with the correct age, 54% underaged, and 14% overaged. The average interpretation deviated from the known age by 1.0 years. Accuracy did not improve when additional fish information was available. General confidence in the interpreter’s age assessments was low and possibly related to quality of the photo images. Interpreters were accustomed to using acetate images with several impressions rather than a single photo image. As a follow‐up, we selected 12 acetate slides used to make the photo images and allowed each interpreter to determine age from the slides. The percentage correctly aged with the subsample improved from 24% to 47% with the acetate slide impressions. Increasing the magnification of acetate slides from 23× to 35× magnification did not affect accuracy. Image quality and multiple impressions can improve accuracy of age determination; however, accuracy was still less than 50%. The low accuracy when interpreting scale annuli of muskellunge 3–10 years of age suggests that managers need to use caution when evaluating data taken from muskellunge scales.

Muskellunge growth potential in northern Wisconsin: Implications for trophy management

The growth potential of muskellunge (Esox masquinongy) was evaluated by back‐calculating growth histories from cleithra removed from 305 fish collected during 1995–2011 to determine whether it was consistent with trophy management goals in northern Wisconsin. Female muskellunge had a larger mean asymptotic length (49.8 in) than did males (43.4 in). Minimum ultimate size of female muskellunge (45.0 in) equaled the 45.0‐in minimum length limit, but was less than the 50.0‐in minimum length limit used on Wisconsin’s trophy waters, while the minimum ultimate size of male muskellunge (34.0 in) was less than the statewide minimum length limit. Minimum reproductive sizes for both sexes were less than Wisconsin’s trophy minimum length limits. Mean growth potential of female muskellunge in northern Wisconsin appears to be sufficient for meeting trophy management objectives and angler expectations. Muskellunge in northern Wisconsin had similar growth potential to those in Ontario populations, but lower growth potential than Minnesota’s populations, perhaps because of genetic and environmental differences.

Validity of age estimates from muskellunge (Esox masquinongy) fin rays and associated effects on estimates of growth

Accurate age estimates are critical for understanding life histories of fishes and developing management strategies for fish populations. However, validation of age estimates requires known-age fish, which are often lacking. We used known-age (ages 1–25) muskellunge (Esox masquinongy) to determine the precision and accuracy of age estimates from fin rays. We also determined whether fin location (anal or pelvic), fin ray number, and preparation methods affected accuracy and precision. Lastly, we determined whether von Bertalanffy growth parameters estimated from fin ray ages were similar to parameters estimated from known ages. Precision and accuracy of age estimates from anal and pelvic rays were similar and estimates were relatively precise (coefficient of variation = 8.5%) and accurate (mean absolute difference from known age = 0.85 years) for ages 4–15, but ages were overestimated for younger fish and underestimated for older fish. Growth models based on estimated age were similar to models based on known age. Anal and pelvic rays offer a nonlethal alternative for age estimation of muskellunge ages 4–15 and for producing reliable estimates of growth.

Comparison of food consumption, growth and metabolism among muskellunge: An investigation of population differentiation

We conducted laboratory evaluations of food consumption, growth, and metabolic rate as functions of water temperature (5–27.5°C) to examine how the young of year in six populations of muskellunge Esox masquinongy (Kentucky, Minnesota, New York, Ohio, St. Lawrence River, and Wisconsin) from three drainages might perform under various thermal regimes. Relative food consumption (g·g−1·d−1) and growth (g·g−1·d−1) were similar among populations at lower temperatures (5 and 10°C, but at higher temperatures (15–27.5°C) fish from Wisconsin and Ohio had higher consumption and faster growth rates than fish from Kentucky and the St. Lawrence River. Metabolic rates increased with temperature from 0.08 mg O2·g−1·h−1 at 5°C to 0.25 mg O2·g−1·h−1 at 25°C, but few differences in metabolic rates were observed among populations at any temperature. Although we found bioenergetic differences among muskellunge from these populations, they could not be explained solely in terms of thermal adaptation or previously defined genetic groupings. Energetic differences among age‐0 muskellunge have important implications for conserving existing esocid populations and managing introduced populations.

Growth and food consumption by tiger muskellunge: Effects of temperature and ration level on bioenergetic model predictions

We measured growth of age‐0 tiger muskellunge as a function of ration size (25, 50, 75, and 100% Cmax) and water temperature (7.5–25°C) and compared experimental results with those predicted from a bioenergetic model. Discrepancies between actual and predicted values varied appreciably with water temperature and growth rate. On average, model output overestimated winter consumption rates at 10 and 7.5°C by 113 to 328%, respectively, whereas model predictions in summer and autumn (20–25°C) were in better agreement with actual values (4 to 58%). We postulate that variation in model performance was related to seasonal changes in esocid metabolic rate, which were not accounted for in the bioenergetic model. Moreover, accuracy of model output varied with feeding and growth rate of tiger muskellunge. The model performed poorly for fish fed low rations compared with estimates based on fish fed ad libitum rations and was attributed, in part, to the influence of growth rate on the accuracy of bioenergetic predictions. Based on modeling simulations, we found that errors associated with bioenergetic parameters had more influence on model output when growth rate was low, which is consistent with our observations. In addition, reduced conversion efficiency at high ration levels may contribute to variable model performance, thereby implying that waste losses should be modeled as a function of ration size for esocids. Our findings support earlier field tests of the esocid bioenergetic model and indicate that food consumption is generally overestimated by the model, particularly in winter months and for fish exhibiting low feeding and growth rates.

Growth and ultimate length of muskellunge from Ontario waterbodies

Growth of muskellunge Esox masquinongy from 12 Ontario sources was investigated by examining 582 samples from the Cleithrum Project archive and other specific studies; 88% of the samples were from angler‐caught “trophy” fish. We detail sampling problems and develop methods for resolving them. Muskellunge from some sources were unsexed; sex was discriminated (probability of correct classification, 98.3%) from the von Bertalanffy growth parameters ultimate length (L∞) and growth coefficient, K. When one sex was inadequately sampled, the von Bertalanffy growth parameters of one sex were used to estimate those of the other. When samples were small and inadequate (<11), we used concordance sum of squares to match growth and give an interim estimate from the adequately sampled source with the best growth match. In Ontario populations, mean ultimate total lengths range widely: from 81.4 to 140.0 cm for females and from 70.7 to 115.9 cm for males. Females can be grouped into three types of growth, producing either large‐, medium‐, or small‐bodied fish (ranging from 140 to 127 cm, 126 to 114 cm, and 113 to 102 cm and smaller, respectively). We describe and categorize growth and growth potential to establish standards for detecting change in exploitation and for reviewing minimum size limits (currently underway) based on growth biology to help sustain and even increase the size of muskellunge populations while producing high‐quality trophy fisheries.

The cleithrum project: An update to 1995

The cleithrum project is an opportunity for individuals to make a significant contribution to the increase in our knowledge of the muskellunge. We need more and continued contributions, particularly cleithra from the largest, oldest individuals which have the greatest amount of life history information. The greatest problem is the obvious one – unlike scales, cleithra cannot be taken from fish that are to be returned alive to the water. The next step we contemplate is an attempt to increase participation from other regions. We would like to encourage contributions from Québec by translating the information package into French and distributing it in that province. There has been a concern in the past about expanding publicity when we had not dedicated technical assistance for fear of receiving a number of cleithral bones which would have to wait some time for analysis and reply.

The potential utility of pelvic fin rays as ageing structures was evaluated for southern US muskellunge Esox masquinongy mm. This evaluation indicated that pelvic fin rays may prove to be a useful, non‐lethal method for ageing muskellunge in southern US waters. Validation studies are still needed to ensure that growth rings form consistently throughout fish’s life span. 14 ± 2 SE) estimated absolute error between observed and predicted length changes for 13 tagged muskellunge was 30 Mitchill populations by comparing age estimates among three readers and against cleithrum estimates, and by comparing observed length changes of tagged fish with changes predicted from growth equations based on pelvic fin ray age estimates. Mean coefficient of variation in age estimates among all readers and between the two readers with prior ageing experience was 17.8% and 5.6%, respectively. Exact and within 1‐year agreement rates between pelvic fin rays and cleithra were 76% and 100%, respectively. Mean (±

Comparative growth, survival and vulnerability to angling of northern pike, muskellunge and hybrid tiger muskellunge in a small lake

Northern pike, muskellunge, and tiger muskellunge fingerlings were stocked in equal numbers for 3 consecutive years in Daggett Lake (14 acres). After 3 years, mean lengths of age-II esocids were as follows: pike, 21.0 inches; tiger muskellunge, 22.9 inches; muskellunge, 28.0 inches. Survival of the three year classes of pike was unusually high (45 to 79%), tiger muskellunge survival was intermediate (9 to 28%), and muskellunge survival was extremely low (0 to 0.4%). It is suggested that the poor survival of muskellunge fingerlings was more likely the result of an inherited intolerance for some environmental factor, rather than the result of differential predation by larger esocids. The total standing crop of esocids in this study was 64.0 pounds per acre; it was 57.8 pounds per acre in a previous study when only pike were stocked in Daggett Lake. From this similarity, it is judged that, in the present study, high survival of pike compensated for low survival of muskellunge, to produce a standing crop of esocids that closely approached carrying capacity for Daggett Lake. In 131.5 man hours of fishing in Daggett Lake, anglers caught 25.1% of the stocked pike and 9.4% of the tiger muskellunge. If it is assumed that all esocids were equally available, it follows that the pike were 2.7 times more vulnerable to angling than were the tiger muskellunge.

Evidence for cogradient growth and survival between two geographically distinct stocks of muskellunge

Intraspecific genetic variation across the distribution of muskellunge Esox masquinongy suggests the existence of divergent stocks among major river drainages. Often, stocks differentiate in response to latitudinal variation in thermal regime. Genetic variation and thermal adaptation may drive fish to either maximize growth during brief growing seasons at higher latitudes (counter‐gradient growth) or experience longer periods of slower growth at lower latitudes (cogradient growth). The strength and direction of these genetic effects matter for fish stocked outside of their native drainage and for populations experiencing changes to regional thermal regimes. We used a replicated pond experiment with uniform initial fish size and environmental conditions to compare the survival and growth of stocked age‐0 Ohio River drainage (OH ) Muskellunge with those of the more northerly upper Mississippi River drainage (MISS ) stock following their first winter and a year after introduction into a common environment. Both stocks had similarly high winter survival, though the MISS stock had a slower growth rate overwinter than the OH stock. Survival during the latter spring‐to‐fall period was greatly reduced for the MISS stock, however, with the differences in growth rate persisting. These findings suggest that cogradient adaptation to temperatures experienced during early life stages in native environments results in a disadvantage for high‐latitude stocks of muskellunge in thermal regimes warmer than those they have historically experienced in their native drainages.