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STUDY AREA
Pool 4 of the Mississippi River is located along the Minnesota and Wisconsin border in Goodhue and Wabasha counties of southeast Minnesota and Pierce, Buffalo, and Pepin counties of western Wisconsin (Figure 1). The MN-WI portion of the Mississippi River is the only water in MN that has a year-round season for all gamefish species. The continuous open season was initiated by Wisconsin in 1967 on its portion of the Mississippi River and Minnesota followed suit in 1969. In 1985 a change in the operating procedure of EXEL’s nuclear power plant allowed warm-water discharge into Pool 3, which has a direct impact on ambient water temperatures in upper Pool 4. The result has been a much longer period of open water below LD 3 during winter months. Consequently, a popular winter open water walleye and sauger fishery has developed below LD 3.
Ten sauger were caught and tagged from depths ranging from 9.1-19.9 m (mean of 13.3 m) in December 2005 and February 2006 (Table 3). The mean length of tagged sauger was 328 mm (SE ± 15.1) and fish ranged from 254 to 409 mm (Table 3). All caught and tagged sauger appeared healthy and had no visual signs of gas bladder overinflation (except for the two fish that were tagged, released, and never regained stability, as described in Methods section). One fish (S74) was never relocated after 24 hours and that fish was excluded from the analysis since it was impossible to determine the fate of that fish (i.e., caught and harvested, tag malfunction, avian mortality, etc).
Tagged sauger generally moved only short distances during the study (overall mean of 70 m), but we were able to detect enough movement to determine whether or not the fish had survived (Table 2; Figure 4; Figure 6). Given that sauger generally concentrate below large river dams in winter, we did not expect to see extensive movement (Pegg et al. 1997; Ickes et al. 1999). Most fish either stayed in the deep hole below the dam or concentrated below wing dams (Figure 6). Based on the movement data we collected, each of the nine remaining tagged sauger survived until the transmitter batteries expired (>21
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days; Table 2). However, the initial mortality we observed during the tagging events (n = 2) indicated a total catch and release mortality rate of 18.2% (2 of the 11). Bettoli et al. (2000) conducted a similar study with similar sized sauger on the Tennessee River and found comparable sauger mortality rates in a 12 d tracking period (Table 3).
Sauger Winter Angling – Net Pen Study
Post release hooking mortality during winter 2005-06 ranged from 12.1-35.1% and averaged 26.7% (Table 3). River discharge during winter 2005-06 was well above normal and net pen location was limited to a 5.5 m slack water area below the island separating the lock and the lift gates, thus substantially increasing sauger transport time to the net pen (Figure 2; Table 3). During the January 2006 angling event, many volunteer anglers participated and we were able to transport fish to the net pen relatively quickly (1-10 min). However, during the March 2006 angling events, fewer volunteer anglers were available and transport time increased substantially (15 min; Table 3). We hypothesized that the increased transport time elevated our mortality estimates during the March angling events; therefore, we repeated three additional angling events in 2006-07 when environmental conditions were more suitable.
Post release hooking mortality during winter 2006-07 ranged from 16.7-32.3% and averaged 26.6% (Table 3). We were able to position the net pen near LD 3 in a slack water area, thus greatly reducing transport time (~ 1.5 min) and providing greater depth availability (8.5 m). Results from winter 2006-07 were nearly identical to those from winter 2005-06. Therefore, we concluded that transport time or depth of net pen did not substantially increase winter sauger catch and release mortality during our study. In addition, Bettoli et al. (2000) held and transported sauger in similar sized coolers for up to 45 min during their study and reported low mortality (Table 3). Combined sauger angling mortality from both winters was 26.6% (57 dead of 214 fish). Because we did not immediately release our fish back into the water, but rather held them in a cooler for 1-15 minutes, our estimates of mortality likely represent the upper limits.
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Only three of the 109 sauger (2.7%) caught in winter 2006-07 suffered from severe gas bladder overinflation and all three fish died. Gas bladder overinflation was evident in 38% of sauger caught on the Tennessee River, but only one of the fish died (Bettoli et al. 2000). We were unsure as to why the frequency of gas bladder overinflation was so much lower and mortality was more common in the Mississippi River, even though fish were caught out of deeper water than that of the Tennessee River study. The results of this study and that of Bettoli et al. (2000) suggest that a series of factors (i.e., water temperature, depth of capture, etc.) regulate gas bladder inflation. Further research examining these relationships using a hyperbolic chamber in the laboratory should be conducted in order to understand the physiology of these processes.
Often, the only time an angler observes physical impairments to deeply caught sauger is when the gas bladder becomes extended through the bucal cavity of the fish. Deflating the gas bladder using a hypodermic needle or “fizzing” has been suggested as a way to improve survival for an array of fish species (Shasteen and Sheehan 1997; Keniry et al. 1996; Lee 1992). Although we did have a few sauger that suffered from gas bladder overinflation, we did not attempt to puncture the air bladder before placing each fish in the net pen. Kerr (2001) suggested that fizzing should be discouraged as significant damage can result from the procedure, particularly if an untrained individual was performing the procedure. According to MNDNR fishing regulations, any fish that is caught and not planned on being harvested must be immediately released, unharmed into the water; therefore any attempt by an angler to deflate the gas bladder would be illegal. The objective of the study was to simulate catch and release fishing as accurately as possible and given that fizzing is not allowed, we did not investigate its effectiveness.
The fate of sauger caught below LD 3 in winter was significantly related to depth of capture (logistic regression; df = 1; P = 0.0006; Table 4). No significant relationship was observed with size of fish (logistic regression; df = 1; P = 0.6237), however, including sauger length in the logistic regression model provided the highest probability level for the goodness-of-fit-test. Therefore, the best model to predict the fate of winter angled sauger was:
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Logit (Y) = 2.6216 + 0.00125(total length) – 0.1365(depth of capture)
For example, using the predictive model and the logit formula, a 305mm sauger caught in the 9.1-11.9 meter depth range would have a 82% chance of survival, whereas the same size fish caught from 18.3- 21.0 meters would have a 57% chance of survival. A 432mm sauger caught from the same depth ranges would have an 85% and 61% chance of living, respectively. Given this model, an angler could predict the probability for released fish survival during winter and make more informed decisions on whether or not to release or harvest the fish.
There were significant differences among the size of sauger caught from shallow ( ≤ 12.0 m) versus deep (> 12.0 m) water (P < 0.05; Table 4). However, the pooled lengths of dead and alive sauger in each depth category were not statistically significant and the size range of dead and alive sauger in each depth range was similar, thus supporting the logistical regression model that stated size was not a significant factor regulating sauger mortality. In addition, the percent angling mortality from sauger caught in the shallowest water (2.4%) was much less than that of the 9.1-11.9 m depth range (21.4%) and size of fish caught was similar, also suggesting that depth of capture was the major contributor to sauger catch and release angling.
Winter creel surveys have been periodically conducted below LD 3 since 1962. Using the six most recent creel surveys, the estimated number of sauger released by anglers from December to March ranged from 11,140 to 86,596 (Stevens 1990; Stevens 1996; Hoxmeier 2002). However, these sauger release estimates apply to the entire open water area in upper Pool 4, which encompasses the main and side-channel habitats (720 ha) and extends to the upper end of Lake Pepin. Although the deep scour hole directly below the dam does attract many anglers, angling pressure occurs throughout the entire open water area. Because the hooking mortality rate we observed was related to depth, we could not accurately extrapolate this rate to estimated number of released sauger from previous creel surveys. Alternatively, we did attempt to extrapolate mortality rates we observed to creel estimates by examining the distribution of depths in the fishable area of Upper Pool 4.
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On a per area basis, only 7.9 ha (1.1%) of the fishable area in winter was 9.1 m or greater and sauger mortality rates were 33% when caught out of those depths. Therefore, if we assume that pressure is directly proportional to area, we would predict about 41-314 dead sauger from ≥ 9.1 m ([0.011 * creel estimate range] * 0.33) and 264-2,055 dead sauger from < 9.1 m ([0.989 * creel estimate range] * 0.024) on an annual basis. It is unlikely that pressure is directly proportional to area, especially when including all depth types. However, if we assume that all fishing occurred in 3.05 m or more, we reduce the fishable area to 231 ha and provide for a more representative estimate. Given this percentage of area
≥ 9.1 m (3.4%), about 383-2,979 sauger die annually from catch and release angling. Even if 10% of the sauger caught and released came from ≥ 9.1 m, annual winter mortality estimates would still be relatively low (608-4,728 fish) when compared to total annual sauger harvest for Pool 4 (36,903-64,430 sauger). Future winter creel survey on upper Pool 4 of the Mississippi River should be designed to incorporate a depth of released fish question in order to accurately estimate catch and release mortality.
Data from the annual Large Lake program does not indicate any decline in walleye or sauger populations since the inception of the continuous open-water fishing season (Hoxmeier 2005). Catch curve analysis from age 2-6 sauger gill netted in Lake Pepin, annual mortality rates range from 26% to 82% and average about 51% (unpublished data). Sauger annual mortality in Pools 11 and 13 of the Mississippi River commonly exceed 80% and more restrictive regulations have been established during the winter angling season to reduce mortality and increase size structure (Kirk Hansen, personal communication). Based on sauger annual mortality estimates and historical gill netting data, we believe that the winter sauger mortality estimates in Pool 4 of the Mississippi River do not substantially influence the entire population. However, anglers can reduce sauger winter angling mortality by limiting the amount of time fished in depths greater than 9.1 m. In addition, anglers that are interested in harvesting quality fish ( ≥ 300mm) would want to avoid fishing deeper ( ≥ 9.1 m) water.
