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Canvasback Mortality from Illegal Hunting on the Upper Mississippi River


Canvasback population

Surveys from aircraft and boats were used to estimate the number of canvasbacks that used Navigation Pool 7 during the 1991 and 1992 waterfowl hunting seasons. We counted canvasbacks separately in areas open and areas closed to waterfowl hunting.

Number of hunters

The number of hunters in our study area were estimated from vehicle counts and boat observations. Vehicles were counted at boat landings and at other points around Lake Onalaska where hunters could access the study area. Vehicles at boat landings near the Lake Onalaska study area were counted on 6 weekend-days and on 5 weekdays during the 17-day hunting period in 1991. All observations were terminated during a severe winter storm on 1 November. In 1992, vehicles were counted on 7 weekdays and 7-weekend days.

An elevated observation blind was used to count boats that entered the northeast portion of Lake Onalaska each day throughout each season. Observers occupied the blind from approximately 45 minutes after sunrise to approximately 20 minutes before sunset and logged information about each boat with hunters that entered or exited the study area. They recorded the time of observation, number of hunters, boat description and registration number, and direction of travel.

Observers compared their records of boats entering and exiting the study area by boat description and registration number. Boats that exited the study area before 1500 hours without a recorded entry time were assumed to have begun hunting at the legal opening time (sunrise in 1991, 0.5 hour before sunrise in 1992). Boats that entered the study area after 1200 hours without recorded return times were assumed to have completed hunting at sunset. For calculations of hunting durations, we assumed hunting began 15 minutes after a boat entered and ended 15 minutes before a boat exited the study area.

Information about hunters

We contacted hunters in the field and at boat landings throughout the 1991-1992 hunting seasons. We collected information on the number of hunters/party, number of vehicles/party, number of shots fired, number and species of harvested or crippled waterfowl, and hunters' knowledge of the local concerns over canvasback food resources on the river and canvasback mortality from hunting.

Observations by law-enforcement personnel

Actions of waterfowlers in the field were monitored by concealed law-enforcement personnel throughout the 1991-1992 hunting seasons. For each hunting party observed, law-enforcement officers recorded each encounter of the hunting party with a duck, goose, American coot (Fulica americana), or common snipe (Gallinago gallinago), time of each encounter, species encountered, flock size, number of shots fired, and the results. An encounter was defined as 1 or more birds that flew within shooting distance of a hunting party. Shooting distance varied and was based on the action of the individual party. For some parties, shooting distance may have been ≥90 m; for others the maximum distance was ≤ 45 m. Results were tallied as passed, missed, downed and retrieved, downed and not retrieved, or hit but kept flying.

Attempts by hunters to shoot canvasbacks

A ratio estimator (Cochran 1977) was used to compute the total number of attempts to shoot canvasbacks. In 1991, observations were divided into 4 strata based on day of the hunting season and time of day. The number of attempts to shoot canvasbacks in a given stratum I (YRi ) was estimated by:

GIF -- Formula to determine the number of attempts to shoot canvasbacks in a given stratum

where yi = total attempts to shoot canvasbacks by parties under observation in stratum i, xi = total hours parties were observed hunting in stratum i, and Hi = total hours all parties hunted in stratum i.

The variance of the ratio estimator (YR ) was estimated by:

GIF -- Formula for variance of the ratio estimator

where yj = total attempts to shoot canvasbacks by party j, xj = total hours party j hunted, H = total hours all parties hunted in strata, n = total parties under observation within strata, and z = the average length of observation for each hunting party. This variance estimate accounts for the portion of the hunting trip observed by the law-enforcement officers.

GIF -- Picture of law-enforcement personnel
Law-enforcement personnel monitored the actions of waterfowl hunters in the field. Photo by J.M. Nissen, 1990.

Assumptions for this model include: the total number of hunting parties and the hours that all parties hunted in stratum i are known; sampling (hunter observation) was conducted at random throughout strata; and observed hunting parties were selected at random. We estimated the total number of hunting parties and hours hunted within a given stratum and used conservative values in our analysis. With these assumptions and preliminary data from 1990 in mind, we developed the following guidelines for scheduling law-enforcement observations in 1991 and 1992:

Hunting effort (total number of hunting parties and hours hunted by all parties) was determined from boat observations and vehicle counts. The average hunt length used to calculate the total number of hours hunted from vehicle count data was based on hunter survey data.

To reduce variability in the model, 1991 data were divided and analyzed by 2 daily time periods (morning and afternoon) and by 2 segments of the hunting season (16 - 20 October; 21 October - 1 Nov), yielding 4 strata. The season division was based largely on canvasback encounter rates because few canvasbacks were encountered during the first season segment (although 10,000 canvasbacks were present on 15 Oct). A noticeable increase in the canvasback encounter rate occurred on 21 October. In 1992, we observed consistent canvasback encounter rates, and no clear seasonal division existed. As a result, 1992 data were analyzed by only 2 daily periods (2 strata).

T-tests using the Satterwaite approximation for degrees of freedom (SAS, PROC TTEST, SAS Inst., Inc. 1989) were used to compare canvasback flock size and encounter-, attempt-, and downed-rates between years. We rejected statistical hypotheses with P≤0.05.

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