Predicting Wood Duck Harvest Rates in Eastern North America

Journal of Wildlife anagement 74(7):1575 1579; 2010; DOI: 10.2193/2009-289 anagement and Conservation Note Predicting Wood Duck Harvest Rates in Eastern North America GREGORY D. BALKCO, 1 Georgia Department of Natural Resources, Wildlife Resources Division, Game anagement Section, Fort Valley, GA 31030, USA PAELA R. GARRETTSON, United States Fish and Wildlife Service, Division of igratory Bird anagement, Laurel, D 20708-4016, USA PAUL I. PADDING, United States Fish and Wildlife Service, Division of igratory Bird anagement, Laurel, D 20708-4038, USA ABSTRACT We developed a method for predicting wood duck (Aix sponsa) harvest rates in eastern North America using waterfowl banding and recovery data, annual indices of hunter numbers, and harvest survey data from the United States and Canada. We predicted that under the current season length (60 days), if hunter numbers remain unchanged, increasing the wood duck bag limit from 2 to 3 would increase harvest of adult male wood ducks in the Atlantic and ississippi flyways by 12.3%, causing an increase in harvest rate of 7.1% from 0.087 to 0.093. The Flyway Councils and the United States Fish and Wildlife Service can consider this information to predict the impacts of regulatory changes. KEY WORDS Aix sponsa, bag limit, harvest rate, multiple regression, regulations, season length, wood duck. Wood ducks (Aix sponsa) are an important waterfowl game species in the Atlantic and ississippi flyways. During the 2007 hunting season in the United States, wood ducks were the Atlantic Flyway s second most harvested species behind the mallard (Anas platyrhynchos) and composed roughly 20% of the flyway s total duck harvest; in the ississippi Flyway, wood ducks were the fifth most harvested duck species and composed roughly 10% of total harvest (Richkus et al. 2008). Wood duck hunting seasons closed in the United States from 1918 through 1940. Hunting seasons reopened in 1941 with a daily bag limit of 1, and in 1962 the daily bag limit was increased to 2 (Bellrose and Holm 1994). Since then, the eastern North American wood duck population appears to have grown steadily (Bellrose and Holm 1994), leading to increasing interest in exploring the feasibility of more liberal hunting regulations for wood ducks. Setting duck hunting regulations in the United States is a cooperative effort by the United States Fish and Wildlife Service (USFWS), representing the federal government, and Flyway Councils that represent state governments (Blohm et al. 2006). Harvest regulations on many waterfowl species are set and evaluated by estimating the current breeding population and habitat condition and then making predictions about the next year s breeding population using known estimates of vital rates such as annual survival, harvest, and reproduction (Nichols et al. 1995a, Williams and Johnson 1995). This method has been used for mid-continent mallards (Runge et al. 2002), eastern mallards (Johnson et al. 2000, 2002), canvasbacks (Aythya valisineria; Silverman 2007), northern pintails (Anas acuta; USFWS 2007), scaup (Aythya marila and A. affinis; Koons et al. 2006, Boomer and Johnson 2007), and western mallards (Johnson et al. 2007). However, attempts to apply these types of population models to wood ducks were unsuccessful, primarily because precise breeding population estimates are not available for wood ducks because they are difficult to count aerially due to 1 E-mail: greg.balkcom@dnr.state.ga.us their use of forested habitats (Bellrose and Holm 1994, Wilkins 2000, Garrettson and Smith 2004). Recently, Garrettson (2007) adapted the Potential Biological Removal (PBR) method (Wade 1998) to estimate allowable harvest rates for wood ducks in eastern North America. This method was originally developed for management of marine mammals and is based on wellestablished ecological and harvest theory (Runge et al. 2004). Potential Biological Removal is usually described in terms of allowable kill, but if a population estimate is not available, as is the case for wood ducks, it can be described in terms of maximum allowable kill rate: k max ~ 1 2 r maxf R where k max is maximum allowable kill rate, r max is intrinsic growth rate of the population at low population density, and F R is a recovery factor that for game species can represent a management objective. The wood duck PBR analysis estimated maximum allowable harvest rate, and the result indicated that wood ducks could tolerate higher harvest rates than are currently realized under liberal (i.e., 60-day) season lengths (Garrettson 2007). However, information about how to set hunting regulations to achieve desired wood duck harvest rates was lacking. Our objective was to predict how changes in season length and daily bag limit would affect harvest rate. ETHODS We estimated annual band-recovery rates for adult male wood ducks in the Atlantic and ississippi flyways from 1971 to 2004. Recent reward band studies found that reporting probability is the same for wood ducks and mallards in eastern North America (P. R. Garrettson, USFWS, unpublished data). Assuming that relationship has not changed over time, P. R. Garrettson (unpublished data) used mallard reporting probability estimates from Henny and Burnham (1976), Nichols et al. (1991, 1995b), and Royle and Garrettson (2005) to develop year-specific Balkcom et al. N Wood Duck Harvest Rates 1575

reporting-probability estimates for wood ducks. We adjusted the annual recovery rates with those reporting-probability estimates to estimate annual wood duck harvest rates. In addition to hunting regulations, the number of people actively hunting ducks in any given year affects duck harvest rates that year; thus, our effort to predict harvest rates had to account for a hunter number effect. Pre- and post-1999 annual estimates of active duck hunters in the eastern United States are not directly comparable, but the annual count of federal duck stamps sold offers a useful surrogate measure of hunter numbers because each adult (.16 yr old) waterfowl hunter in the United States is required to purchase a federal duck stamp. We regressed annual harvest rates of adult male wood ducks against hunting season length (in days) and number of federal duck stamps sold annually in the Atlantic and ississippi flyways (Padding and Klimstra 2008). With the exception of special early seasons in a few states, the daily wood duck bag limit in the United States remained constant at 2 between 1971 and 2004. We modeled harvest rates of adult male wood ducks because estimates are more precise and harvest rates more stable than those for other cohorts (Garrettson 2007). We used multiple linear regression analysis to predict wood duck harvest rates under a 2-bird bag limit as a function of season length in days and hunter numbers, separately and simultaneously (artin and Carney 1977). We conducted statistical analyses using Program R (version 2.6.2,,www. r-project.org., accessed 12 ay 2010). We converted wood duck harvest rates using the arcsine-square root transformation prior to analysis (Sokal and Rohlf 1981). To predict changes in total harvest expected due to changes in the wood duck daily bag limit, we obtained daily bag composition data from 2 surveys conducted annually in North America: Canada s Species Composition Survey and the United States Waterfowl Parts Survey. Both surveys ask samples of waterfowl hunters to submit a wing from every duck they shoot during the entire hunting season, thereby providing species-specific daily duck-harvest records (Cooch et al. 1978, Padding et al. 2006). Daily bag composition data allowed us to estimate the proportion of the total wood duck harvest that was taken as the first, second, third, fourth, fifth, or sixth bird in the bag. We used bag composition data from Eastern Canada (1990 2006), where the bag limit was 6(n 5 12,479 daily bags); the Atlantic (n 5 33,623) and ississippi (n 5 38,333) flyways during regular hunting seasons from 1990 2006, when the daily wood duck bag limit was 2; and special October bag limit liberalizations (n 5 576) in North Carolina and Virginia, USA, when the wood duck bag limit was allowed to equal the daily duck bag of 4 in 1986 and 1987 and 3 in 1988. The latter seasons were limited in scope, lasting only 3 or 4 days per year, but they represented the only experience we had with bag limits.2 during regular duck seasons in the United States. We restricted each data set to include only those daily bags in which 1 wood duck had been harvested, and we counted the total number of bags that included 1, 2, 3, 4, 5, or 6 wood ducks. We assumed that hunters who harvested n wood ducks under a given bag limit n would still have L harvested n 2 1 wood ducks if the bag limit were lowered to n 2 1. Under this assumption, we calculated the total (t n ) and cumulative wood duck harvest (c n ) represented by the nth bird in the bag for bags where n 5 1 to 6. Expected proportional increase in harvest when the wood duck bag limit changes from n to n + 1 is then simply (c n+1 /c n ) 2 1 (artin and Carney 1977). We repeated this calculation for all 4 data sets and finally calculated an average of the 4 data sets, weighted by the sample size (total daily bags) in each. Using harvest estimates from the USFWS Branch of Harvest Surveys (K. Richkus, USFWS, unpublished data), we used linear regression to examine the relationship between actual harvest and harvest rate of adult male wood ducks in the Atlantic and ississippi flyways from 1971 to 2004. We generated an equation to predict proportional changes in harvest rate based on proportional changes in actual harvest from one year to the next (t to t + 1). We generated expected harvest rates using a 3-step process: 1) we predicted a baseline harvest rate based on season length and hunter number, 2) we predicted the expected proportional change in harvest due to bag limit changes, and 3) we determined the proportional change in harvest rate based on the proportional change in actual harvest. RESULTS Between 1971 and 2004, annual harvest rates for adult male wood ducks in eastern North America ranged from 0.040 to 0.114. During the same time period, hunter numbers in the Atlantic and ississippi flyways combined, as indexed by duck stamp sales, ranged from 850,725 to 1,504,507, with the following averages by decade: 1970s 5 1,324,336; 1980s 5 1,042,919; 1990s 5 951,691; and 2000s 5 1,023,153. Duck season length ranged from 30 days to 60 days in 10- day increments, except for a 45-day season in 1973. Hunter numbers and season length affected adult male wood duck harvest rates in eastern North America (r 2 31 5 0.701, P 0.001). Including both season length and hunter 2 numbers provided a better predictor than season length (r 31 5 0.506, P 0.001) or hunter numbers (r 2 31 5 0.529, P 0.001) individually. As expected, harvest rates increased when seasons were longer and hunter numbers were higher. In eastern Canada, cumulative wood duck harvest increased at a declining rate as bag sizes increased (Table 1). Expected harvest increased by 39.8% for a bag limit increase from 1 to 2, by 11.9% for an increase from 2 to 3, and by 4.7%, 1.8%, and 0.5% for incremental increases to 4, 5, and 6 birds per day, respectively. Data from regular hunting seasons in the Atlantic and ississippi flyways indicated that expected wood duck harvest would be 44.9% and 39.8% higher, respectively, under a 2-bird bag than under a 1-bird bag. Wood duck bag data collected during the October liberalizations in North Carolina and Virginia indicated an expected 65.6% increase in harvest when the bag size increased from 1 to 2, a 21.2% increase with a bag size increase from 2 to 3, and a 10.2% increase when the bag size increased from 3 to 4 birds per day. A weighted average of the 4 harvest data sources produced expected harvest increases of 42.0%, 12.3%, 4.9%, 1.8%, and 0.5% with each 1576 The Journal of Wildlife anagement N 74(7)

Table 1. Wood duck bag size data from eastern Canada under a bag limit of 6, which we used to predict changes in harvest with changes in bag limit, 1990 2006. Harvest (H) due to the nth bird in the bag, where n = Wood ducks harvested/bag No. of bags 1 2 3 4 5 6 1 7,510 7,510 2 2,895 2,895 2,895 3 1,164 1,164 1,164 1,164 4 552 552 552 552 552 5 254 254 254 254 254 254 6 104 104 104 104 104 104 104 Total harvest (t n ) due to nth bird: 12,479 4,969 2,074 910 358 104 Cumulative harvest (c n ), due to nth bird: 12,479 17,448 19,522 20,432 20,790 20,894 Increase if bag limit changed to n + 1: 0.398 0.119 0.047 0.018 0.005 incremental increase in wood duck bag limit from 1 to 6 (Table 2). Although actual harvest (H) may not be an accurate predictor of harvest rate (h; r 2 31 5 0.138, P 5 0.031), proportional changes in actual harvest (%DH) were correlated with proportional changes in harvest rate (%Dh; r 2 31 5 0.603, P 0.001; Fig. 1), yielding the following predictive equation: %Dh 5 0.6981 3 %DH 2 0.0150. Using this predictive equation, proportional harvest increases due to each incremental increase in wood duck bag limit from 1 to 6 would yield proportional harvest rate increases of 27.8%, 7.1%, 1.9%, 20.2%, and 21.1%. Because we did not force the regression line to intercept at zero, harvest rate increases become negative as increases in actual harvest approached zero. Expected harvest rates for adult male wood ducks under various season lengths and bag limits ranged from 0.045 to 0.093 (Table 3). We calculated variances of estimates in one of 2 ways: we transformed variance estimates using the delta method (Powell 2007) or we calculated variances of products using the method described by Goodman (1960). Our predicted harvest rates assumed no change in hunter numbers from the recent (2000 2004) average. Under a 60-day season and a 3-bird bag limit, we predicted a harvest rate of 0.093, representing a 7.1% increase over the predicted baseline harvest rate of 0.087 with a 2-bird bag limit. DISCUSSION As expected, harvest rates of adult male wood ducks increased with longer seasons and greater hunter numbers; those 2 factors combined explained 70% of variation in harvest rates. Thus, our analysis for wood ducks indicates that hunter numbers may be a valuable additional parameter to include in modeling harvest of other waterfowl species. Overall number of United States duck hunters has declined since the 1970s, and their inclusion in our model greatly improved our ability to predict harvest rates. For example, as of 2007, wood duck harvest rates were lower than during the shorter seasons of the 1970s, an outcome that would not be predicted by season length alone. However, hunter numbers tended to rise with longer season lengths, so individual effects of these variables can be confounded. Separate analyses suggested that wood ducks could tolerate higher harvest rates than they were experiencing under a 60- day season and a 2-bird bag limit (Garrettson 2007). Through the Adaptive Harvest anagement process, duck hunting season length is determined by the population status of eastern mallards in the Atlantic Flyway and of midcontinent mallards in the ississippi and Central flyways (USFWS 2008). Thus, bag limit changes are the only option for independently increasing or decreasing harvest pressure on wood ducks. Table 2. Expected percentage of change in wood duck harvest based on weighted average of bag size data from 4 sources: Eastern Canada 1990 2006 (n 5 12,479), the Atlantic Flyway 1990 2006 (n 5 33,623), the ississippi Flyway 1990 2006 (n 5 38,333), and the Atlantic Flyway during October liberalizations in 1986 1988 (n 5 576). Bag change CWS a AF b Data source F c AF lib d Average wt 1 to 2 39.8 44.9 39.8 65.6 42.0 2 to 3 11.9 21.2 12.3 3 to 4 4.7 10.2 4.9 4 to 5 1.8 1.8 5 to 6 0.5 0.5 a CWS 5 Canadian Wildlife Service. b AF 5 Atlantic Flyway. c F 5 ississippi Flyway. d AF Lib 5 Atlantic Flyway during the liberalized bag limit experiment. Figure 1. Proportional changes in harvest and harvest rate of adult male wood ducks from one year to the next (t to t + 1) between 1971 and 2004 in the eastern United States. Balkcom et al. N Wood Duck Harvest Rates 1577

Table 3. Predicted harvest rates of adult male wood ducks in eastern North America under various combinations of season length and bag limit based on 1971 2004 harvest rates, 1990 2006 bag composition data, and 2000 2004 average number of hunters. Bag limit Season length 1 2 3 (days) x SE x SE x SE 30 0.045 0.0111 0.057 0.0038 0.061 0.0067 40 0.052 0.0116 0.066 0.0025 0.071 0.0064 50 0.060 0.0124 0.076 0.0025 0.081 0.0068 60 0.068 0.0136 0.087 0.0044 0.093 0.0081 Species-specific duck harvest is often subject to influence by availability and desirability of other species that may be harvested, especially if species-specific bag limits are lower than the total daily duck limit. A higher proportion of bags collected during the 1986 1988 October wood duck liberalization seasons in North Carolina and Virginia contained only wood ducks compared to bags collected during regular duck seasons in the United States or Canada, likely because those seasons were scheduled to provide wood duck harvest opportunity at times when other species were less likely to be present. We suspect this is why data from October liberalizations produced higher expected increases in harvest with increasing bag limits than did other data sources. Although expected harvest increases based on October liberalizations may be biased high, we were reluctant to exclude them because they provided the only United States data on wood duck bag limits.2; we felt that using a weighted average based on all data sources was a reasonable compromise. However, even data from regular wood duck seasons gave expected harvest increases higher than those of several other species. For a bag limit increase from 2 to 3, expected harvest increases derived for mallards and green-winged teal (Anas crecca) were about 8% (P. I. Padding, USFWS, unpublished data) and about 3% for mottled ducks (Anas fulvigula; Wilkins 2007). Our predicted 12.3% increase in harvest is similar to the 13% increase found for scaup during preliminary data analysis for a review of scaup population status (Allen et al. 1999; G. T. Allen, USFWS, personal communication). We suspect that species-specific biology and distribution as well as hunter behavior account for differences in predicted harvest increases among species. anagement Implications Knowing and understanding the impacts of regulations on the harvest of wood ducks is necessary prior to implementing regulatory changes. Season length has been the primary regulatory option for managing wood ducks because bag limits have been stable at 2 since 1962. Season length affected harvest by allowing hunters additional days to hunt, but it also appeared that hunter numbers were higher in years with longer seasons. Because there was little history of wood duck bag limits.2 in the United States, we used bag composition data from the Canadian Wildlife Service to determine the impacts of bag limit on wood duck harvest. By including bag composition data and modeling a combination of season length, hunter number, and bag limit, we were able to predict that increasing the bag limit on wood ducks from 2 to 3 in the eastern United States should not exceed the population s allowable harvest. Acknowledgments We thank D. Holm, Illinois Department of Natural Resources, for his input and support during data analysis;. Gendron, Canadian Wildlife Service, for providing bag check data from eastern Canada; and K. Richkus, USFWS, for providing bag check data from the United States. 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