Mallard Harvest Distributions in the Mississippi and Central Flyways

Transcription

1 Management and Conservation Article Mallard Harvest Distributions in the Mississippi and Central Flyways ADAM W. GREEN, 1 Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701, USA DAVID G. KREMENTZ, United States Geological Survey, Arkansas Cooperative Fish and Wildlife Research Unit, University of Arkansas, Fayetteville, AR 72701, USA ABSTRACT The mallard (Anas platyrhynchos) is the most harvested duck in North America. A topic of debate among hunters, especially those in Arkansas, USA, is whether wintering distributions of mallards have changed in recent years. We examined distributions of mallards in the Mississippi (MF) and Central Flyways during hunting seasons to determine if and why harvest distributions changed. We used Geographic Information Systems to analyze spatial distributions of band recoveries and harvest estimated using data from the United States Fish and Wildlife Service Parts Collection Survey. Mean latitudes of band recoveries and harvest estimates showed no significant trends across the study period. Despite slight increases in band recoveries and harvest on the peripheries of kernel density estimates, most harvest occurred in eastern Arkansas and northwestern Mississippi, USA, in all years. We found no evidence for changes in the harvest distributions of mallards. We believe that the late 1990s were years of exceptionally high harvest in the lower MF and that slight shifts northward since 2000 reflect a return to harvest distributions similar to those of the early 1980s. Our results provide biologists with possible explanations to hunter concerns of fewer mallards available for harvest. (JOURNAL OF WILDLIFE MANAGEMENT 72(6): ; 2008) DOI: / KEY WORDS Anas platyrhynchos, band recovery, Central Flyway, harvest, hunter satisfaction, mallard, Mississippi Flyway, wing receipt. The mallard (Anas platyrhynchos) is widely distributed throughout the world and has been extensively studied in both North America and Europe (Bellrose 1980). Because of its wide range and abundance, the mallard is the most harvested waterfowl in North America at approximately 3.5 million/year (Anderson and Henny 1972). Arkansas (USA) is the leading state for mallard harvest with an average 500, ,000 harvested/year (U.S. Fish and Wildlife Service [USFWS] 2003). A topic of debate among waterfowl hunters in the lower Mississippi Flyway (MF), especially Arkansas, has been whether winter distributions of mallards have recently shifted from traditional wintering grounds in the Lower Mississippi Valley (i.e., AR, LA, MS [USA]) to more northerly states such as Missouri and Illinois, USA (R. A. James, Arkansas Game and Fish Commission [AGFC], personal communication). Harvests during the late 1990s ( ) in the MF and Central Flyways (CF) were much higher than any seen since 1961 when USFWS began the Waterfowl Parts Collection Survey (PCS; wing receipts). However, hunter complaints have increased in recent years as harvest has declined (National Flyway Council and Wildlife Management Institute 2006). Despite increases in complaints, hunters in the lower MF harvested more mallards/year during than throughout much of the 1980s and 1990s. Many hunters and biologists attribute recent declines in mallard harvest in the lower MF to a phenomenon known as shortstopping (R. A. James, personal communication). Shortstopping is most often associated with anthropogenic changes in habitat, including new man-made water areas and a large amount of available waste grain (Yancey 1976). 1 agreen@usgs.gov Shortstopping may evolve as a sequential process as follows: 1) waterfowl delay traditional migrations, 2) a segment of the population begins to winter farther north than traditionally, and 3) birds truncate migration to areas north of traditional wintering grounds. Several studies have suggested that, within the MF, mallards follow the same migration corridors from year to year but environmental variables influence the final latitude of wintering grounds (Lensink 1964, Bellrose and Crompton 1970, Nichols et al. 1983). With increases in habitat and warmer and later winters, it is important to distinguish between annual variations in the distribution of waterfowl among wintering areas versus permanent or semi-permanent changes in distributions due to shortstopping (Gale 1976). We used band recoveries and PCS data to examine changes in mallard harvest distributions in the MF and CF from 1980 to Our objective was to determine whether harvest distributions of mallards have shifted northward since 2000 from those during the 1980s and 1990s and, if we found changes in distributions, to relate them to changes in environmental variables such as habitat, precipitation, and temperature. METHODS Data Sets We obtained band recovery records from the United States Geological Survey (USGS) Bird Banding Laboratory (BBL). We used both direct (recovered during the first hunting season after banding) and indirect band recoveries (recovered during any hunting season following the first hunting season after banding). We restricted recoveries to those of normal, wild mallards (i.e., released in the same 10- min block as captured and held 24 hr) banded during the preseason (Jul Sep) and shot or found dead during the 1328 The Journal of Wildlife Management 72(6)

2 hunting seasons (Sep Feb), , in the MF and CF, including the United States and Canada (N ¼ 238,295). We did not restrict band recoveries to those from particular banding reference areas (BRA; see Anderson and Henny 1972) because we were only interested in the location of recovery and not the derivation of harvest. We also obtained PCS records for mallards shot in the MF and CF during hunting seasons, (K. D. Richkus, USFWS, personal communication; N ¼ 418,521). We pooled data across age and sex cohorts for both data sets and removed recoveries with unknown regions of recovery (i.e., unknown latitude or longitude coordinates). Potential problems in using band recoveries to draw inferences about waterfowl distributions include differences in hunting pressure and reporting rates (the proportion of banded birds taken by hunters and returned to the BBL; Hickey 1951, Crissey 1955). Anderson and Henny (1972) suggested that estimates of band-reporting rate at the state level are too inaccurate and that differences in reporting rates between flyways are too small to affect any results. Henny and Burnham (1976) found no difference in reporting rates for the MF and CF and for bands recovered.80 km from the banding site. In a more recent study, Nichols et al. (1995) did not observe differences in reporting rates between the northern and southern harvest areas within both the MF and CF. However, Nichols et al. (1995) found evidence for sex-specific reporting rates within the CF with rates higher for males than females. Rates did not differ among sexes in the MF. Nichols et al. (1995) suggested using sex-specific reporting rates for the CF but stated that one could argue for using estimates based on models without sex-specificity for the CF. There is also weak evidence for geographic or temporal variation in reporting rates of a similar species, the American black duck (Anas rubripes; Conroy and Blandin 1984). Because we examined band recoveries over a long time period (i.e., 24 hunting seasons) it would be difficult to obtain reporting rates for flyways during each year, much less for flyway subunits (e.g., northern, southern, central) or age- or sexspecific rates. Conroy and Blandin (1984) also caution against the use of reporting rates from several studies because of large standard errors associated with estimates and possible biases due to differing designs and assumptions. We did not adjust band recoveries for reporting rates because of the lack of available data for such a long time period, and of evidence for geographically and temporally consistent reporting rates from several studies throughout the 1970s, 1980s, and 1990s, though we do acknowledge that reporting rates may vary slightly over the study period (Anderson and Henny 1972; Henny and Burnham 1976; Nichols et al. 1991, 1995). Another possible source of information on historical mallard distributions are the midwinter waterfowl inventories (MWI). The MWI were originally designed to provide an index to waterfowl numbers and population trends and to describe winter distribution and habitat use (Smith et al. 1989). However, Eggeman and Johnson (1989) point out several problems with the MWI. Most surveys were designed to count waterfowl at traditional locations of waterfowl concentrations, which assumes that the same proportion of waterfowl wintering within a region use the same areas year after year and does not incorporate a random sample design. Any changes in distribution could potentially affect the proportion of birds available for counting and, therefore, bias the counts. Furthermore, several states change the areas surveyed based on changes in waterfowl distributions or behaviors. For example, Maryland (USA) surveyed more inland agricultural areas in response to increasing goose and swan populations. Most states use fixed-wing aircraft to conduct surveys but some also use helicopters or automobiles and boats. It is likely that each method has a different detectability rate and a detailed study would be necessary to correct for these differences. Eggeman and Johnson (1989) suggest that, due to large unmeasured error, MWI should not be used to compare duck numbers among states or years and Heusemann (1999) goes so far as to suggest eliminating the MWI, at least in the Atlantic Flyway. Statistical Analyses Large sample sizes increase sensitivity of statistical tests so seemingly small differences between distributions become significantly different. Because of large distances migrated by mallards from the breeding grounds in Canada to wintering grounds in the lower United States, small differences between distributions, though statistically significant, may not be biologically significant to the ducks or hunters. Arkansas is the central location of most mallard harvest in the United States with Stuttgart, Arkansas, at the center of this distribution (Munro and Kimball 1982). Stuttgart and the northern border of Arkansas lie at approximately and latitude, respectively. We arbitrarily chose a northward shift of 28 latitude (the distance between Stuttgart and the northern border of AR) as a cutoff for determining biologically significant effects of changes in mallard distributions on harvest availability because we believed that a 28 shift would be sufficient to influence hunter satisfaction. The BBL records band recoveries to the nearest 10-minute latitude longitude block and hunters in the PCS sample were asked to report to the county level where each bird was harvested. We calculated the mean latitude of all recoveries within each year using the coordinates of the southeast corner of each 10-minute block, as recorded in the BBL database for band recoveries, and the mean latitude of the centroids (geographical center) of each county weighted by the mallard harvest estimates. Weights varied by county, hunter, and the sampling effort within a region (P. Padding, USFWS, personal communication). We then regressed mean latitude against year for each data set to determine any overall trends (JMP 6; SAS Institute, Inc., Cary, NC). Harvest in the MF and CF from 1998 to 2000 was much higher than in any other period from 1980 to 2003 and a large decline occurred from 2001 to From information gathered from hunter communication with state agencies, Green and Krementz Mallard Harvest Distributions 1329

3 Table 1. Mean latitudes (8) and their deviation from the long-term average of mallard band recoveries (Band D) and harvest (Harvest D) estimated using Waterfowl Parts Collection Survey data within the Mississippi and Central Flyways (USA) during the hunting seasons (Sep Feb), We used the years as a long-term average and compared to the mean latitudes for Exceptionally high harvests occurred in , with a subsequent decline starting in No year had a mean latitude that deviated.28 from the long-term average. Yr Bands Band D Harvest Harvest D Figure 1. Mean latitudes in degrees N (695% CI) for band recoveries (dashed line) and harvest estimated using Waterfowl Parts Collection Survey data (solid line) for mallards within the Mississippi and Central Flyways (USA) during the hunting seasons (Sep Feb), we considered as high hunter satisfaction years and as low-satisfaction years (R. A. James, personal communication; R. Helm, Louisiana Department of Wildlife and Fisheries, personal communication). We used all individual band recoveries and wing receipts from 1980 to 1997 to calculate a long-term average and compared latitudes for each year during against the longterm average. Other studies have examined the distribution of band recoveries of mallards banded within certain BRA using Mardia s test (Mardia 1967, Munro and Kimball 1982, Nichols et al. 1983, Nichols and Hines 1987). The nonparametric test of Mardia is used to test the hypothesis that 2 bivariate distributions differ. Mardia s test is sensitive to small differences between distributions, and with such large sample sizes, significant differences between distributions may not be biologically relevant (Mardia and Spurr 1973). In addition, all between-year comparisons result in 276 possible tests and, therefore, a high experiment-wise error rate. Even when we used a Bonferonni-corrected alpha value of , we found most (approx. 75%) between-year comparisons to be significant. After attempting to quantify differences between band-recovery and harvest distributions, we decided that Mardia s test was not appropriate. Therefore, we used qualitative methods and visually examined the centroids for any outliers or clumping. We used ArcGIS 9.1 to query, analyze, and map band returns and wing receipts. We then calculated 50% and 95% kernel density estimates (KDE) for band recoveries using fixed-kernel estimation, which assumes a random sample and independence of points (Worton 1989), and visually examined the location of 50% kernels. We used least-squares cross validation (LSCV) to determine smoothing factors in the fixed-kernel estimation procedure because LSCV is less biased and performs better than other methods, especially with sample sizes.50 (Seaman and Powell 1996, Seaman et al. 1999). Kernel methods are also less sensitive to autocorrelation within the data than are other home-range estimators (Swihart and Slade 1997, de Solla et al. 1999). Because of the weights associated with the wing receipts, we could not use KDE to analyze the core areas of harvest. Instead, we created a density map of harvest and visually examined the areas of highest harvest. To determine the relative change in harvest among states, we calculated the percent of total band recoveries and estimated harvest by state for each year. RESULTS Mean latitudes of band recoveries ranged from in 2000 to in 1982 (x ¼ , SE¼ 0.118) and harvest ranged from in 2000 to in 2003 (x ¼ , SE ¼ 0.118; Fig. 1). Mean latitudes for band recoveries (R 2 ¼ 0.007, y ¼ 0.007x þ 51.60, df ¼ 23, P ¼ 0.692) and harvest (R 2 ¼ 0.040, y ¼ 0.02x þ 69.92, df ¼ 23, P ¼ 0.351) did not show a trend across time. The long-term mean latitude ( ) was (95% CL ) for band recoveries and (95% CL ) for harvest (Table 1). Comparison of mean latitudes of recoveries to the long-term average resulted in latitudes during high-satisfaction years centered farther south and latitudes during low-satisfaction years centered farther north than the long-term average, yet none deviated.1.28 from the long-term average. Centroids for bands and harvest during were similar to those during the early and mid-1980s, and the years had centroids located much farther south (i.e., ) than all other years (Fig. 2). Visual examination did not show appreciable change in the size and distribution of band recovery KDEs from 1980 to 2000, and core areas (i.e., 50% KDE) for all years included the lower MF, particularly eastern Arkansas and extreme northwest Mississippi (Fig. 3). During low-satisfaction 1330 The Journal of Wildlife Management 72(6)

4 Figure 2. Centroids (mean latitude and longitude) of (A) band recoveries and (B) harvest estimated using Waterfowl Parts Collection Survey data for mallards in the Mississippi and Central Flyways (USA) during the hunting seasons (Sep Dec), years, the core extended farther north and included most of the Bootheel of Missouri and extreme western Kentucky (USA) but still included the same area as in (i.e., eastern AR and northwestern MS). Despite small changes in core areas, 95% KDEs during expanded to the west and northeast. Density maps of estimated harvest showed patterns similar to band recoveries (Fig. 4). Magnitude of harvest changed from year to year but harvest distributions were consistent across the study period. Kernel-density estimates and density maps for individual years are available on the Arkansas Cooperative Fish and Wildlife Research Unit (2008) webpage. Arkansas hunters averaged nearly twice the percentage of band recoveries/year than hunters from any other state during the study period (x ¼ 18.1%, SE ¼ 0.8%) and reported more bands than any other state in every year (Fig. 5). Louisiana (x ¼ 8.3%, SE¼ 0.5%) and Illinois hunters (x ¼ 7.3%, SE ¼ 0.2%) had the second- and third-highest mean percentages of band recoveries. Arkansas hunters also averaged the highest mallard harvest/year (x ¼ 485,843.1; SE ¼ 54,532.4) followed by Louisiana (x ¼ 227,285.4; SE ¼ 42,344.5) and Minnesota hunters (x ¼ 216,690.4; SE ¼ 25,547.7; Fig. 6). We also observed an increase in proportion of the harvest taken within Arkansas (r 2 ¼ 0.316, y ¼ 0.003x 6.77, df ¼ 23, P ¼ 0.004). We investigated the relationship between the mallard breeding population (BPOP) and estimated harvest to explain some of the variation in harvest within the MF and CF. Changes in BPOP explained much of the variation in total harvest within the MF and CF (r 2 ¼ 0.74, y ¼ 0.50x 889,021.7, df ¼ 23, P, 0.001) as well as within the lower MF (r 2 ¼ 0.83, y ¼ 0.25x 817,548.0, df ¼ 23, P, 0.001). DISCUSSION Our results agreed with those of previous studies throughout the last 40 years on mallard harvest and winter distributions in showing small annual variations but no permanent or semi-permanent shifts in distributions. Examinations of winter band recoveries have also shown annual variations but no permanent changes in distributions (Nichols et al. 1983, Nichols and Hines 1987). Because we found no changes in mallard distributions, we did not examine effects of climate on those distributions. Previous studies contain conflicting results as to whether mallards return to the same wintering areas each year. Several studies examining band returns suggest that mallards return to the area in which they spend their first winter (Munro 1943, Boyd and Ogilvie 1961, Crissey 1965). Others suggest that mallards follow the same migration corridors from year to year but wintering grounds are influenced by environmental factors; this is known as flexible homing behavior (Lensink 1964, Bellrose and Crompton 1970). Nichols et al. (1983) examined winter distributions of mallards in wet versus dry years and warm versus cold years and supported the flexible homing behavior hypothesis finding that mallards tended to winter farther south in both wetter and colder years. Pulliainen (1963) and Nilsson (1976) observed similar patterns of habitat use by European mallards on wintering grounds. Band recoveries and harvest showed no consistent shifts northward throughout the study period. Contrary to the shortstopping hypothesis, mean latitudes of band recoveries within the MF have shown an overall southward shift since 1960 ( J. A. Dubovsky, USFWS, personal communication). Otis (2004) reported a large southward shift during the late Figure 3. Kernel density estimates of mallard band recoveries in the Mississippi and Central Flyways (USA) during the September February hunting seasons: (a) , (b) , and (c) Green and Krementz Mallard Harvest Distributions 1331

5 Figure 4. Density maps of mallard harvest using Waterfowl Parts Collection Survey data in the Mississippi and Central Flyways (USA) during the September February hunting seasons: (a) , (b) , and (c) s and found disproportional increases in harvest throughout the MF, particularly the lower MF, between the periods and Despite large declines in harvest in the lower MF beginning in 2001, harvests during were still greater than harvests throughout much of the 1980s and early 1990s. It appears that hunter satisfaction has declined because of high harvests during the late 1990s. Approximately 31% of waterfowl hunters under the age of 45 began hunting between 1997 and 2004 (National Flyway Council and Wildlife Management Institute [NFC and WMI] 2006) and, therefore, most of their hunting experiences have been during times of high duck abundances. In addition, hunter satisfaction has declined throughout the MF, not only in the southern states. In the middle and upper MF 55% and 66% of hunters, respectively, stated that hunting had gotten a little or much worse in the last 5 years (NFC and WMI 2006). Decreases in the mallard BPOP suggest that there are fewer mallards available throughout the flyway and not that birds within the lower MF are wintering farther north (USFWS 2006). Increases in habitat may also affect the number of ducks killed per hunter and, therefore, be perceived as a decline in the number of ducks available for harvest and may provide a source of hunter dissatisfaction. During dry years, potential waterfowl habitat is limited to those areas that are able to retain water for longer periods. Waterfowl concentrate on the remaining water and hunter success increases. During wet years, waterfowl can disperse away from hunters because of the increase in available habitat (Briggs and Holmes 1988). As waterfowl disperse, hunter success decreases and hunters may perceive this as a decline in the number of ducks available for harvest. It does not appear that any large-scale or permanent shifts have occurred in mallard harvest within the MF and CF. Rather than shortstopping, recent declines in harvest within the lower MF are more likely due to declines in the BPOP. The mallard BPOP has fluctuated over time, ranging from a low of 5 million to a high of slightly.11 million since 1955 and is highly dependent on the breeding ground conditions Figure 5. Proportion of Mississippi and Central Flyway mallard band recoveries by year from Arkansas and Louisiana (USA) during the hunting seasons (Sep Feb), Arkansas hunters recovered a higher percentage of bands than hunters in any other state during all 24 years and Louisiana hunters recovered a higher percentage than those any other state, except Arkansas, in all but 7 years. Figure 6. Mean proportion of mallard harvest/year by state (6SE) within the Mississippi and Central Flyways (USA) during the hunting seasons (Sep Feb), We estimated harvest using Waterfowl Parts Collection Survey Data The Journal of Wildlife Management 72(6)

6 Figure 7. Estimated continental mallard breeding population (BPOP; U.S. Fish and Wildlife Service 2006) and harvest within the lower Mississippi Flyway (i.e., AL, AR, LA, MS, and TN; USA) during the years (USFWS 2006). During years with large numbers of ponds in the Prairie Pothole Region, mallards exhibit higher productivity and during years with fewer ponds, mallards overfly the Prairie Potholes, breed in the boreal forest, and exhibit lower productivity (Pospahala et al. 1974). High harvests during the late 1990s coincided with estimated BPOPs of nearly 11 million, numbers not observed since 1958 (Fig. 7; USFWS 2006). The population then started to decline beginning in 2001, the same year hunter complaints began to increase. It is more probable that recent declines in harvest are due to declines in mallard abundance or shortterm changes in distributions related to extant environmental conditions rather than a large-scale shift in only 3 4 years. MANAGEMENT IMPLICATIONS Based on wing receipts and band recoveries, we found little evidence of a northward shift in winter distributions of mallards in either the Mississippi or Central Flyways. If our results are accurate, then waterfowl biologists should offer concerned hunters alternative explanations to perceived declines in wintering mallard numbers. ACKNOWLEDGMENTS R. A. James, J. E. Hines, P. I. Padding, and J. M. Wilson were instrumental in developing the project and support along the way. We thank S. J. Beaupre, J. D. Cothren, L. W. Naylor, and M. C. Runge for comments on earlier versions of the manuscript. We are grateful to AGFC and the USGS Arkansas Cooperative Fish and Wildlife Research Unit for financial support. LITERATURE CITED Anderson, D. R., and C. J. Henny Population ecology of the mallard: I. A review of previous studies and the distribution and migration from breeding areas. U.S. Fish and Wildlife Resource Publication 105, Washington, D.C., USA. 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