Long-lived organisms are difficult subjects for demographic studies because they may outlive the life span of a researcher (Reed et al. 2002). Although such studies are not frequently undertaken, they are important because the successful conservation of long-lived organisms depends on long-term life history research (Congdon et al. 1993, 1994; Congdon and Gibbons 1996; Congdon and Dunham 1997; Wheeler et al. 2003).

The alligator snapping turtle (Macrochelys temminckii) is an example of a long-lived organism that has been reduced throughout much of its range by commercial harvest (Sloan and Lovich 1995; Reed et al. 2002; Moll and Moll 2004; Pritchard 2006; Ernst and Lovich 2009). Age at first reproduction may not be achieved until at least 11–13 yrs of age (Dobie 1971) and perhaps not until 16–17 yrs of age (Tucker and Sloan 1997). Further limitations to population recovery are that females of this species may not reproduce every year (Dobie 1971; Ernst and Lovich 2009).

Riedle et al. (2005) compared presence/absence data of alligator snapping turtles in Oklahoma in 1997–2000 to the distribution based on county records from 1970. Alligator snapping turtles were trapped in only 5 counties out of the 13 counties that had historical records of the species, indicating a possibly diminishing Oklahoma distribution. While Riedle et al. (2005) demonstrated a possible shrinking of the distribution of the alligator snapping turtle in Oklahoma, a relative abundance comparison of alligator snapping turtles between 2 periods in time at specific sites remains to be explored.

In 1993, the Missouri Department of Conservation (MDC) conducted a survey of 211 trap nights that captured 21 alligator snapping turtles at 4 different sites and found 3 additional turtles at the trap sites (Santhuff 1993). MDC continued surveying the population in 1994, trapping for 275 trap nights and capturing 37 new alligator snapping turtles (plus 2 additional recaptured turtles) at 2 additional trap sites (Shipman and Riedle 1994).

To better understand current status of the species in Missouri for future management decisions, the objectives of our study were 1) to assess if the relative abundance of alligator snapping turtles at 6 sites where alligator snapping turtles had previously been captured in Missouri had changed since 1993–1994 and 2) to determine if there have been changes in the population structure of alligator snapping turtles in southeast Missouri between 1993–1994 and 2009. For objective 1, we predicted that we would trap more turtles per trap night at each site in 2009 than were trapped in 1993–1994. This increase in catch per unit effort (CPUE) was expected due to the protected status of the alligator snapping turtle in Missouri and its protected status in Arkansas, which implemented protection in 1993 (most sites in this study are bodies of water shared between Missouri and Arkansas). For objective 2, we predicted that the population structure would include a greater number of both large adult and small juvenile turtles than it did in 1993–1994 due to protected status in Missouri and Arkansas.

Our ultimate purpose in this study was to better understand the current status of this species with the hope of informing future management decisions that might influence the future of these turtle populations. To meet the 2 objectives outlined above, we retrapped the 6 sites that had been trapped in 1993–1994. By comparing the distributions of the population of alligator snapping turtles in the early 1990s to that of 2009, we expect to learn about shifts in the population that may impact the conservation of this species.

METHODS

RESULTS

Relative Abundance

During 1993–1994 surveys, a total of 60 alligator snapping turtles were trapped, with 2 being recaptures at the same sites. During the summer of 2009, we trapped a total of 43 alligator snapping turtles at 6 sites; of these, 3 were recaptures and were not included in the analysis. Fewer alligator snapping turtles were caught per trap night at all 6 sites in the 2009 survey than in the 1993–1994 surveys (z  =  −2.201, n  =  6, p  =  0.028; Fig. 1) despite the fact that our study used more trap nights.

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Population Structure

There was a significant difference in the size class distributions between alligator snapping turtles caught in southeastern Missouri in 1993–1994 and those caught in 2009 (2-sample Kolmogorov-Smirnov test: z  =  1.885, n₁  =  41, n₂  =  61, p  =  0.002; Fig. 2). The 1993–1994 distribution was normally distributed (1-sample Kolmogorov-Smirnov test: z  =  0.572, n  =  61, p  =  0.899), while the 2009 distribution exhibited a skew toward individuals with a smaller carapace length (i.e., it was not normally distributed; 1-sample Kolmogorov-Smirnov test: z  =  1.443, n  =  41, p  =  0.031).

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The CPUE of adult male alligator snapping turtles in 2009 surveys was significantly less than in 1993–1994 surveys (Wilcoxon signed-ranks test: z  =  −2.023, n  =  6, p  =  0.043; Fig. 3). The CPUE of adult female alligator snapping turtles also was significantly less in 2009 surveys than in 1993–1994 surveys (Wilcoxon signed-ranks test: z  =  −2.032, n  =  6, p  =  0.042; Fig. 3). However, CPUE of juvenile alligator snapping turtles did not differ significantly between the 2 sampling periods (Wilcoxon signed-ranks test: z  =  −0.134, n  =  6, p  =  0.893; Fig. 3).

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DISCUSSION

Fifteen years separated the 1993–1994 survey from the 2009 survey. Our study found significant changes in the relative abundance and population structure of alligator snapping turtles in southeastern Missouri between 2 sampling periods (1993–1994 and 2009). Our findings of a decreased catch per unit effort at each site indicate that these populations may be in decline. Declines in alligator snapping turtle populations have been reported by trapping surveys in other regions of their range. Jensen and Birkhead (2003) compared CPUE of alligator snapping turtles in Georgia watersheds between 1988–1989 and 1997–2001. They found potential population declines based on a decrease in CPUE of alligator snapping turtles compared to previous surveys within Georgia watersheds. In a study in Kansas, Shipman (1993) trapped 600 trap nights and captured no alligator snapping turtles, although previous surveys in Kansas had yielded alligator snapping turtles. An alligator snapping turtle population in Oklahoma was sampled from 1997–2001 and 2010–2011 and has declined over this time period (Ligon et al. 2012). The causes of these declines in numbers of alligator snapping turtles remain to be determined.

In addition to a decline in the relative abundance of alligator snapping turtles in these watersheds in Missouri, the shift in population structure toward smaller nonreproductive individuals may impact alligator snapping turtle populations in the future. One potential explanation of the shift in population structure may be an increase in the recruitment of alligator snapping turtles either by higher nest survivorship or increased survival of juveniles. This explanation would account for the trend toward greater juvenile CPUE in 2009 (0.05 juveniles/trap night) than in 1993–1994 (0.02 juveniles/trap night). However, the increase in number of juvenile alligator snapping turtles was not statistically significant.

The reduced number of adult male and female alligator snapping turtles is of particular concern to their successful preservation in Missouri streams. The survival of adult alligator snapping turtles is presumed to be high in the absence of anthropogenic pressures (Reed et al. 2002; Ernst and Lovich 2009). Using models based on real life history data of eastern and alligator snapping turtles, Reed et al. (2002) demonstrated that the stability of alligator snapping turtle populations depends on the high survivorship of the adults and that lowering survival from 98% to 96% will result in population decreases of 50% in less than 50 yrs. Increased juvenile recruitment is frequently not enough to sustain the population stability of turtles in the event of increased adult mortality (Spotila and Paladino 2004), and the loss of reproductive adults in these organisms is not quickly replaced (Brooks et al. 1991). Juvenile turtles do not reproductively contribute to the population, and a shift to nonreproductive turtles could lead to further decreases in abundance in the future (Brooks et al. 1991).

The causes of the decline of adult alligator snapping turtles in Missouri streams remain to be explored. However, indications of declines in adult turtles have occurred in other regions of the range of the alligator snapping turtle. For example, Riedle et al. (2008) documented a similar lack of large adult turtles in eastern Oklahoma, relative to that reported by historical records. In Louisiana, population structure of alligator snapping turtles exhibited a skew toward immature turtles relative to populations in Arkansas and Florida (Boundy and Kennedy 2006). In a statewide survey, Wagner et al. (1996) reported a lack of sexually mature alligator snapping turtles in Arkansas counties where historical harvest had taken place. A subsequent study in northeastern Arkansas found a paucity of large adult alligator snapping turtles that the researchers attributed to historic harvest (Trauth et al. 1998). Jensen and Birkhead (2003) reported trapping fewer 45–90-kg alligator snapping turtles in the Flint River system of Georgia compared to what a commercial trapper from the area reportedly caught in the 1970s. Many researchers (Reed et al. 2002; Pritchard 2006; Ernst and Lovich 2009) believe that such declines would not take place naturally because, other than humans, there are few natural predators of large adult alligator snapping turtles. In general, the findings in Missouri and other nearby states would seem to forecast a potentially bleak future for alligator snapping turtles unless additional steps are taken to reverse the current trends.

Although the causes of a decline in abundance and changes in population structure are not known with certainty, it is possible that some or a combination of factors may have contributed. For example, illegal harvesting by humans within these areas may account for a reduction in adult turtles. Legal commercial fishing traps do inflict mortality on other turtle species in these waterways (Lescher, pers. obs.) and may also be causing mortality on alligator snapping turtles. Habitat fragmentation and habitat conversion, including agricultural development, channelization, and damming of streams, are present throughout southeastern Missouri and have likely impacted turtle populations in this region.

We acknowledge that this study is representative of only 2 sampling periods (1993–1994 and 2009). It is also possible that stochastic events may have influenced the decreased catch per unit effort of alligator snapping turtles. Nonetheless, the broad consistency of the results of the 6 sites surveyed within this study and the consistency of our results with those of similar studies in other nearby regions strongly indicate that populations of alligator snapping turtles in southeastern Missouri may be under increased negative pressures and need to be monitored more closely. This is necessary in order to determine if declines are indeed occurring and what the causes and results of the apparent sex bias in the populations may be. Indeed, such studies may well be critical to the successful management and conservation of alligator snapping turtle populations.