The alligator snapping turtle (Macrochelys temminckii) is America's largest freshwater turtle (to 80 cm carapace length and over 120 kg) and occurs in the Gulf of Mexico watershed from northern peninsular Florida to eastern Texas and inland in the Mississippi–Ohio River drainage to Missouri and Indiana (Pritchard 1989; Lovich 1993). As a food species, the alligator snapper has been the object of significant harvest throughout much of its range, with recent, large-scale trapping/butchering industries operating in the mid-South (Dobie 1971; Sloan and Lovich 1995; Louisiana Department of Wildlife and Fisheries [LDWF] license data). Anecdotal observations of declines during the past 30 years (Pritchard 1989; US Fish and Wildlife Service [USFWS] 2000) have alerted government agencies and private organizations to the need to evaluate harvest pressure and population status of alligator snappers. The alligator snapper now receives full legal protection in many range states and, as of a December 2004 commercial ban in Louisiana, cannot be commercially harvested from the wild anywhere in its range. Concern about harvest pressure resulted in the initiation of status surveys for alligator snappers in several states, although there is a lack of baseline population data for comparison (see Discussion). The LDWF initiated surveys in 1984 that were completed in northeastern Louisiana in 1988 (Douglas 1989). The USFWS listed the alligator snapper as an Endangered Species Act candidate in 1991 (USFWS 1991), which enabled funding for the present survey. Status surveys in southern Louisiana were begun in 1996, and data from these are compared with survey data from other regions. We also provide an update on the status of the alligator snapper meat industry in southeastern Louisiana.

METHODS

Turtles were trapped at 33 sites in eastern Louisiana between April 1996 and November 1997 (Fig. 1). Traps consisted of single-throated hoop nets made from 4 hoops 3.5 feet in diameter and with 1-inch mesh netting. Traps were submerged to within 4–6 inches of their tops, were set during afternoons or evenings, and were checked the following mornings. Traps were baited with frozen menhaden (Brevoortia), and 20–30 (usually 25) traps were set each night. Trapped turtles were weighed to the nearest pound and measured to the nearest half inch (straight, midline, minimum carapace length) with aluminum tree calipers. All turtles under a 13-inch carapace length (CL) were categorized as immature based on Dobie (1971), and no attempt was made to sex them. Sex of turtles over a 13-inch CL was determined from differences in the position of the cloacal aperture relative to the posterior carapace margin and tail base (Dobie 1971). Weights and lengths were converted to metric units prior to statistical analyses. All turtles were tagged with 1-inch-diameter, numbered, stainless-steel tags attached with stainless-steel wire through a hole drilled in the rear of the carapace.

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Table 1. Trap rate, size, and population structure by site for alligator snapping turtles in southeastern Louisiana.^a

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Water bodies were classified as 1) first- and second-order streams, 2) third-order streams, 3) fourth-order rivers, 4) sloughs and canals with imperceptible directional flow, and 5) lacustrine habitats. Each site was ranked for recent harvest pressure as 1) little or no trapping and/or boat traffic, 2) moderate trapping, or 3) heavy fishing/trapping use. Harvest ranks were based on firsthand knowledge (C.K.), which included over 10 years of trapping experience in the region, as well as comparison of trapping information with other fishermen. We defined harvest pressure to include direct trapping with hoop nets or probable incidental take via catfish traps or trotlines. A moderate harvest rank was assigned to sites in which harvest use was intermediate and was used to separate the high and low harvest ranks to eliminate subjectivity. The intermediate sites were not included in statistical comparisons of harvest effects. Relative population densities of turtles were evaluated by comparing numbers of turtles caught per trap per night (= turtles/trap-night [TTN]).

Percentage of males among adults, percentage of juveniles among all turtles, and TTN were compared after values for each were arcsine transformed and evaluated for normality with a Kruskall–Wallis 1-sided test.

RESULTS

Two hundred alligator snappers were trapped during 3504 trap-nights (mean 0.057 TTN). Turtles were trapped at every site except Ponchatoula Creek, and trap rate ranged from 0 to 0.143 TTN (Table 1). Trap rate varied significantly between sites (F = 3.516, p < 0.0001 at 33, 101 df). Over half the traps at Ruddock Bayou were tampered with on the first night, so trapping there was stopped, and data from that night are not used in the following analyses. Trap rate by harvest pressure differed significantly (F = 4.490, p = 0.037 at 1, 100 df): 0.073 TTN with no harvest pressure, 0.049 TTN with heavy harvest. Trap rate did not differ between water types (F = 0.969, p = 0.427 at 4, 131 df). Significantly more turtles were trapped in fall (September, October, November) than spring or summer: 0.094 TTN vs. 0.057 and 0.044 TTN, respectively (F = 5.869, p = 0.004 at 2, 127 df).

The weight/CL relationship was highly correlated (r = 0.939, p < 0.0001; log CL = 0.400 log weight + 2.673). Male and female weight/length relations were identical, but males grew to a larger size than females (Fig. 2). Thus, adult males averaged significantly larger than females: male weight averaged 21.3 kg (n = 48, maximum 51.4 kg), and females averaged 13.4 kg (n = 56, maximum 27.7 kg). Adult male CL averaged 47.2 cm (maximum 62.2 cm), and females averaged 39.6 cm (maximum 53.3 cm). The weight/CL relationship covaried between low and high harvest sites (Fig. 3): log weight = 2.345 log CL – 2.670 vs. log weight = 2.490 log CL – 2.881, respectively (F = 0.096 at 1, 164 df; p = 0.757, multiple R² = 0.981, 0.983, respectively).

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Turtle weight did not vary by site (F = 0.962, p = 0.532 at 32, 167 df) or by season (F = 0.731, p = 0.483 at 2, 197 df). Turtle weight varied by harvest pressure (F = 4.148, p = 0.043 at 1, 166 df): low harvest mean = 8.85 ± 8.96 kg (n = 84); high harvest mean = 11.99 ± 10.89 kg (n = 84). Turtle weight varied by water type (F = 2.459, p = 0.047 at 4, 195 df), with turtles from lacustrine sites and sloughs/canals weighing significantly more (mean = 12.38 ± 11.11 kg, n = 77) than those from riverine sites (mean = 8.19 ± 8.36 kg, n = 123).

The adult male:female ratio (48 vs. 56) did not vary significantly from 1:1 (χ² = 0.48). Percentage of adult males (thus, sex ratio) did not vary by harvest level (F = 0.220, p = 0.803 at 2, 78 df), water type (F = 0.121, p = 0.975 at 4, 76 df), or season (F = 0.170, p = 0.844). Immature turtles (under 33 cm CL) made up 48% of all turtles trapped. No turtles under 12.5 cm CL or less than 0.75 kg were trapped. Percentage of immature turtles did not vary by water type (F = 2.158, p = 0.080 at 4, 91 df), season (F = 0.451, p = 0.638 at 2, 89 df), or harvest level (F = 1.616, p = 0.208 at 1, 73 df).

Average number of turtles trapped per 25 traps, set on consecutive nights in the same spot, remained somewhat constant through the sixth night rather than showing a marked decrease (Fig. 4): combined trap rate for the first 2 nights was an average of 1.67 ± 1.32 (n = 72) turtles, for the third and fourth nights 1.26 ± 1.34 (n = 47), and for the fifth and sixth nights 1.29 ± 1.54 (n = 14), followed by a sharp decrease on the seventh night. No turtles were trapped more than once.

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One tagged turtle was captured by a fisherman on a fishing line on 19 July 1998 on the Abita River, approximately 1 mile above its confluence with the Bogue Falaya River. It had been initially tagged at Ruddock Bayou on 9 April 1997 and weighed 24 pounds when released and 28 pounds when recaptured. Minimally, during 15 months, the turtle would have entered Lake Pontchartrain from Ruddock Bayou, moved about 20 miles along the lake, entered the Tchefuncte River, and traveled about 10 miles upstream to the point of recapture (travel from site 12 to site 15 in Fig. 1). No other recaptured tagged turtles had been reported as of late 2004.

DISCUSSION

We noted a relative uniformity in population structure based on a lack of differences in sex ratio and percentage of immature turtles regardless of harvest and environmental variables. The trap rate differences may be an effect of seasonal differences rather than differences in harvest pressure. Trap rate did not vary between spring trapping in low and high harvest sites and fall trapping in high harvest sites (mean = 0.066, 0.064, and 0.060 TTN, respectively). However, trap rate averaged 0.128 TTN during fall at low harvest sites. Differences in trap rate at Abita River (0.120 vs. 0.225 TTN) and Tchefuncte River (0.044 vs. 0.200 TTN) in spring and fall, respectively, support the idea that trap rate differences were seasonally biased. More fall trapping was done at low harvest than high harvest sites, which may have accounted for the significant difference between harvest types. The difference in trap rate between seasons may have been related to increased feeding prior to a period of winter inactivity and satiation and decrease in movements during hot weather. Also, trapping in summer may have been less successful because of the frequent afternoon rains that caused rapid changes in water levels, temperature, and turbidity. Other potential factors affecting seasonal trapability (e.g., water temperature, prey availability, nesting movements, photoperiod, etc.) were not evaluated. Conversely, although weight did not vary seasonally, it varied between harvest regimes. However, weight also varied between water types. The effects of water type vs. harvest level on turtle weight could not be evaluated because all low harvest sites were riverine and 12 of 16 high harvest sites were lacustrine and slough/canal sites. The effect that harvest pressure has had on turtle populations in the sites we surveyed could not be deduced because of the confounding effects of season and water type.

Few baseline data are available with which to compare current with past trap rates. Cagle and Chaney (1950) caught alligator snappers at 4 of 12 sites trapped in Louisiana in 1947. No alligator snappers were trapped from ditches at Lutcher, the only eastern Louisiana site, in an estimated 34 trap-nights. Alligator snappers were trapped for an estimated rate of 0.063 TTN in Caddo Lake and Lake Iatt and a rate of 0.75 TTN in Tensas Bayou. The latter rate could reflect historic densities in riverine habitats in the Mississippi floodplain. Most of the sites trapped by Cagle and Chaney were lacustrine and were trapped for only 5–25 trap-nights, which would account for the low trap rates there.

Recent surveys in other states indicate that the trap rate in Louisiana is lower than elsewhere (Table 2), suggesting that populations are relatively low in southeastern Louisiana. Trap rates in protected Louisiana streams are still much lower than nonharvested streams in Arkansas (0.073 vs. 0.170 TTN) and heavily harvested waters in Arkansas and Georgia (0.34 and 0.08 TTN, respectively; Johnson 1989; Wagner et al. 1996). The fact that alligator snappers were trapped at twice the rate in harvested vs. nonharvested sites in Arkansas suggests that factors in addition to, or in spite of, harvest history, such as stream size or trapping methods, may be governing perceived population densities. Trap rate could be reduced because of oversaturation of trap placements (25/night vs. 6–12 elsewhere). In many cases it was necessary for us to place traps in less-than-optimal locations along streams and rivers because of the high incidence of trap theft. Thus, traps were often concealed near shorelines rather than placed near the heads of holes, logjams, or other ideal sites (Harrel et al. 1996). However, the preceding factors do not entirely account for the 3- or 4-fold decrease in trap rates compared to other areas. Interestingly, trap rate increased with net mesh size in 4 studies (Table 2).

Table 2. Recent survey data for alligator snapping turtles trapped with hoop nets in several states.^a

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Aside from trap rate, evidence of exploitation can result in differences in weight and length covariance. Mean weights of harvested sea turtles have been shown to vary with harvest levels over many years (Carrillo et al. 1999), but a harvest-based change in weight–length covariance has not been reported, and none would be expected because most current sea turtle populations are below the level at which any such effect would be seen (K. Bjorndal, pers. comm.). The similarity in covariance between harvested and nonharvested populations of alligator snapping turtles might also suggest a uniform depression of populations regardless of recent harvest effects. Conversely, it could also reflect an absence of density-dependence within southern Louisiana populations or confounding effects such as combining habitat types, season, and so on. Close and Seigel (1997) found that the average CL of Trachemys scripta differed significantly between harvested and protected/public sites, but we did not find a difference for alligator snapping turtles: mean CL 43.3 vs. 43.6 cm for low and high harvest, respectively (F = 0.025 at 1, 88 df, p = 0.874). Average adult turtle weights and lengths are also relatively uniform between southeastern Louisiana and other states (Table 2; Fig. 5). Adult CL was nearly identical between southeastern Louisiana, northeastern Louisiana, western Florida, and Arkansas turtles (F = 0.200, p = 0.8967 at 3, 437 df) and was not significantly different between Arkansas turtles from harvested vs. nonharvested counties (p = 0.56). Because of the close relationship between CL and weight (see also Trauth et al. 1998), it is assumed that weights are also not significantly different. Although average sizes of turtles were similar between regions, growth rates might not be uniform, and the assumption of a uniform age structure between populations may be invalid based on differences in size classes between states (Fig. 5).

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The lack of turtles under 12.5 cm CL in the sample cannot be explained by mesh size because over 800 kinosternids measuring 8–13 cm CL were trapped with the alligator snappers. It is likely that the lure/ambush method of feeding in juvenile alligator snappers precludes them from active foraging that would lead them into hoop nets (Drummond and Gordon 1979). Differences in percentage of immature turtles >12.5 cm CL between studies suggest differences in population structures. The low percentage of immature turtles in Florida was suggested to signal the presence of more adult turtles there because of lack of harvest pressure (P. Moler, pers. comm.), but a low percentage of immatures was also trapped in Georgia in heavily harvested sites. In Arkansas, percentage of immature turtles was similar between harvested and nonharvested sites (0.45 vs. 0.47), as was average CL (26.6 vs. 26.1 cm). Sex ratio was reported only by Trauth et al. (1998), who also found a 1:1 sex ratio in adults (specifically 19 males:17 females). Despite a uniformity of mean CL between surveys, distribution of actual CL between surveys varied (Fig. 5), which is apparently a result of differences in immature/adult ratios and/or growth rates. The differing ratios create ambivalent conservation implications: low immature percentage might indicate reduced reproductive success but could alternatively mean an unreduced adult population.

COMMERCE

Louisiana has been home to several high-volume turtle meat markets that are presumed to have contributed to a depletion of alligator snappers in eastern Louisiana and to have created a sink on populations from Alabama, Mississippi, and Arkansas. Dobie (1971), Pritchard (1989), and Sloan and Lovich (1995) summarized data from operations in Jonesville, Manchac, and New Orleans, the largest of which stopped butchering alligator snappers shortly after Pritchard's report (H. Reno, pers. comm.). Since 1995, only 1 licensed turtle-meat operation, in northern Louisiana, has persisted in butchering approximately 50–200 alligator snappers from central Louisiana per year (anonymous owner, pers. comm.). Between 1996 and 2004, 12 Louisiana dealers were selling snapping turtle meat, but most was comprised of common snapper (Chelydra serpentina) according to supplier reports. Genetic identification of turtle meat found alligator snapper meat in only 1 of 32 samples of “putative turtle meat” from Louisiana and Florida markets (Roman and Bowen, 2000).

The Louisiana Department of Wildlife and Fisheries has required, since 1993, that all alligator snappers sold in commerce be reported to the department, with inclusion of sex, weight, and locality of take, and that only turtles greater than 15 inches CL may be sold. Based on mandatory reporting and solicitation of reports from potential trappers and dealers, 0–5 alligator snappers were reported to be commercially harvested each year from 1996 to 2004 in southeastern Louisiana, but this is likely an underestimate.

Intensive commercial harvest of turtles in the region around the Manchac operations was believed to have resulted in a 95% depletion of alligator snapper populations there based on reports from turtle trappers (Pritchard 1989). Cliff Fontenot (in Pritchard 1989) reported failure to trap any snappers during 169 trap-nights at Manchac in 1988, and Platt (1994) stated that they were extirpated from the area. However, 39 alligator snappers were trapped in 727 trap-nights in the Manchac area in 1997, or 0.054 TTN, which is close to the mean of 0.057 for all of eastern Louisiana. We believe differences in trap results are based on greater efficiency of trapping in the present study, aided greatly by the years of experience of C.K. as a professional trapper.

We agree with Moler (1996) that “anecdotal reports of the status of alligator snapping turtles in an area are of little value.” The belief that snappers have been trapped to depletion in the Manchac–Maurepas area proved erroneous. Trauth et al. (1998) and P. Moler (pers. comm.) found “relatively large numbers” of alligator snappers in some streams thought by trappers to have been “turtled out.” Similar terminology from Louisiana turtle fishermen is quoted by Pritchard (1989), which has been cited by a variety of people and groups to promote a need for total protection for alligator snappers (unpublished appeals on file with the USFWS). This is a problem of semantics: “turtled out” typically means “no longer commercially profitable” to a turtle trapper and should not be considered synonymous with extirpation, near elimination, or unrecoverable depletion of a population. Our data indicate that a fisherman in southeastern Louisiana using 25 hoop nets could trap an average of 23.4 pounds (live weight) of legal-sized (> 15 inches CL) alligator snappers per night. At current prices offered for live alligator snappers, this would bring a trapper about $22 per night. Our trapping results indicate that the same trapping regime would produce, on average, 1.4 Chelydra serpentina, 0.9 Pseudemys concinna, 25.2 T. scripta, and 0.9 Apalone spinifera, which, at current prices for meat or breeding stock, would bring an additional average of $24.50 per night. The tremendous effort needed to profitably trap alligator snappers has resulted in a precipitous decrease in the number of commercial trappers in Louisiana, plus closure of a number of commercial enterprises. Two other factors have contributed to a decrease in commercial take of alligator snappers: prior to an actual ban on commercial take, there was a persistent belief among trappers that the species was protected by law, and some catch techniques, such as “noodling” and poling for turtles in lentic waters, have not been passed on to younger generations of fishermen.

We recommend that some or all of the sites surveyed herein be retrapped within the next several years, using identical techniques. Results of resurveys should provide data on growth rates and movements and may detect changes in population structure and density. Trapping should also be undertaken to determine the effects of season and hydrology types on trap rates and population structure.