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Nutria Survivorship, Movement Patterns, and Home Ranges
Lauren E. Nolfo-Clements

Southeastern Naturalist, Volume 8, Number 3 (2009): 399–410

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2009 SOUTHEASTERN NATURALIST 8(3):399–410 Nutria Survivorship, Movement Patterns, and Home Ranges Lauren E. Nolfo-Clements* Abstract - Sixteen Myocastor coypus (Nutria) were implanted with radiotransmitters and monitored on a freshwater fl oating marsh. Mortality and/or transmitter failure was high, with 5 transmitters failing or being destroyed through predation within 3 days of release. Predation mortality was common, with an additional 5 transmitters recovered from carcasses within 35 days of implantation. The remaining 6 animals survived for a mean of 182 days. After removing first movement distances from the analyses, the mean distance traveled between locations for surviving animals was 77.4 m. Distances varied significantly between seasons, but not between the sexes. The average minimum convex polygon (MCP) for individuals with sufficient data was 28.8 ha and the 95% fixed kernel (FK) estimates averaged 32.7 ha. There was no significant difference between home-range estimates overall or between the sexes. Introduction Myocastor coypus Molina (Nutria or Coypu) is a large aquatic rodent endemic to the Patagonian subregion of South America. This mammal has been transported to various locations around the world as a valued furbearer (Carter and Leonard 2002). Many of these introduced populations have become established, resulting in the institution of population-control measures (Bounds and Carowan 2000, Kuhn and Peloquin 1974). Numerous studies have focused on the movement patterns of the Nutria, both in its native and introduced range (Doncaster and Micol 1989, Guichon and Cassini 1999, Reggiani et al. 1993). In the Nutria’s native range, where these animals are subject to intense hunting pressure, studies are focused on illustrating the Nutria’s innocuous nature and avoidance of human settlements (D'Adamo et al. 2000; Guichon and Cassini 1999, 2005). In its introduced range, studies focus on movement patterns in order to better understand the Nutria’s overall ecology, either to contribute information to aid in control efforts (Reggiani et al. 1995) or to enhance means of increasing harvest efficiency (Coreil et al. 1988, Ryszkowski 1966). In Louisiana, where annual Nutria harvests are in the hundreds of thousands and sometimes millions, management efforts focus on controlling rather then eradicating the population (Scarborough and Mouton 2007). Despite government funded control efforts, most studies involving Nutria in Louisiana focus on herbivory and the effects these rodents have on wetland plant communities (Evers et al. 1998, Ford and Grace 1998, Gough and *Department of Ecology and Evolutionary Biology, Tulane University, 400 Boggs Center, New Orleans, LA 70118; Current address - Department of Biology, Suffolk University, 41 Temple Street Boston, MA 02114; lnolfo-clements@suffolk.edu. 400 Southeastern Naturalist Vol. 8, No. 3 Grace 1998, Randall and Foote 2005, Taylor and Grace 1995). Few studies in Louisiana have focused on the movement patterns and activities of the Nutria (Coreil et al. 1988, Warkentin 1968). The purpose of this study was to uncover seasonal and sex-specific trends in Nutria home ranges and movement patterns. I examined Nutria home ranges and movements both annually and seasonally. This study was conducted in a freshwater fl oating marsh in southeastern Louisiana. To the author's knowledge, no other examination of Nutria movement patterns has been conducted in this type of environment. Field-site Description This study was conducted in the Barataria Unit of Jean Lafitte National Historical Park and Preserve (JLNHPP), located about 24 km south of New Orleans in Jefferson Parish, LA. Wetland habitats comprised about 4900 ha of the total park area of approximately 7500 ha (D.P. Muth, JLNHPP, pers. comm.). The climate was subtropical with annual rainfall exceeding 160 cm and mean annual temperature of 21°C (summer average of 28.5 °C, winter average of 12.2 °C). The growing season typically exceeded 260 days. The study site was at or slightly below sea level (White et al. 1983). The wetland habitats of the JLNHPP included fl oating marsh, spoil banks, and open water habitats. The fl oating-marsh habitat roughly coincided with types 1–5 thick-and thin-mat fresh fl oating marsh, as characterized by Sasser et al. (1994). This habitat was nearly devoid of woody vegetation except for patches of Morella cerifera (L.) Small (Myrica cerifera; Wax Myrtle) distributed across the marsh. Canals that were dug primarily for oil and gas exploration in the 1940s–70s provided access to the marsh. These canals were lined by spoil banks that were constructed from the sediments excavated in the digging of the canals. Woody vegetation dominated this habitat type, in contrast to the herbaceous species that dominated the surrounding canal and marsh communities. Smaller bodies of water that traversed open expanses of marsh are referred to as trenasses, French-Acadian for “trail on the marsh.” Trappers probably dug these waterways as routes for laying trap lines (D.P. Muth, pers. comm.). Trenasses averaged 2–3 m wide and were usually completely covered with fl oating aquatic vegetation during the growing season. For a full description of the wetland habitats of JLNHPP, including a species checklist, see Nolfo-Clements (2006). Voucher specimens from that plant survey are housed at the Tulane University Herbarium. Methods Radiotelemetry Sixteen adult Nutria (8 females, 8 males) were captured off of an airboat and implanted with radiotransmitters (Nolfo and Hammond 2006) during January 2004 (7 animals), December 2004 (2 animals), and January 2005 2009 L.E. Nolfo-Clements 401 (7 animals). Animals were released within 24 hours of surgery at a location in the marsh within 100 m from their point of capture, with one exception. All animals were located at least 2 and no more than 4 times per week using a 3-element Yagi antenna attached to a LA12-Q receiver (AVM Instrument Company, Colfax, CA). Relocations occurred at this interval to avoid autocorrelation of data points (Swihart and Slade 1985). All transportation to the marsh areas of JLNHPP was done in a 14-foot aluminum fl atboat with a 70-horsepower outboard motor within the canals of the Park. Marsh travel was done on foot. All radiolocations were done on foot or from a canoe. Either visual or auditory (movement in brush or splashing) confirmation of an animal’s presence was required to confirm a location. A GPS point was taken at each locality using a Garmin Etrex Venture handheld GPS receiver (Forestry Suppliers, Jackson, MS). Statistical analyses I plotted all Nutria locations on a 1998 digital orthophoto quarter quadrangle (DOQQ) of the study area downloaded from the Louisiana statewide GIS site (www.atlas.lsu.edu) and projected into ArcView GIS, Version 3.2. 1999 from ESRI (Environmental Systems Research Institute, Inc., Redlands, CA). I calculated successive movement distances, minimum convex polygon (MCP), and fixed kernel (FK) estimates with 95, 50, and 25% contours using the animal movements extension for ArcView version 1.1 (Hooge and Eichenlaub 1997). There were insufficient data to calculate seasonal MCPs and FK estimates. I calculated successive movement distances and survivorship for all animals. I calculated the MCP and FK estimates only for animals that survived >60 days and therefore had ≥15 radiolocations. I chose the FK estimate versus the adaptive kernel estimate of home range for these analyses because, although adaptive kernel may give the most accurate picture of actual home range, FK produces the least-biased estimates of home-range area with the lowest error, which is especially important when dealing with low sample sizes (Seaman and Powell 1996, Seaman et al. 1999, Worton 1989). I compared survivorship to mean distance traveled for all animals using correlation analysis and tested for linear relationships using Pearson’s correlation coefficient and linear regression analysis. I did the same between the sexes and for survivorship versus MCP areas and FK 95% contour areas. I used a combined ANOVA to examine the interactions between season, sex, and distance traveled. I then compared distances traveled between seasons and between the sexes using an ANOVA with Tukey’s pairwise comparison. Due to the distance between capture and release sites, all distance analyses were conducted both with and without the first distance traveled included in the analyses. In seasonal analyses, winter was December–February, spring was March–May, summer was June–August, and fall was September–November. All statistical analyses were performed using the Paleontological Statistics Software Package for Education and Data Analysis (PAST) (Hammer et al. 2001). I used P < 0.05 to identify significant differences. 402 Southeastern Naturalist Vol. 8, No. 3 Results I collected 249 radiolocations over the course of the study (146 for males, 96 for females). Five of the implanted animals had either transmitter destruction or total transmitter failure within 3 days of release and did not yield any data. The remaining 11 animals survived a mean of 105 days (range = 5–486 days) and traveled a mean of 91.2 m (range = 1.4–1931.3 m) between relocations (Fig. 1). However, once the first distance traveled between release and first relocation was removed from the analysis, the mean distance traveled dropped to 77.4 m. There was no correlation between survivorship and mean distance traveled. The removal of the first distances traveled measurements only affected the mean distance moved for the winter since that was when all captures and implantations took place. Nevertheless, even with the first distances traveled data removed, the mean distances traveled varied significantly between seasons, with the distances being highest in the winter (mean =113.7.0 m, SE = 21.1 m), lowest in the summer (mean = 35.9 m, SE = 5.6 m), and intermediate in the spring (mean = 72.7 m, SE = 14.6 m) and fall (mean = 54.7 m, SE = 10.3 m) (Fig. 2). There was no significant difference between distances traveled between the sexes overall or seasonally, nor were there any interactions between distance traveled, sex, and season. Only 6 Nutria (3 males and 3 females) survived for >60 days and therefore accumulated ≥15 relocations per individual. The survivorship for these animals was a mean of 182 days. Their mean MCP home-range area was 28.8 ha. Their mean FK estimate contours at 95, 50, and 25% respectively were 32.7, 6.0, and 2.4 ha, respectively (Table 1). There was no statistical difference between MCP or 95% FK areas between the sexes. There was some overlap in both MCP and FK home ranges (Fig. 3). Figure 1. Survivorship in days versus mean distance traveled for male and female Nutria monitored at JLNHPP 2004–2005. 2009 L.E. Nolfo-Clements 403 Discussion Survivorship The low survivorship of the Nutria was not completely unexpected. Due to the size of the transmitters used, animals had to weigh at least 4 kg in order to be implanted (Nolfo and Hammond 2006). Any animal that did not meet this criterion was released upon initial capture. We estimated Nutria of this weight to be approximately 7–9 months of age following Louisiana Nutria growth curves created by Atwood (1950). Although there are records of captive Nutria surviving up to 6 years (Gosling and Baker 1982), data suggests that up to 80% of Nutria in the wild die in their first year (Willner et al. 1983) and that individuals over 3 years of age rarely constitute >15% of a wild population (Guichon et al. 2003b). Because the animals in this study were at least 7 month old, they would probably have succumbed to natural mortality within months even without the implantation. Additionally, although Nolfo and Hammond (2006) recommended that Nutria be held in a predator-free environment for at least 72 hours before release, this was not feasible for this study. Therefore, the high initial mortality may be attributed in part to a protracted recovery time or disorientation Figure 2. Mean distance traveled by season for Nutria monitored at JLNHPP ± 1 SE (n = 78, 108, 37, and 17 for winter, spring, summer, and fall, respectively). First distances traveled after release are omitted from the analysis. Means do not statically differ between the sexes and are therefore combined for each season. Table 1. Sex, identification number (#), survivorship, number of locations, MCP area, and 95% FK area of Nutria monitored at JLNHPP 2004–2005. Sex (#) Survivorship (days) # of locations MCP (ha) 95% FK (ha) Male ( n16) 61 15 23.9 66.8 Female (n5) 80 17 54.2 54.6 Male (n10) 126 32 28.7 16.3 Female(n14) 126 33 9.0 8.3 Female (n7) 212 39 47.1 43.7 Male (n2) 486 97 10.1 6.5 404 Southeastern Naturalist Vol. 8, No. 3 after release. Another factor that must be considered in the high mortality of the Nutria in this study is the density of predators at the study site. Both Alligator mississippiensis Daudin (American Alligator) and Canis latrans Say (Coyote) are very abundant (L.E. Nolfo-Clements, unpubl. data), and Nutria have been reported to fall victim to these predators at other locations (Kinler et al. 1987, Valentine et al. 1972). Movement patterns The seasonal differences in distances traveled recorded in this study coincide with previous results for Louisiana Nutria, but not for Nutria in other parts of the world. Coreil et al. (1988) found that female Nutria in Louisiana maintained much larger home ranges in the winter then in the summer. In contrast Reggiani et al. (1993) and Doncaster and Micol (1989) found that Nutria did not show any seasonal changes in movement patterns in Italy and France. The lack of statistical difference detected in male versus female movement patterns in this study contrasts the findings of previous studies and may be due to small sample size. Gosling and Baker (1989) and Doncaster and Micol (1989) found that male Nutria moved further than females between radiolocations. One of the main reasons for some of the discrepancies in the findings between this study and other Nutria movement studies may be the length Figure 3. 95, 50, and 25% FK contours for male and female Nutria labeled by individual. Male 95% areas are dark, female areas are light. Release points are marked with stars “★.” 2009 L.E. Nolfo-Clements 405 of time between radiolocations. In previous studies, animals were relocated at set time intervals within a 24-hour period (i.e., every 30 minutes; Coreil et al. 1988, Edwards et al. 2001, Foerster and Vaughan 2002, Gosling and Baker 1989, Linders et al. 2004). These studies aimed at pinpointing the length of daily movement or assessing diel patterns of activity. My goal was to assess the long-term movement patterns of the Nutria; therefore, radiolocations were made less frequently, similar to other seasonal movement assessments (Chamberlain and Leopold 2005, Diffendorfer et al. 2005, Kamler et al. 2005, Roth 2005). A factor that may have contributed to a few of the longer distances traveled, especially by the male n10, was the distance between the point of capture and the point of release. However, even after removing the first distances traveled by animals upon release, these animals still exhibited notably long travel distances for this species. All animals except n10 were released within approximately 100 m of their capture site from a centralized location. Animal n10 was released over 1 km from his capture site due to extremely low water levels on his release date that did not allow for his transport to his capture locale. N10 was released at the same point as n14 and n16, but it is clear that he promptly traveled directly back to his area of capture and established his home range in that area (Fig. 3). Although this may have artificially infl ated n10’s MCP home range, it did not have any effect on his 95% FK home range (Table 1). Home range Although other studies have assessed Nutria home ranges using MCP, this is the first study that utilized FK estimates. Past studies on Nutria home ranges have all occurred on introduced populations. There have been no home-range or movement studies conducted in their native South American range. In all of the studies that evaluated both sexes, male Nutria were reported to have larger home ranges then females. In an observational and mark-recapture study of an enclosed, breeding, population of Nutria in Poland, Ryszkowski (1966) found that females tended to have more restricted (smaller) home ranges then males. In Italy, Reggianni et al. (1993) reported significant differences in home-range sizes between radio-collared males and females in the spring but not in the winter. They found that the average home-range area across both seasons and both sites was 5.27 ha for females and 14.90 ha for males. Gosling and Baker (1989) used a combination of mark-recapture and radiotelemetry to evaluate the movements and ranging behaviors of Nutria in Great Britain prior to their eradication. They found that males had signifi- cantly larger home ranges than females at all sites. Additionally, the size of these home ranges varied significantly between sites. In areas with patchy resource distribution and low population densities, the average home-range size was 93.9 ha for males and 46.3 ha for females. In contrast, in an area where resources were more evenly distributed and population densities were higher, the average was 6.8 ha for males and 3.0 ha for females. 406 Southeastern Naturalist Vol. 8, No. 3 In a marsh in central west France, Doncaster and Micol (1989) used radiotelemetry to assess the home ranges of male and female Nutria. They found that males had larger home ranges than females and traveled along greater lengths of the canal in the study site then did the females (average home range of 5.68 ha for males and 2.47 ha for females). In Mississippi, Lohmeier (1981) conducted a radiotelemetry-based study on Nutria and found the mean home range for these animals was 2.31 ha. Using MCP, Denena et al. (2003) found that Nutria inhabiting a reservoir/ recreation area in Texas had a mean home range of 2.7 ha (1.6 ha for females and 3.6 ha for males). In Louisiana, there have been two previous studies that have specifically examined movement patterns of Nutria. Warkentin (1968) used a combination of mark-recapture and visual observations to uncover the behaviors and movements of Nutria in and around man-made ponds adjacent to WWII munitions bunkers. She observed that the majority of those animals remained within approximately 274 m (300 yards) of their original capture site. Coreil et al. (1988) radio-tracked female Nutria in an intermediate marsh habitat. They found that animals had the largest MCP home ranges in the winter (138 ha) and the smallest in the summer (7.2 ha). In comparison to these studies on other introduced Nutria populations, the mean annual MCP and 95% FK ranges for the animals in this study are relatively large (28.8 and 32.7 ha, respectively). This result may be due to one of two factors: 1) the pooling of seasonal data; as observed by Coreil et al. (1988), Nutria in Louisiana appear to have significantly larger home ranges in the winter than in the summer. The combination of data from all seasons due to small sample sizes may therefore have translated these seasonal differences into larger annual averages. 2) Patchy resource distribution and/or low population densities at this site (see discussion above; Gosling and Baker 1989). Another possible explanation for the large home ranges found in this study may be the small sample sizes utilized in the analyses. Although MCP estimates necessarily increase as the sample size increases (White and Garrott 1990:151), the opposite is true of FK, estimates where a smaller sample size sometimes leads to infl ated contour areas (Seaman et al. 1999). This discrepancy is only apparent for one animal is this study. Male n16, who survived for 61 days and had 15 relocations, had a MCP home range that was noticeably smaller then his 95% FK area (Table 1). Overall, the MCP and 95% FK areas found in this study did not statistically differ, and therefore their comparatively large extents are probably not due to insufficient relocation sample sizes. However, the small number of animals used in these analyses (6) calls into question the utility of extrapolating these results to other populations, especially in other wetland habitat types. A noteworthy aspect of these home ranges is the similarity in mean areas for males and females. Nutria have been reported as gregarious in areas of both their native and introduced range with groups usually consisting of multiple females, their young, and a single male (Doncaster and Micol 1989, 2009 L.E. Nolfo-Clements 407 Gosling and Baker 1989, Guichon et al. 2003a). Others have concluded that Nutria were territorial, with a definite dominance hierarchy (Reggiani et al. 1993, Ryszkowski 1966, Warkentin 1968). Most home-range and movement studies report larger home ranges for males, with a single male’s range overlapping that of one or more females (Doncastor and Micol 1989, Guichon et al. 2003a, Reggiani et al. 1993). This study did reveal some home-range overlap. This finding may be due to shared release sites, but could also be due to social interactions between radio-implanted individuals (Fig. 3). For example, the male n16 and female n14 were radio-located together on 2 occasions. The overlap between male n2 and female n7 was due to their utilization of communal burrows below a canal spoil bank, although they were never radio-located simultaneously in this area. There was also home-range overlap of the females n5 and n7, who were located within 75 m of each other on 3 occasions. As stated, this is the first study to examine the movement patterns of Nutria on a freshwater fl oating marsh, which, as its name indicates, is not attached to a substrate for most if not all of the year. The variability in fl oating marsh mat thickness has been noted by Sasser et al. (1994, 1996). These different mat thicknesses are associated with whole suites of vegetative characteristics (Nolfo-Clements 2006). It has also been documented that Nutria utilize areas of different mat thicknesses and plant species composition on a seasonal basis (Nolfo-Clements, in press). Hence, the variability in fl oating marsh habitat adds another dimension to habitat selection and movement patterns for this species; that of mat thickness. I have even witnessed Nutria burrowing through the mat, swimming beneath it, and reappearing in a nearby trenasse or canal, a feat that would prove impossible in an attached marsh. Conclusions This study of the movement patterns and home ranges of Nutria on a fl oating marsh revealed useful information regarding both methodologies and results. Based upon the relatively high mortality and low survivorship of the majority of radio-implanted Nutria, future radiotelemetry studies on this species should focus on short-term movement patterns with more frequent relocations and/or a greater number of radio-implanted animals. Although the home ranges calculated for these animals were larger then average when compared with other Nutria studies, this finding may be attributed to the nature of the unique habitat under consideration, the possibility of patchy resource distribution, and the pooling of seasonal data. This study revealed a small glimpse of the ranging behaviors of Nutria on a freshwater fl oating marsh, one that could be greatly expanded through further study and observation. Acknowledgments I would like to thank the staff of Jean Lafitte National Historical Park and Preserve. A special thanks to N. Walters, who was instrumental to the completion of this project, for GIS assistance and Nutria capture. Thanks to L. Zahm and W. Adams for boat use, maintenance, and field support. Thanks to C.S. Hood for all of his guidance, 408 Southeastern Naturalist Vol. 8, No. 3 support, and editorial comments. Thanks are also extended to the Maryland Cooperative Fish and Wildlife Research Unit for the loan of radiotransmitters. This research was funded by grants from the National Parks Service and the Coypu Foundation. This project was covered under Tulane University Institutional Animal Care and Use Committee protocol # 0230-3-16-082. Literature Cited Atwood, E.L.1950. Life-history studies of Nutria, or Coypu, in coastal Louisiana. Journal of Wildlife Management 14:249–265. Bounds, D., and G.A.J. Carowan. 2000. Nutria: A nonnative nemesis. 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