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Survival and Post-release Movements of River Otters Translocated to Western New York
Romeo M. Spinola, Thomas L. Serfass, and Robert P. Brooks

Northeastern Naturalist, Volume 15, Issue 1 (2008): 13–24

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2008 NORTHEASTERN NATURALIST 15(1):13–24 Survival and Post-release Movements of River Otters Translocated to Western New York Romeo M. Spinola1,3,*, Thomas L. Serfass2, and Robert P. Brooks1 Abstract - Survival and post-release movements of individuals translocated for reintroduction purposes have implications for intra-specific interactions, which are essential for reproduction, and, ultimately, for the success of the reintroduction effort. Between 1997–1998, 28 (14M:14F) Lontra canadensis (river otters) were translocated to the Genesee River, NY, to restore extirpated populations. Otters were implanted with transmitters to determine survival, cause of mortality, and post-release movements. Five (3M:2F) otters died during the study: three (2M:1F) mortalities were caused by collisions with vehicles and two (1M:1F) were from unknown causes. Survival rate during the first year was 0.89 (95% CI = 0.78–1.00); annual survival rate was 0.92 (95% CI = 0.79–1.00) and 0.86 (95% CI = 0.70–1.00) for males and females, respectively. Post-release dispersal distance of 22 (11M: 11F) otters ranged from 1.2 to 54.0 km (mean = 12.5 km, 95% CI = 8.5–23.7 km). Dispersal distance of females was greater than that of males by a mean of 8.7 km (95% CI = 0.1–19.2 km). River otters that dispersed >15 km from the release site experienced higher mortality. Introduction Reintroductions are a common management tool to restore extirpated Lontra canadensis Schreber (river otters) populations (Ralls 1990). These programs are expensive and often involve the release of a limited number of individuals, as releasing larger groups of individuals has been shown to do little to increase translocation success (Griffith et al. 1989). However, small populations are susceptible to deleterious consequences of environmental and demographic stochasticity and inbreeding (Caughley and Sinclair 1994). Consequently, a primary goal of reintroduction programs should be to maximize initial population growth to minimize impacts of stochastic events (Komers and Curman 2000). Therefore, reintroduction programs ideally should seek to establish high levels of reproduction and survival by translocated individuals to enhance likelihood of a successful project. Intra-specific interactions, essential for enhancing reproductive opportunities in a newly reintroduced population, depend on the survival and post-release movements of translocated individuals. For example, high postrelease mortality reduces the number of reproducing individuals. Similarly, extreme post-release movements away from the release area reduce the 1Penn State Cooperative Wetlands Center, The Pennsylvania State University, University Park, PA 16802. 2Department of Biology, Frostburg State University, Frostburg, MD 21532. 3Current address - Instituto Internacional en Conservación y Manejo de Vida Silvestre, Universidad Nacional, Apartado 1350-3000, Heredia, Costa Rica. *Corresponding author - 14 Northeastern Naturalist Vol. 15, No. 1 opportunity for intra-specific interactions, which may lessen the extent of breeding activity among translocated individuals (Erickson and McCullough 1987). Thus, understanding factors affecting survival (i.e., identifying causes of mortality) and examining issues influencing post-release movements are crucial to improving the chances of success in reintroduction efforts. Also, empirical estimates of survival and post-release movements are important in viability analyses of reintroduced populations (White 2001). Previous studies suggest survival of translocated river otters is affected by the extent of post-release movements. In Missouri, otters that dispersed farther from the release site experienced higher human-caused mortality than individuals that remained closer to the release site (Erickson and Mc- Cullough 1987). In Indiana, 2 of 5 otters that died less than 1 year post-release had dispersed 66 and 27 km, reaching sites outside of the primary restoration area (Johnson and Berkley 1999). River otters are capable of extensive post-release movements (Erickson and McCullough 1987, Johnson and Berkley 1999), which increase their likelihood of encountering sub-optimal conditions and higher levels of mortality as they emigrate from the primary restoration area. Thus, reintroduction projects should use a continuum of high quality aquatic habitats to minimize human-caused mortalities (e.g., accidental trapping and collisions with cars) during initial movement of reintroduced individuals. Post-release movements in otters are likely to be influenced by their mating system. Otters are polygynous (Estes 1989, Toweill and Tabor 1982); thus, male spatial pattern is expected to change during the breeding season to maximize mating opportunities (Sandell 1989). Consequently, travel distances of males are predicted to increase substantially during that season. Historically, the river otter occurred in all watersheds of New York (Hall 1981). However, during the 1800s, river otter populations started to decline in some regions of the state and became extirpated from many areas (Gotie et al. 1994). As a result, river otter populations remained mainly in upstate New York, east of the Unadilla River and north of the Mohawk Valley (Gotie et al. 1994). In 1995, a river otter reintroduction program was implemented to restore river otter populations to central and western New York. Documenting fates of translocated individuals is important to evaluate the short-term success of the reintroduction effort and to improve the success of future translocations. Despite this, little information has been published on survival and post-release movements in reintroduced otter populations. In this study, we analyzed survival and post-release movements of river otters translocated to the Genesee River in western New York, where otters have been completely extirpated. Our objectives were to 1) estimate survival rates, 2) assess causes of mortality, and 3) describe post-release movements. Study Area The study area (ca. 1882 km2) is located in the upper Genesee River watershed and includes portions of Wyoming, Livingston, and Allegany 2008 R.M. Spinola, T.L. Serfass, and R.P. Brooks 15 counties in western New York (Fig. 1). The area is dominated by agricultural activities interspersed with residential developments. The landscape consists of rolling hills with most communities and major roads in the valleys. The upper Genesee River watershed contains 2297 km of riverine habitats and 51.3 km2 of lentic habitats including emergent wetlands, lakes, Castor canadensis Kuhl (beaver) impoundments, and man-made ponds. Mean size of lentic patches (n = 2363) was 2.2 ha (SD = 13.2; data were obtained with ArcView 3.2, Environmental Systems Research Institute, Inc., Redlands, CA), which are scattered in the agricultural/development matrix on land cover/land-use coverage digitized for the study area. Average temperatures range from -5.2 °C in January to 21.1 °C in July, with an average annual precipitation of 741 mm and average annual snow fall of 1008 mm (USDA 1999). The region typically is covered with snow from late December to early March. River otters were released at 2 sites on the Genesee River in 1997 and 1998 (Fig. 1). The first was located at Lee’s Landing inside 7000-ha Letchworth State Park (LSP). The park includes about 30 km of the Genesee River, which flows through a steep canyon, extending from a series of 3 waterfalls to the Mount Morris Federal Flood Control Dam. Most of the riparian habitat along the Genesee River within LSP has been disturbed by periodic flooding caused by regulation of water levels in the dam. The second site was located on private property at the confluence of Wiscoy Creek and the Genesee River, 18 km upstream of the LSP site. Riparian habitat in this portion of the Genesee River watershed has been degraded by human use, particularly farming. Figure 1. Otter release sites on the Genesee River and dispersal locations of 20 otters in western New York, 1997–1998. Two otters are not displayed because they dispersed beyond the area represented by the map. 16 Northeastern Naturalist Vol. 15, No. 1 Methods Capture, handling, and release Trappers trained to participate in the New York River Otter Project (NYROP) used foot-hold traps sizes #1 to #2 and #11 to capture river otters in the Adirondack and Catskill regions of New York. Prior to release, all otters underwent a captive management program at Cornell University or The Pennsylvania State University, in which veterinarians provided physical and health evaluations and performed surgeries to implant radio-transmitters (Hernandez-Divers et al. 2001, Serfass et al. 1993). Transmitters were placed into the peritoneal cavity following established surgical procedures and post-operative protocols (Hernandez-Divers et al. 2001, Serfass et al. 1993). Typically, otters were held in captivity for 7–15 days prior to implanting transmitters, followed by a post-surgical convalescent period of about 7 days. All capture and handling procedures were approved by the Institutional Animal Care and Use Committee at The Pennsylvania State University (Permit No. 91R1583D096) and Cornell University (Permit No. 94-108-03). At the LSP site, 14 (8M:6F) and 6 (3M:3F) otters were released in 1997 and 1998, respectively. Eight (3M:5F) otters were also released at the Wiscoy Creek site in 1998. Except for 4 otters (3M:1F) released in July 1997, all releases occurred in October and November. All released river otters were adults assumed to be based on tooth wear and size (body mass and length). Radio-telemetry Each otter was equipped with an implantable transmitter (IMP-200; Telonics, Inc. Mesa, AZ) in the 164–165 MHz range with an expected operational life of 10 mo. During radiotracking, river otters initially were located from a vehicle with a TR-2 receiver (Telonics, Inc.) and an omnidirectional whip antenna. When a signal was detected, the location of the animal was determined by using an “H” antenna and walking to the signal (homing) without relying on triangulation data (White and Garrot 1990). Locations were plotted on 1:24,000 scale topographic maps and digitized using ArcView 3.2 to create a GIS database. We attempted to locate otters daily, but frequency varied with seasonal conditions and access to areas occupied by otters. When otters could not be located by ground tracking, we flew in a single engine airplane (Piper PA- 22-150) equipped with an omnidirectional whip antenna. Specific locations of “relocated” otters were then determined by ground tracking immediately after the flight. Survival We determined if an otter died by radio-monitoring and by verifying reports of road-killed otters reported to the New York State Department of Environmental Conservation (NYS-DEC). We considered an otter to be inactive when signal strength was consistent during the first 60 sec of a radio-monitoring session. If an inactive signal was recorded from the same location during several days, we intensified the monitoring to determine if 2008 R.M. Spinola, T.L. Serfass, and R.P. Brooks 17 the animal was alive. If we suspected that the animal was dead, we made all possible efforts to recover the carcass. Recovered carcasses were sent to the College of Veterinary Sciences at Cornell University for necropsy. We estimated survival functions and annual survival rates with the staggered-entry Kaplan-Meier method (Pollock et al. 1989a, b). We rightcensored (i.e., eliminated the individual for the subsequent analysis) a river otter after losing radio contact with it, assuming it had either left the study area or had an inoperable transmitter. The survival function was calculated for 64 weekly exposure periods. The log-rank test (Pollock et al. 1989b) with 1 degree of freedom was used to compare the survival function between river otters translocated in 1997 and 1998, and between sexes for a period of 64 weeks. We estimated 95% confidence intervals for survival rates based on a log-normal distribution. The lower and upper confidence limits were computed as: where, Post-release movements We measured movements of river otters as the net displacement between radio-locations taken 2–3 days apart. Distances were obtained with the “create polyline from point file” function in the Animal Movement Extension for ArcView (Hooge and Eichenlaub 1997). We analyzed mean distance moved for males and females using a 2 x 5 factorial design. The two factors were sex, with 2 levels (male and female), and season, with 5 levels (Fall I, Winter I, Spring I, Summer I, Fall II; see next section—remarks on data pooling and seasons definition). We used a linear mixed model to account for repeated measures and individuals with missing data to examine the influence of sex and season in the extent of otter movements (Littell et al. 1998). We used a first-order autoregressive variance- covariance structure to specify the variance-covariance matrix for the within-subject effect. We conducted pairwise comparisons for those terms in the model that were significant, adjusting the confidence intervals for the effect size with the Bonferroni method. Distance data were transformed using natural logarithms to meet assumptions of homogeneity of variance (Cochran C19 = 0.13, P = 0.677) and normality. To measure post-release dispersal, we measured the linear distance from the release site to the arithmetic mean of radio-locations observed during the last season the river otter was monitored. Only those otters that were monitored until the transmitter battery expired (n = 17) or known to have died during the study (n = 5) were included in calculating post-release dispersal. 18 Northeastern Naturalist Vol. 15, No. 1 We estimated 95% confidence intervals of means and the mean difference (effect size) by bootstrapping based on the 2.5 and 97.5 percentiles of 1000 replicates. We compared the post-release dispersal distances between males and females using multi-response permutation procedure (MRPP) for univariate-grouped data analogous to the t-test. The MRPP is based on distance functions and does not assume any population distribution (Cade and Richards 1999). Remarks on data pooling and seasons definition Data on post-release movements for 1997 and 1998 were pooled because of insufficient sample sizes (Anderson et al. 2001). Seasons were defined by calendar months (fall: September 21–December 20; winter: December 21–March 20; spring: March 21–June 20; summer: June 21–September 20). Roman numerals as suffix were used to denote a specific season to group animals based on their first or second season regardless of actual year of release. Fall I included data collected during fall 1997 and 1998 for river otters released in 1997 and 1998, respectively; Winter I included data collected during winter 1998 and 1999 for river otters released in 1997 and 1998, respectively; Spring I included data collected during spring of 1998 and 1999 for river otters released in 1997 and 1998, respectively; Summer I included data collected during summer of 1998 and 1999 for river otters released in 1997 and 1998, respectively; Fall II included data collected during fall of 1998 and 1999 for river otters released in 1997 and 1998, respectively. Spring was considered to be the breeding season of river otters (Hamilton and Eadie 1964, Melquist and Hornocker 1983, Toweill and Tabor 1982). Remarks on statistical analysis We used an alpha level of 0.05 for all statistical analyses. The linear mixed model for repeated measures was conducted using PROC MIXED - SAS (SAS Institute Inc., Cary, NC). Data transformation and tests for homogeneity of variance and normality were conducted with R (R Development Core Team 2004). The bootstrapping procedures were performed with S-Plus 6.1 for Windows (Insightful Corporation, Seattle, WA). The multiple response permutation procedure (MRPP) was performed with Blossom Version W2001.07t (Midcontinent Ecological Science Center, US Geological Survey, Fort Collins, CO). Results Survival Five (3M:2F) of 28 translocated otters died during the study. Three mortalities were caused by collisions with vehicles: F790 was struck by a vehicle 37 km north of the release site in March 1998 (5 months post-release), M180 was road-killed 17 km northwest of the release site in March 1999 (19 months post-release), and M590 was killed in a collision with a vehicle 15 km southeast of the release site in March 2000 (32 months post-release). All vehicle-related mortalities were associated with long-distance dispersal from 2008 R.M. Spinola, T.L. Serfass, and R.P. Brooks 19 the release site. The other two mortalities were of unknown causes. The transmitter from M730 was recovered from a beaver pond 5 km west of the release site 10 months post-release, and Female F240 was also found dead in a beaver pond 3.7 km south of the release site in January 2000 (14 months after being released); cause was undetermined for both deaths. Annual survival rate for translocated river otters was 0.89 (95% CI= 0.78–1.0) for both sexes combined, 0.92 (95% CI = 0.79–1.0) for males, and 0.86 (95% CI = 0.70–1.0) for females. Survival rates did not differ between years (χ2 1 = 0.18, P = 0.67) or sexes (χ2 1 = 0.32, P = 0.57). M180 and M590 were killed after the monitoring periods of 52 and 64 weeks considered for the estimation of annual survival rate and survival function, respectively. Consequently, those mortalities were not included in both estimations. Post-release movements We measured 1297 distances moved for the 28 translocated otters. Mean number of distances was 47.5 (SD = 20.9, range = 15–87) and 45.1 (SD = 23.2, range = 16–94) for males and females, respectively. Distances varied seasonally from 0.53 to 1.60 km for males and from 0.55 to 0.89 km for females (Table 1). Maximum linear distance between locations was 17.5 and 9.3 km for male M320 and female F080, respectively. These movements occurred during the first post-release spring and fall for M320 and F080, respectively. The largest mean distance moved for males occurred in the breeding season (Spring I); however, for females, distances were greatest in Fall I (just after being released). Lowest mean distances for both sexes occurred in Fall II (Table 1). Movements did not differ between males and females (F(1,70) = 0.38, P = 0.537), and there was no interaction between sex and season (F(4,70) = 0.31, P = 0.867). However, movements differed by season (F(4,70 )= 3.92, P = 0.006). Pairwise comparisons among all means for seasons showed movements were greater in Fall I than in Fall II, but identified no other differences in movements between seasons (Table 2). Post-release dispersal of 22 (11M:11F) otters ranged from 1.2 to 54.0 km (Figs. 1and 2), with a mean of 12.5 km (95% CI = 8.5–23.7 km). Mean postrelease dispersal distance was 8.1 km (95% CI = 6.0–13.5 km) for males and 16.8 km (95% CI = 8.9–37.5 km) for females and was greater for females than for males (δ = 10.845, P = 0.0532) by a mean of 8.7 km (95% CI = 0.1–19.2 km). Table 1. Seasonal mean distance (km) between radio-locations 2–3 days apart of male and female river otters translocated to the Genesee River in western New York, 1997–1999. Males Females Season Mean 95% CI n Mean 95% CI n Fall I 1.26 0.79–2.48 10 0.89 0.58–1.33 13 Winter I 0.79 0.34–1.80 9 0.61 0.27–1.37 13 Spring I 1.60 0.98–2.59 13 0.74 0.45–1.87 10 Summer I 0.77 0.35–1.16 10 0.85 0.39–1.85 10 Fall II 0.53 0.15–1.11 8 0.55 0.29–0.93 10 Overall 1.049 0.80–1.30 14 0.73 0.56–0.90 14 20 Northeastern Naturalist Vol. 15, No. 1 Seventeen (77%) of 22 otters dispersed <15 km from the release site. Two (12%) of these died, whereas 3 (60%) of the 5 otters that dispersed greater than 15 km died. The proportion of dead otters was greater for the otters that dispersed >15 km than the otters that dispersed <15 km (χ2 = 5.1186, df = 1, P = 0.0237). The greatest dispersal distances from the release site were 21.3, 37.0, and 54.0 km for 3 females. Discussion Our high annual first-year survival rate (89%) was similar to that reported for otters translocated to Missouri (81%; Erickson et al. 1984), Pennsylvania (83%; Serfass et al. 1986), Tennessee (91%; Griess 1987), and Indiana (71%; Johnson and Berkley 1999). Lower survival rates Table 2. Comparisons among seasonal mean distance (km) between radio-locations 2–3 days apart of male and female river otters translocated to the Genesee River in western New York, 1997–1999. Back-transforming the mean difference on the log-scale corresponds to a ratio of the medians on the original scale. For example, the first comparison resulted in a ratio of 2.95. This means that the median extent of movements during Fall I was 2.95 times that of the median of Winter I. Comparison Ratio 95% Confidence IntervalA Fall I–Winter I 2.95 0.76–11.45 Fall I–Spring I 1.26 0.29–5.50 Fall I–Summer I 2.45 0.52–11.47 Fall I–Fall II 6.22 1.25–31.03 Winter I–Spring I 0.43 0.11–1.70 Winter I–Summer I 0.83 0.18–3.91 Winter I–Fall II 2.11 0.41–10.74 Spring I–Summer I 1.94 0.49–7.66 Spring I–Fall II 4.93 1.03–23.48 Summer I–Fall II 2.54 0.60–10.73 AAdjusted with the Bonferroni method. Figure 2. Postrelease dispersal distance (km) of 22 river otters translocated to the Genesee River, NY, 1997–1999. 2008 R.M. Spinola, T.L. Serfass, and R.P. Brooks 21 occurred in otters translocated to Ohio (46%; McDonald 1989), Oklahoma (60%; Hoover et al. 1984), and West Virginia (56.7%; Tango et al. 1991). In most river otter translocation projects, stress-related mortalities occurred shortly after release. In contrast, necropsies for the found carcasses and the timing of mortality of the 5 otters that died during our study yielded no evidence to suggest that capture, transport, captive management, or implantation of transmitters contributed to the mortalities. We believe our captive management program was beneficial to river otters and may have increased their post-release survival. The program facilitated treatment of trap-related injuries, recovery from stress caused by capture and handling (Rothschild 2005), and recovery from surgeries to implant transmitters. A captive management program also enhances the opportunity to detect communicable diseases, such as rabies (Serfass et al. 1995). We recommend other river otter reintroduction projects incorporate similar programs with veterinary supervision into their project protocol. Three of our five mortalities were vehicle-related and coincided with a noticeable change in otter spacing patterns in late winter and early spring, the peak period for reproductive activity. Movements of river otters during this period may be influenced by easier access to new habitats because of ice melt and increased opportunities to enhance reproductive success (i.e., males searching for females). In addition, vehicle-related mortality was greater in otters that dispersed extensive distances from release sites. As individuals move greater distances, they encountered sub-optimal habitats and mortality factors associated with human-dominated landscapes. Although release areas were selected to minimize these impacts, habitat conditions and human activities in surrounding aeas differed from those in release areas. Thus, conflicts related to human activities (e.g., accidental trapping and collision with vehicles) are more likely in river otters displaying long-distance dispersal. Similar fates were reported for otters released in Missouri (Erickson and McCullough 1987) and Indiana (Johnson and Berkley 1999). Movements were greater immediately after release and, excluding males in breeding season, typically decreased thereafter. We suspect this change was related to exploratory movements as newly released otters searched for denning, resting, and foraging sites. Accordingly, establishment and development of a home range by otters coincided with more restricted movement patterns (Spínola 2003). Post-release dispersal distances displayed by translocated river otters were likely affected by habitat characteristics. The wide range in dispersal distances may be due to the patchiness of suitable aquatic habitats along the Genesee River. Most translocated otters left the Genesee River and LSP to inhabit a mosaic of isolated patches of aquatic habitats dispersed throughout a landscape dominated by agricultural activities. Otters often traveled long distances to reach these habitats. In Missouri, Erickson and McCullough (1987) reported shorter dispersal distances for otters released in a palustrine wetland than for those released in a riverine system. 22 Northeastern Naturalist Vol. 15, No. 1 Overall, female river otters dispersed farther than males. We suspect these differences were related to the habitat and spatial pattern requirements of females compared to males. Considering that female river otters carry out the parental care alone, it would be expected that females would occupy higher-quality habitats than males and, thus, travel greater distance after being released in search of appropriate areas. Also, female river otters, unlike males, displayed intra-sexual territoriality (Spinola 2003). These conditions combine to impose specific requirements on habitats and space to enable females to locate high-quality habitats and secluded areas to raise young and avoid conspecific females. The density and distribution of roads in the aquatic-landscape matrix may cause significant mortality among river otters traveling overland. Otters may be more vulnerable to vehicle-related mortality in late winter and spring when movement patterns change, probably in response to breeding and parturition. Males may be particularly vulnerable because they travel farther searching for opportunities to breed. Suitable otter habitats in human-dominated landscapes are disjunct, which may impose greater overland travel, thereby increasing the likelihood of vehicle-related mortality. Because most otter reintroduction programs release few individuals per site, high levels of survival are important to enhance the likelihood of establishing a population. Based on our study, the density and distribution of roads should be considered when evaluating the suitability of a release site. Acknowledgments We thank the Pennsylvania State Cooperative Wetlands Center, the New York River Otter Project, the New York State Department of Environmental Conservation, the Pennsylvania Wild Resource Conservation Fund, the US Army Corp of Engineers, and the New York State Office of Parks, Recreation, and Historic Preservation. We also thank Dr. G. Kollias and the veterinarian staff from Cornell University, and T. Blakenship from Pennsylvania State University for implanting transmitters. Bruce Penrod, Dennis Money, and June Summers provided support, and “Chuck” Green and Robin Holevinsky assisted in field work. Finally, we thank the Rob and Bessie Welder Wildlife Foundation from Texas for providing a fellowship to R.M. Spinola. Literature Cited Anderson, D.R., K.P. Burnham, W.R. Gould, and S. Cherry. 2001. Concerns about finding effects that are actually spurious. Wildlife Society Bulletin 29:311–316. Cade, B.S., and J. Richards. 1999. User Manual for BLOSSOM Statistical Software. 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