Abstract
We evaluated impact of the needle length, sex, and body condition on chemical immobilization induction time in 50 (29 males and 21 females) free-ranging fallow deer (Dama dama) in Sweden, 2006–11. Induction time is probably the single most important factor when immobilizing free-ranging wildlife with the use of a remote drug-delivery system. Induction times should be short to minimize stress and risk of injury, and to ensure that immobilized animals can be found and clinically monitored as soon as possible. We measured the distance between the darting location and where we recovered the immobilized animal and also the time occurring between the two events. We used two types of needles: 2.0 × 30– or 2.0 × 40–mm barbed needles with side ports. The most important result is that a 10-mm-longer dart needle can reduce the retrieval time substantially (>20 min) until an animal is under monitoring. On average after the darting, the retrieval time decreased from 51 to 29 min and the distance decreased from 519 m from the darting location to 294 m. We suggest that a needle length of 40 mm is preferable for immobilization of wild fallow deer, especially for animals in over-average–to–fat body condition.
Induction time is probably the single most important factor when chemically immobilizing free-ranging wildlife by use of a remote drug-delivery system (RDDS) (Kreeger and Arnemo 2012). Induction times should be short to minimize stress, risk of injuries, accidents, and side effects such as hyperthermia and respiratory problems, and to ensure that immobilized animals can be found, handled, and clinically monitored as soon as possible (Caulkett and Arnemo 2007; Kreeger and Arnemo 2012; Arnemo et al. 2014). Dart placement is the most important determinant of induction time with quickest absorption occurring after injection in large muscle masses (Caulkett and Arnemo 2007; Kreeger and Arnemo 2012). Haigh (1991) and Arnemo et al. (2011) reported quicker inductions with animals darted in large muscle groups than individuals with suboptimal hits. Other factors that may influence induction time include dose, the animal's body weight, physical condition, age, gender, reproductive status, and psychological condition. Use of a RDDS appropriate for the target animal may also be a factor to consider by avoiding reduced drug effects due to trauma, bleeding, or leakage of drugs (Cattet et al. 2006). In addition, the choice of needle length is a trade-off (i.e., the needle should be long enough to ensure intramuscular injection without causing serious injury on impact). To test how needle length impacted induction time in free-ranging fallow deer, we compared 30- and 40-mm needles in 50 capture events. Normally, one should expect a positive correlation between the distance an animal moved after darting and the time it took for the person to find it, but this measure is always affected by terrain and other stochastic events. A more precise measure should be the distance between the darting and where the animal is found. Therefore we measured both distance and time.
Our study was conducted in 2006–11 on wild fallow deer in southwestern Sweden at the Koberg estate (58°N, 12°E). Deer were darted with the use of a CO2-powered rifle and 1.5-mL dart syringes (Dan-Inject, Børkop, Denmark) by stalking or from a tree stand, hunting stand, or car. Animals were never chased prior to darting. Darts were fitted with a transmitter (Pneu-Dart Inc., Williamsport, Pennsylvania, USA) for very high frequency (VHF) radiotracking and 2.0 × 30– or 2.0 × 40–mm barbed needles with side ports (Dan-Inject). When possible, two or more people obtained bearings from the VHF transmitter in the dart to determine the animal's rough location by cross bearings or even triangulation. In situations where this was not possible because of geographical constraints we waited 5–10 min before tracking the animal to avoid losing contact with it due to the limited range of the weak transmitter in the dart. Darting range was typically less than 20 m. Animals were immobilized between October and March and equipped with ear tags and GPS collars (GPS-Pro light 2D, Vectronic Aerospace GmbH, Berlin, Germany). The animals were immobilized with a combination of Large Animal Immobilon (Novartis Animal Health UK Ltd., Surrey, UK) and xylazine as Rompun 100 mg/mL dry substance (KVP Pharma and Veterinär Produkte GmbH, Kiel, Germany) mixed to a concentration of 2.45 mg etorphine, 10 mg acepromazine, and 100 mg xylazine/mL. We used 0.8, 1.0, and 1.2 mL of the mixture for small (<60 kg), medium (61–68 kg), and large (>81 kg) individuals, respectively. For reversal, diprenorphine (Large Animal Revivon, Novartis Animal Health UK) and atipamezole (Atipamezole, Orion Pharma, Espoo, Finland) were administrated either intramuscularly in the tongue or intravenously to ensure a rapid recovery and minimize the risk of accidents (mainly drowning) after release. On average 0.9 (0.4–1.2) mL diprenorphine and 1.9 (0.7–2.6) mL atipamezole were used.
We measured the shortest distance between the place of darting and recovery of the animal. Locations of darting and retrieval of the sedated animal were determined by using a hand held GPS (Garmin 60CSx, Garmin, Olathe, Kansas, USA). Age and sex of immobilized animals were determined. Based on subcutaneous fat deposits over the rump and back, body condition was assessed as thin, below average, average, over average, and fat. Also, the level of immobilization was subjectively assessed based on physical response to external stimuli on a six-grade scale ranging from no drug effect to complete immobilization.
This study was approved by the Ethical Committee on Animal Experiments, Gothenburg, Sweden (Dnr: 405-2008). All statistical analyses were performed in Statistica 7 (Statsoft Inc. 2007). The individual animal was always the sampling unit. We tested the hypothesis that the needle length, sex, and body condition influenced flight distance (i.e., that the distance between the darting location to retrieval of the immobilized animal was significantly different between immobilizations performed with different needle length and among animals of different sex or body condition). We tested for normal distribution with Kolmogorov-Smirnov test and used analysis of variance. Spearman rank correlation was used when data were not normally distributed. We used a paired t-test to test differences in body condition between sexes.
Using a longer needle shortened the distance an animal moved after darting (Tables 1 and 2). Specifically, a longer needle (40 mm) resulted in animals being found at a shorter distance from the darting location (Fig. 1 and Table 2). A 30-mm needle resulted in an average recovery distance of 519 m from the darting place, whereas a 40-mm needle resulted in an average recovery distance of 294 m. Also, animals in better condition were found at a longer distance from the darting location (Table 2). Overall, females were in better condition than males (Table 1, paired t-test; P = 0.0013, nmales = 29, nfemales = 21, t = −3.41).
Straight-line flight distance between darting and the recovery site for 50 wild fallow deer (Dama dama), grouped by sex and needle length used for immobilization in Sweden, 2006–11.
Straight-line flight distance between darting and the recovery site for 50 wild fallow deer (Dama dama), grouped by sex and needle length used for immobilization in Sweden, 2006–11.
Average flight distance with 95% confidence interval and condition score for 50 wild fallow deer (Dama dama) divided by dart needle length and sex in Sweden, 2006–11.

Analysis of variance table for straight-line flight distance between darting and recovery sites for 50 wild fallow deer (Dama dama) in Sweden, 2006–11. Explanatory variables are the two needle lengths used for immobilization, body condition, and sex.

There was a positive correlation between the distance between darting place and where the animal was found and the retrieval time (Pearson rank correlation: n = 46; r = 0.40; P = 0.005). The average retrieval time was 40 min. Therefore, needle length influenced the time of retrieval such that an animal darted with a 40-mm needle was found faster (paired t-test; P = 0.0049, n30mm = 13 and n40mm = 30, t = −2.9). The average time to find an animal was 51 min when darted with a 30-mm needle and 29 min when darted with a 40-mm needle. In addition, animals immobilized with the 40-mm needle were more deeply sedated (paired t-test: P = 0.026, n30mm = 13 and n40mm = 28, t = 2.3).
Males and females differ in overall condition during winter and especially during and after the rut. Adult males are reported to lose 22% of body weight during the rut in Bulgaria (Ninov 2003) and in our study area they lose up to 50% of body weight during the rut (P.K. unpubl.). Depending on season of immobilization, we thus would expect that animals would be in different condition and this would likely result in differences in walking distances between males and females.
Independent of the overall study goal in ecological research, an underlying objective of capture and handling in the field concerns minimizing immobilization time and releasing animals as soon as possible (Chalmers and Barrett 1982). Our most important result was that a 10-mm longer dart needle can reduce retrieval time substantially (>20 min) until an animal is under monitoring. Shorter time until retrieval of a sedated animal will result in better monitoring which potentially decreases risk of capture related mortality.
We thank P. Grängstedt for help during fieldwork, the Silfverschiöld family for allowing us to work at their estate, and A. Friberg for his patience with all the interruptions in his daily work. This study was part of a larger project studying roe deer and fallow deer interactions and the ecology of wild fallow deer in Sweden managed by P.K. and was supported by grants from The Swedish Environmental Protection Agency to P.K. and U.A.B., the Swedish Association for Hunting and Wildlife Management to P.K., and the Swedish research council, the Helge Ax:son Johnson foundation, and the King Carl XVI Gustaf foundation for science, technology, and environment to U.A.B.