Pronghorn Migration Corridors - Clear Lake - 2015-2020 [ds2932]

The project lead for the collection of this data was and Richard Shinn. Pronghorn (28 adult females) were captured and equipped with GPS collars (Sirtrack, Havelock North, NZ) transmitting data from 2015-2020. The Clear Lake herd contains migrants, but this herd does not migrate between traditional summer and winter seasonal ranges. Instead, much of the herd displays a somewhat nomadic migratory tendency, slowly migrating north, east, or south for the summer using various high use areas as they move. Therefore, annual home ranges were modeled using year-round data to demarcate high use areas in lieu of modeling the specific winter ranges commonly seen in other ungulate analyses in California. The areas adjacent to both east and west of Clear Lake Reservoir are highly used during winter by many of the collared animals. Additionally, a few individuals persist west of Highway 139 year-round, seemingly separated from the rest of the herd due to this highway barrier. However, other pronghorn cross this road near Cornell and join this subgroup. Summer ranges are spread out, with many individuals moving southeast through Modoc National Forest or as far north as Fremont National Forest in Oregon. A few outliers in the herd moved long distances south, crossing Rt 139 to Oak Ridge, or east into Likely Tables pronghorn herd areas. GPS locations were fixed between 1-6 hour intervals in the dataset. To improve the quality of the data set as per Bjørneraas et al. (2010), the GPS data were filtered prior to analysis to remove locations which were: i) further from either the previous point or subsequent point than an individual pronghorn is able to travel in the elapsed time, ii) forming spikes in the movement trajectory based on outgoing and incoming speeds and turning angles sharper than a predefined threshold , or iii) fixed in 2D space and visually assessed as a bad fix by the analyst. The methodology used for this migration analysis allowed for the mapping of the herd’s home range and the identification and prioritization of migration corridors. Brownian Bridge Movement Models (BBMMs; Sawyer et al. 2009) were constructed with GPS collar data from 23 migrating pronghorn, including 72 migration sequences, location, date, time, and average location error as inputs in Migration Mapper. The average migration time and average migration distance for pronghorn was 12.11 days and 34.18 km, respectively. Corridors and stopovers were prioritized based on the number of animals moving through a particular area. BBMMs were produced at a spatial resolution of 50 m using a sequential fix interval of less than 27 hours. Due to varying fix rates in the data, separate models using Brownian bridge movement models (BMMM), with an adaptable variance rate, and fixed motion variances of 1000 were produced per migration sequence and visually compared for the entire dataset, with best models being combined prior to population-level analyses (68% of sequences selected with BBMM). In general, fixed motion variances were used when BBMM variances exceeded 8000. Home range analyses were based on data from 24 pronghorn and 47 year-round sequences using a fixed motion variance of 1000. Home range designations for this herd may expand with a larger sample, filling in some of the gaps between home range polygons in the map. Large water bodies were clipped from the final outputs. Corridors are visualized based on pronghorn use per cell, with greater than or equal to 1 pronghorn, greater than or equal to 3 pronghorn (10% of the sample), and greater than or equal to 5 pronghorn (20% of the sample) representing migration corridors, medium use corridors, and high use corridors, respectively. Stopovers were calculated as the top 10 percent of the population level utilization distribution during migrations and can be interpreted as high use areas. Stopover polygon areas less than 20,000 m2 were removed, but remaining small stopovers may be interpreted as short-term resting sites, likely based on a small concentration of points from an individual animal. Home range is visualized as the 50th percentile contour of the home range utilization distribution.