How Do Biologists Capture Island Foxes?

Meet guest blogger, FIF Board Member, Jessica Sanchez. 

Sanchez is a wildlife veterinarian and epidemiologist who's been working with Channel Island foxes since 2006. She started her career managing the captive breeding and wild fox monitoring program on Santa Cruz Island. For her master's degree, she researched social interactions among island foxes, and modeled vaccination and monitoring methods to prevent and detect disease outbreaks. She is currently a consulting veterinarian for island foxes across the Channel Islands, providing medical care and advising on biosecurity and fox health research.

Every year, island managers and biologists conduct island-wide capture of wild island foxes to monitor the population size, reproduction, and health of individuals. This is done using the same locations every year, so direct comparisons can be made and trends identified. (counting island foxes)

The first step is finding a safe location to place a box trap–somewhere with flat ground, so it won't roll. It has to be out of the direct sun and wind, away from water with no risk of flooding, and where we can use vegetation to cover and disguise the capture device.

 

We capture foxes using wire, humane, live box traps; the very same ones used to capture feral cats. There is a "treadle plate" at the back that is attached to the door with a stiff wire. When the animal steps on the treadle, the door closes behind them.

We disguise the wire bottom by covering it lightly with grass or other vegetation. This also provides a comfortable bed for the fox to spend the night. We attract the fox with a small cup of fox-safe food at the very back, past the treadle. The fox must enter far enough to trigger the door and close it behind itself. An aromatic scent is also spread on a branch above the box trap–this scent travels longer distances than the smell of the food and attracts the fox to the area.

We cover the box trap with vegetation so the island fox is shaded and protected from any other animals that might come poking around. Then, we leave it overnight and return to check it first thing in the morning.

Sometimes, animals will try to get to the bait without going inside. This trap was disturbed by ravens! Ravens are extremely smart and will learn that the brightly colored "flagging" we put on trees to mark locations means there is an easy meal nearby.

If a fox is in the box trap, we first weigh both the trap and the fox. Once the fox is released, we subtract the weight of just the trap to calculate the fox's weight. An adult island fox will weigh between 1.5–3 kg, or 3–6 lbs, with foxes being slightly different sizes on different islands (largest on Santa Catalina, smallest on Santa Cruz).

We carefully remove the fox from the trap, being sure not to get their tail caught in the door!

The first thing we do once we have the fox in hand is scan it for a microchip tag, just like the ones used in dogs and cats. This tells us that the fox has been previously caught. The microchips are small implants, the size of a grain of rice, and are inserted under the skin near the shoulder blades using a needle.

The microchip provides lifelong identification of an individual so we can track its history–the locations it was captured, its age, any injuries, whether it had offspring, vaccines administered, any blood test results, and radio-collar frequencies.  - Jessica Sanchez

See F257's multi-year story.

The microchip can also alert the biologist that the captured fox has already had a health check that year. If so, it is immediately released.

 

If not, the island fox will receive a full health check. Follow as Jessica provides an island fox with a Health Check Part 2 - body condition. 

Health Check Part 3 - Preventative care 

Other species encountered during health checks

Fox Foto Friday - Counting Island Foxes

Capture cages and ID microchips are key in counting island foxes across the Channel Islands. Camera trap images, like this one, may help provide additional information on island fox behavior. This fox is alone, appears healthy, and shows no sign of injury from interactions with other island foxes. It's also wearing something important around its neck.

Fresno Chaffee Zoo Selects FIF for Five Year Grant

Fresno Chaffee Zoo representatives visiting Santa Cruz Island

The Animal Care Staff from the Fresno Chaffee Zoo has long supported island fox conservation. They first raised funds for radio-tracking collars in 2006.

The Fresno Chaffee Zoo awards conservation grants to a variety of worldwide conservation efforts each year. Over the past eight years, Friends of the Island Fox has received support from the Fresno Chaffee Zoo to fund:

• 20 radio collars 
• 200 vaccinations during our 2012 drive

This summer, we received notification that a five-year grant has been awarded to Friends of the Island Fox for island fox conservation. Of 66 applications only 8 were extend 5-year support. Among the grant recipients, FIF is the only organization working with a North American species in the United States!

We are thrilled by the support from our friends at the Fresno Chaffee Zoo. While Fresno may be over the hills from the Pacific Ocean and the Channel Islands, FIF has participated in Fresno's Earth Day celebration and found their visitors very interested in the island fox and its survival.

The success of island fox conservation is based on local support and dedicated partnerships. The first year of the Fresno Chaffee Zoo grant supported important serology or blood testing looking for the presence of canine diseases among island foxes and the replacement of equipment important to annual counting and health checks.

Their second year of support will continue supporting fieldwork by replacing 43 of the field capture cages that safely enable island foxes to be captured for annual counting and health checks.

During the annual evaluation of the six populations, approximately 100 specially adapted capture cages are employed on each island. To ensure disease is not transferred between islands, each Channel Island has its own set of field equipment. With the financial support from our island fox friends at the Fresno Chaffee Zoo, Channel Island fox conservation enters its 17th year with much needed new equipment.

The Science of Counting Island Foxes

courtesy of National Park Service

In 2000 when there were only 15 island foxes surviving in captive breeding pens on San Miguel, you could count them easily on your fingers.

Today all Channel Island foxes across the six islands where they live are back in the wild. (A few rescued individuals from San Clemente Island can be seen in mainland zoos.) The recovery of the endangered island fox on San Miguel, Santa Rosa, Santa Cruz and Santa Catalina Islands has been record-breaking and the days of counting all of the physical island foxes are long gone.
 

courtesy of National Park Service

The process of counting foxes on each island begins in the fall with special capture cages.

These cages are set up either at the cross sections of grids or along a string depending on the island and its topography (More on strings and grids)

Cages in an area are set out over the course of a specific number of nights and checked each morning. On Catalina, each string is run for four consecutive nights; after that, typically the majority of individuals would be foxes that had already been captured on previous nights. Each island fox is identifiable because it has a Passive Identification Transponder (PIT) microchip. An island fox caught for the first time receives a PIT tag. From that point on the PIT tag allows it to be recognized as a specific individual. It can take weeks for biologists to evaluate each island.

Over the course of all the capture nights, the number of individual island foxes, their gender, ages and location caught are documented. Island foxes are quite territorial, therefore it is unlikely that a fox caught in one area of the island would be found on a different area of the island during the month of counting. (If it is, that is important too.)

The data is input into a modeling program for analysis. Different programs may be used on different islands and over the years land managers may change which program they use. The modeling program calculates the density of island foxes in specific habitats across the island and estimates a total island population.

Sometimes, however, it takes a human brain to see complexity in the numbers. On two islands where recovery has been robust over the past few years, San Miguel and Santa Catalina, the estimated population numbers appear to be higher in 2013 than they were in 2012.

But observations on both islands conclude that drought may have reduced births and did cause starvation in a number of pups last summer, prior to counting. How could the population number increase while the number of surviving pups decreased significantly?

Biologists Calvin Duncan and Julie King from the Catalina Island Conservancy explain that on Catalina they caught a greater number of adult island foxes that had not been counted for a number of years. Young animals are more likely to be curious and challenged with finding food; they venture into capture cages more readily. Once an animal is in the cage, no other can be caught that night. This year, on the fourth night of capturing, they were still finding a high percentage of animals that had not been caught on previous nights.

courtesy of Kevin Schafer

The increase in the population number for Catalina is warranted because of the number of unique individual animals that were counted. But rather than an increase in the population, the number represents a refinement of the estimate toward greater accuracy.

On both San Miguel and Santa Catalina Islands, island fox numbers have surpassed historic population figures. Both islands may have reached carrying capacity, or the number of individual animals that can find the territory and food resources necessary for a healthy life. As the drought continues this summer, there may be further impacts not only on pups, but also on older individuals.

Counting island foxes provides an important picture of island fox recovery, but observations throughout the year, health checks, blood tests, data collected from radio-collared island foxes and necropsies is vitally important to understanding the whole recovery picture.

Counting Channel Island Foxes

How do you count a population of small animals across an island, when they are so well camouflaged?

How long did it take you to find the island fox?   photo courtesy of M. Jakubowski

Each July to December, across the six Channel Islands where island foxes live, biologists put out special capture cages to catch individual animals. These wire box-traps have a door that snaps shut when the fox steps on a trigger plate at the rear of the box.

The number of cages put out each night and the manner in which they are arranged differs between islands and land managers. Tim Coonan, senior biologist for Channel Islands National Park explains that on Santa Rosa and San Miguel "Traps are deployed in small grids." These grids typically are stretched over rolling hills. The grid pattern may be two traps by six traps wide or three traps by six traps wide. The capture-trap locations are spaced approximately 250 meters apart with the capture cages usually under bushes or scrub. The cage is covered with  burlap to further protect the fox from the elements.

Cages are baited with fragrant substances particularly interesting to island foxes. Each cage has food and water. The grid of cages is checked each morning. Foxes in the cages are first scanned for their microchip ID. Biologist Calvin Duncan explains why this is important.

Each individually identified island fox is counted. This year's pups are counted separately to determine an estimated fertility rate. Pups born in April are old enough to be on their own, but they usually are somewhat smaller than adults and will not yet have received an ID microchip. While in-hand, the island fox receives a health check, vaccinations and an ID microchip or radio tracking collar if needed.

On Catalina Island capture cages are put out along lines called "strings." On mountainous terrain, strings of cages are more practical than grids. On all of the islands, actual numbers of individual island foxes caught during the fall are entered into various population modeling software to determine the best estimate for the total population. After strenuous weeks of counting in the field, it takes months in the office to produce reliable numbers. That is why the number of island foxes counted in the fall becomes official in June of the following year.

These are the official island fox population numbers through fall of 2012.

• San Miguel Island - 540, up from a low of 15 in the year 2000
• Santa Rosa Island - 637, up from a low of 15 in the year 2000
• Santa Cruz Island - 1,354 , up from a low of ~62 in the year 2002
• Santa Catalina Island - 1,502 , up from a low of ~103 in 2000

Notice how Santa Catalina and San Miguel both showed slight population drops last year. Biologists believe this a sign the populations are stabilizing at their natural levels. Drought this year may impact populations as well. Notice too how Santa Rosa is finally showing robust population increase. We hope this population will continue to recover. Whether the numbers are up or down, counting island foxes is the only way we can have the best estimate of population recovery.