Each week in June, we eagerly returned to lower Dark Canyon within the San Bernardino National Forest. Treading lightly with eyes carefully scanning every rock, log, and crevice in the creek bed, we walked in and along the creek searching for the 111 young mountain yellow-legged frogs we raised from last year’s breeding season and released in the creek May of this year. Our goal is to catch each one we see and assess how they are doing out in their new home, but that’s easier said than done.
At barely 4 centimeters in length and camouflaged perfectly against the speckled granite rocks, the froglets can be nearly impossible to see. With some luck, the sun is just right and the rocks along the creek are warm, inviting the frogs to climb out into the open to catch some rays. The key to catching the frogs is spotting them well before we approach, and then being very sneaky. The slightest shadow, snapping branch, or abrupt movement can cause them to jump from their sunny basking spot, quickly swim to the bottom of the creek, and wedge themselves out of sight under a stone or in the soft mud.
Once one is spotted, we move closer and try to swiftly catch the frog with our nets before it can jump off and hide. Sometimes the frog gets away and we are left staring down into the water for any hint of movement, hoping we get a chance to try again. More often though, we are victorious and can then proceed to scan the frog’s microchip, take its weight, and measure its body length. We also note the GPS coordinates where the frog was found. Since each frog can be uniquely identified, we are able to track each frog’s growth and assess how far it has traveled from the original release site. After our brief exam, the frog is returned to the spot we found it and we continue our walk up the creek in search of the next one.
The survey path only covers about 300 meters of habitat, but with our slow, deliberate pace each survey takes almost 3 hours from start to finish. At the end, our eyes are tired from the constant scanning of the creek but we’re excited to get home and look at the data. In the last four surveys, we have captured 31 frogs out of the 55 we saw, the grand majority of which were only captured once. This means we were able to collect growth data from about 25% of the frogs we released in May! Quite a feat all things considered! Every frog caught appeared healthy and the data supports that they have grown even in the short time since their release. We’ll continue to survey this new population each month until fall and will add in searching for tadpoles after our July release of tads hatched this spring.
Check back for more updates on our reintroductions and field research during this busy year. If you missed the blog about our reintroduction in May at the Dark Canyon site, check it out here: http://institute.sandiegozoo.org/science-blog/booming-first-half-our-10th-year-mountain-yellow-legged-frog-program