In Part 1 of this blog series we discussed the invasive species management curve, where we introduced the importance of “Early Detection and Rapid Response”. We explained that the early detection process involves conducting call surveys to track American bullfrogs (Rana catesbeiana) throughout the Fraser Valley. In Part 2, we will be discussing what happens when we detect bullfrogs at a high value Oregon Spotted Frog (OSF) location.
Once a bullfrog has been detected at a high value OSF location, the rapid response part comes into play. In the case of bullfrogs, the response is typically to remove them from the system. Two ways of accomplishing this are to hunt the adults at night and to survey for and destroy egg masses in the day. In 2018, the team found that conducting egg mass surveys to be more effective than hunting adults at Morris Valley. Egg mass surveys are done by kayak in teams of two. Surveys should be done every two days during the breeding season as bullfrogs develop from eggs to tadpoles within 48 hours in favourable conditions. The surveyors begin by scanning the water surface throughout the site until an egg mass is discovered. Once a bullfrog egg mass is identified, the surveyors use a hand-pump to pull the egg mass into a bucket. Balancing on a kayak while holding a hand-pump and a bucket can be tricky! The egg masses are paddled to land where they are poured out and left to desiccate. In 2018, we found and destroyed five bullfrog egg masses. Destroying even a single egg mass is considered a victory, as they can contain up to 20,000 eggs. However, this method isn’t 100% effective as some eggs or egg masses will be missed. Therefore, it is only one component of a broader effort to ensure the impacts of bullfrogs on OSF are minimized.
**Important – if you are considering Bullfrog control, make sure you are targeting the right egg masses! Remember: Fraser Valley Bullfrogs and Green Frogs breed in the wetlands and ponds in the summer, while our native wetland/pond-breeding amphibians lay their eggs in the spring. Always consult a guide, (like this one) and if you are unsure, ask for an expert opinion.**
These images show a fresh bullfrog egg mass. It was found on the edge of a channel in one metre deep water. The egg mass was mostly floating on the surface, with some parts attached to emergent vegetation. The underwater image clearly shows the egg mass floating on the water surface.
While surveying for egg masses, the team came across an interesting organism, the magnificent bryozoan (Pectinatella magnifica). This strange looking egg mass is actually a colony of small filter feeding invertebrates. Bryozoans are an ancient clade, dating back to 500 mya.
To learn more, check out this blog post. https://nerdist.com/bryozoan-snot-monster-science-biology-nerdoween/