Egg mass surveys

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Collecting eggs for husbandry. Photo: Isabelle Groc

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Embryos in late development, just before hatching. Photo: Andy Wright

How we do egg mass surveys

Every year in the spring, when breeding time begins, we conduct a detailed census of Oregon Spotted Frog breeding for all known populations in the Fraser Valley of BC. By counting individual egg masses (distinct jelly clusters that can contain up to 1400 individual eggs), we can identify exactly how many females have bred in a given year, and estimate adult population sizes. Although slogging through wetlands counting egg masses can be very labour intensive, it’s the most efficient way of monitoring population trends, and the most basic tool in our toolbox.

We walk or kayak through all wetted areas in and around known breeding sites (they tend to breed in the same locations each year), and as far as we’re able into the surrounding waterways to ensure we’re not missing any egg masses. We collect exact locations and counts of all egg masses and frogs encountered, and return a minimum of three times to ensure we haven’t missed the late breeders.

Despite our best efforts, egg mass surveys are simply not possible in some sites where Oregon Spotted Frogs may lay their eggs. Some suitable habitats are too deep for people in waders, but too thickly vegetated to access by boat. In 2015, we tested out Unmanned Aerial Vehicles (UAV, drones) for egg mass surveys in some known locations, developed methods, and intend to use them for surveys in previously inaccessible locations in future years.

Results

Egg mass surveys began in 2000, with variable effort through the early years due to inconsistent funding, and variable effort in some locations due to inconsistent access permissions. The following results are from sites in which we have been able to consistently survey through the years.

Numbers are low. We see between 60 and 400 egg masses each year, which means that’s how many females are breeding. We estimate 1 adult male per female, meaning we expect there are under 1000 adult Oregon Spotted Frogs in Canada.

Sadly, we watched the Aldergrove population decline from 90 egg masses in 1997 to none in 2007, and have not seen breeding at that location since then. Why? We’re not sure. Perhaps the habitat changed too much; perhaps the invasive American Bullfrog population at the site overwhelmed the Oregon Spotted Frog population; perhaps we started monitoring them when the chytrid fungus arrived, and we simply documented their decline.

Unfortunately we don’t have ‘before’ data to compare with the ‘after’ data. Some individuals from this population were captured and held for breeding at the Toronto Zoo as an assurance population.