Counting tadpoles – one, two, skip a few, ninety-nine, 20 thousand!

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 The Oregon spotted frog is Canada’s most endangered frog, with fewer than 300 breeding individuals in the wild. The recovery plan for Oregon spotted frog calls for conservation breedingheadstarting and release to re-establish viable populations in at least six sites. Wildlife Preservation Canada began coordinating these efforts in 2010, adding a conservation breeding program to the headstarting work taking place at the Greater Vancouver Zoo. This expanded on the breeding efforts underway at the Vancouver Aquarium.

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Three staff from the Oregon spotted frog recovery team spent five days counting tadpoles, coming up with a whopping 20,514 tadpoles! (Give or take a few – you can see how this could be a difficult task!)

We had amazing success with our egg laying females this year with 41 out of 43 successfully laying their eggs! (Read more about how we helped this happen, what we changed and our ideas why here). This is great for our program. It means we have learned at the very least the “how” portion of getting females to release their eggs, even if the “why” is yet to be determined. Getting females to lay their eggs means that we will be able to minimize the number of females with egg binding, where the females retains their eggs through the end of the breeding season. Egg binding either results in infection and ultimately death or would require risky surgery which renders the females sterile. Getting eggs out of the females reliably was step number one.

Now, we dream bigger! What if those eggs are actually fertile? In the past we have had poor fertility in the captive program, with most clusters under 10% fertility. This means that in a typical year, even if we get 30+ egg masses we would see only around 2000 tadpoles. When we are looking at breeding endangered species low fertility could be caused by on overwhelming number of factors.

  • Is the quality of eggs and sperm poor?
  • Is the water chemistry off? If so how is it off?
  • What are the captive compared to wild water parameters?
  • Is the nutrition of the frogs not appropriate? If so how?
  • At what life stage is nutrition most important?
  • Are the genetics of the captive population inbred or otherwise not compatible?

And the list goes on!

Once the eggs are laid all we can do is wait and see. Luckily for us, tadpole eggs are massive compared to some other species, and a lot easier to see since they are external to the animal and encased in clear jelly. In fact we can closely track the stages of the developing embryo through a process called Gosner Staging. This key below helps us to see the sequential progression of the eggs through the defined stages and compare between clutches. So once we have eggs we wait with baited breath, for the signs of development.

The Gosner Staging System for Anurans, an amphibian development chart

Soon we start to see changes. We were thrown off by the very first egg mass where there was hardly any development. Disappointed, we thought, “Oh well, at least we learned how to get the females to lay eggs reliably, that’s a huge achievement.”

But….wait. The first egg mass was a dud, but the next ones are getting a blastopore, which looks like a white blotch (about stage 10), and cell cleavage. Soon the rest of the egg masses had developed to what we affectionately call the “jelly bean” stage (stage 15). And then we thought to ourselves…is this actually happening?!?

Yes, yes, it is! Well, we thought, “At least its April, the weather will be cool and development will be slow. As they start to hatch we can slowly count them and release them, since we can’t possibly rear them all.”

But the weather forecast said – haha yeah right, how about summer in April instead! Daytime temperatures shot to 25+ and nights above 15. This meant our bounty of tadpoles grew very fast, hatched in record speed, and all at once.

The team sprang into action. We arranged a release day in 5 days from first hatch and began to count tadpoles. Three staff members spent the next 5 days counting. Within the first day we had already counted more tadpoles than we would have in a normal year, over 3000, and we hadn’t even made a dent. The next days we would see at least 4000 more per day. If you’re doing the math with me you can see that number going up.

Once the dust had settled and the spread sheet was compiled we had counted a whopping 20,514 tadpoles, give or take a few. It is generally accepted that to establish a species like a frog, it is necessary to release 5,000 animals a year for 5 years. Our production of 20,000 tadpoles this year alone is double the number we released in total over the past 10 years! We are in a much better position to establish new populations of this species as well as supplement exiting populations of this frog.

The release of these animals into our restored site dwarfs all previous releases at this site…combined.

Now with tadpoles counted and 1000 retained to rear throughout summer, we take a deep breath and prepare to drive our precious cargo to the release site for their big day. Stay tuned for our release day blog!

Does anyone know where I can get 20,000 tiny seat belts?

Andrea Gielens

Lead Biologist – Fraser Valley Wetlands Recovery Program

Andrea manages WPC’s captive breeding and release programs for the Oregon spotted frog and the coastal western painted turtle. Andrea has studied at-risk reptiles and amphibians in Canada and abroad, including a term at the Durrell Wildlife Conservation Trust in Jersey. Andrea also manages the Taylor’s checkerspot butterfly recovery program in BC.

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