Bullfrog

Identification & Biology
Latin Name : Lithobates catesbeianus
Category : Amphibians & Reptiles
Description :

The Bullfrog is quite distinctive in all its life stages. It is a very large, robust frog, green or brown in colour and with large golden eyes. Adult female Bullfrogs may reach 20 centimetres in length (not including legs!) and 750 grams in weight. Male Bullfrogs are somewhat smaller, as is usual for amphibians. Both sexes have a large and distinct tympanum (‘ear’) just behind and below the eye. The tympanum is partly surrounded by a fold of skin that runs from the eye down to the shoulder. Males have a tympanum roughly twice the size of the eye, while females have a smaller tympanum that is about the same size as the eye. The sexes may also be distinguished by their throat colour – males have yellow throats, often quite bright, while females have paler cream or white throats.

Green Frogs, another introduced species, also have a conspicuous tympanum, but they do have dorsolateral folds. They do not grow as large as Bullfrogs.

You may often see adult bullfrogs floating just beneath the water’s surface – all that is visible are their eyes and their green snouts. Even more often, you may hear the deep bwaa, bwaa call of the males in the spring and summer; the calls can carry up to a kilometre. Sometimes, when approaching a shoreline, you may hear frogs leaping into the water with loud splashes and “eep!” cries; these are juvenile Bullfrogs, avoiding you as a potential predator. None of the native frogs in B.C. do this, though Green Frogs do.

Bullfrog tadpoles are as easy to spot as their parents. Again, they are quite large – up to 15 cm long! Bullfrogs spend one or sometimes two years in the tadpole stage, compared to just a few months for many other frog species. The tadpoles are very dark green – almost black – in colour, with an arrowhead-shaped body and a dorsal fin that begins behind the body. Newly metamorphosed Bullfrogs are small compared to adult Bullfrogs, but are still as big as the adults of other species. They can be distinguished from native frogs by their large tympanum.

Habitat

Bullfrog populations have become established in B.C. in the Lower Mainland, Lasqueti Island, and Vancouver Island as far north as Campbell River. They have also been reported in the South Okanagan.

Bullfrogs need to live in water and seem to prefer fairly shallow, warm ponds and lakes with lots of vegetation. Sometimes they are seen in ditches and slow streams, but in general they prefer standing water. The breeding ponds must be permanent, due to the long tadpole stage. On occasion, adult or juvenile Bullfrogs may travel short distances over land, dispersing to new ponds; juveniles travel in the fall, shortly after metamorphosis, while adults are more often seen travelling in the spring.

 

Impact & Risks

Because of their large size and voracious appetite, bullfrogs outcompete and prey upon many indigenous species. Adult bullfrogs are highly predatory, consuming insects and other small invertebrates, birds, small mammals, snakes, fish and other amphibians (including smaller bullfrogs). There is evidence that bullfrog colonizations of lakes are followed by declines in the native Red-legged Frog and Pacific Chorus Frog populations. Bullfrog tadpoles are primarily herbivorous, grazing on algae and detritus. Bullfrog tadpoles compete for food and habitat with the tadpoles of native frogs. Bullfrogs can also spread the chytrid fungus, which is responsible for declining amphibian populations.

Treatment & Disposal
  • Do NOT purchase bullfrogs as pets
  •  Never release bullfrogs into the wild
  •  Do NOT transport wild bullfrogs or tadpoles from place to place, it is illegal under the Wildlife act to capture, transport, keep or sell wild animals including bullfrogs.
  •  Large tadpoles can be a tempting pet or a popular addition to your backyard pond, but this has been identified as a primary way bullfrogs have been spreading through the province
  •  “Build it and they will come!” If you want native frogs in your backyard pond the best thing to do is create a welcoming environment for the frogs: water, shelter and insects
    If you see or hear a bullfrog report it to OASISS
Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.

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