Abstract
Consumption of frog legs is increasing worldwide, with potentially dramatic effects for ecosystems. More and more functioning frog farms are reported to exist. However, due to the lack of reliable methods to distinguish farmed from wild‐caught individuals, the origin of frogs in the international trade is often uncertain. Here, we present a new methodological approach to this problem. We investigated the isotopic composition of legally traded frog legs from suppliers in Vietnam and Indonesia. Muscle and bone tissue samples were examined for δ15N, δ13C, and δ18O stable isotope compositions, to elucidate the conditions under which the frogs grew up. We used DNA barcoding (16S rRNA) to verify species identities. We identified three traded species (Hoplobatrachus rugulosus, Fejervarya cancrivora and Limnonectes macrodon); species identities were partly deviating from package labeling. Isotopic values of δ15N and δ18O showed significant differences between species and country of origin. Based on low δ15N composition and generally little variation in stable isotope values, our results imply that frogs from Vietnam were indeed farmed. In contrast, the frogs from the Indonesian supplier likely grew up under natural conditions, indicated by higher δ15N values and stronger variability in the stable isotope composition. Our results indicate that stable isotope analyses seem to be a useful tool to distinguish between naturally growing and intensively farmed frogs. We believe that this method can be used to improve the control in the international trade of frog legs, as well as for other biological products, thus supporting farming activities and decreasing pressure on wild populations. However, we examined different species from different countries and had no access to samples of individuals with confirmed origin and living conditions. Therefore, we suggest improving this method further with individuals of known origin and history, preferably including samples of the respective nutritive bases.
Highlights
The international trade of frog legs encompassed five million tons of meat in 2007 (UN Statistics Division)
We aimed to find a method to and reliably distinguish intensively farmed frogs from those growing under natural conditions
We present evidence that stable isotope analyses may serve as such a useful tool
Summary
The international trade of frog legs encompassed five million tons of meat in 2007 (UN Statistics Division). Different attempts were made to shed light on the identity of traded species, but to know which species are involved does not necessarily allow conclusions if they were farmed or wild-caught (Veith, Kosuch, Feldmann, Martens, & Seitz, 2000). Another promising attempt was measuring femoral bone density, where frogs from captivity show lighter bones (Yang, Huang, Xia, Xu, & Dahmer, 2011). We present a possible approach to answer the question if a traded frog was intensively farmed, reared under natural-like conditions or caught from the wild, by comparing isotopic values from tissue samples of legally traded frog legs. Our hypotheses were the following: (1) the nitrogen isotopic signal from intensively farmed individuals should be lower than expected from naturally growing populations (lower protein source food/trophic level) and (2) the variability in isotopic v alues should be lower in intensively farmed individuals (same conditions for all individuals), compared to those growing under natural conditions
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