Unsustainable harvest of water frogs 367
FIG. 1 Seasonal estimates of population size of Anatolian water frogs (Pelophylax spp.) in the sampling locations in southern Turkey during 2013–2015.
(Ministry of Food, Agriculture and Livestock, 2016). Of 10,295 marked adults, 707 were recaptured. Overall sex ratio among the marked individuals was 1.55 (male:female), with a range of 0.37–1.76 across all locations and years. Population sizes as estimated by CAPTURE analyses
were highest in spring and lowest in autumn, with a slight decrease across years (Fig. 1). Apparent survival rates (f) averaged 0.35 for juveniles and 0.56 for adults, and mean fe- cundity (F × Sa)was 0.38 (Table 1). The eigenvalue of the re- sulting stage matrix is 0.74, corresponding to a 26% decline per year (the eigenvalue gives the proportional change in population size from one year to the next, as in N(t + 1)/N(t), with 1.0 corresponding to no change, and values ,1 cor- responding to population decline). This is a steeper decline than the population reduction based on capture estimates, which was c. 20% per year. The difference is probably be- cause of emigration (i.e. the apparent survival underestimat- ing true survival). Using the 5% upper bound of emigration that we assumed, the stage matrix becomes (Table 1):
0.000 0.381 0.400 0.612
The eigenvalue of this matrix is 0.802, which is more
consistent with the observed declines. Thus, we used the above matrix, with values corrected for emigration, for further analysis. Even with this correction, the 10-year population reduction would be c. 89%(1–0.80210), and 3-generation (c. 13-year) reduction would be .90%. Sto- chastic simulations estimated a 95.6% risk of extinction in 50 years, with a median time to extinction of 36 years, and an almost 100% risk of the population declining to ,1,000 individuals. The analysis of capture efficiency resulted in estimates of
survival rate in a non-harvested population averaging 0.55 for juveniles and 0.85 for adults (after correction for emigra- tion; Table 2). The resulting stage matrix has an eigenvalue of 1.115, corresponding to an 11.5%increase per year in a non-
harvested population. This is a reasonable value, consider- ing that the population must be below carrying capacity as a result of the effects of past harvest, so a moderate popula- tion increase in the absence of harvest would be expected. However, the growth rate may be higher at lower densities, which would influence the estimation of harvest impact. Simulations with harvest mortality (H)of 0–0.3 showed that risk of population decline increases with harvest rate, reaching c. 100% when H.0.2, and the total harvest over the 50-year simulated period reaches a maximum at c. H= 0.05, and declines with increasing H above this value (Fig. 2).
Harvesting history and export statistics
Frog harvest and export data statistics have been recorded by TÜİK since 1991, but the records are not complete until 2002 and inconsistent thereafter. This is probably because of inaccurate and incomplete information provided by the har- vesters and export companies. Nevertheless, it is clear that Turkey plays an important role in the frog trade, especially to European countries. In 2017 alone, nearly 350 t of frogs were exported, worth .USD 3.5 million. During 1991–2017 frog harvests from Seyhan and Ceyhan Deltas comprised 34.4%(nearly 4,500 t) of the total harvest in Turkey (Fig. 3). Since 2012 the average harvest from this area was 327 t per year (TÜİK Fisheries Statistics, 2018). Based on the mean weight of frogs in our study, this harvest repre- sents c. 17 million frogs per year from the Ceyhan–Seyhan region. Frogs are also farmed, where they are raised in man-
made structures and supplemented with feed. This is a relatively new business in Turkey, with c. 12 farms estab- lished within the last decade (Kürüm, 2015). These farms contribute an average of 45 tons annually to frog exports, with a total of 180 tons during 2014–2017 (TÜİKFisheries Statistics, 2018).
Oryx, 2021, 55(3), 364–372 © The Author(s), 2020. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605319000176
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