Population parameters and life-table analysis of two coexisting freshwater turtles: are the Bellinger River turtle populations threatened?

Abstract

Two species of freshwater turtle coexist in the Bellinger River: Elseya georgesi is common but limited to the Bellinger River, whereas Emydura macquarii is widespread but rare in the Bellinger River. The Bellinger River population of E. macquarii has been proposed as a distinct subspecies, so it may be endangered. Survivorship, fecundity, growth, size and age were determined for El. georgesi and the finite rate of increase (λ) was estimated by a life-table analysis using mark–recapture data from surveys between 1988 and 2004. These parameters were compared with those of well studied populations of E. macquarii to assess whether modelling the demographic parameters of El. georgesi could serve as a surrogate for estimating the influences of these demographic parameters on λ in the Bellinger River population of E. macquarii. We estimated that ~4500 El. georgesi inhabit the study area and, despite a size distribution strongly biased towards large individuals, the population is increasing (λ = 1.15) in the best-case scenario, or slightly decreasing (λ = 0.96) in the worst-case scenario. Comparing El. georgesi with E. macquarii from the Bellinger River and elsewhere suggests that E. macquarii grows faster, attains greater maximum size, has a greater clutch size and a higher fecundity than El. georgesi. Hence, El. georgesi does not serve as a good surrogate to determine demographic influences on λ in E. macquarii.

On a razor's edge: Status and prospects of the critically endangered Bellinger River snapping turtle, Myuchelys georgesi

“…Historically, the M. georgesi population in the Bellinger River was dominated by mature individuals. Immatures made up 16% of the M. georgesi captured in 1988–2004 (Blamires et al, ), 4–5% in 2007 (Spencer et al, ; present study), and 23% in the river upstream of Thora in 1988–2008 (Blamires & Spencer, ). The proportion of immatures was also low among diseased turtles in 2015 (8%).…”

“…However, recognition of mature turtles was constrained by the limited available information on size at maturation. Blamires et al () reported a minimum SCL of 154 mm for gravid female M. georgesi , but the maximum size of immature females, the mean size at female maturation, and the minimum, mean, and maximum sizes at male maturation are unknown for this species. In the absence of such information, M. georgesi with SCL ≥ 154 mm and without enlarged tails were assigned as mature females.…”

“…Historically, the population size of M. georgesi in the Bellinger River was estimated at 4,468 ± 1,409 individuals (mean ± SE) by capture–mark–recapture (CMR) analysis of data obtained in 1988–1991 and 2000–2004 (Blamires et al, ). The population size of E. macquarii was not estimated at this time but was probably about 100, as only 11 E. macquarii were captured compared with 466 M. georgesi.…”

“…Subsequently, E. macquarii apparently became more common, with Spencer, Georges, and Welsh () recording 360 M. georgesi and 76 E. macquarii in a survey in 2007. However, 26 of the E. macquarii were caught by dip netting from a boat at night, a capture method not employed by Blamires et al (). Density estimates from surveys in 2007 and 2014 suggested an M. georgesi population half the size calculated by Blamires et al (); that is, about 2,200 individuals (Spencer et al, ).…”

“…However, 26 of the E. macquarii were caught by dip netting from a boat at night, a capture method not employed by Blamires et al (). Density estimates from surveys in 2007 and 2014 suggested an M. georgesi population half the size calculated by Blamires et al (); that is, about 2,200 individuals (Spencer et al, ). The population size of M. georgesi immediately before the disease outbreak is unknown.…”