Impacts of river alteration for flood control on freshwater turtle populations

Abstract

The impacts of artificial river alteration for flood control on the dynamics of freshwater turtle populations were studied in small urban and rural river systems in northwestern Chiba Prefecture Low turtle population densities in other river sections where flood control measures have already been implemented suggest slow population recovery. River alteration practices also altered the age structures, sex ratios, and demographic characteristics of the turtle populations, although the exact consequences for population viability have not yet been determined. Because the turtles select concave submerged riverbanks or bottoms with slow or no current velocity, increased current velocity in the altered river section make the entire riverbank and bottom unsuitable for hibernation. Overall, river alteration for flood control during the winter season when turtles are inactive due to low water temperature negatively impacted freshwater turtle populations.

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

“…River turtles may have poorer body condition when flows are lower or less frequent because of reduced food supply and opportunities for foraging (Bondi & Marks, ; Howard, Beesley, Ward, & Stokeld, ). However, high river flows or velocities can also impair turtle populations (Lenhart, Naber, & Nieber, ; Usuda, Morita, & Hasegawa, ). Although flows in the Bellinger River were sometimes low in spring 2014, the river did not cease to flow or fall below historical low flows.…”

“…Higher temperatures are usually beneficial to freshwater turtles, because warmer waters tend to have greater food productivity, or allow a longer growing season and more rapid food digestion (Ashton, Bettaso, & Welsh, 2015;Frazer, Greene, & Gibbons, 1993;Gibbons, 1970;Thornhill, 1982 (Bondi & Marks, 2013;Howard, Beesley, Ward, & Stokeld, 2016). However, high river flows or velocities can also impair turtle populations (Lenhart, Naber, & Nieber, 2013;Usuda, Morita, & Hasegawa, 2012). Although flows in the Bellinger River were sometimes low in spring 2014, the river did not cease to flow or fall below historical low flows.…”

Species Distribution Modeling Provides New Insights into Different Spatial Distribution Patterns among Native and Alien Freshwater Turtles in Japan

“…Therefore, temperature ultimately determines the distribution and population dynamics of freshwater turtles. Furthermore, micro-habitat environments, such as current velocity (e.g., Buhlmann and Vaughan, 1991;Usuda et al, 2012;Kagayama, 2019), may also be important. For example, Kagayama (2019) surveyed the distribution patterns of three freshwater turtle species in the Yorogawa River system (35°24'2.5842" N, 140°7'46.6896" E) and showed that M. reevesii and T. s. elegans were abundant in the lower regions of the river, whereas M. japonica was abundant in the middle and upper regions of the river.…”

“…It was expected that climatic factors, such as the highest temperature of the warmest month (August), the lowest temperature of the coldest month (January), and total daylight hours limited the distribution of freshwater turtle species because these ectotherms required moderate temperatures and solar radiation to control their body temperature (Vitt and Caldwell, 2014). Furthermore, slope as the topographic factor and precipitation may affect the dispersal or establishment of freshwater turtles in the high-elevation regions because the distribution of turtles may be limited by the velocity of water currents (Buhlmann and Vaughan, 1991;Usuda et al, 2012;Kagayama, 2019). All five variables of grid data were divided by 1 km grid cells using ArcGIS 10.0 (ESRI, CA, USA), and we calculated only the total daylight hours from April to November using ArcGIS 10.0 and the Mesh Climate Value, which is a contiguous nationwide grid consisting of 1 km 2 cells because most freshwater turtles hibernate during winter in the water (Ultsch, 1989).…”

“…A nationwide turtle survey conducted by the Nature Conservation Society of Japan demonstrated that M. japonica had become uncommon in many areas of Japan, and the most abundant freshwater turtle was the introduced red-eared slider T. s. elegans and M. reevesii (The Nature Conservation Society of Japan, 2014). Some studies have suggested that recent population declines of M. japonica have been caused not only by habitat degradation due to river alteration for flood control (Usuda et al, 2012) and predation by the introduced raccoon, Procyon lotor (Kosuge and Kobayashi, 2015), but also by interspecific interactions with alien turtles with similar ecological requirements (Yasukawa et al, 2008).…”