Food Habits of the Chinese Stripe-Necked Turtle, Ocadia sinensis, in the Keelung River, Northern Taiwan

Abstract

-Stomach contents from 186 Ocadia sinensis collected from the Keelung River, northern Taiwan, between July 1995 and April 1997 were examined. Diet composition varied by sex. Females shifted from an omnivorous to a herbivorous diet as they grew, and they consumed more plant material compared to males. Volumetrically, plant matter represented 87.7% of the total stomach contents in females, whereas it only constituted 39.5% in males. Seasonal differences in diets were also found. Animal matter contributed more to cool season diets. Females, especially, tended to take plants in much greater proportion in the warm season (94.8% in volume) compared to the cool season (50.9%). In males, animal matter occupied 54.4% and 71.1% of the total volume of stomach contents both in the warm and cool season, respectively. The percentage of empty stomachs was greater in the cool season (41.2%) than in the warm season (13.3%). The Chinese stripe-necked turtle (Ocadia sinensis) is reported to be largely herbivorous, feeding mainly on plant material both in nature and in captivity (Smith, 1923; Horikawa, 1934; Ernst and Barbour, 1989). Although 0. sinensis is abundant and widespread in rivers, ponds, and lakes of Taiwan (Pope, 1935; Mao, 1971), its life history is poorly known. Until recently, little has been published dealing with its taxonomy and ecology (McCord and Iverson, 1992; 1994; Chen and Lue, 1998). Knowledge of the food habits of a given species may provide insights into the links between animals and their environments. Many studies on turtles have described resource partitioning in terms of food, space, time, or their interaction (see Toft [1985] for review). Morphometrics and growth rates often are associated with feeding niches (Berry, 1975), and dietary shifts with body size have been documented for several turtles (Clark and Gibbons, 1969; Moll, 1976; Georges, 1982; Hart, 1983). As ectothermic organisms, the foraging behavior and diet composition of turtles may also be affected by seasonal changes of ambient temperature and food availability (Parmenter and Avery, 1990; Lagueuex et al., 1995). In this study, we provide data on the dietary habits of 0. sinensis, and examine intersexual, ontogenetic, and seasonal variation in the composition of the food consumed. MATERIALS AND METHODS Study Area-We collected turtles from a 3.5km stretch of the midstream of the Keelung Riv2 Present Address: National Museum of Marine Science and Technology-Preparatory Office, P.O. Box 7202, Keelung 202, Taiwan, Republic of China. er (121?41'E, 2506'N), at 4-6 m elevation. The river in the study area is narrow, meandering, and sluggish, with a width of about 25 m and depth of approximately 1.5 m (Fig. 1). The river bottom mainly consists of silt and sand. The water is usually turbid. Both banks of the river are covered mostly with cultivated bamboo shrubs. Riparian vegetation consists primarily of exotic perennial grasses (including Brachiaria mutica and Pennisetum purpureum). Some non-grass herbaceous plants, including Murdannia keisak (Commelinaceae), Ludwigia octovalvis (Onagraceae), Seigesbeckia orientalis, Elipta prostrata (Compositae), and some species of Polygonaceae, occasionally grow on both banks. Few typically aquatic plants exist in this area. Only some filamentous algae and Lemna aequinoctialis (Lemnaceae) appear in early summer. The average monthly temperature in the warm season (May-November) was 25.4 C (range: 20.5-28.5 C) and 17.1 C (15.2-20.6 C) in the cool season (December-April). According to data collected by the Environmental Protection Administration, average water temperature also changes seasonally, from 20.0-31.4 C in the warm months to 14.5-18.9 C in the cool months. Precipitation is distinctly seasonal, with an average of 218 mm of rain per month during April-August, although typhoons may bring heavy rain in the course of a few days. A total of 320 mm of rain falls per month during the rest of the year (Fig. 2). Discharge in this area was highly variable, with 7.71-20.16 m3/sec during April-August and 25.34-46.76 m3/sec during September-March. In the cool season, changes in water level often result in the inundation of bank vegetation. The water quality in the midstream of the Keelung River was graded This content downloaded from 207.46.13.93 on Tue, 02 Aug 2016 05:30:31 UTC All use subject to http://about.jstor.org/terms T.-H. CHEN AND K.-Y. LUE FIG. 1. Representative view of the study site of Ocadia sinensis in the Keelung River, northern Taiwan. as medium-polluted by organic sewage discharged from the nearby area. Methods-Our study was conducted during both the warm and cool seasons, July-December 1995, February-May 1996, and March-April 1997. Turtles were collected with funnel traps baited with canned cat food, with a long-handled dipnet, or by hand while patrolling along the banks. The bait was enclosed in a wire cage