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Invertebrate Drift in the Tambun Stream in Danum Valley

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Invertebrate Drift in the Tambun Stream in Danum Valley Invertebrate drift in the Tambun stream in Danum Valley Anita Bousa, Wildlife Conservation Society, Lao PDR Chiara De Cesare, University of Innsbruck, Austria Abstract The invertebrate drift is the main food source of stream fish, but do the fish eat just the aquatic animals or do they also eat the terrestrial animals that drop down into the water? The drift composition was measured and fish gut contents were examined in the Tambun stream (Danum Valley, Sabah, Borneo). The results show that invertebrate drift in the Tambun Stream accounted for 41 million potential food particles drifting down the Tambun Stream each 24 hours. The numbers of aquatic animals drifting were greatest at night. The reason may be that the animals are minimising risks of being eaten by fish, which are visual predators. Also, the fish guts content showed that the fish prefer terrestrial and aquatic animal to exuviae. The terrestrial animals drop accidentally into the water and flounder. They are not adapted to the water environment and are therefore more vulnerable to predation in water. The aquatic animals are adapted to living and surviving under these conditions through structural and behavioural adaptation. One of those adaptations could be the voluntary drift during the night, when they are not visible to fish. Such controlled drift allows redistribution with minimum risk. INTRODUCTION Water flow and the swimming fish are visible and easily recordable features of streams. But there are also less obvious movements of smaller cryptic animals in freshwater habitats. Animals are resuspended from the bottom and carried downstream. Are these movements accidental and driven just by the water flow or are they deliberate and adaptive and controlled by natural selection? Forest and freshwater ecosystems are deeply connected. A continuous exchange of energy and nutrients takes place between them. Forests may contribute organic detritus, leaves and twigs and even whole trees that decompose in the water and provide energy for the stream community. In turn, insects may emerge as adults from the stream and provide food for birds, bats, spiders and reptiles in the adjacent forest. There are longer connections also, for example among migratory fish, bears that eat them and then excrete nutrients to the forest floor that the fish have acquired in the ocean. Forest trees take up these nutrients, returning organic detritus to the rivers that ultimately contributes food for invertebrates and then the young salmon. Pristine tropical ecosystems are characterised by low free nutrients and high diversity because on the one hand high rainfall leaches nutrients readily from the system if there are no mechanisms within the forest to retain scarce nutrients and on the other a long history has led to differentiation of many Bousa&deCesare2011.doc 1 Tropical Biology Association species that maintain such mechanisms. Tropical streams are often very low in nutrients and the aquatic invertebrate communities are sparser compared with temperate systems. There is thus more competition for nutrients and also high predation rates because long growth seasons allow reproduction of fish several times per year. All these factors may contribute to the generally low numbers of invertebrates found in many tropical streams (Vannoto (1980); in Fenoglio et al., 2002). Like all predators fish play a very important role in freshwaters. Their feeding behaviour can control the prey-populations (e.g. aquatic stages of invertebrates like mayfly larvae). And they can transfer energy and nutrients to the forest if they are preyed on by terrestrial amphibians (e.g. lizards) or mammals (e.g. bears, humans). The food sources for the fish in a stream may come from both terrestrial and aquatic origins. They include invertebrates, plant material and other fish. There are also detritovore species fish like Lobocheilos bo (Popta, 1904). Food sources may differ in a stream and in a main stem river in both composition and amount; in a big river, for example, there will be more sediment than in a small one but less oxygen and less overhanging vegetation. Fish are visual feeders; hence they can only catch prey if they see it. Aquatic invertebrates have developed various adaptations to cope with the risks of displacement by the water currents that also make them incidentally less visible to the fish. They have a brownish camouflage colour and often flat body shape to cling closely on or under rocks. Their body shape and legs are streamlined and well adapted to the motion of the water. If they move they move quickly. However, sometimes they do become detached and suspended in the currents and are then said to be part of the drift. The drift is the downstream transport of aquatic organism in the river (Fenoglio et al., 2002). It may have a very important colonisation value for example in recovery after disturbance (Fenoglio et al., 2002). Also the gene flow in riverine species is linked to the drift (Chaput-Bardy et al., 2008). The drift of larvae is described as accidental in the literature; but less is known about the possibility of non-accidental drift for deliberate redistribution. In the drift, invertebrates are much more vulnerable to being eaten by fish. Drift at night, when the fish cannot see them, might seem thus to be less risky than drift during daylight. Bousa&deCesare2011.doc 2 Tropical Biology Association Aims This study investigates the food sources for fish in an environment that has only a small invertebrate community, yet a visibly obvious fish population and to determine whether drift of aquatic animals is random or shows patterns that might be adaptive. It was to investigate the feeding preferences of the fish in the Tambun stream in the Danum Valley and the role of terrestrial animals that accidentally fall into the stream compared with truly aquatic food and to seek patterns in the timing of the aquatic animal drift that might avoid the risks of fish predation. METHODS The site of the study is the ca. 300 m long last stretch of the Tambun headstream before the confluence into the Sigama River next to the Danum Valley Field Centre (DVFC) in the Malaysian region of Sabah (Borneo). The Tambun stream is surrounded by primary forest on one side and confines with the DVFC trails and habitations on the other side. About 10 km2 of forest were logged in the 1980s. The Tambun headstream has a conductivity of about 50 µScm-1 and no detectable nitrogen. There is a big overhang of vegetation all along the stream so that there is a great uptake from plant material and animals dropping from the forest into the water The drift experiment This experiment was conducted twice. The first run was conducted in the Tambun stream on 17th October 2010 from 10:00 a.m. until 1:00 p.m. of the 18th October. Four benthos nets with 20 cm x 24 cm openings and 42 cm bag depth of 0.25 mm mesh were set across the Tambun stream to catch aquatic animals drifting in the water and the terrestrial animals that drop into the stream (Figure 1). The four nets were sampled every three hours, over 28 hours. The number of invertebrates (size generally < 2 mm) in each sample were counted), classified into four groups (aquatic animals, aquatic animal exuviae, terrestrial animal and terrestrial animal exuviae) and preserved in ethanol. Exuviae were only counted for the first sample in the first Figure 1. One of the drift nets on run, but for all samples in the second. The second run was conducted the study site. on 21 October 2010 starting at 9:00 am. The same procedure was used but the number of replicates was reduced to three nets, left for 34 hours with samples taken every four hours. On 21st October there was a storm rainfall at 3 p.m. which increased the stream flow greatly for several hours overnight. Therefore a correction of 1.3 (proportion between number of animals found before the storm and after) was attempt for dilution during flood. Bousa&deCesare2011.doc 3 Tropical Biology Association The collected data were tested for significance with One way ANOVA test (Minitab 14) and by regression (Excel X for Mac, 2001). Food particles number To estimate the number of food particles drifting in the stream the discharge was first measured with the formula (equation 1). D = l *x*v (1) D discharge (m3sec-1) l width of the stream bed (m) x average of the depth measured every m along l (m) v velocity of the current (m sec-1) The numbers of animals and exuviae drifting in the stream per 24 hrs were calculated by multiplying D by the total number of animals and exuviae counted per unit volume Fish sampling in stream and river On the 17th October 2010 twenty fish (Nematabramis everetti, Cyprinidae) (Figure 2) were caught by seine netting in the Tambun headstream. Five were dissected and the contents of their guts were counted using the categories used for stream drift. The remaining 13 were kept alive in a running water aquarium for observation of their feeding behaviour. Figure 2. Nematabramis everetti caught on the 17th October 2010 in the Tambun The proportions of the animal and exuviae classes found in the guts and stream. in the nets (drift experiment 2.1) were compared with the Ivlev Selectivity Index (Ivlev, 1961). D=(r-p)/r+p-2rp (3) r, proportion in guts p, proportion in food available A value of -1 means complete discrimination, 0 no selection, and +1 maximum positive selection). Thirteen fishes (one Clarias teysmani (Clariidae); six Paracrosschilus sp; two Lobocheilus bo; one Lobocheilus sp; and two Epalzeorhynchus kalliurus (all Cyprinidae)) were caught in a gill net set overnight on the 18th October 2010 at the confluence of the Tambun headstream with the Segama River.
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