-1 Papyrus wetlands a lunar-modulated refuge for aquatic fauna Bakari Mnaya 1, *, Eric Wolanski 2 and Yustina Kiwango 3 1 Department of Environmental Science and Human Engineering, Saitama University, 255 Shimo-Okubo, Saitama – shi, Saitama 338-8570, Japan; 2 Australian Institute of Marine Science, PMB No. 3, Townsville, MC, Queensland 4810, Australia; 3 Tanzania National Parks, PO Box 3134, Arusha, Tanzania; *Author for correspondence (e-mail: jmnaya@yahoo.com) Received 17 October 2004; accepted in revised form 4 March 2005 Key words: Fish larvae, Low dissolved oxygen, Moonlight, Papyrus wetlands, Predation Abstract A field study was undertaken of the use of papyrus (Cyperus papyrus) wetlands fringing Rubondo Island, Lake Victoria, Tanzania, as a refuge from predators by freshwater shrimps (Caridina nilotica) and tilapia (Oreochromis niloticus) larvae. During the full moon nights, fish larvae took refuge more than 5 m in the papyrus and did not venture out into the lake, while they ventured into fringing lake waters during moonless nights. The freshwater shrimps and lake flies (Chaoborus sp.) did not migrate with the moon. This fauna took refuge in the papyrus wetlands even though the dissolved oxygen concentration was as low as 0.4 ppm (<5% of saturation). Introduction The physical environment (i.e. submerged vegeta- tion, suitable temperature, and availability of dis- solved oxygen of lakeshores has a major influence on the movement and distribution of living organisms (Chapman and Liem 1995). These riparian and littoral zones are important to fish by maintaining bank stability, providing shade and cover, filtering the water to maintain water quality and providing invertebrate food organisms (Platts 1983; Moring et al. 1985; Cummins 1993). Fur- ther, some wetlands such as papyrus (Cyperus papyrus) swamps supply large amounts of organic nutrients to fringing waters, thus allowing an in- crease in animal and plant production at the swamp edge (Gaudet 1980; Moore 1994). Another important role of the vegetated littoral zone is to provide a refuge for plankton from predation by zooplanktivorous fishes because their hunting efficiency is decreased by an increasing vegetation density (Manatunge et al. 2000). As the juvenile planktivorous fish matures to adulthood, its large physical size prevents it to efficiently catch prey within the vegetated littoral zone. It must then leave the protection of the submerged vege- tation in the littoral zone to catch plankton in open waters. Because the vegetation density is very high, large predators cannot enter papyrus wetlands that then may provide a refuge for juvenile catfish (Barbus neumayeri), freshwater shrimp (Caridina nilotica) and possibly Nile tilapia (Oreochromis niloticus) (Balirwa 1998; Branstrator and Mwebaza-Ndaw- ula 1998; Chapman and Liem 1995; Mbahinzireki et al. 1998; Mnaya and Wolanski 2002). However these waters also have very poor water quality, particularly low dissolved oxygen (DO) concen- tration possibly due to accumulation of dead organic matter at the substratum as this bay is poorly flushed (Petr 1973; Mnaya and Wolanski 2002). For air-breathing fish, papyrus swamps are Wetlands Ecology and Management (2006) 14:359–363 Ó Springer 2006 DOI 10.1007/s11273-005-5807-2