Freshwater Crayish 16:93–96, 2008 Copyright ©2008 International Association of Astacology ISBN: 978-0-9581424-8-9 print / 978-0-9805452-0-3 online 93 Jouni Tulonen, 1 esa erkamo, 1 Japo Jussila 2,3,† and ari mannonen 2 1 Finnish Game and Fisheries Research Institute, Evo Game and Fisheries Research, 16970 Evo, Finland 2 Crayish Innovation Center, Meijeritie 1, 17200 Vääksy, Finland 3 Department of Bioscience, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland † Corresponding Author — E-mail: japo.jussila@uku.i Shelter and Depth Use of Adult Noble Crayish (Astacus astacus (L.)) and Signal Crayish (Pacifastacus leniusculus (Dana)) in the Presence of a Predator INTRODUCTION The water levels in large lakes in Finland are regulated for a variety of reasons (e.g., illing of reservoirs for hydro-electric power stations, improvement of agricultural conditions on low- lying land, aquatic ecosystem conservation, and recreational usage of private shoreline properties). The interests of the stakeholders affected by water level regulation are somewhat conlicting and quite often it is hard to ind an acceptable compromise. Until recently, the main criteria for the regulation of water levels has been the needs of power companies to store and use water to generate energy. However, there have been changes in the perceptions and value of these priorities. Recently, plans to alter water level regulation schemes have had to take into consideration the views of conservation groups, the impacts of water level regulation on aquatic ecosystems, and other fauna, including birds. The habitat of crayish is typically the shallows along the shoreline and this is usually heavily affected by any appreciable changes in water levels. Obviously, luctuations in water levels do occur naturally, however, the effects can be more pronounced when there is an artiicial water level regulation scheme. In such cases, the difference between the minimum and the maximum water levels can be between 50 to 100 cm. The distance between these extremes is more pronounced when water levels are regulated to suit the needs of power generation companies, and much of the crayish habitat can become exposed during periods of low water levels. Changes in water level also has a great impact on benthic fauna in the littoral zone (Grimås 1962), and major luctuations in the water level can lead to declines in both the number of species, and their density. Crayish are also more exposed to predators in heavily regulated water bodies (Füreder and Machino 1996). Noble crayish prefer to live in shallow water and catches normally peak at depths less than 5 m, while signal crayish can occupy deeper areas with best catches recorded at water depths from 5 to 10 m (Westman 1985). Thus, one could predict that signal crayish would be less vulnerable to luctuating water levels than noble crayish. Furthermore, these two species can inhabit the same water bodies, as is the case in both Finland and Sweden, and in spite of their different depth preferences, they are in competition at several levels, especially for habitat, hides, and mating partners (Hogger 1988; Söderbäck 1991; Holdich et al. 1995; Vorburger and Ribi 1999; Skurdal and Taugbøl 2002). Furthermore, within mature noble and signal crayish, there are different categories of vulnerability to competition depending on the sex, molting cycle, and reproductive stage of the crayish. The aim of this investigation was to gather information as part of a larger project focusing on the effects of water level regulation on aquatic ecosystems in the large lakes of the Pirkanmaa region in Southern Finland. The wider aim of the project was to develop best practices for water level regulation in the large lakes of this region to ensure minimal damage to the aquatic ecosystems. This particular study was carried out to monitor and compare depth preferences and shelter use of adult crayish in the shallow, shoreline waters. Abstract.— The behaviour and shelter choice of adult noble crayish (Astacus astacus) and signal crayish (Pacifastacus leniusculus) were studied in ponds situated at the Evo Game and Fisheries Research Institute, Finland. Three ponds were constructed to relect typical natural shore morphology, with three layers of artiicial shelters for the crayish at different depths (0 – 35 cm, 36 – 70 cm and 71 – 105 cm). Each pond was divided into two separate compartments, one compartment in each pond was stocked with a predator (pike (Esox lucius) weighing approximately 500 g). Each pond was stocked on 17 May with individually marked crayish of both species. Seven individuals from each of three groups (males, berried females, and skipping (non-ovigerous) females) were stocked, giving a total of 42 crayish in each compartment (2 species x 7 individuals x 3 groups). Their behaviour was monitored, and recorded every weekday morning from 25 May to 10 June, with a video camera. The results showed that both noble and signal crayish occupied shelters at a similar rate with a slight difference in response to the presence of a predator. Noble crayish seemed to prefer shallower water than signal crayish, and we also observed a difference in the preferred water depth between skipping females of the two species. No difference was observed between the species among berried females or males in preferred water depth, though mortality of berried females seemed to be higher. [Keywords.— Astacus astacus; behaviour; Pacifastacus leniusculus; water level regulation].