Introduction
Roach (Rutilus rutilus) are common and widely distributed freshwater fish in northern Eurasia. Although of freshwater origin, roach also thrive in the brackish coastal and estuarine waters of the Baltic, Caspian, Black, and Aral seas (Banarescu and Coad 1991). Adult roach are found in brackish water up to salinities of 10 [per thousand]-14 [per thousand] (Jager et al. 1981; Banarescu and Coad 1991). Like other cyprinids, the roach is not an economically valued target species for the coastal fishery, but it is commonly caught by recreational fishermen, the annual recreational fishing catch being around 800 tonnes (t) on the Finnish coast alone (Anonymous 2004). In some countries, the roach is more valued also in the commercial catches (Erm et al. 1992). The roach is currently among the most abundant species in gill net test-fishing catches in the coastal areas of the northern Baltic Sea (Lappalainen et al. 2001; Adjers et al. 2006), and its ecological importance in aquatic ecosystems is presumed to be considerable (Winfield and Townsend 1991).
The quality and quantity of spawning, larval, and nursery areas are important for fish production, and the habitat requirements of fish are most strict during the early life stages (Urho 2002). For roach, an improvement in reproduction conditions is the likely reason for the recent increase in abundance of the species in the Gulf of Finland. Shallow, sheltered, and vegetated shores are important as spawning and larval areas for roach in lakes (Holcik and Hruska 1965; Lange and Dmitriyeva 1973; Mann 1996). In the Baltic Sea, roach spawn in early May after the ice breakup, and during this time, shores covered by reeds (Phragmites australis) form the main vegetated habitat available for roach in the sheltered littoral shores of the northern Baltic Sea. The perennial reed belts extend from the hyperlittoral zone to a depth of usually 1.5 m, and new growth rises annually (Roosaluste 2007). Winter ice and waves typically cut down and flatten emergent reeds in the outer parts of the belt, forming a sheltered spawning and larval habitat for roach. The abundance and range of reed belts has increased, and they are a dominant feature of sheltered shores along the northern Baltic Sea coast (Roosaluste 2007). Reed expansion has been accelerated by human-induced climate change, by the cessation of coastal meadow management, and especially in the outer areas, by general eutrophication of the Baltic Sea (Roosaluste 2007).
According to laboratory experiments, the early life stages of roach are sensitive to salinity. Salinity is especially critical during fertilization and embryonic development and also for the survival of newly hatched roach larvae (Jager et al. 1981; Klinkhardt and Winkler 1989). According to Jager et el. (1981) and Klinkhardt and Winkler (1989), roach eggs fertilized in water with a salinity of 3.5 [per thousand] or higher have been found to die before the embryos hatch. Even if salinity was initially low and then increased to 3.5 [per thousand] or higher after hatching, the mortality rate of the newly hatched larvae was observed to be almost 100%. At lower salinities (<3.5 [per thousand]), embryo development and hatching were normal. For Caspian roach (Rutilus rutilus caspicus), optimal salinity during reproduction has been reported to be 2.5 [per thousand]-5 [per thousand] (Oliphan 1941), and for Black Sea roach (Rutilus rutilus heckeli), 0 [per thousand]-5 [per thousand] (Karpevich 1975). These subspecies have clearly adapted themselves to more saline environments and, thus, have been experimentally shown to manage to reproduce in salinities up to as high as 7.5 [per thousand]-10 [per thousand] (Oliphan 1941; Karpevich 1975).
Experimental studies have shown that a constant 9[degrees]C forms a critical temperature minimum for the development of roach embryos (Cerny 1974; Gulidov and Popova 1979). Roach abundance at the end of the first summer has also been reported to show a positive correlation with increasing turbidity, and eutrophic conditions have been suggested to be beneficial to roach in the early life stages (Sandstrom and Karas 2002).
Global climate change is expected to cause extensive changes in the aquatic ecosystem and fisheries (e.g., Kennedy 1990; Reist et al. 2006; BACC Author Team 2008). Nonetheless, insufficient information is available on the potential direct and indirect effects of climate change on fish. Attention so far has focused mainly on the effects of climate warming and increasing water temperatures on fish communities and species distributions (e.g., Lehtonen 1996; Reist et al. 2006; Henderson 2007). In the Baltic Sea, however, climate change is expected to influence not only the temperature, but also the salinity conditions (Meier 2006; Meier et al. 2006; BACC Author Team 2008) because of increased rainfall and increased freshwater runoff in the coastal areas (Carter et al. 2005; Silander et al. 2006). In the brackish water of the Baltic Sea, several aquatic organisms are living close to their salinity tolerance limit. Therefore, it appears that future changes in salinity will also considerably affect fish and other aquatic organisms in the Baltic Sea area.
The extent of the reproduction habitats of freshwater fish species such as roach is poorly known in the northern Baltic Sea, and no systematic surveys of the reproduction areas of roach have been carried out in the coastal areas. To date, only a few studies have used statistical modelling combined with geographic information system (GIS) to map essential fish habitats (e.g., Eastwood et al. 2001; Vaz et al. 2005; Bergstrom et al. 2007). However, continuous high-resolution maps of fish reproduction areas could offer versatile tools for efficient management of the coastal zone, and spatial predictive modelling would offer a convenient way to extrapolate the data gained by limited field survey resources. Thus, in this study, we combined remote sensing, field surveys, statistical models, and GIS methods to (i) determine and explain the occurrence of roach larvae along environmental gradients and (ii) produce spatial prediction maps of the potential reproduction areas of roach in an archipelago area in the northwestern Gulf of Finland. Finally, we considered the possible mechanisms by which climate change may affect spring salinities and consequently the extent of roach reproduction areas in the northwestern Gulf of Finland.
Materials and methods
Study area
The study area was located in the Tammisaari archipelago in the northwestern part of the Gulf of Finland (59[degrees]N, 23[degrees]E) and stretched from the innermost archipelago (Pojo Bay) to the outermost mainland (Hanko Peninsula). It was divided into three subareas representing the three archipelago zones: (A) the innermost bay area, (B) the intermediate archipelago, and (C) the outer archipelago (Fig. 1). The total length of the shoreline was 270 km in the inner bay area, 520 km in the intermediate archipelago, and 610 km in outer archipelago. The archipelago in this area is very complex and extensive. Environmental factors such as salinity, temperature, and littoral vegetation differ both spatially and temporally in the various archipelago zones, and strong environmental gradients are formed from the inner bays to the outer archipelago zone. Shores covered with reeds constitute a considerable part of the shoreline, especially in the shallow nearshore waters in the inner bay area: in the study area, reed belts extended along 58% of the shoreline in the inner bay area, 32% in the intermediate archipelago, and 9% in the outer archipelago. The River Mustionjoki, with a mean runoff of 19 [m.sup.3] x [s.sup.-1], is the only large river in the study area and runs into the northern …
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