James D. Reist, Frederick J. Wrona, Terry D. Prowse, Michael Power, J. Brian Dempson, Richard J. Beamish, Jacquelynne R. King, Theresa J. Carmichael and Chantelle D. Sawatzky General Effects of Climate Change on Arctic Fishes and Fish Populations Projected shifts in climate forcing variables such as temperature and precipitation are of great relevance to arctic freshwater ecosystems and biota. These will result in many direct and indirect effects upon the ecosystems and fish present therein. Shifts projected for fish popu- lations will range from positive to negative in overall effect, differ among species and also among populations within species depending upon their biology and toler- ances, and will be integrated by the fish within their local aquascapes. This results in a wide range of future possibilities for arctic freshwater and diadromous fishes. Owing to a dearth of basic knowledge regarding fish biology and habitat interactions in the north, complicated by scaling issues and uncertainty in future climate projections, only qualitative scenarios can be developed in most cases. This limits preparedness to meet chal- lenges of climate change in the Arctic with respect to fish and fisheries. INTRODUCTION Fishes and wildlife intimately associated with arctic freshwater and estuarine systems are of great significance to local human populations (1) as well as significant keystone components of the ecosystems (2). Accordingly, interest in understanding the impacts of climate change on these components is very high. However, in addition to the problems outlined elsewhere in this issue (2), detailed understanding of climate change impacts on higher-order biota is complicated by a number of factors. Firstly, fishes and wildlife will experience first-order effects of climate change (e.g., increased growth in arctic taxa due to warmer conditions and higher productivity). Large numbers of second-order effects will also occur (e.g., increased competition with species extending their distribution northward). The responses of fishes will integrate these effects in complex and not readily discernible ways; further, responses to climate change will be embedded within effects resulting from other impacts such as exploitation and habitat alteration, and it may be impossible to differentiate these. These multiple impacts are likely to act cumulatively or synergistically to affect arctic taxa. Secondly, higher-level ecosystem components affect lower levels in the ecosystem (i.e., top-down control) and in turn are affected by changes in those levels (i.e., bottom-up control) (2). The balance between such controlling influences may shift in indiscernible ways in response to climate change, and this, in turn, will affect fish populations. Thirdly, higher-level ecosys- tem components typically migrate seasonally and/or annually between habitats or areas key to their life histories—arctic freshwater and diadromous (i.e., sea-run) fishes and aquatic mammals may do so locally, and aquatic birds tend to do so globally between arctic and non-arctic areas. Thus, the effects of climate change on such organisms will represent the integrated impacts across numerous habitats that indirectly affect the species of interest. These biotic circumstances increase the uncertainty associated with developing understanding of species-specific responses to climate change, particularly for key fish (Table 1-Box 1) and other aquatic species that are of economic and ecological importance to arctic freshwater ecosystems and the communities of northern residents that depend on them. This contribution provides an overview summary of fresh- water and diadromous (primarily anadromous) fishes of the Arctic, develops information needs required to project respons- es of arctic fishes to climate change, and examines some general effects of climate change on physical habitat in the context of their relevance to fishes. Finally, to promote future research a short review is given of various approaches relevant for projecting climate change effects on arctic fish populations. This treatment is not exhaustive but rather is developed for particular important arctic freshwater and diadromous fishes as examples of the types of changes to be expected. FRESHWATER AND DIADROMOUS FISHES OF THE ARCTIC There are approximately 99 species in 48 genera of freshwater and diadromous (i.e., anadromous or catadromous forms moving between fresh and marine waters) fishes present in the Arctic as used here (2). These represent 17 families (Table 1). Ninety-nine species is a conservative estimate because some groups (e.g., chars and whitefishes) in fact contain complexes of incompletely resolved species. Many species are also represented by local polymorphic forms that biologically act as species (e.g., four morphs of Arctic char, Salvelinus alpinus, in Thingvalla- vatn, Iceland) (6). The most species-rich family is the Salmonidae with more than 33 species present, most of which are important in various fisheries. The next most species-rich family is the Cyprinidae with 23 species, few of which are fished generally, although some may be fished locally. All remaining families have six or fewer species, and five families are represented in the Arctic by a single species. These generalities hold true for the individual ACIA regions as well (Table 1). All of the families represented in the Arctic are also present in lower-latitude temperate and subtemperate regions. Most have 370 Ambio Vol. 35, No. 7, November 2006 Ó Royal Swedish Academy of Sciences 2006 http://www.ambio.kva.se