INDEX WORDS: Ruffe, introductions, native populations, Europe, Lake Constance, Bassenthwaite

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1 J. Great Lakes Res. 24(2): Internat. Assoc. Great Lakes Res., 1998 Recent Introductions of the Ruffe (Gymnocephalus cernuus) to Coregonus and Perca Lakes in Europe and an Analysis of Their Natural Distributions in Sweden and Finland Ian J. Winfield 1 *, Roland Rösch 2, Magnus Appelberg 3, Anders Kinnerbäck 3 and Martti Rask 4 1 NERC Institute of Freshwater Ecology The Windermere Laboratory Far Sawrey, Ambleside, Cumbria LA22 OLP, U.K. 2 Fischereiforschungstelle Baden-Württemberg Mühlesch 13 D Langenargen, Germany 3 Institute of Freshwater Research S Drottningholm, Sweden 4 Finnish Game and Fisheries Research Institute Evo Fisheries Research and Aquaculture FIN Evo, Finland ABSTRACT. Although the ruffe (Gymnocephalus cernuus) is native to many parts of Europe, its distribution has recently increased through accidental introductions. This synthesis reviews this situation and perceived possible consequences for Coregonus spp. and Eurasian perch (Perca fluviatilis), presents case histories from Lake Constance and Bassenthwaite Lake, and examines natural distributions of ruffe, Coregonus spp., and perch in northern Europe. In Lake Constance, a large mesotrophic lake in central Europe supporting whitefish (Coregonus lavaretus) and perch fisheries, the ruffe was first recorded in 1987 and now occupies many areas, feeding extensively on the eggs of inshore-spawning whitefish. In contrast, Bassenthwaite Lake is a small eutrophic U.K. lake without commercial fisheries, but containing perch and vendace (Coregonus albula), where the ruffe was first recorded in 1991 and is now widespread. Although predation by ruffe on vendace eggs was not recorded during spawning in late 1995, it was in late Vendace and perch populations have shown no marked changes between 1991 and 1996, but the cyprinid roach (Rutilus rutilus) has declined. Among 705 lakes in Sweden, differences were found in the features of lakes in which ruffe occur allopatrically and sympatrically with vendace, white-fish, and perch, but nevertheless such coexistence was found in 59, 37, and 210 lakes, respectively. In Finland, ruffe were found to inhabit 427 of 710 lakes, including many in which they coexisted with vendace, whitefish, and perch. Factors influencing the distribution of ruffe in Finland included lake size, acidity, trophic status, hypolimnetic anoxia, and availability of spawning substratum. INDEX WORDS: Ruffe, introductions, native populations, Europe, Lake Constance, Bassenthwaite Lake. *Corresponding author. i.winfield@ife.ac.uk

2 INTRODUCTION The ruffe (Gymnocephalus cernuus (L.)) is a small, bottom-dwelling, and largely benthivorous percid fish found throughout much of central and northern Europe (Fig. 1) where it has long been considered to have little or no value for commercial or recreational fisheries. However, as in North America, recent accidental introductions have raised concerns over potential negative effects on commercially-important or rare fish species, particularly whitefishes of the genus Coregonus and perch of the genus Perca. Effects on the former genus may act through egg predation (e.g., Rosch and Schmid 1996), while those on the latter may involve competition for food (e.g., Bergman and Greenberg 1994). In a recent bibliography of the ruffe, Winfield and McCulloch (1995) recorded 15 publications concerned with such European introductions, of which one was published in the 1960s, four in the 1980s, and 10 in the 1990s. The objectives of the present paper were to assess the ruffe as a problem species in Europe, to review two recent ruffe introductions in Germany and the U.K., and to determine the extent to and conditions under which natural populations of ruffe, Coregonus, and Perca species coexist. Consequently, this three-part synthesis begins by reviewing introductions of the ruffe in Europe and their perceived possible consequences for other fish species, particularly Coregonus spp. and Eurasian perch (Perca fluviatilis), based on the bibliography assembled by Winfield and McCulloch (1995). The second part presents contrasting European case histories from Lake Constance and Bassenthwaite Lake, drawing extensively from the publications of Rösch and Schmid (1996), Schmid (in press), Winfield et al. (1996a), Winfield et al. (1996b), Winfield et al. (in press a), and Winfield et al. (in press b), while the final component explores allopatric and sympatric natural populations of ruffe, Coregonus spp., and perch in Sweden and Finland of northern Europe to examine the locations and morphologies of lakes in which native populations do and do not coexist.

3 OVERVIEW OF RUFFE INTRODUCTIONS TO EUROPEAN LAKES Summary of Introductions In the European mainland, ruffe introductions have been reported from Italy (Chiara 1986), Norway (Kålås 1995), and from Lake Geneva of Switzerland and France (Matthey 1966), and Lake Constance of Germany, Austria, and Switzerland (Berg et al. 1989). The means of introduction are unknown, although in each case they were probably accidental and certainly not part of any formal introduction program. Only the introduction in 1987 to Lake Constance is still being actively studied, as described in detail below. Within the U.K., the ruffe was until recently largely restricted to lowland England. However, within the last two decades it has been introduced to sites in other parts of England, Scotland, and Wales (Maitland et al. 1983, Adams and Maitland 1998, Winfield 1992, Winfield et al. 1996a, Win-field et al. 1996b). Among the nine mainland U.K. lakes containing Coregonus species, the ruffe was first recorded from Loch Lomond in Scotland in 1982, and from Llyn Tegid (Bala Lake) in Wales and Bassenthwaite Lake in England in The Loch Lomond introduction has been the most extensively studied and is considered elsewhere in detail (Adams and Maitland 1998). As in mainland Europe, the means of these ruffe introductions in the U.K. are unknown but are again probably accidental. Escaped or discarded live bait from anglers fishing for pike (Esox lucius L.) seems the most likely source and, in the context of Coregonus conservation, it is notable that three of the four mainland U.K. lakes which hold Coregonus species (i.e., whitefish (Coregonus lavaretus (L.)) or vendace (Coregonus albula (L.))) and pike (Loch Lomond, Llyn Tegid, and Bassenthwaite Lake) have received ruffe introductions in recent years, while the remaining lakes which hold Coregonus species but not pike (Loch Eck, Brotherswater, Haweswater, Red Tarn, and Ullswater) have remained free from introductions. Perceived Possible Consequences Given the recent increase of interest in ruffe introductions evident in the bibliography of Winfield and McCulloch (1995), between 1995 and 1997 over 50 copies of a questionnaire were distributed to researchers in Europe. Table 1, based on 31 responses, shows the answers to three components of this questionnaire. After removing four respondents (from Ireland, Northern Ireland, Portugal, and Spain) for whom the ruffe remains absent from their country, a total of eight respondents (30%) perceived the ruffe to be a problem, among which six cases (75%) were due to introduced populations and only two instances (25%) to native populations. Despite this concern, research on the control of ruffe populations is only in progress in three countries, i.e., Estonia, Germany, and the U.K. TWO CASE HISTORIES OF RUFFE INTRODUCTIONS TO EUROPEAN LAKES Lake Constance Lake Constance, located at an altitude of 400 m on the borders of Austria, Germany, and Switzerland in central Europe (Fig. 1), has a surface area of 54,000 ha, a maximum depth of 254 m, and a mean depth of 120 m. Following a period of eutrophication during the present century, nutrient management measures have resulted in an intensive reoligotrophication since the beginning of the 1980s (IGKB 1995). The fish community of over 30 species is intensively exploited in commercial fisheries, with whitefish and perch the most important species. Ruffe were first recorded from Lake Constance in 1987 (Berg et al. 1989). The western part of the lake was completely colonized by 1990 (Fischer 1994), the eastern part reached by 1991 (Hartmann 1993), and the ruffe is now the most abundant fish species in the inshore areas of the lake (Rosch and Schmid 1996). In late 1994, the ruffe population consisted mainly of fish from 0 to 3 years old, with a mean length of 76 mm being reached during their first year and both sexes maturing at an age of 1 or 2 years (Rosch and Schmid 1996).

4 The major threat posed by the ruffe to the commercial fisheries of Lake Constance is thought to arise from their ability to predate the eggs of inshore-spawning whitefish during their long winter incubation period. Rosch and Schmid (1996) found that in the spawning periods of 1993 and 1994, ruffe switched to whitefish eggs as a main prey, whereas other potential predators such as perch and burbot (Lota lota (L.)) ingested only few eggs. Further sampling during and after the spawning period of 1995 again showed that nearly all ruffe consumed whitefish eggs at this time (Schmid in press). Given that the numbers of eggs predated were such that 300 ruffe could consume in 1 day all of the eggs laid by a single whitefish female, it was concluded that such predation is likely to have negative effects on the natural recruitment of inshore-spawning whitefish. However, because the natural recruitment of whitefish in Lake Constance is supplemented by stocking of unknown efficiency (Eckmann et al. 1988), predictions concerning the influence of ruffe on whitefish overall year class strength are impossible at present. Ruffe may also adversely impact the commercial fisheries of Lake Constance through competitive interactions with perch. Although this topic has not yet been studied in detail, it is a component of a current intensive research program investigating ruffe population dynamics and possible interactions with other species in this lake. Assuming a continued oligotrophication of Lake Constance, the perch population is likely to return to feeding more extensively on the benthos. In this case, competition between perch and ruffe may be more likely (Rösch and Schmid 1996). Bassenthwaite Lake Bassenthwaite Lake, located at an altitude of 68.8 m in the English Lake District of the U.K. (Fig. 1), has a surface area of 528 ha, a maximum depth of 19.0 m, and a mean depth of 5.3 m. The lake is presently highly eutrophic, although a recent improvement in the treatment of a major sewage discharge is expected to reduce the total phosphorus input by at least 50% (see Winfield et al. 1996a). The fish community of Bassenthwaite Lake has increased from six to nine species in recent years following introductions of ruffe and the cyprinids roach (Rutilus rutilus

5 (L.)) and dace (Leuciscus leuciscus (L.)). Although the lake is only exploited by light recreational fishing, it is extremely important in a national context because it contains one of only two extant U.K. populations of vendace and, partly as a consequence, is designated as a National Nature Reserve. Ruffe was first recorded from Bassenthwaite Lake in April 1991 when two individuals were taken in survey gill nets. In October of 1991, ruffe comprised 9% by numbers of all fish taken in survey sampling, with the community being dominated by roach (41%), perch (31%), and vendace (15%) (Fig. 2). By 1996, a similar survey showed marked changes in relative abundance within the fish community, with ruffe increasing (57%), roach decreasing (1%), dace being first recorded (2%), but perch (28%) and vendace (12%) remaining at similar relative levels (Fig. 2). In 1991, ruffe sampled during the fall months ranged between 66 and 153 mm in fork length and 0 and 4 years in age, reaching a mean length of 70 mm during their first year (Win-field et al. 1996b) and both sexes maturing at an age of 1 or 2 years (IJW unpublished data). Vendace ranged between 158 and 261 mm and 2 and 9 years, and perch between 60 and 220 mm and 0 and 2 years. In 1996, these values had changed to between 51 and 124 mm in fork length and 0 and 2 years in age for ruffe, between 156 and 227 mm and 2 and 5 years for vendace, and between 65 and 241 mm and 0 and 5 years for perch. Length frequency distributions did not differ significantly over this period for any of these three species (Fig. 2). Elsewhere in Europe, the predation of incubating Coregonus eggs by introduced ruffe populations has been recorded during the winter in Loch Lomond (Adams and Tippett 1991) and Lake Constance (Rösen and Schmid 1996, Schmid in press).

6 Consequently, the diet of ruffe in Bassenthwaite Lake was studied during the winters of 1995/1996 and 1996/1997, which included the local vendace spawning periods of late November and December. Ruffe were found to be present on the then only known vendace spawning ground on nine out of 10 sampling occasions (Winfield et al. in press b, IJW unpublished data). Vendace eggs were never recorded in the diet of ruffe in the winter of 1995/1996, when chironomid larvae, Asellus sp., and Ostracoda were their most important prey (Winfield et al. in press b). However, ruffe were found to have consumed vendace eggs in the following winter of 1996/1997, when chironomid larvae, Ostracoda, and Chydorus sp. were also frequently consumed (IJW unpublished data). The absence of vendace eggs from the diet of ruffe in the winter of 1995/1996 may have been due to poor spawning by vendace in that period as the latter's recruitment in this very eutrophic lake has certainly been very inconsistent in recent years (Winfield et al. 1996a), although quantitative data on egg abundance are unavailable. Although the poor status of the vendace in Bassenthwaite Lake during the 1990s may be due more to eutrophication than to ruffe (see Winfield et al. 1996a), it is clear that the latter's introduction to Bassenthwaite Lake is just one aspect of a very dynamic and unpredictable fish community. While the relatively small size and shallow depth of Bassenthwaite Lake may have been expected to accelerate any effects of ruffe on vendace or perch through increasing the likelihood of spatial overlaps, the additional introductions of roach and dace greatly complicate the situation and probably preclude any applicability to other lake systems. In particular, it is pertinent to note that the dace has elsewhere been shown to be a predator of the larvae of Coregonus species (Shirobokov 1992). ALLOPATRIC AND SYMPATRIC NATURAL DISTRIBUTIONS OF RUFFE, COREGONUS SPP., AND EURASIAN PERCH IN SWEDEN AND FINLAND Sweden The natural distributions of ruffe, Coregonus species, and perch in Sweden were examined using FIBase, the Swedish national database of fish distribution held at the Institute of Freshwater Research, Drottningholm. This database contains information collated from many years of sampling in 705 lakes. Data on Coregonus species are held as specific to vendace or to Coregonus spp., the latter of which is a grouping of several other species overwhelmingly dominated by the whitefish and referred to as such hereafter. Associated datasets of FIBase contain environmental data on lake temperature, chemistry, and trophic state, but are more limited in coverage than are those concerning physical features. Moreover, the former parameters are also subject to significant changes over time, although extensive relevant time series are available for only a few lakes. Consequently, the present initial analysis is restricted to an examination of the locations and morphologies of lakes containing ruffe, vendace, whitefish, and perch. It thus considers lake latitude, altitude, surface area, maximum depth, and mean depth, variations in which among all the lakes of the database are summarised in Figure 3 and Table 2. Within this set, ruffe, vendace, whitefish, and perch occurred in 211 (30%), 103 (15%), 86 (12%), and 665 (94%) lakes, respectively, for which summary data are also given in Table 2. Of the 211 lakes containing ruffe, vendace occurred in 59 (28%), whitefish in 37 (17%), and perch in 210 (99%) lakes. Differences from the complete lake set for each of the five above lake descriptors were assessed for groups of lakes containing the four above species using Mann-Whitney U tests (Table 3). Lakes containing ruffe were of significantly greater latitude, lower altitude, and greater surface area, while those containing vendace were of significantly greater surface area, greater maximum depth, and greater mean depth. For whitefish, lakes were of significantly greater latitude, greater altitude, greater surface area, greater maximum depth, and greater mean depth. Only the group of lakes containing the nearly ubiquitous perch showed no significant differences from the full set. Comparisons were then made between sets of lakes in which ruffe do and do not coexist with vendace, whitefish, and perch. Lakes in which vendace and whitefish coexist have been excluded from the analysis because such coexistence does not occur in Lake Constance (whitefish only) nor in Bassenthwaite Lake (vendace only), and so such communities are of less interest in the present context. The statistical significances of any observed differences in lake parameters were assessed using Mann-Whitney U tests.

7 Vendace occurred without whitefish but with and without ruffe in 45 and 31 lakes, respectively (Fig. 4 and Table 4). Lakes with ruffe were of significantly lower altitude and greater surface area. Whitefish occurred without vendace but with and without ruffe in 23 and 34 lakes, respectively (Fig. 5 and Table 4). Lakes of the two groups differed significantly only with respect to altitude, which was lower for those with ruffe. Perch occurred with and without ruffe in 210 and 455 lakes, respectively (Fig. 6 and Table 4). Lakes with ruffe were of significantly greater latitude, lower altitude, greater surface area, and greater maximum depth. While extrapolations from the above analyses to lakes elsewhere in the world subject to ruffe introductions must always be made tentatively, it is clear that in Sweden at least natural populations of ruffe commonly coexist with those of vendace, whitefish, or perch in a variety of lakes. With respect to the coexistence of ruffe and vendace in any Swedish lake which is similar to Bassenthwaite Lake in terms of its physical features (defined as plus or minus 20% of the Bassenthwaite Lake value for features shown to be statistically significant above), it is notable that these species do coexist in Skulesjon at an altitude of 55.0 m, surface area of 445 ha, and maximum depth of 41.0 m (no data available for mean depth). A similar analysis reveals coexistence of ruffe and perch in Lake Raslangen at an altitude of 73.0 m, surface area of 498 ha, and maximum depth of 19.0 m (no data available for mean depth). With res-

8 pect to Lake Constance, ruffe and whitefish coexist in the similar Swedish lake of Letten at an altitude of m, surface area of ha, maximum depth of 32.0 m, and mean depth of 10.6 m, but no similar lake contains both ruffe and perch.

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11 Finland The following account of ruffe populations in Finland and their potential interactions with vendace, whitefish, and perch is based largely on the results of a recent mail survey (Tammi et al. unpublished data) on the fish status of 860 Finnish lakes larger than 4 ha in surface area. Information was obtained on 710 lakes, among which the ruffe was present in 427 (60%). Other species found in at least 50% of lakes were perch (93%), pike (91%), roach (78%), and burbot (69%). Among lakes of different surface areas, ruffe occurred in 30% of small (4 to 10 ha) lakes, which rarely contained vendace (1%) or whitefish (11%) but were usually inhabited by perch (85%) as shown in Table 5. The latter also shows that all four species usually coexisted in large (greater than 1,000 ha) lakes. In addition to lake size, the distribution range of ruffe in Finland is also influenced by lake position in the river system, acidity, trophic status, hypolimnetic anoxia, and the availability of spawning substrata. The absence of ruffe from lakes in the headwaters of river systems has been suggested to be due to its poor ability to migrate upstream (Koli 1990), although such lakes also often lack hard bottoms which are the preferred spawning substrata of ruffe in Finland. An extensive survey of Finnish lakes with respect to acidification (Rask and Tuunainen 1990, Tuunainen et al. 1991) revealed that the ruffe is quite tolerant of increased acidity, falling between the tolerances of roach and burbot (critical ph levels 5.5 to 6.0) and perch and pike (critical ph levels 4.5 to 5.0). Some acidified lakes exist with ph levels of ca. 5 where ruffe have been

12 eliminated but perch and/or pike persist, while in other lakes of similar ph the mean size of ruffe is relatively larger than expected in such oligotrophic conditions, suggesting a reduced level of reproduction (Rask and Tuunainen 1990). The recent survey of Tammi et al. (unpublished data) also demonstrated the effect of acidity on ruffe distribution, with this species being present in ca. 30% of acidic lakes of ph less than 5.5, but present in ca. 70% of lakes of ph 6.0 or greater. However, in addition to ph itself this distribution may also be largely influenced by the typically small size and upstream location of acidic lakes in Finland. With respect to trophic status, a slight eutrophication increases the proportion of ruffe in the fish community (Koli 1990), where higher growth rates and larger ultimate sizes are also shown. For example, 5-year-old ruffe in oligotrophic lakes are typically ca. 100 mm in total length, while in more eutrophic conditions such fish are usually ca. 150 mm (Koli 1990, Sarvala et al. 1994). Oxygen availability is of course also linked to the trophic status of a lake and in Finland ruffe are not commonly found in lakes with continuously low levels of dissolved oxygen in the hypolimnion, even if adequate oxygen levels are available in the littoral zone. Although appropriate experimental studies have not been undertaken, it is possible that in such lakes ruffe would suffer from food competition as considered below. Another environmental parameter often linked with trophic status is water transparency, although staining by humic substances largely independent of the degree of eutrophication is also an important factor in water clarity in Finland. Because the ruffe is known to be active under conditions of low light levels (Bergman 1988), it might be expected to be correspondingly successful in humic or otherwise turbid lake waters. While an effect of this nature is apparent with respect to lake nutrient levels as described above, no such trend can be observed in humic lakes, particularly small ones. This is probably because such small and humic lakes

13 also tend to have poor hypolimnetic oxygen conditions. In fact, the survey of Tammi et al. (unpublished data) found ruffe to occur more frequently and more abundantly in clearer lakes. Interactions between ruffe, Coregonus species, and perch have not been extensively researched in Finland, although some studies are appropriate to a consideration of such issues. These may be viewed against the background facts that lake communities in Finland tend to be relatively simple in species composition, and that their structuring by piscivorous species appears to have a lesser role when compared with lakes of the same character in North America (Tonn et al. 1990). In addition, while aquarium experiments in the laboratory have shown that ruffe can have an influence on benthic animal communities, effects in field conditions have not been so clear (Mattila 1992). Vendace have traditionally been the most important species in commercial fisheries in Finnish lakes. Such activities have been concentrated on large lakes which are also inhabited by ruffe. These vendace populations typically vary considerably in abundance and recruitment between years. While many factors have been suggested to be involved in producing such effects (see for example Viljanen 1986), the ruffe has not been among them. Finally, although studies elsewhere have demonstrated a potential for and identified the mechanisms of interactions between ruffe and perch, including competition for benthic and zooplanktonic prey (e.g., Bergman and Greenberg 1994), there is no widespread evidence of negative effects of ruffe on perch in Finnish lakes. In the survey of Tammi et al. (unpublished data) it is clear that ruffe, which occur in 60% of lakes, do not inevitably exclude perch because the latter are found more frequently and occur in 93% of lakes. When considering larger lakes (see again Table 5), it is apparent that similar situations occur with ruffe, vendace, and whitefish. Although the work of Bergman and Greenberg (1994) in Sweden has shown that perch can be a competitive dominant in littoral or epilimnetic areas, with the ruffe favored in profundal or hypolimnetic habitats, these two species usually coexist in Finland if both types of habitats are available in a lake. In addition, if the lake is large then vendace and whitefish are also typically present. CONCLUDING REMARKS This synthesis has shown that despite the concerns provoked by ruffe introductions to new areas of Europe and North America, populations commonly coexist with Coregonus, Perca, and other fish species within the species' natural European range where they are not generally regarded as a problem. Nevertheless, the high population densities and extensive spatial occupations of the introduced ruffe populations in Lake Constance and Bassenthwaite Lake, which are also seen in North America (Mills et al. 1993), are likely to be much greater than those shown by native populations. It is possible that the current high densities of the introduced populations are those typical of recently established species, which may with time show marked declines. While quantitative data on absolute abundance are unavailable to investigate this possibility, the Swedish FIBase does hold relative abundance data for ruffe and other species within the fish communities of its 705 lakes. Future analysis of this information and comparison with equivalent data for introduced populations is highly desirable. In addition to developing the present analysis from a qualitative presence/absence basis to one in which at least relative abundance is considered, it would also be profitable to incorporate effects of lake temperature, water chemistry, and trophic state. Finally, in the context of future research on the fundamental aspects of ruffe introductions, it is notable that small-scale experimental studies of Bergman and Greenberg (1994) have revealed that

14 indirect but important competitive interactions can occur between ruffe, a percid (perch), and a cyprinid (roach), with implications for interactions in larger natural systems. Furthermore, the extensive study of the Loch Lomond ruffe introduction (Adams and Maitland 1998) has shown that such indirect interactions can extend beyond the fish community to impact on the avifauna of a lake. Future research should address such difficult but important issues. Monitoring and other research programs on the introduced ruffe population of Lake Constance will continue, not least because this water body contains extremely valuable commercial fisheries. While effects of ruffe on the whitefish or perch populations may be confounded by changes in the character of the lake as a consequence of its continued reoligotrophication, any strong influences on the natural recruitment of inshore-spawning whitefish are likely to be independent of the lake's trophic status. Bassenthwaite Lake will also experience a period of nutrient reductions over the next few years, although any effects of ruffe on the vendace and perch of this lake may be more intractably confounded by the introduced populations of roach and dace. Nevertheless, a monitoring program for the nationally rare vendace population is now in place and these activities will also allow the development of the ruffe population to be followed. In summary, there is considerable evidence that, at least in northern latitudes in established European communities, ruffe, Coregonus, and Perca species can coexist. However, introduced ruffe populations apparently usually attain very high densities, leading to a dynamic species composition of the fish community, and direct comparisons with long-established communities may be of limited validity. It is possible that during this period of flux, other fish populations may become extinct or their fisheries become uneconomic. Consequently, the introduction of ruffe populations elsewhere should continue to be considered as a potentially grave threat to other fish species and actions should be taken to prevent further occurrences. ACKNOWLEDGMENTS We thank the many people across Europe who responded to our questionnaire. Within the U.K., we acknowledge the assistance of Janice Fletcher, Peter Cubby, and Trevor Furnass, and the funding of the Environment Agency, North West Water Limited, and the Natural Environment Research Council. In Sweden, we thank the Swedish Environmental Protection Agency and the Swedish National Board of Fisheries for their funding of FIBase. In Finland, beneficial discussions with Drs. Lauri Koli, Jukka Ruuhijarvi, and Pekka Tuunainen are gratefully acknowledged, while Jouni Tammi kindly provided preliminary information from the 1,000 Lake Fish Status Survey of Finland. IJW thanks Douglas Dodge for his invaluable contributions to earlier discussions and for his kind supply of relevant information from North America. Finally, we are grateful to two anonymous referees for their constructive comments.