NORTHEASTERN NATURALIST 2008 15(4):577–588 Fish Movement Among Lakes: Are Lakes Isolated? Robert A. Daniels 1,* , Richard S. Morse 1 , James W. Sutherland 2 , Robert T. Bombard 2 , and Charles W. Boylen 3 Abstract - The concept of a lake as an isolated unit is a central theme in research and management of freshwater systems. Support is based on direct observations of lake communities. Studies undertaken in the last several decades lend tacit support because the methods used in both research and management often do not question the underlying notion that lake communities are essentially isolated. In a study of fish as- semblages in interconnected lakes, we noted movement of tagged fish among lakes. We also found that species introduced to one lake were later captured in neighboring lakes. We found that fish species in lake assemblages did not differ from those in inlet and outlet stream assemblages; although relative abundance varied, species richness and composition did not. This finding suggests that fish assemblages in lakes are not isolated. Rather, immigration and emigration from streams and other lakes occurs. Although few individuals migrated to new lakes, any movement can affect popula- tion structure (e.g., through recolonization, gene flow) and management goals (e.g., spread of disease). Consequently, we suggest that methods commonly used to assess fish assemblages in lakes and the concept of the lake as a management unit may need to be reconsidered. Rather than be treated as isolated populations, fishes in lake com- munities may be better treated as a watershed-wide metapopulation. Introduction The belief that a lake is an island, initially presented by Forbes in 1887 (c.f. Forbes 1925), is a fundamental tenet of limnology (Magnuson et al. 1998). For over a century, this concept has guided the work of limnological researchers and lake managers, who often treat lakes as individual, isolated units. The belief that lake-dwelling organisms make up a distinct community is an appealing concept in that it greatly simplifies the way limnological and fisheries data are collected and interpreted. The concept further provides a framework for how such data are ultimately used in managing those commu- nities. However, inherent in the concept are assumptions that are becoming increasingly difficult to support. Simply stated, the concept holds that lakes are habitat islands surrounded by different and disconnected habitats. In this scheme, organisms are con- fined to a single lake; organisms not currently in the lake would not occur there unless they were introduced. Because a lake is functionally isolated, it is implicitly assumed that the species in the lake community must be rela- tively sedentary. Movement within the lake is recognized as possible, but 1 New York State Museum, CEC 3140, Albany, NY 12230. 2 New York State Department of Environmental Conservation, 625 Broadway, Albany, NY 12233. 3 Darrin Freshwater Institute, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. * Corresponding author - [email protected].
Transcript
NORTHEASTERN NATURALIST2008 15(4):577–588
Fish Movement Among Lakes: Are Lakes Isolated?
Robert A. Daniels1,*, Richard S. Morse1, James W. Sutherland2, Robert T. Bombard2, and Charles W. Boylen3
Abstract - The concept of a lake as an isolated unit is a central theme in research and management of freshwater systems. Support is based on direct observations of lake communities. Studies undertaken in the last several decades lend tacit support because the methods used in both research and management often do not question the underlying notion that lake communities are essentially isolated. In a study of fi sh as-semblages in interconnected lakes, we noted movement of tagged fi sh among lakes. We also found that species introduced to one lake were later captured in neighboring lakes. We found that fi sh species in lake assemblages did not differ from those in inlet and outlet stream assemblages; although relative abundance varied, species richness and composition did not. This fi nding suggests that fi sh assemblages in lakes are not isolated. Rather, immigration and emigration from streams and other lakes occurs. Although few individuals migrated to new lakes, any movement can affect popula-tion structure (e.g., through recolonization, gene fl ow) and management goals (e.g., spread of disease). Consequently, we suggest that methods commonly used to assess fi sh assemblages in lakes and the concept of the lake as a management unit may need to be reconsidered. Rather than be treated as isolated populations, fi shes in lake com-munities may be better treated as a watershed-wide metapopulation.
Introduction
The belief that a lake is an island, initially presented by Forbes in 1887 (c.f. Forbes 1925), is a fundamental tenet of limnology (Magnuson et al. 1998). For over a century, this concept has guided the work of limnological researchers and lake managers, who often treat lakes as individual, isolated units. The belief that lake-dwelling organisms make up a distinct community is an appealing concept in that it greatly simplifi es the way limnological and fi sheries data are collected and interpreted. The concept further provides a framework for how such data are ultimately used in managing those commu-nities. However, inherent in the concept are assumptions that are becoming increasingly diffi cult to support. Simply stated, the concept holds that lakes are habitat islands surrounded by different and disconnected habitats. In this scheme, organisms are con-fi ned to a single lake; organisms not currently in the lake would not occur there unless they were introduced. Because a lake is functionally isolated, it is implicitly assumed that the species in the lake community must be rela-tively sedentary. Movement within the lake is recognized as possible, but
1New York State Museum, CEC 3140, Albany, NY 12230. 2New York State Department of Environmental Conservation, 625 Broadway, Albany, NY 12233. 3Darrin Freshwater Institute, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. *Corresponding author - [email protected].
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emigration from the lake is not. This viewpoint ignores characteristics of one prominent component of many lake communities—fi sh. North American fi shes, with few exceptions, are stream dwellers; few are obligate lake dwell-ers (Moyle and Cech 1996). Moreover, the long-held notion that fi sh are relatively sedentary (Gerking 1959) is becoming less defensible (Fausch and Young 1995, Gowan et al. 1994, Rodríguez 2002). With a growing body of information, the concept of the isolated, individual lake has been questioned, and many recognize the connectivity that exists among lakes, streams, and landscapes (Magnuson and Kratz 2000). We report results here that support this contention. Several kinds of studies depend upon and support the notion that distinct fi sh assemblages exist in lakes (e.g., Magnuson et al. 1998). The basic design of such studies is similar. Lakes are either sampled with several types of gear for a relatively short period, or a single lake is sampled repeatedly over a long period (Magnuson and Kratz 2000). Several assumptions about the re-sulting data are adopted, but not often tested. Studies assume that (1) the temporally-limited data fairly represent the species composition of the lake and (2) with these data as a basis, future changes in the species composition of the lake can be explained. Olden et al. (2006) examined these assump-tions and, using a long-term data set from lakes in Wisconsin, found general support for both assumptions. They noted, however, that assessing commu-nity structure in lakes can be complicated by the environmental factors that affect composition and the spatial and temporal extent of the data. Both as-sumptions can be supported as demonstrated by Olden et al. (2006), and are reasonable if the lake is truly isolated. However, possible confounding issues include immigration or emigration of fi sh or the presence of rare species (Magnuson et al. 1994), so results of studies using single-year community assessments need to be carefully assessed and can perpetuate the notion that a lake assemblage is isolated. Since 1995, we have monitored fi sh populations in connected lakes in the Adirondack Mountains in Herkimer County, NY. Short streams link fi ve of our study lakes (Fig. 1). We examined fi sh movement among lakes and between streams and lakes in several ways. If lakes are isolated, individual fi sh should not move among lakes or between lakes and streams. If our re-sults show that fi sh are not confi ned to a single lake, then we should reject this hypothesis. Rejection would necessitate adjustments to many current fi sheries-management practices and research protocols. Researchers and managers would need to view lakes, and the biota found therein, as being part of a broader landscape and biotic community.
Methods
A tagging program was initiated in 1995 to examine annual and seasonal changes in the population size of fi shes in the study lakes: Rondaxe, Dart, and Moss. All are located in the Moose River watershed within the Saint
Lawrence River drainage. All are relatively small (mean surface area = 63 ha) and are situated in a landscape dominated by mixed conifer and decidu-ous forest. Each lake has inlet and outlet streams and the distance between lakes averages 3.5 km. Two other lakes, Big Moose and Cascade Lakes, are also connected to these three lakes by short stream segments; we also sampled these two lakes. Other characteristics of the streams and lakes are included in Figure 1. Fish were captured in trapnets, identifi ed to species, and counted. Fish were measured (standard length [SL], in mm), and individuals in fi ve species over a minimum length were tagged with sequentially numbered anchor tags and released. The fi ve species that we tagged were: Ameiurus nebulosus (Lesueur) (Brown Bullhead, over 80 mm SL), Catostomus com-mersonii (Lacepède) (White Sucker, over 90 mm SL), Ambloplites rupestris (Rafi nesque) (Rock Bass, over 75 mm SL), Lepomis gibbosus (Linnaeus) (Pumpkinseed, over 75 mm SL), and Perca fl avescens (Mitchill) (Yellow Perch, over 90 mm SL). Other species were identifi ed, counted, measured, and released. Sampling was done in spring and autumn so that fi sh would be handled in cooler temperatures and mortality minimized (Stickney 1983). Tagged and recaptured individuals provided one basis for evaluating fi sh movements among lakes and between lakes and streams. In general, anchor tags are retained well (Wydoski and Emory 1983) and were embedded in the musculature of each of these species equally well.
Figure 1. Lakes in the upper Moose River watershed, Adirondack Mountains, Ham-ilton and Herkimer counties, NY. At each lake, the surface area of the lake (ha), the elevation (m), and the area of the drainage basin (km2) are listed. For each stream segment, the average gradient (m/km) and segment length (km) are listed.
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Inlet and outlet streams (Fig. 1) were sampled in summer 1998, 2001, and 2004 with either a backpack electroshocker or 6-m bag seine (bag with 3 mm bar, wings with 5 mm bar). Stream sampling was conducted in reaches from 50 to 100 m in length and that included riffl e, run, and pool habitats. All fi sh in stream collections were identifi ed to species and counted. Most fi sh were returned to the stream. Individuals not returned alive to the lake or streams are vouchered at the New York State Museum. In addition, unanticipated fi sh introductions occurred during our moni-toring studies that allowed us to test the fi sh movement hypothesis using a second, independent source of information. Micropterus salmoides (Lace-pède) (Largemouth Bass) was introduced into Lake Rondaxe in stocking events in 1998 and 1999. This species is not native to Adirondack lakes and was absent from the system until the stocking events occurred. The lake as-sociation reported that 500 fi ngerling fi sh were stocked in the spring each of the two years. Two other species were stocked into the system in the mid-1990s: Osmerus mordax (Mitchill) (Rainbow Smelt) was introduced into Moss Lake by 1995, and Micropterus dolomieu (Lacepède) (Smallmouth Bass) was stocked into Lake Rondaxe in 1997. The introduction of Rainbow Smelt was not sanctioned; our information rests on the date of fi rst appear-ance of the species in catches and/or is gleaned from anecdotal information from local sources.
Results
Recapture of tagged fi sh Recapture locations of previously tagged fi sh provided information on movement within lakes and among them. During the study, we tagged 33,875 fi sh in Moss, Rondaxe, and Dart lakes and recaptured 4649 fi sh at least once. Several individuals were recaptured more than once, making the recapture rate 20.4%. Between 1995 and 2006, 43 individual fi sh moved among lakes. Fifteen White Suckers, 6 Brown Bullheads, and one each of Rock Bass, Pumpkinseed, and Yellow Perch were tagged in a downstream lake and moved upstream. Thirteen White Suckers, one Brown Bullhead, and one Yellow Perch moved downstream from one of the upstream lakes. Three White Suckers and one each of Yellow Perch and Brown Bullhead were tagged in one upstream lake, moved downstream to Lake Rondaxe, and were recaptured in the other upstream lake (Table 1). The shortest time between tagging in one lake and recapture in a different lake was 21 days; this by two White Suckers tagged in late spring 1997. Other fi sh recaptured in a lake different from the one where they were originally tagged were taken from 117 to 1808 days after tagging. Fish tagged in each of the years from 1995 to 2005 were recaptured in a lake different than the one in which it was tagged originally between 1996 and 2006. Of all recaptured fi sh, 0.9% emi-grated from the lake in which it was tagged initially. White Sucker emigrated most frequently, and Pumpkinseed and Rock Bass least frequently. Table 1 provides a summary of fi sh movement among lakes.
Fish movement subsequent to a fi sh introduction In spring 1998, 500 Largemouth Bass were released into Lake Rondaxe; an additional 500 fi sh were stocked in spring 1999. In autumn 1998, we be-gan to catch Largemouth Bass in Lake Rondaxe, and it since has become an important component of the lake assemblage (Fig. 2). In 2000, Largemouth Bass was fi rst collected in the upstream Dart Lake, and the number and rela-tive abundance of Largemouth Bass in Dart Lake has continued to increase (Fig. 2). Largemouth Bass was fi rst collected in Moss Lake in 2003, and the number of individuals and relative abundance increased in subsequent years (Fig. 2). Several size classes have been caught in both lakes, which suggest annual recruitment, although the extent of in-lake reproduction in either up-stream lake is not known. Annual upstream migrations may continue. Smallmouth Bass was captured in Rondaxe Lake in 1997, 1998, 2001, and 2005 but none has been taken in either Dart or Moss Lakes. Rainbow Smelt was collected initially in Moss Lake in 1995. This species was cap-tured in Moss Lake every autumn since 1997. Relative abundance has varied between 0.1 and 0.8%. In 2000, an individual (SL = 106 mm) was captured in Rondaxe Lake.
Stream fi sh assemblage Seventeen fi sh taxa inhabit inlet and outlet streams associated with each of the study lakes. Eighteen taxa have been collected in the three lakes (Ta-ble 2). One species, Culaea inconstans (Kirkland) (Brook Stickleback), has
Figure 2. Relative abundance of Largemouth Bass in three upland lakes in the Moose River watershed, Adirondack Mountains, NY, 1998–2006. Based on catches from summer and fall samples in Rondaxe, Moss, and Dart lakes.
been taken in stream samples only and two (Rainbow Smelt and Salvelinus namaycush (Walbaum) [Lake Trout]), have been collected only in lakes. The species that compose the stream and lake assemblages are similar. Relative abundance varies, however. In the lakes, Brown Bullhead, White Sucker, Luxilus cornutus (Mitchill) (Common Shiner), Pumpkinseed, and Yellow Perch were the species most frequently taken. These species were rarely taken in streams, where Umbra limi (Kirtland) (Central Mudminnow), Semotilus atromaculatus (Mitchill) (Creek Chub), Rhinichthys atratulus (Hermann) (Eastern Blacknose Dace), and Salvelinus fontinalis (Mitchill) (Brook Trout) dominated upstream sites and where the centrarchids and Yellow Perch were more common downstream of Lake Rondaxe (Table 2). Neither black bass species was collected at any stream sites in 1998. By 2001, Largemouth Bass was taken downstream of Lake Rondaxe and in the North Branch, Moose River between Lake Rondaxe and Dart Lake. By 2004, Largemouth Bass was taken downstream of Lake Rondaxe, between Lake Rondaxe and Moss Lake and upstream of Moss Lake.
Discussion
Three different types of information support the contention that Adiron-dack lake fi sh assemblages are not isolated either from the interconnecting streams or nearby lakes: recapture of tagged fi sh, capture data on fi sh move-ment subsequent to introductions, and the composition of the lake and stream assemblages. Overall, 0.9% of recaptured fi sh were taken in lakes other than the one in which they were initially caught (Table 1). White Sucker was the most likely species to be recaptured in a different lake (2.7%). The 43 fi sh with catch histories detailed in Table 1 emigrated from one lake to another. Although a relatively small percentage of the total number of recaptured tagged fi sh emigrate, the presence of any emigration demonstrates that at least a small part of each population regularly disperses, which also has been noted in other fi sh populations (Petty and Grossman 2004, Smithson and Johnston 1999). Fish movement is key to rejecting the idea of lake-specifi c fi sh assemblag-es. Researchers recognize that some fi sh move, although the behaviors often are identifi ed as specialized as noted in Gowan et al. (1994). For example, fi sh are known to migrate to feed or spawn (Josephson and Youngs 1996, Raney and Webster 1942) or minimize threats (Fraser et al. 2006). Certain species move in response to seasonal changes (Josephson and Youngs 1996, Meyers et al. 1992). Our data indicate, however, that the movement behavior of at least some individuals of some species does not fall clearly into these cat-egories. White Sucker, for example, is a fi sh that migrates upstream to spawn (Raney and Webster 1942), and because it was the species that emigrated most frequently in our study, its movement perhaps might be explained as part of its spawning behavior. However, two individuals clearly were tagged after the completion of the spring spawning run and were recaptured in a dif-ferent lake before the beginning of the next run. Furthermore, 13 individuals
migrated downstream rather than upstream, which is unexpected if the emi-gration was accidentally the result of a spawning run. Thus, almost half of the individuals in the sample that were documented to have moved between lakes did not behave in a way typical of spawning White Sucker. Rainbow Smelt migrate from lakes into tributaries to spawn (Langlois 1935). The individual migrant was taken downstream of the source lake—a behavior not associated with a spawning run. Pumpkinseed, Rock Bass, and Brown Bullhead, the other species that migrated between lakes, spawn in nests in lakes (Smith 1985). None of our marked fi sh was young, so the observed behavior was not associated with out-migration of young fi sh, a search for nursery or rearing habitat, or any movement associated with early life history. In fact, all the individual fi sh that immigrated to a second lake were adults when tagged. Although information to the contrary is mounting (e.g., Gowan and Fausch 1995, Neely and George 2006), fi sheries biologists and managers often accept that fi sh, particularly as adults, spend their lives in relatively small areas. This assumed sedentary nature of freshwater fi sh (Gerking 1959), implicit in the belief that lakes are isolated, has infl uenced manage-ment strategies. Our data corroborate other studies that suggest that lengthy movements of individual fi sh, even if only a small part of the population, are not unusual (Coombs and Rodríguez 2007, Gowan et al. 1994). Reports of fi sh movement among lakes have been noted for over a century. For example, what was believed to have been a single point introduction of Yellow Perch in the Moose River watershed, NY, was followed by a rapid expansion into all neighboring lakes within a decade (Mather 1886). Mather (1886) also reported the multi-lake expansion of Smallmouth Bass from a single-lake introduction during the same period. Inter-lake movement of 0.9% of the individuals of a community would seem to have little effect on community structure and the population ecology of any of the study lakes. Other aspects of the biology of the popu-lations involved, however, are affected. For example, dispersal among lakes can maintain genetic similarity among neighboring populations and dispers-ing fi sh can act as vectors in dispersal of diseases and parasites. Dispersing fi sh can replenish declining or extirpated populations, or, as in the case of Largemouth Bass described here, can serve as the vanguard of an invasion of an exotic species. The presence of Largemouth Bass in upstream lakes and the presence of both Largemouth and Smallmouth Bass in stream samples soon after their introduction into Rondaxe Lake is strong evidence of rapid out-migration from a single point source. Although unsanctioned stocking of upstream lakes is possible, the presence of Largemouth Bass in streams be-tween the lakes suggests that migration between lakes is occurring and that any boundary between streams and lakes, if present at all, is porous (Jackson et al. 2001). In effect, the individual lake populations examined here can be consid-ered local habitat patches of a metapopulation, i.e., discrete populations,
Northeastern Naturalist Vol. 15, No. 4586
largely unaffected by each other, but with some inter-population interaction through inter-lake movement of individuals (Hanski 1999). In this defi nition of metapopulation, high extinction rate of local populations is not a defi ning characteristic, which makes the concept more useful in examining communi-ty relationships in aquatic sciences (Kritzer and Sale 2004). As an analytical approach to assess the dynamics of each local patch and also of the regional network, these lakes can serve as an effective metapopulation model because of the demonstrated low level of exchange among the local populations. The model will become increasingly valuable to managers as populations in patches (= lakes) become stressed by environmental disturbances, such as an invasive species or exotic diseases, and protection of individual populations becomes more dependent upon the patch network (= watershed). It is important that managers re-examine their treatment of lakes as man-agement units. Evidence (e.g., the data in this report, Jackson et al. 2001) suggests that the convenience of accepting this approach does not offset the potential damage that can occur from its use. The rapid dispersal of the ex-otic Largemouth Bass into neighboring lakes and streams after an approved introduction into one lake demonstrates the need to consider the effect on the watershed when planning stocking, reclamation, or species protection proj-ects. A second basic consideration is that, when dealing with management of drainage lakes, inlet and outlet streams need to be a part of the management plan. Ability to disperse through streams is dependent upon the character-istics of both the species and the stream (Olden et al. 2001), but effective management of the lake fi shery is tied, in part, to the tributaries. Finally, data should be collected in a way that allows managers to make assessments at the watershed level. Although the need to use different devices to capture dif-ferent species and life stages is widely recognized (e.g., Jackson and Harvey 1997), repeated sampling of the entire assemblage is also needed. In addition, lake surveys often do not include associated stream sampling. Repeated sur-veys of lake and stream sites across the watershed are useful and can provide the temporal and spatial detail needed for development of effective manage-ment plans. This additional sampling may not require additional resources (see Olden et al. 2006), but will require that available resources be used to maximize information obtained from appropriate sampling protocols. Our survey work demonstrates that some fi sh emigrate from the lake in which they were tagged to other lakes in the system. We have also followed the out-migration to neighboring lakes from point introductions of two ex-otic species. Finally, our data suggest that the stream and lake assemblages are generally composed of the same species and, presumably, individual fi sh move between the two macrohabitats in their inter-lake migrations. Future studies should be designed to incorporate methods that allow detection of fi sh movement, including unique marks on individual fi sh and repeated surveys. We suggest that the concept of the lake as an island, or more spe-cifi cally as a unique management unit, is too simple. Lake populations need to be treated as dynamic components of a metapopulation.
We thank all the volunteers, interns, students, and colleagues who participated in the fi eld work over the past decade, particularly B.R. Weatherwax and D.A. Bloomquist. R.E. Schmidt, J.A. Tyler, and T.J. Sullivan reviewed drafts of this manuscript, and we appreciate their valuable comments and suggestions. This work was supported in part by EPA contract 68D20171.
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