Northeastern Naturalist Vol. 22, No. 1 T.M. Evans and K.E. Limburg 2015 69 2015 NORTHEASTERN NATURALIST 22(1):69–83 The Distribution of Larval Sea Lampreys, Petromyzon marinus, and their Nutritional Sources in the Hudson River Basin Thomas M. Evans1,* and Karin E. Limburg1 Abstract - We studied the distribution and food sources of larval Petromyzon marinus (Sea Lamprey) in the Hudson River basin, NY, and found ammocoetes of Sea Lampreys in four tributaries of the Hudson River: 1) Cedar Pond Brook, 2) Catskill Creek, 3) Roeliff Jansen Kill, and 4) Rondout Creek. The largest numbers of Sea Lampreys in the Hudson River basin appears to come from Catskill Creek. Sea Lampreys could increase their range in the Hudson River basin in the near future as barriers to migration are removed. Isotopic analysis demonstrated that Sea Lamprey larvae depended on both terrestrial plant material (i.e., allochthonous) and aquatic primary production (i.e., autochthonous), but that site characteristics influenced the importance of each to nutrition. Larval lampreys from the Kaaterskill Creek depended on allochthonous sources for about half of their nutrition, while those at Cedar Pond Brook obtained only ~1% of their nutrition from these same sources. Gut contents of larval Sea Lampreys were isotopically distinct from filter-feeding macroinvertebrates, suggesting that they exploit food resources differently. Introduction Sea Lampreys in the Hudson River Basin Anadromous fishes spawn in freshwater but migrate to marine or very large freshwater ecosystems for the majority of their growth and maturation, and are important members of many coastal ecosystems. The Hudson River is a biologically diverse ecosystem that currently contains 10 anadromous fish species (Levinton and Waldman 2006) and has been the focus of extensive research (Jackson et al. 2005). Research efforts have not been divided evenly across all species, however, and there are gaps that remain in the current understanding of the migratory fish community (Waldman 2006). Petromyzon marinus L. (Sea Lamprey) are the most poorly studied member of the Hudson River anadromous fish community. Study of Sea Lampreys has been neglected because they are not considered economically or socially important in North America, they are often difficult to sample, and perceptions of this species are dominated by negative attitudes toward invasive populations in the upper Great Lakes. Sea Lampreys belong to the most primitive vertebrate lineage (Class: Cyclostomata), and employ an anadromous, obligatory semelparous life history (Saunders et al. 2006). Unlike most other anadromous fishes, which spend a shorter time in freshwater than in the marine waters, Sea Lampreys spend protracted periods in freshwater (up to 17 years) before a relatively brief time (1–3 years) at sea 1State University of New York, College of Environmental Science and Forestry, Syracuse, NY 13210. *Corresponding author - tevans03@syr.edu. Manuscript Editor: Jay Stauffer Northeastern Naturalist 70 T.M. Evans and K.E. Limburg 2015 Vol. 22, No. 1 (Beamish 1980). An estimation of historical and current (within 100 years) abundance and distribution is difficult due to a lack of information . Stable isotope analysis of lower trophic levels in tributary food webs Larval lampreys are filter-feeders. Although materials they ingest have been documented, the source(s) of the material is still largely unknown (Moore and Mallatt 1980, Mundahl et al. 2005, Sutton and Bowen 1994). Isotopic analysis offers an alternative approach to gut-content analysis to determine nutritional sources. Consumers develop an isotopic signature based upon the isotopic signature of their diet and the proportion of the foods they use for nutrition (Michener and Kaufman 2007, Peterson and Fry 1987). The nutritional sources supporting an organism can be estimated by modeling the contribution of sources to consumer values (Moore and Semmens 2008, Phillips and Gregg 2003). Simultaneous use of multiple natural isotopes can help resolve the dietary and nutritional sources supporting a consumer with greater accuracy than with a single isotope (Caraco et al. 2010, Peterson and Fry 1987). Larval lampreys and many aquatic invertebrates are filter feeders (Mallat 1982, Voshell 2002), but it is unclear if they use food sources in the same proportions. Many macroinvertebrates also filter feed. To date only a single study has analyzed gut contents of larval lampreys and aquatic macroinvertebrates with stable isotopes simultaneously (Bilby et al. 1996). Interestingly, Bilby et al. (1996) found that the δ15N of larval lampreys was most similar to collector-gatherers, but the δ13C of larval lampreys was more similar to shredders and grazers. Although stable isotopes have only been applied in a limited number of studies of larval lampreys (Evans 2012, Hollett 1995, Limm and Power 2011, Shirakawa et al. 2009), isotopes have been widely used to examine macroinvertebrates (Finlay 2001, Vander Zanden and Rasmussen 1999). Therefore, the purposes of this study were twofold: 1) to identify the current presence of Sea Lampreys in tributaries of the Hudson River estuary below the Troy Dam and 2) to determine the sources of organic matter (OM) supporting young-ofyear (YOY) larval lampreys and selected macroinvertebrates utilizing stable isotope analysis. We hypothesized that YOY Sea Lamprey nutrition was predominantly composed of allochthonous materials (Sutton and Bowen 1994), and that larval lampreys were isotopically similar to macroinvertebrate filterer s. Field-site Description We sampled 25 tributaries of the Hudson River estuary below the Troy Dam for Sea Lampreys in June, July, and August of 2013 (Fig. 1). Sampled tributaries were diverse in appearance and included rocky coldwater first-order streams, channelized and highly eutrophic second-order streams, and warmwater third-order streams. Selection of sites was based upon recommendations from New York State Department of Environmental Conservation (NYSDEC), as well as published journals and reports of Sea Lampreys. Sites varied widely in the predominant land use within their watershed (Table 1), but sampling always occurred in areas with freshwater, Northeastern Naturalist Vol. 22, No. 1 T.M. Evans and K.E. Limburg 2015 71 preferably above the head of tide. Sampling was conducted below the head of tide if access was only found there. Methods We used a backpack electrofisher (Haltech, HT-2000) to detect larval lampreys. A low shock (5 Hz, 150 V, 2:2 pulse pattern) incited larval lampreys to the surface, where they were collected. Electrofishing was conducted for 15 minutes at sandy Figure 1. Sampled study sites for Sea Lampreys in Hudson watershed below the Troy Dam. Northeastern Naturalist 72 T.M. Evans and K.E. Limburg 2015 Vol. 22, No. 1 and fine substrate indicative of habitat used by larval lampreys at each site (Potter 1980). Fisherman and local residents were also interviewed to identify appropriate collection sites. Collections for stable isotopes analysis To understand the role Sea Lampreys play in freshwater food webs, we used stable isotope ratio analysis to examine young-of-year (YOY) larval lampreys, collector- gatherer aquatic insects, and primary producers. YOY larval lampreys were collected following the procedure described above. We assumed the smallest size class (16–34 mm) at a site to be YOY (Beamish 1980). Larval lampreys were stored for no more than 24 hours in plastic bottles filled with sand and water from where they were collected in an ice bath (~0 °C) until they could be returned to laboratory to be frozen at -20 °C. Sand was added to the bottles to allow larval lampreys to rest. We collected Hydropsychids and Isonychiids (both collector-filterers)if they were present at each site by kick-netting within riffles <50 m from where larval lampreys were collected, and then hand-picking invertebrates using clean nitrile gloves and forceps. Kick-netting continued until >5 individuals of each group were collected; usually this collection required 1–3 minutes. We stored invertebrates in self-sealing bags with stream water at 0–4 °C for 36–48 hours to allow them to Table 1. Streams sampled for Sea Lampreys in the present study. Watershed area and percent land use within that watershed are included. Watershed Open Stream area (km2) Developed (%) Forest (%) Agriculture (%) water (%) Annsville Creek 53.2 13.0 83.0 2.0 2.0 Black Creek 89.5 7.3 80.4 10.8 1.5 Catskill Creek 128.0 15.2 75.3 8.7 0.9 Cedar Pond Brook 46.0 19.5 76.1 1.4 3.1 Claverack Creek 89.8 12.2 37.7 49.7 0.4 Coxsackie Creek 74.8 14.0 56.3 28.4 1.3 Croton River 134.0 21.8 65.3 5.2 7.8 Furnace Brook 151.0 33.7 49.7 2.5 14.0 Hannacroix Creek 171.0 6.5 77.7 12.1 3.7 Indian Brook 99.2 15.9 73.3 2.8 8.0 Indian Kill 106.0 13.7 61.6 9.9 14.8 Kaaterskill Creek 78.5 9.6 83.1 7.0 0.3 Kinderhook Creek 64.0 13.2 44.6 41.3 0.8 Moodna Creek 116.0 26.6 58.5 12.6 2.3 Minisceongo Creek 49.3 43.2 53.2 0.7 2.9 Muitzes Kill 81.6 11.1 40.4 46.5 2.0 Normans Kill 104.0 37.5 47.2 15.2 0.1 Poesten Kill 44.0 27.0 35.4 37.2 0.4 Quassaick Creek 132.0 28.5 62.9 6.3 2.3 Roeliff Jansen Kill 119.0 6.6 46.3 46.0 1.1 Rondout Creek 91.3 6.4 77.6 15.4 0.6 Saw Kill 68.0 10.9 58.4 30.0 0.7 Stockport Creek 64.0 13.2 44.6 41.3 0.8 Vlockie Kill 52.8 13.8 46.7 29.6 9.9 Vloman Kill 79.3 23.7 48.6 27.6 0.1 Northeastern Naturalist Vol. 22, No. 1 T.M. Evans and K.E. Limburg 2015 73 void their guts. Potential primary food sources were also collected simultaneously. We collected leaves of common terrestrial plants (e.g., Acer spp. [maples], Vitis riparia Michx [Riverbank Grape], Quercus sp. [oaks] and Rosa multiflora Thunb. [Multiflora Rose]) by hand picking within 100 m of the stream both upstream and downstream. and gathered surface soil (0–4 cm) by scraping the surface under the leaf litter. Aquatic plants and algae (e.g., Elodea sp., Potamogeton sp., and Myriophyllum sp.) observed at the site were picked from surfaces and rinsed in stream water before being placed in a self-sealing bag. We excavated undisturbed surface sediments (0–4 cm) from the areas in which larval lampreys were collected. In the laboratory, aquatic plants and algae were washed in deionized water to remove detritus before being processed for stable isotope analysis. To acquire a muscle sample for larval lampreys, we decapitated 3 individuals per site (n = 12) after the seventh gill opening, discarded the head, extruded the contents of the visceral sack and the notochord from the body, and then used the remaining materialfor stable isotope analysis. Macroinvertebrates were thawed and sorted by family. All samples for stable isotope analysis were dried to constant mass at 60 °C, homogenized by grinding in a clean porcelain mortar, and then stored in desiccation chambers. Stable isotope values are calculated using the formula δX = 1000(Rsample/Rstandard - 1), where Rsample is the ratio of the heavy to light isotope in the sample and Rstandard is the ratio in an agreed-upon standard. To eliminate leading zeroes and ease readability, the raw sample value is multiplied by 1000. Values are reported as “per mil” (‰) and designated by the notation δ with a superscript indicating which heavy isotope has been measured. Subsamples of every sample type were packed in clean tin capsules and analyzed for d13C and d15N at the University of California, Davis (using a PDZ Europa ANCA-GSL [EA] attached to a PDZ Europa 20-20 isotope ratio mass spectrometer [IRMS]). Standard deviations for replicate analyses of standards for the instrument were ≤0.2‰ for δ 13C and ≤0.3‰ for δ15N. Stable isotope mixing models The δ13C values are sensitive to the amount of lipid in a sample, which is more negative (i.e., more depleted in 13C) than muscle tissue (Post et al. 2007). Isotopic values were not corrected for lipid content in any sample because δ13C values were not correlated with the C:N, a proxy for lipid content (Kiljunen et al. 2006, Post et al. 2007), in any group (i.e., primary producers, sediments, macroinvertebrates, or larval lampreys; Table 2). Table 2. C:N, δ13C, the number of samples (n), and the significance of the correlation between C:N and δ13C across all sites in the present study. Values (in per mil, ‰) are reported as mean (±SD). Group C:N δ13C (‰) n R2 P-value Primary producers 17.6 (6.5) -27.0 (7.6) 23 0.12 0.11 Soils and sediments 13.5 (3.8) -28.2 (0.7) 8 0.38 0.10 Macroinvertebrates 5.8 (0.6) -27.8 (2.9) 25 0.03 0.31 Larval lampreys 5.7 (1.4) -25.2 (2.9) 12 0.01 0.81 Northeastern Naturalist 74 T.M. Evans and K.E. Limburg 2015 Vol. 22, No. 1 We used the Bayesian stable isotope mixing model MixSIR (Moore and Semmens 2008) to estimate the contributions of potential food sources to YOY larval lamprey nutrition. Bayesian statistics predict the likely occurrences that led to the observation after an event has already occurred. MixSIR applies this statistical analysis to stable isotope mixing models, allowing for the incorporation of the uncertainties around source isotopic values and fractionation estimates to better predict food-source dependence and confidence of the importance of a given dietary item to the organism (Moore and Semmens 2008). We modeled each site separately because of apparent differences in isotopic values of primary producers and consumers. Potential food sources for lampreys and invertebrates at each site were divided into two groups: autochthonous and allochthonous. We considered autochthonous sources to be all types of aquatic plants collected at a site, including algae and macrophytes, and allochthonous sources to be all terrestrial plants collected at a site. Terrestrial surface soils and aquatic surface sediments were isotopically intermediate between terrestrial and aquatic plants. Therefore, these sources were likely amalgamations of terrestrial and aquatic plants, which were already accounted for in the model and were not included explicitly in the final model. The final models included all the measured terrestrial plants at a site as the allochthonous source, and all of the measured aquatic plants, including algae, as the autochthonous source. Isotopic values of larval lampreys, invertebrates, and potential nutritional sources were all derived only from measurements of samples collected in the present study. On the basis of prior work (Sutton and Bowen 1994), we assumed larval lampreys to be one trophic level above primary producers and applied published fractionation values of 0.4 ± 1.3‰ for δ13C and 3.4 ± 1.0‰ for δ15N (Post 2002). We also considered invertebrates to be one trophic level above primary producers and used the same fractionation values for them as for larval lampreys; we called this model the “standard” model. We also tested a “low fractionation value” model because work by Vanderklift and Ponsard (2003) has suggested that detritivores fractionate 15N at lower values than carnivores, herbivores, or omnivores. The low fractionation value model used the following fractionation values: 0.4 ± 1.3‰ for δ13C and 0.5 ± 1.1‰ for δ15N. All models were run with 1,000,000 iterations (i.e., MixSIR attempted to find a solution to the model for each iteration). Results conformed to the recommended guidelines for determining if the model output had estimated true posterior distributions (Moore and Semmens 2008). Results are reported as the posterior median percent contribution of autochthonous and allochthonous sources to a consumer. Results Larval Sea Lampreys were found in four tributaries to the Hudson River: 1) Roeliff Jansen Kill, 2) Catskill Creek, 3) Rondout Creek, and 4) Cedar Pond Brook. We did not find Sea Lampreys in all tributaries with records of thi s species (Table 3). Northeastern Naturalist Vol. 22, No. 1 T.M. Evans and K.E. Limburg 2015 75 Table 3. Streams where Sea Lampreys have been reported in the Hudson River and the 2013 capture confirmation or explanation of discordance. Reason for disagreement = reason for disagreement with historic data if Sea Lampreys were not collected. Lampreys Lampreys observed Stream reported 2013 Source Reason for disagreement Black Creek Y N Schmidt and Limburg 1989 Single larvae captured during large sampling effort, animals may be rare Catskill Creek Y Y Numerous NA Cedar Pond Brook Y Y PIPLON 2010 NA Hannacroix Creek Y N Greeley and Bishop 1933 Historic record (1933), possible that lampreys currently spawn upstream of sampling site Kaaterskill Creek Y Y Schmidt and Cooper 1996, NA Bryan et al. 2005 Poesten Kill Y N HRA 2007a Single record of an adult, possible migrant looking for spawning site Quassaick Creek Y N HRA 2005 Single record of a dead adult, possible migrant looking for spawning site Roeliff Jansen Kill Y Y Brussard et al. 1981, NA Waldman 2006 Rondout Creek Y Y Greeley and Greene 1937, NA HRA 2007b Stockport Creek Y N HRA 2005, Maybe currently extirpated, interview with a local resident suggest this to Interviewee 2013 be the case; no current observations Saw Kill Y N Smith 1985 American Brook Lamprey record, Sea Lampreys may occasionally spawn in mouth of stream Northeastern Naturalist 76 T.M. Evans and K.E. Limburg 2015 Vol. 22, No. 1 Figure 2. Stable isotope ratios of δ13C vs. δ15N for macroinvertebrates, young of year of larval Sea Lamprey, and their potential food sources at (A) Cedar Pond Brook, (B) Kaaterskill Creek, (C) Roeliff Jansen Kill, and (D) Rondout Creek. Points noted with an * had low amounts of C or N and may have less-precise estimates than other samples. Error bars around samples are smaller than the point symbols on the figure. Stable isotope analysis Isotopic values of YOY lamprey larvae were different from that of macroinvertebrates at every site, even though the macroinvertebrate groups measured were filterers (i.e., Hydropsychidae, and Isonychiidae; Fig. 2). Contributions of autochthonous and allochthonous sources to YOY larval lampreys varied by site in both models (Figs. 3, 4). In the standard fraction model, larval lampreys depended on autochthonous sources almost completely (median contribution = 98.7% at Cedar Pond Brook), to less than half of their nutritional needs (median = 40.3% at Kaaterskill Creek; Fig. 3). In the low fractionation model, the median contribution of autochthonous sources to larval lampreys increased by 13.4–24.8%, except at Cedar Pond Brook which only increased by 0.6%, while allochthonous dependence decreased accordingly (Fig. 4). Discussion Sea lamprey distribution in the Hudson River Hannacroix Creek was a tributary historically used by Sea Lampreys (Greeley and Bishop 1933) and is currently surrounded by forest and agricultural lands (Table 1). Only reaches influenced by tidal inputs were sampled, which were not ideal for lampreys. Sea Lampreys may be able to exploit upper reaches, and further efforts should be made to determine if they still use Hannacroix Creek. Stockport Creek is formed at the confluence of its 2 main tributaries, the Kinderhook and Claverack Creek. Water temperatures in the Kinderhook and Claverack Creek near their confluences in July were close to the lethal limit for Sea Lampreys (30 °C in stream, the lethal limit is 31°C; Jobling 1981). Therefore, lower reaches Northeastern Naturalist Vol. 22, No. 1 T.M. Evans and K.E. Limburg 2015 77 Figure 3. Median percent contributions of autochthonous sources to macroinvertebrates and Sea Lamprey larvae at different streams calculated by the Bayesian model MixSIR (Moore and Semmens 2008). Lower and upper error bars correspond to the 5% and 95% posterior proportional contributions, respectively. Figure 4. Median percent contributions of autochthonous source contributions to diet of larval Sea Lampreys with a standard fractionation model (Δδ15N = 3.4‰, white bars) compared to a low fractionation model (Δδ15N = 0.5‰, grey bars) calculated by the Bayesian model Mix- SIR (Moore and Semmens 2008). Lower and upper error bars correspond to the 5% and 95% posterior proportional contributions, respectively. Northeastern Naturalist 78 T.M. Evans and K.E. Limburg 2015 Vol. 22, No. 1 were unlikely to support larval lampreys, and adults would need to migrate further upstream to reproduce. Dead juvenile Sea Lampreys were observed at the hydropower facility in Kinderhook, NY (Anonymous, pers. comm, Columbiaville, NY). He also reported not having seen any for “a couple of years”. Surveys for Sea Lampreys above the dams in Valatie, NY, were carried out, but no larvae were found. Habitat upstream looked to be of high quality, and if Sea Lampreys could access the habitat they would undoubtedly use it. The long, protracted larval period may allow rare adult penetrations into upper reaches to produce successful migrants for long periods. Highly visible migrating adults would be attracted to the scent of these larval lampreys (Fine et al. 2004) and congregate below successful spawning areas, wasting their reproductive output. Neither adults nor larvae of Sea Lamprey were observed in Black Creek in the summer of 2013, though the site was visited during the spawning period in June and then searched for larvae in July. Sea Lampreys may still be present at this stream, although they must be rare. Large stretches of appropriate habitat were available for both adults and larvae throughout the stream. In addition, a large Alosa pseudoharengus Wilson (Alewife) spawning run is still present in Black Creek (Schmidt and Limburg 1989), and Anguilla rostrata LeSueur (American Eel) are also common (Bowser et al. 2013). It is unclear why Sea Lampreys are not more common (if they are present), or why they are not exploiting this habitat (if they are absent); Sea Lampreys may do well in Black Creek if restoration efforts were carried out. A single adult lamprey has been reported from Poesten Kill, Quassaick Creek, and Saw Kill (Table 3), but no larvae or adults were observed during the present study. Poesten Kill and Quassaick Creek are both highly urbanized streams (Table 1); the adult Sea Lampreys observed recently at each (HRA 2005, 2007a) were likely searching for appropriate spawning sites. In the Saw Kill, the habitat available for larval lampreys below the first impassable barrier (a natural falls) is extremely limited and would not be a successful rearing area for Sea Lamprey larvae. It is possible that low-density populations of Sea Lamprey larvae were not detected at sites from which they have been recorded because of their cryptic nature and the difficulty in detecting them. However, long stretches (>100 m) of stream where ideal habitat for larvae was located immediately downstream of potential adult spawning grounds were searched at every site. At sites where Sea Lampreys were found, larvae were always found within 15 minutes of searching, usually within 10 minutes, and often at the first sand bar . Stable isotope analysis Larval lampreys and measured macroinvertebrates were isotopically well explained by the collected food sources at all sites except for two larval lampreys at Cedar Pond Brook (Fig. 2). At Cedar Pond Brook, lampreys were primarily supported by autochthonous sources (Fig. 3), because aquatic primary producers had isotopic values high enough to explain the high isotopic values of lampreys (in respect to both δ13C and δ15N). Although the model explained larval lamprey isotopic Northeastern Naturalist Vol. 22, No. 1 T.M. Evans and K.E. Limburg 2015 79 values with autochthonous sources, lampreys at this site may actually be receiving N and C contributions from anthropogenic waste sources, such as sewage or leaky septic systems; however, these sources were not sampled. Anthropogenic waste is more positive with respect to both δ13C and δ15N than natural sources (Cravotta 1997, McClelland et al. 1997), and larval values (range of δ13C = -27 to -23 and δ15N = 6 to 9) would be easily explained by this source (range of δ13C = -25 to -20 and δ15N = -2 to 13; Cravotta 1997). The stream is immediately downstream of a county park, residential housing, and an urban center. Based on MixSIR modeling, predicted contributions of sources to measured filter feeders at Cedar Pond Brook suggested nearly entire reliance (median contribution > 95%) on autochthonous sources (Fig. 3). The macroinvertebrates measured at Kaaterskill Creek were isotopically similar to larval lampreys. Predicted source contributions to different groups at Kaaterskill Creek were similar to one another, but the uncertainty was large for all estimates (Fig. 3). A single aquatic algae sample had a δ13C signature similar to terrestrial plants, which resulted in a large standard deviation for autochthonous sources in the model. The model suggests that animals at Kaaterskill Creek appear to be more dependent on allochthonous sources than larval lampreys at all other sites. The water at the site was moderately turbid, and the bottom of the stream was not readily visible at all depths. The turbidity appeared to be the result of suspended particulate matter, which may have been the result of runoff from local fields transporting soil material. If this is the case, the system may be more dependent on allochthonous carbon because particulates shade algal growth and can provide a source of nutrition (Bartels et al. 2012, Rounick et al. 1982). At the Roeliff Jansen Kill, larval lampreys were dependent approximately equally on autochthonous and allochthonous sources (Fig. 3). Source contributions to larval lampreys at this site were not similar to Isonychiidae. Larval lamprey filtering/ collecting is generally assumed to be similar to other invertebrates, but may not be similar to other groups (Mallatt 1982). Lampreys are unlike all other filter feeders and have no good proxy among any extant group (Mallatt 1982). Part of the success of lampreys as a group may be their ability to exploit a habitat and feeding style that no other group has adapted to fill. At Rondout Creek, autochthonous sources were more important than allochthonous sources for all groups (Fig. 3). Lampreys were more similar to Hyrdropsychiidae (a sedentary collector-filterer), than the more-mobile collectorfilterer Isonychiidae. Rondout Creek is wide (>10 m) and slow moving above a barrier to Sea Lamprey migration, immediately upstream of the sample site, which likely promotes algal growth in that section. Large larvae of Simulidae, which are also filter feeders, were also very common at the site, often completely covering rocks in areas of high current. Lampreys throughout Rondout Creek may not be as dependent on autochthonous sources. When a low-fractionation model was used to determine dependence of larval Sea Lamprey on autochthonous sources, the importance of those sources increased (Fig. 4). Larval Sea Lampreys may fractionate 15N at lower rates than Northeastern Naturalist 80 T.M. Evans and K.E. Limburg 2015 Vol. 22, No. 1 other groups because they rely on a low-quality and nutritionally sparse diet (Sutton and Bowen 1994). Controlled laboratory feeding experiments of larval Sea Lampreys will be able to test this assumption (J. Jolley, US Fish and Wildlife Service, Vancouver, WA, pers. comm.). If the low fractionation values of 15N are supported by laboratory studies on larval lampreys, this finding would suggest that lampreys are heavily reliant (>50% of nutritional support at all sites; Fig. 4) on the intermittent production of autochthonous sources (likely algae) as other workers have found (Yap and Bowen 2003). Therefore, although algae may constitute a minor portion of the gut content (Mundahl et al. 2005, Sutton and Bowen 1994), it may be more important to larval lamprey growth and development than the more abundant detrital fraction. Larval Sea Lampreys appear capable of exploiting a variety of sources to meet nutritional requirements. In addition, larval lampreys may be sensitive to some types of human pollution, which could be detected with isotopic analysis. Further work is needed to determine if they could be useful as bio-monitoring tools. Their limited distribution and the difficulty of sampling for larval lampreys may prevent wide-scale use, but they could offer unique information about the environment at sites in which they occur. Larval lampreys were also isotopically unique from the surrounding macroinvertebrate community and appeared to rely on a different proportion of autochthonous and allochthonous sources. Acknowledgments We thank the Tibor T. Polgar Fellowship Program of the Hudson River Foundation and the Edna S. Bailey Sussman Foundation for funding and support of this project. We also thank K. Hattala, R. Adams, and other members of the NYSDEC for advice and guidance on selecting locations and choosing streams to survey. J. Waldman, R. Schmidt, D. Yozzo, and E. Kiviat provided helpful advice during the project. D. Strayer and two anonymous reviewers provided comments on an earlier draft. C. Eger provided critical logistical support in the field. Literature Cited Bartels, P., J. Cucherousset, K. Steger, P. 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