NotesNon-lethal sampling of lake sturgeon for stable isotope analysis: Comparing pectoral fin-clip and dorsal muscle for use in trophic studies
Introduction
Owing to the predictable changes that occur in stable isotope ratios as energy flows through food webs, stable isotope analysis (SIA) has become a commonly used method for obtaining time-integrated information about feeding relationships in food web studies (Peterson and Fry, 1987, Finlay and Kendall, 2007). When used to study the feeding ecology of fish, dorsal white muscle is commonly sampled because it has an intermediate turnover rate, low isotopic variability, and frequently has low lipid content (Pinnegar and Polunin, 1999).
Obtaining dorsal muscle tissue for SIA can pose problems for many species as laws or management plans may constrain, or prohibit, the lethal sampling needed to obtain the tissue (e.g., Haley, 1998, Brosse et al., 2002). On larger species, the utilization of muscle biopsy plugs has been applied (Hanisch et al., 2010, Nelson et al., 2011, Carlisle et al., 2012) as one means of reducing the need for lethal sampling, but biopsies are often viewed as undesirable for juveniles or species of special concern (COSEWIC, 2007).
In place of muscle tissue, a number of studies have considered alternative non-lethal sampling including: mucus (Church et al., 2009), scales (Sinnatamby et al., 2008) and fin clips (e.g., Willis et al., 2013). Fin clip tissue has been considered a suitable surrogate to dorsal muscle tissue in several reviews including: Australian tropical and temperate fishes (Jardine et al., 2011), European freshwater fishes (Tronquart et al., 2012) and North American freshwater species (Willis et al., 2013). Within North American studies salmonids, in particular, dominate in study comparisons (42%) as a result of their threatened status and the associated demands for non-lethal sampling alternatives (Sanderson et al., 2009, Hanisch et al., 2010).
While there is some evidence that regional multi-species models for fin–muscle relationships may be applied in fish food web related studies (Jardine et al., 2011, Tronquart et al., 2012), the fin–muscle stable isotope relationship has frequently been shown to vary by species (Kelly et al., 2006, Hanisch et al., 2010, Willis et al., 2013). Furthermore, fin–muscle relationships often fail (28% of test cases) to exhibit a 1:1 slope indicative of an unbiased conversion of one tissue SIA value to another (Willis et al., 2013). Therefore, while strong relationships between the isotope signatures of fin clip and muscle tissues available in the literature demonstrate that fin clips can be used in place of muscle tissue, evidence of conversion biases and the dominance of salmonid studies indicate the need for further studies on non-salmonid species.
One species for which tissue comparisons have not been made to date is lake sturgeon (Acipenser fulvescens). Lake sturgeon population sizes have decreased in recent decades due to poor water quality, overfishing and damming (COSEWIC (Committee on the Status of Endangered Wildlife in Canada), 2006, Fisheries and Oceans Canada, 2008), with recent studies having identified the need for further non-lethal sampling information on sturgeon diet and foraging in areas where populations are affected by invasive species (McCabe et al., 2006) or other anthropogenic stressors (e.g., Haxton and Findlay, 2008). Furthermore, lake sturgeon represent a unique fin–muscle comparison given the high muscle tissue C:N ratios (typically > 5) as compared to previously examined species (e.g., Tronquart et al., 2012 C:N ≈ 3.5).
A strong relationship has been found between the C:N ratio and lipid content in animals (Post et al., 2007). Lipids are isotopically depleted in carbon relative to carbohydrates and proteins due to differing fractionation during lipid synthesis (DeNiro and Epstein, 1978, Griffiths, 1991). As such, some researchers have employed direct chemical extraction of lipids or mathematical normalization techniques to minimize the bias caused by heterogeneous lipid content among tissue samples (McConnaughey and McRoy, 1979, Sweeting et al., 2006, Hoffman and Sutton, 2010), with the technique being recommended when the C:N ratio is relatively high (above 3.5) or highly variable (Post et al., 2007).
In light of the above, the aim of the present study was to explore the use of non-lethal sampling for SIA of adult lake sturgeon. A specific objective was to test the hypothesis that there is a significant relationship between the stable isotope measures of pectoral fin-clip and dorsal muscle tissue samples. Furthermore, we sought to address a critical analytical issue (e.g., Arrington et al., 2006, Kiljunen et al., 2006, Logan et al., 2008, Fagan et al., 2011) by evaluating the influence of lipid extraction and mathematical normalization models on the obtained pectoral fin-clip and muscle tissue stable isotope comparisons as a means of determining whether lipid extraction and/or correction is routinely required for lake sturgeon stable isotope related studies.
Section snippets
Methods
The lake sturgeon used in the study were captured on the Rainy River, Ontario (48°36′ N, 93°24′ W) immediately below the International Falls Dam. The Rainy River is located on the Minnesota–Ontario border and flows westward from Rainy Lake to the south end of Lake of the Woods. The fish were collected by the staff of Fisheries and Oceans Canada by boat electro-fishing at night or with 25.4 and 30.5 cm mesh gill nets set overnight in early to late May 2012 and 2013. Nets were set following
Results
Lake sturgeon sampled in this study were primarily adult fish with a mean age (± standard deviation) of 18.4 (± 6.5) but covered a significant length range (42.5–168.5 cm) (Table 1). Of the 68 lake sturgeon sampled for both muscle and fin tissue, 30 (12 in 2012 and 18 in 2013) yielded sufficient muscle tissue (approximately 1 mg) for the SIA of both dorsal muscle (DM) and lipid extracted dorsal muscle (DME) tissue from which the C:Nprotein and ∆δ13Clipid parameters required for the mass balance
Discussion
Stable isotope values obtained from pectoral fin clip tissue of lake sturgeon were significantly related to paired samples of dorsal muscle and provided a reasonable surrogate for muscle tissue SIA values as evidenced by the 1:1 slope of the comparative regressions. Relationships, however, explained less than 50% of the variation in the data given the high variation in the C:N ratio between individuals. The C:N ratio is typically used as a surrogate measure of lipid content. While lipid
Acknowledgments
Samples were obtained in the field with the help of the Fisheries and Oceans Canada personnel, notably: William Gardner and Evan Timusk. Jeff Murihead also assisted with sampling. Funding was provided by the International Joint Commission, Fisheries and Oceans Canada, HydroNet and an NSERC operating grant to Power. NSERC also provided an industrial graduate scholarship to Smith through the kind co-operation of the St. Lawrence River Institute of Environmental Science.
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