Article pubs.acs.org/est
Mercury Elimination by a Top Predator, Esox lucius Jillian L. A. Van Walleghem,†,‡,∥ Paul J. Blanchfield,*,‡ Lee E. Hrenchuk,‡ and Holger Hintelmann§ †
Department of Zoology, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Experimental Lakes Area, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba, Canada R3T 2N6 § Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario, Canada K9J 7B8 ‡
ABSTRACT: Top-level piscivores are highly sought after for consumption in freshwater fisheries, yet these species contain the highest levels of the neurotoxin monomethylmercury (MMHg) and therefore present the greatest concern for MMHg exposure to humans. The slow elimination of MMHg is one factor that contributes to high levels of this contaminant in fish; however, little quantitative information exists on elimination rates by top predators in nature. We determined rates of MMHg elimination in northern pike (Esox lucius) by transferring fish that had naturally accumulated isotope-enriched MMHg (spike MMHg) through a whole-lake Hg loading study to a different lake. Over a period of ∼7 y, pike were periodically recaptured and a small amount of muscle tissue was extracted using a nonlethal biopsy. Spike total mercury (THg) persisted in muscle tissue throughout the entire study despite discontinuing exposure upon transfer to the new lake. Spike THg burdens increased for the first ∼460 d, followed by a decline to 65% of original burden levels over the next 200 d, and subsequently reached a plateau near original burden levels for the remainder of the study. We estimated the half-life of muscle THg to be 3.3 y (1193 d), roughly 1.2- to 2.7-fold slower than predicted by current elimination models. We advocate for further long-term field studies that examine kinetics of MMHg in fish to better inform predictive models estimating the recovery of MMHg-contaminated fisheries.
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INTRODUCTION Large-bodied piscivorous fish are the mainstay of sport, subsistence, and commercial fisheries in North America, yet these species are the most highly contaminated by monomethylmercury (MMHg) (e.g., ref 1), a potent neurotoxin.2 Because humans are primarily exposed to MMHg through eating fish,2 concern about MMHg toxicity has resulted in widespread fish consumption advisories.3,4 Fish primarily accumulate MMHg via their diet (>80%),5 and because MMHg biomagnifies (i.e., increases with each trophic level),6 predatory fish frequently accumulate MMHg to levels exceeding recommended consumption limits. In addition, predatory fish are typically long-lived (>5 y), and exposure to MMHg in prey throughout their life results in noticeable increases in MMHg concentration with body size7 and age.8 The level of MMHg present in fish tissues at any time is a balance between uptake and elimination from the body.9 Laboratory studies suggest that fish lose MMHg slowly (halflife from 16−1030 d; reviewed in refs 10 and 11). However, estimates of MMHg elimination by fish are primarily founded on short-term laboratory studies that have involved a single (acute) exposure to MMHg.10,11 Some fish tissues absorb and lose MMHg quickly (e.g., blood, liver, kidney), whereas others respond more slowly (e.g., muscle).12 As a result, studies that involve long-term (chronic) exposure lead to a greater proportion of the compound being stored in slow elimination compartments such as muscle.10 Consequently, long-term studies (>90 d)10 of fish muscle tissue most accurately reflect the slow phase of MMHg elimination and provide a benchmark © 2013 American Chemical Society
for understanding recovery trajectories of MMHg-contaminated fisheries. Recently, several long-term studies have converged on the finding that published elimination models10 overestimate fish MMHg elimination rates by several fold (∼3−6×).11,13,14 The consistently slower rates of MMHg elimination found in these field and laboratory studies indicate that additional long-term investigations would improve predictions of MMHg elimination by fish. Moreover, studies under natural conditions are especially needed. Field studies of chronically exposed fish have involved transporting highly contaminated fish to lakes that have lower MMHg levels in the food web.15,16 The continued exposure of fish to ambient (or background) MMHg present in all lakes could have biased the resulting estimates of elimination. A lack of extended studies with realistic levels of MMHg exposure, along with the issue of continued mercury uptake in past field studies, prevents direct application of previous elimination estimates to predatory fish in natural settings.11 Mercury elimination rates derived under natural conditions are critical when attempting to estimate recovery timelines of contaminated fisheries in cases such as following flooding or after reductions in deposition of Hg.17 We monitored the elimination of mercury by free-ranging northern pike (Esox Received: Revised: Accepted: Published: 4147
October 29, 2012 April 5, 2013 April 9, 2013 April 9, 2013 dx.doi.org/10.1021/es304332v | Environ. Sci. Technol. 2013, 47, 4147−4154
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lucius), a large, widely distributed, predatory fish species. Our study design involved the transfer of northern pike that had naturally accumulated MMHg enriched in a stable isotope (referred to as spike Hg) in a whole-ecosystem study18 to another boreal lake to which no spike Hg had been added. Pike were recaptured periodically over ∼7 y and concentrations of spike total mercury (THg; representative of MMHg)19,20 were measured to monitor the kinetics of MMHg in fish muscle tissue after exposure to spike MMHg had stopped. Our primary objective was to determine rates of MMHg elimination from muscle tissue. We focus on muscle because it is the main storage site of MMHg in fish, it is the tissue that humans consume, and it is the target of Hg contamination monitoring programs.3,21 A second objective was to compare our findings to published equations of MMHg elimination.
open-water period from 2001 to 2007.18,22 Fish in Lake 658 naturally accumulated spike Hg,5,18 which can be analytically distinguished from ambient Hg.23 After 3−4 y of exposure to spike Hg, resident northern pike were transferred to nearby Lake 240 (44 ha), where ambient THg concentrations in young-of-year and age-1 yellow perch were 1.5-fold and 2.5-fold lower respectively than in Lake 658. Therefore, pike transferred from Lake 658 to Lake 240 were adjusting to a complete cessation of exposure to spike MMHg and roughly half the dietary exposure concentration for ambient MMHg. Collection and Analysis of Fish. We captured a total of 17 pike from Lake 658 by angling and trap nets and transferred these fish to Lake 240 in 2004 (June 22 to August 3) and 2005 (June 3−15). On the basis of body size, lack of previous capture, and low levels of ambient and spike Hg, we suspect that four of these northern pike had entered Lake 658 from Winnange Lake during a high water period in the spring of 2004 (part a of Figure 1). We estimate that pike originally from Winnange Lake had been present in Lake 658 for 2−14 mo; long enough to accumulate detectable levels of spike Hg prior to transfer. Pike were transported in coolers (95 L) filled with lake water and held in Lake 240 overnight in a wire mesh pen (3 m × 1.3 m × 1.2 m). Pike were anaesthetized with 0.06 g·L−1 tricaine methanesulfonate (MS 222), measured for fork length (mm), weighed (1 g), and a small biopsy of dorsal muscle (mean ±1SE = 0.054 ± 0.0031 g) was collected using a dermal punch.24 This nonlethal method allowed for repeated sampling of individual fish over time. Fish were marked with PIT tags (Passive Integrated Transponder; Biomark, Inc.) to identify individuals upon recapture. Northern pike not previously surgically implanted with acoustic temperaturesensing transmitters (22 g in air, 62 mm × 16 mm diam, Model CTT-83-3, 3 y battery life, Sonotronics) underwent this procedure following standard methods.25 Pike recovered from anesthesia in a tub of fresh lake water before being released into Lake 240. Pike were periodically monitored to quantify swimming activity,26 a variable known to influence metabolism and Hg kinetics in fish.14,27,28 Activity tracks lasted for approximately 1 h, during which time tagged pike were located at ∼5 min intervals. Overall, 79 activity tracks were completed (n = 5−13 per fish), which occurred sufficiently long after capture events (5−25 d) to not influence behavior. In total, 11 of the 17 northern pike were recaptured (Table 1). The day in which a pike was released into Lake 240 was considered day 0 for that individual. For statistical comparisons,
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EXPERIMENTAL SECTION Study Location. We conducted this study at Lakes 658 and 240 at the Experimental Lakes Area (ELA) in northwestern ON, Canada (49° 39′ 15″ N, 93° 43′ 35″ W; Figure 1). Both
Table 1. Mean (± 1SE) Concentrations of Spike and Ambient THg and Body Masses (Wet Weight, w.w.) of Northern Pike (Esox lucius) Recaptured at Intervals after Transfer from Lake 658 to Lake 240
Figure 1. (a) Location of Lake 658, site of a mercury addition study, and Lake 240 at the Experimental Lakes Area (ELA) in northwestern Ontario, Canada. (b) Acoustic telemetry was used to monitor habitat use of northern pike (Esox lucius) in Lake 658 and after transfer to Lake 240. Locations of the same individual pike in both lakes are shown (Lake 658, n = 220 positions; Lake 240, n = 145 positions).
lakes have a similar fish community with northern pike as the top predator and yellow perch (Perca f lavescens) as the dominant prey species. Lake 658 (8 ha) is the site of METAALICUS (Mercury Experiment To Assess Atmospheric Loading In Canada and the United States), a whole-ecosystem study where mercury enriched with ∼90.8% 202Hg isotope (spike Hg) was added to the lake surface biweekly during the 4148
sampling period (d since transfer)
n
spike THg (μg·g−1 w.w.)
ambient THg (μg·g−1 w.w.)
fish weight (g w.w.)
0 80 320 460 660 1015 1420 1720 2480
11 5 6 2 6 5 1 1 1
0.080 0.060 0.076 0.054 0.026 0.029 0.047 0.037 0.044
0.773 0.572 0.840 0.638 0.603 1.074 1.259 1.123 1.812
920 (62) 1159 (117) 1315 (63) 1572 (38) 1641 (125) 1637 (95) 1916 1739 2537
(0.013) (0.016) (0.019) (0.046) (0.006) (0.008)
(0.057) (0.063) (0.109) (0.191) (0.059) (0.127)
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because THg concentration has been shown to be higher in muscle tissue.11,32 At t0, northern pike ranged in body size (752−1233 g) and in muscle spike and ambient THg concentrations (Table 1). Further, only a subset of these pike was recaptured during each sampling period. To account for these differences, we normalized our data to examine the mean percentage of original (t0) spike THg burden present in individual northern pike on each subsequent sampling day (ti):17
recapture periods from time of transfer (range of days shown in brackets) were categorized as follows: 80 d (75−88 d), 320 d (296−345 d), 460 d (446−473 d), 660 d (630−684 d), 1015 d (989−1027 d), 1420 d, 1720 d, and 2480 d. Fish were located using a hydrophone (Model DH4, Sonotronics) and receiver (USR-5W)25 and captured with seine nets, trap nets, and shortterm (5−30 min) gill net sets (multipanel nets with mesh from 1.5−6.5 cm). We anaesthetized northern pike as described above before collecting a muscle biopsy and measuring fish weight and fork length. Muscle samples were inserted into 0.6 mL plastic vials, immediately put on ice, and frozen within 30 min. Muscle tissue samples were weighed to the nearest 0.0001 g (Mettler AE 163) before and after freeze-drying to obtain wet and dry sample masses. We measured ambient and spike THg, which includes both inorganic and MMHg, after previous verification that >90% of Hg was in the form of MMHg for both spike and ambient Hg in yellow perch from Lake 658.11 We therefore expect that almost all Hg will be in the form of MMHg for northern pike because the percentage of MMHg in muscle tissue increases with trophic position.1,20 Hg species in the freeze-dried samples were quantified by inductively coupled plasma mass spectrometry (ICP-MS) at Trent University, Peterborough, ON.23 Samples were digested with HNO3/H2SO4 (7:3 v/v) and heated at 80 °C until brown NOx gases no longer formed. THg of sample digests was reduced by SnCl2 and determined by ICP-MS (ThermoFinnigan Element2) using a continuous flow cold vapor generation technique. To correct for procedural recoveries, all samples were spiked with 201HgCl2 prior to sample analysis. Method blanks (mean ±1SD: 8.6 ± 1.4 ng·g−1; n = 3) and certified reference materials were measured for each batch of samples. Reference materials (NRC DORM-2: 4.730 ± 0.100 μg·g−1; n = 3) were not statistically different from certified values (4.640 ± 0.260 μg·g−1). To quantify spike THg concentrations in fish, we measured individual THg isotope abundances in each sample and compared them to natural ambient THg isotope abundances.23,29 The precision of this isotope ratio measurement was typically better than 1% (1 RSD). Natural variation in Hg isotope ratios observed in a variety of environment samples30,31 is well below the measurement precision of the instrumentation used here such that natural variations would not affect the findings of this study. Statistical and Model Analyses. Analyses were completed with Statistica (6.1, Statsoft, Inc.) and SigmaPlot (11.0, Systat Software, Inc.). Assumptions that residuals were normally distributed with homogeneous variance were tested with Shapiro-Wilk’s and Bartlett’s tests. We used an α level of 0.05 unless otherwise noted. We compared muscle concentrations of spike and ambient THg for individual fish at each recapture period to a predicted concentration assuming fish had not gained or lost any THg after transfer to Lake 240 using paired ttests (Bonferroni adjusted α = 0.0125). Predicted concentrations ([THg]pred) were determined by dividing the original THg burden (time zero, t0) by fish mass upon recapture for each sampling period (ti):
%t0 burden = (THg burden ti ÷ THg burden t0) × 100%
We used a least-squares regression (quadratic polynomial fit) of mean data to calculate the half-life (50% of original burden) of spike THg in northern pike omitting days when a single fish was captured (Table 1). We compared our estimate of half-life to those predicted by chronic and acute exposure models.10 These general elimination models are based on a variety of marine and freshwater fish species, Hg exposure regimes, field and laboratory settings, and fish Hg measurements (muscle or whole body).10 In these studies, water temperature and fish weight were the best predictors of MMHg elimination rate,10 likely because of their role in metabolism, a driving factor for Hg elimination in fish.10,27 The equations we used are as follows: chronic: ln(K ) = 0.066T − 0.20(ln(W )) − 5.83
acute: ln(K ) = 0.066T − 0.20(ln(W )) − 6.56
where K is the MMHg elimination rate coefficient (per day), T is water temperature (°C), and W is fish mass (g w.w.). Water temperature profiles were collected biweekly at 1 m depth intervals throughout the open water season and several times over the winter in Lake 240 (2004−2006) and Lake 239 (2004−2011), a long-term ELA reference lake upstream of Lake 240 (part a of Figure 1; 2004−2009: Flett Research Mark II thermistor; 2010−2011: RBR CTD Model XRX 620). We used water temperature data from Lake 239 in the elimination equations because of the longer record in this lake and because water temperatures were not different from Lake 240 for the shallow depths occupied by northern pike (0−5 m: t-tests, P’s > 0.05), as determined from temperature-sensing telemetry data (∼80% of >1000 detections were at 0−5 m). We estimated T by calculating the mean daily temperature for the upper 5 m of the water column for each temperature profile. Regression (polynomial if n > 2, linear if n = 2) of observed growth data for individual fish was used to estimate W for these same days. After calculating K for each time-step (∼biweekly on water temperature profile days), we calculated the percent of original spike THg burden that would be retained by individual fish on each sampling day. To permit the direct comparison of empirical percent t0 burden in northern pike with modeled values, mean model predictions for each sampling day were calculated with the exact subset of fish for which there were empirical data and using the in situ water temperature that fish were exposed to as described above. We used exponential decay regression of mean modeled percent t0 spike THg burden to determine half-life of spike THg for the chronic and acute models. The elimination models presented here were designed to predict MMHg loss from whole fish.10 Because the relationship between muscle and whole body mercury concentrations is linear,32 absolute predicted MMHg burden values may be different for muscle and whole fish; however, predictions of the
[THg]pred ti = THg burden t0 ÷ fish mass ti
We calculated body burdens of spike and ambient THg by multiplying THg concentration (μg·g−1 wet weight, w.w.) by the fresh weight of the whole fish.15 This calculation of burden is a relative measure of the mercury content in the whole fish 4149
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proportion of t0 MMHg still present at ti should be similar for muscle and whole fish as long as temperature and mass inputs do not differ.
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RESULTS AND DISCUSSION Fish Growth and Behavior. Overall, northern pike transferred to Lake 240 showed positive growth and exhibited habitat choices and behavior typical of this species. Northern pike dispersed from the point of release and established core areas of use in the littoral zone (part b of Figure 1). Detailed monitoring of pike showed they were often stationary for extended periods33 as is typical of this ambush predator.34 Estimated activity rates were highly variable (0.003−0.207 m·s−1) and not significantly different among individuals (Kruskal−Wallis ANOVA: H7 = 6.84, P = 0.45), therefore we did not further examine the potential influence of activity on mercury elimination rates. Northern pike grew substantially following transfer from Lake 658 to Lake 240 increasing from an initial mean starting body mass of 920 g (±1SE = 62 g, n = 11) by an average of 25% within the first growing season (day 80), and nearly doubling in size by the end of the third year (day 1015: increase of 95 ± 17%; part a of Figure 2). Biopsy wounds healed quickly and did not appear to affect fish survival (also ref 24). Fish captured on the spawning grounds were found in reproductive condition. THg Concentrations and Burdens. Initial THg concentrations in muscle of northern pike collected from Lake 658 were, on average, 0.080 μg·g−1 w.w. (±1SE = 0.013 μg·g−1 w.w.) for spike THg and 0.773 ± 0.057 μg·g−1 w.w. for ambient THg (Table 1). Therefore, experimentally added spike THg accounted for, on average, 9% (±1.4%) of the THg (spike + ambient) present in Lake 658 northern pike muscle tissue. Northern pike introduced to Lake 240 ranged in age from 4 to 8 y and would have all had the same length of exposure to spike THg for fish transferred in 2004. Variability in starting spike THg concentrations among fish was likely due to exposure differences related to diet,35 and duration in Lake 658 for pike that immigrated from Winnange Lake or were transferred in the second year of study (2005). After transfer to Lake 240, northern pike from Lake 658 were adjusting to partial and complete reductions in exposure to ambient and spike THg, respectively. Changes in concentrations of ambient and spike THg in northern pike after transfer were similar for the first 660 d that fish were in Lake 240 (parts b and c of Figure 2) exhibiting a general pattern of a decrease then increase followed by another decrease. The initial decrease was much less pronounced for ambient THg than for spike THg, and after 660 d, ambient THg concentrations steadily increased. Relative ambient THg concentrations in pike muscle were significantly greater on days 320 (t1,5 = 2.84, P = 0.04), 660 (t1,5 = 4.32, P = 0.01), and 1015 (t1,4 = 5.61, P = 0.01) than those predicted had there been no accumulation of ambient THg following transfer (part b of Figure 2) indicating continued accumulation of ambient THg in Lake 240. Changes in ambient THg concentration on days 460, 1420, 1720, and 2480 also appeared to be greater than predicted values; however, it was not possible to statistically compare these results because too few fish were captured on these days (Table 1). The continued uptake of ambient mercury by northern pike after transfer to a lake with lower prey concentrations highlights a concern for past estimates of MMHg elimination generated using this approach (e.g., refs 15 and 16).
Figure 2. Mean (±1SE) change in (a) body mass (i.e., growth), (b) muscle ambient total mercury (THg) concentration, and (c) muscle spike THg concentration of northern pike (Esox lucius) over ∼7 y following transfer from Lake 658 to Lake 240. Closed circles represent mean observed values for each sampling day. Open circles represent predicted concentrations on the same sampling days if fish had not accumulated or lost any mercury after transfer to Lake 240 (Methods for calculations). Asterisks (*) indicate a significant difference between observed and predicted values on that sampling day. Note that predicted values on sampling day 0 (t0) are equal to observed values and are therefore hidden.
Ecosystem level additions of spike THg ensured natural accumulation of MMHg (i.e., chronic exposure)10 through both diet and respiration pathways5 by northern pike over several years while in Lake 658 and the complete cessation of exposure upon transfer to Lake 240 allowed for a detailed estimation of mercury elimination not possible through analysis of ambient mercury. Concentrations of spike THg in northern pike muscle declined after day 660 (part c of Figure 2), but this was primarily due to growth dilution,36 rather than elimination from muscle tissue, because observed changes in spike THg concentration were similar to those predicted due solely to changes in body mass (THgpred; 80 d: t1,4 = −1.38, P = 0.24; 320 d: t1,5 = 1.41, P = 0.22; 660 d: t1,5 = −3.01, P = 0.03; 1015 d: t1,4 = 0.17, P = 0.87). Predicted spike THg concentrations were also similar to observed values for sampling periods when few fish recaptures did not permit statistical comparisons (days 460, 1420, 1720, and 2480; Table 1). Spike THg was detectable in northern pike muscle tissue 7 y after exposure to isotopic mercury demonstrating the persistence of this compound in top predatory fish. Divergence in spike and ambient THg 4150
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Figure 3. Individual northern pike (Esox lucius) body burdens of spike total mercury (THg, black circles, left y axis) and ambient THg (gray circles, right y axis) over ∼7 y following transfer from Lake 658 to Lake 240 in June 2004. Three pike (i, j, and k) were transferred from Lake 658 a year later. Some fish (b, j, and k) were suspected to have entered Lake 658 from neighboring Winnange Lake (W). Two fish were confirmed dead during the study (a on day 311, g on day 632). Fish within years are ordered in increasing body size, and fish weights on sampling day 0 (t0) are noted in the top left corner of each panel.
concentrations over time demonstrates the strength of being able to account for individual growth rate concomitant with
changes in MMHg concentrations and that such an approach is critical for examining MMHg elimination in natural settings. 4151
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Individual northern pike exhibited a variety of patterns in spike THg burden over the course of the study (Figure 3). For the three fish that were frequently recaptured (>4 times), we observed a general pattern of increase, decrease and leveling off in spike THg burdens over time (parts b, e, and h of Figure 3). A number of other fish were not recaptured frequently enough to elucidate this complete pattern although the available data are suggestive of a similar trend. For example, some fish showed initial increases in spike burden but were not recaptured at a later time to determine whether this declined and leveled off (parts f and g of Figure 3). Other fish were not recaptured during periods when their conspecifics showed peak spike burdens but did exhibit a pattern consistent with the plateau phase, whereby burdens were similar to, or slightly less than, those measured at t0 (parts c, d, and j of Figure 3). Two fish that were recaptured only once, both 80 d after transfer to Lake 240, did not show any consistent temporal patterns in spike THg burden (parts a and i of Figure 3). Known mortalities during the experiment provide a partial explanation for the limited data obtained for some fish (parts a and g of Figure 3). One pike showed a rapid decrease in spike burden that was not consistent with other fish (part k of Figure 3). A period of initial increase followed by a decline (or plateau) in ambient THg burden was observed for several fish (parts b, e, f, and h of Figure 3) over the first 660 d, after which burden increased. Similar patterns of ambient and spike THg burdens over the first 660 d may indicate the time required for northern pike to respond to lower levels of MMHg exposure in Lake 240. Importantly, our data show that top predatory fish do adjust to lower levels of mercury exposure, although the time required to attain a new steady state (∼2 y) appears to be lengthier than most previous studies of MMHg elimination.10 After normalizing to remove variation among individual t0 northern pike, we found that relative spike THg burden increased over the first ∼460 d (1.3 y) in Lake 240, to a level roughly 3-fold greater than at t0 (Figure 4). Relative spike THg burden of pike rapidly declined for the next 200 d, to a level two-thirds (65%) of their original burden. Over the next 3 y (1060 d), relative muscle spike THg burden remained comparable to original burden (85−110%). Similar to absolute burden data for individual fish (parts b, e, and h of Figure 3), the percentage of original spike THg burden retained by northern pike after ending exposure to this form of mercury is suggestive of a pattern of MMHg increase, loss, and plateau (Figure 4). Spike THg burden of the individual captured at 2480 d was higher (155%) than its original burden. In this instance, the fish was the largest northern pike in our data set and was a gravid female such that a large portion of its mass was in the form of eggs. MMHg concentrations in eggs are typically very low relative to concentrations in axial muscle tissue or carcass,37 which may have upwardly biased the burden estimate. Our experimental design did not allow us to measure THg in tissues other than muscle; however, the lack of continued exposure coupled with significant increases in spike THg muscle burden while in Lake 240 may indicate the slow process of internal redistribution. This trend is not commonly reported for muscle tissue but has been observed in organs such as liver, heart, intestine, and spleen after acute exposure to MMHg38,39 where: (i) initial exposure causes MMHg levels to increase in tissue, (ii) a period of loss follows because exposure has ended and MMHg is transferred out of the organ, and (iii) a plateau is reached consisting of tightly bound MMHg or sustained by continuous redistribution of low MMHg levels among tissues.12
Figure 4. Mean (±1SE) percentage of original spike total mercury (THg) burden retained by northern pike (Esox lucius) following transfer from Lake 658 to Lake 240 (closed black circles). The half-life of spike mercury was determined to be 1193 d based on the polynomial equation (y = 104.8823 + 0.4314x − 4E−4x2, R2 = 0.33, P = 0.55) excluding days with a single sample. For the same fish, mean (±1 SE) estimates of percent spike burden retained calculated using chronic (open gray circles) and acute (closed gray circles) mercury elimination models10 are shown for each sampling day. Exponential decay regressions of these modeled data yielded estimates of the halflife of spike THg of 438 d (chronic; y = 96.4362*exp(−0.0015*x), R2 = 0.98, P = 0.0001) and 963 d (acute; y = 98.1043*exp(−0.0007*x), R2 = 0.99, P < 0.0001).
In muscle, we suggest that the initial increase may result from transfer of MMHg out of organs and the bloodstream into muscle where it is stored12,18,38 bound tightly to cysteine in protein.40 A similar pattern of redistribution was observed in a long-term companion study of age-1 yellow perch where spike THg muscle burdens increased for 6 mo after transfer from Lake 658 to Lake 240 concurrent with declines in spike THg in the liver and carcass (not including muscle); the subsequent decline of spike THg in muscle occurred after 1 y.11 Similar muscle THg kinetics have been observed in other long-term (>250 d) studies.21,22,38 Our present findings for northern pike show that complete transfer of spike Hg into muscle lasted at least 2.5 times longer than for yellow perch,16 which may be related to the differences in body size or metabolism of these species.41 Estimation of THg Elimination and Models. A goal of this study was to determine rates of MMHg elimination (K) by northern pike. The half-life of spike THg in northern pike muscle was best approximated using least-squares regression (quadratic polynomial) of normalized mean percent t0 burden data. Whereas spike THg burdens of pike captured during the final sampling days (days 1420, 1720, and 2480) demonstrate the persistence of THg in fish muscle tissue, our analysis included only days for which multiple fish were captured to avoid bias that might be introduced by lack of replication. The half-life of spike THg in northern pike was estimated to be 1193 d (3.3 y, Figure 4), although a longer half-life (1628 d or 4.5 y) when all but the last data point were included in the analysis cautions that further long-term data are still needed. Half-life estimates of spike THg determined using exponential decay regressions of chronic and acute elimination model output data, matched for the same individuals captured on each sampling day were 438 and 963 d, respectively (Figure 4). The 4152
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limitations of our data set did not allow for a robust estimate of error associated with half-life predictions for northern pike. Nonetheless, the modeled half-life estimate of THg in pike muscle tissue was 2.7-fold faster than observed for chronic exposure and 1.2-fold faster for acute exposure. The magnitude of the difference in elimination rate between modeled and observed estimates is consistent with a previous field study on yellow perch (chronic, 3.1×; acute, 1.5×),11 as well as for longterm laboratory trials conducted on lake whitefish, Coregonis clupeaformis (chronic, 2.4×; acute, 1.2×),19 and lake trout, Salvelinus namaycush (chronic, 5.5×; acute, 2.6×).14 Moreover, overestimation is consistently twice as great for chronic exposure than for acute exposure scenarios, likely related to the application of longer-term studies in development of the acute equation.10,11 Whereas we readily acknowledge the uncertainty of our findings related to small sample sizes, a key finding from this study and others11,14 is that rates of MMHg elimination by fish are slower than what models predict. This result highlights the general nature of these elimination models, which were created using combined data from a diverse array of studies.10 It is probable that a model designed to estimate MMHg elimination in large-bodied, northtemperate fish species would reflect the observed results of the present study more closely. It is worthwhile noting that rates of mercury elimination by northern pike in the present study were based on burdens calculated using muscle concentrations, in contrast to other studies that used whole body burden measurements. At present, evidence of whether THg concentrations in fish muscle tissue differ from whole fish is equivocal. Some studies have suggested that muscle and whole fish THg concentrations are equal,14,28,42 yet others show substantially lower THg concentrations in whole fish relative to muscle.32,33 It was not possible to directly determine whole fish THg concentrations in the present study due to the nature of the multiple-biopsy design. Conversion of our data to whole body THg concentrations using a published relationship for northern pike32 yielded 30−40% decreases in THg concentrations and body burdens, as expected from the ratio of muscle/whole body THg concentration but no change in THg elimination patterns. The resulting whole body half-life estimate was 1096 d, which is
