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Raptor Research Center

Background:

Walker Lake, in west central Nevada, is a troubled desert terminal lake with an uncertain future. Decreased water flows from the Walker River, largely caused by diversions for upstream use over the past hundred years, have greatly reduced its volume and degraded its water quality.  This, in turn, is seriously threatening the lake's fishery. The loss of Walker Lake's fishery could have a devastating effect on bird species that rely upon the fish for food. 

Among these species is the common loon (Gavia immer). Each spring and fall, large numbers of common loons migrate to Walker Lake, resting and feeding there before continuing their migration. Because biologists have had no information regarding the breeding and wintering grounds of the loons that use Walker Lake, it has been impossible to estimate the effects that the loss of these food resources would have. Likewise, planning management strategies that might mitigate the effects of such a loss in resources was futile without first determining the importance of the fishery in the ecology of the loon.

The loons at Walker Lake may represent a significant percentage, if not the total sum, of a specific population of interior breeding loons. Therefore, the loss of Walker Lake's fishery could have an adverse effect on a significant portion of the western North American loon population. By capturing and radio marking sentinel individuals with satellite-received Platform Transmitter Terminals (PTTs), we have a mechanism to study the movements of loons and thus begin to address these issues. We found high levels of mercury in blood from loons we captured on Walker Lake during our first field season. Thus, another goal became identifying the source of this contamination. Managers can use the information obtained via radio tracking to address the greater implications of the possible loss of this at-risk migratory cohort.

Our project includes staff of the Raptor Research Center of Boise State University, the BioDiversity Research Institute (BRI), the US Geological Survey (USGS), the US Fish and Wildlife Service (USFWS), and the Nevada Division of Wildlife (NDOW).

Our co-investigators for research of mercury contamination  in the loons are Dave Evers, Jim Paruk and other staff of the BioDiversity Research Institute, and Mike Lico, USGS Water Resources Division, Carson City, Nevada, and Stan Wiemeyer, USFWS, Reno.

Links to:

Map of loon movements  1998

Map of loon locations at Walker Lake Spring 2000 

Map of loon migration Spring 2000

Map of loon locations in Saskatchewan  Summer 2000

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Research Effort 1998-99:

We launched a cooperative study to determine the loons’ nesting and wintering locales by using implanted satellite-received telemetry. During the 1998-1999  pilot study we captured six loons by dip net from NDOW boats, on nights with little moonlight. We implanted three PTTs subcutaneously, and although shortcomings in transmitter function soon became evident we were able to track one individual to its breeding ground in west central Saskatchewan. We obtained blood samples from six loons and analyzed them for environmental contaminants. The results revealed blood mercury levels placing four of the six in a high risk category (3.0 ppm and above) as established by the Northeast Loon Study Working Group. 

We initially suspected this would be indicative only of contaminants acquired on wintering grounds and brought along to Walker Lake. However, data from previous investigation revealed elevated mercury levels in a 1996 composite sample of Lahontan tui chub (Gila bicolor obesus) from the lake. This fish is eaten by loons. In April 1999 we conducted another round of loon blood sampling, capturing 20 more individuals. Seven of those subjects met the high-risk criteria of 3.0 ppm mercury. NDOW concurrently sampled six chub and six Lahontan cutthroat trout (Salmo clarki henshawi) from the lake. Chub samples averaged 0.59 ppm mercury and trout 0.68 ppm, both below the current U.S. Food and Drug Administration immediate action level (1.0 ppm) for mercury in fish. Although this was good news for public health, BioDiversity Research Institute studies have shown reproductive impairment in common loons feeding on fish carrying 0.30 ppm mercury and no reproduction in those feeding on fish carrying 0.40 ppm.  

Thus, the loons, and perhaps other fish eating birds, that use Walker Lake face a double threat:  loss of the food base and environmental contamination from the food that is available now.

Research Effort 2000:

Accordingly, in 1999 we  submitted a joint proposal with USFWS, USGS, and BRI to continue the loon study and to investigate availability and sources of mercury in the Walker River Basin. That proposal received partial funding. Teaming again with NDOW, our protocol for 2000 included the capture, blood sampling, and radio marking of 10 loons (three with improved PTTs) in early April. 

On the nights of 6-7 April 2000 we captured and blood sampled 23 loons at Walker Lake. Three received implanted PTTs and VHF transmitters attached to color leg bands; seven others received the VHF leg bands alone. We returned to attempt recaptures three weeks later when the moon phase was once again conducive to the effort. By then few loons remained on the lake and two of the three bearing PTTs were already far to the north.  High winds from the north limited our time on the lake to less than four hours during the two nights. We did not locate any of the original subjects and captured only two more loons for sampling. 

Our analyses of the 25 blood samples collected in spring 2000 placed 10 of those individuals in the high risk category for mercury, with one individual's concentration of 9.46 ppm being the highest of the 51 loons sampled to date.

PTTs implanted during the current phase of the study are programmed to transmit eight hours and be inactive for 72 hours repeatedly until the batteries are exhausted. We expect to receive positions into March of 2001, which would allow us to define breeding and wintering locales for these three subjects. The loon bearing PTT #11979 left Walker Lake between 11 and 14 April, spent at least 10 days in Montana, and arrived at its expected breeding locale by 2 May. The loon bearing PTT #11989 left Walker Lake between 14 and 17 April, arriving at its expected breeding locale by 24 April. The loon bearing PTT #19214 left Walker Lake between 28 April and 1 May, arriving at its expected breeding locale by 6 May (click for map). Like the one subject tracked successfully in 1998, all three went to west central Saskatchewan, indicating that loons breeding in that area may comprise a significant proportion of the Walker Lake migrants.

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Research Effort 2001:

Our goal for spring 2001 was to collect stomach contents from captured loons for mercury analysis. This would assure that we were acquiring mercury levels in the prey loons actually consume at Walker Lake, rather than analyzing fish of an appropriate size from the lake and applying those results. After lengthy consultation, we arrived at a protocol that called for anesthesia with isoflurane gas and recovery of stomach contents through a tube via gastric lavage. We secured the services of a veterinarian and a vet student from UC-Davis, and the town of Walker Lake donated the use of its community center/firehouse for this process.

On the nights of April 18-19 we conducted loon capture efforts at Walker Lake. Our Nevada Division of Wildlife colleagues were unable to participate, and the only boat available that could operate on the lake those nights was the one provided by Stillwater NWR personnel. Both evenings began with high winds that made it impossible to proceed at dark. Each night, however, we were able to launch the Stillwater boat just before midnight. Four loons were secured the first night and six the second.

Unfortunately, the lengthy delay each evening hampered our efforts to collect stomach contents. Since the loons had fed before dark and metabolize rapidly, there was nothing for our veterinary staff to collect by the time we could put birds in their hands. We were able to add ten blood and feather samples to our database.

Our goal for fall 2001 was to compare blood mercury levels among migrant loons on Topaz Lake, far upriver in the basin but only 40 air miles to the west of Walker Lake. Topaz dates to 1922 and is connected to the river via a canal. Because of the lakes’ proximity we thought it useful to compare blood results among migrants. If Topaz migrants display similar blood Hg levels to those at Walker, the major source of acquisition is probably outside the Walker Basin.

We conducted fall fieldwork Oct. 22 (Topaz Lake) and Oct. 23 (Walker Lake) nights. Loons were present at Topaz but capture attempts were hampered by high ambient light levels (partial moon before it set, casino lights) and calm conditions (amplified boat noise). Most loons dove when approached, but we did manage to capture and sample two individuals. The following evening we reassembled at Walker Lake. Windy conditions prevented the use of one NDOW boat, but we did well using the two other craft. Sixteen loons were captured and sampled.

Accordingly, we added another 18 blood samples to our accumulated database. We have now captured exactly 100 common loons (61 spring, 39 fall), including four recaptures in subsequent migrations of loons we banded during the course of this study.

Two individuals we captured and banded were also recovered dead in 2001 at other locations. A loon we banded on October 26, 2000 was recovered, presumably in its breeding area at Iroquois Lake, Saskatchewan on June 14, 2001. Including the four loons we successfully tracked by satellite, all five Walker Lake migrants for which we now have breeding locales fall within a circle having a radius of about 90 miles in Saskatchewan.

A loon we banded on April 7, 2000 was also recovered at Mica Bay, Idaho on November 4, 2001. Attempts to acquire remains of both recovered birds for analysis were unsuccessful.

Our USGS-WRD partners had extensively sampled sediments and water at stream sites, reservoirs, and the lake itself. Results showed generally low Hg concentrations in the upper basin, generally higher concentrations in the middle basin, and yet higher concentrations in the lower basin and lake. Their interpretation, bolstered by sediment core results, is that natural background Hg concentrations in sediments entering the lake were ~30-40 ng/g (nanograms per gram-equivalent to parts per billion) prior to mining in the basin. During the late 1800s and early 1900s mining activities, especially in the Aurora and Bodie Districts, considerably increased the input of Hg to streams in the basin. Much of this Hg was transported into Walker Lake, resulting in concentrations >1,000 ng/g in sediments. Current Hg concentrations in sediments entering Walker Lake are between 100 and 400 ng/g.

Sampling by our USFWS partners found that aquatic invertebrates showed elevated Hg levels at some stream sites downstream of historic mining operations; no elevated levels were detected at similar sites where upstream mining was not documented. Stan Wiemeyer also sampled additional tui chub from Walker Lake and compiled his results and those available from previous efforts. Chub >20 cm in length showed a quantum increase in Hg level over those <20 cm in length, to levels that are demonstrated to affect loons adversely. We interpret this as an artifact of chub >20 cm beginning to feed on smaller individuals and thereby concentrating Hg more rapidly.

We have analyzed blood samples of all 98 common loons captured at Walker Lake (61 spring, 37 fall). Mean total blood Hg was 3.0 ppm (range 0.15 - 9.46 ppm). Based on these results, blood Hg levels of Walker Lake migrant loons exceed those documented anywhere else in the United States. Our mean level actually matches the established 3.0 ppm high risk threshold, and 45% of our individuals meet that criteria. Based on Wiemeyer’s results, Hg levels in tui chub at Walker Lake generally increase with fish size. If loons are acquiring significant amounts of Hg from these chub, it would follow that larger loons (capable of consuming larger fish) would display higher Hg blood levels. In fact, that is the exact pattern our results show. The blood Hg of smaller female loons (2,665-4,000g, n=46) is 2.11 ppm, and only 22% of individuals meet the 3.0 ppm high risk level. Individuals of unknown sex (4,075-4,400g, n=28) had a mean blood Hg level of 3.72 ppm, and 61% were “high risk”. Mean blood Hg of males (4,450-4,850g, n=24) was 3.84 ppm, and 71% met the high risk threshold.

Repeated migratory visits to Walker Lake could also result in cumulative increases in blood Hg for individual loons if, as we suspect, migrants are acquiring significant amounts during their stay. We have captured a single individual during the springs of 2000 and 2001, and again during the fall of 2001. Over the course of roughly 18 months, this individual’s  went from 1.74 to 4.72 to 8.15 ppm. Although the two blood samples from Topaz Lake migrants comprise a wholly inadequate sample size, it must be noted that those two individuals had very low (mean=0.66 ppm) blood Hg levels.

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Research Effort 2004

Support from the Nevada BioDiversity Initiative of the USFWS, North Star Science and Technology, and NDOW enabled the work in October.  Numbers of migrants present were reduced from previous levels, but we captured six and implanted PTTs.  The PTTs soon began to go silent and sensor data confirmed two individuals had either died or shed the transmitters.  By December 26, we were receiving locations from only one PTT and that loon remained at Walker Lake throughout the winter.  This individual departed to the north on April 13 and the PTT battery finally failed in early May as the loon staged in Montana.

No definitive cause of PTT failures could be determined.  Conjecture is that a slightly thicker PTT profile affected mobility in the days immediately following the implant.  At the same time, foraging prospects at Walker Lake had been reduced by the inability (due to declining water quality) of Lahontan tui chub to reproduce in significant numbers since 2002.  Blood mercury levels of our six 2004 subjects were reduced (mean 2.26 ppm) from previous levels.  We suspect that loons are no longer spending weeks at the lake during migration.  Most probably arrive, forage for a few days, and then move on when their efforts are unrewarding.  Thus, it is also likely the Hg uptake among those we captured was reduced since they were probably recent arrivals.  Recent sharp declines in numbers of loons counted at the lake in spring and fall may be explained by this reasoning.

Status:

We continue to circulate proposals to complete determination of the yearly movement pattern and distribution of the migratory loons that use Walker Lake and to determine the primary source of Hg contamination in this cohort.  We will relate these findings to the threats posed by contamination of the loons and the potential loss of the Walker Lake fisheries. 

Specifically, we will:

• Capture and analyze blood samples from up to 30 more Walker Lake migrant common loons during an additional fall and spring effort and radio mark 12 (8 fall, 4 spring) with satellite-received telemetry as employed in 1998 and 2000.  Given the probability of reduced foraging prospects at the lake, we propose to transplant subjects to Topaz or Pyramid Lake to enhance prospects of success.

• Track these individuals to wintering and nesting areas using the Argos satellite system.  Further sample subjects and/or associates, fish, water, and sediments at these locales.

• Attempt to capture and analyze blood samples from an additional 10 Topaz Lake migrant common loons.

• Determine impacts of MeHg body burden for the common loon.

• Provide natural resources managers an appraisal of the Hg toxicity problem within the Walker River Basin.

In addition, based on our supposition of reduced foraging prospects at Walker Lake, we propose to assess the extent of that phenomenon through extensive observation and tracking of sentinel individuals via VHF telemetry.

Results can be used to develop management plans assuring the Walker Basin's suitability for native and transient wildlife as well as humans.  Partial funding of previous proposals has provided a firm foundation of knowledge and allowed development of the field techniques required to accomplish these objectives.

Links to:

Map of loon locations at Walker Lake Spring 2000 

Map of Loon migration Spring 2000

Map of loon locations in Saskatchewan  Summer 2000

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Acknowledgments

All involved wish to thank U.S. Sen. Harry Reid (D, NV) and his staff for their commitment and assistance. The wholehearted support of our NDOW colleagues, led by Larry Neel and Mike Sevon, has been integral to the success of the project. Bill Henry of Stillwater National Wildlife Refuge (US Fish and Wildlife Service), Mary Jo Elpers (USFWS, Reno), Graham Chisholm of The Nature Conservancy and the Great Basin Bird Observatory, Bob Goodman of Desert Images, and various members of the Walker Lake Working Group have also given freely of their time and talents. We also appreciate the support of Earthspan.

Photos copyrighted by Bob Goodman of Desert Images and may not be reproduced without written permission.
Desert Images, 7900 N. Virginia #193, Reno, NV  89506.  Phone:775-972-7848.

Yates, Mike. 1999b. Walker Lake Satellite Telemetry Common Loon Study. Great Basin Birds 2:68-69.

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  This page was last revised on 17 December 2007.