A Modified Floating-Fish Snare for Capture of Inland Bald Eagles

Ronald E. Jackman, W. GralngerHunt, Daniel E. Driscoll BIoSystems Analysis, Inc. 303 Potrero St., Suite 29-203 Santa Cruz, CA 95060 J. Mark Jenkins Pacific Gas & Electric Company Technical and Ecological Services 3400 Crow Canyon Rd. San Ramon, CA 94583

In our studies of the foraging ecology of Bald We occasionallycaptured eagles with certaincon- Eagles (Haliaeetus leucocephalus)in California figurations,including those described by previous and Arizona (Hunt et ah 1992a, 1992b, Jenkins authors;however, we abandoneda particularde- 1992), we usedvarious floating-fish snares to cap- sign after multiplefailures or refusals. A single- ture 30 adult Bald Eagles for transmitterattach- noosedesign, similar to Robards'(1966) method, mentand colorbanding. Problemswith approach- was rarelysuccessful for us (n=35 attempts,9% ing wary eagles by boat withoutflushing or alert- capturerate). We had moderatesuccess (n=10 ing them, combined with their aversion to attempts,20% capturerate) with anotherdesign: monofilament nooses on inland reservoirs, led us a single-nooseattached to the side of a small (10 to modify previouslyreported techniques. Cain cm) sunfish(Centrarchidae). The configuration and Hodges(1989) describeda 4-noose,floating- described in this paper attracted and captured ' fish snare modifiedfrom the originalsingle noose eagles most consistently. floatingfish used by Robards(1966) inAlaska and a bi-lateral,2-noose system used by Frenzel and We prepared floatingfish with carved styrofoam Anthony(1982) in Oregonto captureBald Eagles plugs using methods described by Frenzel and on open water. We experimentedwith these de- Anthony(1982) and Cain and Hodges (1989); signs, but were unable to achieve reportedsuc- however,we insertedthe floatationplug mostly into cess rates (>50%; see Cain and Hodges(1989) the anteriorportion, rather than the entire body for reviewof reportedsuccess rates). cavity,allowing the tail of the fish to droopmore deeplybelow the surfaceof the water (Figure1). Duringefforts to constructa reliable floating-fish For each noosewe tied a slip knot, shown in Fig- snare,we triedmany other designs (n>30) by vary- ure l a (R. Frenzel, pers. comm.), at one end of ing the following: size of bait fish (10 cm to over approximately1.5 m of 18 kg (40 lb) test lightgreen 500 cm total length),bait fish shape (deep-bod- monofilament,leaving a 2 cm end tab. The end ied, cylindrical,flat-bottomed), number of nooses tab providedenough line for this knot to tighten (1-6), nooseposition on the fish (both in and out completely,after the eaglewas snared. When tied of the waterand at variousangles to the longitudi- correctly,slip knotsheld the nooseshape, but slid nal axisof the fishand to the sudaceof the water), along the monofilamentwith minimumfriction, and noosesize (approx.8 cm to 15 cm), monofilament completelyunraveled if pulledthrough. Thus, breaking strength (14-27 kg 'test), and unsnaredeagles could more easilyseparate the monofilamentcolor (clear, green, brown). Noose fishfrom the nooses than with previously described positionand numberseemed particularly impor- knots(Cain and Hodges 1989). This allowed tant since the eagles were apparentlytrying to uncaptured,monofilament-wary eagles to be re- avoidthe. monofilament (see below),regardless wardedwith the fishmaking them more inclined to of color. seize another bait fish. Page98 NorthAmerican Bird Bander Vol.18 No.3 We placedtwo nooses in an alternate/lateralposi- swimmingto shoreon the narrow reservoirs. The tion (Figure lb) that allowedmaximum coverage anchorsystem consisted of a 4.5 kgbarbell weight ofthe exposedfish .surface with the fewest nooses. connectedto a 1.5 m lengthof heavy(1.1 cmdia.) Noose diameters were similar to those used by nylon-jacketedshock cord with an overhandknot previousauthors (10 - 12 cm). We useda large tiedat the freeend (Figurelc). The largeshock upholsteryneedle to feed the free end of each cordhelped alleviate problems with eagles break- noose into the ventralsurface of fish and through ingnooses in shallow water (.5 to 3 m deep).Also the styrofoamplug to exitdorsally. A slightbend- connectedto the anchorweight was about 25 m ing of the monofilamentnear the slip knot and of brown nylon cord tied to a section of placementof small staples (one per noose, see monofilament(approximately 5 m of 11 kg (25 lb) Cain and Hodgas 1989) kept the noosesflat on test)attached to a largecork (Figure lc). Thecork the fish's ventral surface. Slight bendingdid not and cordallowed us to retrieveanchors if the eagle noticeablyweaken monofilament during our infor- escapedwith the baitfish, and facilitatedanchor mal tests of breakingstrength; the monofilament placementin deep water. We stacked anchor and invariablybroke at a knot, characteristicallythe lines,cork end first,into large (approx.14 cm x 18 weakestpoint. Riskingentanglement, we left all cm x 30 cm) army surplusammo boxesto trans- fins intact for a more natural presentationof the portsets and facilitatebait placement. bait; however,we sewed the left pectoralfin to the bodywith light monofilament to helpprevent it from Duringbait placement,we tied 27 kg (60 lb) test entanglingthe anterior noose (Figure lb). Bait monofilamentfrom a largespool to the free end of fish were stored in a small cooler and positioned the shockcord below the knot usingan improved to allow nooses to remain flat against the ventral clinchknot with 3.5 twists(a commonfishing knot; surface. Kreh and Sosin 1972). Holdingthe nyloncord, we lowered the anchor weight out of the boat to We used large (approx.40 cm total length),suck- the lake bottom. We achieved a stable anchor ers (Catostomusspp.) or catfish (Ictalurusspp.) positionby soundingwith the anchorline to finda as our preferredbait. We believe that coveringa relativelyflat bottomprofile and allowedthe an- large amountof the fishes'flat ventralsurface lat- chor to settle into mud or sand without contacting erally with the nooses (distance "x" in Figure lb) rocks(the sound of the ironanchor contacting rocks and presentingonly the noosedportion of the fish is audibleeven throughdeep water). Becauseof at the surface (taildrooped) contributed to our suc- the possibilityof an eagle dislodgingthe anchor cess with this design. In our experience,the use and sinkinginto deeper water when captured,we of larger fish forced eagles to grasp baits more avoided areas with steep or uneven bottom pro- deeply; small fish appeared to be taken just with files. We nexttied adouble surgeon'sloop (double talons, not fullygrasped by the eagles. These fac- the line, make an overhand knot, pass the loop tors seemed to allow the noose to fully snare the throughthe knotagain and tighten;Kreh and Sosin distalphalanx. In addition,we observedthat large 1972) to the end of the 27 kg test monofilament fish were usually ignored by common non-target and tied the two noose ends from the bait fish to bird species (e.g., Osprey Pandionhaliaetus, gulls this loop using improvedclinch knots with five Larusspp., Common MergansersMergus mergan- twists. To avoid abrasion, all knots were moist- ser) knownto foul floating-fishsnares (Frenzel and ened before tightening. Knot type and quality are Anthony1982, Cain and Hodgas 1989,A. Harmata importantwhen working with monofilament;see pers. comm.). Kreh and Sosin (1972) or consulta fishingguide or expert for more information. To increase our chances of an early strike and re- duce the potentialfor interferenceby non-target In deep water, we left about 1 m of slack in the species,we typicallyset outtwo or threefish snares shocksystem to allow for wave action and water just beforedawn at habitualforaging sites, observ- level fluctuations on reservoirs. In shallow water able from one location in a boat. We used fixed the shock cord itself was also lightlyweighted or anchors to avoid drifting sets or trapped eagles coveredby thebottom substrate (i.e., sand or mud) to reducevisibility. We placedthe retrievalcork as July-Sept.1993 North American Bird Bander Page 99 far from the bait as possible. Once captured,we ACKNOWLEDGMENTS used a largesalmon landing net to carefullyscoop the eagle into the boat. We wish to thank L. S. Youngfor introducingus to floating-fishsnares. B. Quigleyprovided us with The alternate/lateral floating-fishsnare was suc- tips on handlingmonofilament. Field personnel cessfulin capturingBald Eagles in 17 of 43 events includedC. Himmelwright,J. Linthicum,G. Sand- whereeagles tried to graspthis bait (40% capture ers, L. Spiegel, P. Hunt, R. Lehman, L. Small- success). We eliminated nine additional events Jackman, G. Beatty,and M. Cross. Techniques from the success rate calculationswhere eagles were developedduring studies of Bald Eagle ecol- purposelystruck the fish, apparentlyattempting to ogy funded by the Pacific Gas and ElectricCo. dislodge the monofilamentnooses. When exhib- and the U.S. Bureauof Reclamationand managed itingthis behavior,eagles hit the bait fish with their by C. Thelander. We thank S. Cain, A. Harmata, talonswithout grasping it, often rippingportions of and P. Bloomfor commentson the manuscript. the flesh and sometimesflipping the fishout of the water. Eagles seizing the fish but not caught in the nooses were rewarded with the fish (n=9); LITERATURE CITED snappednooses when grasping the bait (n=4, usu- ally in shallow water); could not separate the fish Cain, S. L. and J. I. Hodges. 1989. A floating-fish from the nooses(n=7, e.g., due to snared fin); were snare for capturingBald Eagles. J. Raptor caught briefly but escaped (n--2, noose probably Res. 23(1):10-13. closedon talononly); or the baitdisappeared (n=4, probablyunseen forages by eaglesor intederence Frenzel,R.W. and R.G. Anthony. 1982. Method by other species). for live-capturingBald Eagles and Osprey over open water. U.S.D.I. Fish and Wildl. The modificationswe describedto the floating-fish Serv. Res. Infor. Bull. 82-13. 2 pp. snare were designed as a compromisebetween circumventingthe aversion that certain resident Hunt, W.G., J.M. Jenkins, R.E. Jackman, C.G. adult Bald Eagles have toward monofilamentand Thelander and A.T. Gerstell. 1992a. For- achieving a reasonable capture rate. Shyness aging ecologyof Bald Eagles on a regu- towardmonofilament possibly derives from previ- lated river. J. Raptor Res. 26(4):243-256. ous encounterswith fishingline attachedto fish or entangledon shorelineperches. Eagles also be- Hunt, W.G., D.E. Driscoll, E.W. Bianchi, and R.E. came wary of noosedfish from previouscaptures Jackman. 1992b. Ecologyof Bald Eagles and near captures,or when they had difficultypull- in Arizona. Report to U.S. Bureau of Rec- ing fish off the snare line. In additionto attempts lamation, Contract 6-CS-30-04470. Bio- to dislodge monofilament, some monofilament- SystemsAnalysis, Inc., Santa Cruz, CA. wary eagles apparentlytried to avoid the nooses when taking the bait fish, as evidencedby their Jenkins,J.M. 1992. Ecologyand behaviorof a flightpatterns around the bait set priorto grasping residentpopulation of Bald Eagles. Ph.D. it. Otherbirds avoided floating-fish snares entirely. dissertation,University of California,Davis. To limit visibilityof monofilament,yet maximize coverageof exposedsudaces, we used only two Kreh, L. and M. Sosin. 1972. PracticalFishing nooses,configured alternate/laterally. By droop- Knots. Crown Publishers, Inc. New York, ingthe posteriorsection of a largefish, we reduced New York. 160 p. the eagle's access to the only portionof the fish not covered by nooses. We set baits in shaded Robards,F.C. 1966. Capture,handling and band- areas when possible;trapping was most effective ing of Bald Eagles. U.S.D.I. Fish and Wildl. duringearly morning hours before sunlight illumi- Serv. Unpublishedreport. Juneau,Alaska. nated the monofilament. 25 p.

Page100 NorthAmerican Bird Bander Vo1.18No.3 Figure 1. a) slip knot b) alternate/lateralnoose configuration c) anchored floating-fishsnare

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