]. RaptorRes. 25(4):109-112 ¸ 1991 The Raptor ResearchFoundation, Inc.

THE DIET OF OSPREYS AND THEIR IMPACT ON THE LOCAL FISHERY

PETER K. MCLEAN • AND MITCHELL A. BYRD Departmentof Biology,College of William and Mary, Williamsburg,VA 23785

ABSTRACT.--Ospreys(Pan dion haliaetus), were observedat sevennests located in southwesternChesapeake Bay, for 642 hr between21 May and 25 July 1985. On average5.4 fish/day were deliveredto the nests. The size of fish deliveredranged from 4 to 43 cm, and the mean weight of fish deliveredwas 156.9 g. Atlantic (Brevortiatyrannus) accounted for nearly 75% of the diet, although White Perch (Motone americana),Atlantic Croaker (Micropogoniasundulatus), Oyster Toadfish (Opsanustau), and American Eel (Anguilla rostrata)also were commonprey. ChesapeakeBay Ospreys,estimated at 3000 breedingpairs, would be expectedto eat about 132 171 kg of fish during the 52-day nestlingperiod. This "harvest"represents 0.004% of the annual ChesapeakeBay commercialharvest and likely has a minimal impact on the local fishery.

La dietade fixguila Pescadora (Pan dion haliaetus) en la BahiaChesapeake y su impacto en la pescalocal EXTR^CTO.--fixguilasPescadoras (Pandion haliaetus) en sietenidos ubicados en la zonasudoeste de la Bahia Chesapeake,fueron observadas por 642 horasentre el 21 de mayo y el 25 de julio de 1985. Un promediode 5.4 pesces/diafueron 11evadosa cada nido. E1 tamafio del pescadoque rue 11evadoal nido oscil6entre 4 y 43 cm, y el pesomedio fue de 156.9 g. Los pecesde la especieBrevortia tyrannus constituyeroncerca del 75% de la dieta,aunque peces de las especiesMorone americana, Microp.ogonias undulatus,Opsanus tau, Anguilla rostrata tambign fueron presa comfin. Se estimaque 3000 paresde Aquilas Pescadorasde la Bahia de Chesapeake,en la •poca reproductiva,podrian comeraproximadamente 132 171 kg de pescadodurante los 52 dias del perlodoen que las crias estrinen el nido. Esta "cosecha"de pecesrepresenta 0.004% de la pescacomercial anual de la Bahia de Chesapeakey al parecer tiene un impactominimo en la industriade pescalocal. [Traducci6n de Eudoxio Paredes-Ruiz]

Few studieshave reportedon the feedingecology to the nest.We estimatedthe size of fish using reference of Ospreys(Pandion haliaetus) in the Chesapeake pointsin and around the nest includingthe residentOs- prey'starsus. We alsoaffixed a woodenrod, graduatedat Bay (Stinson1978, McLean 1986). BecauseOspreys 12 cm intervals,to the nestto improveour size estimates. might competewith commercialfishermen for the We later convertedsize estimatesto gramsusing length- bay'sever-diminishing fish populations,this paper weight relationshipsspecific for each fish (Table 1), and reportson the foodhabits of Ospreysof southwestern we basedour calculationof speciescomposition in the diet ChesapeakeBay and their bearingon the bay'sfish- on wet weight values. To furthersubstantiate diet composition, we visitedeach ery. nesttwice a week to collectprey remains.Later, usinga MATERIALS AND METHODS referencecollection at the Virginia Institute of Marine Science and the assistanceof the collection curator, the Between21 May and 25 July 1985, we observedseven remainswere identified. Diet compositionwas basedon Ospreynests located in Mathews and LancasterCounties, frequencyof occurrenceof the prey item. Virginia. Nests were approximately 25 to 125 m from shoremaking them easyto observeand accessibleby boat. At three sites,nests were closeenough to allow two to be RESULTS AND DISCUSSION observedsimultaneously. Ospreys were observed4 d/wk. Each day includedtwo 7.5 hr observationperiods (0530- We observed378 fish being deliveredin 642 hr 1300 and 1300-2030 H) which were arranged system- of observation,giving an averageof 54 fish/nest(SD atically to allow 16 hr of observationper nest. We used = 12.5, N = 7) for the 10 wk. This deliveryrate is 20 by 60, 40 by 60 and 40 by 80 spottingscopes for equivalentto 5.4 fish/d given one observationday observationof the number, speciesand sizeof fishdelivered for each nest per week. Fish lengthsranged ap- proximately4-43 cm. During one nestvisit, we no- 1 Present address:St. Andrew's School, Middletown, DE ticed the occupantseating a very large Menhaden 19709. (Brevoortiatyrannus) measured later at 43 cm, but

109 110 PETER K. MCLEAN AND MITCHELL A. BYRD VOL. 25, NO. 4

Table 1. Length-weightrelationships of fish eatenby ChesapeakeBay Ospreysin 1985.

SPECIES EQUATIONa Menhaden (Brevoortiatyrannus) Ln W = -12.075 + 3.215 Ln fork L b Eel (Anguillarostrata) Log W = -6.56 + 3.34 Log L c Hogchoker( Trinectesmaculatus) LogW = -3.71095 + 2.65844Log L a Perch (Morone americana) Log W = -5.172 + 3.190 Log L e Flounder (Paralichthysdentatus) LogW = -5.8759 + 3.3238Log L f Catfish (Ictaluruscatus) Log Y-- 1.9791 + 0.1689 Log Xg Oyster Toadfish ( Opsanustau) LogW = -5.223 + 3.223Log L h Seatrout(Cynoscion nebulosus) Log W = -4.423 q- 2.861 Log L i Butterfish(Peprilus triacanthus) LogW = -5.1852 + 3.2646Log LJ W or Y: Weight (g), L or X = Length (mm). FromJ. Merriner(National Marine FisheriesService, unpubl. data). From Harrell and Loyacano(1980). From Dawson(1962) andD. Haven(Virginia Institute of Marine Science,unpubl. data). From St. Pierre and Davis (1972). From Lux and Porter (1966). From Jachowskiand Schwartz(1965). FromSwartz and van Engel (1968). From Mercer (1983). From Dupaul andMcEachran (1973). most large fish were American Eel (Anguillaros- would be rejectedby the feedingOsprey. Also, food trata), White Perch (Moroneamericana), White Cat- itemsfound in the nestmay havebeen nest material. fish (Ictaluruscatus), Atlantic Croaker (Micropogo- The large (Pomatomussaltatrix) cranium nias undulatus), or Spotted Seatrout (Cynoscion foundin oneof the nestswas probably collected from nebulosus). the shorenearby. In other partsof the bay, Ospreys Conversionsof fish lengths to weights revealed havebeen observed gathering Bluefish remains from that Atlantic Menhaden accountedfor nearly 75% the beach(P. $pitzer, pers.comm.). of the diet (Table 2). White Perchrepresented over The diet of Ospreysin southwesternChesapeake 7% of the diet, whereas Atlantic Croaker, Oyster Bay appearsto reflectlocal prey availability;these Toadfish (Opsanustau), and American Eel each resultsare similar to thoseof a recentstudy of Flor- comprisedabout 3% of the diet. During the 10 wk ida Ospreys(Edwards 1988). In ChesapeakeBay, of observation,we recorded15 speciesdelivered to Menhaden are plentiful and representover 80% of the nest(Table 2). The meanweight of fishdelivered the commercialcatch (Thompson 1984). Because during the nestlingperiod was 156.9 g (SD = 167.1, Menhadenschool near the water'ssurface, they make N -- 246). Diet compositionvaried amongbroods, attractive prey. On two occasions,we observeda however,nearly all broodsreceived at least 50% male Ospreyclutching two Menhaden, one in each Menhaden. set of talons.American Eels were huntedprimarily Analysisof prey remainsindicated that Menha- over shallow water. Though a significantdiet item den constitutedalmost 65%, whereasOyster Toad- in this study,they reputedlyare unimportantin the fish, Atlantic Needlefish, White Perch, Croaker, and diet of populationselsewhere (P. $pitzer, Sunfishtogether comprised 23%. Menhaden, in the pers.comm.). Needlefish (Strongylura marina), Oys- form of opercula,pectoral and caudal fins, and scales, ter Toadfish, Summer Flounder (Paralichthysden- predominatedin the remains taken at each nest. tatus)and Hogchokers(Trinectes maculatus) are typ- Mandibles, craniums, and bills of Needlefish and icallybottom dwellers but alsoare found occasionally jaws of Oyster Toadfishwere particularly well rep- in the shallows,especially at high tide. Under these resented at two nests. The few American Eel remains conditions,Ospreys can effectively capture these fish reflecteda biasin determiningdiet compositionfrom about 0.5 m beneaththe water's surface(pers. ob- prey remains;some prey (e.g., eel) were eatenmore servation). In , Edwards (1988) demonstrat- easily and had fewer bonesand hard parts which ed that Ospreyspreferred Sunfish (Lepomis spp.) WINTER 1991 CHESAPEAKEBAY OSPREYS 111

Table 2. The dietof Ospreysin southwesternChesapeake Bay basedon the meanweight of fishdelivered to the nest.

SPECIES WEIGHT (g)a % OF DIET Menhaden (Brevoortiatyrannus) 152.5 (134.4, 255) 74.69 White perch (Moroneamericana) 290.0 (366.0, 13) 7.24 Atlantic croaker(Micropogonias undulatus) 185.9 (117.2, 11) 3.93 Oystertoadfish (Opsanus tau) 133.7 (52.8, 13) 3.34 Americaneel (Anguillarostrata) 93.0 (90.6, 16) 2.86 Hogchoker( Trinectesmaculatus) 120.5 (68.8, 8) 1.85 Summer flounder (Paralichthysdentatus) 82.0 (82.2, 11) 1.73 White catfish (Ictaluruscatus) 223.2 (17.82, 4) 1.71 Spottedseatrout ( Cynoscionnebulosus) 410.0 (278.6, 2) 1.58 Harvestfish( Peprilus alepidotus ) 228.8 (--, 1) 0.44 Butterfish( Peprilus triacanthus) 222.8 (--, 1) 0.43 Needlefish( Strongylura marina) 54.6 (--, 1) 0.10 Cutlassfish( Trichiuruslepturus) 22.9 (25.0, 2) 0.09 Sunfish(Lepomis macrochirus) 15.2 (--, 1) 0.03 Spanishmackerel (Scomberomorus maculatus) 45.7 (--, 1)b -- Unknown 32 c -- Total 100.02

a Mean (SD, N). b Length(cm); uncertain of length-weightrelationship. c Total number of unidentifiablespecies.

when theywere moreabundant and Shad(Dorosoma ACKNOWLEDGMENTS spp.) when Sunfishabundance declined. SteveSmith and Paul Gerdeshelped us identify common In terms of fish size and the averagenumber of fishspecies of thebay. Dr. Tom Monroeguided us through fish delivereddaily, our findingsare consistentwith prey-remainsidentification. Chris McLean assistedwith data collection.Ralph Dimmick, Chris Stinson,Steve Poz- someothers' (Stinson 1978, Hakkinen 1977, Prevost zanghera,Carol Ann Pala, and severalreferees commented 1982, Henny 1988). In an earlier studyof Chesa- on earlier draftsof this manuscript.The projectwas fund- peakeBay Ospreys,Stinson (1978) reporteda mean ed by the Virginia Commissionof Game and Inland Fish- fish size of 237.1 g (SD = 160.0). Prevost(1982) eries, Non-game and Endangered SpeciesProgram, the Williamsburg Bird Club, and the Collegeof William and observedAfrican Ospreyswith fish as large as 740 Mary. g, thoughmost fish generallyweighed between 200 and 400 g. In Finland, Hakkinen (1977) foundthat 5.2 (SD = 1.0) fish were deliveredper day. LITERATURE CITED It is likely that the Ospreys'impact on a fishery DAWSON,C.E. 1962. Length-weightrelationship of South is insignificant.Hakkinen (1977) calculatedthat the Carolina Hogchokers,Trinectes maculatus. Trans. Am Ospreypopulation in Finland consumed0.6% of the Fish Society91:89-90. total Finnish freshwater fish catch. In the Chesa- DUPAUL,W.D. ANDJ.D. MCEACHRAN.1973. Age and peakeBay, Ospreys consume 5.4 fish/d per breeding growthof the butterfishPeprilus tricanus, in the lower pair includingyoung. Given a mean fish weight of York River. ChesapeakeScience 14:205-207. 156.9 g, the ChesapeakeBay Osprey population EDWARDS,T.C., JR. 1988. Temporal variationin prey preferencepatterns of adult Ospreys.Auk 105:244- (estimatedat 3000 breeding pairs) would be ex- 251. pectedto eat approximately132 171 kg of fish over HAKKINEN,I. 1977. Food catchof the Osprey (Pandion the 52-d nestlingperiod. This Osprey"harvest" rep- haliaetus)during the breedingseason. Ornis Fenn. 54' resents0.004% of the annual ChesapeakeBay com- 166-169. mercialharvest of nearly 300 million kg (Thompson HARRELL,R.M. AND H.A. LOYACANO,JR. 1980. Age, 1984). Clearly, the Ospreys'influence on the bay's growth,and sex ratio of theAmerican eel in the Cooper fishery is negligible. River, South Carolina. Proceedingsof the Annual Con- 112 PETERK. MCLE^N AND MITCHELL A. BYRD VOL. 25, NO. 4

ferenceof the SoutheasternAssociation of Fish and Wildlife Carolina Developmentof Natural Resourcesand Com- Agencies34:349-359. munity Development.Morehead City, NC. HENNY,C.J. 1988. Osprey.Pages 73-101 in R.S. Palm- PREVOST,Y.A. 1982. The wintering ecologyof Ospreys er, [ED.], Handbook of North American birds. Yale in Senegambia.Ph.D. thesis,University of Edinburgh, University Press,New Haven, CT. Edinburgh, Scotland. JACHOWSKI,R. AND F.T. SCHWARTZ.1965. The age, STINSON,C.H. 1978. The influence of environmental growth and lengthweight relationshipsof the Patuxent conditionson aspectsof the time budgetsof breeding River, Maryland Ictalurid white, catfishIctalurus catus. Ospreys.Oecologia 36:127-139. ChesapeakeScience 6:226-237. ST. PIERRE,R.A. ANDJ. DAVIS. 1972. White perch in Lux, F.E. ANDL.R. PORTER,JR. 1966. Length-weight the James and York Rivers. ChesapeakeScience 13: relationship of the summer flounder, Paralichthysden- 272-281. tatus.U.S. Department of the Interior Burr. Comm. SWARTZ,R.C. AND W.A. VAN ENGEL. 1968. Length- Fish. 55 R-Fish. 531:1-5, Washington,D.C. weight and girth relationsin the toadfish,Opsanus tau. McLEAN, P.K. 1986. The feedingecology of the Ches- ChesapeakeScience 9:249-253. apeake Bay Ospreysand the growth and behaviorof THOMPSON,B.C. 1984. FisheryStatistics of the United their young.M.A. thesis,College of William andMary, States 1977. Statistical Digest No. 71, U.S. Depart- Williamsburg, VA. ment of Commerce,Washington, DC. MERCER, L.P. 1983. A biological and fisheriesprofile of , Cynoscionregalis. Report No. 39, North Received26 October 1990; accepted5 April 1991