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Flexible foraging tactics by a large opportunistic seabird preying on forage- and large pelagic fishes

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Flexible foraging tactics by a large opportunistic seabird preying on forage- and large pelagic fishes
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  MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog SerVol. ■ ■ doi: 10.3354/meps08006 Published ■ ■ INTRODUCTION Predators behave opportunistically to take advan-tage of changing prey and environmental conditions.Many marine apex predators exhibit flexible foragingbehaviour to exploit shifting prey diversity and theirchanging spatial and temporal distributions (Croxall1987, Bowen et al. 2006, Watanuki et al. 2008). Thisbehavioural flexibility is mediated by the decisionmaking of individual predators each being constrainedby its own cognitive and physical capabilities. Individ-uals bring both transient and steady-state behaviour tonavigate their daily lives in dynamic marine environ-ments (Ollason et al. 2006). Breeding seabirds are fur-ther constrained by working to meet the increasingenergy demands of offspring while foraging from afixed colony site (Orians & Pearson 1979)Among large seabird predators, gannets employflexible tactics to capture a variety of ephemeralpelagic prey that can vary in mass from tens to manyhundreds of grams (Montevecchi & Berruti 1991,Bunce 2000, Hamer et al. 2001). They also scavenge © Inter-Research 2009 · www.int-res.com*Email: mont@mun.ca M  8  0  0  6  1  9 M ai  2  0  0  9  C E : L B T  S : B N eP P :  S  J  S M ont   ev  e c ch i  WA  ,B  env  en ut  i   S  , G ar t  h  e S  ,D  av  or  en GK  ,F i  f  i   el   d D  Flexible foraging tactics by a large opportunisticseabird preying on forage and large pelagic fishes W. A. Montevecchi 1, *, S. Benvenuti 2 , S. Garthe 3 , G. K. Davoren 4 , D. Fifield 1 1 Cognitive and Behavioural Ecology Program, Memorial University, St. John’s, Newfoundland and Labrador A1B 3X9, Canada 2 Department of Ethology, Ecology and Evolution, University of Pisa, Via Volta 6, 56126 Pisa, Italy 3 Research and Technology Centre (FTZ), University of Kiel, Hafentörn 1, 25761 Büsum, Germany 4 Department of Zoology, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada ABSTRACT: Generalist and opportunistic marine predators use flexible foraging behaviour to exploitprey bases that change in diversity and spatial and temporal distributions. Behavioural flexibility isconstrained by characteristics such as individual cognitive and physical capabilities, age, reproduc-tive condition and central place foraging. To assess flexibility in the foraging tactics of a marine bird,we investigated the diets and foraging behaviour of the largest seabird predator in the North AtlanticOcean. Northern gannets Sula bassana exploit a broad spectrum of pelagic prey that range in massby more than 2 orders of magnitude. We investigated their foraging activity at their largest offshorecolony in the western Atlantic Ocean during 1998 to 2002, when they preyed primarily on shoals ofspawning and post-spawning capelin Mallotus villosus  , a small forage fish (~15 g), and also on amuch larger pelagic fish, post-smolt Atlantic salmon Salmo salar  (~200 g). Inter-annual dietary varia-tion is associated with gannet and prey fish distributions. Landings of capelin at the colony by gan-nets were correlated with returns of larger foraging flocks from inshore, whereas landings of Atlanticsalmon were associated with smaller flocks returning from offshore. Maximum foraging trip dis-tances ranged from 20 to 200 km and averaged 57 ±12 (SE) km, consistent with distances to inshorecapelin aggregations. When capelin abundance was low (in 2002), more gannets foraged offshore,preyed on large pelagic fishes (mostly Atlantic salmon) and exhibited the greatest dietary diversity.Though the outbound portions of foraging trips were more sinuous than inbound routes, individualgannets exhibited general fidelity to foraging sites. These large avian predators used flexible forag-ing tactics to adjust to changing prey conditions and generate longer-term strategies to take advan-tage of diverse trophic interactions over a range of ocean ecosystems.KEY WORDS: Foraging tactics· Seabirds· Forage fishes· Diets· Capelin· Gannet· Atlantic salmon·Ecosystem Resale or republication not permitted without written consent of the publisher  PROOF ONLYNot for redistribution  Mar Ecol Prog Ser ■ ■ discards and offal from fishing vessels (Burger &Cooper 1984, Garthe et al.1996). The largest gannetand the largest North Atlantic seabird, the northerngannet Sula bassana delivers loads of from one to tensof prey items (numbers inversely related to prey massthat range from <10 to >500 g) to rapidly growing,lipid-loading chicks during a 13 wk nestling period(Montevecchi et al. 1984). Chicks are fed primarilylarge oily pelagic fishes (e.g. Atlantic mackerel Scomber scombrus  , Atlantic herring Clupea harengus  )and at times small forage fishes (e.g. capelin Mallotus villosus  , sand lances Ammodytes  spp.; Nelson 1982,Montevecchi & Myers 1995, Garthe et al. 2007b).Associated with a centennially anomalous cold-water perturbation in the northwest Atlantic Oceanduring 1991 (Drinkwater 1996), northern gannetsswitched from preying on large migratory, warm-water species (Atlantic mackerel, Atlantic saury Scomberesox saurus  , short-finned squid Illex illece-brosus  ; their dominant prey during the late 1970s and1980s) to cold-water fishes, primarily capelin, forlonger than a decade (Montevecchi 2007). Throughoutthis period, gannets have successfully provisioned off-spring, and their populations in the northwest AtlanticOcean have grown substantially (Chardine 2000).In this paper, we integrate different levels of dietaryand behavioural analyses to highlight the foraging tac-tics of northern gannets when preying on small foragefishes (capelin) and large pelagic fishes (e.g. Atlanticsalmon Salmo salar  ) during 1998 to 2002. Inter-annualvariation in diets is compared with concurrent avianand capelin distributions and densities obtained fromvessel surveys during 1998 to 2002. Off the northeastNewfoundland coast, Canada, capelin shoals exploitedby avian and mammalian predators often persist infixed locations (Davoren et al. 2003b), and we assessedthe gannets’ fidelity to foraging areas (e.g. Hamer et al.2001, Watanuki et al. 2003) by comparing the direc-tions and distances of successive trips by the sameindividuals equipped with compass loggers. Thereturning directions and sizes of foraging flocks wererelated to landings of capelin and of large pelagicfishes, mostly Atlantic salmon, and foraging trip pat-terning was explored by comparing the fractal dimen-sions of outbound and inbound routes. Becauseseabirds often use different foraging strategies whenprovisioning themselves versus when gathering foodfor offspring (Weimerskirch et al. 1994, Davoren &Burger 1999), we compared the distances from thecolony of terminal diving bouts on a foraging trip(chick provisioning dives) with the distances of previ-ous diving bouts (presumably those for self-provision-ing). We use this information to assess the flexible for-aging tactics used by the largest seabird predator inthe North Atlantic Ocean. MATERIALS ANDMETHODSStudy site and period. Research was carried out onFunk Island (49°45’N, 53°11’W), a small (800 × 400 m)flat granite rock in the northwest Atlantic Ocean (Mon-tevecchi & Tuck 1987), about 50 km off the northeastcoast of Newfoundland (Fig. 1). The island has an esti-mated population of 9800 pairs of northern gannets(Chardine 2000) and is the 4th largest and mostoceanic of the 6 gannet colonies in North America.Researchers were on the island from 4 to 17 August1998, 26 July to 5 August 1999, 5 to 13 August 2000,30July to 7 August 2001 and 6 to 13 August 2002. Diet sampling. Food samples were obtained byapproaching roosting gannets that often regurgitatedfood as they moved away from researchers (Montevec-chi & Myers 1995). Samples were also obtained frombirds captured for data logger attachments andremovals and from discarded regurgitations andscraps in the colony. While there are likely differencesbetween samples collected in roosts and in the colony,samples from these sources are comparable (W. A.Montevecchi unpubl. data), and we opted to minimizedisturbance to breeders by collecting regurgitations atroosts well outside the colony. Regurgitated prey itemswere identified to species level, fresh fish sampleswere measured for total length and very fresh fishwere weighed with spring scales. Prey landings arepresented as percentages of total regurgitations duringeach year. Scans of 360°for flocks returning to colony. Thenumbers of gannets in incoming flocks were recordedby means of scans at various times of day. Counts wererecorded within each 45°sector of a 360°scan per-formed from the island’s highest point near the north-east end with compass-directional binoculars thatwere held with the horizon at mid-diameter as theobserver slowly scanned the sector for 1 min. Three fullrotations (24 min) comprised a scan session. Windspeed and direction were recorded with a hand-heldanemometer and the directional binoculars at the out-set of scans and, if conditions changed, also at the end.Two or three observers counted each year, usuallyworking in pairs with one person counting and theother recording; counts were recorded on a portabletape recorder when a single observer counted.Because observers worked in multiple years most oftenin pairs of the same observers, and because data wereanalyzed in 90°quadrants, influences of individual dif-ferences were minimized. Frequencies of returningforagers within 90°quadrants were compared on dailyand annual bases with chi-square tests, and numberswere plotted as percentages of total counts. Compar-isons of the percentages of gannets returning frominshore (135°to 315°) and offshore (315°to 135°) direc- 2  Montevecchi et al.: Foraging tactics of gannets on pelagic fishes tions were correlated with the numbers of regurgita-tions of the 4 most common prey (capelin, Atlanticsalmon, Atlantic saury, Atlantic herring) landed in thecolony on the same day. The average sizes of flocksreturning from inshore and offshore were comparedwith 1-way ANOVA. Vessel surveys and spatial distribution calculations. A mesoscale (1 to 800 km) survey aboard the 23 mCanadian Coast Guard research vessel ‘Shamook’ wasconducted over 5 d within avian foraging ranges andprime capelin spawning and staging areas to the southand west of Funk Island during July and August 2000to 2002 (Fig. 1). Nine east–west cross-shelf transects9km apart on north–south axis were run during the12h d –1 that the ‘Shamook’ operated. Surveys wereperiodically interrupted to identify the species compo-sition of acoustic signals using a modified shrimp trawl.The majority of fish sampled by mass were capelin ineach year (2000: 96%; 2001: 71%; 2002: 55%).During transects, vessel speed (11 to 16 km h –1 ) washeld constant and signals of prey species wererecorded continuously using 2 hydroacoustic systems.For the 2000 survey, a EQ100 system (Simrad) wasoperated through a hull-mounted single 38 kHz beamtransducer that operated at 1 ping s –1 over a range of250 m with a bandwidth of 0.4 to 0.6 ms. The trans-ducer was at a depth of 3 m and acoustic signals werereliable at 8 m and deeper. The sample depth of theacoustic system (8 to 250 m) and vessel speed (14 to16km h –1 ) were held constant throughout all surveys.Echograms were continuously printed and the relativeabundance of fish was quantified by estimating thepercent cover of the prey image in each 250 × 10 m ver-tical bin (Piatt 1990). An index of percent cover of preyin each bin was estimated from 0 (no prey) to 9 (nearsaturation). Values were squared before analysis (0 to81) to account for the non-linear change in sounderintensity relative to fish school density (Piatt 1990), giv-ing a relative measure of acoustic prey abundance.During 2001 and 2002, a DT 6000 hydroacoustic sys-tem (BioSonics) operated through a 38 kHz split-beamtransducer in a towed body. The transducer had a 2-way beam angle of 22.671 dB and the echo sounderwas operated at 1 ping s –1 , a bandwidth of 5.1 kHz anda pulse duration of 0.4 ms. The transducer was at adepth of 5 m and the upper resolution of the acousticsignals (8 to 10 m) was well within the diving range ofnorthern gannets to 20 m (Garthe et al. 2001). Vesselspeed was maintained between 11 to 14 km h –1 . Rawhigh resolution acoustic data (volume backscatteringcoefficients, s  v  ) were recorded continuously and usedto derive capelin biomass estimates (g m –2 ; seeDavoren et al. 2006). Variability in the quality of hydro-acoustic systems used and data processing techniquesprecluded comparing capelin abundance amongyears, but allowed inter-annual comparisons of distrib-utional patterns.During acoustic transects, seabirds were countedcontinuously by a single observer in a 90°arc out to300 m from the bow to the port side of the ship usingstandard strip methods (Tasker et al. 1984). Countswith behavioural descriptions (on water, flying, feed-ing) were entered into a laptop computer with count-ing software (D. Senciall, Birds & Beasty Counter, Fish- 3Fig. 1. Study area showing Funk Island and vessel survey routes off the northeast coast of Newfoundland  Mar Ecol Prog Ser ■ ■ eries and Oceans Canada, v. 1.0 [1998]) connected tothe vessel’s navigational system. A latitude–longitudeposition was appended to each sighting. Capture of birds. Adult gannets with 4 to 6 wk oldchicks in their nests were captured with a telescopingnoose pole from a small portable blind to minimize dis-turbance to neighbouring birds. We avoided capturingpairs with younger poikilothermic chicks as this wasmore disturbing to parents, and because parents witholder chick had greater energetic demands. Pairs onthe colony’s periphery were used to avoid disruption tothe interior colony; to reduce possible sampling bias inpotential differences between peripheral and interiornesting pairs, we captured birds in the 3rd or 4th rowsfrom the colony edge. Breeding success was high at thesites where we worked (W. A. Montevecchi & S. Garthepers. obs.). Birds were captured, marked on the headand neck with colored markers for identification from adistance, equipped and banded with US Fish andWildlife Service (USFWS) bands issued by the Cana-dian Wildlife Service usually within 5 to 10 min. Weheld the necessary university, provincial and federalpermits, and birds were treated in accordance with theguidelines of the Canadian Council on Animal Care. Data loggers. Four types of data loggers were usedon 24 breeding adult gannets for a total of 49 foragingtrips. Durations of attachments varied from 27 to 75 h.We attached LTD-100 tags (Lotek Marine Technolo-gies) to 7 gannets that made 9 foraging trips in 1999.Dall’Antonia (DA) activity–depth recorders weredeployed on 7 gannets that completed 16 foraging tripsin 2001, DA compass loggers were attached to 7gannets that completed 14 foraging trips in 2002, andcompass Tlog devices (65 mm length, 16 mm diameter,14.5 g, 2 MB memory; Earth & Oceans) with horizontaldirection recorders were also used to track 10 succes-sive foraging flight paths of 2 gannets in 2003 and 1 in2004. The data recording intervals set on each bird-borne device (below)allowed continuous recording forup to 3 to 4 d, and the weight of each device weighed ≤ ~1% of a gannet’s body mass. With the exception ofthe LTD-100 tags (see ‘LTD loggers’ ) , all loggers wereattached with tesa tape to feathers on the lower backabout 5 cm above the uropygeal gland or atop 4 centraltail feathers. When an equipped gannet was recap-tured, devices were removed quickly, and data weredownloaded into a laptop computer. LTD loggers:  LTD-100 tags (57 mm length, 18 mmdiameter, 16 g, 1 MB memory) were attached with tesatape to 2 plastic bands on one leg. Activity was inferredfrom thermal fluctuation records taken every 30 s (fol-lowing Wilson et al. 1995, Garthe et al. 1999); divedepth data are reported elsewhere (Garthe et al. 2000,2003). When temperature indicated sea surface tem-perature (SST) and remained constant, the bird wasconsidered to be swimming or resting on the waterwith the logger submerged. When temperature variedslightly within the range of recorded air temperature(some degrees higher than SST), the bird was consid-ered to be flying. When the temperature varied inhigher ranges (as the logger was warmed by the bird’sand chick’s body heat and often solar and terrestrialradiation) and with other diurnal thermal rhythms, thebird was considered to be in the colony (i.e. at the nestmost of the time). These occurrences were validated byobservations using a spotting scope from outside thecolony to check when marked birds carrying loggerswere present at nest sites (Garthe et al. 2003). Dall’ Antonia device activity-depth loggers:  Dall’Antonia (DA) devices (80 mm length, 22 to 31 mmwidth, 13 to 18.5 mm height, 28 g, 128 kB memory)recorded activity (from a motion sensor) every 6 s anddepth (from a pressure sensor) every 4 s (Dall’Antoniaet al. 1993, Benvenuti et al. 1998). In previous research(e.g. Garthe et al. 2000, 2007), we demonstrated thatU-shaped dives for capelin and V-shaped dives forlarge pelagic fishes like post-smolt Atlantic salmon aretypically in excess of 4 s, and we are confident that werecorded the foraging dives by the gannets in ourstudy. The activity sensor (a 5 mm diameter modifiedmicrophone membrane) was activated by flight signalsproduced by accelerations and/or body vibrationscaused by wing beats. During diving, signals are pro-duced by wing and/or foot propulsions. Flight activitywas identified on graphs as regular high-level deflec-tions, easily distinguishable from the low frequencysignals produced when the bird is on the sea surfaceand from weak irregular or absent signals when thebird is at the nest. Devices were attached either tofeathers on the lower back about 5 cm above theuropygial gland or the 4 central tail feathers with tesatape. Compass loggers:  DA compass loggers (93 mmlength, 22 to 33 mm width, 15 to 20 mm height, 33 g,128 kB memory) with direction recorders, flight andpressure sensors and one with a flight recorder but nopressure sensor were used to track flight directionsand activities (Benvenuti et al. 1998, 2001). Intervalsbetween successive recordings were 6 to 8 s for boththe compass logger and flight sensor and 4 s for thepressure sensor. Used in conjunction with satellitetags, these loggers indicated similar general foragingareas (Falk et al. 2001). Compass Tlog devices (65 mmlength, 16 mm diameter, 14.5 g, 2 MB memory) withhorizontal direction recorders were also used to trackforaging routes. Intervals between successive direc-tional records were 6 s. Foraging routes, ranges and diving activity. Twenty-four compass routes from 10 parental gannetswere plotted; 1 of these birds had 4 consecutive forag- 4  Montevecchi et al.: Foraging tactics of gannets on pelagic fishes ing routes recorded, 3 had 3, 5 had 2, and 1 had 1 routerecorded. Foraging routes and ranges were recon-structed by multiplying an estimated average flightspeed of 14.9 m s –1 (54 km h –1 ; Pennycuick 1997) by thesummed flight time along outbound and inbound for-aging routes, time on water, a corrected magnetic dec-lination (–28°) and wind influences (wind speeds anddirections were recorded at the colony [see above] andalso obtained from the Environment Canada weatherstation at Pool’s Island near the mouth of Bonavista Bayand within the gannets’ foraging range). Potentialsources of error include imperfect alignment of adevice with a gannet’s body axis, drift when birds areon water and variable flight speeds in different windconditions (Grémillet et al. 2004, D. Fifield & W. A.Montevecchi unpubl. data). For the 7 gannetsequipped in 2002, we estimated distances to all divingbouts (groups of dives, excluding those shallower than1 m, separated by long intervals of flying or swimming)and from the last dives of a foraging trip to the colony.Points of maximum trip distance were mapped for all10 compass-equipped birds by extrapolating distancesand directions from Funk Island on a 1:250000 map ofthe northeast Newfoundland coast.Scanned images of foraging trip routes during 2002printed on 27.9 × 43.2 cm paper were subjected to box-counting fractal analysis using the HarFa Harmonicand Fractal Image Analyzer (available at: www.fch.vutbr.cz/lectures/imagessci/download/harfasp.html).Fractal dimensions of the (1) entire route, (2) outboundroute to the most distant point, (3) outbound route tothe last or terminal dive, (4) inbound route from themost distant point and (5) inbound route from the lastor terminal dive. Fractal dimensions of (2) versus(4)and of (3) versus (5) were compared with t  -tests forrelated samples and with binomial tests of differences(Siegel 1957). We describe the number and temporaland spatial patterning of diving bouts performedthroughout trips; depth profiles of different diving pat-terns are reported by Garthe et al. (2000). We alsoassessed the consistency of foraging route directionson successive trips by 10 ind. gannets. RESULTSPrey landings Capelin comprised 77% of the prey loads returned tothe colony, ranging from 51 to 100% on a per annumbases from 1998 to 2002 (Table 1). Atlantic salmon wasthe next most common prey with high annual variationranging from 0 to 34% of landings. Atlantic saury,Atlantic herring and Atlantic mackerel made up rela-tively minor dietary components. During 1998 to 2000,capelin comprised more than 80% of prey landings,though the situation was somewhat different in 2001and very different in 2002, when Atlantic salmon madeup significant dietary components and when dietarydiversity was greatest (Table 1). Northern gannet land-ings of capelin contained 6 to 20 fish compared withlandings of salmon that contained 1 to 3 much largerfish. Directions and sizes of returning flocks and preylandings at colony The flight directions of incoming gannets are shownin Fig. 2. In all years, most birds returned to the colonyfrom inshore foraging sites southwest of the island,except during 2002 when most returned from offshoresites north of the colony ( χ 2 = 902.7, df = 12, p < 0.001).Comparisons of the directions of returning birds andproportions of prey species landed at the colonyrevealed significant relationships between inshorereturns and landings of capelin (y  = 0.49 x + 38.5, n =12, R 2 = 0.33, p = 0.049) and between offshore foragingand landings of Atlantic salmon (y  = 1.15 x + 19.1, n =12, R 2 = 0.60, p = 0.003; Fig. 3). No relationships forother common prey (Atlantic herring, Atlantic saury)were detected (p > 0.05). The sizes of flocks returningfrom inshore were significantly larger than thosereturning from offshore in each year and in all yearscombined (Fig. 4). Flock sizes were lowest in the year2002 when capelin was least abundant. 5Prey199819992000200120021998–2002 combinedCapelin84 (384)87 (86)100 (183)72 (153)51 (145)77 (951)Atlantic salmon1 (5)0025 (54)34 (95)12 (154)Atlantic saury 12 (56)10 (10)0 1 (3)3 (9)6 (78)Atlantic herring3 (15)3 (3)02 (4)3 (8)2 (30)Atlantic mackerel00009 (25)2 (25)Totals100 (460)100 (99)100 (183)100 (214)100 (282)99 (1238)Table 1. Sula bassana. Percentagesof each prey species in regurgitations (with numbers in parentheses) by northern gannets on Funk Island, 1998 to 2002
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