Swordfish are known to undergo large diel vertical movements from surface waters at night to \u3e 300 m depth during the day. Evidence presented over the past several deeades suggests the lunar cycle affects these vertical migrations. This study collected data concurrently from 7 swordfish throughout 3 consecutive lunar cycles using pop-up satellite archival tags. All individuals demonstrated an inverse relationship between recorded nocturnal depths and lunar illumination

Loefer, J.L.

McGovern, J.C.

Sedberry, George R.

English

Loefer, Joshua K.

McGovern, John C.

Aquila Digital Community

Gulf and Caribbean ResearchVolume 19 | Issue 2January 2007Nocturnal Depth Distribution of Western North Atlantic Swordfish (Xiphias gladius,Linnaeus, 1758) in Relation to Lunar IlluminationJoshua K. LoeferSouth Carolina Department of Natural ResourcesGeorge R. SedberrySouth Carolina Department of Natural ResourcesJohn C. McGovernNOAA Fisheries ServiceDOI: 10.18785/gcr.1902.10Follow this and additional works at: http://aquila.usm.edu/gcrPart of the Marine Biology CommonsThis Article is brought to you for free and open access by The Aquila Digital Community. It has been accepted for inclusion in Gulf and CaribbeanResearch by an authorized editor of The Aquila Digital Community. For more information, please contact Joshua.Cromwell@usm.edu.Recommended CitationLoefer, J. K., G. R. Sedberry and J. C. McGovern. 2007. Nocturnal Depth Distribution of Western North Atlantic Swordfish (Xiphiasgladius, Linnaeus, 1758) in Relation to Lunar Illumination. Gulf and Caribbean Research 19 (2): 83-88.Retrieved from http://aquila.usm.edu/gcr/vol19/iss2/1083Gulf and Caribbean Research Vol 19(2), 83-88, 2007 Manuscript received November 17, 2006; accepted March 29. 2007 NOCTURNAL DEPTH DISTRIBUTION OF WESTERN NORTH ATLANTIC SWORDFISH (X/PH/AS GlAD/US, LINNAEUS, 1758) IN RELATION TO LUNAR ILLUMINATION Joshua K. Loefer', George R. Sedberry1, and John C. McGovem2 ISouth Carolina Department of Natural Resources, Marine Resources Research Institute, PO Box 12559, Charleston, South Carolina 29422-2559, USA *Corresponding author. Phane (843) 953-9060, FAX (843) 953-9820, E-mail /oeferj@dnr.sc.gav 'NOAA Fisheries Service, 263 13th Avenue South, Saint Petersburg, Florida 33701 USA ABSTRACl' Swordfish are known to undergo large diel vertical movements from surface waters at night to > 300 m depth during the day. Evidence presented over the past several deeades suggests the lunar cycle affects these vertical migrations. This study collected data concurrently from 7 swordfish throughout 3 consecutive lunar cycles using pop-up satellite archival tags. All individuals demonstrated an inverse relationship between recorded nocturnal depths and lunar illumination. RESUMEN Los peces espadas son reconocidos por experimentar extensas ntigraciones verticales deade ]a superfi-cie durante ]a noehe, hasta aguas profundas (> 300 m) durante el dfa. La evidencia presen!ada durante las pasadas decadas expone que el cicio lunar afecta eslas. Este estudio presenta ]a infurmaci6n que ha sido recopilada reci-entem.ente de 7 peces espadas durante 3 ciclos lunares consecutivos utilizando etiquetas de archivos que resurgen a ]a superficie y envian toda ]a informaci6n al sat6lite, conocidos en ingl6s como ''pop-up satellite archival tags." Todos los individuos demostraron una relaci6n inversa entre los datos obtenidos para profundidades documentadas en las horas noctumas e. INTRODUCTION The swordfish (Xiphias gladius, Linnaeus, 1758) has a worldwide distribution in tropical, sub-tropical and temperate oceans, and its palatable flesh makes it a highly valued commodity throughout its range. The commercial importance of this species has lead to a substantial amount of research on life history, movements, stock identifica-tion, and bycatch issues (Ward et al. 2(00). Swordfish are known to undergo large diel vertical migrations cbaracterized by descent to depths of up to 600 m or more during daylight bours and ascent to shallower waters at night (Carey and Robison 1981, Carey 1990). Therefore, most swordfish fisheries target the species at night when they are generally closest to the surface. Draganik and Cholyst (1988) and di Natale and Mangano (1995) noted variation in swordfish catch rates during the full moon in central Atlantic and Italian waters, respec-tively. de la Serna et al. (1992) found CPUE maximums during the new moon in the Strait of Gibraltar and adjacent Mediterranean and Atlantic waters. Moreno et al. (1991), however, found no relation between moonlight intensity and swordfish abundance, and Manday (1964) also con-cluded there was no lunar effect on depth-related catch in Cuban waters. Though some of these reports document changes in swordfish catch rate or abundance in relation to the lunar cycle, the methodologies used cannot provide demonstrative evidence that these variations are a direct result of behavioral changes in swordfish. Variations in CPUE or total catch may be due to designed or uninten-tional variation in depth of the fishing gear, changes in gear configuration related to tidal currents associated with lunar phase, visibility of bait that might be affected by lunar phase, or lunar periodicity in the behavior of the swordfish or its prey. Electronic tagging provides one of the best available methods to document vertical movements in large pelagic predators such as swordfish and investigate long-term (> 30 d) cyclical behavioral changes. There have been several published studies on elec-tronic tagging of swordfish (Carey and Robison 1981, Carey 1990, Sedberry and Loefer 2001, Takahashi et al. 2003). However, the study by Carey and Robison (1981) was the ouly one to mention changes in vertical movement patterns associated with the lunar cycle. They noted that of 6 swordfish tracked using acoustic telemetry, the 3 fish tracked during the full moon occupied deeper nocturnal depths than individuals tracked during other lunar phases. Limitations in study duration imposed by acoustic track-ing, however, precluded tracking an individual specimen for an entire lunar cycle. Therefore, their observations on lunar differences had to be inferred from differences in behavior between different individuals, tracked at different times, and, in some cases, different locations. Pop-up satellite archival tags (PSATs) allow for lon-ger-tenn fishery independent data collection from highly-migratory fish species than was previously feasible. These devices allow researchers to collect large amounts of vertical movement data from individuals over large time 84LoEPER Ef AL. TABLE I Tag deployment and pop-up information for all swordfish in the stody. DAL = days at large (days from tog deploy-ment to first satellite contact), LJFL = lower jaw fork length. Study Est. interval Deployment Pop-up LJFL TagID# (mos) Deployment location date 28662 1 32°6.4'N,78"34.2'W 411812002 28663 1 32"2.8'N,78"2S.1'W 411612002 28664 1 32°4.0'N,78"39.2'W S/1712002 2866S 1 32°3.4'N,78°43.8'W Sn12oo2 28666 2 32°4.5'N, 78°43.5'W Snl2OO2 28668 2 32"O.0'N, 78"23.8'W 4/1712002 28669 2 32"7.1'N,78"33.3'W 4/1812002 28670 3 32"2.4'N,78°41.3'W SI1712002 28671 3 32"7.6'N,78"32.1'W 4/1712002 3OO3S 3 32"7.0'N,78°40.9'W Sn12oo2 30036 3 32°4.1 'N, 78"24.5'W 411612002 30037 4 32°3.7'N,78"37.2'W 4/1712002 30038 4 32°1.9'N, 78°42.4'W S/1612002 30039 4 31°S8.9'N,78°4S.8'W Snl2OO2 30040 4 31°S8.9'N,78°4S.8'W Snl2OO2 periods without the need for recapture. This study presents PSAT data collected concurrently on several swordfish through 3 consecutive lunar cycles in the western North Atlantic. MATERIALS AND METHODS Swordfish were captured with pelagic longline gear deployed from a research vessel between 16 April and 17 May 2002. A total of IS swordfish with estimated lower-jaw fork lengths from 81 to 213 cm were fitted with satellite pop-up tags. Tags were attached to the subjects by harpooning a 6.2S cm titanium M-type dart anchor into the dorsal musculature about S cm below the midline of the base of the dorsal fin. The anchors were inserted using a 20 em titanium tagging needle protected by a 6 em diameter rubber stopper that lintited penetration depth to 12 em. Tags were tethered to the anchors with 30 cm of 1.66 mm diameter fluorocarbon monofilament leader (100 kg tensile strength). Swordfish were not removed from the water during tagging, and were released by cutting the monofila-ment hook leader within a few em of the hook. Satellite pop-up tags (model PTT-100 Archival pop-up tag, manufactured by Microwave Telemetry, Inc., Columbia, MD, USA) were programmed to jettison at intervals of 30 (n=4), 60 (n=3), 90 (n=4) or 120 (n=4) days following tagging. The tags archived temperature and Pop-up location date DAL (em) 32°19.6'N,78°4S.4'W 04128/02 10 91 34°17.6'N,74°4S.7'W 04121102 S 183 32"21.2'N, 78°16.5'W 06/17/02 31 IS2 3S"2.0'N, 74°S2.9'W 06/07/02 31 122 34°31.6'N, 7S"37.0'W 07/07/02 61 137 No contact 112 32°S0.9'N, 77"28.7'W 04/27/02 9 213 31°37.0'N,78"29.8'W 08/17/02 92 107 34°40.1'N,7S"32.9'W 0412S/02 8 102 32°14.3'N,78°S.6'W OSI12102 S 122 28°S9.0'N,79°S8.1'W 07116/02 91 81 33°31.9'N,7s014.6'W 04123/02 6 107 32°1.3'N, 79"O.0'W 09116/02 123 107 37°46.7'N,70"S1.6'W 08/17/02 102 122 33°49.9'N,76"30.2'W OS/l6/02 9 122 pressure readings once each hour until the release date was reached. A more complete description of the programming and function of the model PTT-I00 PSAT can be found in Loefer et al. (2OOS). Depth data retrieved from 7 of the IS tags were combined and the resulting dataset was filtered so that ouly depth readings recorded between OSOO and 0900 GMT during the period of greatest temporal data overlap between individuals (8 May 2002 to IS August 2002) remained. The OSOO to 0900 GMT timeslot was selected because depth data were consistently shallowest during tltis time period. A mean was calculated for each hourly timeslot in the filtered dataset. Each hourly mean was calculated from 2 to 7 concurrent depth readings from individual PSATs. These data were then compared to a cor-responding lunar illumination schedule calculated by the United States Naval Observatory (http://aa.usno.navy.mil). Mean nightly depths were then regressed on percent lunar ilIuntination and a correlation coefficient was calculated. RESULTS The IS tags deployed in tltis study collected a total of S33 d of data while attached to the study anintals, of which 462 d (87%) were retrieved via satellite. Six of the IS tags stayed attached throughout the entire study interval and released on schedule. A total of 8 tags released before the 85w w FIOiIdi ... .... ...... "OW , , , . .. ..... ... .. . .... w w Tagging locations • Pop-up locations Days at large 0 .... " ~-0 · ... C} 120doyt. "., JlpNI.lw •• ' taaIaI-tq ...... ' ff=z(oo='!-u.pIII __ n.(r.-. ...... IoAN 0).+ 1101, __ -... ~ wi,.,l"W dale, ud ODe til _ DIIde I&IelllIe ........... fu""';";em uI. dm. tr...1IIc H cmy-rdcuc: tap rqpt-cd Jb.t 3 wmc ckic to i"""""i* po.t...u.x ...mlity, 4 were"'" 10 tIJ !hcdctjDI (fmm. 410 \III d.tler _chmMlI), ad liz ......, fur 1IIc cwl.y ~ of IliIC 111 IXII1ld DDI be dr(ennj'" from liz .ewfeml. <1m. (1ltb1e 1). ~IIP Inre!jew <1m. fur liz (j run-dit .. tap jadice!erl vwia-lim ia JCIICnII dirccJioaI aI.....- One IiIh mmcd IIl1iIII (360 tm IIniJht-linc ~), 2 mmcd IIDrlbcul (390 11I1d 490 tm), ad 3 __ witbin. SO tm of ... t:Igiq loc:lltiew afIw"\IP to 120 d 111; lap (Fis- 1). ~ dIta faom ti1e (j fwll-i I 6'" tap, u well • tile tac whicll IIIcd afIw" 98 d of _clmtNt Ihowed II rIIIIIII pdlm of die! ~ mI.Jndoaa. SWord&Ir. occwp!ed eha1Ja ____ from. 0 to 160 III. 111; 1Il&ht. ad IIIIIfIII1ed to depIU uI. 3SO 10 770 III. durlq tile day. -rm,- bcpa. 10 ucead tr... depIb. aouad 2-3 h beIore _ (bctwoett 0000-00'lS GMT ill the Itudy _), IIIId 1IIcy mmwi-' in ~~ __ fu aboat -4-6 h. S.....JIiIh ......uy bcpa to dolCClld beIore 1IIc lint po.t-IIJIIri& huudy """""" (1IIEIri&: 100II-1040 GMT). 11Ic IilI<Dod (OSOIJ-tJ\lOO OM!") dwIa dcm!mI1nlaI. _ iIMnc ....n.JiDa betwoettmcm1llJCbmull dcptluI mil. ~I..­jlbwnjMljew lIIauaJlooullllc 3_1DII Itudy period. (Rprc 2). NocIamIl. dcp8uI =mtccl durlq pcriodI uI. ~ 9M' I..-jlhwnjnet;ew raqcd. from. 0 10 172 m wilila_ ofTI.o ID. (II '" 443), ~ o:t.ptbl ~ ..... lunIr ilhwnj_ lion. _ ,.;:10" __ bltw_ 0 11I1d 142 m wi .. II ....... of 34.3 III. (II'" Q, Hpre 3). LineIr recr-i0" of ....... 1Ii&hdY depthI 011. pem!III. ~ j1lpmjnotjon ~II ~j"'at relarlew""p (II_ 99, P < 0.001) "Wid!. II cor-reladoa coetftdN ' of -0.66. 86ABLunarilluminationDepth (m)Depth (m)0204060801001200255075100125150100%50%0%05/20 06/03 06/17 07/01 07/15 07/29 08/12.... ~ .. .. s.:., ~ . 'e.:-. • ••• .. • "-!  • •• JII..-ep ••• ••• ~~-~ ...... , .. ;fl ...... ~. ! ...:,. ... . ..... "'!. ""'~ .. . -.-" ..... -:;.: .. .. . 1.-"'... ... . . --- _.. . . ... .~ ... • •• • •• • • • F'Iaure 2. LIIIIU' UlUlldaatlcm !lClwhJle c:ompared to (A) hourly IIUIUl olllOCtarlW deptlu (II • 501) aad (B) ...... tly depth IIHllD (it-99). DISClJ8SlON Stillwell ODd KOOlcr (1985) found Ihat swordJish diet in the WCItem North Atlantic coa.siJts primarily of ccpha-Iopodo (82% by frequency of oc<w • ...,.) ODd IOIcoo<s (53%), many taxa of which are known to make:ooctumaJ. migrations into the shallow mixed layer to feed (Brodziak and Hendrickson 1999, Kinzer and Sbulz 1985, Roper and Young 1975, Walker and Nichols 1993). Changes in noctuma1 swordfish behavior, therefore., may be a result of active predation OIl vertically migrant midwater cepha-lopods and fiBhcs. Swordfiah may al»o follow iaolumct to immene tbemaelves in aD environment for wbich their eyes are IIdapted for maximum visual. acuity aDd bunting 5UCCtaS (Wammt 20(4). Regardless of the rcuOD for these cyclical changes, the theory that swordfish follow isolumee: and that their vmical migrations are affc:ctcd by the lutllr cycle is DOt ocw. D bas Ioq been oonsiden:d. common knowledge among commr;r-da1 fiII:Iermen and fisbcry wor:kcn thai: swooIfiIh are found fW1h« from doc _ .. Digh. during doc full mocm !ban durinJ other lunar phases. However, thia conclusion has been bued on data collected over a small time frame Q.ess than ODe full lunar cycle, Carey IIOd Robison 1981), on com-parisons between different individuaJs observed at different ..... (Carey ODd Rolli"", 1981). on ""'"}'Sis of CPUE_ (llo>gom;k ODd QooI,... 1988. Mon:no" 01. 1991. di Nodale IlJld Manpno J m). or 011 aneodoIaJ obaemdi.ona from fi.Ibenooa (MIDday 1964. di Nalale ODd Manpno 1995~ In contrut. the data fur this study were collcdcd concum:ntly from mnltiple indiviW.al fish throughout 3 CODIeCUtive hmar 870 20 40 60 80 100 120020406080100120140160180Depth(m)FrequencyNOCTERNAL DISTRIBUTION OF SWORDFISH IN WESTERN NORTII ATLANTIC Figure 3. Depth frequeney distribution for noctnmal depths (n = 851) recorded between 0500 and 0900 GMT during new moon ('; 10% lunar illumination, black burs) and full moon (;' 90% lunar illumination, white burs). cycles. The described behavior was ubiquitous despite the movement of several anllnals away from the tagging area during the stody. It is also interesting to note that the correla-tion between uightly depth and lunar illumination remained intact even when the moon was not visihle in the stody area (had not risen or had previously set) during the 0500 to 0900 GMT window examined. The data presented thus provide confirmation of the long accepted, yet unproven, theory that changes in noctumal swordfish depth distributions are cor-related with the lunar cycle. ACKNOWLEDGEMENTS We acknowledge the efforts of D.Russ, S. Meister, G. Ulrich, S. Burns, P. Mikell, B. White, D. Oakley, and the crew of the RN Palmetto for field assistance. Constructive criticism of the manuscript was provided hy M. Reichert and J. Geddings, while I. Bagnarnanay La Torre Torres and G. Perez assisted with Spauish translation. The research described herein complies with all current laws and animal welfare policies of the Uuited States. Permits for the captore and tagging of swordfish were provided by the US Department of Commerce, National Oceauic and Atmospheric Administration Office Sustainable Fisheries (permit no. HMS-SRP-02-00l). Funding was provid-ed by NOAA Fisheries grant numbers NA07FL0497, NA16FL2814, and NA03NMF4720321 (G. Sedberry, Principal Investigator). This is Contribution Number 583 from the South Carolina Marine Resources Center. LITERATURE CITED Brodziak, 1. and L. Hendrickson. 1999. An analysis of environ-mental effects on survey catches of squids LoZigo pealei and [llex illecebrocus in the northwest Atlantic. Fishery Bulletin 97:1)...24. Carey, EG. and B.H. Robison. 1981. Daily patterns in the activi-ties of swordfish, Xiphias gladius, observed by acoustic telemetry. Fishery Bulletin 79:277-292. Carey, EG. 1990. Further acoustic telemetry observations of swordfish. In: R. H. Stroud, ed. Planning the Future of Billfishes: Research and Management in the 90s and Beyond. Proceedings of the Second International Billfish Symposium, Kailua-Kona, Hawaii, 1988. Part 2: Contributed Papers. National Coalition for Marine Conservation, Inc., Savanuah, GA, USA, p. 103-122. de la Serna, 1.M., E. Alot, and E. Rivera. 1992. Analysis of CPUEs by size group of the swordfish (Xiphias g/adius) caught by drifting gillnets in the area of Gibraltar Strait, dur-ing the years 1989-1990). Collected Volume of Scientific Papers, ICCAT 39:62tHi34. di Natale, A. and A. Mangano. 1995. Moon phases influence on CPUE: A first analysis of swordfish dtiftnet catch data from the Italian fleet between 1990 and 1991. Collected Volume of Scientific Papers, ICCAT 44:264--267. Draganik, B. and 1. Cholyst. 1988. Temperature and moonlight as stimulators for feeding activity by swordfish. Collected Volume of Scientific Papers, ICCAT 27:305-314. Kinzer, 1. and K. Shulz. 1985. Vertical distribution and feeding patterns of midwater fish in the central equatorial Atlantic (I. Myctophidae). Maline Biology 85:313-322. Loofer, 1.K., G.R. Sedberry, and 1.C. McGovern. 2005. Vertical movements of a shortfin mako in the western North Atlantic as detemtined by pop-up satellite tagging. Southeastern Naturalist 4(2):237-246. Manday, D.G. 1964. Biologic pesquera del emperador 0 pez de espada, Xiphias g/adius Linnaeus (Teleostomi: Xipbiidae) en las aquas de Cuba. Poeyana, Serie B:1:1-37. 88LoEPER Ef AL. Moreno, S., J. Pol, and L. Munoz. 1991. Influencia de la luna en la abundancia del emperador. Collected Volume of Scientific Papers, ICCAT 35:508-510. Roper, C.F.E. and R.E. Young. 1975. Vertical distribution of pelagic cepbalopods. Smithsonian Contributions to Zoology 209:1-51. Sedberry, G.R. and J.K. Loefer. 2001. Satellite telemetry tracking of swordfish, Xiphias gladius off the eastern United States. Marine Biology 139:355-360. Stillwell, C.E. and N.E. Kohler. 1985. Food and feeding ecol-ogy of the swordfish Xiphias gladius in the western North Atlantic Ocean with estimates of daily ratioo. Marine Ecology Progress Series 22:239--247. Takahashi, M., H. Okamura, K. Yokawa, and M. Okazaki. 2003. Swimming behavior and migration of a swordfish record-ed by an archival tag. Marine and Freshwater Research 54:527-534. Walker, M.G. and J.H. Nichols. 1993. Predation on Benthosemn glaciale (Myctophidae) by spawning mackerel (Scomber scombrus). Journal of Fish Biology 42:618-620. Ward, P., J.M. Porter, and S. ElscoL 2000. Broadbill swordfish: status of established fisheries and lessons for developing fisheries. Fish and Fisheries 1:317-336. Warrant, E. 2004. Vision in the dimmest habitats on Earth. Journal of Comparative Physiology 190:765-789. 

Nocturnal Depth Distribution of Western North Atlantic Swordfish (Xiphias gladius, Linnaeus, 1758) in Relation to Lunar Illumination

Aquatic Commons

Gulf and Caribbean Research Vol 19(2), 83–88, 2007 Manuscript received November 17, 2006; accepted March 29, 2007
83
NOCTURNAL dEPTH dISTRIBUTION OF WESTERN NORTH ATLANTIC 
SWORdFISH (XIPHIAS GLADIUS, LINNAEUS, 1758) IN RELATION TO 
LUNAR ILLUMINATION
Joshua k. Loefer1*, George R. Sedberry1, and John C. McGovern2
1South Carolina Department of Natural Resources, Marine Resources Research Institute, PO 
Box 12559, Charleston, South Carolina 29422-2559, USA
*Corresponding author. Phone (843) 953-9060, FAX (843) 953-9820, E-mail loeferj@dnr.sc.gov
2NOAA Fisheries Service, 263 13th Avenue South, Saint Petersburg, Florida 33701 USA
ABSTRACT Swordfish are known to undergo large diel vertical movements from surface waters at night to > 300 m 
depth during the day. Evidence presented over the past several decades suggests the lunar cycle affects these vertical 
migrations. This study collected data concurrently from 7 swordfish throughout 3 consecutive lunar cycles using 
pop-up satellite archival tags. All individuals demonstrated an inverse relationship between recorded nocturnal 
depths and lunar illumination.
RESUMEN Los peces espadas son reconocidos por experimentar extensas migraciones verticales desde la superfi-
cie durante la noche, hasta aguas profundas (> 300 m) durante el día. La evidencia presentada durante las pasadas 
décadas expone que el ciclo lunar afecta estas. Este estudio presenta la información que ha sido recopilada reci-
entemente de 7 peces espadas durante 3 ciclos lunares consecutivos utilizando etiquetas de archivos que resurgen 
a la superficie y envían toda la información al satélite, conocidos en inglés como “pop-up satellite archival tags.” 
Todos los individuos demostraron una relación inversa entre los datos obtenidos para profundidades documentadas 
en las horas nocturnas e.
introduCtion
The swordfish (Xiphias gladius, Linnaeus, 1758) 
has a worldwide distribution in tropical, sub-tropical and 
temperate oceans, and its palatable flesh makes it a highly 
valued commodity throughout its range. The commercial 
importance of this species has lead to a substantial amount 
of research on life history, movements, stock identifica-
tion, and bycatch issues (Ward et al. 2000).
Swordfish are known to undergo large diel vertical 
migrations characterized by descent to depths of up to 600 
m or more during daylight hours and ascent to shallower 
waters at night (Carey and Robison 1981, Carey 1990). 
Therefore, most swordfish fisheries target the species 
at night when they are generally closest to the surface. 
Draganik and Cholyst (1988) and di Natale and Mangano 
(1995) noted variation in swordfish catch rates during the 
full moon in central Atlantic and Italian waters, respec-
tively. de la Serna et al. (1992) found CPUE maximums 
during the new moon in the Strait of Gibraltar and adjacent 
Mediterranean and Atlantic waters. Moreno et al. (1991), 
however, found no relation between moonlight intensity 
and swordfish abundance, and Manday (1964) also con-
cluded there was no lunar effect on depth-related catch in 
Cuban waters. Though some of these reports document 
changes in swordfish catch rate or abundance in relation 
to the lunar cycle, the methodologies used cannot provide 
demonstrative evidence that these variations are a direct 
result of behavioral changes in swordfish. Variations in 
CPUE or total catch may be due to designed or uninten-
tional variation in depth of the fishing gear, changes in gear 
configuration related to tidal currents associated with lunar 
phase, visibility of bait that might be affected by lunar 
phase, or lunar periodicity in the behavior of the swordfish 
or its prey. Electronic tagging provides one of the best 
available methods to document vertical movements in 
large pelagic predators such as swordfish and investigate 
long-term (> 30 d) cyclical behavioral changes.
There have been several published studies on elec-
tronic tagging of swordfish (Carey and Robison 1981, 
Carey 1990, Sedberry and Loefer 2001, Takahashi et al. 
2003). However, the study by Carey and Robison (1981) 
was the only one to mention changes in vertical movement 
patterns associated with the lunar cycle. They noted that 
of 6 swordfish tracked using acoustic telemetry, the 3 fish 
tracked during the full moon occupied deeper nocturnal 
depths than individuals tracked during other lunar phases. 
Limitations in study duration imposed by acoustic track-
ing, however, precluded tracking an individual specimen 
for an entire lunar cycle. Therefore, their observations on 
lunar differences had to be inferred from differences in 
behavior between different individuals, tracked at different 
times, and, in some cases, different locations. 
Pop-up satellite archival tags (PSATs) allow for lon-
ger-term fishery independent data collection from highly-
migratory fish species than was previously feasible. These 
devices allow researchers to collect large amounts of 
vertical movement data from individuals over large time 
loefer et al.
84
periods without the need for recapture. This study presents 
PSAT data collected concurrently on several swordfish 
through 3 consecutive lunar cycles in the western North 
Atlantic.
matErials and mEthods
Swordfish were captured with pelagic longline gear 
deployed from a research vessel between 16 April and 
17 May 2002. A total of 15 swordfish with estimated 
lower-jaw fork lengths from 81 to 213 cm were fitted with 
satellite pop-up tags. Tags were attached to the subjects by 
harpooning a 6.25 cm titanium M-type dart anchor into the 
dorsal musculature about 5 cm below the midline of the 
base of the dorsal fin. The anchors were inserted using a 20 
cm titanium tagging needle protected by a 6 cm diameter 
rubber stopper that limited penetration depth to 12 cm. 
Tags were tethered to the anchors with 30 cm of 1.66 mm 
diameter fluorocarbon monofilament leader (100 kg tensile 
strength). Swordfish were not removed from the water 
during tagging, and were released by cutting the monofila-
ment hook leader within a few cm of the hook.
Satellite pop-up tags (model PTT-100 Archival pop-
up tag, manufactured by Microwave Telemetry, Inc., 
Columbia, MD, USA) were programmed to jettison at 
intervals of 30 (n = 4), 60 (n = 3), 90 (n = 4) or 120 (n = 4) 
days following tagging. The tags archived temperature and 
pressure readings once each hour until the release date was 
reached. A more complete description of the programming 
and function of the model PTT-100 PSAT can be found in 
Loefer et al. (2005).
Depth data retrieved from 7 of the 15 tags were 
combined and the resulting dataset was filtered so that 
only depth readings recorded between 0500 and 0900 
GMT during the period of greatest temporal data overlap 
between individuals (8 May 2002 to 15 August 2002) 
remained. The 0500 to 0900 GMT timeslot was selected 
because depth data were consistently shallowest during 
this time period. A mean was calculated for each hourly 
timeslot in the filtered dataset. Each hourly mean was 
calculated from 2 to 7 concurrent depth readings from 
individual PSATs. These data were then compared to a cor-
responding lunar illumination schedule calculated by the 
United States Naval Observatory (http://aa.usno.navy.mil). 
Mean nightly depths were then regressed on percent lunar 
illumination and a correlation coefficient was calculated.
rEsults
The 15 tags deployed in this study collected a total of 
533 d of data while attached to the study animals, of which 
462 d (87%) were retrieved via satellite. Six of the 15 tags 
stayed attached throughout the entire study interval and 
released on schedule. A total of 8 tags released before the 
TABLE I
Tag deployment and pop-up information for all swordfish in the study. dAL = days at large (days from tag deploy-
ment to first satellite contact), LJFL = lower jaw fork length.
 Study      Est. 
 interval   deployment  Pop-up  LJFL
Tag Id#  (mos) deployment location date Pop-up location date dAL (cm)
28662 1 32º6.4′N, 78º34.2′W 4/18/2002 32º19.6'N, 78º45.4′W 04/28/02 10 91
28663 1 32º2.8′N, 78º25.1′W 4/16/2002 34º17.6′N, 74º45.7′W 04/21/02 5 183
28664 1 32º4.0′N, 78º39.2′W 5/17/2002 32º21.2′N, 78º16.5′W 06/17/02 31 152
28665 1 32º3.4′N, 78º43.8′W 5/7/2002 35º2.0′N, 74º52.9′W 06/07/02 31 122
28666 2 32º4.5′N, 78º43.5′W 5/7/2002 34º31.6′N, 75º37.0′W 07/07/02 61 137
28668 2 32º0.0′N, 78º23.8′W 4/17/2002  No contact  112
28669 2 32º7.1′N, 78º33.3′W 4/18/2002 32º50.9′N, 77º28.7′W 04/27/02 9 213
28670 3 32º2.4′N, 78º41.3′W 5/17/2002 31º37.0′N, 78º29.8′W 08/17/02 92 107
28671 3 32º7.6′N, 78º32.1′W 4/17/2002 34º40.1′N, 75º32.9′W 04/25/02 8 102
30035 3 32º7.0′N, 78º40.9′W 5/7/2002 32º14.3′N, 78º5.6′W 05/12/02 5 122
30036 3 32º4.1′N, 78º24.5′W 4/16/2002 28º59.0′N, 79º58.1′W 07/16/02 91 81
30037 4 32º3.7′N, 78º37.2′W 4/17/2002 33º31.9′N, 75º14.6′W 04/23/02 6 107
30038 4 32º1.9′N, 78º42.4′W 5/16/2002 32º1.3′N, 79º0.0′W 09/16/02 123 107
30039 4 31º58.9′N, 78º45.8′W 5/7/2002 37º46.7′N, 70º51.6′W 08/17/02 102 122
30040 4 31º58.9′N, 78º45.8′W 5/7/2002 33º49.9′N, 76º30.2′W 05/16/02 9 122
noCtUrnal distribution of swordfish in western north atlantiC
85
scheduled date, and one tag never made satellite contact. 
Examination of data from the 8 early-release tags suggest-
ed that 3 were due to immediate post-release mortality, 4 
were due to tag shedding (from 4 to 98 d after attachment), 
and the cause for the early release of one tag could not 
be determined from the recovered data (Table 1). Pop-up 
location data for the 6 full-retention tags indicated varia-
tion in general directions of movement. One fish moved 
south (360 km straight-line distance), 2 moved northeast 
(390 and 490 km), and 3 were within 50 km of the tagging 
location after up to 120 d at large (Figure 1). 
Archived data from the 6 full-retention tags, as well 
as the tag which shed after 98 d of attachment, showed 
a strong pattern of diel vertical migrations. Swordfish 
occupied shallower waters from 0 to 160 m at night, 
and migrated to depths of 350 to 770 m during the day. 
They began to ascend from depth around 2–3 h before 
sunset (between 0000–0025 GMT in the study area), and 
they remained in near-surface waters for about 4–6 h. 
Swordfish usually began to descend before the first post-
sunrise hourly record (sunrise: 1008–1040 GMT). The 
filtered (0500–0900 GMT) data demonstrated an inverse 
variation between mean nocturnal depths and percent lunar 
illumination throughout the 3-mo study period (Figure 2). 
Nocturnal depths recorded during periods of ≥  90% lunar 
illumination ranged from 0 to 172 m with a mean of 77.0 
m (n  =  443), while depths recorded when lunar illumina-
tion was ≤ 10% were between 0 and 142 m with a mean 
of 34.3 m (n  =  408, Figure 3). Linear regression of mean 
nightly depths on percent lunar illumination demonstrated 
a significant relationship (n  =  99, P  <  0.001) with a cor-
relation coefficient of – 0.66.
Figure 1. Swordfish tagging and tag pop-up locations (n = 6). depth contours (from inshore to offshore) represent 100, 200, 500, 
and 1000 m.
loefer et al.
86
disCussion
Stillwell and Kohler (1985) found that swordfish diet 
in the western North Atlantic consists primarily of cepha-
lopods (82% by frequency of occurrence) and teleosts 
(53%), many taxa of which are known to make nocturnal 
migrations into the shallow mixed layer to feed (Brodziak 
and Hendrickson 1999, Kinzer and Shulz 1985, Roper 
and Young 1975, Walker and Nichols 1993). Changes in 
nocturnal swordfish behavior, therefore, may be a result 
of active predation on vertically migrant midwater cepha-
lopods and fishes. Swordfish may also follow isolumes 
to immerse themselves in an environment for which their 
eyes are adapted for maximum visual acuity and hunting 
success (Warrant 2004).
Regardless of the reason for these cyclical changes, the 
theory that swordfish follow isolumes and that their vertical 
migrations are affected by the lunar cycle is not new. It has 
long been considered common knowledge among commer-
cial fishermen and fishery workers that swordfish are found 
further from the surface at night during the full moon than 
during other lunar phases. However, this conclusion has 
been based on data collected over a small time frame (less 
than one full lunar cycle, Carey and Robison 1981), on com-
parisons between different individuals observed at different 
times (Carey and Robison 1981), on analysis of CPUE data 
(Draganik and Cholyst 1988, Moreno et al. 1991, di Natale 
and Mangano 1995), or on anecdotal observations from 
fishermen (Manday 1964, di Natale and Mangano 1995). In 
contrast, the data for this study were collected concurrently 
from multiple individual fish throughout 3 consecutive lunar 
Figure 2. Lunar illumination schedule compared to (A) hourly mean of nocturnal depths (n = 501) and (B) nightly depth mean 
(n = 99).
A
B
Lu
na
r
ill
um
in
at
io
n
D
ep
th
 (m
)
D
ep
th
 (m
)
0
20
40
60
80
100
120
0
25
50
75
100
125
150
100%
50%
0%
05/20 06/03 06/17 07/01 07/15 07/29 08/12
noCtUrnal distribution of swordfish in western north atlantiC
87
cycles. The described behavior was ubiquitous despite the 
movement of several animals away from the tagging area 
during the study. It is also interesting to note that the correla-
tion between nightly depth and lunar illumination remained 
intact even when the moon was not visible in the study area 
(had not risen or had previously set) during the 0500 to 0900 
GMT window examined. The data presented thus provide 
confirmation of the long accepted, yet unproven, theory that 
changes in nocturnal swordfish depth distributions are cor-
related with the lunar cycle. 
aCknowlEdgEmEnts
We acknowledge the efforts of D.Russ, S. Meister, G. 
Ulrich, S. Burns, P. Mikell, B. White, D. Oakley, and the 
crew of the R/V Palmetto for field assistance. Constructive 
criticism of the manuscript was provided  by M. Reichert 
and J. Geddings, while I. Bagnamanay La Torre Torres 
and G. Perez assisted with Spanish translation. The 
research described herein complies with all current laws 
and animal welfare policies of the United States. Permits 
for the capture and tagging of swordfish were provided by 
the US Department of Commerce, National Oceanic and 
Atmospheric Administration Office Sustainable Fisheries 
(Permit no. HMS-SRP-02-001). Funding was provid-
ed by NOAA Fisheries grant numbers NA07FL0497, 
NA16FL2814, and NA03NMF4720321 (G. Sedberry, 
Principal Investigator). This is Contribution Number 583 
from the South Carolina Marine Resources Center.
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0 20 40 60 80 100 120
0
20
40
60
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140
160
180
D
ep
th
(m
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Frequency
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Nocturnal depth distribution of western North Atlantic swordfish (Xiphias gladius, Linnaeus, 1758) in relation to lunar illumination

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