Seasonal distribution of cephalopod species living in the central and southern North Sea

2005·2.1 MB·English

by  De Heij, A

BASTERIA,69: 91-119, 2005
Seasonal distributionofcephalopod species living in
the centraland southernNorth Sea
A. de Heij
Celebesstraat 5,NL6707ED Wageningen,The Netherlands;[email protected]
&
R.P. Baayen
GijsbrechtvanAmstellaan 18,NL 3703BD Zeist,TheNetherlands;[email protected]
Duringthe International BottomTrawl Surveys and International Beam Trawl Surveys from
1996to 2003, tencephalopodspecies wereencountered inthe centraland southern NorthSea
insixfamilies: Loliginidae(Alloteuthissubulata,Loligoforbesi,Loligovulgaris),Sepiolidae(Sepiola
atlantica, Rossia macrosoma, Sepiettaoweniana), Sepiidae (Sepia officinalis), Ommastrephidae
(Todaropsis eblanae),Onychoteuthidae (Onychoteuthisbanksii), and Octopodidae (Eledone cir-
rhosa).ApartfromA.subulata,noneofthe specieslived inthe investigatedareainlargenum-
bers.Ingeneral,the centraland southernNorthSeaisnotafavourable habitatforcephalopods
due tothe shallowness ofthewater.The occurrenceofindividual speciesis furtherrestricted
bywater temperatureorsalinityrequirements,asshownby theirseasonal migrationpatterns.
These parametersdependonsea depth, time ofthe year, and influx ofwater from outside.
Inside the centraland southernNorthSea,deepwatersarerelativelycool insummerbutrela-
tivelywarm inwinter,while the shallow coastal waters ofBelgiumand The Netherlands are
warminsummer andcoldinwinter.This explainsthe seasonal migrationofAlloteuthissubula-
ta,aspeciesindigenoustothe NorthSea thatprefersrelatively warmwaters.Itmigratesnorth-
westwards beforewinterand southeastwards inlate spring.Asimilarmigrationpatternexists
forthe two otherspeciesintheLoliginidae,Loligoforbesiand L. vulgaris.Temperaturerequire-
mentsofEledonecirrhosadiffer,andsodoesitsmigrationpattern.Forothercephalopods,water
salinityrather thantemperatureis critical. Rossiamacrosomaisrestricted tothe northern range
ofthe centralNorth Seabecause itrequireshighsalinities. Moderate salinities areenduredby
Todaropsiseblanae,aspecies thatoccursinmoderatenumbersinthe westernpart ofthe central
North Sea.Lowsalinities areendured bySepiaofficinalis and Sepiolaatlantica,which spendpart
oftheirlife intheestuaries and mayevensurviveinwaterwith asalinity ofonly27‰.While
SepiolaatlanticalivesintheentirecentralandsouthernNorthSea,Sepiaofficinalisisrestricted to
the coastal waters and estuaries. Numerous other cephalopodspecies occur in the deeper
waters ofthe Atlanticand the Channel. Incidental findings ofunusual cephalopodspeciesin
the centralandsouthern North Sea,such asthoseofOnychoteuthisbanksiiinthepresentstudy,
arelikely due to animalsbeingtakenalongwith theinvadingcurrentsoftheGulfStream.
Keywords: Cephalopoda,North Sea,lifecycle,migrationpatterns,seasonal distribution,water
salinity, water temperature.
92 BASTERIA, Vol. 69,No. 4-6,2005
INTRODUCTION
Underauspices of theInternationalCouncilforthe Exploration of the Sea(ICES), a
numberof European countries perform surveys of the marine faunaof the North Sea.
Fishery surveysof thecentraland southernNorthSeaare conductedby theNetherlands
Institute for Fisheries Research (RIVO), IJmuiden, as part of the InternationalBottom
TrawlSurvey (IBTS)andoftheInternationalBeamTrawlSurvey (BTS).Themainfocusof
the surveys isto collectdataonanumberof commercialfishpopulations forannualfish-
eries managementadvice. Biodiversity studies are also carried out. In this study, the
occurrence and seasonal distributionof cephalopod species in the centraland southern
NorthSeawasexaminedbetween1996and2003.
Theaverage depthofthecentralandsouthernNorthSea,partof thecontinentalshelf
(51°-58°N), is 45 m. Locally, close toScotland, depths of morethan 100 m exist.The bot-
tomofthecentralandsouthernNorthSeaconsistsof sandwithbanksof shingle or clay.
The cephalopod species living in this part ofthe NorthSea thereforearemainly neritic
species, favouringshallowwaters,and livingeitheron(benthicspecies) orjustabovethe
bottom (demersal species). Eighteen species haveso farbeen reported from the central
andsouthernNorthSea(Tesch, 1908;Grimpe, 1925;Stevenson, 1935;Kaas, 1939;Jaeckel,
1958;Kristensen, 1962; Muus, 1963;Lacourt & Huwae, 1981).The dominantcephalopod
species (>90% ofall catches) in the North Seais Alloteuthissubulata(Lamarck, 1798), a
small squid belonging to theLoliginidae. A. subulatais a permanentinhabitantof these
waters. MigrationpatternsofA.subulatawithinthecentralandsouthernNorthSeawere
studied by DeHeij & Baayen (1999). During autumn and early winter, juvenile animals
migrate from thecooling southeastern coastalwaters to the deeper, in winter relatively
warmer, watersof thecentralNorthSea. In spring, theyoungadultsreturn to thewarm-
ing shallowwaters along theDanish, German, Dutch, Belgian, and southeasternBritish
coasts.After spawning, mostoftheadultsdie(DeHeij&Baayen, 1999).
The presentstudy considers all cephalopod species encounteredbetween 1996and
2003in the centraland southern NorthSea. Data are provided on their occurrence and
migration in relationtowater temperatureand salinity.Aimofthe study wastoelucidate
in howfarthe cephalopod species living in thecentraland southernNorthSeadifferin
ecological nicheandseasonalmigrationpatterns.
MATERIALANDMETHODS
Since the1960s, theNetherlandsInstituteforFisheriesResearch (RIVO) participated
in the InternationalBottom Trawl Survey in the central and southern NorthSea. The
research area coveredICES fisheriesareas and divisionsIVb and IVc, and smallpartsof
IVa and Vlld.From 1991 to 1995these surveys were carried out four times a year, in
February, May, August, andNovember. Astandardbottomtrawl(GOV-trawl) wasused
asthesurvey gear.From 1996,thebottomtrawlsurvey was only carried outin February,
while abeamtrawlsurvey was carried outfrom mid-August to mid-September. In this
study, datawere usedfrom 1996 to 2003,and, as asupplement, datafrom other surveys
from1994 to 1996.
For thesampling gridofthesesurveys, theNorthSeawas dividedintorectangles of
onedegree longitude by halfa degree latitude, equalling about30* 30nautical miles.In
every rectangle oneor more 30minuteshaulswere made.For each haul the actualsea
depthwas recorded.Also,water temperatureand salinity wererecordedat5 m depth as
wellas atthebottom.Dataontemperatureandsalinity are notavailableforallhauls(for
De Heij& Baayen: Seasonaldistributionofcephalopod species in theNorth Sea 93
details, see tables). In this study, we used temperature and salinity at fishing depth.
Fishing wasrestrictedtorectangles inopenseauptosix milesfromtheshore.InFebruary,
about 50 out of the 110 rectangles were sampled with the fisheries research vessel
"Tridens"(length 73 m), using aGOV-trawl. Thisis abottomtrawl witha high vertical
net-opening of5to6mand ahorizontalopening ofapproximately20m.Thecod-endhas
a mesh sizeof20mm (stretched mesh). The GOV-trawlis especially used forsampling
gadoids and herring.
In August-September 85-90 rectangles were sampled, by "Tridens" and by "Isis"
(length 27 m). "Isis" mainly coveredthesouth-eastern NorthSea withtwoto fourhauls
perrectangle. The remainingpartsofthe centralandsouthern NorthSeawerecoveredby
"Tridens" (onehaulperrectangle). In thesummer survey bothships usedtwo8 mbeam
trawlswitha60cm highnet-opening and4cmmesh-sizeinthecod-end. Beamtrawlsare
used especially designed forcatching flatfish.Duetotherelatively low verticalnet-open-
ing, they aresuboptimal forpelagic cephalopods. Nevertheless, anindicationofcephalo-
pod occurrencewas thusobtained, thanksto the shallownessofthe NorthSea,the ten-
dency of manyspecies to stay closeto thebottomat daytimeand swim upwards during
thenight, andthelarge numbersofhaulsper year. Onlycatchesfromthestarboard nets
ofbothships were fullyanalysed. Onboardof "Tridens",cephalopod catchesfromstar-
board and rare cephalopod species fromport sidewere collectedand frozenfor further
examinationin the laboratory. Cephalopods caught by "Isis" were not collected due to
staffandtime shortage.
Cephalopods were identifiedaccording toRoper etal. (1984).For allspecies found,
numbersofanimalscaught perrectangleper30minuteswereanalysed. Incaseofanimals
brought ashore, sex was identifiedin sofaraspossible, andmantlelengthwas measured
forallindividuals.
RESULTS
Eight cephalopod species were encounteredin the February surveys between 1996
and 2003 (table 1): Alloteuthissubulata(Lamarck, 1798),Eledone cirrhosa (Lamarck, 1798),
Loligoforbesi Streenstrup, 1856, L. vulgaris Lamarck, 1798,Onychoteuthis banksii (Leach,
1817), Sepia officinalis Linnaeus, 1758, Sepiola atlantica d'Orbigny, 1840, and Todaropsis
eblanae(Ball, 1841). Highbutfluctuating numbersofA. subulatawereencounteredinthis
timeof theyear, from atotalof 1233animalsin 1996to 20,590animals in 1998(approxi-
mately60,000animalsin totalover the8-year survey period). The numberofspecimens
ofthe otherspecies was low,fromamere395animalsofS. atlanticacaught in the8-year
survey period down to 2animalsof O.banksii caught in 1999butnotencounteredin any
otheryear.Usingdataonthenumberofhauls,hauldepth,salinityand temperature(table
2), frequency distributionswere calculatedforeachspecies according to depth, tempera-
ture andsalinity (table3).
Nine cephalopod species were encountered in August - September surveys by
"Tridens"between 1996and 2003 in deep water: A. subulata,E.cirrhosa, L. forbesi, L. vul-
garis, Rossia macrosoma (Delle Chiaje, 1829), S. officinalis, Sepietta oweniana (d'Orbigny,
1840),S.atlantica, and T. eblanae(table4). Thenumberofanimalsperspecies was low (A
subulata, E. cirrhosa, L.forbesi, S. atlantica) toextremely low (L. vulgaris, R. macrosoma, S.
officinalis,S.oweniana,T.eblanae). Fiveofthesespecies werealsoencounteredinthecatch-
es of"Isis" in shallower water: A. subulata, E. cirrhosa, Loligo spec., S. officinalis, and S.
atlantica. In theseshallowerwaters, highernumbersof A. subulataand S. officinalis were
encounteredthanin deeper waters, whilevery lownumbersor noneatallwere encoun-
94\1 BASTERIA, Vol. 69, No. 4-6,2005
Plate 1. North Sea cephalopodscollected duringthe RIVO surveys. 1, Alloteuthis subulata (Lamarck,
1798),a female, b male. 2, Loligoforbesi Steenstrup, 1856.3,LoligovulgarisLamarck, 1798. 4, Todaropsis
eblanae(Ball, 1840).
De Heij& Baayen: Seasonal distributionofcephalopod species intheNorth Sea 95\2
Plate2.NorthSea cephalopodscollected duringtheRIVOsurveys. 1,Sepiolaatlantica d’Orbigny,1840,a
dorsal,b ventral. 2,Rossia macrosoma(DelleChiaje,1829).3,Sepiaofficinalis Linné, 1758.4,Eledonecirrhosa
(Lamarck, 1798).
96 BASTERIA, Vol. 69,No. 4-6,2005
teredforE.cirrhosa,R. macrosoma,S. oweniana,andT.eblanae.Totalnumbersofanimalsfor
thenine species caught inAugust and Septemberby "Tridens"and "Isis"revealed very
lownumbersofA. subulatainSeptember compared withFebruary (table 1).Numbersof
E. cirrhosa, R. macrosoma, and S. officinalis were higher in September than inFebruary.
Using dataonthe numberof hauls, haul depth, salinity and temperature(table 5), fre-
quencydistributionswere calculatedfor eachspecies according tohaul depth, tempera-
tureand salinity (table 6).
Ecological and seasonal distributionpatterns forthe ten species encounteredinthis
study are as follows. In the February surveys, A. subulata, E. cirrhosa, L. forbesi, and T.
eblanaeweremainly orexclusively (Ei.cirrhosa) presentin deeper water(depth class80m).
In thistime oftheyear, L. vulgaris andS. atlanticapreferred moreshallow waters (depth
classesfrom23 to 60 m). Thespecies also differedintemperaturepreference. A. subulata,
L.forbesi, and T.eblanaepreferred water temperaturesof 8 °C. L. vulgaris and S.atlantica
preferredwatersof8-9.5°C, whileE.cirrhosapreferred relativelycool waters(68 C). Most
species were foundat high salinities(35.2 %o), except forS. atlantica, which wasfound at
salinitiesofbetween34.4and 34.8 %o. T.eblanaewas almostexclusively foundatthehigh-
est salinity(35.2 %o).In theAugust andSeptember surveys, A. subulatawas mainly pres-
ent inshallowwater (23 m and 40m depth classes) at 15-18 °C.Asimilar preference was
foundforL.forbesiand L. vulgaris,species thatwerefound atmoderatesalinities(34.4-34.8
%o) E.cirrhosa, R. macrosoma, S. atlantica, and T.eblanaepreferred deep waters (80 mand
100m depth classes) withatemperatureof6-12°C andasalinity of34.8-35.2%o.
LOLIGINIDAE Steenstrup, 1861
Alloteuthissubulata(Lamarck, 1798)(Plate 1 fig. 1a,b)
InFebruary, large numbersofA.subulatawere foundin deepwater (depth class80m)
in the central NorthSea and close toScotland, at temperaturesaround8 °C and a high
salinity (35.2 %o) (tables 1,3; fig. 1).Specimens caught in February wererelatively young
(mantlelength2-14 cm, mostly4-6cm). Sexidentification,basedonthemorphology ofthe
hectocotylus, is possible foranimalsof6 cmandlarger.Malesgenerallywerelarger than
females; at8 cm dorsal mantlelength, femaleswere dominant,whileat 11-14 cm dorsal
mantlelengthnearly onlymaleswerefound(table7).InAugust-September, lownumbers
of A.subulatawere encountered,mainly in shallowwater (23m and 40 m depth classes)
at 15-18 °Cand 34.4%o salinity (tables4, 6).
Loligoforbesi Steenstrup, 1856,andL. vulgaris Lamarck, 1798 (Plate 1 figs2, 3)
InFebruary,moderatenumbersofL. forbesiandL. vulgaris werecaught, partof which
were incomplete and couldnotbeidentified(table 1).L.forbesi was foundat80m depth,
8.0°Cand35.2%o salinity (table3), atsimilarplaces asA.subulata(fig-2). L. vulgaris was
found ataslightly widerrange of conditions(table3), in the southernNorthSearather
thanclose toScotlandas forL.forbesi (fig.3). Specimens ofL. forbesi wereslightly larger
(11-43cm) than those ofL. vulgaris (7-34 cm); in bothspecies, maleswereslightly larger
than females (table7). In September, both species werefound at23-40 m depth, attem-
peratures of 15-18 °C and salinitiesof 34.4-34.8%o (table 6), in similar numbers as in
De Heij& Baayen: Seasonaldistributionofcephalopod species in the NorthSea 97
February. Animals caught in winter were mostly adult (table 7). Animals caught in
August-September were mostlyjuveniles, withtheexceptionof afewspecimens ofLoligo
forbesi (coexistence oftwo generations). InAugust-September, most animalswere caught
in the coastal waters of The Netherlands, Germany and Denmark by "Isis" but could
unfortunately notbetakenashoreforidentificationatspecies level(fig.4,table4).Ingen-
eral, theecological preference ofboth species was similarto thatofA. subulata(tables3,
6), butin winterL. forbesi findsthepreferred conditionsin thenorth-westernpartof the
centralNorthSea, whileL. vulgaris additionally occurs intheEnglish Channel(figs 2, 3).
SEPIOLIDAELeach, 1817
Sepiola atlantica d’Orbigny, 1840 (Plate2 fig. 1a,b)
In all years of the study, moderate numbers were caught in February (table 1).
Animalswere found at all depths, temperaturesand salinities, although warmer waters
(9.5 °C) were favoured as well as moderatesalinities (34.4-34.8 %o) (table 3). S. atlantica
was foundin theentire centralandsouthernNorthSea,particularly closetoScotland and
in theChannel, butnot in thecolder waters to the northof theWaddenSea (fig.5). In
August-September, similarnumbersofanimalsasin February were caught insomeyears,
whileinotheryearsvery fewanimalswerecaught ornoneatall(table4). Thisspecies was
encounteredagainoverthefullrangeofdepth,temperatureandsalinityconditions(table
6), with apreference for deeper waters (100 m) as present close to Scotland (fig. 5). In
February, size ranged from 1.0to2.5cm,in August-September from1.0to3.1 cm. Males
andfemaleswereequally sized and presentin equal proportions (table8). Frequency dis-
tributionsfortemperatureand salinity classes in August-September suggested that two
populations may beinvolved, oneconsisting of larger animals(on average2.5 cm) pre-
ferringcolderwaters(9.0-12.0°C), theotherconsisting of smallerones(onaverage 1.9cm)
preferring higher temperatures(21.0°C), andbothliving at34.7-34.9 %o average salinity
(table 6).
Rossiamacrosoma (Delle Chiaje, 1829)(Plate 2fig.2)
R. macrosomawas notencounteredin February. In August-September, animals were
caught incidentally in deeper (100 m) and colder(6.0-12.0 °C) waters witha high salinity
(34.8-35.2 %o) (tables 4, 6). Theanimals werein thesize-range of 3-7 cm and consisted
mainly of females (table 8). Differences in size between males and females were not
observed.R. macrosoma wasonly foundin thenorthernpartofthesurvey area(fig. 6), in
therectangles fishedin August-September butnot in February.
Sepietta oweniana(d’Orbigny, 1840)
S. oweniana was not encounteredin February. Four females sized 3.5-4.0 cm were
caught inAugust-September 1999, close to thesouthern coastofNorway, in 60m deep
waters at6.0-9.0°C (tables 4, 6and 8). S. owenianawas only found in thenorthernrange
ofthe survey area (fig.7). Sincethese rectangles were not fished in February,no conclu-
sions canbe drawnonseasonaldifferencesindistribution.For thesecatchesnoinforma-
tiononsalinity isavailable.
98 BASTERIA, Vol. 69, No. 4-6,2005
SEPIIDAEKeferstein, 1866
Sepiaofficinalis Linnaeus,1758(Plate 2fig.3)
InFebruary, fourincidentalfemalesof S.officinalis sized 11-23 cm were found atvar-
iouslocationsand conditions(tables 1,3;fig.8). S.officinaliswas moreregularly caughtin
August-September, in shallow (23-40 m depth) and warm (18.0-21.0°C) coastal waters
and in the Channel (tables 4, 6; fig. 8). The distribution over the size classes (table 7;
including datafrom"Isis" from 1995)revealed thepresencein August-Septemberof two
generations: large numbersofjuveniles (2-8 cm; too smallforsex identification),andlow
numbersofadults(11-20cm,bothmalesandfemales). S.officinalis inhabitswarmer south-
ernwaters duringwinter,and migrates toshallow,warm coastalwaters duringsummer.
OMMASTREPHIDAE Steenstrup, 1857
Todaropsis eblanae(Ball, 1840)(Plate 1 fig.4)
InFebruary, T. eblanaewas incidentally found,except for an invasion ofanimals in
1998(table 1).Theanimals(5-11 cmlong; too smallforsex identification) livedin deeper
waters(60-80m)ofthenorthwesternpartofthecentralNorthSea(fig.9), mainly at8.0°C
and 35.2 %osalinity(table 3). InAugust-September, lownumbersof T.eblanaewerefound
in mostyears;invasions werenotobserved(table4). Theanimalslivedinslightly deeper
waters (80-100 m) in the same region and at similar temperaturesand salinities as in
February (table 6; fig. 9). Females were more abundant andslightly larger thanmales
(table7).
ONYCHOTEUTHIDAECray, 1849
Onychoteuthis banksii (Leach,1817)
Thesingle findings ofO. banksiiwere madein February 1999.Oneanimalof11.5cm
(sexunidentifiable) was caught inthecentralNorthSeaat84 m depth in waterof7°Cand
35.0%o at 56°07' N2°8' E.Asecond,femaleof 17.0 cm was caught in theentrance of the
Channel at37m depthin water of7.4°Cand35.2%osalinity at51°25'N23' E(fig. 10).
OCTOPODIDAE d’Orbigny, 1845
Eledonecirrhosa(Lamarck, 1798)(Plate 2fig.4)
InFebruary, E. cirrhosawas incidentally encounteredclosetoScotland, at80m depth,
6.0-8.0 °C and salinitiesof 34.8-35.2%o (tables 1, 3; fig. 11). In September, considerably
largernumberswereencounteredinthesameareaand deepersouth and eastintheNorth
Sea,inwaters withsimilardepth(60-100 m), temperature(6.0-12.0°C)andsalinity (34.8-
35.2%o) asin February (tables4,6; fig. 11).Thesmallcatchesin February arelikely dueto
theuseof abottom trawlthattouches theseabottom muchmore lightly than the beam
trawlsusedinAugust-September, thus allowing animalsto escape.The seasonal distri-
butionpatternsuggested temporarysouthwardmigrationofE. cirrhosafromthenorthern
DeHeij& Baayen: Seasonaldistributionofcephalopod species in theNorthSea 99
NorthSeaintothenorthernpartofthecentralNorthSea duringspring and summer.The
animalscaughtinAugust-September weremainly females.Twenty ofthe22males found
overallyearswerecaught in asingle year(2000),despite similartotalcatchesofE.cirrhosa
inautumn of1998,1999and2000(table4).Theanimalsmainly hadadorsalmantlelength
of5-8 cm,incidentally upto 14cm. Individualweight was alsorecorded,becauseofdif-
ficultiestomeasurethedorsalmantlelength accurately. Animalweightincreasedconsid-
erably with size (table7).
DISCUSSION
Ten cephalopod species were encounteredin thecentraland southernNorthSeain
the IBTS and BTS surveys from 1996 to 2003. These species belonged to six families:
Loliginidae (Alloteuthis subulata, Loligo forbesi, L. vulgaris), Sepiolidae (Sepiola atlantica,
Rossia macrosoma, Sepietta oweniana), Sepiidae (Sepia officinalis), Ommastrephidae
(Todaropsis eblanae), Onychoteuthidae (Onychoteuthis banksii), and Octopodidae (Eledone
cirrhosa). Of these ten species, R. macrosoma was encounteredonly at the northernmost
partoftheinvestigated area,whileO. banksiiand S.owenianawerevery rareand seemed
to have entered the centraland southernNorth Sea accidentally. Despite 459 hauls in
February 1996-2003(equalling 230 fishing hours) and 1114hauls (557 fishing hours) in
August-September 1996-2003,nomorethan2and 4animals, respectively, werecaught of
thelattertwospecies. Theremaining species (A.subulata,L.forbesi, L. vulgaris, S.atlantica,
S.officinalis, T.eblanae,E.cirrhosa) maybe consideredinhabitantsofthecentralandsouth-
ern NorthSea. Of these,only A. subulata livesinthe investigated area in largenumbers.
The same seven species were also consideredregularly present in the "southern North
Sea" (=thecentralandsouthernNorthSeaof thepresentstudy) by Muus(1963).The lat-
ter authorconsideredOctopus vulgaris Lamarck, 1798to alsoinhabitthis area, as recalled
by later authors (Janssen, 1975; Lacourt & Huwae, 1991). Grimpe (1925) reported the
species only from coastal waters in Belgium and The Netherlands. Kristensen (1966)
believedthat O. vulgaris wasa rare invaderofthe southernNorth Sea. O. vulgaris seems
to have disappeared fromthe centraland southernNorthSea around 1965(Boer, 1971)
andwasnotrecordedinthepresentstudy.
ThecentralandsouthernNorthSeaformsa very shallowpartoftheEuropean conti-
nentalshelf. Thecephalopod species livingthereare mainly neritic,andoftenbenthicor
demersal,species. Of the11orders, 50families, 154generaandapproximately 718species
inrecent Cephalopoda (Nesis, 2003),7 families(2partially),42 generaand 371 species are
consideredneriticor partly neritic.Five of the 7 neritic familiesinhabitthe centraland
southern North Sea (neritic: Loliginidae, Sepiolidae, Sepiidae; partially neritic:
Ommastrephidae, Octopodidae). TheOnychoteuthidae, represented in our study by two
isolatedfindingsof Onychoteuthis banksii, areoceanic(Nesis, 2003).TheOmmastrephidae
arepartially neriticand includeseveral very largespecies believedtobelongtotheDutch
fauna(Janssen, 1975;Lacourt & Huwae, 1981) but not encounteredin this study. All
reportedfindings ofsuch large animalspertain toaged animalswashedashore,possibly
after spawning. Such animals have likely entered thewaters ofthecentraland southern
NorthSeaaccidentally (Jansen, 1975).
Thecephalopod species inhabitingthecentralandsouthernNorthSea(A. subulata,L.
forbesi L. vulgaris, S.atlantica, R.macrosoma, S.officinalis, T.eblanae,E.cirrhosa)cover amere
1%ofallcephalopod species andnomorethan2.5%of theneriticcephalopod species.The
centralandsouthernNorthSea apparentlyarenot afavourablehabitatforcephalopods:
only one species (A. subulata) occurs in large numbers,theothers occurinrelatively low
100 BASTERIA, Vol. 69, No. 4-6,2005
numbers.The quantities of squid caught by commercialfisheriesin the northernNorth
Seaand northwestof Scotlandare far higher thanthosecaught inthe centralandsouth-
ern North Sea; still larger quantities of squid and also cuttlefishare landed from the
English Channel(Pierceetal., 2002).Landings of E. cirrhosa andO. vulgaris fromthecen-
tralandsouthern NorthSea are nearly zero. In contrast, 35 species ofcephalopods have
been reported by Muus (1963)from thecoastalarea of the northeasternAtlanticOcean.
Ecological constraintsare the shallownessof the water,salinity, temperatureand depth.
Many cephalopods live in deep waters,and theshallowwaters of theinvestigated parts
of theNorthSea donot seemto offerfavourable conditions.According to someauthors,
animalsmay encounter problems withthemudonthe seabottomcoming fromthe rivers
and settling in shallow waters (Jaeckel, 1957). Remarkably, the Myopsida [including
Loliginidae and Sepiidae (Jaeckel, 1957)], with protective eye lids, dominatein these
waters. Asimilarprotective, moveableeye lid (Jaeckel, 1957)ispresent in E.cirrhosa, the
singlememberof theOctopodidae encounteredinthisstudy. Waterswithhighertemper-
atures(such as theMediterraneanSea) or greaterdepths(such as theAtlanticOcean) are
inhabitedby farhigher numbersofcephalopod species (Muus, 1963; Collinsetal., 2001).
Theoccurrence, seasonaldistribution, ecological nicheandmigrationpatternsof the
ten species encounteredinthis study arediscussedbelow.
Alloteuthissubulata
A. subulata is a neritic, demersalspecies associated withsandy and hardbottoms
(Roperet al., 1984).Jaeckel(1958) considersit an necto-pelagic species, living to a depth
of200mand preferring sandy, muddy orhardseabottoms.Stevenson(1935) considered
A. subulatato be thecommonest cephalopod in the deep waters [40 fathoms(=72 m) or
more] under the Yorkshire coast in winter andearly spring. This agreeswith datafrom
thepresent (fig. 1) and a previous study (De Heij & Baayen, 1999). Large numbers of
younganimalswere caught in February, whilemoderatenumbersof full-grown animals
wereencounteredin August-September, inaccordance withGrimpe (1925).Mostanimals
caughtinwinterhadprobably diedbyAugust-September, afterspawning in MayorJune.
Juvenileanimalshatchedfromtheeggswere too smallinAugust-September tobecaught;
they are foundfromNovember onwards(De Heij &Baayen, 1999).During autumn and
early winter, thejuveniles migrate fromthecooling southeastern coastal waters tofeed-
ing grounds in the deeper, in winterrelatively warmer, waters of the centralNorthSea
and eastof Scotland (DeHeij & Baayen, 1999).In spring, the youngadultsreturn to the
warming shallow coastalwaters along theDanish, German, Dutch, Belgian andsouth-
easternBritishcoastsforspawning. Ecological niche and seasonal migration in relation to
water temperaturehavebeendiscussedin detailby DeHeij& Baayen (1999).A.subulata
isfullyadapted totheecological conditionspresentin thecentralandsouthernNorthSea
and completes itslifecyclein thesewaters.
Loligoforbesi
L.forbesiis anecto-benthicspecies thatprefers deeperwaters (to 400m ormore)than
L. vulgaris (Jaeckel, 1958). Grimpe (1925) consideredit to inhabitthe entire North Sea,
exceptclosetothe coastoftheNetherlands.On theother hand,Kristensen(1966) report-
ed thatL.forbesimigrates inspring fromnorthern waters towardsthe coastalwatersofthe
Netherlandsto depths of 30m or,when water salinity is sufficiently high, closerto the