Pack o r s aff ec studied in f i
nFdll*rc,*
Thesisl &s.t i-t;utt for Fi&eribioZogio Untverslty of Bergen, S p r i w * 1979,
October l900
Thesis, Xns-titutt fox f i slcoribiologZ,
Univ* Bergen,
8pri- m ..--
FACTORS UPTCTING PmP1SHmG OF N0RW.Y LOBSTER w n o n r w . ) ANU PINK SHRISP ~ C q d ~ e x ) ~ STUDIED I1T 1~1SIIILJG ADD
BESEnVIOUR E X P m m T S o
-
by Jon dlmund B j ordalCOPJTIQTTS as i n Momre
P o r i g i n a l
FOnmORD
TEE P m
am1m
l, Introduction
2, ?ile.thods, material aml r e s u l t s
2 I B a i t experiments i n m aquarium 2,2, Pot expeximen-t s i n an a q u a r i u m 2, 30 Fishing experirnelit s
3, Discussion PTORIiJAY LOBSTER 1, Int zooduct ion
2, Tib-kerial a d methods 2,1, Fishing experimen-'c s
2, 1 l Ekpeximen-l; a1 areas 2,1,2 Vsasels
2,1,3 G w r a i d Pisliing method- 2,1.4. B a i t and b a i t i n g method 2,1,5 Fi sliiqy t51:le
2* 2, Biological investigations 2,3, Observation of behaviour
30 RESULTS
30 1 o Fishing experiment S
30 101 Grounds
3.1*2 Catch r e s u l t s f o r t h e d i f f exent types of pot SO 1 o 3 C o r ~ p r a t i v e f i s h i n g experiment S
30 1 0 4 Comparative b a i t experiment S and bait: i n g met hod
30 1 5 Seasonal v a r i a t i o n 383.6 Fishing time
3 ~ 1 6 7 Distribution accordixg t o aex
3 o 1.8 Distribution acoording t o length and weight 3, 'l Bycatch
/
fawa composition3* 1 10 PIPat chinessqp (pakclUr d i s t r i b u t i o n ) 3010'1q Tagging
302 Observation of behaviour
3a20 1 Gwaexal p a t t e r n of movemant 3* 2,2 31iwaal a ~ t i v i t y
3,203 Angle of incidence i n r e l a t i o n t o d i r e c t i o n of current
3 0 2 0 4 Behaviour i n r e l a t i o n t o pots
3r 2,5 Obsomation of ?k?om?raty l o b s t e r s i n o r near burrows
3.2* 6 Bai-t: i n g method
3*2.'f Disouasion of d e ~ i * of experiments 40 Discussion
4*1 Grounds
2 Factors af f oc-'c ing cat clning cf f ick ency
4.-
2* 'l Factors r e l a t i n g t o Xomay l o b s t e r$*2*2 Factors r e l a t i n g t o c m r e n t d02,3 Factors r e l a t i n g t o b a i t
&2.4 Factors r e l a t i n g t o pot
4 , 3 S i g i f i o m c o of f a c t o r s r e l a t i n g t o t h e pot from point of v i e w of t h e rala-tive catching efficiency of the various types of pot
4#4
Fishing time4.5
Distribution accordiag to s i z e a d sex4e6 Tagging
4*7 S% mlcs (%@at ion)
5*
Smnmy6 Aolmotd. edg emeiik a
7. L i t erat w e ( ~ i b l i o ~ r a ~ l l ~ )
30
~ u ~ o & ,
a~ ~ o )
-*a "WO
3)oo.r catch resuPts f o r pirilc shrimps cml be a t t r i l m t e d t o a nxn'ner of different factors: type of pot b a i t g seasonal v a ~ a t i o n p depth, populaticju density, behaviour and n u t r i t i o n , S w o r a l .i;3rpne of' pot were t e s l e d , inclucling t h r e e giving catches of pink shrimps i n an a c ~ a r i z m and ono t hat provided tmrt 111&ile oa,tches of t h e same species i n Alaska, (BAM 1970)~ fn. A2aslca sorae of t h e ccxtohes were obtcined at the bo-t-l-orn (90 m ) m d some were polaglo catches from d i f f ezaen-l- dep-lhs, with h@xaring as 'baitu Trav~ling operations i n t h e sane a r e a yielded- catchea of pi& shritnpa a n c u ~ t i . ~ to ~ibou-G 50 per +t;rawlj.ng h m r r . The same -l;ype of pot t h a t vms usd. i n Kiaslca was tes.ted i n Eywe~!ms where
trawling catches of shrimps came t o qfi-2Cj k g per h o w of:' trawling i n t h e s m ~ j periodo Yypica3. t ~ a w l c3tches of rrk~irn:p i n Rornadalsf j o r d and i n t h e Sergeu, a r e a were even l e s s (noma,~ t ? a ~ I c8t'cchea i n Ravaefjord a r e g-10 1% p 3 r hot?:-
\ \
(B* TQnt cvik, Xnatitutt f o r rnarinkriologl, Espeg~or~d., persox aZ conaw~ictikia~i,
,
So a r e l a t i v e l y low popzitation d e ~ . s i t y mar have becm an im$or.i;ayst f a c t o r with regmd -to t h e poor catch rest~Z"c~ But tke i n s i g n i f i c a n t pot c a t o h ~ s , compared t-j-ith t h e reatilt S froin R L a ~ l c a ~ a r e ov.-t of a l l proportion t o t h e dlPf erevlc e in t rap31 catchex,
Herring were useci as b&-i; for %lie fishing exjprimen-ta i n L.yngen. Othax* types of b a i t were a l s o t r i e d , but t h i s dlid not r e a u l t i n b e t t e r catches, Thert?
was no tendency f o r catches -to increase 8t &iffarent Limes o f yearo The
f iehing expzrimen-t S were c a r r i od out at great er dept ha t haul t h e com~e~pdnding t r i a , l s i n f l L q ! k ~ ~ , , . and -this m.sy h8.w had a cr3rLd11 effect on catch r s s u l t s ,
WiR (2970) supgestec? t h a t t h e ext enaiva vor.tica1 migration dbscived i n +he
oase 4kf pidc shrimps i n ICacbema,?~ Btiy ~l>ms rieinly a feolttng mfgra,ti~n. He showed that t h e d i e t of s . b i m y s i n t h e 3xoa laiygoly consieted of eooplmiLc-kon, espscf a l l y crab larvaeo Ilnxlyeia 0.k' fitomach contm", f m m pink ahimps 1x1 l!J~m?~ic?;n wat ore poin-Ls t o a Inore bot,tomorientm-t B& k)anf c d i e t 8 p a r t s of oopepods, almelids, bolo-Lhwians, Ra,llLolaria, Foraminifera, apoMgaso green algae, Mat oast Perid-inia a3.d Tintinni-d-m, a l l inixocl with mud (woLLEBE~~I~' 1 903) 0 These s t ~ ~ . d i e s pcss?-bly kniLica,t e t h a t C i f f erence i n d i e t a d behaviour i n
oowlec-;ion l.;i-tl.- f nud intake 111ay be rel.evmt -to t h e low catch r e s u l t s obtained f o r pidr ~b-impu oa -the NoX%~egic7,ul coast,, ca~njpared wit11 those i n Alaska*
4@1. Grounds
-1DrrrLL*LQY..P.mm.=a
The Nomay 'lobster has, googxizphically.,, a ;ri.d~ azLr.?a of dis-bribu-tion, with s
depth range of from 20 t o 800 m (FIGUFLF:EDQ & EIO?':diS 1 9 6 7 ) ~ This mems thzl;
+;he species poseesseu relatj.vely largo Loloraxlce thresholds i n r e s r s c t of' such 'hydrographic indices as t emperattwe, salini-t y and oxygen conC en-t a The B o ~ t t W Iobstox a l s o has an extremely vai*ied d i e t , wi-i;h p o 3 . y c h a e t ~ ~ , ou~xs-LaceanS ail mollt~scs as it tx riain food ( ~ 1 3 0 f . 1 ~ ~ ~ 6; DAVIC301T 9?G2)*
The Nomay l o b s t e r digs tmmel-shaped burrot~s in tlie bottom substrata w d s t a y s inside t h m when i t i s not mov5ng about on t h e bottom (-IXBEXQl' e t ale 1965:
FdU2.T~~ 19749 RICE E: CCHRPb!fA'N 1971 ),
Dwii1.g t h e f i s h i n g experiinents Nontag 2 0 b 8 . t ~ ~ ~ were only caught at gmunds
whew t h e bo-t.hm rias ~ o P t (~l.a,~/rnud. b o - t t ~ ~ ) ~ IjllaPysi~ of sedi.rfi3n t from a bot-torn a m p l e Ldcen. Prom Rawnof jord showod t b t .the bottom subst r a t e at a IYoP~nray
l ~ b s t e x grow& consists of eilL, This i s i n agxeomenl; with s i s i 2 a 8tud.ies r e l a t i i g t o Borway l o b s t e r g;ylou.~ds in t h s J r i ~ h Sea (F~'sRT~~ER 'l9751)~
The Normy l o b s t e r appears -l;o be dependent on i h a t type of bottom, especially fort t h e pwpoaas of being able .to dig bwrowa, and t h e other f a c t o r s a f f e c t i n g 6f s t ribut ion are probably 09 lninor impou"canoeg compare6 with t h e cornposit ion of t h e 'bottom substrate,
U - 2 - i -
BRTJfiYP (1372) describes t h e chasacterisfics of po-~/trra,~ gear as f o l l o t r ~ : OPTheso a r e fmplerncnts i n whicli t h e fish enters volvxtarily but i s liampored f ram coming out
.
A few sots o f po-t goay arc: b,wed ~n t h e idea o f t h e victil?l going i n with t h e ass5.sta:nce of t h e 7-eading net or owing Ls ,the presence of s t r u c t w o s
simula-t; ing it S habit at, whereas other% are equipped wi-th sbimulux sowces, sucl? it^ b a i h liighko etc., t h a t can lure the victim over relatively l a r g e distanceso In th:is s-tud.y pots ?rere used w i t l n bskt as t h e stimulus t o ~ v t t r a c t Bornray lobst erss The f i s h i n g method u a n made up of two components: b a i t
f l u r e ) m& P3-k (39 f i s h i n g
The catching procesa w i t h t h i s method 02' ?ishi112 can be divided up i n t o t h r e e stages:
1) Luring stage
-
t h e IJorway l o b s t e r is lured t o t h e pot by t h e b a i t a c t i n g a s stimulus,2 ) % t r y stage t h e Norway l o b s t e r i s i n t h e immediate proximity of t h e pot m d possibly e n t e r s it.
3 ) Escape stage,
The catch (C) i s t h e difference between t h e number of Norway l o b s t e r s gcing i n t o and escaping Prom t h e pot: C = I
-
E* both I and E ( i n and escape) being i n proportion t o t h e number of Emway l o b a t e r s a t t r a c t e d t o the pot(X), The catch equation can therefore ba ct;nire&ed a s f012.0~~1:
X E
C = (i) N ( c )
.
N = ( i - e ) ~ = c.
N, wbere (3.) =3 ,
( e ) = -- N znd cexpresses t h s combined catching effPc,le~cy of pot and b a i t ,
The c n a r a c t e r i s t f c s of t h e baiL and t h e pot are f,wtors t h a t can be controlled, but catch ad.catchine; efficiency a l s o depend on Nexterr.alw f a c t o ~ s , such as t h e current and t h e behaviour p a t t e r n of t h e Norway Lobster, The o w r e n t i s vezy important indeed S ~ o m t h e pairit of view of t h e s i ~ e cf t h e area %hat can be covered by t h e spreadk~ig arms of t h e b a i t , whereas t h e likelihood of a response t o a b a i t stirc!.us cf a p a r t i c u l a r streungth clepe~da on t h e Norway lobst e r f s degrec of n;otivr;t ion ( f a c t o r s such as seasal, time of day aYld n u t r i t i o n a l s l a t e being i n v ~ l v e d ) ~
I w i l l now t r y t o evaluate t h e individual Pac-tors thwt can, be presumed t o be of importmce during t h e 3 stages of t h e catcinb~g pzocess.
The Narway l o b s t e r spends considerable p a r t s of t h e day (i.e., t h e 24 hours) i n I t s bwlrow, and a c i i v i l y outside -the bwrcrir c m be mainly linked with i t s xea;*ch f o r food f ~ l 3 X k ~ u l . i & RICE 1971, C I - S A m 9 JSHDSTOTTTE & RICE 1975).
Cztching af Norway lobstcrx i n pots w i l l probably be confined t o periods of a c t i v e searching f o r food,
Tim_a
of
i(i. CHAPW a t al. (1975)and C I M W & HOIrJARD f 1979) s t a t e t h a t t h c Namay lobstervs pexbiods of d i w n a l a c t i v i t g can be rela-ted t o optirriurn l i g h t i n t e n s i t y of from 1 t o 1oW5 Lux (on t h e bottom). This optimum le'ral of illumination w i l l occur* a%
d i f f e r e n t depths at d i f f e r e n t times of day, so t h e Norway l o b s t e r w i l l display n o c t w n a l a c t i v i t y i n r e l a t i v e l y sBilJ-low water (30 daytime a c t i v i t y a t f a i r 1
separat e periods
However, tCe Norway l o b s t e r stocks investigated i n Lysofjord i n J u l y ;5tt a 4epth of '1'15 m showed a marked yattcrrl of nocturnal a c t i v i t y , rrld t h i s i s noi;
i n keeping withthe theory of daytime a c t i v i t y i n f a i r l y deep wateY, It i~
therefore reasonable t o assume t h a t a,ctLvity of Iforway l o b s t e r s is low i n t h e daytime, even a-t considerable depths, Acccr?Suyf t o studies of behaviour ( i n 3uly), catching of Norway l o b s t e r s i n pots w:i.ll be confined t o t h e
period between 16 hours and 07 hours, with strongest probability of capt7ne between sunset and sunrise.
Season: Light i n t e n s i t y on t h e bottom w i l l vary according t o season of
-
t h e yearo Long diurnal periods with a r e l a t i v e l y high illumina%ion level.w i l l probably r e s u l t i n low a c t i v i t y of Gomay l ~ l ~ s t e r s i n t h e s u i e r semester ~0mparad with t h e wintor senlcster, %lis may be t h e reason f o r reduced catches i n summer,
m s t a ter M.ACKIE & SIEL'PQl'J (1 972) sllo~fcd t h a t uulde~~f ad lobst szrs dj.splayed a signif i c m t i y g r *at er respdnse t o d i f f e r m t concentrations of b a i t material than l o b s t e r s having azcess t o an abundant food supplye T ~ G same could be asawned t o be true 02 iT,rm~ay l o b s t e r s f so t h e degree of
motiviation f o r a response t o t h e bd-i; stimulus increases as t h e f e e l i n g of s t a r v a t ion increaseso
I n h i b i t i w factors: ff t h e Norway l o b s t e r f e e l s motivation inducing i t t o search f o r food, i t w i l l react p o s i t i v e l y t o a b a i t stimulus n';. moving towards t h e source of t b c stimulus, A t t h e l u r i n g and ontry utagas t h e l e v e l 02 motivation c m be reduced by t h e influense of a variety of
disturbing f a c t o r s o i n t h e form of inter- o r i n t r a s p e c i f i c influences o r a scaring e f f e c t produced by t h e pot structure, etso An i n t r a s p e c i f i c
v were influence was observcd durirlg t h e behaviour study when Norway lobster-
on several occasions driven out of t h e f i e l d of vision by others, Such confro.iitations would seem t o be commonest among Bowny l o b s t e r s of
approximately t h e same size, Even t h e pot i t s e l f can have a scaring e f f e c t which w i l l r e a w e t h e l e v e l of motivation, so t h e >Torway l o b s t e r w i l l not t r y t o go any oloser .to t h e b a i t (go i n t o t h e p o t ) o This can be a t t r i b u t e d t o t h e repel3 ent e f f e c t of aromatic substanoes emmlating from t h e pot
material with w h i ~ h t h e n e t t i n g is imp~egnat ed, *%me11 of l plasticvt, &co )*
When a preWcor i s faced with an ~mfarnl'l-iar prey these i s i,Pten a 1atal-t period of varying length (time between location o f prey and succ,e~r;.Cxl.
attack), HABE (1971 ) and GODIN (1978) indiczted t h a t such l a t e n t periocls occurred i n t h e case of salmono Typical predator reactions when confronted with an unusual prey a r e f e a r and apx:oaoh/retreat responses.
Smell of b a i t (froin t h e d i f f e r e n t types of b a i t t e s t e d ) i s probably not Eel-!.
as wlusual by t h e Norway lobster, T b s pot, cm t h e other hand, i s an
unaccustomed struct~re i n connection with t h e pre,y (S t h e b a i t ) , Reactions of f e a r were observed i n rela-tion t o -the pot (spreading of t h e l a r g e pincers), Approach and r e t r e a t responses were alsq comon, and a l a r g e proportion
(43$) of t h e Xomay l o b s t e r s nnder obeervation kept at a distance from t h e pot and were not i n physical con-bacf w:&th it, This could be a reaction t o unfamiliar prey and may be regazdec as a v i t a l i n h i b i t i n g f a c t o r a f f e c t i n g t h e c a t c h i ~ g prccuas,
2,2
-
F a c z r s r e l a t i n g .to currentSpread of t h e b a i t aroma determines t h e s i z e of t h e area round t h e pot where t h e Norway l o b s t e r can be a f f e c t sd bjr chemical stimuli Pfom the b a i t , Bait aroma material is spread i n two d i f f e r e n t ways: l ) by d i f f u s i o n and 2 ) ky tha current* Spreading by means of diffusion i s a slow process 3nd i s praobably of l i t t l e p r a c t i c a l importance, The distance over which t h e b a i t aroma material is spread, would therefore be depend-ent on t h e currento
m::
The number of Norway lcbs'iars lurzd t o t h e pot (B) w i l l be i n p:.-oportion t o t h e ax-ea over which t h e b a i t aroma material i s spread, That area w i l l i m r e a s ~ , wY.th increasing speed of current, The concentration of b a i t aroma material pep u n i t of volrune of s e a water passing t h e b a i t w i l l , on t h e other hande ?.':nS.riish a s t h e speed of current increases,Assuming t h a t t h e Nomay lobfit er shows a respcnse -to a b a i t stimulus at iz particu1a.1~ threshold value f o r t h e conceltration of b a i t aroma inxterial, it i s reasonable t o enppose t h a t t h e r e is an optimal. speed ( r a t e ) of o w r e ~ t f o r a maximum T\J valueu
Direction: Direction of t h e cwz'ent has :m e f f e c t on catching efficiency
W--
when several p a t s a r e put out i n a s t r i n g o r lino, The current done; -tne a t r i n g o f pots i s t h e l e a s t favourable, as t h e individual pots ir, each s e t trill w e r l a p one ailother as f a ss spreadfng of b a i t aroma material is concernedo The degree of overlapping d.ependa on pot distance and t h e s i z e of t h e ef f ect i v e f'is:ling area of -the inCivid.t?.al pot,
A t grounds %ii$b. n r e l a t i v e l y uniform c m r w ~ t d i r e c t i o n i t !.S therefore a n advantage if t h e s t r i n g of po-ts i s placed across t h e ourrent i n o r d e ~ t o achieve a m a x i m u l l sf f e c t i v e f i s h i n g area.^ Studies on behaviour i n Lysef ;jo.cd showed that positionj.ng of a s t r i n g of pois at t h a t ground, i n r e l a t i o n t o d i r e c t i o n hardly mattered a s v a r i a t i o n s i n current d i r e c t i o n were qul-ka considerable.
Constancy of direction: If t h e current cl.,.ar.ges d i r e c t i o n i n t h e coarse c;f
t h e f i s h i n g perioa, t h e r e is m increaso i n t h e a r e a over which t h e b a i t erorn?.
spreads. So vaYriation i n t h e d i r e c t i o n of t h e current w i l l r e s u l t I n an inoroase i n t h e e f f e c t i v e f i s h i n g a r e a m d consequent;ly a higher value %W t h e number of Norway l o b s t e r s t h a t can be lured t o t h e pot, But t h e current
should show a c e r t a i n constancy and p e r s i s t i n a partioular direction, so t h a t most of t h e Norway l o b s t e r s r e a c t i r g t o t h e b a i t stimulus w i l l manage t o reach t h e pot before any change i n c w r e n t oc,cwso
c
Studies i n behaviour have shown t h a t most lTorw~1y l o b s t e r s approached t h e pot by going against t h e currerit
,
It could trhex*sfore b e assumed that t h e b a i t plays a v i t a l part i n l u r i n g t h e Norway 10bs.t er t o t h e pot and. t h a t , by and largo, Norway l o b s t e r s must make t h e i r way zig3inst t h e c w r e n t i n order t o l o c a t e t h e s o w c e of stimulation,CIUPlt7fiT & 130kiJARD (1979) a l s o drew at t e n t i o i t o t h e importame of t h e 'bait f o r luring Norway lobsters, They show~d t h a t t h e b a i t does not stimulate t h e Bornay l o b s t e r , inducing it t o leave i t s burrow, dming periods wh.en i t i n normally not active, but a t t r e c t s t h e fTar~my lo b s t e r when i t i s already out of it S burrow,
B e f o ~ e a Nomay lobstt-.L can respond t o EL b a i t stimulus at a d i ~ t a n c i ? t h a t we coulcl ref ex t o as (8) f . ~ 3 1 n t h e pot t h e strength of stimulation at (d) mudt exceed a cortCin mini,neq value. The strength of stimnulation Is determined by t h e power of a t t r a o t i ~ n of t h e b a i t ( i o c,, t h e extent t o which b a i t s
&rouse it S aPpet i t 0) w d t h e concentrat ion o f %,it aroma material.
Fmer of a t t r a c t i o n : Tlzo p o w r of a;t;.tmc-tloi~ I s q u i t e independci~b ot' t h a
- X -
concentrationp as it i s possible f o r an czi;tra,ctive b a i t t o produce -ths sil,fib
st rength of a t imulat ion with small. conccn-t rat ions as a r a t h e r unattrac.i;im b a i t having l a r g e concent rat ions of 'bait aroina mat e r i a l B
In experiment at ion with lobst era l*!ACZCJE (I 973) ind.i c a t ed a highe-? rexposse t o s.t i m f l i from c u t t l e f i a h extract than f o r aif f s r e n t synthetic co~ilpona~t a of t h e same extract (at a concentratior. 02 2 a. 1 0 Jl), ~ ~ As t h e
concentr&ion of b a i t aroma material was t h e same* dispari-ties i n rsspont;e/
S-lyrength of stimulation can only be explained b j a d i f f eroncc i n -the power of a t t r a c t i o n ,
The power a f a t t r a c t i o n possessed by t h e b a i t ~~rmld seem t o be p a r t i c u l a r l y important at t h e entry stageo J u s t beside t h e pot t h e concentration 1-35 b a i t aroma material i s approximately at i t s maxinun, and so m y increa-e i n t h e strength of ~;timula-i;ion can only b e s?hiav.od by an increase i n t h e po%ae;?
of a % t r a c t i o n o Only ($ of tkLe iTorway 1 0 b ~ t or9 observed duTing t h e s t u d i e s on behaviour were cawht, This l m ~ nntry perce~.tage c m p a r t l y be accounted f o r by technical f a c t o r s concexned with t h e o o r ~ s t ~ u c t i o n of t h e pots, But a reduced l e v e l of mo-tivation i s probably another importmt causeo as 35$
of t h e Nomay l o b s t e r s vcncler observa-tion were not i n physical contao-t with t h e pot, We c m blame t h e scaring effect produced by t h e pates strwcture a,nd possible d i s t wbing fac-t oss (men-bioned i n t h e section on %?"ot o r s
L ,ctlon r e l a t i n g t o Bornmy l o b s t 6 9 ) 4. b a i t poss~xsing a high power of attM-- i s therefore e ~ s o n t i a l i2 sxch a deficiency i n motivation i s t o b e ovarccxe, I e e The dif f w e n t substances i n t h e b a i t w i l l a l t e r a s a
-
r e s u l t of chemical prosc,:sc;q, After n o e r t a i c -time i n t h e sea t h e bait t r i l l"turn so^^^^, w d t h s r o I,II : -l. thorefore be ,l c:?ange i n i t s power of attrac-tionA T t i s not c l e a r whether 2 ~ 1 a l t e m t i o n of t h i s kind i n t h e q u a l i t a t i v e compos- i t i o n of t h e b a i t affec-L7 i t a power of a t t r m t i o n f o r t h e Nomay lobster*o S o l 1 The amount of .the p a r t i c l e s fr*ola t h e b a i t aroma material that
YPa-
i s released depends mainly on th.e s o l u b i l i t y of t h e b a i t , d b a i t of l2.g-h sohnbility wiil provide relatively l a r g e ooncentrations of b a i t apoma material mc1 consequently a l a r g e r a r e a i n which t h e wtrortgth of stimulation i r ;
s u f f i c i e n t l y high t o p~orluce a response iii t h e Norway lobster,
Amount'. The c o n c e n t r ~ t i o n of bai-t arorna rna.i;02i~.J. m u s t be presumed t o be i n
*DP-
proportion .to t h e amowrt cf b a i t o its a largo quantity of b a i t wil.1 have a l a x e r surface f o r t h e reJease of b a i t aroma g a r t i c l c a , I have not c a r r i e d out any experkmerits with d i f f e r e n t amounts of bait. The p r a c t i c a l
significance of d'ifferent amounts of b a i t from t h e point of view of catching efficienoy i s not very clear, but in f i s h i n g t r i a l s with sea t r a p s f o r
t h a t catch was increased wb.en oztching rusk
W )
t h e r e were indicationut h e g.c.mtity of b a i t w a s l a r g e r ( v A L D E B ~ ~ u s ~ .197';)0
-met
hod: When po-t-fishing f o r Norway l o b s t e ~ s (at t h e grounds included i n these investigations) i t was neccssmy Lo use p e r f o ~ a t e d b a i t recep.tacles t o prevent t h e b a i t from belng eaten up by hagfish* Ithen a b a i t container i a used, t h e b a i t can be f i n e l y chopped t o give i-t a l a r g e s t u . ~ ~ c c and increase t h e sepaz'ation of b a i t a,rorna p a r t i c l e s , But i.n s p i t e of plex~ty of perforations t h e surface of contact betweea -the b a i t and t h e brater ilowiue past t h e bakt container i s small, and t h i s leads t o a reduction i n t h e
concuntratian of b a i t arorna material* Bo experiments were c a r r i e d out .ta
t e s t i hat ef f eot
.
Experiment S adopting a ciornblnecl b a i t 2% met hod (unpro t ec; t cd b a i t and b a i t i n a b a i t container ) d.wing s t u h i es on b@havioux did, h o ~ r e v c r ~r e s u l t i n a considerably higher value f o r t h e number of l o b s t e r s observed per hour t h m was %he case ill experiments with a b a i t container.
S i t i n p * The speed of t h e otzrrent i a of-G~YL reduced t o some extent, i n t h e l
--,-%-aa l
layer Just above t h e bottom o w i n g t o t h e creation of L~buZencea The spread. l of t h e b a i t aroma may therefore be l e s s i f -the b a i t i s placed on t h e bottom
of t h e pot, BormaZly t h e b a i t should be placed. f a r enough away from t h e tunnel(s) f o r -the ITort+~c'j,y 1 6 5 ~ ; t ~ z ~ not t o be a b l e -to reC?x:h i t from t h e inside openiqy 0% t h e twmelr
A 1 1 Lypes of b a i t t h a t .sTaro used prwed t o be e f f e c t i v e for l u r i n g Wor:~~a,y lobsters, so t h e strength of stimdat i on at t h e l u r i n g S-tcags may be described a s 'being r e l a t i v e l y got?, Strength of stimulation at t h e entry s t ~ e , on t h s other had, i s not v e q satlsfac-toxy* ju6ging by t h e low entry rateeo So types of b a i t ppossecsing a greatrer power of at--ircxtion will. be essen.ti.sl if
catching efficiency i s t o be increasedo
L i-r on2 es Findings from t h e fishing t r i a l s do nclt indicate any s i g n i f i c a n t dik':'', .
i n strength of stirnula-tion between t h e d i f f w e n t types of b a i t ,, The r e s u l t s revealed b e t t e r catches f a r pots b a i t e d with m8ckerel than f o r pots baited with t r o u t food (feed meal). ItASOFl (1 965) c a r r i e d out compara-Live b s i t i ~ l q trials u i t h herring meal, sttlt mackerel, skate and plaice, with herring meal giving signif icaultly poorer catch r e s u l t s ( f o r crabs) than ~9na,tzr~,lq9 b a i t types, R r t i f i c i a l b a i t i n t h e form o f feed meal i s therefore not a
s u b s t i t u t e f o r na-turd b a i t , but it i s of good lreeping q u a l i t y and easy t o s t o r e on board shipp so it could Be an i d e a l thing t o use as sqrc?serve 3ai-L??,
G
Zn assessing t h e importance of f a c t o r s relaking t o t h e potp I have divicled up .the pot i n t o two component parts:
a ) t h e "pot housing o r casing@' and b ) t h e twsneZo a) The pc* c a s i w
The pots used i n t h e invesiigation c m be divided i n t o two types according t o She coverfW material8
1 ) Closed pots
-
pats cove;.ed i n a s o l i d m3terial (excop-t i n the t m e l : : ) , 2 ) Open pots pots covered with l~ettiirlg,One condition governing the choice of covering material i s ishat i t should g i v e hagfish a good opportunity of escc7,ping0 I f not, t h e gear m.d -the catcka w i l l become s o i l e d ' ~ j i t h .the slime secreted by t h e hagfish when the pot i e hauled in. This was often a psofslsrn in t h e case of po-L l 7 ( c l a l ~ e d pot).
,"
Cloc;ed pots can be an advantage undea> special conditiona, e.g., i n tropics?!
regions where f i s h ancl or~:slaceauls e n t e r cove-ed trzpa i n seazcl.1 of
protection from t h e , kro- i;.ght of -the suno BUTLER ( l 963) showed t h d po-i; S covered with metal I "i. 4 e s ( s h e ~ t s , of metal) on i;ho sides wcl. t;j.-th s n e t t i n g twmel. at each end g;,ge b c t t e r oatch r e s u l t s f o r spot shrimps*
m=)
than e i t l i ~ ~ completely open o r completely covered pots. I E s conclusion i s t h a t %'l:: shrimps 60 not enter a covered pot i n soarch bf pro-bection, but that iho good catch r e s u l t f o r t i ( i h t pot/open tunre1 i s duo t o t h e fac-i %hat t h e b a i t aroma i s c01iconl;sated i n t h e tunnel openings, so skrimnps w i l l more rsaclily f i n & t h e i r tirqy i n t o the poto 3Ludics on beliav-ioux showed t h d -b",h%-t effect wauld I.i.ardZy b e of any grua% importinlce as Pa-r aa ps b f i s h i n g f o r i?Jcr.tra,y 1sba.i; srs i s conc?erne~I, as Ihe Xomay l o b s t e r sobserved. 1;eafde pot
17
d i d not displzy my p a r t i c u l a r i n o l i n a t i o n t o ri~dce f o r tlie t m e 1 op~ning,The covering material can, on ,the other hand, have a& important e f f e c t on t h e spreading of b a i t aroma, as open pots have a r ~ e l a t i v e l y good throughwflow
of water, compared with closed pots, under otherwise s i m i l a r c u r r a t conditionse Apart from pot 22, t h e closed pot Nor 17 achieved poorer catch r e s u l t s than open potso Experience from "Le Faxoes a l s o showed closed pots t o be less e f f i c i e n t than open ones (11. libghammer, fisherman, personal communication).
I"c must f herefore be assumed t h a t pots having a dense covering material possess a l o ~ f a r catch efficiency rating,
m t The shape of t h e pot would seem t o have l i t t l e e f f e c t on ca-tching efficiency, Generally speaking* any scaring ef f a c t (shown by a pot ) w i l l bo independent of pot shape*
Size: Pot s i z e w i l l notplay m y v i t a l part e i t h e s from t h e point of view of
91or
catching efficiency, There has t o be a c e r t a i n minimum s i z e f o r t h e i n s i d e volume of t h e pot t o prevent s a t u r a t i o n of t h e p o t ( p o i n t of saturat5on::
asymptotic value f o r ntmber of Nom~a;y l o b s t e r s t h a t w i l l e n t e r t h e p o t ) + The maximm catch i n f i s h i n g t r i a l s was 8 Norway l o b s t e r s i n one pot (pot 20).
During t h e s t u d i e s on beliaviour a Norway l o b s t e r entered t h e same type of pot af-tep t l ~ o r e were 12 specimens (individusls) i n t h e potg placed t h e r e i n
& v ~ c ~ c + In t h e Faroes a oatoh of 45 Nomay l o b s t e r s has bean achieved in a pot of type 24 (sliglatly l o s s than a pot of type 20), and catches of arounct 20 individuals per pot arc3 not uncommon (FIo ~b~hammcr, fisherman, personal c o m m i c a t i o n ) ,
45
Nomrrzy l o b s t e r s i n pot 24 would correspond -to about one individtwl per dm 2 pot basooThe point of saturatiou: w P l ~ pzaobably vaxy i n t h e d i f f e r e n t types of pot, and i t i s d i f f i c u l t t o ostiir?atc exact valueo. But as far as t h e catch r e s u l t obtained i n t h i s investigation i s conccm.cd (maximurn of 4 Dart~ay l o b s t e r per
5 elm 2 of base)# t h e pot s a t u r a t i o n e f f e c t has probably had no impact on catching ef ficioncya
The tunnel i s t h e functional part of t h e pot, intended t o give t h e Norway l o b s t e r easy access t o t h e i n s i d e of t h o pot and. a t t h e same time a c t a s an eff 00% i v e b G v r i e r o preventing uscapor
?*lost pots bad funnel-shapad tunnels, whereas pot 23 had a t o p tunnelo However, s t u d i e s on behaviour showed t h a t Norway l o b s t e r s normally .go up along t h e tunnel s t ~ ~ o o x t t when they enter t h e poto The tunnel caa therefore be regarded as t h e f'unctional part of t h e tunnel at t h e entry s t a g e and cavr be viewed as an inclj.ned plane with a d i f f e r i n g wgle of inclina-tion, Pot 23 c m be compared on tha,t b a s i s with t h e other pot types,
Norway l o b s t e r s usually t r a v e l over r e l a t i v e l y f l a t m d l e v e l surfaces.
The s t u d i e s on behaviour indicated t h a t Morway lobsters usually avoid
tackling st oep obst aclas, although isolated. o'bsorvat ions have revealed t h a t
%hey do possess f a i r l y good climbing a b i l i t i e s , T-t can therefore bo assumed t h a t catching officiency increases as t h e angle of i n c l i n a t i o n f o r t h e
tunnel f l o o r diminishes. Catching efficiency would seem, morewcrp t o be i n v w s e l y proportional t o t h e leWh of -the tunnel floor. This was most c l e w l y evident during. behaviour s t u d i e s with pot 23, which has a rc;lativcl.y long tunnel f l o o r , 35 cm, as13 of 25 l!omay l o b s t e r s t h a t begm t o go up along t h e ttwnol f l o o r -turned back before -they reached t h e tunnel opening*
To reduce t h e lilrelihoocl of escape, t h e i n s i d e tunnel opening shouldl be a c e r t a i n height above t h e tunnel f l o o r * .!h optin~wn qginclino6-plme - t m e l q s w i l l theraf ore be a compromise between a minimum angle of i n c l i n a t i o n f o r tlie -tunnel f l o o r and a minimum tunnel f l o o r length,
Idat e r i a l : THOIUS (1953) showed tha-t t h e mesh wiath i n t h e tunnel affected
--I-*-
catching efficiency a s f a r as l o b s t e r s were concerned, Pots with fine-meshed n e t t i n g (mesh t?~id.th of '17 nm) i n t h e tu~nnel fl o o r provided b e t t e r catches than pots with a mesh widt4; of' 75 m. 1210st pots i n t h e stud-y ( i n t h i s investigation) had n e t t i n g with a m e ~ h ~~i-<:th of 30 mm i n t h e " c e 2 floor, Howevsr, the
s t u d i e s of behaviour t:Fowed t h a t mesh width could hardly be a l i m i t i n g f a c t o r with ragarc1 t o catching efficiency i n t h e p o t s tha-6 were under cxmi.nation, a s Norway l o b s t e r s move about with ease both on n e t t i n g having a mesh wid-th of 30 mm and n e t t i n g of 10 mm (pat 23),
On t h e other handd, r i g l d i t y i n tile material used f o r tlio tunnel c m have an effect on t h e catclling of Borv~ay lo b s t e r s * Pot 23 wax lcept distended by means of a f l o a t anEl so t h e n e t t i n g i s not completely taut. lie get t h e inlpressiaul -{;hat Xomrsy lobsbere often rem-l; od by retreating vshcn theyf er-t the sw4ace giving under thcm (slack n e t t i w ) * It i s therefore reasonable t o assume t h a t Laut n e t t i n g o r a r i g i d . mat eria2 i n t h e tunnel would be (m
B:
According t o experience gained from behaviour trials, t h e tunnel&ould be placed laa enough t o allow t h e tunnel f l o o r to s l a n t r i s h t up f rorn t h e undersurface (base),
Number: Pot types 20 and 21 were constructed w i t h
4
tunviels so tha% t h e EJommy l d b s t e r ~i~.eald be sure t o come across a tunnel irrespective of t h e w g l e of incidence, Hovmrero obsemrations of behaviour showed t h a t Norway l o b s t e r s seldom entered Whc f i r s t an4 bestw t u m ~ e l ~ but went on a number of d e t a i l e d oxploratosy t r i p s round t h e pot before possibly making t h e i r wayinside.
Z-f; would t h e r e f o r e appeax t h a t t h e positioning of t h e t m e l i n r e l a t i o n t o t h e angle of incidence ( d i r e c t i o n of current) i s not of twny v i t a l impor.tmce.
For a l l t h a t * judging by t h e search p a t t e r n observed i n Norway l o b s t e r s , i t tvould be ~un acJ,vantage t o havo several tunnelso a s t h e probability of coming across a tunnel while roving round t h e pot increased i n propor-Lion t o t h e number of tunnels.
- 0
" I-S" t h e Xort- ay l o b s t e r i s i n -the entrance area of t h e pot
( t h e part of t h e pot periphery t h a t i s oovored by t h e outside opening of t h e tunnel) t h e r e i s a c s s t a i a degree of probability (depending on t h e shape of t h e t w m e l ) t h a t it w i l l . go i n s i d e t h e pot. At a l l other positions along t h e periphery of t h e put t h e probability of entry i s n i l o The probability of entry i n respect of a p a r t i c u l a r type of pot w i l l therefore be
proportional t o t h e r e l a t i v e o n t r ~ x c e ler-g-cli of -the pot (length of m t r m a o m e a i n r e l a t i o n t o t o t a l circlmfereme of t h e pot ). 1dc have attempted t o i l l u s t r a t e t h i s i n Pigr. 41 f o r pot types '17, 20 and 23,
Length of entrance as .X prcportion of t o t a l poriphezy (circumf eroulce) ranges from 10 -to 10%: <or t h e various types of pot
able
1 6 ) ~Table 16: Relative enbr3ance length (length of owtside ~ t m e l opening) i n
v
rel-ation .to t h e c i r c ~ t i ~ ~ f e r e l i c e of t h e poto
a ) 3 a;nd 2 tzanvlelxo respectively,
Basing ourselves on t h e t y p i c a l semcb p a t t e r n of t h e lTomm3r l o b s t e r
(searching rouncl tlie pot)B i L i s r ~ a s o n a % l e f o r us t o a s s m e t h a t catching efffcienoy increases as t h e entrance length incrsasos,
m
The studies on behaviour showed t h a t t h e escape r a t e eias r e l a t i v e l y lowe
~ d i i l s t t h e e n t r m c e r a t e ( i ) ( s i n ) appeared t o be t h e l i m i t i n g f a c t o r a s far as catching efficiency was concerned, So t h e krsxequisite f o r increased oat ohes i s an improvenlent i n t h e f a c t o r s influencing catching efficiency at t h e a n b j s t s e .
Spreading of b a i t aroma i s pro'bably sa-bisfactoxy i n t h e case of a l l t h e open pota (pots covered with net-ting). So what t h e catching efficiency of pots of t h i s type w i l l . be t h e c ~ a o t e r i s t i c s of t h e tunnel.
Pot 14 t That pot was only ueed 4 times i n f i s h i n g t r i a l s f o r -the capture
.IIIQI*L.
of N o m y lobsters* Several fa.0-tors slzould mdcs t h i s an i d e a l pot f o r catching Nonmy lobsters, eogo, i-ts considarable entrance l m g t h ( B @ ) , t h e low position of t h e tunnels il31d t h e small mglc: of i n c l i n a t i o n f o r t h e t m o l f 1 0 0 r e HowevoPg pot 11 was constructccl am ;I,shrimp poto and t h c i n s i d e ( r e c t i t n ~ a ) tunnel opening was r e l a t i v e l y small (6 X 7 cm) and not vcry f l a i b l o ( s t e o ~ wire)* IQ nssuniption i s therefore t h a t t h e s i z e of t h e
i n s i d e t ~ m e l opening, especially t h e width, was a l i m i t i n g f a c t o r f o r t h a t type of pot.
".hat pot was a l s o constructed f o r tlie oap-turu of shrimps, but acfiievcd rcla-l;ively good. catches of Norway lobst or, Like po-t I l, pot q5 has a $=go ontxlmce litngih (.i~t.$), q u i t e a 1014 angle of inclinsation f o r 'the tunnel f l o o r aad a loilj I;w~r,el. positiono The inside t m e l opening i s extremely rr~idc (35 cm), That probably h.as a posit i v s wf'f act on catching cfficicncy, whereas t h , tunnel height (4. cm) must bo regarded as a l i m i t i n g f a c t o r as far as cntuy rate i s oovlccrned, The catching africioncy of pot 15 could probably bc i~clraascd by making the i n s i d e . t ~ m c l opening higher,
8 Of a11 t h e open-type pots, apart from pot 22$ pot 19 gave t h e poorest catch r e s u l t s c The pot had only ono "cunncl. The small angle of inclina-Lion f o r t h e t m e l f l o o r and t h e l m r position of -khc tamel. should have made it i d a a l f o r catching ITommy lobs-t crs, But t h e entrmoc l w g b h of t h c pot was small (I @)$ and tha-t probably had a nega-tivs effect on t h e entry r a t e * This pot w a s collapsible so tha-L; i t would tdce up l e n s spaco on board, wlim folded* Hotrovero t h c collapsing rnochcmism proved -to be r a t h e r
a ~ ~ h i a r d -to use and t h e f r m e was o f t a s l i g h t l y bent p so t h e tunno1 was no%
propcrly cart endedo Slaok notking i n t h c t t m e l portion may therefore have reduced i t S catching off icicncy*
16,
Pot 20 o, That was t h e pot t h a t gave t h e best r e s u l t s during t h e f i s h i n g
..IpuuI
t r i a l s * But s t u d i e s on behaviour showed t h a t only
5
of 61 Norway l o b s t e r s t h a t wcre observed went insf de;. t h e pot So, i n s p i t e of having a high catching efficiency compared with tlie other typoa of potp pot 20 has a low r a t i n g a s far as absolute catching efficiency i s concerned* The main reasons f o r t h i s would appom t o b e i t s r a t h e r small entrance length (33%) and t h e excessj=vely l a r g e angle of i n c l i n a t i o n i n t h e out e r part of t h e tunnel floor.But t h a t type of pot nevertheloss gave b e t t e r catch r e s u l t s than %he other open types of pot* T h i s nay be due t o t h e follob~ing pointst
l ) The s i z e of t h e i n s i d e tunno1 opening d.id not appe'w -to be a l i m i t i n g f a c t o r , as i n tlie case of pots 1 l and 15, and 2) a l a r g e r ontrance area
(4 tunnels) than pot 19 (l tunnel),
Pot 21 : Thxk type of pot i s a colLapaible version of p05 20, and no
y4.por
difference was detected between those types as f a r a s catching efficiency was concerned, Pot 21 takes up much l e s s space on board and is e a s i e r t o handle than pot 20. Bu-t; pot 21 could probably not b e used at grounds having a f a i r l y strong bottom current which mieht a f f oct %he f l o a t system, so preventing t h e pot from being properly extcndod Discrepanoies i n t h e speed of current mig1it b e a possible reason for t h e Zower average catches ob.bained with pot 21, compared x i t h pot 20, a;t t h e ground, i n Romsdslrs-f;iordp whereas t h e opposite w a s t h e case i n Lysef jord,
Pot 22 : O n l y 1 TJortr~ay l o b s t e r was caught i n -the coarse of 20 hauls using
ry*reu
type of pot ( a t ( a ) rclcL.lively good Norway l o b s t e r gromd(s)). That r e s u l t i s much powrer -tk?~.n WO might have expec-Led from an assessment of the ptws charaol; o r i a t i c s (with r e g a d t o covering ma-t e r i a l , shape of tunnels, entrance length, ctc*). Hotilwer, pot 22 i s c o l l a p s i b l e sad i~ kept ex-tended by a f l o a t a n d sinker, The #inker was attmhed. t o t h e bottom of t h e pot
( i t was covered with f a i r l y ~1acI.c n e t t i n g * see Figo 22)# and t h i s may have meant that t h e pot was not r i g h t down on t h e bottom, Th& may have been t h e probable c a m e of %he poor catch r e s u l t *
: T112.t type of pol; was constructed f o r S-tuclies on behaviour on t h e b a s i s of pxcsrious experience gained from fishing tria1.s.
d mexinlum (very high) coefficient f o r entrance area* a small a r g l e of i n c l i n a t i o n f o r t h e pot f l o o r and a r e l a t i v e l y large i n s i d e t w i e l opening wore a l l f a c t o r s t h a t would havo lecl us t o expect a high entry r a t e fop t h a t
t y p e of poto However, the studies on behaviour sl~owed th a t t h e inclined p l m e up towards t h e tunnel opening t ~ ~ s too long aria t h e material used f o r t h e i n c l i n e d plane was probably too slack* The pot ahould therefore have had a firm frame construction o r a more p m ~ e r f u l f l o a t system vri-th a heavier bottom frame cmd mwre buoyancy i n t h e f l o a t . I n addition, t h e length of t h e inclined p l m e should probably be reduced t o some exkent (at t h e expense of an increased of i n c l i ~ t i o n ) , ,
The s t u d i e s an behaviour a l s o showed. t h a t i t ms too easy t o escape from the pot Tl1a.t c m b e attributecl i n part t o t h e special conditions operating during -the behaviour t r i a l s (contact between f l o a t and calnera mounting,
%to,), as pot 23 d i d catch Xorvray l o b s t e r s during t h e f i s h i n g trials, t T h a t 'ty-pe of po-G has made extremely good catalies of N~rway l o b s t e r i n t h e Faroeso 9311t r e s u l t s of t h e compsr&ivo f i s h i n g experiments indicnt ed a 8ornmha.t lowex* catching effioiency f o r t h a t pot than f o r pot 200 That may hc due t o Lhc f a c t t h a t t h e r e a r e fewer t m c l s (roducsd o ~ t r a n c s length) i n pot 24* On the other hand, r i g i d t m e l materialy together with low tunnel position, should h w c a p o s i t i v e e f f e c t on catching cfficiency i n comparison with pot 20.
Studios on behaviour showed tksa-t t h e number of Nmway lob& e r s a t t r a c t e d per unit of time was l e s s f o r closed than f o r open pots.
r An assort ion has baon mad0 (by experienced pot-fishcman, mongst o'claers) -i;l?a;t closed pots a r e b e n e f i c i a l because t h e Bait aroma w i l l bo
concentreV-tod at t h c tunnel opening* so it w i l l bo c m i o r ,to bring t210 prGy -to t'4.1~ -tunnel* I?o such cffcct was o b s w e d i n -the cozwso of t h e behaviov-r t r i a l s . 2Torwa.y lobst c r s d-isplzyed t h e same 'behaviour p a t t e r n during -the soarching stage with pot l 7 aw they d i d i n prosclulca of a ~ n d y p c pots
(search o r explora.tion rouy1d t h e pot tri'cho~~t any special. a i t r a c t i o n effeci;
note& i n front of t h e t m o l opening).
In addition t o poor spreading of b a i t aromao pot 17 i s chtwaoterized by a r a t h e r small e n t r w c o lcngth (21%)~ The small B ~ ~ I o of i n c l i n a t i o n f o r t h e tunnel f l o o r ha8 probably been a p o s i t i v e f a c t o r from the poin-t of v i w of catching efficiencyo
Pot 18 o Pot 18, l i k e pot 17, i s covered with a dense material, btvt i t hag
-
a m tunnel at each end. The f a c t t h a t i t provides b e t t e r spreading of b a i t arotna and has a gree,ter entrance length (27%) has probably l e d -to t h e sornet~ht higher cat ching cf f i c i m y of pot 18*Results of trials involving d i f f e r e n t periods of fishing (pot 20) revealed t h e g r e a t e s t incroaso i n catches i n -the f i r s t 1-2 da~rs (24-liow period-S, and a r e l a t i v e l y modest increase i n catch per d w r i ~ s t h e remainder of tlie f i s h i n g timeo T l i s i s probably due t o rcduc-tion i n t h e strength of tlae b a i t stimulus a f t ftcr about 2 &ys of f i s h i n g time because i t had been v~aslied amay o r t h e b a i t had been eat 6n by fish-lice, hagfish, etc, A r i s e i n catches beyond a 2 4 a y period ( ( f o r ) up t o 9 days) inilicates t h a t -tlie b a i t may have a reduced e f f e c t f o r a r e l a t i v e l y long time and t h a t t h e escape r a t e f o r pot 20 i s f a i r l y l m ~ (according t o t h e studies on behaviour),
The Uffcronce i n average s i z e df male0 and females i n t h e c ~ t c h e s of Non~ay lo b s t e r s may be regarded a s r e p r o s ~ n t i ~ a r e a l d i f f erencc i n
d i s t r i b u t i o n according t o s i z e within t h e population. The smaller a v e r w e s i z e of females i s due t o reduced growth r a t e of s e m l l y mature females 8rlien they WO carrying eggs (STORROW 1912),
s~~GM~Y.uJDEBsER ( l 962) and CIURIZAN Q HOkJRRD ( l 979) have shown t h a t l a r g e Norway lobstex~s spend longer periods of a c t i v i t y involving migrations f o r food outsjdo t h e i r burrows than srnall specimens,, T h a t i s probably due t o wider l i m i t s x f t olermco f o r l i g h t i n t ensi-ty ancl reducod predation st r o s s
i n t h e case of l a r g e Norway lobsters* I!& own studies on behaviour confirm i t
-
small No:rway l o b s t e r s were only 0bse:rved a t night, t~hereas specimens obwcrvod i n t h e daytimu ( b o t ~ ~ e c n m r i s o and sunset) were ro9atively large.Large Norway l o b s t e r s (chicfly males) a r e therefore more exposed t o possihlo c a p t w o than amal1 ones. This ac3comts f o r t h e predominance of malos i n catches f o r t h e suumer semestor, There was a more evon d i s t r i b u t i o n according t o sex i n t h e catches f o r t h e rPsint e r szmcs-tero T l x t may b e due t o t h e r e l a t i v e l y longor periods of a c t i v i t y fbr small 3Torwajr l o b s t e r s
~ l t txibutable t o a reduction I n l i g h t i1l-i; ensitye
Tagging trials showed t h a t t h e t w g i n g mothod was worlcing $ a t i s f a c t o r i l y , a s a l l t h e Borway l o b s t e r s t h a t kad mbu1"ced ( 2 at t h e f i s h i n g grounds and 1 i n t h e aquarium) retained t h e tags a f t e r ccdysis.
JEJSEX (1965) c a r r i e d out comprohensivc! t m g i n g ex~erimentn i n t h e skigem&!/
Kattegat area which showed t h a t Mommy l o b s t e r s do not un8-ertake ;my importa1t migrations, 114y own t a g g i r g trials w u l G appear t Q confirm t h i s , a s 2 repeat catches wore jnado just b0sid.e -tlio tagging s i t e o whereas one Bornray lobster had t r a v e l l e d about 500 m,
Two of t h e 'JYornrny l o b ~ t o r s c a ~ p h t twicc had rnoul-Lod. Length of carapace had increased by 11% i n the case of botlz stpecimenc. l?ARfqBR (1973) quotes t h e follotdng a s t h e r e l a t i o n between carapace length before and aFt;er moult:
y l a0144
+
0+18@ cm9 m d gives t h e incxeaao i n carapace length per rnouli f o r r e l a t i v e l y l a r g e males (carapace Zongth of55
rm) as G. I$* According t o l%,xrnerts equation f o r -the increase i n leiigth per moult, one Borv~ay lo b s t e r would- have moulted twice and t h e othor 3 times, bo-th of them i n t h e course of 15-16 months, H O V J W O ~ ~ t h e p e r c e n t u d incrca,qo 5x1 length indicated by him x~ould point t o 2 nloults f o r b&h, S & M R + ~ ~ S E R S ~ (1962) indicated -th.at 75% of males underwent e o Q s i s every year ( i n t h e ~ m o e s ) , It i s therefore reasonable t o assume t h a t t h e 2 IYom~~qy l o b s t e r s m y have moulted twice before -they were caught f o r a seoond time; 3 rnoults within that spaceof time wou1.d.seem r a t h e r u n r e a l i s t i c , -
)
The average oztch f o r pot 24 i n Lysof jord (g~oundls L2 and 2 3 ) was 2.2 I!Torzaay Lobsters per pot haul, as against 9,4 f o r t h e same type of pot i n t h e S o ~ u ~ d e i n t h e Faroes (MORU 19'78)~
Tlia density of %he Nomm,y l o b s t e r population
7
16 cm i n tho Sounds i n 1937 was estimated at 118 lee per trawling h o w ( x ~ ~ ~ u L R z ) . . ? ~ D ~ s B J 1962). Although t h ~ s o investigations took place a long time <agot t h e r c i s reason t o b e l i e v e t h a t tlie population aensi-by a-t; that ground. i s q u i t e considerable.The difference i n pot ocltclles indicates t h a t t h e population d.ensity i a about 4. times grexter .thm i n Lysof jord. However, t h e lSl0may l o b s t o r g r o d i n -the So~mds i s r e l a t i v e l y shal loi,~ compared with Lysef jord- (4.040 as against
1 I F 1 7 0 m). The smaller ca-Lches i n Lysef j o d can therefore b e ascribecl t o differences i n b a s i c d i e t md! pattern of b e h a v i o ~ ~ r i n c o n j m c t i s n with food inta3ro (feeding) of Nor~rog lo b s t e r s at g~?c?ater depths.
30 ma
Norway l o b s t e r s were caught both at t r a d i t i o n a l shrimp grounds and at grounds t h a t had not been t r i e d before. Catch r e s u l t s were best a t grounds with a s o f t bottom a t a depth of from 115 t o 170 m e The highest average catch per pot haul f o r a s i n g l e s e t of pots was 3.4 'Norway l o b s t e r s o The maximum catch per pot haul was 8 (in~liwid.uals).
Fishing t r i a l s were c a r r i e d out t o compare 8 d i f f e r e n t types of poto Catching efficiency was s i g n i f i c a n t l y improved with a pot covered i n n e t t i n g aYld
having 4 entrances, compared with a t r a d i t i o n a l boxlpot equipped with one tunnelo
Different types of k i t were tested, and a l l of them appeared t o exert an adequate power of a t t r a c t i o n a s f a r a s Norway l o b s t e r s were concerned. From t h e available material it was not, however* possible t o show any difference between t h e d i f f e r e n t types of b a i t o
The best catches were cbtained i n February~I~larch-April.
Tagging t r i a l s were c a r r i o d out,, The tagging method used appeared t o work s a t i s f a c t o r i l y o as Norway l o b s t e r s caught f o r a second time had retained t h e t a g s a f t e r 2 moults, The tagging t r i a l s indioated, f u r t b r m o r e , t h a t t h e Norway l o b s t e r i s a r e l a t i v e l y s t a t i o n a r y creature,
Males were predominant i n t h e catches f o r t h e summer amester,, Females w i t h eggs were caught i n t h e period from July t o ?(archo They made up only 3%
(on numerical b a a i s ) of t h e catches per tmmam.
Biometric r e l a t i o n s between carapace length* overall length and t o t a l weight were calculat ed.
Underwater t e l e v i s i o n apparatus was used t o study t h e behaviour of Norway l o b s t e r s i n r e l a t i o n t o t h e pot gear a t t h e grounds* The diurnal rhythm of Norway l o b s t e r s was indicated, with a c t i v i t y at night a 9% of t h e Norway l o b s t e r s observed approached t h e pot against t h e currant ( ( i n ) 1 ~ 0 0 s e c t o r t o t h e c ~ r e r l t ) ~ a, point that I l l u s t r a t e s -the importance of t h e b a f t as a source 09 s-t;imdLa-tion. S-t~tdies o-n behaviour shotp~ed that t h e type of pot giving t h e best r e ~ u l t ; during f i s h i n g trials only caug1i-l;
5
of 61 Norway l o b s t e r s t h a t had been observed*h c r e a s e d oatchin4 efficiency c a ~ i probably be achievsd by:
l) Using b a i t with a greater power of a t t r a c t i o n ;
2) Increasing t h e entrance length of t h e pot ( t h e part of Lhe pot circumference tlmt ie covered by f.he outside tunnel opening);
3) Reducing t h e angle sf inclination md/or length of -Lrmel floor,
