FlBdevrgen rapportser., 1, 1984, LSSN 0 3 3 3 - 2 5 5 4 The Propagation of Cod zc,dus m c r ; L i a L -
GROWTH, ENERGY CONSUMPTION AND PREY DENSITY REQUIREMENTS IN FIRST FEEDING LARVAE OF COD ( G a d v s rnorhva L.)
T. Solberg and S. Tilseth
Institute of Marine Research, P.O. Box 1870, 501 1- NORDNES Norway
ABSTRACT
Solberg, T. and Tilseth, S., 1984. Growth, energy consump- tion and prey density requirements In first feeding larvae of cod ( G a d u s p,,huo L.). In: E. Dahl, D.S. Danielssen, E. Hoksness and P. Solemdal (Editors), The Propagation of Cod S o d b s r n o r i ~ u a L. Fl0devigen rapportser., 1, 1984:145-166.
Growth, oxygen uptake, swimmrng activity, feedlng abrlity and energy content were investigated ln cod larvae from several groups under different experrmental condltlons.
At yolk absorption the larvae consumed 0.090-0.120 vl O i larvaihour depending on the larval group examined. T6e larval energy consumption either measured as oxygen uptake or calculated from larval welght decrement, was of the same magnxtude. The oxygen uptake was 20-30% hlgher under 100 lux illumination than rn darkness, a dlfference whlch was of the same rnagnrtude (20%) as the dlfference in werght decrement between larvae held at a constant 500 lux and larvae held In constant darkness. Lrght-adapted larvae showed a 6-10 trmes higher swlmming frequency (bursts of swmmrng per mln) than dark-adapted ones.
The overall utllizatxon of yolk for somatic growth varled between 65 a ~ d 70% depending on the larval group examined.
The utllizatlon seemed rndependant of temperature and llght lntenslty when these parameters were kept wlrhin the natural range observed during hatchlng In the Lofoten area.
The speciflc energy content of eggs and larvae showed a sready decrease through the experimental perlod from 5.8 cal/
mq ash free dry welght ln newly fertilrzed eggs to 4.8 cal/
mg ash free dry werght In larvae 12 days after hatchlng.
The larvae showed maxlmum swimming speeds at days 5, 6 and 5 pas$-'riatchlvg (10-20 c m / m l r ? i . T i l e r e was a srgnlzlcart correlatron between mean swrmnlng speed (cm/mrnl and ne?n
swlnrlnc frequency (riuwber of swlmnlng oursts/mln) throuqllodt the experlmentai perlod.
Tne l a ~ v a e showed naxlmum feedrng success (22%) at day 7 post-hatching.
The results are discussed In relatlon to larval prey denslty requirements at the onset of feedlng.
INTRODUCTION
The objective of the present study was to determine the energy demand and food requirements in first-feeding larvae of cod ( G a d u s rnarhua L.) held in the laboratory. The parame- ters investigated were growth, oxygen uptake, feeding ability and swimming ability. The behaviour/energetics approach has been employed in several larval species in order to elucidate the question of larval survival potentials through the first-feeding stage (Lasker, 1963; Rosenthal and Hempel, 1970; Blaxter and Staines, 1971; Hunter, 1972; Laurenoe, 1978; Hunter and Kimbrell, 1980; Houde and Schekter, 1980, 1983). Calculated values of larval food density requirements at first-feeding vary among the species examined but gene- rally tend to be higher than the average concentrations present in the sea. In a review, Theilacker and Dorsey (1980) question the extrapolation of experiments under laboratory conditions to events in the open sea. In the present investigation some of the parameters tested were measured under different experimental conditions, in order to evaluate some of the effects of different laboratory condi- tions on larval behaviour and energy demand.
MATERIALS AND METHODS
Biological material
Eggs and sperm were stripped from ripe cod, caught off the western coast of Norway. The eggs were artificially ferti- lized in the laboratory and incubated in 10 1 aquaria at 5 C , 0
a s d e s c r s b e d by T ~ l s e t b and E l l e r t s e n (1980). T h e Light
~ n t e n s l t y Has a d - j u s t e a t o L O O l u x by r e u t r a l f i l t e r s rrr a 1 2 / 1 2 h i - g h t / d a r k c y c l e , q e a s u r e n e n t s b e l n g made a t r h e w a t e r s u r f a c e ( T e k t r o n l r J l 6 p h o r o m e t e r , r l i u m l n a n c e p r o b e J . 6 5 1 1 , optimum s e n s l t l v i t y 550 nm). Food was n o t a d d e d t o t h e r e a r l n g a q u a r l a .
A l t o g e t h e r 1 6 g r o u p s from d l f f e r e n t f e m a l e f l s h w e r e u s e d designated A t o P ) . I n o n e g r o u p t h e e g g s and l a r v a e w e r e r e a r e d a t t h r e e d l f f e r e n t t e m p e r a t u r e s ( 3 , 5 and ~ O C ) and I n two d l f f e r e n t l l g h t r e g l m e s (500 l u x c o n s t a n t l i l u m x n a t l o ~ and c o n s t a n t d a r k n e s s ) . The l a r v a e s t u d l e d d u r l n g f e e d l n g s u c c e s s and swlmmlng s p e e d e x p e r l r n e n t s w e r e h a n d l e d t h r o u g h - o u t t h e h a t c h i n g p e r l o d by t h e methods o f T r l s e t h and E l l e r t - s e n ( 1 9 8 4 ) , s o t h a t a l l l a r v a e s t u d l e d were o f t h e same a g e .
Growth
The d r y w e l g h t o f e g g s , l a r v a e and l a r v a e w r t b d ~ s s e c t e d y o l k s a c s was determined rn s e v e r a l g r o u p s . Eggs and l a r v a e were q u l c k l y r l n s e d i n d l s t l l l e d w a t e r , d r l e d t o c o n s t a n t w e l g h t s a t 102OC, and l n d l v l d u a l l y w e l g h e d on a Cahn e l e c t r o b a l a n c e . D u r l n g t h e e g g s t a g e t h e embryonic t l s s u e , w h l c h 1s v e r y f r a g l i e , was s t a b l i l z e d f o r l h I n 4% f o r m a l l n I n 10O/oo s e a w a t e r b e f o r e removlng t h e c h o r l o n and y o l k s a c . Cod l a r v a e p r e s e r v e d In f o r m a l l n f o r more t h a n o n e month l o s e approximately 20% o f t h e l r d r y w e l g h t ( S o l b e r g , 1 9 8 0 ) . However, o n e h preservation o f e g g s and l a r v a e d r d n o t l e a d t o a n y s l g n i f l c a n t i v e l g h t reduction.
Oxygen u p t a k e
The oxygen consumption was m e a s u r e d m a n o m e t r l c a l l y by means o f a h a r b u r g r e s p l r o r n e t e r (Towsen and Mercer S e r . 11) following s t a n d a r d m a n o m e t r l c techniques (Urnbrelt e t a l . , 1 9 6 4 )
.
The f l a s k volumes w e r e a p p r o x i m a t e l y 25 m1 and t h e t e m p e r a t u r e was k e p t a t 5 . 0 O ~ . Ten t o P % e n t y l a r v a e were t r a n s f e r r e d from r h e r e a r l n g a q u a r l a t o e a c h V a r b u r g f l a s k ~n5 nil sea water. after a l h acclimation the flasks were closed and the experiment started. Each experiment lasted 15-20 h and the light intensity was adjusted ta l00 lux at the surface of the water bath throughout the experiment. The influence of light on larval oxygen consumption was tested by covering some of the flasks with black plastic.
Energy content
The speclflc energy content of eggs, chorlons and Larvae was determrned rn one groap by a Phrllrpson mlcrobomb calorl- meter accordrnq to the method of Palne (1971) and Preus (1975). The chorrons were removed from the eggs by squeezing the eggs on a fllter paper and washing wlth dlstllled water.
The eggs, c h o r l o ~ s and larvae were freeze-drled to constant welght (20 h), around rr? a mortar and pressed to a prll of 3-5 mg dry welght. The speclflc energy content rs glven as calorres/mg ash free dry welghc. The ash conteqt was found by burnlng known amounts of eggs, chorrons and larvae at 6 0 0 ~ ~ rn a furnace.
Feeding success, perceptive field
The experlmencs were performed ~?rth larvae from one group In a feedlng aquarlum wrth a temperature of ~ O C and the llght rntensrty adlusted to 103 lux at the water surface. The feedlng success was defrned as the percentage of successful captures of prey organrsm to the total number of observed attacks.
Flve larlrae were srmuitaneously transferred to the aaua- rlum. The frrst iarva that started to attack prey was observed for 15 mrn. The rernalnrng four were drsregarded.
The number of attacks were recorded and the larva preserved In 4% formalln rn i0O/oo sea water for gut content analysis.
Durlng each attack the larval reactlve perceptrve dlstance was estrmated visually relatrve to t%e larval scandard length whlch was measured rmmedlateiy after the 1 5 rcln observatron
p e r i o d . The r e a c t i v e p e r c e p t i v e d i s t a n c e was d e f i n e d a s t h e d i s t a n c e from t h e l a r v a l e y e t o t h e p r e y o r g a n i s m a t t h e moment when t h e l a r v a a l t e r e d i t s swimming p a t t e r n t o c h a s e i t s p r e y .
Swimming s p e e d
The l a r v a l sorrwmrng s p e e d was measured i n t v ~ o g r o u p s . Twenty l a r v a e w e r e s s m u l t a n e o u s l v t r a n s f e r r e d from t h e i n c u b a t o r t o a c l r c u l a r a q u a r i u m o f 3C cm d x a m e t e r and 5 c m d e e p . The t e m p e r a t u r e was ~ O Cand t h e l r g h t r n t e n s r t y a d l u s t e d t o approximately L O O l u x a t t h e w a t e r s u r f a c e . The l r g h t s o u r c e was p l a c e d a t a 45O a n g l e t o t h e w a t e r s u r f a c e (Fig. l), whlck- made r t p o s s l b l e t o e v a l u a t e t h e p o s x t ~ o n o f t h e l a r v a e r e l a t i v e t o t h e b o t t o m and s u r f a c e by o h s e r v l n g t h e d i s t a n c e between t h e l a r v a e and t h e l r shadows. L a r v a e s t a y i n g a c t h e b o t t o m o r s u r f a c e were n e g l e c t e d . The s w r m - mrng a c t r v r t y k a s r e c o r d e d on video t a p e by a TV c a m e r a f o r
15 mrn. An a d a p t a t r o n p e r z o d o f 30 mln was a l l o v e d a 5 e r t r a n s f e r . The s w r m i n g s p e e d o f e a c h l a r v a was c a l c u l a t e d bv n e a s u r r n g t h e swrnmxng d r s t a n c e on t h e TV-monrtor s c r e e n a f t e r p l a y b a c k o f t h e v r d e o t a p e . The swimmrng d r s t a n c e was c a l r b r a t e c i t o a c e n t m e t r e m a r k e r p l a c e d -in t h e a q u a r r u r r . The sicrmmrnq a c t r v l t y was a l s c o b s e r v e d r n d a r ~ n e s s b y a r n f r a r e d s e n s l t l r r e TV-camera, u s l n g a Kodak W r a t t e n g e i a t r n f i l c e r n o 8 7 (>7G0 nm) b e t w e e n t h e l r g h t s o u r c e a r d t h e swrmning a a u a r l u m t l r g h t r n t e n s l t y 0 - 1 l u x ) .
F i g . l. O u t l i n e o f t h e o b s e r v a t i o n s y s t e m f o r r e c o r d i n g o f cod i a r v a l swimming s ~ e e d a = swimming a q u a r i u m , b= t h e r r n c s t a t c o n t r o l l e d w a t e r b a t h , c= TV c a m e r a , d= l i q h t s o u r c e , e = m o n i t o r , f= v i d e o r e c o r d e r , G= l i g h t p r o o f box.
Cod larvae show locorrotory act~vrty in brlef Ivrense bursrs of siwlrnnlng {Ellertsen er al., '1980), The frequency of these bursts vas calculated Surlng the swmtmlng speed recordrngs.
Both larval standard length and myotone herght were measured dally In 20 larvae, The helght of the myotome was measured just behrnd the anus.
RESULTS
Growth and yolk utilization efficiency
Fig. 2 shows the dry weights of eqqs, chorions and larvae of group A and B reared at ~ O C in a 12/12 h liqht (100 lux)/dark cycle. Although the eggs from the two groups at fertilization and hatching were of the same weight, they gave rise to larvae of different maximum weight and length. The overall conversion efficiency of yolk into somatic tissues was 64 and 71% In groups k and B , respectively, while the efficrency at hatching was 74% I n both groups.
Fig. 2. Standard length (AA), dry weight of eqqs (TV) chori- ons (+Q), whole larvae (WO), and larvae with dissected yolk sacs (00) of cgroup A (--AV.+--) and B (--hVoO--). SD=t2-3% of values. t=5 C. Light intensity 100 lux in a 12/12 hour
light/dark cycle.
P s g . 3 shows t%e dry werghts of larvae from group C reared a-c three dlfferent temperatures ( 3 O , 5O and ~ O C ) and two dlfferent light regimes at each temperature (constant dark- ness and constant 500 lux illumination).
-3
POST FERTILIZATION (days)
Fig. 3. A: Time of hatching against rearing temperature and B: Dry weight of whole larva: and larvae with dissected yolk sacs of group C reared at 7 C (eo)
,
5'~ (AA) and ~ O C ( * Q ) . --.A+-- larvae reared at constant darkness, --COO-- larvae reared at constant 500 lux illumination. Arrows indicate time of 50% hatching in light-( 9 )
and dark-( t )
adaptedeggs. (SD=i2-3% of values)
.
The overall conversion efficiency of yolk mass into somatic tissues was higher in dark-adapted larvae reared at
~ O C (74%) than in the other rearing categorres (68-69%)
.
The weight data of whole larvae from group C were fitted by the linear regressions given in Table l.
At all rearing temperatures the data indicate an approxi- mately 20% higher weight reduction rate (curve slope) in light-adapted larvae compared to dark-adapted ones. The differences were statisticallv significant at all three temperatures (t-test)
.
TABLE l
The table shows rhe regressron lrnes fittrng the data of toral larval dry weight of group C glven In Flg. 3. Data for the three frrst days after hatching are omltted rn all calculatrons.
temp. light regime regression L
7Oc darkness y= 83.1 - 1 . 7 6 ~ 0.99 500 lux y= 90.4 - 2 . 2 6 ~ 0.99 5Oc darkness y= 87.5 - 1 . 5 3 ~ 0.99 500 lux y= 92.3
-
1 . 8 4 ~ 0.99 3 OC darkness y= 91.3 - 1 . 2 2 ~ 1.00 500 lux y=100.0 - 1 . 5 3 ~ 0.99The temperatures of the rearrng aquarla are grven 13 Table 2. Due to tne evergy from the lrghc, the surface tempera- tures of the lrght exposed aquarla were constantly 0.2-0. ~ O C hrgher than rn the correspondlvg dark exposed aquarra. Ffter hatchlng, the larvae moved lnto the water column where the temperatures of corresponding lrght and dark exposed aquarla were slmilar.
The peak hatchlng (50%) occurred earller rn lrght adapted eggs than rn dark-adapted ones. The data of rearzng tempera- ture (y) versus days from fertrlrzatron to peak hatchrna (X)
-5.56. X
flts the exponentral functlon y=268.e wlth an r2 of 1.00. The hatching success was better tha? 95% in lrght- adapted eggs at all temperatures. In dark-adapted eggs the hatchrng success was better than 9 5 % at ~ O C , 85-90% at 5 C 0
and 75-80% at 3Oc.
TABLE 2
R e a r l n g t e m p e r a t c r e s i O C ) r n l r g b r (500 Lux) and d a r k e x p o s e d a q u a r r a d u r l n g t h e e g g a n e l a r v a l s t a g e s of g r o u p 5.
T h e r m o s t a t s e t t e m p e r a t u r e 3O d e v e l o p -
m e n t a l e g g l a r v a l e g g l a r v a l e g g l a r v a l s t a g e
5 0 0 l u x 3.1'0.2 2.9'0.2 5.220.2 5.0'0.2 6.920.3 6.910.3
d a r k n e s s 2.910.2 2.9r0.2 5.010.2 5.050.2 6.710.3 6.9i0.3
Oxygen c o n s u m p t i o n
Fig. 4 shows the oxygen u p c a k e r n l a r v a e from g r o u p s D , E a2d F .
POST HATCHING iaaysl
F l g . 4 . Oxygen u & t a k e (STPD) rn l a r v a e of g r o u p D (0)
,
E (V) and F (I) a t 5.0 C and 100 l u x l r g h t I n z e n s l c y . Number o f m e a s u r e m e n t s r s marked a t eacb p o i n t . B a r s a r e SD.After an initial rise, the uptake stabilized at a constant level between 0.090 and 0 -120 u 1 0 /larva/h. By the end of
2
the yolk sac stage (6-7 days post-hatching), the larvae reached their maximum tissue weight, 51.2 r 1.3 pg, 56.4 r 2.0 pg and 58.2 i 1.6 pg dry weight/larva in groups D, E and F, respectively, giving a weight specific uptake of 1.8, 1.9 and 2.0 u1O2/rng dry weight/h.
Fig. 5 shows the oxygen uptake of larvae from groups G and H in constant darkness and in constant 100 lux illumination, and Fig. 6 the activity of larvae from groups K , L, M and N in 100 lux illumination and 0.1 lux red light. Five and six days post-hatching the light-adapted larvae consumed 20-30%
more oxygen than dark-adapted ones, while the difference in swimming frequency (bursts of swimming per rnin) was 6-10 fold between light-(100 lux) and dark-adapted (0.1 lux red light) larvae.
POST HATCHiNG ldoysl POST HATCHfNG !days1
Fig. 5. Oxygen uptake in larvae Fig. 6. Swimming activity of group G(eo) and H(AA) at in larvae of group K(oo)
,
constant darkness (.A) and con- I, ( A A )
,
M (+O) and N (.R) at stant 100 lux illumination (oh) constant darkness (.A+ D )(0.1 lux red light) and con- stant 100 lux illumination
( O A O C I ) . Feeding success, perceptive field
Table 3 gives the feeding success of larvae from group J.
The v a r l a t l o n I n cod l a r v a l f e e d r n g s u c c e s s f r o m t h e o n s e t o f e x o g e n o u s f e e d r n g a t a g e 5 d a y s a f t e r hatching, t o t n e e n d o f y o l k a b s o r p t r o n a t a g e 8 d a y s a f t e r h a t c h r n g ,
( X : mean p e r c e n t f e e d i n g s u c c e s s , SD; s t a n d a r d deviation, SL; mean l a r v a l s t a n d a r d l e n g t h ( m m ) . W ; number o f l a r v a e examined.
Age F e e d i n g s u c c e s s SL
X - S D mm
The l a r v a e o b t a i n e d t h e l r maximum f e e d i n g s u c c e s s a t d a y 7 p o s t - h a t c h i n g . The p r e y o r g a n r s m s were made u p o f copepod n a u p l i i ( 7 4 % of t o t a l ) and b i v a l v e v e l i g e r s ( 2 6 % o f t o t a l ) . The a v e r a g e s i z e o f a l l p r e y i n g e s t e d was 193i39um.
The r e a c t i v e p e r c e p t i v e d i s t a n c e v a r i e d from 0 . 5 t o 1 SL ( s t a n d a r d l e n g t h ) . No c o r r e l a t i o n was f o u n d b e t w e e n r e a c t i v e p e r c e p t i v e d i s t a n c e a n d s i z e o f t h e p r e y . The l a r v a e d i d , however, r e a c t t o p r e y b o t h a b o v e and b e l o w t h e h o r i z o n t a l a x i s o f t h e l a r v a l body.
Swimming s p e e d
F i g . 7 gives t h e s t a n d a r d l e n g t h , myotome h e r q h t , swlmming s p e e d and swrmminq f r e q u e n c y r n l a r v a e o f g r o u p 0 and P.
I n b o t h g r o u p s t h e myotome h e i g h t and swimming s p e e d r e a c h e d t h e l r maxlmurn 1-2 d a y s b e f o r e y o l k absorption. The g r o w t h I n l e n g t h , however, d l d n o t c e a s e u n t l l , o r e v e n a f t e r , y o l k a b s o r p t r o n . C o r r e l a t r o n o f corresponding mean v a l u e s o f s w r m l n g s p e e d a n d swmriilng f r e q u e n c y , g l v e n i n F i g . 6 , showed a l e v e l o f s i g n i f i c a n c e b e t t e r t h a n 99.9% r n b o t h g r o u p s ( t - t e s t ) , whrch means t h a t d l s t a n c e t r a v e l l e d p e r b u r s t o f swimming d r d n o t d i f f e r much b e t w e e n l a r v a e o f d i f f e r e n t age.
F i g . 7 . S t a n d a r d l e n g t h , myotome h e i g h t , swimming f r e q u e n c y and swimming s p e e d I n g r o u p O ( c ) and P ( o ) l a r v a e . EYS = end o f y o l k s a c . C u r v e s f i t by e y e .
Energy c o n t e n t and e n e r g y b a l a n c e
T a b l e 4 g r v e s t h e e p e r g y c o n t e n t / m g a s h f r e e d r y w e r g h t I n e g g s , c h o r r o n s , y o l k mass a n d l a r v a e o f g r o u p B a t d l f f e r e n r d e ~ i e l o p m e r t a i s t a g e s . The a s h c o n t e r t xn n e w l y - f e r t x l l z e d e g g s was 8.9%, and l n y o l k s a c l a r v a e 8 . 3 % .
F l g . 8 shows t h e b a l a n c e between y o l k e n e r g y r e s e r v e s and c a t a b o l l c e p e r g y demand I n l a r v a e o f g r o u p B.
A t d a y 5 p o s t h a t c h l n g t h e e n e r g y r e s e r v e s o f t h e y o l k s a c w e r e l e s s t h a n t h e l a r v a l d a l l y c a t a b o l l c e n e r g y demand, a n d t h e l a r v a e e n t e r e d a s t a t e o f n e g a t l v e e n e r g y b a l a n c e .
Speclfrc energy content of whole eggs, chorrocs, yolk mass and larvae from group B [cal/mg ash free dry werght).
Energy content of yolk 1s caiculated from values of whole eggs and ckorlons at day l.
Whole eggs Chorions Yolk Larvae
Age, days post- l l2 1 12 1 20 30 fertilization
Energy content 5.8 5.6 5.5 5 . 5 5 . 9 5 . 2 4 . 8
Fig. 8. Energy content of yolk mass ( 0 ) and daily energy consumption (o) in larvae of group B reared at 5OC.
DISCUSSION
Biological material
Larval groups from drfferent female frsh were studled separateiy. The data showed srgnlflcant dliferences both In larval rvetabolrc rate and growth, even between grouDs of eggs wrth srmllar werghts. The egg groups seem therefore to reClect different g r o ~ t h and energetrc potentlals depend~ng
on parental background. kccordrnglv
,
a mlxinq of groups would have increased the varlance of che measured parameters and masked small dally changes wlthrn each group.Hatching success
The lowered hatching success found in the dark-adapted eggs of group C at 3 and 5OC might result from reduced mus- cular activity in these embryos. The light-adapted eggs hatched somewhat earlier than corresponding dark-adapted ones. The difference, however, might be fully explained by the slight deviation in rearing temperature between light and dark aquaria.
Growth and yolk utilization
The overall utilization of yolk for growth of somatic tissues varied between 65 and 74% depending in the larval group examined. The utilization in light-adapted larvae of group C seemed independent of rearing temperature. As discussed by Howell (1980)
,
several larval species show temperature dependent yolk utilization. These include Atlantic salmon ( S a Z m o saZar) !Hayes and Pelluet, l945), herring (Clupza harengus) {Blaxter and Hempel, 1966), plaice (~l~v.ronec?-Lzs platessa ) (Ryiand and Nichols, ISET), tautog (907~toga on i t i s ) (Laurence,
1973) and yellowtail f launder(3-lmnnda fei-rug-Lnec) (Howell, 1980)
.
Some of the authors,however, reported temperature independence over a specific interval of the temperature range tested (Hayes and Pelluet, 1945; Blaxter and Hempel, 1966; Laurence 1973).
The temperature of the surface layers in the Lofoten spawning area during April to mid-May varies between 3 and 6OC [F?idttun, 1975). The 68-69% yolk util-ization registered both in light- and dark-reared larvae of group C at 3 and ~ O C therefore might be an optimized adaptation within the natural temperature range, srhile the ?4"stilization registered in dark-reared larvae at ~ O C , is probably a reaction to an
that natural selection would favour organisms maximizing their growth. The present observed deviation, however, is not necessarily a benefit to the larvae as the yolk volume available for swimming performance is reduced.
1
Oxygen uptakeManometric methods are frequently used when measuring oxygen consumption in fish larvae (Holliday et al., 1964;
Lawrence, 1969, 1978; Hunter and Kirnbrell, 1979; Cetta and Capuzzo, 1982), but are not considered as particularly
I
satisfactory with these animals (Hunter and Kimbrell, 1979, Theilacker and Dorsey, 1980).The uptake at yolk absorption, however, showed a good correlation with the catabolic energy production calculated from the larval daily weight decrement. This is to be expected provided the only source of energy is of endogenous origin. A similar correpondence was reported in larvae of
I
Pacific sardine ( ~ ~ r d i n o p s caeruZea ) (Lasker, 1962) and largemouth bass (Micropterus satmoides ) (Laurence, 1969).The weight-specific oxygen uptake at yolk absorption also showed a good correlation with the uptake registered in larvae of the Arcto-Norwegian cod at the same developmental stage (1.9-2.0~11 02/mg dry weight) measured by means of oxygen electrodes (Davenport and LGnning, 1980).
The increasing oxygen uptake measured during the first l
l
couple of days after hatching was probably related to growth1
of somatic tissues and increased swimming activity.The 20-40% difference in oxygen consumption measured
I between light- and dark-adapted larvae was moderate compared with the 6-10 fold difference in swimming frequency (bursts of swimming/min) registered under similar light conditions.
l The impact of activity on oxygen consumption in cod larvae thus seemed low compared with the 3.5 fold difference regis- tered between active and inactive larvae of Pacific sardine (Lasker and Theilacker, 1962) and the 9-10 fold difference
registered between active and inactive larvae of herring (Holliday et al., 1964). As discussed by Hunter and Kimbrell (1979) and Theilacker and Dorsey (1980)
,
the larval activity might have been depressed in the Warburg flasks, thus leading to an underestimation of the impact of activity on oxygen consumption. The difference in weight reduction rate, however, between light- and dark-adapted larvae, was no more than 20%, thus confirming the relatively low contribution of activity to routine metabolism in larvae of cod.Swimming activity
Maximum swimming activity occurred simultaneously with maximum somatic tissue weight. The swimming speed was probably somewhat underestimated as the measurements were only made in the horizontal plane. The deviation, however, is probably minor, as cod larvae examined at similar ages tended to maintain their depth when the light was kept constant at intensities normally experienced at sea (Tilseth and Str@mme, 1976). Hunter and Kimbrell (1979) and Weihs
(1980) proposed that the intermittent swimming pattern of fish larvae has a respiratory function, each burst of swimm- ing being triggered by a fall in oxygen tension around the larvae. In cod larvae this seems unlikely since the swimming speed dropped to nearly zero during darkness.
Feeding success
The 11-22% feeding success found at yolk absorption was higher than in most other larval species examined, such as herring (Blaxter and Staines, 1971; Rosenthal and Hempel, 1970)
,
anchovy ( E n g r a u Z i s m g r d a x ) (Hunter, 1972) and corego- nid larvae (Braum, 1967). The feeding ability was more in accordance with the 32% feeding success registered in first- feeding larvae of plaice (Blaxter and Staines, 1971) which was attributed to their greater manoeuvring ability comparedwith herring. The same ability was registered in larvae of cod (Ellertsen et al., 1980).
Energy content and energy balance
The decrease in specific energy content of eggs and larvae measured during the experimental period was probably due to conversion of yolk rich in energy into larval somatic tissues of lower specific energy content.
As shown in Fig. 8 , the larvae after day 5 entered a state of negative energy balance. At this stage the larval somatic tissue weight and swimming activity was at its maximum, as also was the larval feeding incidence (Ellertsen et al., 1980). The physiological condition of the larvae therefore seems optimized for survival during this period.
Food density requirements
One of the objectives of the present study was to deter- mine the larval energy and food density requirements at the onset of feeding. The density might theoretically be calcu- lated from the following equation:
N = number of prey organisms11
ER = energy requirement in first-feeding larvae EC = energy content of prey organism
SV = larval search volume FS = feeding success
The main prey organism of first feeding cod larvae are nauplii of CaZanus finmarchicus with mean carapace length 250pm (Wiborg, 1948; Ellertsen et al., 1976). As far as is known data of weight and energy content of copepod nauplii are unavailable. Volumetric considerations, combined with data of specific weight (Gross and Raymond, 1942), and water
content (Tande, 1979), indicate a dry weight of a 2 5 0 ~ m nauplii of 0.3~9. This is close to the 0 . 2 9 ~ 9 average weight of copepod nauplii offered first feeding larvae of three subtropic marine fishes by Houde and Schekter (1980). The specific energy content was taken as 5.5 cal/mg dry weight, a mean of the values used by Lasker (1962) and Houde and Schekter (1983), giving an energy content of a 250 Pm nauplii of 1.7x10-~ cal. At yolk absorption the larvae consumed oxygen equivalent to 0.010-0.013 cal/larva/day (4.6 cal/ml 02, Brett and Groves, 1979). This, however, only accounts for the catabolic energy. When exogenous food is offered, some of it will be used for growth. During the yolk sac stage the larval daily growth rate was 5-7% (dry weight). If this also applies to first-feeding larvae, the anabolic requirement will correspond to 0.03-0.04 cal/day. The utilization of exogenous food for growth and metabolism is not known in cod larvae, but is probably less than 50%, as found in several other species of marine fish larvae (Cetta and Capuzzo, 1982; Houde and Schekter, 1983). In the present calculations it is taken as 40%. The data compiled thus indicates a total larval demand for exogenous food at the onset of feeding corresponding to 0.09-0.13 cal/larva/day.
In May, the light intensity in the Lofoten area is sufficient for feeding 24 h/day (Gj6sather and Tilseth, 1982). The calculated food density requirements are given in Fig. 9.
The calculated minimum values are higher than the average densities (<20/1) obtained by a in situ particle counter and pump in the Lofoten area in April-May (Tilseth and Ellertsen, 1984a). Similar estimates of critical prey densities in first-feeding fish larvae based on laboratory examination of larval food, searching potential and energy demand, tend to give higher values than generally observed at sea (Rosenthal and Hempel, 1970, 1971; Hunter 1972; Houde and Schekter, 1983). As pointed out by Houde and Schekter (1983), the calculated values are based on average performances. The possibility therefore exist that only exceptionally fit larvae are able to survive under the normally observed
margrnal fooa densrtres, arid tnat large scale survl-ial 1s
dependent on pacchy food d ~ s t r l b u t l o n ~ as proposed for larvae of northern anchovy ay Vlymen (1977) and Lasker and Zwelfel
(1978). Usrng an In sltu partlcle counter Tllseth and Ellertsen (1984a) observed patchy drstrlbutlon of copepod naupllr ln the Lofoeen area at densltles of 100-300/1.
POST HATCHING (days)
Fig. 9. Food d e ~ s l t y requirements frrst feedlng larvae of cod from the onset of exogenous feedlng (day S), TO yolk absorptron (day 6 ) . The curves are based on rnaxlnum ( M ) ,
) and mlnlmum (M) values of swlmnlng speed, feedrng success and catabolic energy demand.
As pointed out, however, by Blaxter and Staines (1971), Rosenthal and Hempel (1970) and discussed by Houde and Schekter (1980), major errors may occur in calculated larval search volumes which are very sensitive to deviations in the observed perceptive distances. Likewise, the swimming speed of cod larvae in a 5 cm deep laboratory aquarium containing - .
rlltered sea water is not necessarily the same as in the open sea. Von Westernhagen and Rosenthal (1959) reported lower swimming speeds In laboratory-reared herring larvae compared with "wild" ones, and Hunter and Thomas (1974) reported a change in the swimming pattern of the larvae of Pacific mackerel ( ~ c o r n b e r ,fapon-lcu; ) when offered food. Accordingly the present calculated values should be regarded as prelimi- nary estimates.
isle wish to tilank Mr. A. Xassel for continuing advice during the writing. The work was in part supported by The Norwegian Marine Pollution Research and Fionitoring Programme.
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