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Fl@devigen rapportser., 1, 1984. ISSN 0333-2594 The Propagation of Cod

G a d u s m o r h u a

L.

A CASE STUDY ON THE DISTRIBUTION OF COD LARVAE AND AVAIL- ABILITY OF PREY ORGANISMS IN RELATION TO PHYSICAL PROCESSES IN LOFOTEN

B. Ellertsen, P. Fossum, P. Solemdal, S. Sundby and S. Tilseth*

Institute of Marine Research, P.O.Box 1870, N-5011 NORDNES Norway

ABSTRACT

Ellertsen,

B.,

Fossum, P., Solemdal, P., Sundby, S. and Tilseth, S., 1984. A case study on the distribution of cod larvae and availability of prey organisms in relation to physical processes in Lofoten. In: E. Dahl, D.S.

Danielssen and E. Moksness and P. Solemdal (Editors), The Propagation of Cod

Gadus m o r h u a

L. Fl@devigen rapport- ser.,

1 ,

1984: 453-477-.

The distribution of cod larvae and copepod nauplii off the Lofoten islands was studied in the years 1979-82. The highest concentrations of cod larvae and copepod nauplii were found in the Austnesfjord, H@lla-Henningsver and the Vester- alsfjord area.

The feeding condition of cod larvae was studied in re- lation to prey abundance, and a direct proportionality was seen between gut content and prey abundance in the larvae at the end of the first feeding period.

Vertical migration of cod larvae was observed at a diurnal station in the VesterSlsfjord under calm weather conditions.

The feeding condition of the cod larvae were bad under the prevalent prey density (<l0 prey org. per 1) at this station.

At a diurnal station in the Austnesfjord, the onset of turbulent mixing lowered the prey and cod larval densities and created a homogeneous vertical distribution of cod larvae

"Authorship equal

and prey animals. This was reflected in the gut content analysis when the percentage of cod larvae with gut contents, and the number of prey organisms per larval gut and the percentage of newly taken prey organisms fell to a critical

level.

INTRODUCTION

It is generally accepted that the variations in stock sizes of various fishes are determined early in their life history, and that the survival of the broods depends to a certain extent on the quantity of food available for the larvae in the plankton.

G.O. Sars (1879) considered wave action and the drifting ashore of eggs and larvae as the main mortality factor in North-east Atlantic cod (Gadus morhua L.). The later hypo- thesis by Hjort (1914) on a "critical period" in the early life of fish larvae, suggesting a lack of available food organisms at the time of yolk absorption as the predominant factor causing high mortality, has been the subject of several investigations.,- May (1974) on the basis of 11 investigations reported in the literature, concluded that no conclusions could be drawn about the existence of a critical period in fish larvae due to inadequate sampling methods.

Sampling methods have been improved and it is now possible to collect larvae and their food organisms from the same water masses at discrete depths (Ellertsen et al., 1981a; Lough, 1984; Fridgeirsson, 1984; Solemdal and Ellertsen, 1984).

Investigations on cod larvae and their food organisms have been carried out in the field by Wiborg (1948) , Marak (1960), Bainbridge and McKay (1968), Ellertsen et al. (1977) , Last (1978), and others,

providing information

on prey type selec- tion, prey size, etc.

Apart from lack of food several other factors which lead

to a high mortality during a "critical period" have been

suggested. A correlation between SW winds and the subsequent

year-class of Arcto-Norwegian cod has been indicated by

Rollefsen (1930; 1932), an effect of freshwater outflow in

the coastal current has been suggested by Gran (1923; 1930), Sund (1924) and Skreslet (1976).

The physical environment may cause distributions of eggs, larvae and prey organisms which are not optimal for the feeding of fish larvae. During moderate weather conditions cod larvae are found in the upper 20 m and their main prey items, copepod nauplii (Wiborg, 1948), have a vertical distribution with a clear maximum at depths of 5-15 m in daytime and the upper 10 m at night (Ellertsen et al., 1981a), and the nauplii concentrations may be high enough to provide sufficient food for growth and survival of the larvae. The horizontal distribution of eggs and larvae is affected by the weather conditions (Ellertsen et al., 1981b;

Rae, 1957); the effect of wind on the vertical distribution of eggs is shown by Solemdal and Sundby (1981). To what extent the wind conditions influence feeding in cod larvae is the subject of the present investigation, which is designed to provide information on the feeding conditions of cod larvae sampled in areas differing physicallv with regard to wind and wave exposure.

MATERIALS AND METHODS

The distribution of cod larvae in the Lofoten area was studied in the three first weeks of May in 1979-1982. Cod larvae were sampled with Juday nets (0.5 m2 opening and 375 um mesh size) and modified conical nets (0.75 m 2

,

¹ rn2 and 2 m opening and 375 um mesh size) in 50-0 m vertical hauls 2

at 200 stations off the Lofoten archipelago. The largest nets were used in areas where the abundance of cod larvae was low. The vertical distribution of cod la'rvae was investigated by the submersible electric pumps described by Solemdal and Ellertsen 1984. Samples were taken at 5 , 10, 15, 20, 25, 30 and 35

m

depth. Fifteen

rn3

of sea water were sampled at each depth. The die1 vertical distribution of cod

larvae was investigated both in the sheltered Austnesfjord (in 1980) and the open bay of the Vesteralsfjord (in 1982).

The larvae were preserved in 4% formalin in loO/oo sca- water for morphometric measurements and gut content analysis.

After dissection, the rest of the gut and liver were removed.

The larvae were rinsed in fresh water, dried to constant weight (min. 24 h) and weighed on a Cahn electrobalance to the nearest pg. The use of formalin preserved larvae to estimate dry weight has some disadvantages as indicated by Theilacker and Dorsey (1980). Loss of fat can reduce the weight up to 30%. In spite of this we have used formalin preserved larvae. The reason is that we tried to see some trends in the weight distribution in relation to prey abun- dance. The same age groups of larvae are compared, the same preservation procedure followed, and the most fat containing organ, the liver, is removed together with the gut during dissection.

The vertical distribution of microzooplankton was investi- gated by using a small submersible electric pump (Flygt 2051, 250 l/min)

.

Samples were pumped on deck through a 50 m long and 5 cm in diameter hose. Samples were taken from following depths: 0 , 2.5, 5 , 7.5, 10, 12.5, 15, 20, 25, 30 and 40 m.

The samples were collected in calibrated tanks (23.7 l), fil- tered through 90 pm mesh size nylon gauze and the zooplankton preserved in 4% formalin. The whole zooplankton sample was counted and identified under a binocular microscope. In 1981 and during the diurnal station in 1980 the abundance of microzooplankton was estimated with the "in situ" particle counter system described by Tilseth and Ellertsen (1983).

Zooplankton samples were taken in connection with cod larval sampling at every second station on each transect of stations, and every second hour through the 24 h period at diurnal stations (the Austnesfjord and the VesterBls- fjord)

.

Salinity and temperature data were collected by a CTD- sonde. The static stability of water masses is expressed by

I,

the squared Brunt-VaisSlS frequency (Phillips, 1977) and

computed for every

5

m intervals for the diurnal stations.

The vertical mixing of the upper layer is largely a function of the energy input by the action of wind (proportional to the squared mean wind speed), and the stratification of the water column. In 1980 a Wolfe wind recorder was used to read the mean wind speed per h. In 1982, the wind speed was manually measured every 30 min.

A study of the vertical migration of cod larvae and their prey organisms was carried out at two diurnal stations in Austnesf jord (1980) and Vesterslsf jord (1982) . ^Vertical

migration of planktonic organisms is influenced by the buoyancy of the organisms and the vertical movement of the water masses. The most important cause of vertical movement of the water masses is vertical turbulence which is a func- tion of the wind mixing and stratification of the water masses. In the diurnal station, the effect of energy impact from wind stress (given as the squared wind speed) on the static stability of the water masses, expressed as the square1 Brunt-VaisXlX frequency, was studied.

The spawning activity of the North-east Atlantic cod was studied for the period 1975-1983. The maximum and 50%

spawning were correlated to the mean sea temperature to estimate the correct time of the "first feeding period" of the larvae. In recent years, maximum spawning occurred at the end of March or the first days of April (Pedersen, 1984).

First feeding of cod larvae occurred in the first two weeks of May.

The resorption of the epithelia1 cells surrounding the yolk sac was used as an index of larval age. The yolk sac staging system described by Ellertsen et al. (1980) was extended with three more stages (Fig.

1).

In stage

8

the epithelial cells which surrounded the yolk

sac are reduced to a string under the stomach. In stage

9

this string is broken up and in stage 10 the epithelial cells

are absent. The duration of stages

8

+ 9 is about

2

weeks

and independent of larval growth. A comparison of larvae in

the same stage will therefore be a comparison of larvae in

the same age groups within the temperature range experienced in different parts of the Lofoten area at this time of the year.

Fig.

1.

A staging system of cod larvae which focus on the resorption of the yolk sac content and development of gut, liver and jaw apparatus, st. 1-7 (Ellertsen-&A,

1980).

The larvae in stage

8-10

are staged on the resorption rate of epithelia1 cells surrounding the yolk sac (see text).

This was verified by comparing two groups of cod larvae.

One was fed a maintenance ration in the laboratory and did not grow at all (t=6O~); the other was a fast growing group in a concrete enclosure experiment (t=7-8Oc) (Ellertsen et al., 1981~). No significant difference in the resorption of the epithelia1 cells surrounding the yolk sac was seen.

However, more experiments at different feeding and tempera- ture regimes must be carried out to verify this.

In the gut content analysis the same stages were compared.

The yolk sac stages representing cod larvae in the first

exogenous feeding stages

(5

and

6)

were pooled, as were the

larvae expected to be in the most critical stages (7, 8 and 9). Samples were analysed from several time periods from the different areas. Means are given with + ^SD.

l

Distribution of cod larvae and copepod nauplii

l

I

The distribution of cod larvae in the Lofoten area during the first weeks of May is dependent on the time and location of spawning, and the physical condition and velocity of the water masses in the drift route of the eggs. The North-east Atlantic cod spawn mainly in the eastern part of Lofoten as shown in Fig. 2. The drift route is shown in Fig. 2, and a

l vertical section on the western side of the Lofoten islands is presented in Fig. 3. The eggs float in the upper 50 m in the near land zone (Sundby, 1983), in 1982 this was in water with a temperature of 5.0°c and a salinity below 34.0~100.

The velocity of this drift depends on the formation of eddies, the bottom topography and the wind stress.

Since the incubation time, and therefore the length of the drift, depends on the water temperature, the whole area from the inner parts of Lofoten to Langby (Fig.

2)

must be sur- veyed to ensure that the main hatching and first feeding areas are covered. The distributions of cod larvae in 1979 and 1982 are shown in Fig.

4.

In 1979, most of the eggs hatched in the Vesterblsfjord-Lang@y area and in 1982, most eggs hatched on the eastern side of the Lofoten islands. The occurrence of cod larvae during the same period in 1980 and 1981 was low and larvae were only observed in a few Juday net hauls. Characteristic of 1979 and 1982 was a similar pattern of distribution in the coastal current close to the shore, with the highest concentration of cod larvae in the Austnes- fjord, Henningsvaer-Hblla area and the Vester5lsfjord area.

The density of cod larvae in the Austnesfjord was high in all

four years.

The distribution of copepod nauplii, mainly CaZanus fir: - m a r c h i c u s , in the Lofoten area was investigated in the years 1980, 1981 and 1982. The results are presented in Fig.

5

as the mean density of nauplii per 1 in the upper 40 meters of the water column. The abundance of copepod nauplii varied from year to year; 610 nauplii per 1 in 1980, ~ 4 0 nauplii per 1 in 1981 and S20 nauplii per

1

in 1982. The distribution was similar to that of cod larvae, with highest densities of nauplii occurring in the Austnesfjord, H0lla-Henningsvar and Vesterglsfjord areas.

20 COD EGGS PER M' l0I SURFACE

F i g . 3. D i s t r i b u t i o n o f cod e g g s , t e m p e r a t u r e and s a l i n i t y on a s e c t i o n of t h e w e s t e r n s i d e of t h e L o f o t e n i s l a n d s , 17 May 1982.

R e l a t i o n between f e e d i n g r a t e s and n a u p l i a r abundance

I n 1982, g u t c o n t e n t a n a l y s e s were made on l a r v a e from f o u r l o c a l i t i e s h a v i n g d i f f e r e n t s t r e s s e s o f wind and c u r - r e n t s and abundances of p r e y organisms ( F i g . 5) ; t h e s h e l - t e r e d A u s t n e s f j o r d had 21.0 n a u p l i i p e r l , w i t h a r a n g e of 17-24 n a u p l i i p e r 1 (N=2) t h e ocean bay of V e s t e r g l s f j o r d 5.8k4.4 n a u p l i i p e r 1 w i t h a r a n g e of 0-23 n a u p l i i p e r l (N=23) and t h e e a s t e r n and w e s t e r n p a r t s o f L o f o t e n had 5 . 6 t 2 . 3 w i t h a r a n g e of 2-11 ( N = l l ) and 4.3k3.0 w i t h a r a n g e of 1-10 (N=22) n a u p l i i p e r 1 r e s p e c t i v e l y . L a r v a e from t h e s e a r e a s were compared w i t h r e g a r d t o t h e i r d r y w e i g h t and g u t f u l l n e s s .

The r e s u l t s a r e p r e s e n t e d i n F i g . 6. a s t h e cod l a r v a l f e e d i n g i n c i d e n c e ( p e r c e n t a g e of l a r v a e w i t h g u t c o n t e n t ) and f e e d i n g r a t i o (number o f food i t e m s p e r l a r v a ) ( T i l s e t h and E l l e r t s e n , 1 9 8 3 ) . I n t h e A u s t n e s f j o r d t h e cod l a r v a l f e e d i n g i n c i d e n c e was 89+6% f o r s t a g e 7 , 8 and 9 l a r v a e . The young- e s t l a r v a e ( s t a g e 5 and 6 ) showed a f e e d i n g i n c i d e n c e of 43+4%. The f e e d i n g i n c i d e n c e o f t h e o l d e s t l a r v a e ( s t a g e 7 , 8 , 9 ) from t h e V e s t e r A l s f j o r d was 8 9 + 7 % . The f e e d i n g i n c i -

Fig. 4.

surf ace,

The horizontal distribution of cod larvae, .n May 1979 and May 1982.

F i g . 5. The h o r i z o n t a l d i s t r i b u t i o n o f copepod n a u p l i i ( N / 1 ) i n May 1980, May 1981 and May 1982.

dence o f t h e y o u n g e s t l a r v a e ( 5 and 6 ) was, however, o n l y 21+21%. The f e e d i n g i n c i d e n c e o f t h e o l d e s t l a r v a e ( s t a g e 7 , 8 and 9 ) from t h e e a s t s i d e o f t h e L o f o t e n i s l a n d s was 7 9 + 1 3 % ) t h e f e e d i n g i n c i d e n c e o f t h e y o u n g e s t l a r v a e ( s t a g e

MEAN NAUPLIAR DENSITY IN SEA 21 NIL 6 NIL 6 NIL 4Nll

AUSTNES

VESTERALS

LOFOTEN LOFOTEN

FJORD FJORD EAST WEST

SIDE SIDE

F i g . 6 . The f e e d i n g i n c i d e n c e ( p e r c e n t a g e of l a r v a e w i t h g u t c o n t e n t , F I % )

,

and f e e d i n g r a t i o ( f o o d i t e m s p e r l a r v a , FR) i n cod l a r v a e from f o u r s e l e c t e d a r e a s .

5 , 6 ) was 1 5 * 1 0 % i n t h i s a r e a . The f e e d i n g i n c i d e n c e i n s t a g e 7 , 8 and 9 l a r v a e from t h e w e s t s i d e o f t h e L o f o t e n i s l a n d s was 5 9 ? 4 % . The y o u n g e s t l a r v a e ( s t a g e 5 and 6 ) i n t h e s e samples had n o t been a b l e t o c a p t u r e n a u p l i i a t a l l , e x c e p t f o r l a r v a e i n one sample on 9 May when 2 2 % of t h e y o u n g e s t l a r v a e were found w i t h g u t c o n t e n t s . The cod l a r v a l f e e d i n g r a t i o h a s been c a l c u l a t e d o n l y f o r t h e o l d e s t l a r v a e from t h e d i f f e r e n t a r e a s . The r e s u l t s a r e p r e s e n t e d i n F i g . 6 , showing a cod l a r v a l f e e d i n g r a t i o of 3 . 2 k 1 . 0 n a u p l i i / l a r v a l g u t i n samples from t h e A u s t n e s f j o r d , 2 . 7 k 0 . 3 n a u p l i i / l a r v a l g u t i n t h e V e s t e r A l s f j o r d and 1 . 7 k 0 . 7 n a u p l i i / l a r v a l g u t i n samples from L o f o t e n e a s t s i d e and 1 . 2 i 0 . 1 n a u p l i i / l a r v a l g u t i n samples from t h e w e s t s i d e of t h e L o f o t e n i s l a n d s .

The d r y w e i g h t s of cod l a r v a e s t a g e s 6 , 7 and 8 from t h e f o u r d i f f e r e n t a r e a s a r e compared i n F i g . 7 . Too few l a r v a e i n s t a g e 9 were found i n t h e A u s t n e s f j o r d and L o f o t e n e a s t s i d e t o be p r e s e n t e d i n t h i s f i g u r e . The mean d r y w e i g h t s o f

MEAN NAUPLIAR DENSITY IN SEA 21 N I L 6 N I L 6 N I L 4 N I L

o?- _{6 1 8} --..-- 6 1 8 9

-

6 1 8 6 1 8 9

NZ 71 N.329 N=152 N.82

AUSTNES VESTERALS LOFOTEN LOFOTEN

EAST WEST

FJORD FJORD SIDE

F i g . 7 . The d r y w e i g h t of cod l a r v a e i n s t a g e 6 , 7 and 8 i n f o u r s e l e c t e d a r e a s .

s t a g e 8 l a r v a e was t h e same i n a l l a r e a s . The l a r v a e i n s t a g e 6 were h e a v i e r i n t h e most exposed a r e a , t h e w e s t e r n s i d e o f L o f o t e n , having mean w e i g h t s of 46 ug. The l a r v a e i n s t a g e 6 weighed 3 9 ug i n t h e A u s t n e s f j o r d , 37 pg i n t h e e a s t e r n p a r t o f L o f o t e n and 40ug i n t h e V e s t e r 5 l s f j o r d .

E f f e c t s o f mixing

V e r t i c a l p r o c e s s e s were r e l a t e d t o t h e v e r t i c a l d i s t r i - b u t i o n of copepod n a u p l i i and cod l a r v a e a t t h e two d i u r n a l s t a t i o n s i n A u s t n e s f j o r d on 12-15 May 1980 and V e s t e r S l s f j o r d on 12-13 May 1982. The p h y s i c a l s i t u a t i o n i n t h e A u s t n e s - f j o r d a f t e r a p e r i o d of calm w e a t h e r c o n d i t i o n s , t y p i c a l f o r t h i s s h e l t e r e d l o c a t i o n , was one of h i g h s t r a t i f i c a t i o n ( F i g . 8 ) . Three d i f f e r e n t w a t e r masses were p r e s e n t i n t h e f j o r d ,

-

120. 12. MAY 1980

40 - 60 - r

1.8 0 - t ° C

4 "

100-

120- 15. MAY 1980 1407

F i g . 8 . The v e r t i c a l s t r u c t u r e o f t e m p e r a t u r e and s a l i n i t y i n A u s t n e s f j o r d a t t h e o n s e t o f t u r b u l e n t mixing 12 May and a f t e r two d a y s w i t h t u r b u l e n t mixing 15 May 1 9 8 0 .

V e s t f j o r d w i n t e r w a t e r was found between a n u p p e r l a y e r and V e s t f j o r d b o t t o m w a t e r . There was a h i g h abundance o f copepod n a u p l i i and cod l a r v a e , and t h e p l a n k t o n i c o r g a n i s m s were p a t c h i l y d i s t r i b u t e d i n t h e w a t e r masses. Then a heavy impact of e n e r g y , from s t r o n g winds blowing a g a i n s t t h e head of t h e f j o r d , c r e a t e d t u r b u l e n t m i x i n g , and t h e V e s t f j o r d w i n t e r w a t e r was p r e s s e d o u t of t h e s y s t e m between t h e b o t t o m w a t e r and t h e u p p e r l a y e r s . A s a r e s u l t t h e r e was a t r a n s -

F i g . 9. The s q u a r e d wind s p e e d ^{( A ) ,} t h e s q u a r e d runt-~aisala f r e q u e n c y ( B ) , t h e d e n s i t y of m i c r o z o o p l a n k t o n g i v e n a s p a r - t j c l e s p e r l i t e r ^{( C )} and d e n s l t y o f cod l a r v a e a s l a r v a e p e r m ( D ) i n t h e d i u r n a l s t a t i o n i n t h e A u s t n e s f j o r d 12-15 May 1 9 8 0 .

p o r t o f n a u p l i i a n d l a r v a e o u t o f t h e s y s t e m a n d p a t c h e s i n t h e u p p e r w a t e r m a s s e s w e r e b r o k e n u p a n d t h e d i s t r i b u t i o n o f o r g a n i s m s became more homogeneous. F i g . 9 B shows t h e s t r a t i -

I,

f i c a t i o n o f t h e w a t e r m a s s e s g i v e n as t h e Brunt-VaisEila f r e q u e n c y , and t h e wind s i t u a t i o n ( F i g . 9 A ) . T h e r e w e r e v e r y s t a b l e w a t e r m a s s e s a t t h e b e g i n n i n g o f t h e s a m p l i n g p e r i o d ,

,I

r e p r e s e n t e d by a h i g h Brunt-VaisSlEi f r e q u e n c y . A f t e r t h e wind s r a r t e d t o blow t h e s t a b i l i t y was b r o k e n down.

F i g . , , l O . The s q u a r e d wind s p e e d ( A ) , t h e s q u a r e d ~ r i i n t - V a l s a l a f r e q u e n c y ( B ) , t h e d e n s i t y o f m i c r o z o o p l a n k t o n g i v e n a s na p l i i p e r l i t e r ( C ) and d e n s i t y o f cod l a r v a e a s l a r v a e p e r

' m

( D ) i n t h e d i u r n a l s t a t i o n i n t h e V e s t e r Z l s f j o r d 10- 12 May 1 9 8 2 .

High c o n c e n t r a t i o n s o f copeppd n a u p l i i w e r e f o u n d i n t h e t r a n s i t i o n l a y e r s b e t w e e n t h e u p p e r w a t e r a n d t h e V e s t f j o r d w i n t e r w a t e r a t t h e b e g i n n i n g o f t h e s a m p l i n g p e r i 0 d . A h i g h e n e r g y i m p a c t a t a b o u t 1700 h c r e a t e d a l o t o f t u r b u l e n t m i x i n g a n d t h i s i s s e e n a s a d e c r e a s e o f t h e B r u n t - V a i s z l i i

,,

f r e q u e n c y . T h e r e was a f a l l i n g c o n c e n t r a t i o n o f copepod n a u p l i i a n d cod l a r v a e t h r o u g h o u t ( F i g . 9C, D ) a n d t h e upward m i g r a t i o n o f c o d l a r v a e was h i n d e r e d by t h e t u r b u l e n t m i x i n g . A t t h e d i u r n a l s t a t i o n i n V e s t e r a l s f j o r d t h e r e was h i g h s t r a t i f i c a t i o n o f t h e u p p e r w a t e r m a s s e s ( F i g . 10B) w i t h v e r y l i t t l e i m p a c t o f e n e r g y from wind s t r e s s ( F i g . 10A) and s i g n i f i c a n t v e r t i c a l m i g r a t i o n s t o w a r d s t h e s u r f a c e o f t h e cod l a r v a e and copepod n a u p l i i w e r e s e e n i n t h e m i d d l e o f t h e n i g h t ( F i g . 1 0 D ) .

AUSTNESFJORD

-NO MIXING : MIXING

1980 1700 HOURS 2000 HOURS 2300 HOURS 0200 HOURS 0500 HOURS 1000 HOURS

W0

³

gsplpO

^{3 3} 3

3a+\+PfFA

o - - -

.-

FEEDING RATIO

'-7 FEEDING INCIDENCE

1982 2300 HOURS 0100 HOURS 0300 HOURS 1200 HOURS 2100 HOURS 0200 HOURS

F i g . 11. The f e e d i n g i n c i d e n c e ( p e r c e n t a g e o f l a r v a e w i t h g u t c o n t e n t , F I % ) , and t h e f e e d i n g r a t i o (number o f f o o d i t e m s p e r l a r v a e , FR) i n t h e d i u r n a l s t a t i o n s i n t h e A u s t n e s - f j o r d ( 1 9 8 0 ) and V e s t e r a l s f j o r d ( 1 9 8 2 ) .

The r e s u l t s f r o m t h e g u t c o n t e n t a n a l y s e s o f c o d l a r v a e from t h e d i u r n a l s t a t i o n i n t h e A u s t n e s f j o r d a r e p r e s e n t e d a s l a r v a l f e e d i n g i n c i d e n c e s and f e e d i n g r a t i o s i n F i g . 11. The f e e d i n g i n c i d e n c e ( p e r c e n t a g e o f l a r v a e w i t h g u t c o n t e n t ) v a r i e d b e t w e e n 73% a n d 1 0 0 % (mean f e e d i n g i n c i d e n c e was 9 4 + 3 . 7 % ) i n s a m p l e s t a k e n b e f o r e a n e f f e c t o f v e r t i c a l m i x i n g c o u l d b e d e t e c t e d i n t h e f e e d i n g c o n d i t i o n s o f t h e l a r v a e . The l a r v a l f e e d i n g r a t i o (number o f f o o d i t e m s p e r l a r v a e ) was 2.220.5. The f e e d i n g i n c i d e n c e v a r i e d b e t w e e n 4 % a n d 9 2 %

i n s a m p l e s t a k e n a f t e r t h e m i x i n g o f t h e s u r f a c e l a y e r s ( 6 9 + 2 2 % ) . The f e e d i n g r a t i o was < 2 p r e y / l a r v a l g u t i n a l l s a m p l e s a f t e r t h e m i x i n g o f t h e u p p e r 40 m o f t h e w a t e r m a s s e s ( 0 . 9 2 0 . 4 ) . The h i g h e s t c o d l a r v a l f e e d i n g r a t i o o b s e r v e d a f t e r t h e e v e n t o c c u r r e d was 1 . 7 p r e y / l a r v a l g u t .

TABLE 1.

The f e e d i n g i n c i d e n c e ( p e r c e n t a g e o f l a r v a e w i t h g u t c o n t e n t ) ( F I ) and t h e p e r c e n t a g e o f l a r v a e w i t h newly c a p t u r e d n a u p l i i (NC) i n t h e g u t , a t two d i u r n a l s t a t i o n s ; A u s t n e s f j o r d ( 1 9 8 0 ) a n d V e s t e r d l s f j o r d ( 1 9 8 2 ) .

D a t e Hours Year NC F1 Number o f

l a r v a e

The d a k a from 1980 i s f r o m T i l s e t h a n d E l l e r t s e n ( 1 9 8 3 ) . T h i s was f o u n d i n o n e s a m p l e f r o m 25 m d e p t h a t 1000 h. The h i g h e s t cod l a r v a l f e e d i n g r a t i o s were f o u n d i n s a m p l e s f r o m 20-30 m d e p t h s .

The g u t c o n t e n t a n a l y s e s o f cod l a r v a e f r o m t h e d i u r n a l s t a t i o n i n t h e V e s t e r 5 l s f j o r d a r e a l s o p r e s e n t e d i n F i g . 11.

The d e n s i t i e s o f cod l a r v a e w e r e much l o w e r t h a n i n t h e A u s t n e s f j o r d and numbers s u f f i c i e n t f o r g u t c o n t e n t a n a l y s e s (N220) w e r e n o t f o u n d i n s a m p l e s from a l l d e p t h s . The c o d

larval feeding incidence varied between 35% and 61% (48+11%).

The highest larval feeding incidence (60%) was found in samples from 20 m depth at 2300 h and 10 m depth at 1230 h.

The cod larval feeding ratio varied between 0 prey/larval gut to 1.5 prey/larval gut with a mean of 0.820.4. Larvae with empty guts were found in one sample from 20 m depth at 0130 h, and the highest feeding ratio was found in larvae from 10 m depth at 1230 h. The percentage of larvae with newly captured nauplii in the gut are presented in Table 1.

18.626% of the gut content was newly taken nauplii in larvae from Austnesfjord before the turbulence had broken down the stability of the water masses. Later the percentage fell to 7+5% throughout the diurnal station. In Vesterblsfjord, gut contents contained 9+11% newly captured prey organisms, in 4 samples, there were no newly captured nauplii, while about 20% newly captured nauplii were found the first evening and in two samples the next day.

The light situation in Lofoten this time of the year is described in Gj@sater and Tilseth (1982).

DISCUSSION

The egg size and the dry weight of the newly hatched cod larvae is variable due to intraspecific and spawning time effects on the egg size (Solemdal, 1970; Solemdal and Sundby, 1981). The mean weight of a sample of larvae in stage 6 (beginning of the first feeding period) is expected to be equal in different first feeding areas in Lofoten. In the Austnesfjord, the east side of Lofoten and in the Vesterblsfjord, however, the weight of stage 6 larvae was lower than on the western side of Lofoten. The larvae in stage 8 were of equal weights in the four areas.

An explanation for the variable dry weight of stage 6 larvae could be that the smallest larvae are lost from the population in the area most exposed to wind and current.

Owing to vertical turbulence, the prey organisms in this

area are often evenly distributed, and in such an area, a large larva with a large search volume would have a great advantage. Smaller larvae will easily be runts and may more easily be predated out of the population while a yolk sac reserve remains. The effects of time and the egg size provide another possible explanation for the variation in dry weight. The biggest eggs are spawned at the beginning of the spawning season (Solemdal 1970). At hatching these eggs will have the most northern distribution, and the larvae will be found in the exposed areas on the western side of the Lofoten islands.

The dry weight of the larvae is a conservative parameter which reflects the egg size and the feeding history of the larvae during the last few days, while the gut content re- flects the feeding history of the larvae in the hours prior to the catch, and can be correlated with the corresponding abundance of microzooplankton. Tilseth and Ellertsen

(1984)

gave some indications of criteria of gut contents of larvae in good feeding condition. A feeding incidence >go%, a feeding ratio >3.0 and a percentage of newly ingested nauplii above 10%, indicate a good availability of food to first feeding cod larvae. In the lab the larvae start to feed in stage

5

or 6 (6-7 days old), but will not be in a critical stage deprived of food until until they reach stage

8.

In the surveys feeding incidences (FI%) and ratios (FR) indicating a good availability of food organisms were only found in the Austnesfjord and in

2

out of 3 surveys in the VesterAlsfjord, and this variable feeding response in the larvae reflects the abundance of prey in the four different areas.

At the two diurnal stations in the Austnesfjord and

VesterAlsfjord, good feeding conditions were seen in the

Austnesfjord before the onset of turbulent mixing. More than

60 copepod nauplii per 1 occurred in pump samples from the

transition layer between the upper layers and the Vestfjord

winter water. Micropatches with even higher prey densities

were probably present in this layer. The turbulent mixing lowered food availability and this was clearly reflected in the gut content analyses, both the

F I % ,

the

FR

and the percentage of newly taken prey organisms decreased.

The critical prey density for first feeding cod larvae has been found to be between 20 and 190 nauplii per 1, depending on the swimming speed and feeding success (Solberg and Tilseth, 1984). This corresponds well with the findings in the Austnesfjord with cod larvae in good feeding condition at the prey densities present.

The Vesteralsfjord diurnal station shows that a prey density <l0 nauplii per 1 is insufficient for adequate feeding of cod larvae. A fifty percent feeding incidence, 1.0 copepod nauplii per gut and a variable percentage of newly taken prey organisms, indicate that most of the larvae were starving. This was seen in spite of calm weather where stratification and the creation of patches should be maxi- mized.

This contrasts with the pelagic prey densities observed in pond and concrete enclosure experiments. Biestad and Moks- ness (1981) and Kvenseth and Biestad (1984) report high feeding incidences of herring larvae in an enclosure and of cod larvae in a pond at similarly low prey densities of (10 per litre.

An explanation of these discrepancies could be that in ponds and enclosures higher concentrations of prey develop near the bottom or the walls, or that undetectable micro patches develop in the water mass because of the special physical conditions. Biestad and Moksness (1981) report that prey densities increased near the bottom of enclosures.

When the static stability of the water masses is high, a

maximum layer of microzooplankton with dense patches of

copepod nauplii will migrate vertically in response to die1

changes in light intensity, as described by Tilseth and

Ellertsen (1983). The cod larvae may tend to stay in this

layer by altering their searching behaviour inresponse

to

the

prey density, as described for anchovy larvae (Hunter and

Tomas, 1974). Vertical migration was seen in Vesterdlsfjord giving distinct maxima in the surface layer during night time. The highest concentrations of copepod nauplii were also found near the surface during night time. However, turbulent mixing will hinder this migration and distribute the larvae and the prey organisms evenly in the water masses, like the situation during the diurnal station in the Aust- nesf jord.

Our observations support the hypothesis of Vlymen (1977) that survival of fish larvae in the open sea depends on the existence of small scale patchiness of forage. Only in two samples from Vesteralsfjord were larvae found in good feeding conditions when the prey density was low. At such low prey densities, some larvae will always find sufficient prey for survival, but most of the larvae will be runts that starve to death or are eaten by predators. Successful first-feeding of cod larvae, the first and perhaps the most important step towards creating a good year class, appears to be dependent on both biological and physical factors affecting the abun- dance and availability of microzooplankton. These factors must be considered on a population wide basis and include the timing of reproduction of cod and

C a l a n u s

, weather condi- tions, and the stability of the water column.

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