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HAVFORSKNINGSINSTITU~TETS EGG- OG LARVEPROGRAEI ( HELP

FACTORS AFFECTING THE VERTICAL DISTRIBUTION OF EGGS

Svein Sundby

Institute of Marine Research Division Marine Environment

P.O. Box 1870 Nordnes

5024

BERGEN

Norway

ABSTRACT

The spatia1 distribution of eggs and larvae is a function of the pro- perties of the ambient water, i.e. the density, current and turbulent diffusion, and of the physical properties of the eggs, i.e. the buoy- ancy and dimension. The study of the vertical distribution is the first step to understanding the horizontal transport of eggs and larvae. Two models for the vertical distribution of eggs are applied to demonstrate how the physical and biological conditions influence the vertical distribution for the three main categories of eggs, here defined as pelagic, bathypelagic and bottom eggs. In particular, the physical conditions affecting the distribution of bathypelagic eggs are studied. The wind induced turbulence is the most important ambient factor for the vertical distribution of pelagic eggs and larvae. It contributes to mixing the buoyant eggs and larvae through the wind mixed layer. The vertical spreading of bathypelagic eggs depends mainly on the buoyancy distribution of the eggs. It is demonstrated from the model results that non-adhesive demersal eggs will be partly mixed into the water column. This mechanism contributes to the

horizontal transport of demersal eggs.

The s p a t i a 1 d i s t r i b u t i o n of eggs and l a r v a e i s i n f l u e n c e d by a s e t of b i o l o g i c a l and p h y s i c a l p r o c e s s e s , which t o g e t h e r c o n t r i b u t e t o d e t e r - mine t h e f a t e of t h e y e a r c l a s s . The importance of t h e d i s t r i b u t i o n and t r a n s p o r t of t h e e a r l y s t a g e s f o r r e c r u i t m e n t of f i s h s t o c k s has been p o i n t e d o u t by many a u t h o r s : I t has been suggested t h a t unfavor- a b l e d r i f t of eggs and l a r v a e beyond t h e a p p r o p r i a t e d i s t r i b u t i o n a l a r e a w i l l cause permanent l o s s from t h e p o p u l a t i o n ( H j o r t ,

1914).

On t h e o t h e r hand i t has been shown t h a t anomalous t r a n s p o r t from one r e g i o n t o a n o t h e r may t r a n s f e r r e c r u i t s from one s t o c k t o a n o t h e r . T h i s was d e s c r i b e d by Hansen and Buch

(1986)

who found t h a t e x p o r t of cod l a r v a e from t h e I c e l a n d t o t h e Greenland a r e a i n c r e a s e d t h e

Greenland cod s t o c k . The importance of l a r v a l r e t e n t i o n w i t h i n s p e c i - f i c g e o g r a p h i c a l r e g i o n s d u r i n g c r i t i c a l p e r i o d s h a s been o u t l i n e d by I l e s and S i n c l a i r (1982). Considering t h e l a r g e v e r t i c a l v a r i a t i o n s of t h e h o r i z o n t a l flow f i e l d , i t i s e v i d e n t t h a t t h e v e r t i c a l d i s t r i b u - t i o n of eggs and l a r v a e i s important f o r t h e i r h o r i z o n t a l t r a n s p o r t and s p r e a d i n g and t h e i r f a t e with r e s p e c t t o s u r v i v a l . Cushing (1982) gave an example of how t h e v e r t i c a l d i s t r i b u t i o n i t s e l f d i r e c t l y i n f l u e n c e d t h e r e c r u i t m e n t : The r i s i n g h a l o c l i n e i n t h e B a l t i c brought up cod eggs and l a r v a e c l o s e r t o t h e p r o d u c t i v e l a y e r s where t h e i r

food i s produced. Consequently, one i m p o r t a n t s t e p towards

understanding t h e r e c r u i t m e n t p r o c e s s e s i s t o d e s c r i b e and understand t h e v e r t i c a l d i s t r i b u t i o n of eggs and l a r v a e . This paper demonstrates how t h e p h y s i c a l p r o p e r t i e s of eggs and t h e ambient p h y s i c a l f o r c e s i n f l u e n c e t h e v e r t i c a l d i s t r i b u t i o n . Two models f o r t h e v e r t i c a l d i s t r i b u t i o n of eggs (Sundby, 1983; Westgård,

1989)

a r e a p p l i e d t o demonstrate t h e t h r e e main types of v e r t i c a l d i s t r i b u t i o n : p e l a g i c , b a t h y p e l a g i c and bottom d i s t r i b u t i o n s . The p h y s i c a l c o n d i t i o n s f o r each type of d i s t r i b u t i o n a r e analysed.

RESULTS AND DISCUSSION

Basic Equation

The v e r t i c a l p r o c e s s e s which i n f l u e n c e d i s t r i b u t i o n of e g g s , a r e h e r e s t u d i e d , and i t i s assumed t h a t a l l h o r i z o n t a l g r a d i e n t s a r e z e r o . The b a s i c e q u a t i o n i s then reduced t o t h e v e r t i c a l component of t h e

d i f f u s i o n e q u a t i o n :

where

C ( z , t ) i s t h e c o n c e n t r a t i o n of eggs i n numbers p e r u n i t volume w ( z , t ) i s t h e v e r t i c a l v e l o c i t y of t h e eggs

K ( z , t ) i s t h e v e r t i c a l eddy d i f f u s i v i t y c o e f f i c i e n t S ( z , t ) i s t h e spawning production of eggs

M ( z , t ) i s t h e egg m o r t a l i t y

z i s t h e v e r t i c a l component, p o s i t i v e towards i n c r e a s i n g depth t i s time.

To s o l v e t h e e q u a t i o n , t h e v e r t i c a l v e l o c i t y of t h e e g g s , w ( z , t ) , and t h e v e r t i c a l eddy d i f f u s i v i t y c o e f f i c i e n t , K ( z , t ) , must be known. The v e r t i c a l v e l o c i t y i s expressed w = f ( d , A e , v ) , where d i s t h e diameter of t h e egg, Ae i s t h e d i f f e r e n c e i n d e n s i t y (buoyancy) between t h e egg, e e , and t h e ambient w a t e r , ew, and v i s t h e molecular v i s c o s i t y of t h e w a t e r . Hence, t h e two p h y s i c a l p r o p e r t i e s o f t h e e g g s , t h e buoyancy and t h e diameter a r e key parameters t o model t h e v e r t i c a l d i s t r i b u t i o n .

I n t h e f o l l o w i n g s e c t i o n s i t w i l l be shown t h a t changes of t h e buoy- ancy, Ae = s w - @ e , r e s u l t s i n t h e most pronounced changes t o t h e s o l u t i o n s of t h e b a s i c e q u a t i o n ( 1 ) . For t h i s reason i t i s u s e f u l t o c l a s s i f y eggs i n t o t h r e e main groups (See F i g u r e 1 ) :

I n t h i s paper group A w i l l be d e f i n e d a s " p e l a g i c " e g g s , group B as

"bathypelagic" suid group C a s "bottom" e g g s , S i n c e t h e r e a r e few

d e s c r i p t i o n s of t h e v e r t i c a l d i s t r i b u t i o n of eggs i n r e l a t i o n t o t h e i r buoyancy, much of t h e l i t e r a t u r e i s n o t c o n s i s t e n t i n t h i s q u e s t i o n . It seems t h a t eggs a r e o f t e n c a l l e d " p e l a g i c " i f t h e y a r e found i n t h e water column and "demersal" i f they a r e found on t h e bottom. However, t h e s o l u t i o n t o e q u a t i o n (1) w i l l show t h a t a l l t h e t h r e e groups, d e f i n e d above, a r e found i n t h e water column. Even demersal eggs, given they a r e non-adhesive, w i l l be mixed i n t o t h e water column. I n a d d i t i o n t o t h e t h r e e main types of eggs i n F i g u r e 1 , t h e r e a r e a l s o

mlxed t y p e s , I n p a r t i c u l a r , s p e c i e s with a buoymey distribution between p e l a g i c and b a t h y p e l a g i e have been r e p o r t e d by Coombs - - e t a l ,

(1985) ^a

F i g u r e

1.

Buoyancy d i s t r i b u t i o n of t h r e e c a t e g o r i e s of eggs (upper p a r t of t h e f i g u r e ) i n r e l a t i o n t o t h e s a l i n i t y p r o f i l e (lower p a r t of t h e f i g u r e ) . A : p e l a g i c eggs, B: b a t h y p e l a g i c e g g s , C : bottom eggs.

Egg buoyancy

The g e n e r a l knowledge of f i s h eggs' buoyancy h a s i n c r e a s e d

s u b s t a n t i a l l y d u r i n g r e c e n t y e a r s , i n p a r t i c u l a r a f t e r t h e d e n s i t y g r a d i e n t column was introduced by Coombs

(1981).

T h i s instrument e n a b l e s us t o measure t h e s p e c i f i c g r a v i t y of i n d i v i d u a l eggs with a high accurracy and r e s o l u t i o n . The p h y s i o l o g i c a l c a u s e s of buoyancy i n marine f i s h eggs have been s t u d i e d by Craik and Harvey (1987).

P e l a g i c eggs have a s p e c i f i c d e n s i t y which i s lower than t h e d e n s i t y of t h e upper mixed l a y e r of t h e s e a . Hence they tend t o r i s e towards t h e s u r f a c e . Only a small f r a c t i o n of such e g g s , however, i s found a t t h e very s u r f a c e , because t h e t u r b u l e n t f o r c e s of t h e mixed l a y e r c o u n t e r a c t t h e buoyant f o r c e s of t h e eggs. Depending on t h e magnitude of t h e c o u n t e r a c t i n g f o r c e s t h e eggs w i l l be more o r less concentrated towards t h e s u r f a c e . Examples of eggs with such d i s t r i b u t i o n s a r e t h e North Sea p l a i c e eggs (Pommeranz,

1973).

t h e North Sea mackerel eggs

( I v e r s e n , 1973; Coombs, 1981) and North-east a r c t i c cod eggs (Solemdal and Sundby, 1981). The v e r t i c a l d i s t r i b u t i o n s of t h e s e s p e c i e s were

Bathypelagic eggs have a h i g h e r s p e c i f i c d e n s i t y t h a n t h e d e n s i t y of t h e upper mixed l a y e r of t h e s e a , b u t lower than t h e d e n s i t y of t h e bottom l a y e r . Hence they a r e d i s t r i b u t e d a t mid-depths and q u i t e f r e q u e n t l y i n t h e pycnocline. Examples of s p e c i e s having t h i s type of egg d i s t r i b u t i o n s a r e P a c i f i c h a l i b u t (Hippoglossus s t e n o l e p i s )

(Thompson and van Cleve, 1936). t h e B a l t i c cod ( K h d l e r ,

1949)

and t h e A t l a n t i c h a l i b u t (Hippoglossus hippoglossus L . ) found i n Norwegian f j o r d s and c o a s t a l waters (Haug e t a l . , 1984, 1 9 8 6 ) . Kendall and K i m

(1989)

demonstrated how t h e b a t h y p e l a g i c eggs from t h e walleye p o l l o c k (Theragra chalcogramma) s u b s t a n t i a l l y changed t h e i r v e r t i c a l

d i s t r i b u t i o n mainly due t o changes i n t h e buoyancy d u r i n g t h e development.

Bottom eggs have a s p e c i f i c d e n s i t y which i s h i g h e r than t h e d e n s i t y of t h e bottom l a y e r . I n p r i n c i p l e , demersal eggs i s one kind of bottom eggs. However, i t i s e s s e n t i a l t o d i s t i n g u i s h between adhesive and non-adhesive eggs. With r e s p e c t t o t h e n a t u r e of t h e v e r t i c a l f o r c e s a c t i n g on t h e eggs, t h e r e i s no d i f f e r e n c e between non-adhesive demersal eggs and p e l a g i c eggs. Non-adhesive demersal e g g s , u n l e s s t h e y a r e b u r i e d i n t h e s e a bed, w i l l be more o r less mixed i n t o t h e water column depending on t h e magnitude of t h e bottom t u r b u l e n t mixing. One s p e c i e s having non-adhesive demersal eggs i s t h e s a f f r o n cod (Eleginus g r a c i l i s ) i n t h e n o r t h - e a s t P a c i f i c Ocean (Dunn and Matarese,

1986).

Even adbesive eggs may b r e & l o o s e from each o t h e r and t h e s e a bed and be mixed i n t o t h e bottom l a y e r s and t r a n s p o r t e d away. T h i s has been r e p o r t e d f o r t h e Barents Sea c a p e l i n eggs (Bakke and B j ~ r k e ,

1973).

The d e n s i t y d i s t r i b u t i o n of an egg p o p u l a t i o n can o f t e n be d e s c r i b e d by a Gaussian d i s t r i b u t i o n f u n c t i o n . The e g g s from t h e North-east a r c t i c cod a r e n e u t r a l l y buoyant a t an average s a l i n i t y of 31.0 and t h e s t a n d a r d d e v i a t i o n i s 0.55 (Solemdal and Sundby, 1 9 8 1 ) . Eggs from t h e A t l a n t i c h a l i b u t a r e n e u t r a l l y buoyant a t an average s a l i n i t y of 34.2 w i t h a s t a n d a r d d e v i a t i o n of 0.52 (Haug e t a l . ,

1986).

The

buoyancy of s p r a t eggs o f f t h e s o u t h e r n c o a s t of Great B r i t a i n ranged o v e r about

8

s a l i n i t y u n i t s d u r i n g t h e end of t h e egg s t a g e (Coombs ^- e t a l . ,

1985).

Consequently, i t i s expected t h a t t h e r e i s a l a r g e

d i f f e r e n c e of t h e v e r t i c a l d i s t r i b u t i o n of t h e h e a v i e s t and t h e l i g h t e s t f r a c t i o n s of an egg population. I n F i g u r e 1 t h e n e u t r a l

buoyancy distributions of three different egg population ase drawn together with

a

vertical salinity profile, Distribution

A,

which is consistent with the eggs of North-east arctic cod, will appear as pelagic eggs. Distribution B, which is identical with the Atlantic halibut eggs, will appear as bathypelagic eggs and distribution C will appear as bottom eggs. The latter is a hypothetic distribution, since no reports of quantitative density measurements of non-adhesive bottom eggs are available.

There are, however, examples of species which is a mixture of the main types described above. Coombs et al., 1985 investigated buoyancy and vertical distribution of eggs from sprat (Sprattus sprattus) and pilchard (Sardina pilchardus) off the south coast of Great Britain.

These eggs were a mixture of pelagic and bathypelagic eggs. In addition, the specific gravity of the eggs changed through

development, and the ambient salinity varied through spawning season.

This will result in a variety of possible vertical distributions, partly with peak concentrations in the surface, and partly in the pycnocline.

Vertical eddy diffusivity'coefficient

The other parameter which influences the vertical distribution of eggs is the vertical eddy diffusivity coefficient. Depending on the depth, wind velocity, stratification, tida1 energy and bottom stress it varies over approximately

5

orders of magnitude, It is largest in the mixed layer and decreases to a minimum in the pycnocline due to the stratification which reduces the vertical transport of turbulent energy,

A

slight increase occurs below the pycnocline due to the weaker stratification followed by a pronounced increase in the bottom layer due to bottom friction. Figure 2 shows qualitatively how the vertical eddy diffusivity coefficient may vary through a water column.

Estimates of the mixed layer eddy viscosity coefficient are difficult partly due to the great problems of resolving wave motion from

turbulence. Sverdrup et al. (1942) derived estimates of the eddy viscosity coefficient from the Ekman theory. Sundby (1983) estimated over-all eddy diffusivity coefficients for the mixed layer from a model on the vertical distribution of pelagic eggs. Thorpe (1984) made estimates of the eddy diffusivity coefficient in the surface layers based on a model for the vertical distribution of air bubbles in the

s e a , ALthough t h e i r r e s u l t s t o same e x t e n t d i f f e r , i t may be concluded

2 - 1

t h a t t h e eddy d i f f u s i v i t y c o e f f l c i e n t ranges from about 10 cm s a t 3 2 - 1

wind speed n e a r z e r o t o about 10 cm s a t wind speed of approximately 20 ms- l

.

F i g u r e 2 . Ranges of t p e - p e r t i c a 1 d i s t r i b u t i o n of t h e eddy d i f f u s i v i t y c o e f f i c i e n t , K , i n cm s ( r i g h t p a r t ) f o r a hydrographic p r o f i l e

( l e f t p a r t ) i d e n t i c a l with t h e p r o f i l e i n Figure 1.

I n t h e pycnocline t h e eddy d i f f u s i v i t y c o e f f i c i e n t i s i n v e r s e l y r e - l a t e d t o t h e s t r a t i f i c a t i o n and d i r e c t l y dependent on t h e energy i n - , p u t . However, t h e f u n c t i o n a l r e l a t i o n s h i p t o t h o s e parametres i s s t i l l unknown. S e v e r a l a u t h o r s have e s t i m a t e d t h e v e r t i c a l eddy d i f f u s i v i t y i n d i f f e r e n t f j o r d s and c o a s t a l waters e . g . Gade (1970) f o r O s l o f j o r d - e n , Kullenberg

(1971)

f o r shallow c o a s t a l w a t e r s , Svensson (1980) f o r a Swedish f j o r d and Buch (1982) f o r two-layered Scandinavian f j o r d s . G a r g e t t (1984) reviewed t h e l i t e r a t u r e on t h e v e r t i c a l d i f f u s i v i t y c o e f f i c i e n t i n s t r a t i f i e d systems. Depending on t h e degree of s t r a t i f i c a t i o n , t h e eddy d i f f u s i v i t y c o e f f i c i e n t ranges from 0 . 5 x

2 - 1 2 -1

cm s t o about 1

- 4

cm s

.

The bottom t u r b u l e n c e , which normally e x t e n d s s e v e r a l meters above t h e bottom, i s mainly dependent on t h e boundary l a y e r v e l o c i t y and t h e bottom roughness

.

^Bowden

⁽¹⁹⁶²⁾

r e p o r t e d v a l u e s from s e v e r a l a u t h o r s

.

I n a r e a s of s t r o n g t i d a 1 mixing t h e eddy d i f f u s i v i t y c o e f f i c i e n t may

2 - 1 2 -1

exceed 100 cm s , although 1 cm s i s more common above t h e s e a bed i n deep o c e a n i c a r e a s .

If the terms spawning production, S(z,t), and mortality, M(z,t), are neglected in equation (l), and stationary conditions are considered, the diffusion equation reduces to:

Equation (2) applies to the fraction eggs which has reached the steady state distribution after being spawned at some depth. We assume the buoyancy distributions shown in Figure 1 and a variation of the eddy diffusivity coefficient as shown in Figure

2

and apply the models for the vertical distribution (Sundby, 1983; ~estgård,l989). The three categories of eggs, pelagic (A), bathypelagic (B) and bottom eggs (C), will appear as shown in Figures 3a and 3b.

Figure 3a shows the distribution for a situation of low eddy diffusi- vity coefficients in the water column, corresponding to the lower range of eddy diffusivity profile in Figure

2.

The value of the mixed

2 - 1

layer is 75 cm s corresponding to wind speed of O ms-l (according to Sundby (1983)). In the pycnocline the minimum eddy diffusivity

2 - 1

coefficient is 0.01 cm s , and the value of the bottom layer is 6

2 - 1

cm s .

Figure 3b shows the distribution for a situation of high eddy diffusivity coefficient, corresponding to the higher range of eddy diffusivity profi'le in Figure 2. The mixed layer eddy diffusivity

2 - 1

coefficient is 585 cm s corresponding to wind speed of 15 øs-' (according to Sundby (1983)). The minimum value of the pycnocline is

2 - 1

0.5 cm s , and the value for the bottom layer coefficient is 90

2 - 1

cm s .

Figures 3a

and

3b show that the vertical distribution of pelagic eggs (A) is very sensitive to variations in the wind induced turbulence, as also demonstrated by Sundby (1983). The figures also show that varying bottom turbulence levels influences the vertical distribution of

bottom eggs (C). However, the vertical distribution of bathypelagic eggs (B) confined to the pycnocline is rather insensitive to

variations in the pycnocline turbulence. The pycnocline eddy

diffusivity coefficient is increased by a factor of

50

from Figures 3a

to 3b, but the vertical spreading is not substantially changed.

Consequently, the vertical spreading

in

the pycnocline dependc

mainly

on the density distribution of the eggs and the density profile, and not very much on the level of turbulence.

Figure 3 a. Vertical distribution of pelagic (A), bathypelagic (B) and bottom eggs (C) during low turbulence (See numerical values in the text). The neutral buoyancy distributions, salinity profile and eddy diffusion profile as shown in Figures 1 and 2.

Figure 3 b. Vertical distribution of pelagic (A), bathypelagic (B) and bottom eggs (C) during high turbulence (See numerical values in the text). The neutral buoyancy distributions, salinity profile and eddy diffusion profile as shown in Figures

1

and

2.

To show this we solve the diffusion equation for eggs of a given

density in a linear pycnocline. The eddy diffusivity coefficient can

tben be considered as constant with respect to depth, In a binear pycnocline,

e ( z ) =

k

z +

b, the vertical velocity,

w(%),

varies line- arly within the Stokes regime. The vertical velocity may therefore be written:

where m is a constant, and

zA

is the level where

Ae(z) =

O, i.e. the level of neutral buoyancy of the egg. Equation (3) is inserted into equation

( 2 ) :

The solution to equation (4) is:

where

CA

is the concentration of eggs at the depth of neutral buoyan- cy,

z A .

According to eq. (5) the eggs are vertically distributed as a normal distribution, where the standard deviation:

When the velocity of the eggs is confined within the Stokes regime, the Stokes equation for the terminal velocity is valid and the expression for m becomes:

where d is the diameter of the egg and N is the Brunt-Vaisala frequency. The value of the molecular viscosity,

v,

is tabulated by Riley and Skirrow (1975) (e.g. at 5

o C

and salinity 30 the molecular

- 1 - 1

viscosity is 0.016 gcm s

) .

We take, as an example, the vertical distribution of Atlantic halibut in the pycnocline of northern Norway fjords described by Haug et al.

(1986). where the squared ~runt-vaisgl& frequency,

_{N * ,}

ranged from

O. ~ X I O - ~ to

2 .

O X I O - ~ s - ~ . From the above mentioned literature on the

influence of stratificati~n

_cn

the turbulence, the eddy diffusivity

2 - 1

c o e f f i c i e n t range f r o n 0-1 t o 0 , 5 cm s

.

When t h e s e v a l u e c a r e in- s e r t e d i n t o e q ,

6

and

7 ,

t h e s t a n d a r d d e v i a t i o n , a , of t h e v e r t i e a l s p r e a d i n g of one buoyancy group of h a l i b u t eggs w i l l range from 0 . 4 t o

1 . 6

meters. According t o Haug - - e t a l . (1986) t h e o l d e r eggs (which d e f i n i t e l y have come t o a s t e a d y s t a t e v e r t i c a l d i s t r i b u t i o n ) extend over a 150

-

250 m water column. Consequently t h e l a r g e v e r t i c a l s p r e a d i n g of h a l i b u t eggs i n t h e water column is due t o t h e n e u t r a l buoyancy d i s t r i b u t i o n of t h e eggs a l o n e and n o t t h e due t o t h e v e r t i c a l t u r b u l e n c e .

The v e r t i c a l d i s t r i b u t i o n of eggs w i t h a d e n s i t y h i g h e r t h a n t h e bot- tom water l a y e r , h e r e d e f i n e d a s bottom eggs ( d i s t r i b u t i o n C i n Figs.

3 a , 3 b ) , has t h e i n v e r s e d i s t r i b u t i o n of t h a t of p e l a g i c eggs. S i n c e t h e bottom t u r b u l e n c e g e n e r a l l y i s much lower than t h e t u r b u l e n c e i n t h e upper mixed l a y e r , t h e bottom eggs w i l l be more c o n c e n t r a t e d towards t h e boundary than t h e p e l a g i c eggs. For low l e v e l s of bottom t u r b u l e n c e ( F i g . 3 a ) , l e s s than

3 %

of t h e eggs a r e mixed more than

3

m above t h e s e a bed. However, i n shallow r e g i o n s where t h e t i d a 1 induced bottom t u r b u l e n c e i s h i g h , and where t h e mixed l a y e r may even extend t o t h e bottom, t h e bottom t u r b u l e n c e c o e f f i c i e n t may exceed 100

2 - 1

cm s

,

and t h e eggs w i l l be d i s t r i b u t e d a s C i n F i g u r e 3b. Here 40

%

of t h e eggs are more than

3

m above t h e s e a bed.

Development of s t e a d y s t a t e egg p r o f i l e s

I n t h e p r e v i o u s s e c t i o n d i s t r i b u t i o n s based on b a l a n c e between t h e buoyancy f o r c e of t h e eggs and t h e v e r t i c a l t u r b u l e n c e f o r c e s were s t u d i e d , i . e . when 6C/6t = O ( s t e a d y s t a t e ) . The t i m e i t t a k e s t o reach s t e a d y s t a t e d i s t r i b u t i o n , depends on t h e spawning d e p t h , t h e buoyancy and t h e t u r b u l e n c e . Figure

4

i l l u s t r a t e s how t h e mixed l a y e r t u r b u l e n c e i n f l u e n c e t h e time t o reach t h e s t e a d y s t a t e p r o f i l e f o r p e l a g i c cod eggs spawned a t 120 m d e p t h . The buoyancy d i s t r i b u t i o n of t h e s e eggs e q u a l s t h o s e of t h e cod eggs shown i n F i g u r e 1. The f i g u r e shows t h e c o n c e n t r a t i o n p r o f i l e f o r every 6 t h hour a f t e r spawning n e a r t h e bottom. I t t a k e s

48

hours t o reach t h e s t e a d y s t a t e d i s t r i b u t i o n d u r i n g calm wind c o n d i t i o n s , while i t t a k e s only 30 hours t o reach s t e a d y s t a t e a t wind speed of

15

m / s . I t can a l s o be shown t h a t t h e r e a r e l a r g e v a r i a t i o n s i n t h e time t o reach s t e a d y s t a t e f o r t h e heavy f r a c t i o n and t h e l i g h t f r a c t i o n of t h e eggs. While t h e buoyancy of p e l a g i c eggs i s c o n s t a n t a s they r i s e through t h e mixed l a y e r , t h e buoyancy of b a t h y p e l a g i c eggs d e c r e a s e s a s they move toward t h e depth

CONCENTRATION INo/m3)

W=15 m l s

0 500 O 500 O 500 0 500 O 500

CONCENTRATION lNo/rn3)

Figure 4. Development of the vertical profile of pelagic eggs (A) from an initial distribution, i.e. spawning near the bottom, to steady state for two events of mixed layer turbulence. Upper partllwind velocity, W

=

O ms-l. Lower part: wind velocity, W

= 15

ms .

of equilibrium. It implies that the vertical velocity also decreases, and it takes a relatively long time to reach steady state for

bathypelagic eggs.

CONCLUSION

The vertical distribution of pelagic eggs is mainly influenced by the wind induced mixing. The buoyancy distribution determines the vertical distribution of bathypelagic eggs, while the vertical spreading in the pycnocline of these eggs is essential insensitive to vertical mixing.

Model results show that non-adhesive demersal eggs partly will be found in the water column. This will contribute to advection of demer- sal eggs. The time it takes to reach a steady state vertical egg

distribution depends on the spawning depth, buoyancy distribution of

the eggs and the vertical mixing.

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Oversikt over

Nr. 1 P. Solemdal og P. Bratland: Klekkeforløp for lodde i Varangerfjorden 1986.

Nr.

2

T. Haug og S. Sundby: Kveitelarver og miljø.

Undersøkelser

på

gytefeltene ved Sør~ya.

Nr. 3 H. Bjørke, K. Hansen og S. Sundby: Postlarveundersøkelser i 1986.

Nr. 4 H. Bjørke, K. Hansen og W. Melle: Sildeklekking og seigyting på Møre 1986.

Nr. 5 H. Bjørke and S. Sundby: Abundance indices for the Arcto- Norwegian cod in 1979-1986 based on larvae investigations.

Nr. 6 P. Fossum: Sult under larvestadiet - en viktig rekrutterings- mekanisme?

Nr. 7 P. Fossum og S. Tuene: Loddelarveundersøkelsene 1987.

Nr. 8 P. Fossum, H. Bjørke and R. Sætre: Studies on herring larvae off western Norway in 1986.

Nr. 9 K. Nedreaas and O.M. Smestad: O-group saithe and herring off the Norwegian coast in 1986 and 1987.

Nr.

10

P. Solemdal: Gytefelt og gyteperiode hos norsk-arktisk hyse.

Nr. 11 B. Ellertsen: Kopepodnauplier på Møre våren 1986 - ^nærings-

tilbudet til sildelarver.

Nr. 12 H. Bjørke, P. Fossum, K. Nedreaas og R. Sætre:

Yngelunders~kelser - ^1985.

Nr, 13 Faglig profil og aktivitetene i 1986-87,

Nr. 14 H. BjØrke, K. Hansen, M. Johannessen og S. Sundby:

Postlarveunders~kelser

-

juni/juli 1987.

Nr, 15 H. 3j~rke: Sildeklekking

på

MØre i 1986-87.

Nr. 16 H. BjØrke, K. Bakkeplass og K. Hansen: Forekomster av fiskeegg fra Stad til Gims~y i februar-april 1987,

Nr. 17 T. Westgård: A model of the vertical distribution of pelagic fish eggs.

A computer realization.

Nr. 18 T. Westgård, A. Christiansen og T. Knudsen: Forskerkart.

EDB-presentasjon av marine data.

Nr. 19 R. Sætre og H. Bjørke: Oljevirksomhet på Møre. Konsekvenser

for fiskeressursene,

Denne rapportserien har begrenset distribusjon. Opplysninger om programmet og rapportene k m rettes til

Programledelsen for BELP

Fiskeridirektoratets Havforskningsinstitutt Postboks 1870

5024 Bergen

Nr, 20

S ,

Mehl, K, Nedreaas, O,M, Smedstad a d

T ,

Westgård: Q-group saithe and herring off

the

Norwegian coast in April-May 1988.

Nr, 21 P, Fossum: Loddelarveunders~kelsene 1988,

Nr. 22 R. Sætre, H, Bj@rke and P , Fossum; Studies on herring larvae off western Norway in 1987,

Nr,

₂₃

Aktivitetene i I988

Nr, 24 C. Olsen

arad

A, Vold Soldal: Coastal concentrations of O-group NE-Arctic cod,

Nr, 25 P, Solemdal, T. Knutsen and H, Bj~rke: Spawning areas and spawning period of the North-East Arctic haddock (Melanogram- mus aeglefinus L.),

Nr, 26 P. Fossum og K , G, BAkeplass

:

Loddelarveunders~kelsene 1989.

Nr. 27 K , Nedreaas, H, Senneset og O.M, Smedstad: Kartlegging av O- gruppe fisk utmror norskekysten

i

april-mai 1989*

Nr, 28 H. Bj~rke, B, ELlertsen, K, Hansen og K, Bakkeplass: Yngel- undersokelser i juli-atimst i 1988 og 1989 utenfor

Norskekysten,

Nr,

29 S ,

Sundby m d P , Fossum: Feeding eonditions of Arcto-

norwegå-ni cod larvae compared to the Rotschild-Osborn theory on small-scale turbulence and plankton contact rates.

Nr. 30 Aktivitetene

i

1989

Nr, 31 P. Fossum: The condition

of the

herring larvae off Western Norway

in

the period 1985-87.

Nr,

32

H, BJØrke, B. Ellertsen, P, Fossum og R , Sætre: Sildelarve- unders8kelsene

i

1988,

Nr, 33 V. Øiestad: Petroleumsvirksomhet utenfor kysten av Midt-Norge,

Konsekvenser for fi.skerecsursene.