re-estimation of three parameters, associated with ...

NOAA Technical Memorandum NMFS

MAY 1983 'I

RE-ESTIMATION OF THREE PARAMETERS ASSOCIATED WITH ANCHOVY EGG AND LARVAL ABUNDANCE: TEMPERATURE DEPENDENT INCUBATION TIME, YOLK-SAC GROWTH RATE AND EGG AND LARVAL RETENTION IN MESH NETS

Nancy C. H. Lo

NOAA-TM-NMFS-SWFC-3

1

U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Marine Fisheries Service Southwest Fisheries Center

N O A A Technical Memorandum NMFS

The National Oceanic and Atmospheric Administration (NOAA), organized in 1970, has evolved into an agency which establishes national policies an manages and conserves our oceanic, coastal, and atmospheric resources. A n organizational element within NOAA, the Office of Fisheries is responsible for fisheries policy and the direction of the National Marine Fisheries Service (NMFS). In addition to i t s formal publications, the NMFS uses the N O A A Technical Memorandum series to issue informal scientific and technical publications when complete formal review and editorial processing are not appropriate or feasible. Documents within this series, however, reflect sound professional work and may be referenced in the formal scientific and technical literature.

NOAA Technical Memorandum NMFS This TM series is used lor documentallon and timely communication 01 preliminary results. imterim reports. or Special purpose inlormation.and have not received complete formal review. editorialcontrol. or detailed editlng

MAY 1983

RE-ESTIMATION OF THREE PARAMETERS ASSOCIATED WITH ANCHOVY EGG AND LARVAL ABUNDANCE: TEMPERATURE DEPENDENT INCUBATION TIME, YOLK-SAC GROWTH RATE AND EGG AND LARVAL RETENTION IN MESH NETS Nancy C. H. Lo National Marine Fisheries Service Southwest Fisheries Center 8604 La Jolla Shores Drive La Jolla, California 9 2 0 3 8

NOAA-TM-NMFS-SWFC-3

1

U.S. DEPARTMENT OF COMMERCE Malcolm B a l d r i g e . S e c r e t a r y

National Oceanic and Atmospheric Administration John V. Byrne. Administrator

National Marine Fisheries Service William G. Gordon. Assistant Administrator for Fisheries

-1-

ABSTRACT

The estimates o f t h r e e parameters r e q u i r e d f o r n o r t h e r n anchovy ( E n g r a u l i s mordax) abundance assessment were updated u s i n g new data from 1981 and 1982.

These t h r e e parameters a r e i n c u b a t i o n time, y o l k - s a c l a r v a l growth,

and r e t e n t i o n o f eggs and l a r v a e i n v a r i o u s mesh nets.

The growth data o f t h e yolk-sac anchovy l a r v a e were updated by i n c l u d i n g a s e t o f new data c o l l e c t e d i n 1981.

This new data s e t c o n s i s t e d o f l a r v a l

l e n g t h and age c o l l e c t e d a t t h r e e temperatures:

13.5,

15.0,

and 16.5"C.

The

i n c u b a t i o n time was r e l a t e d t o t h e water temperature i n a negative exponential form.

A Gompertz model was used t o describe t h e growth o f yolk-sac anchovy

l a r v a e , i n which t h e temperature-dependent growth r a t e has an exponential f orm.

The r e s u l t s o f a t h r e e - v e r t i c a l - n e t experiment conducted i n March 1982 showed t h a t 0.333 mm mesh was n o t e x t r u s i o n - f r e e .

The r e t e n t i o n s o f 0.333 mm

N i t e x mesh r e l a t i v e t o 0.150 mm and 0.075 mm meshes are 0.91 f o r eggs, 0.63 f o r larvae < 4.0 mm (preserved standard l e n g t h ) , 0.95 f o r l a r v a e = 4.5 mm, and 1 f o r l a r v a e > 4.5 mm. The r e t e n t i o n o f eggs and l a r v a e f o r l a r g e meshes can

be c o r r e c t e d accordi n g l y

.

- 2-

1.

INTRODUCTION AND DATA SOURCES

Q u a n t i t a t i v e assessment o f egg and l a r v a l abundances from ichthyoplankton data depends on t h e use o f a p p r o p r i a t e c o r r e c t i o n f a c t o r s o r weights f o r v a r i o u s parameters, as i t i s impossible t o have a b i a s - f r e e sampler.

These

parameters were most r e c e n t l y estimated by Zweifel and Smith (1981) f o r t h e n o r t h e r n anchovy.

I n o r d e r t o c o r r e c t f o r t h e biases, Z w e i f e l a p p l i e d a

negative binomial weighted model developed by B i s s e l (1972) ( Z w e i f e l and Smith 1981; H e w i t t 1982) w i t h each observation weighted by i t s e f f e c t i v e sampler size.

Since t h e completion o f t h e i r work, new and more complete data have

become a v a i l a b l e a t t h e Southwest F i s h e r i e s Center (SWFC) f o r t h r e e o f these parameters--incubation time, growth i n l e n g t h o f egg through yolk-sac stage l a r v a e , and e x t r u s i o n o f eggs and l a r v a e through t h e meshes o f plankton nets.

New data were c o l l e c t e d from a l a b o r a t o r y experiment conducted i n 1981 on temperature s p e c i f i c i n c u b a t i o n r a t e s and yolk-sac l a r v a growth r a t e s o f n o r t h e r n anchovy. I n a d d i t i o n , an experiment a t sea was conducted i n March 1982 t o determine r e t e n t i o n o f anchovy eggs and l a r v a e i n t h r e e i d e n t i c a l n e t frames, each w i t h a d i f f e r e n t mesh net.

Data from o l d e r sources a r e combined

w i t h these data t o p r o v i d e new parameter estimates.

To improve estimates o f t h e r e l a t i o n s h i p between temperature and b o t h i n c u b a t i o n t i m e and yolk-sac l a r v a l growth and t o e s t a b l i s h c r i t e r i a f o r developmental stages o f yo1 k-sac anchovy, a 1aboratory experiment was conducted a t t h r e e temperatures (13.5, from a c a p t i v e brood stock.

15.0 and 16.5"C)

i n 1981 u s i n g eggs

These data were used along w i t h those c o l l e c t e d

-3b y David Kramer and employed by Zweifel and Lasker (1976) and Zweifel and Hunter (MS) ( a t temperatures 14, 17 and 20°C) t o develop a r e v i s e d model f o r expressing t h e r e l a t i o n s h i p between yolk-sac l a r v a l growth and temperature.

Estimates o f r e t e n t i o n o f anchovy eggs and l a r v a e i n small mesh n e t s were i n v e s t i g a t e d d u r i n g a week-long f i e l d experiment conducted i n March 1982 i n t h e Los Angeles B i g h t a t a s i n g l e s t a t i o n .

One hundred f o r t y - t w o v e r t i c a l n e t

tows were made u s i n g t h r e e 25 cm diameter frames each w i t h a N i t e x n e t o f d i f f e r e n t mesh s i z e (0.333,

0.150 and 0.075 mm) mounted on t h e same wire.

These p l a n k t o n n e t s a r e deployed t o 70 m depth and r e t r i e v e d v e r t i c a l l y . a r e r e f e r r e d t o as CalVET n e t s (CalCOFI v e r t i c a l nets, H e w i t t

i n press).

They The

anchovy eggs taken i n each o f t h e n e t s were s o r t e d according t o t h e i r developmental stages and t h e standard l e n g t h o f t h e anchovy l a r v a e was measured.

The r e t e n t i o n o f anchovy eggs and l a r v a e i n l a r g e mesh n e t s (0.55 and 0.505 mm vs 0.333 mm) was reexamined.

Data from 0.333 mm mesh CalVET tows and

0.505 mm mesh Bongo tows (McGowan and Brown 1966) taken d u r i n g January-April

i n 1979-81 a l o n g w i t h those used by Lenarz (1972) were i n c l u d e d i n t h e a n a l y s i s t o r e d e f i n e t h e r e t e n t i o n o f eggs and l a r v a e i n various mesh s i z e s o f p l a n k t o n nets.

2.

METHODS

2.1

Temperature Dependent I n c u b a t i o n Time and Growth o f Yo1 k-Sac Northern Anchovy

- 4I n t h e 1981 l a b o r a t o r y experiment anchovy eggs were incubated a t t h r e e nominal temperatures (13.5,

15 and 16.5"C)

i n 10 l i t e r c o n t a i n e r s i n

temperature c o n t r o l l e d water baths w i t h 20 eggs taken p e r hour u n t i l hatching. A f t e r hatching, 20 yolk-sac l a r v a e were taken every 4 hours and 10 o f these were measured t o determine mean standard l i v e l e n g t h .

The time o f t h e f i r s t

hatch and approximate time o f l a s t hatch were recorded.

Because eggs a r e

l o c a t e d a t t h e s u r f a c e w h i l e l a r v a e a r e u s u a l l y i n t h e middle depths, b o t h s u r f a c e and m i d d l e depth temperatures were recorded.

A t 16.5OCY l a r v a e o l d e r than 4.14 days (3.78-3.96mm)

were excluded

because they began t o s t a r v e and would b i a s t h e growth curve downward.

The

temperatures used i n t h e a n a l y s i s were geometric mean surface temperature b e f o r e h a t c h i n g and middle depth temperature a f t e r hatching, measured every 4 hours.

The elapsed time from f e r t i l i z a t i o n , and t h e measured mean standard

l i v e l e n g t h o f anchovy l a r v a e were used along w i t h Kramer's l a r v a l data set, r e p o r t e d i n Zweifel and Lasker (1976), t o f i t a temperature dependent growth curve f o r yolk-sac anchovy l a r v a e (Table 1 ) .

2.2

Retention of Anchovy Eggs and Larvae i n Various Mesh Nets

I n e s t i m a t i n g t h e abundance o f eggs and l a r v a e , one o f t h e sources o f b i a s i s t h e i r e x t r u s i o n through t h e meshes o f t h e net.

E x t r u s i o n i s measured

by t h e r a t i o o f c a t c h r e t a i n e d i n a l a r g e mesh n e t t o t h a t taken i n a s m a l l e r mesh net.

R e t e n t i o n o f anchovy eggs and l a r v a e i n 0.333 mm N i t e x mesh compared t o

-5-

Table 1.

Live length (mm) and age from f e r t i l i z a t i o n (days) o f yolk-sac (The predicted length larvae a t various temperatures ( " C ) . was computed from equation ( 3 ) ) . AGE (days)

OBSERVED PREDICTED LENGTH LENGTH (mm) (mm)

SE

TEMP. O C

DATA SOURCE

- 6s m a l l e r N i t e x meshes (0.150 and 0.075 mm) was i n v e s t i g a t e d w i t h the p r e v i o u s l y mentioned 1982 f i e l d experiment.

The t h r e e 25 cm n e t s were randomly assigned

t o p o r t , c e n t e r and starboard p o s i t i o n s , and every 6 hours, two o f t h e n e t s were switched according t o a predetermined system t o e l i m i n a t e p o s s i b l e b i a s due t o n e t p o s i t i o n .

Retention o f anchovy eggs and l a r v a e i n 0.505 mm N i t e x mesh compared t o 0.333 mm N i t e x mesh was estimated from t h e l a r v a e taken d u r i n g January-April i n 1979-81 i n t h e 0.505 mm mesh Bongo n e t and t h e 0.333 mm mesh CalVET n e t a t s t a t i o n s where both nets were towed.

Retention o f eggs and l a r v a e i n 0.55 mm

s i l k mesh (1 m diameter n e t ) compared t o t h a t i n 0.333 mm N i t e x mesh (0.5 m diameter n e t ) was discussed by Lenarz (1972).

I n t h e same study t h e eggs

taken i n a 0.505 mm N i t e x mesh n e t were compared t o those taken i n a 0.333 mm N i t e x mesh n e t (Smith MS) although most o f the l a r v a e were n e i t h e r counted nor measured.

3.

RESULTS

3.1

Incubation Time as a Function o f Temperature

The i n c u b a t i o n time of eggs was f i r s t analyzed by Zweifel and Lasker (1976) and was found t o be i n v e r s e l y r e l a t e d t o t h e temperature, i.e., h i g h e r t h e temperature, t h e s h o r t e r t h e i n c u b a t i o n time.

the

The new data on

i n c u b a t i o n time o f anchovy eggs discussed i n t h e Methods s e c t i o n and those used by Zweifel and Lasker (1976) were used t o f i t an exponential equation expressing t h e r e l a t i o n between i n c u b a t i o n time and i n c u b a t i o n temperature ( F i g . 1)

-7-

0

m

Predicted

.

a

0

Observed

0

* e

0 M

0

cu

0 8

r(

11

I

I

I

I

1

1

I

12

13

14

15

16

17

18

19

Temperature Figure 1. Incubation time (days) o f anchovy eggs a s a function of temperature ( "C)

.

I

I

20

21

-8-

where tI i s t h e i n c u b a t i o n time i n days and temp i s t h e temperature i n "C (Fig. 1).

Equation (1) can be used t o estimate anchovy egg production by

a d j u s t i n g t h e standing stock o f eggs f o r given average water temperature (Lo I n prep.).

3.2

Temperature Dependent Growth o f Yolk-Sac Northern Anchovy Larvae

A f t e r t h e onset o f feeding, growth i n sea caught anchovy l a r v a e appears t o be independent o f water temperature w i t h i n season (Methot and Kramer 1979; Methot 1981).

Thus I have f o l l o w e d t h e procedure o f Methot and H e w i t t (1980)

and have assumed t h a t o n l y t h e egg and yolk-sac stages o f anchovy growth are temperature dependent and growth over t h e r e s t o f t h e l a r v a l p e r i o d (4.1-27 mm) i s temperature independent o r food dependent.

I combined t h e new data on

growth o f yolk-sac n o r t h e r n anchovy w i t h t h e data o f Zweifel and Hunter (MS) and estimated t h e r e l a t i o n between l i v e l e n g t h and age a t 6 temperatures u s i n g t h e Gompertz growth equation ( L a i r d 1969).

where Lt i s t h e l i v e l e n g t h (mm) a t age t days from f e r t i l i z a t i o n .

,L

i s t h e asymptotic l e n g t h (mm) a t t h e age of f i r s t feeding,

Lk i s t h e l i v e l e n g t h (mm) a t age t k

- 9tk i s e i t h e r t h e age a t hatching, which i s temperature dependent

i n c u b a t i o n time ( t T ) o r age 0 ( t o ) and ‘temp

i s t h e growth c o e f f i c i e n t which i s temperature dependent.

Equation ( 2 ) was d e r i v e d from t h e assumption o f t h e exponential form of t h e age-dependent instantaneous growth r a t e .

i.e.,

(2.1)

where

i s the

LkeX/aand L&,

growth r a t e a t age tko S u b s t i t u t i n g ,L i n eq (2.1) produces eq ( 2 ) .

for

for

The values o f the

c o e f f i c i e n t s f o r each o f 6 temperatures (see Table 1 f o r data) a r e w i t h t k = tI, t h e i n c u b a t i o n time:

L tempOC 13.47 14.00 15.03 16.23 17.00 20.00

n

(SEI (mm)

4.166 (0.117) 4.21 (0.29) 4.328 (0.208) 4.287 (0.467) 4.546 (0.108) 4.152 (0.040)

The c o e f f i c i e n t f i t t o (dtemp,

3.11 ( 0.0 347

2.27 (0.33) 2.86 (0.078 ) 2.84 (0.0566) 2.406 (0.104) 2.4 (0 .O 734

0.551 (0.090 1 0.78 (0.38) 0.556 (0.1555) 0.9438 (0.417) 1.1543 (0.026) 1.71 (0.027)

dtemp i s a f u n c t i o n o f temperature.

30

7 30

11 8 10

An exponential curve was

temp) g i v i n g t h e f i n a l v e r s i o n o f t h e length-age r e l a t i o n f o r

yolk-sac anchovy l a r v a e l e s s than 4.1 mm w i t h t k = to and Lk = Lo

- 10Equation ( 3 ) was simultaneously f i t t e d t o a l l the d a t a collected a t 6 temperatures and resulted i n the following estimated parameters (Table 1 and

Fig. 2):

Parameters

Estimates

(SE)

4.25 mm

0.093 mm

LO

0.32 mm

0.17 mm

A

0.11

0.02

B

0.12

0.006

LOO

Retention Coefficients f o r Anchovy Eggs and Larvae

3.3

T h i s section discusses the results of a s e r i e s of simultaneous v e r t i c a l

tows of 0.333, 0.150 and 0.075mm mesh conducted i n March 1982, the estimation of egg retention by nets of two l a r g e mesh s i z e s compared t o smaller meshes, and the estimation of the retention of anchovy larvae as a function of

preserved standard 1ength.

The following notations a r e used t h r o u g h the text f o r the retention r a t e calculation:

YiL:

catch per u n i t volume of water f i l t e r e d (0.05 m3/m

-

depth) f o r mesh i and egg and larval length L.

R i j L = YiL/Y.jL:

the retention r a t e of mesh i compared t o mesh j.

The sample s t a t i s t i c s a r e denoted as

jiLand

h

R for

YiL

and R

where

0 f o r eggs

L = l a r v a l length (mm) (preserved standard l e n g t h ) ,

-11-

Length (mm) 2.0

I

2.5 I

3.0 1

3.5 1

4.5

4.0 I

U

0

N

0

v 0

I P

00

0

Figure 2. Growth curves o f yolk-sac stage o f anchovy larvae a t various temperatures.

5.0 1

-12-

f o r 0.075 and 0.150mm N i t e x meshes,

i and j = 1 2

f o r 0.333mm N i t e x mesh,

3

f o r 0.505mm N i t e x mesh Bongo

4

f o r 0.505mm N i t e x mesh 1 m r i n g net,

5

f o r 0.55mm s i l k mesh 1 m r i n g net.

and

RCFijL = l/RijL:

r e t e n t i o n c o r r e c t i o n f a c t o r , which was used as a

m u l t i p l i e r t o c o r r e c t egg and l a r v a counts.

The b a s i c procedure f o r computing r e t e n t i o n o f eggs and l a r v a e o f l e n g t h L from mesh i (compared t o mesh 1) f o r meshes .505 mm and .55 mm ( i > 3) is:

and

2.

The r e t e n t i o n of l a r v a e ( R i l ~ )was a l s o described as a

f u n c t i o n o f i t s preserved l e n g t h (L) f o r a given mesh s i z e i:

The c o e f f i c i e n t s Ai

and Bi a r e estimated i n l a t e r sections.

-133.3.1

Retention C o e f f i c i e n t s o f Eggs and Larvae i n 0.333mm Mesh From t h e Simul taneous Three-Vertical-Net Tows

I n o r d e r t o e v a l u a t e whether t h e t h r e e meshes (0.075,

0.150 and 0.333 mm)

r e t a i n t h e same amount of eggs and l a r v a e , s t a t i s t i c a l t e s t i n g procedures were s e l e c t e d a f t e r c a r e f u l examination o f t h e data.

A l l t h e egg and l a r v a samples

were f i r s t s o r t e d and t h e l a r v a e measured t o t h e nearest 0.5 mm. sample c o n t a i n s counts o f eggs and l a r v a e o f 2.0, t o t a l o f eleven groups. than 6.5 mm.

2.5,

3.0,

Thus, each

..., 6.5+

mm, a

The l a s t l e n g t h group c o n t a i n s l a r v a e equal o r l a r g e r

I examined t h e b a s i c s t a t i s t i c s (sample mean and standard

d e v i a t i o n ) , t h e frequency d i s t r i b u t i o n o f counts i n each group by mesh s i z e (Fig. 31, and t h e c o r r e l a t i o n between groups w i t h i n mesh size.

The b a s i c s t a t i s t i c s i n d i c a t e d t h a t w i t h i n l a r v a l e n g t h group, t h e sample standard d e v i a t i o n ( s ) was p r o p o r t i o n a l t o i t s sample mean );( e.g.,

the r a t i o ,

e t c . (Table 4 ) . symmetric.

s/j,

among meshes,

was 1.11 t o 1.15 f o r 2.0 mm, 0.83 t o 0.97 f o r 2.5 mm

F o r each mesh s i z e , t h e d i s t r i b u t i o n o f egg counts was f a i r l y

The d i s t r i b u t ons o f l a r v a counts between 2.0 and 4.0 mm were

skewed, w i t h a t most 0.40 o f samples c o n t a i n i n g zero catch, whereas t h e d i s t r i b u t i o n s o f l a r v a e > 4.5 mm were h i g h l y skewed, w i t h 0.69 t o 0.97 of t h e samples c o n t a i n i n g zero c a t c h and t h e maximum count being 8 (Table 2 and F i g . 3).

Because a l l t h e l a r v a e d i s t r i b u t i o n s were skewed, t h e l o g a r i t h m i c

t r a n s f o r m a t i o n was a p p l i e d t o l a r v a data, x = I n ( y + l ) , p r i o r t o t h e computation o f c o r r e l a t i o n c o e f f i c i e n t s .

Significant correlation exists

< 4.0 mm. between a d j a c e n t groups f o r eggs and l a r v a e -

The c o r r e l a t i o n between

a d j a c e n t groups seems s t r o n g e r i n meshes 0.075 and 0.150 mm than 0.333 mm.

As

a r e s u l t , a m u l t i v a r i a t e a n a l y s i s o f variance (MANOVA) was chosen f o r eggs and

-14EGG AND LARVA COUNTS Lr

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g 3 3

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f

f

3 3

3 3

F i g u r e 3 A . Frequen y d i s t r i b u t i o n s o f anchovy eggs and l a r v a e by leng-h groups (2-4 mm) f o r each o f t h r e e mesh s i z e s (0.075, 0.150 and 0.333 mm) based upon t h r e e - v e r t i c a l - n e t experiment, 1982. Sample means a r e denoted by ' M I . Each d o t represents one count. The number a t t h e end i s t h e t o t a l frequency f o r t h a t category.

-15LOG (egg and larva counts + 1)

.. .. **

............. ............ ............ .......... .,.. ...... .. .. ..

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1

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W

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.

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x3

.

P A VI

0

3 3

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.................. ............... .............. ........... .......... ......... ....... .....

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................ ............... ............ ........... ........... ......... ....... ...... .....

3 3

v

0

3 3

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0 DIMOULDmONLDmOULB~o,~)DmONP-

.............. ........... ........... ........... .......... ........ ....... ..... .. .. .. .. .... ....

oGooooooooooooooooooooooGoz OoOOOOOOOOoOO~OOOOOOoooooo~

...........................

I

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F i g u r e 3B. Frequency d i s t r i b u t i o n s o f anchovy eggs and l a r v a e by l e n g t h groups (2-4 mm) f o r each o f t h r e e mesh s i z e s (0.075, 0.150 and 0.333 mm) a f t e r l o g t r a n s f o r m a t i o n based upon t h r e e - v e r t i c a l - n e t experiment, 1982. Sample means a r e denoted by 'MI. Each d o t represents one count. The number a t t h e end i s t h e t o t a l frequency f o r t h a t category.

-16-

Table 2. Number of samples w i t h zero catch from a t o t a l of 140' samples for anchovy eggs and larvae by length group, the maximum catch for each of three mesh sizes and the chi-square values for larvae > 4.5mm based upon three-vertical-net-experiment, 1982. Number of samples w i t h zero catch 0.075mm

0.150mm

Maximum catch

0.333mm

chi-square

0.75mm

0.150mm

0.333mm

d.f.=2

0

0

0

2.0 mm

42

38

33

2.5

16

10

9

3.0

0

0

2

3.5

7

8

18

4.0

27

24

52

4.5

103

97

97

8

3

5

0.83

5.0

111

107

103

5

3

5

1.27

5.5

131

133

136

2

1

1

2.0

6 .O

132

130

133

2

1

1

0.6

6.5+

117

102

118

2

3

2

7.24*

Eggs

11.94 ( d . f . = l O )

'Two samples were discarded because they were spoiled. *Significant a t 5% level.

E

-17l a r v a e < 4.5 mm and a chi-square t e s t on t h e p r o p o r t i o n o f samples w i t h zero c a t c h among t h r e e meshes was chosen f o r l a r v a e > 4.5

mm t o d e t e c t t h e

r e t e n t i o n d i f f e r e n c e o f t h r e e meshes.

For eggs and l a r v a e < 4.0 mm, a two-way ( f a c t o r ) MANOVA was conducted t o t e s t whether t h e mesh s i z e s o f 0.075,

0.150 and 0.333 mm and t h e p o s i t i o n o f

t h e t h r e e n e t s (starboard, c e n t e r and p o r t ) had any s i g n i f i c a n t e f f e c t on t h e i.

catches.

Because t h e sample standard d e v i a t i o n o f l a r v a counts was

p r o p o r t i o n a l t o i t s sample mean w i t h i n t h e l e n g t h group, and t h e frequency d i s t r i b u t i o n s were skewed, a l o g a r i t h m i c t r a n s f o r m a t i o n x = l n ( y + l ) was a p p l i e d t o l a r v a data t o e q u a l i z e t h e variances and reduce t h e skewness of t h e data. The constant "1" was added t o t h e o r i g i n a l data f o r zero observations (Scheff; 1958; Snedecor and Cochran 1969; L i 1965; A i t c h i s o n and Brown 1973). Comparison o f t h e variances o f transformed data showed t h a t except f o r l a r v a e o f 2.0 mm, t h e variances w i t h i n l e n g t h groups among meshes were n o t significantly different.

Moreover, t h e d i s t r i b u t i o n s of the transformed data

were more symmetric than those o f t h e o r i g i n a l data (Fig. 3).

The r e s u l t s

from t h e two-way MANOVA i n d i c a t e d t h e n e t p o s i t i o n made no s i g n i f i c a n t d i f f e r e n c e i n c a t c h (p=0.83),

< O.OOl),

w h i l e mesh s i z e made a s i g n i f i c a n t d i f f e r e n c e (p

as d i d t h e i n t e r a c t i o n o f mesh s i z e and n e t p o s i t i o n ( p < 0.001).

Because t h e i n t e r a c t i o n o f mesh s i z e and n e t p o s i t i o n was s i g n i f i c a n t , a separate MANOVA was r u n f o r each o f t h e t h r e e p o s i t i o n s .

It was found t h a t

t h e o v e r a l l mean catches o f eggs and l a r v a e among meshes were s i g n i f i c a n t l y d i f f e r e n t a t each p o s i t i o n .

The u n i v a r i a t e t e s t s show t h a t t h e mean catch was

s i g n i f i c a n t l y d i f f e r e n t f o r eggs and l a r v a e < 4.5 mm w i t h t h e exception o f the mean c a t c h o f l a r v a e o f 2.0 mm and 2.5 mm on t h e starboard s i d e which was n o t s i g n i f i c a n t l y d i f f e r e n t among meshes.

The cause o f t h e i n t e r a c t i o n i s

-18-

unknown.

Since t h e p o s i t i o n o f t h e n e t d i d n o t a f f e c t t h e catch, a one-way MANOVA was used t o e v a l u a t e t h e d i f f e r e n c e among meshes.

The mean c a t c h o f anchovy

eggs and l a r v a e taken i n small mesh n e t s (0.075 and 0.150mm) was compared t o t h a t taken i n 0.333mm mesh net, a f t e r t h e mean catch was compared between small meshes (0.075 and 0.150mm).

The mean c a t c h o f eggs and l a r v a e i n t h e

0.075mm n e t was s i g n i f i c a n t l y d i f f e r e n t from t h a t i n t h e 0.150mm mesh n e t (p=0.04).

However, t h e s i g n i f i c a n t d i f f e r e n c e was m a i n l y due t o egg counts

( c a t c h i n 0.150 mm was s l i g h t l y h i g h e r than t h a t i n 0.075 mm mesh (p=0.03)) whereas a l l t h e l a r v a e catches seemed t o be t h e same between these two meshes.

.

Therefore, t h e catches from these two small meshes were combined t o be compared w i t h t h a t o f 0.333 mm mesh.

The mean c a t c h i n t h e 0.333 mm mesh n e t

was d i f f e r e n t from b o t h o f t h e small mesh n e t s (0.075 o r 0.150mm) (Table 3 ) .

( p < 0.001)

Hence, eggs and l a r v a l anchovy 4.5 mm among 3 mesh sizes, a 2x3 contigency t a b l e was f i r s t c o n s t r u c t e d f o r each o f t h e 5 groups where t h e f i r s t a t t r i b u t e was mesh s i z e (.075, c a t c h ( z e r o and non zero). chi-square values.

0.150 and 0.333 mm) and t h e second was

A l l b u t t h e l a s t l e n g t h group had i n s i g n i f i c a n t

Because l i t t l e c o r r e l a t i o n e x i s t s among t h e groups, t h e

sum o f t h e 5 chi-square values was used t o t e s t t h e o v e r a l l r e t e n t i o n d i f f e r e n c e among t h e mesh sizes.

The chi-square value (11.94) was

i n s i g n i f i c a n t a t 5% l e v e l (Table 2).

Thus t h e r e seems t o be no e x t r u s i o n o f

> 4.5 mm from 0.333 mm mesh net. larvae -

-19-

Table 3.

One-way a n a l y s i s o f variance o f r e t e n t i o n o f anchovy eggs and l a r v a e ( < 4.0 mm, l o g t r a n s f o r m a t i o n ) i n t h r e e meshes -comparison between catches i n 0.075 mm and 0.150 mm meshes, and comparison between catches i n 0.333 mm and small meshes.

Effect .075 mm vs. .150 mm

Variate

Sum of squares

A1 l 1

Mean squares

TSQ = 13.67

U n i v a r i a t e Tests 3793.13 eggs 1.68 2.0 mm 2.69 2.5 0.25 3 .O 0.33 3.5 0.11 4.0 ,075 mm & .150 mm vs. .333 mm

d.f.

1 1 1 1 1 1

2.25 (6,397) 3793.13 1.68 2.69 0.25 0.33 0.11

(1,402) 5.01 1.44 2.96 0.68 0.44 0.25

4419.34 2.93 7.19 18.89 15.79 15.79

18.79 (6,397) (1,402) 5.84 2.52 7.93 51.35 20.95 20.95

TSQ = 114.14

All'

Univariate tests 4419.34 eggs 2.93 2.0 mm 7.19 2.5 18.89 3.0 15.79 3.5 11.71 4.0

1 1 1 1 1 1

F (d.f.)

Error eggs 2.0

2.5 3.0 3.5 4.0

304141.94 467.95 364.70 147.90 303.01 177.77

lMul t i v a r i a t e a n a l y s i s . 2p