ICHTHYOPLANKTONIC AND ACOUSTIC BIOMASS ESTIMATES OF ANCHOVY IN THE AEGEAN SEA (JUNE 2003 AND JUNE 2004)

S. SOMARAKIS1, A. MACHIAS2, M. GIANNOULAKI 2, A. SIAPATIS2, M. TORRE3, A. ANASTASOPOULOU2, V. VASSILOPOULOU2, A.

KALLIANIOTIS3, C. PAPACONSTANTINOU2

1 University of Patras, Department of Biology, Patras2 Hellenic Centre of Marine Research, Athens and Iraklion Subdivisions

3 Fisheries Research Institute, Kavala

T h e D E P M m o d e l

B = ( k . P . A . W ) / ( R . F . S ) w h e r e , B = s p a w n i n g s t o c k b i o m a s s i n m e t r i c t o n s , k = c o n v e r s i o n f a c t o r f r o m g r a m s t o m e t r i c t o n s , P = d a i l y e g g p r o d u c t i o n ( n u m b e r o f e g g s p e r s a m p l i n g u n i t , m 2 ) , A = t o t a l s u r v e y a r e a ( i n s a m p l i n g u n i t s , m 2 ) , W = a v e r a g e w e i g h t o f m a t u r e f e m a l e s ( g r a m s ) , R = s e x r a t i o ( f r a c t i o n o f m a t u r e f e m a l e s b y w e i g h t ) , F = b a t c h f e c u n d i t y ( m e a n n u m b e r o f e g g s p e r m a t u r e f e m a l e s p e r s p a w n i n g ) , S = f r a c t i o n o f m a t u r e f e m a l e s s p a w n i n g p e r d a y ( s p a w n i n g f r e q u e n c y )

)2)()()()(( 22222 COVSSCVFCVWCVPCVBVarB

Egg samplesEgg samplesEstimatorsEstimators

Daily Egg Production Method (DEPM)Daily Egg Production Method (DEPM)

Systematic adaptive sampling

Post-stratification

00

11 P

A

AP

A

AP

)()(

11)( 0

01

1 PVarA

APVar

A

A

nPVar

Adult samplesAdult samples

Sampling with probability related to stock abundance

y

m y

m

i ii

n

ii

n

1

1

Var y

m y y

m

nn n

i ii

n

i

i

n ( )

( )

2 2

1

1

2

1

Variance of Biomass estimateVariance of Biomass estimate

VarB B CV P CV W CV F CV S COVS

2 2 2 2 2 2( ( ) ( ) ( ) ( ) )

WP2 stations

23 24 25 26

39

40

41

23 24 25 26

39

40

41

23 24 25 26

39

40

41

2 3 2 4 2 5 2 6

3 9

4 0

4 1

20042003

Strymoniko s & Kavala Gulfs

Evoikos Gulf

Thracian Sea

Thermaikos Gulf

LemnosPlateau

AEGEAN SEA

Strymoniko s & Kavala Gulfs

Evoikos Gulf

Thracian Sea

Thermaikos Gulf

LemnosPlateau

AEGEAN SEA

Stratum I

Stratum II

Stratum I

Stratum II

Egg surveys

10 x 5 nm grid4 x 4 nm in the Evoikos Gulf

2 3 2 4 2 5 2 6

3 9

4 0

4 1

2 3 2 4 2 5 2 6

3 9

4 0

4 1

Pelagic trawl Purse-seine

2003 2004

Strymoniko s & Kavala Gulfs

Evoikos Gulf

Thracian Sea

Thermaikos Gulf

LemnosPlateau

AEGEAN SEA

Stratum I

Stratum II

Adult surveys

• separate biomass estimates for the eastern (Stratum I) and western (Stratum II) part of the surveyed area, based on distribution patterns of fish and their eggs, as well as regional differences in hydrologic regimes

• the surveyed area did not cover the entire spawning area in the east (Turkish territorial waters)

• vertical plankton tows (WP2, 0.200-mm mesh, mouth area: 0.255 m2)• stratification of survey area into negative and positive strata • use of both eggs and yolk sac stages to increase precision in daily egg production

estimates • plankton station weighting proportional to their representative area• fitting of an exponential mortality model to the counts of embryos on their age to

derive the daily egg production (weighted non linear regression) • use of both commercial and experimental adult samples • use of the ratio estimator for adult parameters and their variances • fit of a linear model regressing batch fecundity on ovary-free weight• Use of POF-1 for spawning fraction calculations• bias-correction of spawning fractions to overcome the problem of oversampling

active (Day-0) spawning females

DEPM application characteristics

2 3 2 4 2 5 2 6

3 9

4 0

4 1

2003

3.92 – 1239.22 eggs/m2

2 3 2 4 2 5 2 6

3 9

4 0

4 1 3.92 – 1498.02 eggs/m2

Negative stratum

2004

•Three main areas of increased egg abundance•Low egg abundance in Thermaikos Gulf during 2004

2 3 2 4 2 5 2 6

3 9

4 0

4 1

1 5

1 8

2 1

2 4

2 7

2 3 2 4 2 5 2 6

3 9

4 0

4 1

2003Temperature (5m)

Hydrology (June 2003)

2 3 2 4 2 5 2 6

3 9

4 0

4 1

3 0

3 2

3 4

3 6

3 8

4 0

2 3 2 4 2 5 2 6

3 9

4 0

4 1

2003 Salinity (5m)

The eastern part of the surveyed area(Stratum I) was cooler and less saline due to BSW influence

2 3 2 4 2 5 2 6

3 9

4 0

4 1

1 5

1 8

2 1

2 4

2 7

2 3 2 4 2 5 2 6

3 9

4 0

4 1

2004 Temperature (5m)

Hydrology (June 2004)

2 3 2 4 2 5 2 6

3 9

4 0

4 1

3 0

3 2

3 4

3 6

3 8

4 0

2 3 2 4 2 5 2 6

3 9

4 0

4 1

2004Salinity (5m)

The eastern part of the surveyed area(Stratum I) was cooler and less saline due to BSW influence

Gelatinous zooplankton

23 24 25 26

39

40

41

Anchovy eggs

3.92 - 188.24 Ctenophora/m 2

Ctenophora

Beroe sp.

2004

Atresia

High incidence of state-1 and state-2 atresia in Thermaikos Gulf (Stratum II) during June 2004

Comparison of Pelagic trawl – Purse seine samples

Adult parameters – Stratum I

0

0. 1

0. 2

0. 3

0. 4

0. 5

0.4

0.5

0.6

0.7

0.8

0.9

1

3200

3700

4200

4700

5200

5700

6200

14

16

18

20

22

24

26

0

0.2

0.4

0.6

0.8

1

0.19

0.29

0.39

0.49

0.59

5200

6200

7200

8200

9200

10200

12 14 16 18

20 22

24

12

15

18

21

24

27

0

0.2

0.4

0.6

0.8

1

0

0.1

0.2

0.3

0.4

0.5

3200

5200

7200

9200

11200

Γρι-γρι Πελαγικ ή τράτα Γρι-γρι Πελα γική τράτα Γρι-γρι Πελαγική τράτα

R

W

S

F

2003 2004 2003+2004

*

Batch fecundity

Stratum I

Hydrated oocyte method

2004F = 390.1 W*

R2 = 0.41n = 92

2003F = 265.36 W*

R2 = 0.27n = 45

0

2000

4000

6000

8000

10000

12000

14000

0 5 10 15 20 25 30

W* (g)

Αρ

ιθμ

ός α

βγώ

ν (F

)

2004

2003

Relative batch fecundity (eggs/g) was significantly higher in 2004

Density dependence?

Spawning frequency (S)

Post ovulatory follicles (POFs)

Day-0

00.00

06.00

Day-1

00.00

06.00

The degeneration of POFs is faster during night-time (vertical distribution of adults)

The POF-1 are small at about age-30 hours

No POF-2 are present in early night – POFs resorption <42 hours

2003 2004 East West East West Number of plankton samples 111 92 110 94 Number of adult samples 23 12 17 21 Survey area (A, km2) 17962 12870 17657 13304 Positive area (A1, km2) 15613 10427 12333 9111 Daily egg production in the positive area (P1, eggs m-2)

63.50 (CV=0.190)

84.13 (CV=0.266)

51.91 (CV=0.210)

42.48 (CV=0.176)

Daily egg production in the surveyed area (P, eggs m-2)

55.17 (CV=0.204)

68.16 (CV=0.296)

36.25 (CV=0.239)

29.09 (CV=0.214)

Average weight of mature female (W, g) 15.36 (CV=0.022)

17.57 (CV=0.049)

18.92 (CV=0.039)

16.73 (CV=0.029)

Weight specific sex ratio (R) 0.648 (CV=0.038)

0.453 (CV=0.097)

0.630 (CV=0.077)

0.571 (CV=0.044)

Spawning fraction of mature females (S) 0.340 (CV=0.062)

0.341 (CV=0.095)

0.436 (CV=0.051)

0.125 (CV=0.243)

Batch fecundity (F, number of eggs) 3936 (CV=0.049)

4446 (CV=0.063)

7053 (CV=0.047)

5482 (CV=0.083)

Spawning stock biomass (SSB, t) 17600 (CV=0.220)

22442 (CV=0.316)

6251 (CV=0.247)

16548 (CV=0.320)

Total SSB (East + West) 40042 (CV=0.202)

22799 (CV=0.242)

Comparison of Acoustics and DEPM estimates

0

10000

20000

30000

40000

50000

60000

70000

80000

2003 2004 2003 2004 2003 2004

East West Total

Bio

mas

s (t

)

DEPM SSB Acoustics TSB

•The Acoustic estimates are always slightly higher, except for the western stratum during 2004•Both methods show a marked decrease in biomass during 2004 for the eastern stratum

Comments and conclusions

• The distribution pattern of anchovy from the acoustic sampling as well as the pattern of egg distribution in the surveyed area suggest that the anchovy stock forms three major aggregations in the Aegean Sea: (a) The Thracian Sea aggregation distributed in waters that receive the direct influence of Black Sea waters (b) the Thermaikos Gulf aggregation inhabiting the semi-enclosed and productive Thermaikos Gulf and (c) the N. Evoikos Gulf aggregation with fish located in a highly enclosed area, which is also known to be very productive.

• Marked differences in certain DEPM parameters were observed between the two years in both the eastern and the western part of the Aegean Sea which deserve to be examined with regard to environmental differences (mainly zooplankton biomass, gelatinous zooplankton abundance) and density- or size- dependent differences in anchovy reproductive performance.

• The DEPM SSB estimates were quite close to the acoustic TSB estimates except in the western part of the survey area during June 2004. In the 2004 DEPM parameter estimation, data from the Thermaikos Gulf (characterized by very low egg abundances, low spawning frequency and high proportion of atretic females) were pooled with data from the Evoikos Gulf (high egg abundances high spawning frequency). This might have posed problems in the DEPM parameter estimates, which were hard to solve due to the small size of the Evoikos Gulf.

• The estimates accumulated from the application of DEPM and acoustics to the Aegean Sea stock during the Greek Extended Program should be used to tune assessment techniques such as the statistical catch-at-age methods. This will greatly help in the management of this important stock of the Aegean Sea.

• Stock size estimates in the eastern part of the Aegean Sea are probably under-estimates of the true stock sizes due to the inability of the Greek scientists to sample the Turkish territorial waters.