Presentation on theme: "ICHTHYOPLANKTONIC AND ACOUSTIC BIOMASS ESTIMATES OF ANCHOVY IN THE AEGEAN SEA (JUNE 2003 AND JUNE 2004) S. SOMARAKIS 1, A. MACHIAS 2, M. GIANNOULAKI 2,"— Presentation transcript:
ICHTHYOPLANKTONIC AND ACOUSTIC BIOMASS ESTIMATES OF ANCHOVY IN THE AEGEAN SEA (JUNE 2003 AND JUNE 2004) S. SOMARAKIS 1, A. MACHIAS 2, M. GIANNOULAKI 2, A. SIAPATIS 2, M. TORRE 3, A. ANASTASOPOULOU 2, V. VASSILOPOULOU 2, A. KALLIANIOTIS 3, C. PAPACONSTANTINOU 2 1 University of Patras, Department of Biology, Patras 2 Hellenic Centre of Marine Research, Athens and Iraklion Subdivisions 3 Fisheries Research Institute, Kavala
Egg samples Estimators Daily Egg Production Method (DEPM) Systematic adaptive sampling Post-stratification Adult samples Sampling with probability related to stock abundance Variance of Biomass estimate
WP2 stations Stratum I Stratum II Stratum I Stratum II Egg surveys 10 x 5 nm grid 4 x 4 nm in the Evoikos Gulf
Pelagic trawlPurse-seine 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, mm mesh, mouth area: m 2 ) 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
– eggs/m – eggs/m 2 Negative stratum 2004 Three main areas of increased egg abundance Low egg abundance in Thermaikos Gulf during 2004
2003 Temperature (5m) Hydrology (June 2003) 2003 Salinity (5m) The eastern part of the surveyed area (Stratum I) was cooler and less saline due to BSW influence
2004 Temperature (5m) Hydrology (June 2004) 2004 Salinity (5m) The eastern part of the surveyed area (Stratum I) was cooler and less saline due to BSW influence
Gelatinous zooplankton 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 *
Batch fecundity Stratum I Hydrated oocyte method Relative batch fecundity (eggs/g) was significantly higher in 2004 Density dependence?
Spawning frequency (S) Post ovulatory follicles (POFs) Day Day 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
Comparison of Acoustics and DEPM estimates 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 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.