1. Ecosystem Based Management and Monitoring in the deep Med
Ecosystem Based Management and Monitoring in the deep Med. & Atlantic, August NUI Galway
PRELIMINARY STOCK ASSESSMENT OF BLACKSPOT RED SEABREAM, PAGELLUS BOGARAVEO, IN THE IONIAN SEA (EASTERN MEDITERRANEAN) AND POTENTIAL HARVEST CONTROL RULES
Haralabous J., Damalas D., Mytilineou Ch.*, Kavadas S., Dokos J., Anastasopoulou A., Bekas P., Christidis G. & Dogramazti K. Hellenic Centre for Marine Research, P.O. BOX 712, Anavissos, Athens, Greece
The blackspot red seabream, Pagellus boga-raveo, and its fishery in the E. Ionian Sea consist a data poor situation, as the species is not included in the targets of the National Fisheries Data Collection Regulation (DCR). Analytical stock assessment has never been carried out for this species in the Eastern Me-diterranean Sea and particularly the Eastern Ionian Sea. A first attempt has been carried out in the framework of the DEEPFISHMAN Project using pseudo-cohort analysis & sur-plus production approaches on the landings, effort and size composition data for the pe-riod Abundance indices from surveys available from 1998 to 2008 were al-so used for a non-equilibrium surplus produ-ction model.
1. Pseudo-cohort analysis
The total catch for all gears fishing black-spot red seabream obtained by the DCR (Table 1) and the length frequency distribution of the catch, derived from commercial fishery on-board sampling (Fig. 1) were used to produ-ce the catch-at-age distributions from all ge-ars combined by year and in average (Fig. 2). Pseudo-cohort analysis was applied on each year's catch-at-age separately and on the a-verage catch-at-age of the period
as follows:
Average F= 0.306
Average stock in N= 7.37x106
Average stock biomass= t
The assessment revealed that the immature part of the population does not suffer from the intense fishing pressure as the older one.
it (Pella and Tomlinson, 1969). Hence, the pa-rameter values are adjusted to provide the best fit of the predicted to observed time se-ries of relative abundance. This model gave the following results (Fig 5):
MSY=161.5 t, Bmsy=633.4 t & Fmsy=0.255
Fig. 5. Non-equilibrium surplus production models outputs for the Eastern Ionian Sea blackspot red seabream stock.
Fig. 3. Pseudo-cohort results of P. bogaraveo stock in the Eastern Ionian Sea (average ). Natural losses of the stock, catch, survivors and fishing mortality (F) by age are also shown.
Catches in 2008 dropped below MSY after 3 successive years of exceeding it ( ). At the same period, the biomass level followed a declining trend approaching Bmsy, which may indicates possible appearance of over-exploi-tation in the near future.
2. Surplus production models
Schaefer & Fox surplus production models were applied to determine the optimum le-vel of effort (fMSY) producing the maximum sustainable yield (MSY). The input data, total catch and total effort for all gears covered the period (DCR data).
Schaefer’s model results (Fig. 4):
MSY=168 t & fMSY=26,017 days-at-sea.
Fox model results (Fig 4):
MSY=157 t & fMSY=20,790 days at sea
Potential Harvest Control Rules
The above values could serve as the source for the estimation of Biological Reference Points and Harvest Control Rules. The study of the ab-ove for P. bogaraveo in the E. Ionian Sea was based on the framework proposed by Froese et al. (2011).
According to the proposed rules (Fig. 6):
MSY=161.5t, 0.91MSY=147t, 0.75MSY=121t, Bmsy=633.4t, 1.3Bmsy=823.4t, 0.5Bmsy=316.7t.
the stock was close to overfishing the first fifth year period ( ). The subsequent ye-ars the picture was improved. Since the bio-mass of tons in 2008 (last assessed ye-ar) did not fall below Bmsy and the yield of tons was below 91% of MSY no reduc-tion in catches is needed for the present.
Table 1. Landings, Discards and Total Catch (in tons) of P. bogara-veo in the Eastern Ionian Sea. GR: Greece; OTB: Otter trawl; GNS: Gill-nets; LLS: Long-lines; GTR: Trammel-nets
Fig. 4. Surplus production models outputs for the Eastern Ionian red blackspot seabream stock ( ).
Both surplus production models suggested that the level of exploitation in 2008 seemed to be within sustainable limits, although in the previous three years the stock could be characterised more or less as overexploited.
3. Non-equilibrium surplus
production models
In the present work, we suggest an approach to utilize survey abundance indices (MEDITS) & catch data from commercial fisheries for a non-equilibrium surplus production model as applied by Tserpes et al. (2007). Assessment estimates were based on the Schaefer logistic model. The analysis assumes that population follows the logistic growth and the model pa-rameters a non-equilibrium approach. We fol-low an observation error time-series fitting method, which assumes that the underlying production relationship is correct, and that all the errors occur in the relationship between true stock size and the index used to measure
Fig. 1. Average length frequency distribution of P. bogaraveo in the total catch by gear. OTB: Otter trawl; GNS: Gill-nets; LLS: Long-lines; GTR: Trammel-nets
Fig. 6. Relative catch and biomass of P. bogaraveo stock in the Ionian Sea shown in the framework of the proposed by Froese et al harvest control rules. Average values for the first 5 years of the studied period ( ), the subsequent 5 years ( ) and the last year (2008) are depicted with different symbols.
References
Froese R., Branch T.A., Proelß A., Quaas M., Sainsbury K. and C. Zimmermann Generic harvest control rules for European fisheries. Fish and Fisheries, 12:
Pella J.J. and Tomlinson P.K A generalized stock-production model. Bull. Int. Am. Trop. Tuna Com. 13:
Tserpes G, J. Haralabous and C. Maravelias A non-equilibrium surplus production model approach utilizing MEDITS data. GFCM workshop on trawl survey based monitoring fishery system in the Mediterranean, Rome, 2007.
Acknowledgements: The analysis included in this work was financed by DEEPFISHMAN project (European Community's FP7, grant N° ). Co-financing was provided to HCMR from the Greek General Secretariat for Research and Technology.
Fig. 2. Annual catch-at-age distributions (all gears combi-ned) of P.bogaraveo in the Eastern Ionian Sea ( ).
Pseudo-cohort results can be seen graphi-cally in Fig 3. The average fishing mortality, stock in numbers and weight were estimated