G. Daskalov

Centre for Environment, Fisheries and Aquaculture Science, Lowestoft, England, United Kingdom

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Publications (10)18.03 Total impact

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    ABSTRACT: It is well known that human activities, such as harvesting, have had major direct effects on marine ecosystems. However, it is far less acknowledged that human activities in the surroundings might have important effects on marine systems. There is growing evidence suggesting that major reorganization (i.e., a regime shift) is a common feature in the temporal evolution of a marine system. Here we show, and quantify, the interaction of human activities (nutrient upload) with a favourable climate (run-off) and its contribution to the eutrophication of the Black Sea in the 1980s. Based on virtual analysis of the bottom-up (eutrophication) vs. top-down (trophic cascades) effects, we found that an earlier onset of eutrophication could have counteracted the restructuring of the trophic regulation at the base of the food web that resulted from the depletion of top predators in the 1970s. These enhanced bottom-up effects would, however, not propagate upwards in the food web beyond the zooplankton level. Our simulations identified the removal of apex predators as a key element in terms of loss of resilience that inevitably leads to a reorganization. Once the food web has been truncated, the type and magnitude of interventions on the group replacing the apex predator as the new upper trophic level have no effect in preventing the trophic cascade. By characterizing the tipping point at which increased bottom-up forcing exactly counteracts the top-down cascading effects, our results emphasize the importance of a comprehensive analysis that take into account all structuring forces at play (including those beyond the marine system) at a given time.
    Global Change Biology 02/2011; 17(3):1251 - 1265. · 8.22 Impact Factor
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    ABSTRACT: After its establishment in 2008 the STECF working group on Mediterranean stock assessments SGMED, covered by its mandate, also assesses exploited living resources in the Black Sea. STECF during its last plenary in July 2008 rejected the working group’s recommendation regarding specific fisheries management options for 2009. It was decide d a dedicated meeting to be held in the first quarter of 2009 to review these recommendations under specific terms of reference (ToR). The meeting took place in the hotel Belvedere in Ranco, Italy, 23-27 March 2009.
  • Georgi M. Daskalov
    02/2009: pages 418 - 433; , ISBN: 9781444302653
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    ABSTRACT: Fishing mortality and primary production (or proxy for) were used to drive the dynamics of fish assemblages in 9 trophodynamic models of contrasting marine ecosystems. Historical trends in abundance were reconstructed by fitting model predictions to observations from stock assessments and fisheries independent survey data. The model fitting exercise derives values for otherwise unknown parameters that specify the relative strength of trophic interactions and, in some instances, a time series anomaly for changes in primary production. We measured how much better or worse were model predictions when bottom-up forcing by primary production were added to top-down forcing by fishing. Searching for cross system patterns, the relative contribution of fishing and changes in primary production, mediated through trophic interactions, are evaluated for the ecosystems as a whole and for selected similar species in different ecosystems. The analysis provides a simple qualitative way to explain which forcing factors have most influence on modeled dynamics. Both fishing and primary production forcing were required to obtain the best model fits to data. Fishing effects more strongly influenced 6 of 9 of the ecosystems, but primary production was more often found to be the main factor influencing the selected pelagic and demersal fish stock trends. Examination of sensitivity to ecological and model parameters suggests that the results are the product of complex food-web interactions rather than simple deterministic responses of the models.
    Ecological Modelling. 01/2009;
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    ABSTRACT: The European Community is expected to establish long-term management plans (LTMP) for relevant Mediterranean demersal and small pelagic fisherie s based on precautionary approach and adaptive management in taking measures designed to protect and conserve living aquatic resources, to provide for their sustainable exploitation and to minimise the impact of fishing activities on marine eco- systems. The plans shall include conservation reference points such as targets against which measuring the recovery to or the maintenance of stocks within safe biological limits for fisheries exploiting stocks at/or within safe biological limits (e.g. population si ze and/or long-term yields and/or fishing mortality rate and/or stability of catches). The management plans shall be drawn up on the basis of the precautionary approach to fisheries management and take account of limit reference points as identified by scientists. The quantitative scientific assessment should provide sufficiently precise and accurate biological and economic indicators and re ference points to allow also for an adaptive management of fisheries. Stating clearly how stocks and fisheries will be assessed and how deci sion will be taken is fundamental for proper and effective implementation of management plans as well as for transparency and consultations with stakeholders. Demersal and small pelagic stocks and fisheries in the Mediterranean are evaluated both at national and GFCM level; however these evaluations are often not recurring, are spatially restricted to only some GFCM geographical sub-areas (see attached reference map), covering only partially the overall spatial range where Community fishing fleets and stoc ks are distributed, and address only few stocks out of several that may be exploited in the same fish eries. Limited attention is also given to technical interactions between different fishi ng gears exploiting the same stocks. A limited, although fundamental, scientific contribution of EU fishery scientists to the GFCM assessment process is increasingly affecting the capacity of this regional fisheries management organization to identify harvesting strategies and control rules and to adopt precautionary and adaptive fisheries management measures based on scientific advice. Anyhow, GFCM and most of the riparian countries co nsider that management measures to control the exploitation rate and fishing effort, complemented by technical measures, are the most adequate approach for multi-species and multiple-gears Mediterranean fisheries. Nevertheless, provided that scientific advice unde rlines to do so, also output measures may be conceivable to manage fisheries particularly for both small pelagic and benthic fish stocks. Coherence and certain level of harmonization between Community and multilateral framework measures are advisable for effective conservation measures and to enhance responsible management supported by all concerned Parties and stakeholders in the Mediterranean. STECF can play an important role in focusing great er contributions of European scientists towards stocks and fisheries assessment, in identifying a common scientific framework regarding specific analyses to advise on Community plans and to be then channelled into or completed by the GFCM working groups. STECF was requested at its November plenary sess ion to set up an operational workprogramme for 2008, beginning in the 1st quarter of 2008, with a view to update the status of the main demersal stocks and evaluate the exploitation levels with re spect to their biological and economic production potentials and the sustainability of the stock by using both trawl surveys and commercial catch/landing data as collected through the Community Data Coll ection regulation N° 1543/2000 as well as other scientific information coll ected at national level. Within this work-programme STECF is also requested to provide its advice on the status of the main small pelagic stocks and to evaluate the exploitation levels with respect to their biological and economic production potentials and the sustainability of the stock by using both echo and/or DEPM surveys and commercial catch/landing data as collected through the Community Data Collection regulation N° 1543/2000 as well as other scien tific information collect ed at national level. STECF should take into consideration the data that Member States have been collecting on a regular basis both via monitoring fishing activities and carrying out direct surveys. STECF, in replying at the following terms of reference, should also take into consideration chapter 7 of the 26th STECF Plenary session of 5-9 November 2007, as well as the re port of the STECF working group on balance between fishing capacity and fishing opportunities. STECF shall contribute to identify and setup a ad visory framework regarding low risk adaptive management by identifying and using appropriate risk assessment methods in order to understand where we stand with respect to sustainable explo itation of ecologically and economically important stocks and what additional manage ment actions need to be taken. On the basis of the STECF advice derived at the April 2008 plenary the Commission launched an official data calls to EU Member States requesti ng submission of data collected under the Community Data Collection regulation N° 1543/2000. STECF is requested in particular: 9 to advice whether the data availability may allo w the development of a precautionary conceptual framework within which develop specific harvesting strategies and decision control rules for an adaptive management of demersal and sma ll pelagic fisheries in the Mediterranean; 9 to set up a conceptual, methodological and oper ational assessment framework which will allow STECF to carry out in a standardized way both stoc ks assessment analyses and detailed reviews of assessments done by other scientific bodies in the Mediterranean. The selected assessment methods shall allow estimating indicators for measuring th e current status of demersal and small pelagic fisheries and stocks, the sustainability of the e xploitation and to measure progress towards higher fishing productivity (MSY or other proxy) with respect to precautionary technical/biological reference points relating to MSY or other yield-base d reference points, to low risk of stock collapse and to maintaining the reproduc tive capacity of the stocks; 9 to set up a conceptual, methodological and oper ational assessment fram ework which will allow STECF to identify economic indicators and referenc e points compatible with economic profitability of the main fisheries while ensuring sustainable exploitation of the stocks in the Mediterranean; 9 to indicate whether age/length-based VPA or statistical catch-at –age/length methods are adequate modelling tools to estimate precautionary indicat ors and reference points measuring the current status and future development of multispecies/multig ears Mediterranean fisheries. STECF shall also provide a conceptual and operational framework to use, if advisable, these methods for demersal and small pelagic Mediterranean fisheries; to identify adequate empirical modelling approaches that are adequate to estimate precautionary indicators and reference points measuring th e current status and future development of multispecies/multigears Mediterrane an fisheries. STECF shall also provide a conceptual and operational framework to use, if advisable, these methods for demersal and small pelagic Mediterranean fisheries; 9 to identify the decision-making support modelling tools that are adequate for the Mediterranean fisheries and that will produce outputs that supp ort sustainable use of fishery resources recognizing the need for a precautionary framework in the face of uncertainty and that may allow to provide projections of alternative scenarios for short- medium and long term management guidance; 9 to provide either a qualitative or quantitative understanding of the level of precision and accuracy attached to the estimation of indicators and refere nce points through the different modelling tools; 9 to identify which decision-making support mode lling tools may help in setting up stock-size dependent harvesting strategies and respective decision control rules; 9 to provide information on the data and standardised format needed for each of the decision-making support modelling tool which will be used to launch official data calls under the DCR n° 1543/2000. STECF should also indicate criteria to ensure quality cross- checks of the data received upon the calls. STECF is requested to review the report of the Bl ack Sea working group which worked in parallel to the STECF April plenary and SGMED-08-03 of June 9-13 (Barza, Ispra) meetings, evaluate the findings and make any appropriate comments and re commendations. STECF is requested in particular to advice on 2009 catch limitations for turbot and spra t as well as on any other technical measures that is considered adequate for sustainable exploitation of these stocks. For the year 2008, the European Community adopted catch limitations and associated technical measures for sprat and turbot fisheries in the Black Sea. With a view to update the assessments and catch forecast of the concerned stocks and fisheries in the area an ad-hoc STECF working group on Black Sea was convened.
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    ABSTRACT: Large-scale transitions between alternative states in ecosystems are known as regime shifts. Once described as healthy and dominated by various marine predators, the Black Sea ecosystem by the late 20th century had experienced anthropogenic impacts such as heavy fishing, cultural eutrophication, and invasions by alien species. We studied changes related to these "natural experiments" to reveal the mechanisms of regime shifts. Two major shifts were detected, the first related to a depletion of marine predators and the second to an outburst of the alien comb jelly Mnemiopsis leidyi; both shifts were triggered by intense fishing resulting in system-wide trophic cascades. The complex nature of ecosystem responses to human activities calls for more elaborate approaches than currently provided by traditional environmental and fisheries management. This implies challenging existing practices and implementing explanatory models of ecosystem interactions that can better reconcile conservation and ecosystem management ideals.
    Proceedings of the National Academy of Sciences 07/2007; 104(25):10518-23. · 9.81 Impact Factor
  • Georgi M. Daskalov, Elchin V. Mamedov
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    ABSTRACT: Daskalov, G. M., and Mamedov, E. V. 2007. Integrated fisheries assessment and possible causes for the collapse of anchovy kilka in the Caspian Sea. – ICES Journal of Marine Science, 64. Anchovy kilka ( Clupeonella engrauliformis ) is the most abundant species of fish in the Caspian Sea. An integrated catch-at-age analysis based on commercial catch data and relative abundance indices from research surveys over the period 1991–2004 indicated two main periods. First, a period of high catches, from 1991 to 2000, sustained by relatively good recruitment and high spawning-stock biomass. At the end of this period, catches peaked at up to 271 400 t, fishing mortality reached 1.8 y−1 in 1999, and overfishing seems to have taken place. Over the next period, 2001–2004, the stock virtually collapsed, recruitment failing in 2001 and remaining very low thereafter. In 2005, total catches dropped to a historical minimum of 54 300 t. The most likely primary cause of the stock collapse is the invasion and spread of the ctenophore Mnemiopsis leidyi in the Caspian Sea. The dramatic recruitment failure from 2001 to 2004 is primarily attributed to competition/predation by this ctenophore, although other factors, including overfishing, likely contributed.
    01/2007;
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    Ices Journal of Marine Science - ICES J MAR SCI. 01/2005; 62(3):430-442.
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    ABSTRACT: Scientific information about the Black Sea ecosystem with respect to jelly plankton – fisheries interactions is reviewed. Long time-series of the main components of pelagic community: fishes, invertebrates, discussed trends, and relationships between them are analysed. The amount of information and results from the analyses will be used to formulate hypotheses about the linkages between gelatinous plankton and the other pelagic populations and the effects on fish stocks. These data will be used as a background study for the following stages in the Eurogel project: experimental work, data analyses and modelling. Дана научная информация об экосистеме Черного моря, относящаяся к взаимодействию между желетелым планктоном и рыболовством, проанализированы многолетние ряды наблюдений главных компонентов пелагического сообщества: рыб и беспозвоночных, обсуждены их тренды и связи между ними. Итоговая информация и результаты анализа будут использованы для формулирования гипотез о связях между желетелым планктоном и другими пелагическими популяциями и их воздействием на запасы рыб. Эта статья может быть использована как фоновое исследование для последующих стадий проекта «EUROGEL»: экспериментальных работ, анализа данных и моделирования.
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Publication Stats

253 Citations
18.03 Total Impact Points

Institutions

  • 2009
    • Centre for Environment, Fisheries and Aquaculture Science
      • Lowestoft Laboratory
      Lowestoft, England, United Kingdom