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  • 2020-2024  (1)
  • 2015-2019  (6)
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  • 1
    Online Resource
    Online Resource
    Wissenschaftliche Grundlagen für ökosystem-orientiertes Fischereimanagement in den Gewässern vor Grönland (2017)
    Kiel : Inst. für Meereskunde
    Details Availability
    Keywords: Report
    Type of Medium: Online Resource
    Pages: Online-Ressource (134 Seiten, 4 MB) , Diagramme, Karten
    Series Statement: Berichte aus dem Institut für Meereskunde an der Christian-Albrechts-Universität Kiel 234
    URL: https://oceanrep.geomar.de/25474/
    URL: http://dx.doi.org/10.3289/ifm_ber_234
    DOI: 10.3289/ifm_ber_234
    Language: German
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  • 2
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    Food for Thought : Eastern Baltic cod in distress: biological changes and challenges for stock assessment (2015)
    Oxford Univ. Press
    In:  ICES Journal of Marine Science, 72 (8). pp. 2180-2186.
    Details
    Publication Date: 2020-07-29
    Description: The eastern Baltic (EB) cod (Gadus morhua) stock was depleted and overexploited for decades until the mid-2000s, when fishing mortality rapidly declined and biomass started to increase, as shown by stock assessments. These positive developments were partly assigned to effective management measures, and the EB cod was considered one of the most successful stock recoveries in recent times. In contrast to this optimistic view, the analytical stock assessment failed in 2014, leaving the present stock status unclear. Deteriorated quality of some basic input data for stock assessment in combination with changes in environmental and ecological conditions has led to an unusual situation for cod in the Baltic Sea, which poses new challenges for stock assessment and management advice. A number of adverse developments such as low nutritional condition and disappearance of larger individuals indicate that the stock is in distress. In this study, we (i) summarize the knowledge of recent changes in cod biology and ecosystem conditions, (ii) describe the subsequent challenges for stock assessment, and (iii) highlight the key questions where answers are urgently needed to understand the present stock status and provide scientifically solid support for cod management in the Baltic Sea.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1093/icesjms/fsv109
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Reproduction of Baltic cod, Gadus morhua (Actinopterygii: Gadiformes: Gadidae), in the Gotland Basin: Causes of annual variability (2015)
    Societas Scientiarum Stetinensis
    In:  Acta Ichthyologica et Piscatoria, 45 (3). pp. 247-258.
    Details
    Publication Date: 2019-01-21
    Description: Background. The Gotland Basin spawning ground is one of three main spawning areas of Baltic cod, Gadus morhua Linnaeus, 1758. The threshold water parameters for cod development are the salinity exceeding 11‰ and the oxygen level above 2 mL· L–1. Such conditions are only present when the 11‰ isohaline is above the 2 mL· L–1 isooxygen. In such situation the water volume between the isolines is called the “suitable reproduction volume”. When the position of the isolines is reversed, the salinity and the oxygen level of the water layer demarcated by them are below the required thresholds and as such the water is unsuitable for the cod development. We refer to it as the “unsuitable reproduction volume”. The main aim of the presently reported study was to examine whether variations in suitable and unsuitable reproduction habitat estimates could explain the fluctuations in cod recruitment. Materials and Methods. The suitable and unsuitable reproduction volumes in the Gotland Basin were estimated based on single point observations at three oceanographic monitoring stations using the contouring software Balthypsograph. To test the spatial hydrological heterogeneity in the Gotland Basin we used 15 observations during 1969–1995 on four stations in each of two transects. The oceanographic monitoring and demersal trawl research survey data from 1974–2012 have been used in analyses. Results. The sufficient reproduction conditions in the southern Gotland Basin persisted only until 1981. In later decades the reproduction potential of the Gotland Basin has decreased. The latest major North Sea water inflows in 1993 and 2003 in the Central Gotland Basin formed the suitable reproduction volume below the layers where cod eggs are floating. A significant relation between the recruit abundance and suitable and the unsuitable reproduction volumes was found only for the Southern Gotland Basin (P 〈 0.03). Conclusion. We demonstrated that single-point estimates sufficiently quantify the reproduction conditions in the Gotland Basin and thus can be used for estimation of the reproduction volumes. The unsuitable reproduction volume concept can be used as an ecological indicator for egg survival probability in the Gotland Basin. However, it is too premature to re-define the concept of the suitable reproduction volume because it is applicable only to the Gotland spawning ground.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3750/AIP2015.45.3.04
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Eastern Baltic cod recruitment revisited—dynamics and impacting factors (2017)
    ICES
    In:  ICES Journal of Marine Science, 74 (1). pp. 3-19.
    Details
    Publication Date: 2020-02-06
    Description: The Eastern Baltic cod abundance started rapidly to increase in the mid-2000s as evidenced by analytical stock assessments, due to increased recruitment and declining fishing mortality. Since 2014, the analytical stock assessment is not available, leaving the present stock status unclear and casting doubts about the magnitude of the recent increase in recruitment. Earlier studies identified main factors impacting on cod reproductive success to be related to the loss of two out of three spawning areas in the 1980s caused by lack of major Baltic inflows with a concurrent reduction in salinity and oxygen. Other important factors include prey availability for first-feeding larvae, egg predation by sprat and herring and cannibalism on juveniles, all in one way or the other related to the prevailing hydrographic conditions. These factors cannot explain increased reproductive success in the last decade, as the period was characterized by an absence of large-scale Baltic inflows since 2003 and persistent anoxic conditions in the bottom water of the deep Baltic basins. This questions the perception of the increased recruitment in later years and challenges our present understanding of cod recruitment dynamics in the Baltic Sea. In this contribution, we review evidence from the recent literature supplemented by information from latest research cruises to elucidate whether cod reproductive success indeed has increased during the last decade, and we suggest the key processes responsible for the recent dynamics in cod recruitment and outline directions for future research.
    Type: Article , PeerReviewed
    Format: text
    Format: archive
    Format: image
    Format: image
    Format: text
    DOI: 10.1093/icesjms/fsw172
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Quantifying predation on Baltic cod early life stages (2017)
    NRC
    In:  Canadian Journal of Fisheries and Aquatic Sciences, 74 (6). pp. 833-842.
    Details
    Publication Date: 2020-02-06
    Description: Predation on cod eggs by sprat and herring is known to be one of the processes influencing reproductive success of the Eastern Baltic cod, and has been reported to have contributed to lack of recovery of the stock in the 1990s. This study quantifies the predation on cod eggs in the Bornholm Basin, the major spawning area of cod in the central Baltic Sea, in the 1990’s in comparison to the second half of the 2000s. The analyses involve estimating daily consumption rates of predator populations, which are then compared with corresponding daily egg production rates. As a methodological advancement compared to earlier studies, spatially resolved information on predator distribution and abundance is utilized in quantifying predator stock size. This resulted in more realistic consumption estimates in relation to overall egg production, compared to earlier studies that consistently overestimated predation pressure by clupeids. Our results suggest a generally lower predation pressure on cod eggs in the mid- to late-2000s, due to a combination of reduced predator abundance and lower daily rations by individual predators.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1139/cjfas-2016-0215
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Egg production methods applied to Eastern Baltic cod provide indices of spawning stock dynamics (2020)
    Elsevier
    Details
    Publication Date: 2023-02-08
    Description: Highlights: • Egg production methods have been used for assessing the Eastern Baltic cod stock. • The analysis contributed to the re-establishment of an analytical assessment for the stock. • Annual and daily egg production methods gave similar results. • Results confirmed trends from trawl surveys, questioned earlier because of potential change in catchability. • Several biological processes impact on absolute spawning stock estimates and research needs are identified to improve these. Abstract: Egg production methods (EPM) provide fishery independent estimates of spawning stock sizes and dynamics of fish populations. Such methods are commonly used for short-lived pelagic species, less so for demersal fish such as cod. In this paper, we apply EPMs on cod in the eastern Baltic Sea, using a long time series of ichthyoplankton data. Stock assessment of Eastern Baltic cod has been challenged due to changing productivity of the stock invalidating some of the standard procedures, e.g. age determination and input variables, e.g. natural mortality. We demonstrate that EPMs, based on other data and assumptions than standard stock assessments, provide useful information on stock status and dynamics. We apply both the annual and daily egg production methods, which yielded similar results and were in line with stock trends derived from bottom trawl surveys. However, the EPM based spawning stock estimates were consistently lower compared to results from the latest analytical stock assessment. We identified processes introducing uncertainties in EPM estimates and their effects on the resulting estimates, and conclude that they mainly affect the absolute estimates but less the relative trends in stock dynamics. Therefore, we consider that EPMs are useful for providing relative indices for stock assessment purposes, with the Eastern Baltic cod being the first case where such indices are included in an official stock assessment of a demersal gadoid species. We also identify knowledge gaps in order to be able to derive absolute stock size estimates from EPMs in the future.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Designing spawning closures can be complicated: Experience from cod in the Baltic Sea (2019)
    Elsevier
    In:  Ocean & Coastal Management, 169 . pp. 129-136.
    Details
    Publication Date: 2022-01-31
    Description: Highlights: • Designing spawning closures requires consideration of the mechanisms through which the closures can affect the fish stocks. • Small area closures may have unintended negative effects to the stocks due to fishing effort reallocation. • Closures covering most of the stock distribution are more robust to gaps in biological knowledge than small area closures. Abstract: Fisheries management measures often include spatio-temporal closures during the spawning period of the fish with an overarching aim of improving the stock status. The different mechanisms how a spawning closure potentially can influence the stock are often not explicitly considered when designing such closures. In this paper, we review and synthesize the available data and knowledge on potential effects of the implemented spawning closures on cod in the Baltic Sea. The Baltic cod example represents a relatively data rich case, which allows demonstrating how a closure might affect different parameters of stock status via different mechanisms, including potential unintended negative effects. We conclude that designing relatively small area closures appropriately is highly complex and data demanding, and may involve tradeoffs between positive and negative impacts on the stock. Seasonal closures covering most of the stock distribution during the spawning time are more robust to data limitations, and less likely to be counterproductive if suboptimally designed.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1016/j.ocecoaman.2018.12.018
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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