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  • 1
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    Danish Institute for Fisheries Research
    In:  Dana, 10 . pp. 179-201.
    Publication Date: 2018-08-17
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2019-01-07
    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
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  • 3
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    Oxford Univ. Press
    In:  ICES Journal of Marine Science, 57 . pp. 300-309.
    Publication Date: 2017-03-07
    Description: Cod is the top piscivore predator in the Baltic Sea ecosystem. Based on stomach content data from 62 427 cod collected during 1977–1994 and food consumption rates, cannibalism in the Eastern and Western Baltic cod stocks has been quantified using multispecies virtual population analysis. In the Eastern Baltic stock, depending on model assumptions, an average of 25–38% of the 0-group and 11–17% of the 1-group were removed by predation by adults. Thus, between age 0 and age 2 a year class may lose on average about 31% and 44% of the initial number as a result of cannibalism. Cannibalism is lower in the Western Baltic. On average, 19% of the 0-group and 9% of the 1-group are consumed per year, i.e. 24% of the initial cohort is eaten before reaching age 2. Predation was most intense in 1978–1984, a period with high juvenile abundance and large adult stock sizes in both areas. Subsequently, stock, recruitment, and cannibalism declined steadily until the early 1990s and then increased again. Problems identified in relation to data compilation and estimation procedure are discussed with respect to their impact on estimates of cannibalism and stock– recruitment relationships
    Type: Article , PeerReviewed
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  • 4
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    GLOBEC International Project Office
    In:  GLOBEC International Newsletter, 10 (1). pp. 15-17.
    Publication Date: 2018-10-11
    Type: Article , NonPeerReviewed
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  • 5
    Publication Date: 2018-03-20
    Description: Fish stock development in the Central Baltic Sea (1976-2000) in relation to variability in the environment - DTU Orbit (15/04/14) Fish stock development in the Central Baltic Sea (1976-2000) in relation to variability in the environment - DTU Orbit (15/04/14) Köster F, Möllmann C, Neuenfeldt S, Vinther M, St. John M, Tomkiewicz J et al. Fish stock development in the Central Baltic Sea (1976-2000) in relation to variability in the environment. I C E S Marine Science Symposia. 2003;219:294-306
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2017-01-09
    Description: At present, several cod stocks are outside safe biological limits and are managed under recovery plans. For these stocks Total Allowable Catches (TAC's) are generally low and quotas are accompanied by a broad variety of technical measures influencing the fishing patterns. Consequently, the input data to stock assessment models relying on catch statistics from the commercial fisheries is potentially biased and the perception of stock status may be incorrect. Egg production methods (EPM) provide a fishery independent alternative. Additionally, they provide better estimates of stock reproductive potential (SRP). Eastern Baltic cod (Gadus morhua callarias L.) has severely declined throughout the 2nd half of the 1980s and 1st half of the 1990s due to climate-driven adverse hydrographic conditions and high fishing intensity. Since 2007 the stock is managed under a long-term management plan and showed signs of recovery in most recent years. Since 1986, egg surveys have been carried out regularly in the Bornholm Basin, the most important spawning area of Eastern Baltic cod since mid-1980s. In the present paper the robustness of EPM towards simplification of spawning parameters and towards reduction of the number of egg surveys is tested applying three different methods requiring different numbers of egg surveys. We applied the annual egg production method (AEPM) requiring full egg survey coverage of the spawning season to estimate cod abundances in the Bornholm Basin. In addition, the daily fecundity reduction method (DFRM) and the daily egg production method (DEPM) were tested, the latter two methods requiring only single egg surveys, but require more complex reproduction input parameters. All three methods provided a comparable result, which was also expected as many spawning parameters were derived from the same underlying data sets. In a sensitivity analysis several input parameters were varied simultaneously up to 20% in both directions. EPM were especially sensitive towards changes in proportions of mature females at age, whereas changes in the various fecundity parameters and spawning fraction were less influential. EPM results followed the large scale spawning stock trends of the Baltic International Trawl Survey index, whereas the year to year variations of the index were not captured to well. EPM yielded spawning stock sizes in the same order of magnitude as provided by a spatially down-scaled multi-species stock assessment model.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2019-01-21
    Description: The eastern Baltic cod stock has recently started to recover, after two decades of severe depletion, however with unexpected side effects. The stock has not re-occupied its former wide distribution range, but remains concentrated in a limited area in the southern Baltic Sea. The biomass of forage fish, i.e., sprat and herring, is historic low in this area, which in combination with increasing cod stock results in locally high predation mortality of forage fish and cannibalism of cod. In line with low prey availability, body weight and nutritional condition of cod drastically declined. In the southern Baltic Sea, cod competes with pelagic fisheries for the limited resources of sprat and herring, while the largest biomass of these species is currently found outside the distribution range of cod. Accounting for spatial overlap between species is crucial in developing ecosystem based fisheries management to enhance the recovery of predator stocks.
    Type: Article , PeerReviewed
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  • 8
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    In:  [Other] In: Pan-Baltic workshop co-organised by NORDEN and BIO C3, 04.-05.09.2014, Charlottenlund Castle, Denmark .
    Publication Date: 2015-02-12
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 9
    Publication Date: 2019-02-01
    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
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  • 10
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    NRC
    In:  Canadian Journal of Fisheries and Aquatic Sciences, 74 (6). pp. 833-842.
    Publication Date: 2019-02-01
    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.
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