| 1. |
- Almqvist, Gustaf, et al.
(författare)
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Report of the Benchmark Workshop on Baltic Cod Stocks (WKBALTCOD)
- 2015
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The ICES Benchmark Workshop on Baltic Cod Stocks (WKBALTCOD), chaired by External Chair Jean-Jacques Maguire, Canada and ICES Chair Marie Storr-Paulsen, Denmark, and attended by two invited external experts Verena Trenkel, France and Meaghan Bryan, USA met in Rostock, Germany, 2–6 March 2015 with 39 participants and six countries represented. The objective of WKBALTCOD was to evaluate the appropriateness of data and methods to determine stock status and investigate meth-ods appropriate to use in the single-stock assessment for the cod stock in SD 22–24 and cod in SD 25–32 in the Baltic. Participants in the workshop were a large group with diverse backgrounds representing the industry, fisheries, NGOs, managers and scientists.The single-stock analytic assessment of the eastern Baltic stock was not accepted by the assessment working group (WGBFAS) in 2014 due to severe problems with the input data. The advice for the eastern Baltic cod was, therefore, based on the ICES approach for data-limited stocks. As an outcome ICES decided to establish a bench-mark for both cod stocks and to scope an integrated assessment for the Baltic cod stocks. The first meeting (WKSIBCA) was therefore meant to introduce the interces-sional work conducted since the assessment working group in April 2014, and to reach some conclusions on how to proceed both in the short term (Benchmark in March 2015) and longer term (2–3 years) and was seen as a data compilation work-shop, there is produced a separate report from this workshop. The WKBALTCOD was the 2nd meeting in the benchmark process and was intended to come up with a final stock assessment method, stock annex and input data for both stocks. As it was not possible to reach conclusive decision on the final model to be used for the east Baltic cod stock during the benchmark meeting and as more work on the preferable models was needed, it was decided by the ACOM leadership to prolong the bench-mark process until the assessment working group meeting in April 2015. This deci-sion has led to a relatively long process partly mixed with the assessment working group WGBFAS.It became clear during the benchmark process that although large effort has been put into explaining the underlying processes leading to the changes in the Baltic ecosys-tem, there is still some lack of understanding of the present situation in the eastern Baltic cod stock. Therefore, it was not possible to reach firm conclusions on the final model to be used and therefore not possible to set reference points. It was decided to continue to explore the most promising models and to continue to improve the input data until the assessment working group started in April.The main challenges still to be solved for the Eastern Baltic cod stock is the quantifi-cation of increased natural mortality and decrease in growth. Through several presentations during the workshop (both WKSIBCA and WKBALTCOD) it became clear that natural mortality very likely has increased in later years, due to decreased condition and increased parasite infection. A decrease in growth also seems plausible duo to a decrease in condition and/or selectivity-induced mortality of the largest in-dividuals. However, as none of these parameters are easily estimated, especially with the severe ageing problems, different model assumptions made the output very shaky.For the western Baltic cod, stock identification issues were examined in area SD 24, the intermediate area: based on otolith characteristics and genetics. Due to the results showing a large proportion of east cod in this area, it was decided to split the catch2 | ICES WKBALTCOD REPORT 2015and survey from SD 24 into either the western or eastern Baltic cod stock. It was pos-sible to derive proportions of eastern and western cod in SD 24 back to the mid-1990s.For the western Baltic cod stock a modelled survey indices was included in the as-sessment covering the western part of SD 24 and Area 22+23 and based on a smoothed ALK.Both cod stocks have in the past used commercial tuning fleet to have a better cov-ered of older age groups. It was decided to abound this time-series duo quality issues such as a limited coverage and problems with technical creeping.WKBALTCOD was not able to explore and define reference points for the Western Baltic cod stock during the meeting due to time constraints, but these were calculated and decided by correspondence after the meeting. The recent protocols on estimation procedures developed by WKMSYREF3 for stocks with a full analytical assessment and for data-limited stocks served as objective guidelines to obtain reference point estimates.
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| 2. |
- Nissling, Anders, et al.
(författare)
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Vertical distribution and egg survival of Baltic sprat in relation to temperature and oxygen conditions - implications for stock development
- 2002
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Sprat, Sprattus sprattus, is a key species in the Baltic Sea. Besides being a target species in the Baltic mixed clupeoid fishery it is a major zooplanktivore and dominates, together with herring, one of four trophic levels in the Baltic Sea food web. Occasionally strong year classes of Baltic sprat are followed by a number of year classes below average. During the 1990s however a series of year classes above average were produced and the stock reached historically high levels with implications for the Baltic food web. Spawning stock biomass and recruitment are poorly related suggesting that environmental conditions have a great impact on the year-class strength. In the main spawning areas, i.e. the Baltic deep basins, egg development occurs at highly varying environmental conditions. In the present investigation egg specific gravity and variation in vertical egg distribution as well as egg survival in relation to temperature and oxygen conditions were studied. Incubation of eggs revealed significantly lower viable hatch at <5 degree C. Egg occurrence in relation to oxygen concentration suggested lower survival at <2 ml/l. Egg specific gravity averaged 1.00858 plus or minus 0.00116 g/cm3 during peak spawning but was significantly higher early in the season and significantly lower towards the end of the spawning period. In general, eggs kept buoyant in the deep layers at low oxygen conditions early in the season, developed at more favourable oxygen concentrations but at poor temperature conditions during peak spawning, and were distributed at both more favourable temperature and oxygen conditions towards the end of the spawning season. Using conditions at in average occurrence of eggs during peak spawning for the period 1970-2000 the relative importance of temperature and oxygen conditions was evaluated. Results suggested that variation in temperature is the most important factor affecting egg survival in the Bornholm basin (SD 25), that mainly oxygen conditions determine the survival rate in the Gotland basin (SD 28) whereas variation in both temperature and oxygen conditions influence survival in the Gdansk Deep (SD 26). In conclusion, opportunities for egg survival are influenced by both temperature conditions following mild or severe winters as well as by frequency and magnitude of saline water inflows into the Baltic Sea.
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