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Effects of population size and environmental factors on habitat choice and migration patterns of the Eastern Baltic Sea cod (Gadus morhua callarias): consequences for stock assessment
Södertörn University, School of Life Sciences.
2012 (English)Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
Abstract [en]

The subject matter of this Bachelor´s thesis is the Eastern Baltic Sea cod. A literature review has been done regarding how environmental factors such as salinity, oxygen supply and temperature but also the size of the cod populations affects the migration patterns and the habitat choice of the cod. The result of the review shows that, when the eastern Baltic cod population is small, its area of distribution is restricted to the southern parts of the Baltic Sea. Moreover, high salinity leads to a larger area of distribution, while low oxygen content in the Deep Basins leads to vertical migrations, as well as further migrations south and north, to areas with better hydrological conditions. Finally the the cod tend to prefer habitats with warmer water, if the oxygen and salinity conditions are suitable, at least during the spawning period.

This thesis also include estimation on how well the annual scientific trawl surveys performed by coastal Baltic countries, reflects the actual cod population size. This estimation is partly based on a critical review of available information about the trawl survey design in relation to cod migration patterns, distribution and habitat choice. Furthermore, a test has been done of how well the annual stock size estimates for the latest year, presented in the International Council for the Exploration of the Sea (ICES) Annual Reports of the Baltic Fisheries Assessment Working Group (WGBFAS), between the years 1996-2009 reflected the best available stock size estimates from 2010. The results indicate that the scientific trawl surveys might not work satisfactory. This indicates that further research must be done in the field, therefore possible alternatives such as comparisons of positioning data of commercial and scientific trawls and interviews with fishermen are suggested.

Abstract [sv]

Denna uppsats handlar om torsken i det östra Östersjöbeståndet. En litteratursammanfattning har gjorts gällande hur miljömässiga faktorer såsom salthalt, syretillgång, temperatur samt storleken på bestånden påverkar migrationsmönster och habitatval hos torsken. Resultaten visar bland annat att när torskbestånden är små blir torskens utbredningsområden begränsade till de södra delarna av Östersjön. Höga salthalter leder till större utbredningsområden medan låga syrehalter i djupbassängerna leder till vertikala migrationer såväl som längre migrationer söder och norr om bassängerna, till områden med bättre syretillgång. Torsken verkar dessutom föredra habitat med varmare vatten om syre- och salthaltsförhållandena är gynnsamma, åtminstone under lekperioden.

Vidare inkluderar denna uppsats en bedömning av hur väl de årliga vetenskapliga trålningsundersökningarna som utförs av länder med kust mot Östersjön, återspeglar den faktiska storleken på torskbestånden. Denna bedömning baseras på en kritisk granskning av den information som finns tillgänglig gällande de vetenskapliga trålningsundersökningarna, i förhållande till torskens migrationsmönster, distribution och habitatval. Dessutom har ett test gjorts av hur väl de årliga beståndsuppskattningarna för 1996-2009 presenterade i International Council for the Exploration of the Sea (ICES) Annual Report of the Baltic Sea Fisheries Assessment Working Group (WGBFAS), reflekterar de bästa tillgängliga beståndsuppskattningarna från 2010. Resultaten indikerar att de vetenskapliga provtrålningsundersökningarna inte fungerar tillfredsställande, vilket visar på att vidare studier måste göras inom området. Möjliga alternativ, såsom jämförelser av positioneringsdata för kommersiell- samt vetenskaplig trålning, samt intervjuer med yrkesfiskare presenteras.

Place, publisher, year, edition, pages
2012. , 31 p.
Keyword [en]
Feeding Migration, Spawning Migration, Salinity, Oxygen supply, Temperature, Scientific trawl surveys, ICES assessment reports
Keyword [sv]
Födomigration, Lekmigration, Salthalt, Syretillgång, Temperatur, Vetenskapliga trålningsundersökningar, ICES uppskattningsrapporter
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:sh:diva-17228OAI: oai:DiVA.org:sh-17228DiVA: diva2:561923
Subject / course
Environment and development
Uppsok
Life Earth Science
Supervisors
Available from: 2012-10-22 Created: 2012-10-22 Last updated: 2012-10-22Bibliographically approved

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