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Fish, seabirds and trophic cascades in the Baltic Sea
Stockholm University, Faculty of Science, Department of Systems Ecology. Stockholm University, Stockholm Resilience Centre, Baltic Nest Institute.
2006 (English)In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 323, 233-238 p.Article in journal (Refereed) Published
Abstract [en]




In the relatively simple Baltic Sea ecosystem, zooplankton-feeding sprat



Sprattus sprattus

is a major food source for breeding seabirds and piscivorous fish, and an important resource for

commercial fisheries. Large-scale and long-term ecosystem changes resulting mainly from over fishing

and recruitment failure of cod



Gadus morhua

, which is the main fish predator of sprat, have

affected natural-history patterns in a piscivorous seabird, the common guillemot



Uria aalge

, in a complex

way. As the sprat stock increased, leading to lower energy content of fish, common guillemot

chick body mass at fledging decreased. However, chick fledging body mass recovered in recent years

as the sprat stock diminished, which brought about corresponding increases in sprat weight-at-age

and energy content. The cod and sprat fishery affect the common guillemots in the Baltic Sea, but the

effects differ depending on the management strategy.

Place, publisher, year, edition, pages
2006. Vol. 323, 233-238 p.
Keyword [en]
Cod · Baltic Sea · Fisheries management · Gadus morhua · Common guillemot · Sprat ·
National Category
URN: urn:nbn:se:su:diva-25873OAI: diva2:200716
Part of urn:nbn:se:su:diva-953Available from: 2006-04-05 Created: 2006-04-05 Last updated: 2010-11-03Bibliographically approved
In thesis
1. Complexity and Change in a Simple Food Web: Studies in the Baltic Sea (FAO Area 27.IIId)
Open this publication in new window or tab >>Complexity and Change in a Simple Food Web: Studies in the Baltic Sea (FAO Area 27.IIId)
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

An influence at one trophic level can result in dynamic impacts also on other components of a food web. These dynamics are known as trophic cascades, and can be both top-down and bottom-up. After a near-collapse of the Baltic cod Gadus morhua stock in the 1980s, its main prey sprat Sprattus sprattus increased dramatically. The main food of sprat, marine copepods, decreased during the same time period, likely a combined effect of increased predation pressure from sprat and decreasing salinities. This shortage of food for sprat resulted in decreasing quality of sprat as a food source for common guillemots Uria aalge. However, a recent increase in fishing for sprat has again resulted in better feeding conditions for guillemots.

Human impacts on this simple food web can be complex. In the early 20th century, marine mammals were abundant and nutrient levels were low in the Baltic Sea. This thesis illustrate that this situation corresponded to lower fish biomass. A reduction of seals early in the century led to reduced top-down control, which resulted in increasing fish stocks. Later, in the 1950s, the largest inflow of salt water during the century mobilized accumulated phosphorus from the deep sediments, which stimulated nitrogen fixation. Combined with increasing anthropogenic nutrient loads, this led to increased primary production and a rapid change from an oligotrophic to a eutrophicated state. This change can be termed a regime shift, which also stimulated fish production. Subsequent over-fishing of cod likely caused a second regime shift, from a cod- to a clupeid- dominated state, which led to the described effects on the common guillemots.

Several factors affect the life-history of this long-lived seabird. Bycatches in gillnet fisheries is one factor directly affecting guillemot survival, and the proportion of bycatches increased during a period of increasing fishing effort. Surprisingly, avian cholera, a previously undocumented disease in common guillemots, was found at times to cause considerable adult mortality. Common guillemot life-history information can communicate the diversity of factors influencing marine ecosystems – hopefully this can increase our understanding of how complex even "simple" food webs are.

Place, publisher, year, edition, pages
Stockholm: Institutionen för systemekologi, 2006. 36 p.
Avian cholera, Bottom-up, Bycatch, Clupea harengus, Ecosystem, Food web, Gadus morhua, Halichoerus grypus, Regime shift, Sprattus sprattus, Top-down, Trophic cascade, Uria aalge
National Category
urn:nbn:se:su:diva-953 (URN)91-7155-225-1 (ISBN)
Public defence
2006-05-19, sal G, Arrheniuslaboratorierna, Svante Arrhenius väg 14-18, Stockholm, 10:00
Available from: 2006-04-05 Created: 2006-04-05Bibliographically approved

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Österblom, Henrik
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