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 deep sediments, which stimulated nitrogen fixation. Combined with increasing anthropogenic nutrient loads, this led to increased primary production and a rapid change from a 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, avain 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.
Keywords: Avian cholera, Bottom-up, Bycatch, Clupea harengus, Ecosystem, Food web, Gadus morhua, Halichoerus grypus, Regime shift, Sprattus sprattus, Top-down, Trophic cascade, Uria aalge
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