Marine Research Findings of the VECTORS Project

This website provides access to the research results of the VECTORS project, which can be used to support marine management decisions, policies and governance as well as future research and investment. VECTORS was a large scale project that brought together more than 200 expert researchers from 16 different countries. It examined the significant changes taking place in European seas, their causes, and the impacts they will have on society.

Reconstructing the population dynamics of sprat in the Baltic Sea in the 20th century

Sprat currently constitutes the largest biomass and is one of the most important fish species in the food web of the open Baltic Sea. Present knowledge of sprat dynamics is largely based on a pronounced increase in biomass in the 1980s to a record high stock size in the mid-1990s, due to reduced cod predation and favourable temperature conditions for sprat reproduction. It is largely unknown how the population would develop under different combinations of climate, predator abundance, and human pressures. The first objective of this study was to gather the information scattered in various national and international publications and reports, in different languages, that could be used to quantify sprat stock dynamics prior to 1974. In a second step, these data were used to produce quantitative estimates of stock size from the 1970s (when the ICES stock assessment starts) back to the early decades of the 20th century.

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We identified peaks in sprat biomass in the beginning of the 1930s, 1960s, and 1970s. The exploitation rate of sprat was low until the development of pelagic fisheries in the 1960s. These results facilitate new analyses of the effects of climate, predation, and anthropogenic drivers on sprat.

Long time-series of population dynamics are increasingly needed in order to understand human impacts on marine ecosystems and support their sustainable management. The extended analytical assessment of sprat in the Baltic Sea identified peaks in biomass in the beginning of the 1960s and 1970s at 900 kt, which is similar to the spawner biomass estimated for most of the 2000s. In the mid-1960s, the spawner biomass was 50% lower, at 400 kt. The proxies for spawner biomass derived from egg abundance estimates confirmed the relatively higher sprat stock in the early 1970s compared with the mid-1960s. For the years 1945–1955, as well as for the late 1930s, egg abundance data suggest a relatively low spawner biomass, at 300–500 kt. The high average egg abundance in 1931 corresponds to a spawner biomass at 850 kt, i.e. similar to the analytical estimates for the early 1960s and 1970s. In the years 1956–1974, the strongest year classes were formed in 1955, 1957, 1959, and 1967. The estimates of sprat biomass relative to landings suggest a low overall exploitation rate until the 1960s, when it gradually started to increase, corresponding to an increase in total landings. The main contribution of this study is integrating the previously fragmentary and qualitative information on historical stock developments of sprat into quantitative estimates covering the entire Baltic Sea.

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Spatially resolved analyses of long term dynamics of sprat show that in the 1960s, the largest biomass of sprat was found in the northern Baltic Sea, which is similar to the situation in recent years. These results facilitate new analyses towards understanding the mechanisms behind changes in sprat distribution.

In the 1960s, the largest biomass of sprat was found in the northern Baltic Sea, after which the biomass in this area drastically declined to a record low level in the 1980s. In the period from the 1970s to the 1980s, biomass also declined in eastern and southwestern areas, but less dramatically due to a previously relatively lower biomass in these areas. Sprat biomass in southwestern Baltic Sea started to recover in the second half of the 1980s, whereas in the northern Baltic Sea, the biomass did not increase until the 1990s. In the mid-1990s, the biomass reached a peak in all sub-regions, resulting in a record high overall stock level in the Baltic Sea. The biomass in southwestern areas in the mid-1990s was particularly outstanding, being several fold higher than in any other period from the 1960s to the present. From the second half of the 1990s to the 2000s, the biomass in this area rapidly declined. In contrast, the biomass in eastern and northern areas has been relatively stable, exhibiting only a minor decline since the mid-1990s to the present. New quantitative evidence of sprat dynamics and spatial distribution under various combinations of natural and human drivers can contribute to developing an ecosystem-based approach and setting long-term management strategies for the Baltic Sea, taking into account changes in spatial distribution of the resources.

Lead Author:

Margit Eero
(mee@aquanospam.dtu.dk)
Danmarks Tekniske Universitet (DTU-Aqua)
Date of research: March 2012

Related articles:

Fish distributions and spatial management measures 

Functional responses of herring and sprat to prey 

Vital rates of fish larvae 

Changes in herring larvae and environment 1957-2010

Early life stage survival of Baltic cod

Impact of jellyfish on fisheries and tourism 

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This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 266445
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