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Annual variation in productivity on the Faroe Shelf during the 20th century


Una Matras



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In order to manage the fisheries resources on the Faroe Plateau in a sustainable way, it is of crucial importance to know how the productivity on the Faroe Plateau ecosystem has varied interannually back in time. This is because the ecosystem seems to support larger fish stocks and higher catches when the productivity is high and vice versa. Reference points, with regard to stock size and fishing mortality, are established for cod, haddock and saithe in Faroese waters, but they refer to the 1961-2008 period, and the use of these reference points in the future depends critically upon the representativeness of the 1961-2008 period. The Faroe Plateau ecosystem may be divided into two components: the Faroe Shelf ecosystem (shallower than around 130 m) and the Deepwater ecosystem (down to around 500 m depth). The productivity of the Deepwater ecosystem seems to be goverened by large-scale oceonographic features, i.e. the dynamics of the Subpolar gyre, and seems to affect the productivity of the saithe stock at the Faroes. The productivity of the Faroe Shelf ecosystem seems to vary negatively with the productivity of the Deepwater ecosystem and affects the stock dynamics of e.g. cod and haddock. While there exists and index of productivity of the Deepwater ecosystem back to 1960, direct information about the productivity of the Faroe Shelf ecosystem extends only back to 1990, although indirect measures (cod growth and recruitment) extend as far back as 1925. Arctica islandica is a long-lived (> 100 years) demersal scallop that may regarded as a recorder of ecosystem productivity back in time. It grows throughout its lifetime and the width of the annual growth rings is positively correlated with feeding intensity that most likely is a reflection of the primary productivity of the ecosystem. The method of sawing the shell and reading the thickness of the growth rings is well established. The 1-year project aims to sample specimens of Arctica islandica from shallow waters (< 130 m) on the Faroe Shelf and to construct a preliminary productivity-chronology back in time. The productivity may not be uniform on the Faroe Shelf, i.e. there might be a southern component (off Suðuroy) and a northern component (off the other islands). Also, the productivity in enclosed bays and fjords might be different from off shore locations. Hence, it will be necessary to sample Arctica islandica from different locations in order to investigate spatial variations in productivity and – based on this information – to select those sites/individuals that are suitable to be used to the chronology of ecosystem productivity. The valitity of the chronology will be tested against phytoplankton productivity for 1990-2008 as well as indirect measures of productivity (cod recruitment 1925-2008 and cod growth 1961-2008). If there is a high correlation between the Arctica islandica chronology and the other measures of productivity, then it will be applied for a PhD project. The results of the PhD project could represent crucial information about reference points for sustainable fisheries on the Faroe Plateau as well as information about global warming and climate change.


The ocean quahog (Arctica islandica) is long-lived (up to 500 years) and may be regarded as a living ‘monitoring station’ of the environmental conditions in the vicinity of the individual. The annual variation in growth of the ocean quahog in Faroese waters is investigated. The annual growth varies. A large annual increment indicates that the environment in the growth period has been favorable. Ocean quahog feeds on phytoplankton, and the growth of ocean quahogs should therefore be a good proxy for the primary production.

This project has proved it feasible to get ocean quahogs more than a hundred years old. This makes it possible to reconstruct the primary production over the last century.

Results from this study indicate a growth difference between the three sampled areas as well as between the individual clams. This may be caused by local effects like fresh water run-off and the difference in exposure (sheltered or exposed). Such local differences may have an effect on the productivity in the area. The results indicate that the number of specimens examined is too low to give reliable statistically results on the growth differences between the sites.

The ocean quahogs sampled in Oyndarfjørður seem to be the most appropriate of the sampled areas when compared to parameters like primary production and cod growth. Oyndarfjørður is situated northerly and is a rather open inlet with no big rivers entering. Therefore Oyndarfjørður might be a good reference of the primary production on the continental shelf.

The Arctica-based growth index reveals that favorable growth years precede year with high on-shelf primary production by a year or two. Comparing the shell growth of ocean quahogs to the growth of cod reveals that the growth of ocean quahog is one year earlier than cod growth, but biomass of cod seems to follow the growth of ocean quahog with a lag of about three years. A high growth is observed in the ocean quahog one year after the passage of saline oceanic anomalies. Thereby, the growth of ocean quahogs may not only tell something about the past, but about the future as well. There is a need for further studies to confirm or reject a possible connection between the oceanic marine climate and the on-shelf, growth of ocean quahogs and cod growth.

Further studies on growth increments of ocean quahog could represent crucial information about reference points for sustainable fisheries on the Faroe Plateau as well as information about global warming and climate change.

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