Can cod help to clean up the Baltic Sea?

Picture this scene a year or two from now: we are on a fishing boat, far out in the Baltic Sea. The boat is rolling lazily in the swell and two fishermen are sweating on deck, gutting cod and putting them in ice-filled boxes to keep them fresh for market.

But what’s unusual on this boat is that rather than throwing all the fish offal back overboard, for the squabbling gulls, the fishermen are removing it and putting it in a plastic box.

When the boat is back in port the cod will be sold at the market and the contents of the plastic box will be sent off for testing and then incineration. As well as providing someone with a tasty meal, the cod will have done their bit to help clean up the Baltic Sea, but how?

Cod

PCBS
The Baltic Sea is still one of the most heavily polluted seas in the world. Among the chemical contaminants are long-lasting pollutants such as PCBs (polychlorinated biphenyls) which are poisonous compounds that can impair reproduction.

PCBs were initially manufactured in the 1930s and chiefly used in electronic equipment as they are resistant to high temperatures and do not break down easily. They have entered the Baltic Sea via the atmosphere (73%) and rivers (27%), and although manufacture of PCBs stopped in 1976, they are still in the marine environment and likely to be there for many years to come.

The problem is that because PCBs are easily absorbed by marine organisms, they tend to get magnified up the food chain through the microscopic plants and tiny animals of the plankton into fish and seals and seabirds, which end up getting the biggest doses.

As PCBs tend to “stick” to lipids and fats, doses are usually highest in fat-rich species high in the food chain, such as fish and marine mammals; particularly strong concentrations are found in cod livers and the blubber of seals.

PCBs in fish
Concentrating on fish in the Baltic Sea, we know that PCB levels in individual cod and other fish were very high in the 1970s & early 1980s and that they have fallen since then.

We also know that the fish community in the Baltic was large during that period and the amount caught by fishing boats was also very large—for instance around 800,000 tonnes were landed annually, during the period 1990-1998.

Baltic cod

Because the fish had high levels of PCBs and large numbers of them were removed from the sea, we wondered whether this might have any effect on the total amount of PCBs in the Baltic Sea. In particular, we wondered whether removal of fish might actually have a secondary effect of helping to reduce the amount of PCBs in the Baltic.

To find out we combined annual landings data for the different fish species, with PCB concentrations measured in the tissues of these same species. The results of these calculations showed that the main fish community of the Baltic Sea contained 260 kg of PCBs in the late 1980s–early 1990s.

Figure 1. Click on the figure to enlarge

During the same time period, fishing for herring, sprat, cod, and salmon removed on average at least 31 kg of PCBs per year. The rate at which PCBs were removed depended on both the PCB concentrations in various fish tissues (e. g., muscles, liver), and on the landings of the different fish species. For example, during the early to mid–1970s when PCB concentrations were higher, the herring and sprat fisheries each removed 40–70 kg PCB annually (Fig. 1).

And what was particularly interesting was that our calculations showed that if all cod livers associated with the Baltic cod fishery during the mid-1970s to early 1980s had been retained instead of discarded back to the sea, approximately 80 kg of PCB could have been removed each year (Fig. 2). Cod liver and other organs can have high fat contents and therefore high PCB concentrations.

So fishing does remove PCBs from the Baltic Sea, and in fact the true removal rate is certainly higher than our estimate. This is because we had to exclude from our calculations some fish species (e. g., flatfishes) and fish body parts (e. g., internal organs and skin) for which there are no measurements of PCB concentrations; as a result our calculations underestimate the total removal of PCB due to fishing.

Figure 2. Click on the figure to enlarge

The next step was to see how the removal of PCBs through fishing compared with natural removal processes in the Baltic Sea. To assess this we compared our fishing-related export rates with the inputs and outputs of a previously published model of PCB dynamics in the Baltic Sea (Wania et al., 2001).

To our surprise, this comparison showed that fishing removed as much or more PCBs from the Baltic than some other natural processes known to eliminate PCBs (e. g., break-down in the water column, export to the North Sea by ocean currents).

The comparison also showed that fishing represents 3.5% of the total known PCB removal. And it confirmed that fish and fishing are definitely as important for Baltic PCB dynamics—and probably other similar compounds e.g. dioxins—as other processes and should be included in new models and budgets of Baltic PCB dynamics.

Because fishing also appears to be the only method of PCB removal over which we have some control, one might ask whether environment and fisheries agencies around the Baltic should consider banning the discard of cod liver and other fish organs at sea.

Instead the fish offal could be brought ashore where it could be disposed of or treated to destroy the PCBs. Fishing regulations already require that all cod intended for human consumption must be cleaned (i.e., internal organs removed) before they are landed at fishing wharves so this could, in theory, be introduced.

Obviously making such a waste-management system work in practice will require much more planning and study (e. g., cost-benefit analyses) to decide whether the effort will be worthwhile. But taking the view that “every little helps” the fact is that if the PCBs in fish offal were removed from the Baltic ecosystem, over time PCB concentrations would fall a little bit faster than if left entirely to processes controlled by nature.

As a final point, preventing the cod population from falling to critical levels would have the dual benefit of not only ensuring its viability, but could also contribute to an ongoing reduction in the PCB contamination of the Baltic Sea.

By Brian R. MacKenzie, Sture Hansson , Lisa Almesjö1 and Neil Fletcher

For further information please contact:
Brian R. MacKenzie
Department of Marine Ecology and Aquaculture
Danish Institute for Fisheries Research
Kavalergården 6
DK-2920 Charlottenlund
Denmark
Tel.: +45-3396-3403
E-mail: brm@dfu.min.dk

Or see:
MacKenzie, B. R., Almesjö, L., and Hansson, S. 2004. Fish, fishing and pollutant reduction in the Baltic Sea. Env. Sci. Technol. 38: 1970-1976.
Wania, F., Broman, D., Axelman, J., Näf, C., and Agrell, C. 2001. A multicompartment, multi-basin fugacity model describing the fate of PCBs in the Baltic Sea. In A systems analysis of the Baltic Sea. pp. 417–448. Edited by F.V.Wulff, L.A.Rahm, and P.Larsson. Springer-Verlag, Berlin Heidelberg.

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