35 
respectively, slightly higher values are 
observed. At the end of this assessment 
period, means of measured values between 
1 and 10 Bq kg 1 wet weight are observed. 
For samples of the edible parts of herring 
(head without entrails), which were analysed 
mainly from basins with box numbers from 
83 upwards, slightly lower 137 Cs activity 
concentrations were found compared to the 
values for combined round fish muscle shown 
in Figure 2. 
Figure 3 shows the 137 Cs time series for 
the flat fish group, consisting of flounder 
(Platichthys flesus), plaice ( Pleuronectes 
platessa) and dab ( Limanda limanda), in 
the western and southern Baltic Sea areas. 
Samples of fillets/flesh were used for these 
measurements. At the end of the assessment 
period, the values were below about 8 Bq kg 1 
wet weight. 
In the coastal areas of Baltic Sea basins 
around Finland (boxes 83, 85, 87 and 89), 
freshwater fish, i.e. pike (Esox lucius), have 
also been used for monitoring measurements 
since 1985. Due to larger freshwater contents 
in this seawater, their concentration factors 
are larger, resulting in slightly larger 137 Cs 
values. Figure 4 shows the corresponding 
comparison of measured and modeled 
137 Cs in fillets of pike; the trends are clearly 
decreasing. By the end of the assessment 
period the annual means approached values 
between about 10 and 25 Bq kg 1 wet weight. 
The corresponding measurement/model 
comparison for 90 Sr in pike muscle tissue 
is illustrated in Figure 5. The levels of 
activity concentrations in pike, which in the 
assessment period are about 200-fold smaller 
than those of 137 Cs, are slowly decreasing. 
By the end of the assessment period, levels 
of about 0.02-0.03 Bq kg 1 wet weight were 
measured. As 90 Sr is a strongly bone-seeking 
radionuclide, its concentrations in fillets are 
rather small. The presence of even small 
amounts of bone in the fillet samples will result 
in increased observed concentration values. 
This explains why fluctuations are observed 
in some cases. As suggested by the trends 
of the model curves in Figure 5, the impact 
of Chernobyl fallout was much less than for 
137 Cs. 
Bladder wrack, Fucus vesiculosus, 
accumulates radionuclides (IAEA, 2004) to 
such an extent that it is a good measurable 
indicator, especially for gamma-emitting 
radionuclides. Fucus can be collected over 
the whole Baltic Sea area. 137 Cs in Fucus has 
been monitored since 1984 in practically all 
considered Baltic basins, from the Kattegat 
(71) to the Gulf of Finland (89); with the 
exception of the Bothnian Bay, where no 
samples were taken. 
The corresponding measurement/model 
comparison for 137 Cs in Fucus vesiculosus 
is shown in Figure 6 for seven Baltic Sea 
basins. The trends within the period of this 
assessment in the various basins show a slow 
decrease. By the end of this period, activity 
concentrations in Fucus approached values 
(in Bq kg -1 dry weight) of 5 (Kattegat), 7 (Belt 
Sea) and 20 in the eastern and northern 
basins (Baltic Sea east and west, Bothnian 
Bay, Gulfs of Riga and Finland). 
The modeled curve for the Gulf of Finland 
(box 89) shows in 1986-2006 some deviation 
from measured values during this time, initially 
(until 1990) underestimating the measured 
values, while after about 1998 producing 
overestimates. This discrepancy will have to 
be considered for future modeling work. 
3C.4.5 Conclusions 
Within the current assessment period, 
Chernobyl-derived 137 Cs continued to be 
the most dominant man-made radionuclide 
in Baltic Sea fish regarding activity 
concentrations. By the end of the period, 
mean values of 1 -10 Bq kg 1 wet weight were 
found in (marine) round fish (cod, herring, 
whiting) in various Baltic Sea basins, while 
freshwater pike sampled in Finnish coastal 
waters, due to its higher concentration factors, 
showed values of 10-25 Bq kg 1 wet weight. 
In (marine) flat fish (plaice, flounder, dab), 
slightly lower mean values were found than in 
(marine) round fish. 
Baltic Sea basin specific concentration 
factors of 137 Cs and 90 Sr have been derived 
for the three above-mentioned groups offish 
and for bladder wrack, Fucus vesiculosus, 
with the latter covering practically the whole 
of the Baltic Sea. The findings were based 
on seawater and biota data compiled by 
the MORS-PRO Group over the years 
1988/1990-2006. 
Activity ratios 238 Pu/ 239 ' 240 PU anc | 241^ m /239,240p u 
were evaluated in samples of sprat, blue 
mussel and Fucus vesiculosus, collected in 
the Belt Sea and the Southern Baltic Proper. 
Baltic Sea Environment Proceedings No. 117