New insights into the recent collapse of Eastern Baltic cod from historical data on stock health.

The Eastern Baltic cod (Gadus morhua) stock is currently in a very poor state, with low biomass and adverse trends in several life history and demographic parameters. This raises concern over whether and to what level recovery is possible. Here, we look for new insights from a historical perspective, extending the time series of various stock health indicators back to the 1940s, i.e. to the beginning of intensive exploitation of the Eastern Baltic cod. The historical data confirm that the stock deterioration in recent years is unprecedented, as all indicators are presently in their worst states on record. Cod body condition and energy reserves were equally low in the 1940s-1950s, accompanied by high parasitic liver worm infection, comparable to that measured in recent years. However, other stock parameters (size structure, size at maturity, stock distribution) are currently in their worst states over the past 80 years. In contrast, the state of cod in the 1970s to early 1990s that is often perceived as a desirable target, was exceptional, with the most favorable indicator levels in the time series. Long-term observation data reveal concurrent or asynchronous trends in different indicators of stock health and to what extent these have coincided with changes in possible external drivers. In this way, the extended time series contribute to ongoing research on understanding the collapse of the cod and its recovery potential.

Seeking the true time: Exploring otolith chemistry as an age-determination tool.

Fish otoliths' chronometric properties make them useful for age and growth rate estimation in fisheries management. For the Eastern Baltic Sea cod stock (Gadus morhua), unclear seasonal growth zones in otoliths have resulted in unreliable age and growth information. Here, a new age estimation method based on seasonal patterns in trace elemental otolith incorporation was tested for the first time and compared with the traditional method of visually counting growth zones, using otoliths from the Baltic and North seas. Various trace elemental ratios, linked to fish metabolic activity (higher in summer) or external environment (migration to colder, deeper habitats with higher salinity in winter), were tested for age estimation based on assessing their seasonal variations in concentration. Mg:Ca and P:Ca, both proxies for growth and metabolic activity, showed greatest seasonality and therefore have the best potential to be used as chemical clocks. Otolith image readability was significantly lower in the Baltic than in the North Sea. The chemical (novel) method had an overall greater precision and percentage agreement among readers (11.2%, 74.0%) than the visual (traditional) method (23.1%, 51.0%). Visual readers generally selected more highly contrasting zones as annuli whereas the chemical readers identified brighter regions within the first two annuli and darker zones thereafter. Visual estimates produced significantly higher, more variable ages than did the chemical ones. Based on the analyses in our study, we suggest that otolith microchemistry is a promising alternative ageing method for fish populations difficult to age, such as the Eastern Baltic cod.

Effects of freezing on length and mass measurements of Atlantic cod Gadus morhua in the Baltic Sea.

An aggregated sample of 925 Atlantic cod Gadus morhua collected by four countries in different regions of the Baltic Sea during different seasons were measured (total length, LT = 161-890 mm and weighed (mass, M = 45-6900 g) both before freezing and after defrosting. The cod were found to decrease significantly in both LT and M following death and frozen storage. There was an average (±SD) change in LT of -2.91% (±0.05%) following freezing, independent of starting LT . Total M changed by -2.65% (±0.14%), independent of starting mass. Shrinkage of LT and M did not differ significantly between 1 and 4 months frozen storage, though LT shrinkage was significantly greater after 1 or 4 months in the freezer compared with after 5 days. There was significant variation in LT and M shrinkage between regions of capture. A significant negative relationship between condition of cod and LT or M change was also observed. Equations to back-calculate fresh LT and M from thawed LT , M and standard length (LS ), gutted LT , gutted LS and gutted M are provided.

Marine litter on the seafloor of the southern Baltic.

Marine litter occurrence and composition were investigated during routine bottom trawl fish surveys type BITS performed in the Polish Maritime Areas (the southern Baltic Sea). Sampling covered a distance of 325km and an area of 16km2 at a depth range of 19-110m. Litter densities varying between 0 items/ha (34% of tows) and 2.23items/ha with a mean of 0.20items/ha (SD=0.30) are at the bottom range of densities reported from other shelf habitats worldwide at similar water depths. The majority of the items (40%) were found at a depth range of 51-60m. Overall, plastic was the most common litter type (67% of all items) found in all tows with litter. The results of this study indicate that despite the Baltic being a semi-enclosed basin, with a densely populated coastline and extensive shipping, marine litter pollution of the southern Baltic seafloor is low compared to other coastal areas.

Inter-tissue distribution and evaluation of potential toxicity of PCBs in Baltic cod (Gadus morhua L.).

PCBs measured in the muscle, liver, and gonads of cod collected from two locations in the southern Baltic were evaluated as total (summation operator PCBs), indicator (summation operator PCB(7)), and dioxin-like (summation operator DL-PCBs) concentrations. Gender, tissue lipid content, and collection site were important determinants of PCB levels. Irrespective of collection site, lipid-normalized summation operator PCBs, summation operator PCB(7), and summation operator DL-PCBs in females were the lowest in the muscle, and of similar levels in liver and gonads. In males the concentrations had a decreasing order of liver >muscle >gonads. Despite the gender differences in tissue levels, the profiles were fairly similar in both genders and sites. Significant relationships were found for both summation operator DL-PCBs and summation operator TEQ(DL-PCB) and concentrations of summation operator PCBs and summation operator PCB(7). The levels of summation operator TEQ(DL-PCB) in the livers of both genders and in female gonads, in addition to the total PCBs load in these organs, raise concern and suggest that the species can suffer from PCBs toxic effects.