Spatial distribution of cod in the Baltic Sea in relation to abiotic factors - a question of fish-age and area

No abstracts are to be cited without prior reference to the author.

The eastern Baltic cod (Gadus morhua) is the main piscivore in the Baltic Sea. Unfortionatly, it has decreased during the last two decades. The decrease is thought to be due to a combination of high fishing pressure and possibly suboptimal environmental conditions. One specific problem in the Baltic Sea is that inflows from the North Sea determine the water turnover and oxygen and saline concentrations, but also the yearly nutrient loading. During the last 15 years only two major inflows have occurred and as a consequence, areas with hypoxic and anoxic conditions have commonly occurred. In this study, we investigate the distribution of 1-, 3- and 5-year old cod in relation to oxygen, salinity, and temperature between 1988 and 2004 (first quarter) in subdivisions 25-28 in the Baltic Sea. We explored the spatial distribution of cod analytically but also graphically. The highest abundance of cod of all ages during all years was found in Subdivision 25, independently of oxygen and total biomass of cod (based on XSA estimates). At a smaller spatial scale, within subdivision 25, 3- and 5-year old cod had similar distributions, offshore in deep waters, whereas 1-year old cod occurred at more coastal areas. Within the other three subdivisions explored, no clear patterns could be detected. We found a positive relationship between oxygen concentration, as well as salinity, and the total abundance of cod (all age-classes), independently of total biomass of cod and recruits. An additional exploration confirmed oxygen to be the main factor affecting cod density and distribution. Furthermore, there is a negative correlation between oxygen concentration and a scaled abundance measure of 3- and 5-year old cod indicating that oxygen affects the survey abundance index used in cod assessment. Therefore, an increased awareness of the Baltic cod distributional response to a decreasing abundance and fluctuations of hydrographical conditions may enable an increase accuracy of stock size estimations, which may ensure the future of the Baltic cod and the Baltic cod fishery.