To achieve healthy marine ecosystems, an ecosystem based approach to fisheries management has been widely acknowledged as the fundamental principle for sustainable resource. The underlying aim of this approach is an ecologically sound resource management that responds to ecosystem processes.
With increasing use of the marine environment, spatial aspects in marine management are getting increasingly important including conservation issues which are becoming an integral part of the overall spatial planning. Implementation of ecosystem approach to fisheries management should be seen in conjunction with this process, given the spatial heterogeneity in fish populations. Especially in a multispecies context, when several species interact in a system (e.g. through predator-prey interactions or competition), it is of fundamental importance to understand the spatial patterns of the different species and the causes and consequences of their distribution changes.
Within species, information on the population structure is pivotal for sustainable management. Such information is, however, still lacking for several commercially important species of fish.
The Baltic Sea ecosystem has undergone major structural changes including a more than 10-fold decline in cod and herring biomass from the late 1980s to the 1990s, and a 5-fold increase in the biomass of sprat after the early 1990s. Baltic cod, sprat and herring have dramatically changed their spatial distributions during the past three decades, and therefore the interactions between them (in terms of predation and competition) have also changed.
Spawning stock biomass (SSB) of cod, sprat and herring, including the proportion of catch (shown as sectors of SSB), by area (ICES Subdivisions [SD]), in 2010. Clisk on the figure to see larger version.
Concomitant with its collapse during the late 1980s, the cod population has contracted in ICES Sub-division (SD) 25, where most of the adult cod is still currently concentrated in (STECF 2012, WKMULTBAL 2012). The stocks of sprat and herring have, on the other hand, increased in the northern Baltic since the early 1990s. Here, a sharp decrease in condition has been observed which is probably due density dependence driven by competition for zooplanktonic food. Hence, only about 10–15% of the herring and sprat biomass is currently distributed in SD 25. Largest concentrations of both species are found in SDs 28–32, i.e. beyond of the current distribution area of cod. The causes of the spatial changes in distributions are still unresolved, but could be related to both demographic and hydro-climatic factors, intertwined with fishing pressure.
Since 2007, the eastern Baltic cod population has started to recover, partly due to effective management measures. However, the recent recovery of cod is concentrated only in SD 25 where the abundance has reached the highest values recorded in this area since the 1970s. Recent observations show that the cod has become lean in SD 25, creating problems for the fishers and producing high rates of discarding and high-grading from the fishery. The reasons of the decrease in cod growth in SD 25 are still unclear. Likewise, the reasons of a lack of expansion of cod population into the north areas, rich of pelagic food, are still unclear but could be related to adverse hydrological conditions in the north.
In summary, the Baltic has to face a largely modified spatial ecosystem set-up affecting the biological interactions among species. At the present distribution of the fish stocks in the Baltic Sea, intensive predator-prey and competitive interactions are taking place in limited areas in the south-western and north-eastern Baltic Sea. The present ecosystem setup may not be any longer structured to sustain large cod stocks. At the same time, fisheries are competing with cod for the limited resources of sprat and herring in the southern area, which results in a substantially higher mortality on these local stock components of pelagic fish compared to the abundant resources distributed in the northern Baltic Sea.
The present situation in the Baltic Sea calls hence for a spatially explicit assessment and management of these resources. For example, a relatively higher fishing pressure on clupeids in the north could release clupeid competition in these areas, and at the same time release prey-to-predator feedback loops favoring cod recruitment in the north and a re-expansion of the stock distribution into northern areas. However, before such a management can be designed and implemented, more knowledge is needed on the processes generating spatial heterogeneity, i.e. to be able to identify processes which operate on a local scale, but potentially have wider ecosystem consequences.
Especially, the basic ecological processes of recruitment, predation, migration and exploitation have to be re-interpreted in their spatial context in order to create the knowledge-base necessary to implement a spatially-explicit ecosystem-based fisheries management. The Baltic Sea is currently chosen as a pilot case for taking into account biological interactions in the new fisheries management plans being under development in the European Commission. Thus, the experiences from the Baltic Sea can serve as a basis for similar developments elsewhere.
The stock dynamics of flatfishes in the Baltic Sea, including the commercial most important flatfish species, the flounder, are largely unknown. To date, analytical assessments are hampered by the lack of understanding spatial distribution, stock separation, and recruitment mechanisms in flatfish. Flounder in the Baltic are hypothesized to belong to different populations, producing either pelagic or demersal eggs. There are most probably differential recruitment rates between the two populations which thus contribute differently to the exploitable stock. In addition several separate sub-populations have been identified within these spawning types but their number is too high to be used as units in routine assessment. Merging of many biological populations into few assessment units can however have detrimental effects on the stocks. Hence there is a need to increase the understanding of stock structure and spatial distribution of flounder to develop analytical assessment tools for a sustainable management.