Implications of stock recovery for a neighbouring management unit: experience from the Baltic cod

Abstract

Cod in the Baltic Sea is assessed and managed as two separate stocks, i.e. eastern and western Baltic cod. The eastern Baltic cod has recently started to recover after several decades of severe depletion. In the present study, we suggest that the recovery of the eastern Baltic cod population has also substantially increased cod abundance in a specific area of the adjacent western Baltic management unit. This is investigated through long time-series of spatially resolved stock assessment data supplemented by genetic analyses of origin of the cod currently found in the transition area between the two populations. Due to immigrating cod from the east, there are currently large spatial differences in cod abundance and mean weight in the western Baltic management unit that raise new management concerns. First, the high abundance of cod of eastern origin found in the western Baltic management unit can mask the relatively poor state of the western Baltic cod population. Second, the current fishing mortality estimates for the entire western Baltic management unit, used as basis for fisheries management, are difficult to interpret as these are highly influenced by mixing of biological populations and the spatial distribution of fisheries.

Genetic analyses reveal complex dynamics within a marine fish management area

“…Our data from archived otoliths collected in 2003 and 2004 indicate that mixing is not a recent phenomenon. This implies that eastern fish were also present in the area at times with much smaller population sizes of eastern Baltic cod than estimated for late 2000s to early 2010s (Eero et al, ), as also suggested by morphological data collected from otoliths (Hüssy, Hinrichsen et al, ). Our data further suggested the presence of a strong east–west gradient in mixing proportions in samples from 2003 to 2004, similarly to the pattern observed in contemporary data collected after 2011, although it should be noted that the samples represent much lower geographical resolution than the contemporary data set.…”

First evidence of spawning of eastern Baltic cod (Gadus morhua callarias) in the Belt Sea, the main spawning area of western Baltic cod (Gadus morhuaL.)

“…(, ) for the 1990s and Eero et al. () for the spawning season 2012, also found no evidence of spawning activities of EBC in the Belt Sea.…”

“…(), Eero et al. () and Hüssy et al. () to the conclusion, that currently seasonal spawning migrations from the Bornholm Sea to the Arkona Sea take place.…”

“…(, ) and Eero et al. () described the Belt Sea as an area in which mainly cod with the genetic signature of WBC spawn. However, as already discussed by Berner and Müller (), low mixing of individuals from the Kattegat occurs.…”

“…Whether the spawning activities of EBC in the Belt Sea can be lead back to the exceptional hydrographic situation in March 2010, or whether it is a result of a temporary “spill‐over” (Eero et al., ) into the Belt Sea, or is a combination of the two, can currently only be suspected. For the ICES SD: 25–32, the traditional main distribution area of EBC, in 2010, the highest index for cod ≥38 cm has been demonstrated since 2004 (ICES ) on the basis of the data of the Baltic International Trawl Surveys.…”