Estimating abundances of 0-group western Baltic cod by using pound net fisheries

Nearshore 0-group western Baltic cod are frequently caught as bycatch in the commercial pound net fishery. Pound net fishermen from the Danish Isle of Funen and Lolland and the German Isle of Fehmarn have recorded their catches of small cod between September and December 2008. Abundance patterns were analysed, particularly concerning the influence of abiotic factors (hydrography, meteorology) and the differences between sampling sites. Catch per unit effort (CPUE) differed by site and location, whereas CPUE were highest at Lolland. Correlation between catch and wind/currents were generally weak. However, wind directions and current speeds seem to affect the catch rates. Finally an algorithm was developed to calculate a recruitment index for western Baltic cod recruitment success based on previous analyses

Is there a limit to the potential effects of shortening lastridge ropes on the size selectivity of diamond mesh codends?

Shortening codend lastridge ropes can be an effective fishing gear modification to improve the size selection properties in diamond mesh codends. Lastridge ropes attached to codend selvedges withstand the longitudinal forces created by the catch building up and therefore, prevent the codend meshes from closing. However, the extent to which the lastridge ropes should be shortened to maximize the effect of this measure is unclear. Besides opening codend meshes, shortening lastridge ropes can also lead to net folding, which can potentially have negative consequences for size selectivity. In the present study, we tested the size selective properties of a 129 mm diamond mesh codend in three different configurations: 0 %, 15 % and 30 % shortened lastridge ropes. Selectivity data were collected for cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and redfish (Sebastes spp.) in the Barents Sea gadoid bottom trawl fishery. Shortening the lastridge ropes by 15 % had a significant effect on the release efficiency of haddock between 35 and 50 cm, whereas to obtain a similar result for cod, the lastridge ropes had to be shortened by 30 %. However, the use of shortened lastridge ropes significantly increased the retention of fish below 35 cm for both species, especially when the lastridge ropes were shortened from 15 % to 30 %. The effect on redfish size selectivity was in general limited. Exploitation pattern indicators showed that there was no added benefit from shortening them further from 15 % to 30 % for any of the three species. This study concludes that, while shortening lastridge ropes can contribute to improved size selection of diamond mesh codends, reducing them beyond 15 % is not recommended because it can substantially increase the retention of undersized fish, probably due to net folding.publishedVersio

Egg mortality: predation and hydrography in the central Baltic

Cod and sprat are the dominant fish species in the Baltic pelagic ecosystem, both of great economic importance and ecologically strongly interlinked. Management of both species is challenged by highly variable recruitment success. Recent studies have identified predation and hydrographic conditions during the egg phase to be of critical importance. Two years of extensive field investigations in the Bornholm Basin, central Baltic Sea, were undertaken. In 2002, a typical stagnation situation characterized by low salinity and poor oxygen conditions was investigated, and in early 2003, a major inflow of North Sea water completely changed the hydrographic conditions by increasing salinity and oxygen content, thereby altering ecological conditions. The goal was to quantify egg mortality caused by predation and hydrography, and to compare these estimates with independent estimates based on cohort analysis. Results indicated high intra-annual variability in egg mortality. Cod and sprat egg mortality responded differently to the major Baltic inflow: mortality related to hydrographic conditions increased for sprat and decreased for cod. On the other hand, predation mortality during peak spawning decreased for sprat and increased for cod

The distribution of Baltic sprat, Sprattus sprattus L.- causes and consequences

This thesis shows different aspects of the distribution of adult sprat in the central Baltic Sea. The seasonal development of sprat abundance in the Bornholm Basin was studied under the influence of the major Baltic Inflow 2003. Additionally paramters, determining the vertical distribution of Baltic sprat were determined. This new knowledge was applied and two models were presented to describe the vertical distribution of sprat. The found distribution patterns were combined to investigate the impact of sprat on the zooplankton community