Reduktion af sælers indvirkning på fiskeriet

Recent years have seen an increase in conflicts between fisheries and the rising number of seals in Danish waters, resulting in both direct and indirect effects on various fisheries. It is primarily passive gear fisheries like gillnets, longlines, pound nets and fyke nets that are suffering from the seal attacks. DTU Aqua has in recent years developed seal-safe cod pots to a level where it is possible to conduct a sustainable fishery with these pots, but they are not a solution for all fisheries. In the present project we have looked at whether other mitigation methods could help some of these fisheries. We have also looked into the hidden losses in gillnet fishing, into the consumption by grey seals of cod in the Baltic Sea and into the hidden bycatch of marine mammals in gillnet fisheries.

In the first work package we tested a novel seal scarer at an aquaculture facility in Horsens Fjord. The novelty of this scarer is that it works through the startle respons of the seals, which should avoid problems with habituation. Unfortunately, the effect of the seal scarer was very small and not sufficient to justify the cost of the scarer.

The second work package tested whether a stop net could keep seals from entering pound nets and eating or injuring the fish caught in the net. We tested two different types of stop nets, both with 150 mm mesh size. Although they were very effective at keeping seals out of the pound nets, the catches of the target species, garfish, were reduced by around 60 %.

In the third work package we investigated how much cod grey seals remove without trace from gillnets around Bornholm. We found that on average 48 % of the fish we placed in the nets were removed without trace, and on some days all the cod in the nets were removed by the seals. The ratio between removed cod and damaged cod was 18, compared to around 7 in similar trials conducted in Sweden.

The scope of the fourth work package was to demonstrate how the use of an agent-based model (ABM) could be applied to mimic the foraging behaviour of grey seals in the southern parts of the Baltic Sea in order to predict spatial and temporal predation intensity of grey seals on cod populations. The model describes the movement and foraging behaviour of seals during stays in the South and Western Baltic Sea. Seal foraging on cod is described as partly a function of the cod biomass interpolated from DTU AQUA's biannual survey data, and partly as a function of the agents' memory. Preliminary results from the model estimate a consumption rate of approximately 1,400 tonnes of cod per year for grey seals from the four haul-outs in the area.

The fifth work package was aimed at determining the drop-out rate for marine mammals bycaught in gillnets. Drop-outs are bycaught animals that fall out of the nets during hauling before the nets break the surface and therefore are not recorded by REM-systems or onboard observers. We used an ROV to record the number of bycaught marine mammals in gillnets set for lumpsucker in the Øresund. The nets were investigated immediately before the fisherman hauled the nets. Comparing the ROV recordings with the recordings made by the REM system onboard the fishing vessel would yield the drop-out ratio. Unfortunately, the fishing vessel recorded only one marine mammal – a harbour porpoise – on the REM-system during the 15-day cruise in 2022, and this porpoise was also recorded by the ROV. We also compared bycatches of seabirds between REM and ROV and both systems recorded the same 15 birds. We conclude that the method worked really well in this area and should be repeated in fisheries with high bycatch rates.