The waters of the outer Oslofjorden have always been an important source of food for the local population. Mackerel arrive here each year in May/June. Around the year 1900 there was also a rich herring fishery here. These days the herring normally head elsewhere, but they still appear here from time to time. The big schools of bluefin tuna that were caught in great numbers in the 1950s and 60s have also gone. Coastal cod stocks have also fallen sharply in recent years, although the reasons for this remain unclear. The shrimp and langoustine fisheries remain important, however. Professional fishers still operate here, and there is a fish depot at Sandøsund on Hvasser.

The lobster population has fallen sharply over recent decades, primarily due to overfishing. However, the creation of a protected area by Vestre Bolæren in 2006 has produced spectacular results. The population has risen by 350 percent, with significant increases also seen in adjacent areas. The plan is now to establish additional protected areas within the national park. If adopted, this will see the size of the protected areas for lobsters rise from the current 70 hectares to 1,400 hectares.