Benthic fish also acquire greater parasite loads, due to their tendency to prey on epibenthic intermediate hosts or other paratenic hosts. Therefore, the composition of a fishery, such as a lake, could have an important influence of A. crassus within a particular locality. Second, A. crassus larvae have the ability to infect several species of freshwater cyclopoid copepod, as well as estuarine copepods e.g. Eurytemora affinis. This allows transmission of the parasite within a wide range of aquatic habitats. Third, the larvae remain for longer within the host's swimbladder wall whilst developing into the fourth larval stage, as opposed to moving directly through it (as occurs in Pacific eels). Effects on hosts Individual hosts The hosts' swimbladder wall becomes inflamed as cells undergo fibrosis, which can prevent further invasion by A. crassus larvae. High parasitic loads (>10 adult nematodes per eel) can reduce the proportion of oxygen in the swimbladder by about 60% in comparison to uninfected eels. Structural changes include possible alterations in the epithelial cells, hindering processes involved in acidification of the blood and leading to a decline in the rate of gas deposition. Mortality may also be linked to secondary bacterial infection, particularly in intensive eel farms. Infected eels are also less resistant to stress, with infections causing large increases in serum cortisol levels (an important primary messenger of stress response in teleost fish). Populations Mortalities are both more intense and identifiable within eel farms than in the wild. Thus, it is difficult to compare losses between wild and farmed eels. The best documented cases of mass A. anguilla mortalities are from the summer of 1991, in Lake Balaton, Hungary, and the Vranov reservoir in the Czech Republic. during 1994. Both displayed similar characteristics involving low water oxygen levels, coupled with high temperatures, plus high eel densities and A. crassus infection levels. These conditions are the same outlined as ideal for epizootics by Barus & Prokes, 1996. It has been suggested that the highly stressful conditions in the lakes were compounded by the intense parasite presence (in Lake Balaton, burdens were as high as 30-50 adults and 200 larvae per eel). Parallels between Lake Balaton and another water body, the Neusiedler See in Austria could counter the extreme effects of A. crassus on eel populations under these conditions. The small size of the eels (<50 cm long) due to the high eel density in the lake actually