prevented high parasite loads, and the absence of mosquito insecticides in this case is thought to help explain the lack of mass mortalities. Migration A. crassus has been repeatedly blamed for the dramatic drop in eel recruitment during the 1980s, although this level of blame has receded in recent years. The effects of A. crassus on eel populations are not isolated, but are part of a synergistic effect composed of factors including over-fishing of elvers, habitat loss, global warming and pollution all have significant effects on eel recruitment. Doubt over mostly attributing this drop to A. crassus stemmed from an observation that during the period A. anguilla suffered a large drop in recruitment, the American eel Anguilla rostrata experienced a similar decline (98%) in North America, despite A. crassus having not been introduced to that ecosystem at that point. The parasite could possibly impair the migration of eels to the Sargasso Sea, where European and American eels spawn. For European eels, this migration involves a journey of about 5500 km. Research has been carried out to precisely quantify the possible damage a given parasite load could have in the ability of a silver eel to migrate and reproduce. In an experiment to assess the energetic cost of the migration in general, it was shown that eels with less than 13% fat reserves would not be capable of reaching their spawning ground. Looking at the effect of an A. crassus burden, however, showed that heavy parasite burdens do affect swimming performance and reproductive output following migration. References Further reading External links A. crassus as a parasite of eel swimbladders (pdf) "Anguillicola crassus (Kuwahara, Niimi and Hagaki, 1974)" at Internet Archive JNCC website Spirurida Nematodes described in 1974