Panmictic refers to a random mating strategy, frequently employed by fish, where breeding is just as likely to occur between any two individuals in a population as between any two others. Mating in this way is not influenced by any any environmental (e.g., geographic proximity), hereditary (e.g., timing of spawning), or social interaction (e.g., polygamous mating systems).
American Eels have a panmictic population.
In a population genetics context, if a species is a panmictic population, there is no genetic evidence of population structure throughout its range. For example, American Eels (Anguilla rostrata) are catadromous fish found from the Caribbean to Greenland. However, they all migrate to breed in the same location in the Sargasso Sea so genetic samples from throughout their North American distribution range show a complete lack of genetic differentiation, or complete panmixia.