Stream Water Creates a Discernable Odor Gradient that Migratory, Juvenile American Eels May Follow Inshore
Peter W. Sorensen
Young, migrating anguillid eels face the monumental challenge of locating productive coastal and associated inland freshwater regions after they metamorphose over the continental shelf. Olfactory cues are efficacious in this challenge because of the distances they can travel, the quantity of information they can convey, and the remarkable sensitivity of fish olfactory systems. An example of the power of olfaction is found in adult sea lamprey Petromyzon marinus, which can locate spawning streams by detecting subpicomolar concentrations (grams in tens of billion liters; Sorensen et al. 2005) of sulfated steroids released by stream-resident larvae. Creutzberg (1961) appears to have been the first to suggest that innately recognized natural odors emanating from freshwater ecosystems guide young anguillid eels to natural freshwaters from the ocean. Testing various natural waters in a circular maze, he discovered that changes in water chemistry cause elvers to either surge forward with the water flow or reside on the bottom, mediating selective tidal transport. Cruetzberg (1961) also found that salinity gradients alone could not explain this phenomenon as charcoal filtration removed the attractive properties of freshwater. Later, Miles (1968) discovered that the attractive components found in stream water were biodegradable and recognized innately. Subsequently, Sorensen (1986) tested plant extracts and films, conspecific washings, and minerals on elver movement to demonstrate that the freshwater attractants are composed of multiple cues originating from microflora and fauna, which correlated with stream productivity. Although a few candidate compounds have been suggested, definitive biochemical identification has not been performed (Sola 1995). The possible role of freshwater attractants on the actual ability of recently metamorphosed anguillid eels to locate natural inland waters has not yet been described. Here, I describe a study that addressed the latter issue by testing the attractive properties of estuarine waters versus natural stream water, and the ions they contained.