The hydrographic features of anguillid spawning areas: potential signposts for migrating eels

Area of Science:

• Marine Biology
• Oceanography
• Ecology

Background:

• Catadromous eels (Genus Anguilla) undertake long migrations from freshwater/estuarine habitats to marine spawning grounds.
• Satellite tagging reveals eels perform diel vertical migrations, reaching depths of 600-800m during the day and ascending to 150-300m at night.
• The precise navigation mechanisms used by eels to locate their spawning areas remain largely unknown.

Purpose of the Study:

• To describe the hydrographic conditions (salinity, temperature, currents) in and around known and hypothetical anguillid eel spawning areas.
• To investigate how these physical oceanographic features might serve as navigational cues for migrating eels.

Main Methods:

• Analysis of salinity, temperature, and geostrophic currents between 0 and 800m depths.
• Examination of conditions within two confirmed and three hypothetical anguillid spawning areas during their likely spawning seasons.
• Correlation of hydrographic features with known eel migration patterns and depths.

Main Results:

• Subducted 'Subtropical Underwater' with elevated salinity (~35.0 to 36.8) at ~150m depths was identified in four of the five studied ocean gyres.
• These high-salinity cores, potentially carrying odor plumes, are located near western ocean margins where subduction occurs.
• Spawning conditions may involve temperatures of 16-24°C with a deepened thermocline, weak westward currents at depth, and eastward surface countercurrents.

Conclusions:

• Hydrographic features, particularly the Subtropical Underwater and associated salinity gradients, likely act as crucial signposts for anguillid eels during their oceanic migrations.
• Eels' sensory capabilities enable them to detect and utilize these features for navigation to their spawning grounds.
• Understanding these oceanographic cues is vital for comprehending eel reproductive strategies and migration patterns.