We aimed to investigate movement of endemic New Zealand black flounder (Rhombosolea retiaria) between the sea and freshwater through otolith microchemistry. Most flounder are entirely marine, but the black flounder is unusual because it is classified as a freshwater fish but as adults can inhabit the sea, estuaries, lakes, or rivers. Its migration downstream is thought to occur in winter, with juveniles returning to freshwater in spring, but very little is known about its life cycle. To clarify the importance of diadromy in its life history we used laser ablation inductively-coupled mass spectrometry to examine the otoliths of freshwater-caught black flounder.
Black flounder from the Mokau River, Taranaki, North Island West Coast, had high strontium with low barium at the beginning of the life cycle, indicating life in the estuary. A drop in strontium accompanied by a rise in barium in later life shows movement into full freshwater. Black flounder from the Clive River, Hawkes Bay, North Island East Coast, however, showed quite complex life histories, with several movements between the estuary and fresh water. In the adjacent Ngaruroro River, black flounder showed low concentrations of strontium throughout life compared to flounder from other rivers, indicating an entirely freshwater existence even when the primordium was laid down during egg development.
We conclude that black flounder show a variety of life histories, sometimes with complex patterns of occupancy of the estuary and fresh water, with possible freshwater spawning.