Thursday, September 30, 2010
The Magic of Strontium
While I attempt to stay afloat during this seemingly endless torrential downpour Eastern North Carolina is experiencing I decided to finally take the time to write about anadromy and how it can be examined using otolith microchemistry.
Anadromous fish spend most of their adult life in the ocean returning to freshwater to spawn, the classic example of anadromy is Pacific salmon. While it is all well and good to say that a fish moves between salt and freshwater the timing and frequency of these movements can have implications on the management of anadromous species. The goal of tracking migrations between the marine and freshwater environment can be accomplished through traditional mark and recapture studies, however, these lack corresponding age data and fail to provide a complete history of the fishes migrations the way otolith analysis can.
In studies of anadromy, it seems as though the element strontium (Sr) is some sort of miracle element. Although exceptions do exist Sr is typically more highly concentrated in marine waters than freshwaters (Limburg 1995). In addition to showing differences between marine and freshwater environments Sr also incorporates into the otolith quite nicely, perhaps becaucuse Sr is similar in size to Ca, and otolith Sr usually reflects Sr in the ambient water (Campana 1999).
If everything stated previously holds true (there is higher concentrations of Sr in saltwater than freshwater, and Sr is incorporated into the otolith in ratios similar to the ambient water) then the pattern of Sr in an otolith should increase and decrease when a fish moves between saltwater and freshwater. The classic pattern of anadromy can be illustrated by this picture (although this picture may or may not have been for the purpose of investigating anadromy) from Dr. Norman Halden at the Univeristy of Manitoba
The bottom plot shows Sr. The center, or core, of the otolith, representing growth when the fish was young shows low Sr. This pattern could be interpreted as the fish was remaining in a low Sr environment (freshwater). As this fish gets older there are peaks and declines in Sr. This could be interpreted as the fish moving between salt (peaks) and freshwater (declines). This pattern is pretty typical of what you would see from an anadromous fish species. The Sr pattern for a fish that spends its whole life in freshwater would theoretically be relatively flat, without the peaks and declines in Sr.
However, there are limitations to this method and the pattern of Sr is not always clear cut, as I am coming to find out with my own data. Dealing with an anadromous species like river herring I would expect to see a pattern of fluctuating Sr. River herring are born in freshwater (where you would expect low Sr), then move to the ocean (where you would expect high Sr). River herring then move between the ocean and freshwater in order to spawn, therefore, I would expect to see low Sr at the core of the otolith and then fluctuations throughout its life. However, I have not necessarily seen this pattern, and at least in some of the fish I have looked at the pattern appears to be backwards. I think this is pretty interesting, and am still trying to determine if it is the result of instrument error or some natural phenomenon.
Campana, S. (1999). Chemistry and composition of fish otoliths:pathways, mechanisms and applications Marine Ecology Progress Series, 188, 263-297 DOI: 10.3354/meps188263
Limburg, K. (1995). Otolith strontium traces environmental history of subyearling American shad Alosa sapidissima Marine Ecology Progress Series, 119, 25-35 DOI: 10.3354/meps119025