The Gallery
Cod in the Baltic Sea.
These data visualizations show analyses done by Yvette Heimbrand and me at Cornell's synchrotron. Yvette organized them geographically, from west to east, and also down toward the right-hand corner, it's further into the Baltic Proper. You can see that the otolith growth bands are more clear on the left side. Those on the right are not growing as well.
The next set of images shows maps of strontium. Reds/yellows are higher concentrations. In the Baltic Sea, strontium is more or less correlated with salinity, so each otolith becomes a fish's personal salinometer. Those in the upper left lived in saltiest water, while those on the right and lower right lived in lower salinity water.
Finally, the set of maps below the strontium are the trace element manganese. This element seems to be taken up in dissolved form, entering the bloodstream via the fish's gills, and then ending up in endolymph in which the otoliths form. The dissolved form of manganese occurs when dissolved oxygen is very low . Thus, it is a signature of hypoxia (note - it can also dissolve in acidic water, such as in blackwaters of the Amazon basin).
In this example, more manganese is seen in otoliths to the right - that's where hypoxia is worst in the Baltic.
Curvina (an Amazon basin freshwater drum fish)
Ted Hermann and I have quantified trace elemental distribution in a number of fish species in the Amazon basin. The maps you see here were made on a large otolith from a curvina. It's in the Scianid family; most members are marine, but a few are freshwater like this one. The numbers along the left and bottom are millimeters; the scales are the mass concentrations of Mn and Sr, respectively. The striking images show the irregularity of uptake around the growth bands. But they also reflect the differences in habitat used by the fish throughout its life. We think that the elevated manganese (Mn) was incorporated wen the fish was in "black water" - the tea-stained backwaters that leach organics out of vegetation during the wet season. The patterns of elevated strontium (Sr) are a bit harder to interpret, but might come from the fish having moved into brackish water.
It is hard not to be struck by the beauty of these patterns!
"Eel Dreaming."
These are Baltic Sea eel otoliths, with a photo cut-out of an eel mounted in between them. The otoliths show their strontium concentrations. Red is high, blue is low, and the scale corresponds to salinity. The red dots are elevated Sr in their cores, formed at birth in the Sargasso Sea. This is work I did with Lars Westin, Håkan Wickström, and Henrik Svedäng.
The otolith on the left is from an eel that was captured as a glass eel, stocked into fresh water (low Sr), but then migrated into the Baltic Sea (yellowish blues). The otolith on the right is from an eel that grew first in the ocean, then moved into the Baltic, and spent some years moving in and out of streams, feeding and growing.
We don't really understand the selenium (Se) enough to interpret the patterns, except to say that it appears to have been taken up in freshwater and brackish environments, thus presumably within the Mekong River and Mekong Delta, respectively.
A new project: I'm working with An Vu and Lee Baumgartner from Charles Sturt University in Australia. They are looking for anadromous species in the Mekong River in Vietnam. One of the few known anadromous species is a giant ("goliath") catfish known as Pangasius krempfi.
This collage compares two individuals. The otoliths and maps to the left are from a fish caught in fresh water. Blues are low concentrations, yellows and reds are higher. The strontium tells most of the story here. You can see a small dot of blue in the core of the Sr map. That was when this fish was born > 700 km from the sea, below Khone Falls. Then, as a tiny fry, it was swept down stream to the estuary, where it spent a year, then moved into the ocean, where it spent a couple of years before migrating back into fresh water, where it was caught two years later.
In contrast, the otolith on the right tells a different story: this fish shows no evidence of experience in fresh water. It was caught 35 km off shore in the ocean. If true, this is a new discovery.