In my opinion one of the most interesting questions asked about fish is where are you from? Or where have you been? Unfortunately, fish don’t carry around birth certificates that make answering these questions easy and uncomplicated…or do they?
Enter otolith microchemistry. In a sense the elemental makeup of the core of an otolith can act as a “birth certificate” for a fish. The idea is that the elemental composition at the core of the otolith will reflect the chemistry of the water it was born in. If there is enough differentiation between different habitats you should be able to see clear separation between fish born in different areas. The clearest examples of this technique have been in looking at fish from different rivers, and estuaries.
An example of this technique put to use is Thorrold et al. (1998). In this study elemental and isotopic ratios in otoliths were used to classify juvenile weakfish to natal estuaries along the east coast of the United States. Fish from different estuaries showed pretty clear differences in elemental and isotopic ratios, which allowed accurate classification of these fish to their natal estuaries.
|Me with weakfish|
So why is any of this important? Well, believe it or not investigating and understanding the life history of a fish is really hard. Traditional mark and recapture studies, which usually involve sticking a tag into a fish, releasing and then hoping to recapture it at some point are often ineffective and typically wind up relying on pretty low recapture rates. Also, tagging young juvenile fish can be tedious and unfortunately usually has fairly high mortality. However, if you are able to identify a reliable elemental signature for a river that is reflected in fish otoliths it acts as a “natural tag”. Therefore, if in the future you capture an adult fish you can examine the elemental signature at the core of that otolith to determine where that fish was born. This is what Thorold et al. (2001) did as a follow up to Thorrold et al. (1998).
This information is useful in looking at natal homing; straying rates between spawning locations, stock discrimination, movements and migrations, and by catch rates. This technique forms the basis for my project. I am attempting to classify juvenile river herring to natal watersheds within the Albemarle Sound NC. While this area isn’t on the same scale as some of the studies mentioned previously, I think I should be able to find some differences in the elemental composition of fish otoliths from these waters.
Brazner, J., Campana, S., Tanner, D., & Schram, S. (2004). Reconstructing Habitat Use and Wetland Nursery Origin of Yellow Perch from Lake Superior using Otolith Elemental Analysis Journal of Great Lakes Research, 30 (4), 492-507 DOI: 10.1016/S0380-1330(04)70365-2
Thorrold, S., Jones, C., Swart, P., & Targett, T. (1998). Accurate classification of juvenile weakfish Cynoscion regalis to estuarine nursery areas based on chemical signatures in otoliths Marine Ecology Progress Series, 173, 253-265 DOI: 10.3354/meps173253
Thorrold, S. (2001). Natal Homing in a Marine Fish Metapopulation Science, 291 (5502), 297-299 DOI: 10.1126/science.291.5502.297
Walther, B., Thorrold, S., & Olney, J. (2008). Geochemical Signatures in Otoliths Record Natal Origins of American Shad Transactions of the American Fisheries Society, 137 (1), 57-69 DOI: 10.1577/T07-029.1