Tracking without tags

Posted on June 4, 2011

Our understanding of recruitment processes, migration behaviour and population connectivity all rest on a knowledge of how and when organisms move between different areas.  While a number of techniques are available for tracking aquatic animals, most of them are difficult to use with very small individuals.  Electronic transmitter tags have minimum size limits and may affect the physiology and/or behaviour of small-bodied species, and recapturing individuals with passive tags, such as coded wire tags or tattoos, from large populations of tiny juveniles can be akin to retrieving a needle from a haystack.  An alternative approach that doesn’t involve the capture, marking, release and recapture of individuals relies on the use of stable isotopes.   Because  the levels of stable carbon and nitrogen isotopes in an organism reflect those in its diet, its movement patterns can be inferred by reference to known differences in the isotopic signatures of two or more locations.  French investigators interested in tracking local-scale movements of two small fish species (Leuciscus souffia and Alburnoides bipunctatus) between theDuranceRiver and one of its tributaries first analysed caddis fly samples to derive unique background isotopic references for the two locations.  Then they compared  these baseline values with those from fish collected at the same places.  For caddis, stable nitrogen values were significantly higher in the main river than the tributary.  At both sites, fish could be separated into two groups which clearly differed in terms of their nitrogen signatures.  In this particular study, isotope tracking allowed the investigators to conclude that high-nitrogen individuals in the tributary fed and grew in the main river in spring and summer before moving to the tributary. 

Reference:  Durbec, M., Cavalli, L., Grey, J., Chappaz, R. & Nguyen The, B.   2010.  The use of stable isotopes to trace small-scale movements by small fish species.  Hydrobiologia  641, 23–31.

Posted in: behaviour, fish