Guest post by Gretchen Grammer, Gretchen is a PhD student in the Marine Biology Program, School of Earth & Environmental Science at The University of Adelaide. Her current research centres on the use of fish otoliths as a proxy to examine oceanographic processes in relation to climate variability.
She is also interested in the use of sclerochronology and sclerochemistry to further understand the biology and ecology of marine species and the past environments they have encountered. Gretchen is under the supervision of Prof. Bronwyn Gillanders and Dr Chris Izzo.
What is a chronology? Not to mention, how does it fit with fish earbones? Fish HAVE earbones?? These are some of the questions you may ask yourself when reading the title of this research post. Fish do indeed have earbones or more appropriately, earstones. Officially called otoliths, they are calcium-based structures located in the fish’s head and used for hearing and balance. When sectioned with a low speed saw or even broken in half, growth bands can be seen within the otolith’s interior. These growth bands are akin to the annual rings found in trees and can be used in very similar ways.
The science of dating tree rings and analysing the growth bands is called dendrochronology, while the analysis of growth rings from hard parts of aquatic animals is called sclerochronology. Besides otoliths, other structures that contain growth bands are vertebra of sharks, coral skeletons, mollusc shells, and mammal teeth. A variety of information can be gleaned from these hard structures. Besides information about the amount of annual growth of the organism, we can also gain insight into environmental conditions that the animal encountered during that period of its life. This can be done by assigning each growth band a year through a technique called crossdating which results in a chronology or sequence of events through time. This chronology can then be compared to environmental indices recorded by instruments (e.g. sea surface temperature, Multivariate El Niño Southern Oscillation Index, Southern Oscillation Index, etc.) or the composition of trace elements in the growth band. By combining these factors with the long-term growth chronology, multi-decadal reconstruction of environmental records and climatic change through time can be deduced from organisms’ growth patterns.
For my research, I have targeted a long-lived, bottom dwelling fish called ocean perch (Helicolenus percoides) to use as proxy species to examine oceanographic processes in relation to climate variability in the southern hemisphere. Ocean perch live in the deeper waters on the continental shelf off southern Australia and New Zealand. Some of the criteria to be considered when selecting a species for use in developing a marine growth chronology are: longevity, occurrence and range, site-fidelity, ease of capture, and readability of the otoliths. Ocean perch live 40+ years, are fairly common with a wide distribution, have high site-fidelity (non-migratory), are caught both commercially and as bycatch in a variety of fisheries, and growth rings can be seen in their otoliths.
To begin developing a growth chronology, you first have to catch the fish (or find some old, archived otoliths…)! The next step is to dissect out the otoliths from the fish and embed them in epoxy resin. The otoliths are then thin sectioned on a low-speed saw, polished and mounted on a microscope slide. High resolution microscopic images are taken so the annular growth bands can be measured. The measurements are used to visually crossdate the growth patterns across individual fish in search of “signature years”, extremely narrow or wide bands compared to neighbouring bands. Crossdating is a dendrochronological technique that helps to assign correct calendar years to each growth band. Synchronous growth band width patterns are cross-matched across multiple samples at a certain time and place. Only the otoliths with the clearest bands are included in a chronology. After the chronology has been visually crossdated, it is statistically verified, detrended and correlated with the various environmental climate indices mentioned above.
Thus far, I have examined the otoliths of 36 ocean perch collected offshore of South End, South Australia in October – November 2011 and have found potential signature years to be 1998 (narrow), 2006 (wide), 2007 (narrow), and 2009 (wide). I have not correlated these with the environmental indices yet, so I am not sure what is happening in the fish’s environment to either stunt (narrow bands) or increase (wide bands) its growth in a certain year. The next step with my research is to create an overall master chronology (by adding more fish) from the area and link it to environmental data in order to study climate-growth relationships and how they related to oceanographic processes along the southern Australia coast as well as the effects of climate variability on fish growth.
Stay tuned for future results!
Guest Post by Gretchen Grammer, if you would like to contribute your research to a guest post on The Environment Institute Blog email firstname.lastname@example.org.