Otoliths are hard, calcium carbonate structures located directly behind the brain of bony fishes. Sometime also referred to as ‘ear stones’, they are used by fish for hearing and balance and help the fish know which way is up.
Otoliths range in size from one-tenth of an inch to one inch long and are found in the heads of all fishes except sharks, lampreys and rays. There are three types of otoliths; sagitta, asteriscus and lapillus with the sagitta being the largest of the 3 pairs of otoliths.
As a fish grows, deposits of calcium carbonate are laid down in rings on the otoliths. The otolith on the right is that of a 3-year old Tiger Flathead (Neoplatycephalus richardsoni or Platycephalus richarsoni), one of the most important quota species caught in the South East Trawl Fishery. The dark translucent zones represent periods of fast growth and the white opaque zones (called annuli) represent periods of slower growth. Fisheries scientists count the annuli in the sagitta to calculate the age of fish – just like counting rings on a tree to determine its age!
This age data allows fisheries scientists to estimate growth rates, maximum age, age at maturity, and trends of future generations. It is also used in age-based stock assessment models, to estimate mortality and population structure, to follow cohorts through time, to know species’ longevity and much more.
But these tiny, structures can provide a lot more information than just a fish’s age!
A study by Steven Campana (2007) from the Bedford Institute of Oceanography in Nova Scotia found that as otoliths grow, they incorporate specific chemical elements and isotopes that are unique to their surrounding aquatic environment. This can result in an otolith ‘elemental fingerprint’ that can act as an environmental tag of groups of fish that hatch from different environments and could potentially be used to differentiate between fish stocks of the same species. In lake systems for example, otolith elemental fingerprints could be used to quantifying the relative contributions of different wetland nursery areas to recruitment in adjacent lake populations.
In the case of the Patagonian toothfish (Dissostichus eleginoides), another important Commonwealth quota species in the Antarctic Fishery, scientist demonstrated that otolith microchemistry can be used to examining population structure and provenance.
Ashford and Jones (2007) from the Center for Quantitative Fisheries Ecology, Old Dominion University, in Virginia used carbon and oxygen isotope (slightly different types of carbon and oxygen) ratios in otoliths to distinguish between toothfish that were caught in Patagonian shelf water off South America and those caught in South Georgia (Antarctic waters).
Understanding the geographical range of different stocks of the same species is critical in assessing them. It can also hep fight illegal, unreported and unregulated commercial fishing.