Fish and Otolith Science: A subject and article review by Justin Londer

A short review of "Otolith science entering the 21st century," a paper written in 2005, published by the journal "Marine and Freshwater Research (Steven E. Campana, 2005. 56:485-495) follows. By Justin Londer

Simply put, we live and study in a region that presents amazing opportunities for research in aquatic sciences, Ichthyology and fisheries. The Great Lakes and associated watersheds are unparalleled in and home to hundreds of fish species that warrant considerable research. Much of this research has already been and should continue be related to the examination of the otoliths and inferences that can be made from them that will help preserve native biodiversity and improve fisheries.

As Campana notes in his paper, literature regarding otolith study is being published at five times the rate that it was in 1970's and approximately 200 papers per year are being published in the primary literature. In addition, Campana notes that while traditional otolith research seemed focused on annual age and growth estimates other non-aging studies have increased markedly. He notes that while aging today makes up 40% of otolith studies, that this percentage is drastically down from levels in recent history and that the drop is accounted for by a rise in the other fields of otolith study. In addition, the balance may even be shifting to this direction more. Besides aging, Campana identified fourteen other research applications for otoliths. Some of these I have listed below with brief explanations. There is some overlap between the different areas as I describe them, however, Campana in more detail separates these categories based on more detail.

Otolith microstructure can be used to sequence daily growth increments, hatch date, growth rate, or duration of residency of pelagic larvae. Microstructures can also be useful for comparing growth rate and hatch rate of different cohorts as a function of different environmental conditions. Otolith microstructure can help determine population dynamics by permitting back calculations of growth to assess the influence of size selective mortality on previous life history stages. This information can be of great use when thinking about how fishing pressure can affect both the direction and magnitude of size selective mortality.

Hearing and balance, or the function of otoliths has also been studied at a small very level by using otolith structure. As Campana notes, the low level of study in this area seems ironic since hearing and balance are so critical to fish.

Otolith allometry and growth, or factors that influence the shape or allometry between otolith and somatic growth has been studied. The unique shape of otoliths between species of fish makes them a good candidate for identification of fish when other more normal means of identification cannot be used for various reasons. Studying the shape of otoliths has revealed that both genetics and the environment play a role in the shape of the otolith. Using the otolith for species identification can be useful when looking at what fish another fish may be eating via its stomach contents or feces.

Another area of research related to otoliths is that of tracer applications that allow researchers to determine if different fish across different years might be experiencing the same environmental conditions. Campana notes that an exciting area of developmental research has emerged that involves certain otolith markers used in freshwater fish to determine the movement patterns and origins of fishes over time to be reconstructed with high fidelity.

Mass marking techniques that involve induced temperature fluctuations or calcium-binding chemicals in the water can be used to induce distinct chemical or visual markings in the otoliths at the time of application. In turn, these distinct markings can be looked for in and be beneficial when recapturing stocked fish. The benefit largely lays in the potential to mass-mark many fish at once at a potential low cost

Trace element analysis in otoliths can be studied and provide information about the level of pollution that fish have been exposed to. This can be important when considering what type of pollutants and at what levels a fish was exposed to in its lifetime.

Stable isotopes and radioisotope composition in otolith have been studied as well. The analysis of these components can play a role in determining what a fish ate and at what levels the fish ate that food item at over its lifetime.

Environmental reconstruction from otolith structure can permit a clearer picture of the environment in what a fish lived.

Otoliths can be used when studying fish fossils.

Methodological advances in all of these areas of otolith study have also been researched.

Physiological studies have revealed that certain biological compounds in fish can play an important role in the development and seeding of the otolith.

Unfortunately for fish the examination of otoliths requires the sacrifice of the fish. Often (and as I have personally witnessed at my place of research) otoliths are not utilized from fish that sacrificed for other areas of research such as fecundity, sexual dimorphism and diet. Because so many opportunities to learn about fish from otoliths exist, I believe that fisherman, research scientists and anyone else who has an opportunity to save an otolith and document its origin should do so. This is particularly true because storing otoliths does not take up much space and is easy to do so that they do not degrade over time. Even if individuals themselves do not plan to use the otolith, there are many scientists who could. And, in using the otolith we waste less of the resource we are interested in as scientists fisherman. Lastly, the examination and dissemination of information on otoliths would seemingly only work to everyone’s advantage since it could provide vital information of the sustainability of fisheries and the like. Perhaps the best solution would be an otolith repository program in which people could send otoliths through the mail to a central location where they could remain preserved for later processing.

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