SEGH Articles

# Otoliths: The little “White” box recorders of the fish world

01 February 2013
The use of fish otolith (ear bone) microchemistry has enabled scientists understand better fish migratory patterns and stock identification

Andy Marriott is in his final year studying for a PhD in the use of biogeochemical tags (primarily otoliths) to determine the origins and movement patterns of fishes. This NERC-funded research project at Bangor University, North Wales is supervised by Dr. Ian McCarthy and Prof. Chris Richardson of the School of Ocean Sciences at Bangor University, Dr Simon Chenery at the British Geological Survey and Dr Mike Armstrong (Case partner) from the Centre for Environment, Fisheries and Aquaculture Science.

Since graduating from Bangor University in 2007 where Andy studied the population biology of the red gurnard (Chelidonichthys cuculus) in inshore waters of Eastern Anglesey his interest in fish otoliths, or ear stones as they are more commonly known, has grown. Otoliths are paired calcified structures, situated in semi-circular canals either side of the brain and are used by the fish as auditory and/or balance organs. Their use as indicators in identifying the age of fish has been well documented with the first observations of otolith growth increments “annuli” by Reibisch in 1899. These annuli grow throughout the life of the fish creating banding patterns and can be a reliable way of estimating daily, weekly, monthly and yearly growth patterns. However, it is their use as an environmental recorder and biogeochemical tag which has accelerated the use of these metabolically inert structures during the last decade. The use of otolith microchemistry has enabled scientists to get a better understanding of fish migratory patterns, stock identification and the detection of Diadromy (fish moving between fresh and marine waters) with the reconstruction of temperature and salinity histories.

Variations observed in the chemical composition of fish otoliths, such as changes in trace elements incorporated within the otolith matrix, has enabled the discrimination between the life histories of fish species which spend some part of their life residing in water bodies which differ in their chemical composition. For example, moving between marine and freshwater (in either direction), or moving between bodies of water in either the marine or freshwater environment with distinct (natural or anthropogenic) water chemistry. These elemental tags may provide valuable information on movement patterns of larval and juvenile fish, may enable the identification of distinct “groups” of fish based on spawning or nursery location and allow the study of connectivity and habitat utilisation in fish populations.

Working with the BGS, one part of Andy’s PhD project has been to examine whether otolith microchemistry can be used to identify the rivers of origin of juvenile brown trout parr (Salmo trutta) collected from 36 rivers in NW England, Wales, Isle of Man, SW Scotland and the east coast of Ireland which drain into the Irish Sea. The ultimate aim of this research being to determine whether river- or region-specific chemical tags exist in the otoliths which may be used to identify sea-caught sea trout (the form of S. trutta that migrates to sea to feed) back to their region or river of origin.

Otoliths removed from trout parr have been analysed using solution-based inductively-coupled plasma mass spectrometry (sb-ICP-MS) to measure the element: Ca ratios of Mg, Mn, Sr and Ba and data analyses are currently underway to determine whether these biogeochemical tags can be used to classify the trout parr back to source. Relating the otolith chemical tag in a given catchment to the underlying geological bedrock formations and water chemistry may also assist in our understanding of why the trout parr are being classified back to particular locations. “Otoliths may be acting as little “white box recorders” of the entire life history of the fish with the differences in ambient water and various environmental conditions experienced by the fish all stored within the calcified structures. The exciting bit is trying to untangle the secrets held within the otoliths to tell us the life story of that fish”.

Future work involving the analysis of otoliths collected from adult sea trout may allow the identification of those adults back to their regions (or even rivers) of origin using the chemical tags identified in the juvenile trout parr. The validity of using trace elements as biogeochemical tags found within fish otoliths of brown trout parr may also assist in future conservation and management planning for this species.

Andy Marriott, Andrew.lewis.marriott@gmail.com

Nuffield Fish Laboratory, Bangor University, UK.

References:

Marriott, A. L., Latchford, J. W. and McCarthy, I. D. 2010. Population biology of the red gurnard (Aspitrigla cuculus L.; Triglidae) in the inshore waters of Eastern Anglesey and Northwest Wales. Journal of Applied Ichthyology. 26, 504-512.

http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0426.2010.01455.x/full?globalMessage=0

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