Otolith – Fish ageing
Big Mamma: The Story of a Legendary Tigerfish
One of the most unforgettable moments from my fieldwork came when we encountered a remarkable tigerfish we nicknamed "Big Mamma." Weighing in at a staggering 19.8 pounds, one of the largest recorded individuals in Lake Jozini.
The Mavungana Flyfishing team tried to revive her by gently moving her through the water to reoxygenate her gills. Unfortunately, after a while, her gills stopped moving altogether. When we opened her up, we discovered she was a gravid female, full of eggs. Tragically, part of her reproductive system had ruptured, rotting in her abdominal cavity, and she succumbed to septicaemia.
Although we lost her, Big Mamma gave us something incredibly valuable—her story. We removed her otoliths, the tiny ear bones inside a fish’s head, to help us determine her age and life history.
What are otoliths, and why do they matter ?
Otoliths (the inner ear bones of fish) are paired calcified structures used for balance and hearing in fish and are primarily composed of calcium carbonate minerals (Campana 1999). Otoliths form by adding alternating layers of protein (otolin) and calcium carbonate (Gillanders and Kingsford 2000; Paxton et al. 2013). What makes them fascinating is that they grow in distinct layers throughout a fish’s life, much like the rings in a tree trunk. These layers form due to environmental changes and seasonal growth patterns. Otoliths have continual growth from before the time of hatch to the time of death, thus, otoliths provide a chemical chronology over the entire lifetime of a fish (Campana 1999; Campana et al. 2000).
By examining these layers, scientists can determine not just the age of a fish, but also clues about its growth rate, maturity, and life history. This method of ageing fish is widely accepted in fisheries science.
So how old was Big Mamma ?
After preparing and sectioning the otoliths, we counted 16 distinct growth rings—meaning Big Mamma was likely around 16 years old. That’s an impressive age for a tigerfish and gives us critical insight into the longevity of these top predators.
Her loss was heartbreaking, but her legacy will live on through the data she provided, helping us better understand and conserve the species in the years to come.
BLOG BY : Angelica Kaiser - Junior Researcher, Ph.D. Candidate
References:
Campana, S.E. 1999. Chemistry and composition of fish otoliths: Pathways, mechanisms and applications. Marine Ecology Progress Series, 188: 263–297.
Gillanders, B.M. and Kingsford, M.J. 2000. Elemental fingerprints of otoliths of fish may distinguish estuarine ‘nursery’ habitats. Marine Ecology Progress Series, 201: 273–286.
Paxton, B.R., O’Brien, G.C., Slingsby, J.A. and Copin, R. 2013. Validation of the periodicity of growth increment deposition in otoliths from the larval and early juvenile stages of two cyprinids from the Orange-Vaal river system, South Africa. African Journal of Aquatic Science, 38: 49–54.
Campana, S.E., Chouinard, G.A., Hanson, J.M., Fréchet, A. and Brattey, J. 2000. Otolith elemental fingerprints as biological tracers of fish stocks. Fisheries Research, 46: 343–357.