Otoliths are dense calcium carbonate structures found within the cranium, that put pressure on cilia and stimulate sensory hair cells, enabling a fish to hear. The chemical structure of otoliths is typically aragonite, which is a more dense form of calcium carbonate that allows the otolith to put pressure on the cilia that enable the fish to hear. Aragonite otoliths can shift to the less dense polymorph, vaterite. The otolith then becomes irregularly shaped and impairs the fish’s ability to hear. Previous studies found that otoliths can shift their chemical structure, especially in recirculating aquacultured settings. Many studies have suggested possible factors such as increased growth, temperature, and genetic predisposition. However, evidence suggests that increased levels of dissolved carbon dioxide (DCO2) may be the causative agent in the shift of otolith composition .
A preliminary study was conducted to demonstrate the effects of DCO2 in juvenile rainbow trout (Oncorhynchus mykiss), which are commonly aquacultured. Nine fish tanks (110L) were set up to test the effects of DCO2 on the otolith’s structures in rainbow trout. CO2 was dispersed through airflow meters, and three tanks each received 0mg/L, 15mg/L, and 30 mg/L of additional CO2 . Weight and length were measured every two weeks. Sagittal o tolith samples were taken at the start of the experiment and every 4 weeks over the course of the 8-week experiment.
A Micro Raman Spectrometer (WiTec Alpha 300 Raman, equipped with a 785 nm excitation laser) was used to determine the chemical structure of the otoliths. The number of fish in each treatment displaying the vateritic signature (Figure 1) was quantified and analyzed using an ANOVA for each sampling point (start, 4, and 8 weeks). Vateritic otoliths were seen as early as 4 weeks in the highest CO2 treatments . Growth, both in terms of length and weight, were also significantly affected by higher levels of DCO2. The analysis of this preliminary data strongly suggests that DCO2 is a causative agent for vateritic otoliths in recirculating aquaculture systems and that more emphasis should be placed on the significance of DCO2 to improve the welfare of aquacultured fish . This may be especially important in fish used for restocking programs and may help to improve the success rate.