The potential impact of ocean acidification upon eggs and larvae of yellowfin tuna (Thunnus albacares)

Anthropogenic carbon dioxide (CO₂) emissions are resulting in increasing absorption of CO₂ by the earth's oceans, which has led to a decline in ocean pH, a process known as ocean acidification (OA). Evidence suggests that OA may have the potential to affect the distribution and population dynamics of many marine organisms. Early life history processes (e.g. fertilization) and stages (eggs, larvae, juveniles) may be relatively more vulnerable to potential OA impacts, with implications for recruitment in marine populations. The potential impact of OA upon tuna populations has not been investigated, although tuna are key components of pelagic ecosystems and, in the Pacific Ocean, form the basis of one of the largest and most valuable fisheries in the world. This paper reviews current knowledge of potential OA impacts on fish and presents results from a pilot study investigating how OA may affect eggs and larvae of yellowfin tuna, Thunnus albacares. Two separate trials were conducted to test the impact of pCO₂ on yellowfin egg stage duration, larval growth and survival. The pCO₂ levels tested ranged from present day (~400μatm) to levels predicted to occur in some areas of the spawning habitat within the next 100 years (<2500μatm) to 300 years (~<5000μatm) to much more extreme levels (~10,000μatm). In trial 1, there was evidence for significantly reduced larval survival (at mean pCO₂ levels≥4730μatm) and growth (at mean pCO₂ levels≥2108μatm), while egg hatch time was increased at extreme pCO₂ levels≥10,000μatm (^*intermediate levels were not tested). In trial 2, egg hatch times were increased at mean pCO₂ levels≥1573μatm, but growth was only impacted at higher pCO₂ (≥8800μatm) and there was no relationship with survival. Unstable ambient conditions during trial 2 are likely to have contributed to the difference in results between trials. Despite the technical challenges with these experiments, there is a need for future empirical work which can in turn support modeling-based approaches to assess how OA will affect the ecologically and economically important tropical tuna resources.