An experimental approach to assessing climate sensitivity in juvenile Yellowbelly Flounder (Rhombosolea leporina): Methodological insights and applications

Abstract

As anthropogenic CO₂ emissions continue to rise, marine ecosystems face the dual threats of ocean acidification and ocean warming. A number of studies have examined the effects of ocean acidification and warming on coastal marine fin fish species, but little attention has been paid to estuarine species, the assumption being that they are likely to be tolerant to fluctuations in seawater parameters given freshwater and large tidal dynamics. This thesis investigates the impacts of these stressors on the early life history of the estuarine yellowbelly flounder (Rhombosolea leporina), an ecologically and culturally significant taonga species in New Zealand.

Research was conducted in two phases; a larval pilot study focusing on the transition to exogenous feeding, and a behavioural study on juveniles assessing olfactory mediated responses to food and predator cues.

The larval pilot study, carried out in aquaria, revealed an extreme developmental bottleneck at approximately 14 days post-hatch (dph). Mass mortality occurred across all treatments, coinciding with the onset of metamorphosis and the “critical period” of first feeding. The extent of the mortality event masked any possible treatment effects imposed in terms of pH and temperature combinations. These results highlight the high physiological sensitivity of R. leporina during early ontogeny, independent of climate stressors.

In the second phase, juvenile flounder were exposed to a fully-crossed factorial design of pH (~7.3 & ~8.0) and temperature (20°C & 25°C). Behavioural trials using a two channel choice chamber measured the proportion of time spent in stimulus streams (Perna canaliculus food cues & Notolabrus celidotus predator cues). A three-way ANOVA revealed a significant main effect of stimulus type (p = 0.017) and temperature (p=0.040), but no significant effect of pH and no interactive effects. Interestingly, 66.7% of individuals remained in the neutral mixing zone, particularly during predator trials and under elevated temperatures.

These findings suggest that while R. leporina larvae remain highly vulnerable to intrinsic developmental bottlenecks, juveniles exhibit a degree of behavioural resilience to reduced pH, potentially due to their evolved tolerance of fluctuating estuarine environments. The preference for remaining stationary (crypsis) in the neutral zone is interpreted as a natural, risk-averse strategy for this benthic species. This research underscores the importance of considering species-specific ecology and life stage transitions when predicting the impacts of climate change on New Zealand fisheries.

Overall, this research apparently represents the first study to experimentally examine the effects of ocean acidification and warming on estuarine flatfish species across critical early life stages. Although no clear pH-driven effects were detected in the juvenile behavioural trials, the study remains valuable in testing hypotheses around the resilience of marine species that naturally experience highly variable environmental conditions. In particular, species with significant estuarine habitat phases may possess inherent tolerance to fluctuations in seawater chemistry, yet may still face vulnerability during key developmental transitions. Investigations such as this are therefore important for improving our understanding of how climate change stressors interact with species-specific ecology and life history bottlenecks, and for informing future research on the adaptive capacity of estuarine-associated marine fishes.