Individual variability in the thermal limits of juvenile dusky kob Argyrosomus japonicus from a permanently open estuary

Abstract

Globally, fish are facing the pressure of increasing thermal variability, which challenges the physiological capacity and resilience of species at the level of the individual, population and community. As transition zones, estuarine ecosystems are becoming increasingly thermally dynamic, which affects their condition and function as nursery habitats. Assessing the thermal tolerance of estuary-associated species is critical to understanding their adaptive capacity to these increasingly variable environments. This study aimed to assess the thermal tolerance of dusky kob Argyrosomus japonicus to understand their capacity to adapt to thermal variability. Thermal tolerance was estimated for individual fish using a repeated measures (CT_min and CT_max) dynamic method, while physiological breakpoints were estimated by running a piecewise linear breakpoint analysis on estimates of the respiration rate (opercula beat counts) at every 1 °C degree change. Among 12 individuals, the thermal endpoint ranged from 8.1 °C to 32.6 °C, with an overall thermal breadth of 24.5 °C. Piecewise linear breakpoint analysis of respiration rate identified a lower breakpoint at 13 °C and an upper breakpoint of 28 °C, suggesting an optimal thermal range between these temperatures. There was considerable variability in the thermal endpoints and breakpoints of the individuals. Using a percentile ranking method, fish were categorised as broadly tolerant, intermediate or narrowly tolerant thermal performers. The broad thermal breadth and diversity in thermal physiological phenotypes suggests that this species may have considerable capacity to adapt to an increasingly variable environment. However, since environmental temperatures may exceed the upper breakpoint in upper reaches of an estuary and fall below the lower breakpoint near the mouth of the estuary in summer months, it is likely that further increases in variability may have implications for the way in which the species utilises the estuarine environment in the future.