An eco-physiological investigation of fisheries-induced evolution : comparing the resilience of larvae from exploited and unexploited commercial reef fish populations to projected ocean acidification
Authors:   , Muller Cuen 
Published: April 2022

It is now accepted that anthropogenic-induced climate change is resulting in unprecedented rates of change to marine environments. Marine organisms are being challenged by rapidly increasing temperatures, acidification, expansion of oxygen dead zones, and higher frequencies and magnitudes of extreme weather events. Exploited fish populations are also undergoing selective harvesting. Certain traits, such as large size, fast growth, and/or bold/active behaviours, are being actively targeted and removed from the population gene pool. This selective removal of individuals may compromise the capacity of fish populations to resist or recover from environmental disturbances and reduce their ability to adapt to a changing environment as many of these traits are heritable. As most marine fishes' embryonic and larval stages represent the period when individuals are most sensitive to environmental disturbances, they are a critical bottleneck to population persistence in the face of exploitation and climate change. This thesis aimed to quantify and compare the metabolic physiology, growth, and development of an exploited and endemic sparid, the roman seabream Chrysoblephus laticeps, during the early larval stages under 1) ocean acidification conditions expected by the year 2100 and 2) from populations experiencing dissimilar rates of exploitation. To quantify and compare the physiology of larvae, adult C. laticeps from an exploited population were captured and field-spawned. Fertilised eggs were placed into either control/present-day conditions (pH = 8.03, pCO2 ≈ 420 μatm) or high-pCO2/hypercapnic treatment conditions (pH = 7.63, pCO2 ≈ 1400 μatm). The metabolic physiology of individual larvae was determined using a novel rolling-regression technique on static respirometry data. Here, estimates of the minimum and maximum oxygen consumption rates (VO2) could be determined with high test-retest reliability. The very early developmental stages (yolk-sac stage) appeared resilient to high pCO2 conditions despite being exposed to treatment conditions throughout the embryonic stage. Preflexion larvae showed sensitivity to treatment conditions by exhibiting reduced metabolic and growth rates, consistent with metabolic depression, associated with environmental stress. However, by the onset of flexion, which coincides with gill development, acid-base regulation, and muscle differentiation, metabolic and growth rates of treatment larvae were significantly greater than that of controls. This suggests that acid-base regulation imposes a high cost to maintain internal pH homeostasis. Importantly, these elevated metabolic costs were likely mediated through increased feeding rates in experimental conditions where food was ad libitum. In natural conditions, where food availability may be varied, high pCO2 conditions could be associated with higher mortality rates. Based on evidence that protected/unexploited populations are more genetically diverse and are composed of individuals representing a greater range of metabolic phenotypes, offspring were collected from a protected population experiencing otherwise similar environmental conditions to the exploited population. Metabolic rates of control larvae were generally similar to those of the exploited population. However, minimum rates of VO2 were typically higher for larvae from the protected population at comparable life stages. Preflexion treatment larvae from the protected population did not appear to undergo a period of reduced metabolism or growth compared to their control counterparts. While metabolic rates at the onset of flexion were significantly higher for treatment larvae, this was not associated with growth differences. Growth over-compensation following periods of growth depression is often associated with deleterious effects, such as organ damage and body or developmental malformations. This suggests somewhat improved resilience to ocean acidification conditions. This thesis found evidence that larval C. laticeps are sensitive to ocean acidification conditions expected by 2100. When this stressor is combined with increasing thermal variability, changing current coastal regimes, and heterogeneous food availability, also expected to occur by 2100, ocean acidification may compromise the population persistence of this species. However, an energetics approach to stress-tolerance suggests that larvae from the protected population may inherently show greater resilience to climate change-related environmental stressors. Evidence that exploitation affects the resilience of fish larvae to climate change highlights the need for an evolutionary approach to fisheries management and the importance of spatial protection in maintaining larger and more resilient populations while providing the raw material essential for adaptation.