Amoebic gill disease increases energy requirements and decreases hypoxia tolerance in Atlantic salmon (Salmo salar) smolts.

Globally, Atlantic salmon (Salmo salar Linnaeus) aquaculture is now routinely affected by amoebic gill disease (AGD; Neoparamoeba perurans). The disease proliferates throughout the summer and is implicated in decreasing tolerance of salmon to environmental perturbations, yet little empirical evidence exists to support these observations. Using salmon acclimated to 15 or 19 °C, our aim was to determine the effects of clinically light-moderate (industry-relevant) AGD on metabolism (MO2rest and MO2max), aerobic scope (MO2max - MO2rest), excess post-exercise oxygen consumption (EPOC), and hypoxia tolerance. An increase in MO2rest (~8% and ~ 13% increase within the 15 and 19 °C acclimation groups, respectively) with increasing disease signs demonstrated an increase in baseline energy requirements as the disease progressed. Conversely, MO2max remained stable at both temperatures (~364 mg O2 kg-1 h-1), resulting in a decline in aerobic scope by 13 and 19% in the 15 and 19 °C groups, respectively. There was evidence of a decrease in hypoxia tolerance as the dissolved oxygen concentrations at loss of equilibrium increased by ~8% with more severe lesion coverage of the gills. These results suggest an increase in basal energy requirements and reduction in hypoxia tolerance as AGD proliferates, lending support to the idea that AGD reduces environmental tolerance. However, the lack of an effect of acclimation temperature indicates that the temperature-disease interaction may be more complicated than currently thought.

Aerobic and anaerobic movement energetics of hybrid and pure parental abalone.

The underlying mechanisms controlling growth heterosis in marine invertebrates remain poorly understood. We used pure blacklip (Haliotis rubra) and greenlip (Haliotis laevigata) abalone, as well as their hybrid, to test whether differences in movement and/or aerobic versus anaerobic energy use are linked to a purported increased growth rate in hybrids. Abalone were acclimated to control (16 °C) and typical summer temperatures (23 °C), each with oxygen treatments of 100% air saturation (O2sat) or 70% O2sat. The experiment then consisted of two phases. During the first phase (chronic exposure), movement and oxygen consumption rates (MO2) of abalone were measured during a 2 day observation period at stable acclimation conditions. Additionaly, lactate dehydrogenase (LDH) and tauropine dehydrogenase (TDH) activities were measured. During phase two (acute exposure), O2sat was raised to 100% for abalone acclimated to 70% O2sat followed by an acute decrease in oxygen to anoxia for all acclimation groups during which movement and MO2 were determined again. During the chronic exposure, hybrids and H. laevigata moved shorter distances than H. rubra. Resting MO2, LDH and TDH activities, however, were similar between abalone types but were increased at 23 °C compared to 16 °C. During the acute exposure, the initial increase to 100% O2sat for individuals acclimated to 70% O2sat resulted in increased movement compared to individuals acclimated to 100% O2sat for hybrids and H. rubra when compared within type of abalone. Similarly, MO2 during spontaneous activity of all three types of abalone previously subjected to 70% O2sat increased above those at 100% O2sat. When oxygen levels had dropped below the critical oxygen level (Pcrit), movement in hybrids and H. laevigata increased up to 6.5-fold compared to movement above Pcrit. Differences in movement and energy use between hybrids and pure species were not marked enough to support the hypothesis that the purportedly higher growth in hybrids is due to an energetic advantage over pure species.