M. Sackville

Physiological consequences of the salmon louse (Lepeophtheirus salmonis) on juvenile pink salmon (Oncorhynchus gorbuscha): implications for wild salmon ecology and management, and for salmon aquaculture.

Pink salmon, Oncorhynchus gorbuscha, are the most abundant wild salmon species and are thought of as an indicator of ecosystem health. The salmon louse, Lepeophtheirus salmonis, is endemic to pink salmon habitat but these ectoparasites have been implicated in reducing local pink salmon populations in the Broughton Archipelago, British Columbia. This allegation arose largely because juvenile pink salmon migrate past commercial open net salmon farms, which are known to incubate the salmon louse. Juvenile pink salmon are thought to be especially sensitive to this ectoparasite because they enter the sea at such a small size (approx. 0.2 g). Here, we describe how ‘no effect’ thresholds for salmon louse sublethal impacts on juvenile pink salmon were determined using physiological principles. These data were accepted by environmental managers and are being used to minimize the impact of salmon aquaculture on wild pink salmon populations.

Swimming performance and associated ionic disturbance of juvenile pink salmon Oncorhynchus gorbuscha determined using different acceleration profiles.

Swimming performance was assessed in juvenile pink salmon Oncorhynchus gorbuscha (body mass<5.0 g) using five different protocols: four constant acceleration tests each with a different acceleration profile (rates of 0.005, 0.011, 0.021 and 0.053 cm s(-2)) and a repeated ramped-critical swimming speed test. Regardless of the swim protocol, the final swimming speeds did not differ significantly (P>0.05) among swim tests and ranged from 4.54 to 5.20 body lengths s(-1). This result supports the hypothesis that at an early life stage, O. gorbuscha display the same fatigue speeds independent of the swimming test utilized. Whole body and plasma [Na+] and [Cl-] measured at the conclusion of these tests were significantly elevated when compared with control values (P<0.05) and appear to be predominantly associated with dehydration rather than net ion gain. Given this finding for a small salmonid, estimates of swim performance can be accurately measured with acceleration tests lasting<10 min, allowing a more rapid processing than is possible with a longer critical swim speed test.

Hagfish: Champions of CO2 tolerance question the origins of vertebrate gill function

The gill is widely accepted to have played a key role in the adaptive radiation of early vertebrates by supplanting the skin as the dominant site of gas exchange. However, in the most basal extant craniates, the hagfishes, gills play only a minor role in gas exchange. In contrast, we found hagfish gills to be associated with a tremendous capacity for acid-base regulation. Indeed, Pacific hagfish exposed acutely to severe sustained hypercarbia tolerated among the most severe blood acidoses ever reported (1.2 pH unit reduction) and subsequently exhibited the greatest degree of acid-base compensation ever observed in an aquatic chordate. This was accomplished through an unprecedented increase in plasma [HCO3−] (>75 mM) in exchange for [Cl−]. We thus propose that the first physiological function of the ancestral gill was acid-base regulation, and that the gill was later co-opted for its central role in gas exchange in more derived aquatic vertebrates.

Water pH limits extracellular but not intracellular pH compensation in the CO2 tolerant freshwater fish, Pangasianodon hypophthalmus

ABSTRACT Preferentially regulating intracellular pH (pHi) confers exceptional CO2 tolerance on fish, but is often associated with reductions in extracellular pH (pHe) compensation. It is unknown whether these reductions are due to intrinsically lower capacities for pHe compensation, hypercarbia-induced reductions in water pH or other factors. To test how water pH affects capacities and strategies for pH compensation, we exposed the CO2-tolerant fish Pangasianodon hypophthalmus to 3 kPa PCO2 for 20 h at an ecologically relevant water pH of 4.5 or 5.8. Brain, heart and liver pHi was preferentially regulated in both treatments. However, blood pHe compensation was severely reduced at water pH 4.5 but not 5.8. This suggests that low water pH limits acute pHe but not pHi compensation in fishes preferentially regulating pHi. Hypercarbia-induced reductions in water pH might therefore underlie the unexplained reductions to pHe compensation in fishes preferentially regulating pHi, and may increase selection for preferential pHi regulation. Summary: Low water pH limits extracellular pH compensation in a CO2-tolerant fish. This may increase selection for a more robust CO2 defence strategy where intracellular pH is preferentially regulated.

Case Study: Gill Plasticity in Larval Fishes

Adult teleosts possess an impressive degree of plasticity with respect to gas exchange and ion regulation. The gill is central to this plasticity, serving as the dominant site for both of these processes. Indeed, reversible changes to gill morphology can modify the oxygen and ion transport cascades such that flux is maintained despite changes to supply and demand. Gill plasticity thus provides clear selective benefits, allowing teleosts to thrive in habitats with fluctuating levels of oxygen and ions, and likely contributing to their broad distribution. However, this differs markedly in early life stages. Here, we use rainbow trout as a case study to review gill plasticity during larval development with respect to gas exchange and ion regulation. Surprisingly, the larval gill exhibits little plasticity relative to the adult form. Gill morphology is mostly fixed early on, and plasticity appears limited to changes in convection and protein function (hemoglobin isoform, ion affinity, Na+, K+ ATPase activity, tight junction permeability). Larvae thus have a reduced capacity to maintain trans-epithelial gas and ion flux in hypoxic and ion-poor water, respectively. Interestingly, larval development is slowed by hypoxia, but not by reduced ion uptake in ion-poor water. We propose that this limited gill plasticity is associated with selection for rapid development and minimal endogenous reserves when larvae rear in a stable environment on a fixed energy budget. As a consequence, this limited gill plasticity may contribute to the survival bottleneck often associated with larval development. However, these ideas are untested and based on few studies. We therefore conclude this review with several research directions aimed to provide a more comprehensive understanding of gill plasticity in larval rainbow trout specifically, and teleosts generally.