World Aquaculture Magazine - June 2020

54 JUNE 2020 • WORLD AQUACULTURE • WWW.WA S.ORG been realized, as exemplified by almost half a century of R&D with Atlantic salmon (Benfey 2016). Among other things, this has been attributed to reduced temperature and hypoxia tolerance (Sambraus et al. 2017, 2018). A well-documented outcome of triploidy in fish is a dramatic increase in cell volume that is accompanied by a reduction in cell numbers, resulting in a lower ratio of cellular surface area to volume and, hence, less total cellular surface area for a given tissue type (Benfey 1999). This is predicted to affect any processes that are limited by cellular surface area, including respiratory gas exchange. To develop a better understanding of the oft- reported reduced performance of triploids, we are investigating how the interaction between temperature and oxygen availability affects metabolism and environmental tolerances in triploids, using the brook charr Salvelinus fontinalis as a model salmonid. One of our current approaches examines the effect of environmental temperature (9-18 C, i.e., both below and above the optimum 15 C for diploids) on aerobic scope, which is calculated as the difference between standard metabolic rate (SMR; when the animal is undisturbed, minimally active and in a post-absorptive state) and maximummetabolic rate (MMR; when the animal is exercised to the point of exhaustion). Aerobic scope is thus a useful measure of the ability to cope with stress. Both SMR and MMR are determined by measuring mass-specific oxygen consumption rates of individual fish within swim-flume respirometers (Fig. 6) that can be monitored remotely to minimize stress on the animals and have adjustable circulation rates to regulate swimming speed. Preliminary results indicate that triploids, on average, have lower aerobic scope than diploids at all environmental temperatures and, critically, that the difference between triploids and diploids increases with increasing temperature. Another of our current approaches examines the effect of environmental temperature on hypoxia tolerance in fish that have been acclimated to 15 or 18 C. Using the methodology outlined by Benfey and Devlin (2018), we rapidly raise water temperature using a standard critical thermal maximum (CTmax) protocol in an insulated chamber and then determine the oxygen tension at which fish lose equilibrium when compressed nitrogen is used to drive oxygen out of the water (Fig. 7). Preliminary results show that triploids are, on average, less tolerant of hypoxia than diploids at any temperature above 18 C and, just as with aerobic scope, the divergence between ploidies increases with increasing temperature. Independent of ploidy, acclimation to 18 C improved hypoxia tolerance at high temperatures, with triploids acclimated to 18 C outperforming diploids acclimated to 15 C. Taken together, these results highlight that triploids are fundamentally different from diploids in their ability to function at elevated temperatures and that, just as for diploids, their ability to cope with elevated temperatures can be improved through temperature acclimation. Sterility Induction by Disrupting Early Germ Cell Development Ten-Tsao Wong and Yonathan Zohar In conjunction with growing aquaculture activities, an effective containment strategy for large-scale commercial aquaculture operations is desperately needed to achieve environmental sustainability. The most proficient genetic containment strategy possible is the use of reproductively sterile fish. Sterility carries environmental significance along with economic benefits. Escaped infertile individuals are not able to propagate and/or interbreed with wild stocks. In addition, sterilization minimizes energy input toward gonadal growth while enhancing muscle (flesh) development and promoting overall health. Many farmed aquatic animals attain sexual maturity before reaching market size. Sexual maturation is associated with a substantial decrease in somatic growth due to the diversion of energy into development of gonads. The period of intensive gonadal growth also results in deterioration of flesh quality and suppresses the immune systems, increasing susceptibility to stress and disease (Manzoor Ali and Satyanarayana Rao 1989, Zohar 1989). As a consequence, the incidence of sexual and early maturation can have significant economic consequences for aquaculture companies. Furthermore, sterility is a means for FIGURE 6. Adult male brook charr in a swim-flume respirometer. FIGURE 7. Experimental set-up for regulating water temperature and oxygen tension for assessing hypoxia tolerance (A = calibrated thermometer, B = temperature probe, C = in-flow for compressed nitrogen, D = oxygen probe for calibrated dissolved oxygen meter, and E = immersion heater).