Thi Hong Gam Le, Montgomery Daniel W, Laronde Daniel S, Mackinnon Rachael, Richards Jeffrey G, Brauner Colin J
Department of Zoology, The University of British Columbia, Vancouver, British Columbia, Canada.
J Fish Biol. 2024 Oct 8. doi: 10.1111/jfb.15957.
There is a growing interest in Atlantic salmon (Salmo salar) aquaculture to extend the time fish are reared in freshwater (FW) recirculating aquaculture systems (RAS), producing larger FW salmon that can then be induced to undergo smoltification before transfer into marine net pens for grow-out and harvest. Smolts can be produced by photoperiod (PT) manipulation in RASs, but little is known about how delaying smoltification to larger body sizes affects susceptibility to elevated CO levels (hypercapnia), which can occur at high stocking densities in FW RAS or during transport from FW RAS rearing facilities to marine net pens. To address this, Atlantic salmon were reared from hatch to one of three different sizes (~230, ~580, or ~1300 g) in FW (3 ppt) under continuous light (24:0, light:dark). Once fish reached the desired sizes, a group of salmon were maintained on continuous light 24L:0D to serve as a control salmon. A second group of salmon were exposed to 8 weeks of 12L:12D and then to 4 weeks of 24L:0D to serve as PT treatment salmon, which is the PT manipulation commonly used in Atlantic salmon aquaculture to induce smoltification. At the end of PT manipulation, both control and PT treatment salmon were exposed to 0% or 1.5% CO (30 mg/L) for 96 h in FW and then transferred to air-equilibrated seawater (SW, 35 ppt, normocapnia). Salmon were sampled at the end of the 96-h FW CO exposure and at 24 h and 7 days in SW for measurements of blood ion/acid-base status, muscle water content (MWC), and gill and kidney Na/K ATPase (NKA) activity. Exposure to 96 h of CO in FW resulted in acid-base disturbances in fish from all three size classes, with decreases in blood pH and increases in blood PCO and plasma [HCO ] but no mortality. Despite these large acid-base disturbances in FW, after transfer to normocapnic SW, there were no significant effects of CO exposure on extracellular blood pH, intracellular red blood cell pH, or plasma osmoregulatory status for all three sizes of post-smolt salmon. In general, SW transfer was associated with significant increases in plasma ions and osmolality, as well as gill and kidney NKA activity after 24 h and 1 week in SW with no significant impacts between different sizes of salmon. Thus, exposure to 30 mg CO/L that mimics levels experienced during transport from FW RAS to an SW transfer site may have minimal effects on Atlantic salmon smolts up to 1300 g.
人们对大西洋鲑鱼（Salmo salar）养殖的兴趣日益浓厚，希望延长鱼类在淡水循环水养殖系统（RAS）中的饲养时间，培育出更大的淡水鲑鱼，然后诱导其进行降海洄游，再转移到海洋网箱中进行育肥和收获。可以通过在RAS中操纵光周期（PT）来培育降海型幼鲑，但对于将降海洄游推迟到更大体型时如何影响其对高二氧化碳水平（高碳酸血症）的易感性知之甚少，高碳酸血症可能发生在淡水RAS的高放养密度下，或在从淡水RAS养殖设施运输到海洋网箱的过程中。为了解决这个问题，将大西洋鲑鱼从孵化开始在淡水（3‰）中连续光照（24:0，光:暗）下饲养到三种不同大小（约230、约580或约1300克）中的一种。一旦鱼达到所需大小，一组鲑鱼保持连续光照24L:0D作为对照鲑鱼。第二组鲑鱼先暴露于8周的12L:12D，然后暴露于4周的24L:0D作为PT处理鲑鱼，这是大西洋鲑鱼养殖中常用的诱导降海洄游的PT操纵方法。在PT操纵结束时，对照和PT处理鲑鱼都在淡水中暴露于0%或1.5%的二氧化碳（30毫克/升）96小时，然后转移到空气平衡的海水（SW，35‰，正常碳酸血症）中。在96小时的淡水二氧化碳暴露结束时以及在海水中24小时和7天时对鲑鱼进行采样，以测量血液离子/酸碱状态、肌肉含水量（MWC）以及鳃和肾脏的钠/钾ATP酶（NKA）活性。在淡水中暴露于96小时的二氧化碳导致所有三个大小类别的鱼出现酸碱紊乱，血液pH值降低，血液PCO和血浆[HCO]升高，但没有死亡。尽管在淡水中出现了这些严重的酸碱紊乱，但转移到正常碳酸血症的海水中后，对于所有三种大小的降海型幼鲑，二氧化碳暴露对细胞外血液pH值、细胞内红细胞pH值或血浆渗透压调节状态均无显著影响。一般来说，转移到海水中与血浆离子和渗透压显著增加以及在海水中24小时和1周后鳃和肾脏NKA活性增加有关，不同大小的鲑鱼之间没有显著影响。因此，暴露于30毫克/升的二氧化碳（模拟从淡水RAS运输到海水转移地点时经历的水平）对重达1300克的大西洋鲑鱼降海型幼鲑可能影响最小。
