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|Title:||The Physiological Adaptations of Two Salmonids, the Rainbow trout (Oncorhynchus mykiss) and the High pH Tolerant Lahontan Cutthroat Trout (Oncorhynchus clarki henshawi), to Highly Alkaline Environments|
|Authors:||Wilkie, Patrick Michael|
|Advisor:||Wood, Chris M.|
|Abstract:||<p>Efforts to stock salmonids into alkaline waters have had little success. Accordingly,<br />this thesis examined the survival and physiology of two salmonids, the rainbow trout (RBT;<br />Oncorhynchus mykiss) and the high pH tolerant Lahontan cutthroat trout (LCT; Oncorhynchus<br />clarki henshawi) experimentally exposed to alkaline pH. ExPosure of RBT (acclimated at pH<br />8.0) to pH 9.5, for up to 4 weeks, led to minimal mortality and was characterized by<br />transiently inhibited ammonia excretion rates (JAmm), and chronically elevated plasma and<br />white muscle total ammonia stores (TAmm = NH₃ + NH₄⁺). Elevated plasma TAmm facilitated recovery of JAmm by maintaining positive NH₃, gradients across the gill epithelium. Temporarily elevated urea excretion rates prevented internal TAmm from reaching toxic levels<br />while JAmm was inhibited. A chronic respiratory alkalosis (decreased blood Pcon) rapidly developed at high pH, but a counterbalancing metabolic acidosis (decreased blood [HCO₃⁻]), which initially resulted from elevated white muscle lactic acid production, stabilized blood pH after 1 day. Transient and chronic decreases in gill Cl⁻ transporter (J^cl max) and Na⁺ transporter (J^Na max) number, respectively, resulted in decreased Cl⁻ and Na⁺ uptake and led to reduced plasma [Cl⁻] and [Na⁺]. A complete recovery of J^cl max (by 3 days) and counterbalancing reductions in gill Na⁺ losses, re-estahlished internal ion balance beyond 3<br />days. In contrast, the LCT (reared at pH 8.4) never before exposed to alkaline pH, rapidly<br />adapted to the alkaline (pH 9.4) waters of Pyramid Lake, Nevada, their natural adult habitat.<br />These adaptions included permanently reduced basal ammonia production which was reflected in chronically lowered JAmm and minimal, temporary increases in plasma TAmm. Chronic elevations in plasma NH₃, were not required for maintenance of JAmm in the LCT. The LCT also underwent a chronic respiratory alkalosis and a transient metabolic alkalosis but increases in gill chloride cell exposure quickly mitigated the continual base load by facilitating HCO₃⁻ excretion. No disturbances to internal ion balance were observed. The responses of LCT to challenge in more alkaline (pH 10) lake water resembled those of RBT at pH 9.5 (15ºC) and greatly increased plasma NH, and reduced plasma ion concentrations, resulted in 50 %<br />mortality by 72 h. In conclusion, alterations in N-waste metabolism and gill functional<br />morphology facilitate salmonid survival at high pH. However, the ability of LCT to rapidly<br />make these adjustments, with minimal disblrbance to its pre-exposure physiological status,<br />enables it to survive in an environment which is unsuitable for other salmonids.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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