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.

Water balance trumps ion balance for early marine survival of juvenile pink salmon (Oncorhynchus gorbuscha).

Smolting salmonids typically require weeks to months of physiological preparation in freshwater (FW) before entering seawater (SW). Remarkably, pink salmon (Oncorhynchus gorbuscha) enter SW directly following yolk absorption and gravel emergence at a size of 0.2 g. To survive this exceptional SW migration, pink salmon were hypothesized to develop hypo-osmoregulatory abilities prior to yolk absorption and emergence. To test this, alevins (pre-yolk absorption) and fry (post-yolk absorption) were transferred from FW in darkness to SW under simulated natural photoperiod (SNP). Ionoregulatory status was assessed at 0, 1 and 5 days post-transfer. SW alevins showed no evidence of hypo-osmoregulation, marked by significant water loss and no increase in gill Na⁺/K⁺-ATPase (NKA) activity or Na⁺:K⁺:2Cl⁻ cotransporter (NKCC) immunoreactive (IR) cell frequency. Conversely, fry maintained water balance, upregulated gill NKA activity by 50 %, increased the NKA α1b/α1a mRNA expression ratio by sixfold and increased NKCC IR cell frequency. We also provide the first evidence of photoperiod-triggered smoltification in pink salmon, as fry exposed to SNP in FW exhibited preparatory changes in gill NKA activity and α1 subunit expression similar to fry exposed to SNP in SW. Interestingly, fry incurred larger increases in whole body Na⁺ than alevins following both SW and FW + SNP exposure (40 and 20 % in fry vs. 0 % in alevins). The ability to incur and tolerate large ion loads may underlie a novel mechanism for maintaining water balance in SW prior to completing hypo-osmoregulatory development. We propose that pink salmon represent a new form of anadromy termed "precocious anadromy".

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.