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".