Hypo-osmoregulatory capacity during smolting of endangered inner Bay of Fundy Atlantic salmon and other eastern Canadian stocks

Inner Bay of Fundy (iBoF) Salmo salar are at risk of extinction due to high losses during the marine phase; cause unknown. As a first step towards testing the hypothesis that compromised smoltification is implicated, we investigated whether genetic differences in hypo-osmoregulatory ability exist between two iBoF stocks (Stewiacke and Gaspereau) and two other Nova Scotia stocks (Sackville and Medway), all reared under the same conditions from the egg stage. In addition, LaHave and Saint John stocks were included, though their rearing history differed from the other four stocks. Plasma osmolality and muscle water content following biweekly 96 h 35 ppt salinity challenge tests (constant 6 °C), was significantly affected by the interaction between genotype and challenge test date. iBoF Stewiacke stock exhibited relatively poor hypo-osmoregulatory ability during April and early May compared to the other five stocks, indicating a later completion of smolting. By comparison, the iBoF Gaspereau stock exhibited similar temporal changes in hypo-osmoregulatory ability to the other stocks. From late May to early July, all stocks exhibited similarly good hypo-osmoregulatory ability. This unusually long smolt window was associated with rearing at 6 °C in well water. Establishing the osmoregulatory capacity of iBoF smolts under ‘ideal’ lab conditions is useful baseline knowledge that may assist future studies distinguish normal from abnormal among wild or restocked fish.