Comparative migration success of Atlantic salmon smolts through three large natural lakes

Migration through standing waterbodies can be challenging for Atlantic salmon smolts and has often been associated with delay, disorientation and elevated mortality rates. Despite the large number of watersheds featuring stillwaters, data on smolt migration through lakes are relatively limited, particularly in relation to large natural lakes. The current study investigated movements of Salmo salar smolts on seven tributaries across three large lakes in the north of Ireland. Migration success through lakes was highly variable across the study systems, ranging from 24% to 81%. Individual smolt length was not a significant predictor of successful lake migration, whilst tributary of origin was a significant predictor for successful lake passage. Minimum lake migration speed varied and ranged from <1 to >6 km day^-1 across tagging groups. The probability of successful lake migration diminished with increasing time in the lake, and the probability of successful migration from most tributaries reduced to 0.5 after lake residence of 7.6-8.3 days. Successful migration probability was higher for Roogagh river smolts transiting Lough Melvin, with a 0.5 probability of success retained after 13.8 days of lake residence. The average time of entry into Loughs Neagh and Erne was not random and displayed a unimodal tendency towards late evening. The Lough Melvin tributaries displayed a more scattered pattern of lake entrance, with smolts emanating from the Ballagh tributary revealing a uniform time distribution (Rayleigh Test; mean resultant length = 0.21; p > 0.05). The differences in survival and phenology between the study lakes are likely the result of differential background predation levels, with an influential keystone predator species, Esox lucius, notably absent from Lough Melvin.