Severe thiamine deficiency in Baltic salmon coincides with low wild recruitment, increasingly so at long migration...

Thiamine deficiency is a condition implicated as a driver of population declines across continents and taxa. In salmonids, symptoms of this deficiency are often observed in compensatory hatcheries, where large parts of the clutches display severe neurological disorders and die. However, whether hatchery-quantified thiamine deficiency is associated with reduced wild salmonid recruitment has rarely been tested. Here, we combine two long-term monitoring datasets: (i) a 30+ year time series of thiamine deficiency status in eight Atlantic salmon (Salmo salar) rivers draining into the Baltic Sea (called M74), and (ii) wild recruitment data for each corresponding river estimated by electrofishing. We investigated how wild recruitment varied with M74 incidence and further tested whether within-river migration length affected the strength of this relationship. We found that severe thiamine deficiency coincided with reduced wild recruitment. Furthermore, longer within-river migration steepened the negative association between thiamine deficiency and recruitment. As a control comparison, we did not find any evidence that thiamine status, as quantified in the Baltic, was associated with salmon recruitment on the Atlantic coast. These results indicate that M74, which is mainly observed in hatcheries, may reflect the conditions experienced by wild salmon in the rivers.