Yes, this is one of the factors at play but there are no proof that seasonal depression is the main one. All of northern countries have this issue without having such high suicide rates.

This issue is a sociocultural one. There has never been any proof that Seasonal affective disorder (SAD) can increase suicide rates that much. Living up north, I can assure you that everyone is aware of SAD and is taking steps to minimize it. Omega-3, vitamin supplements, light therapy, etc.