One possiblility could be the dreaded Red Tide algal blooms which poison all sea creatures with their powerful neurotoxins.
Red tides effect the whole marine food chain and can lead to Neurotoxic poisoning in humans. There are also other types of algal blooms that can poison humans and other animals.
I could imagine a scenario in the late Mesolithic where a series of large algal bloom events occurred over a number of seasons, decimating the wildlife living around the coasts of Northern Europe. The human population who relied solely on the sea as their main source of food could have faced a potential famine situation. In step the first farmers/pastoralists and we've never looked back since...dental caries , diabetes, coronary artery disease and all that good shit was soon to follow.

Its interesting to note that an increase in Red Blooms was observed during the last El Nino event.
Just scanning through some red tide info
Last year the US Government closed 13000 sq miles of shellfish beds of Massachusets due because of Red Tides.
In 1999 600 miles of Chinese coastline were affected by Red Tide.
In 1998, again in China, a Red Tide lasted 40 days and affected 3125 square mile of coast.