Once a cheap store-cupboard staple, sardines are fast becoming a luxury food item. Last year, #SardineGirlSummer saw influencers promoting the health benefits of sardinemaxxing, while sardinecore became the must-have fashion trend. Now, demand is accelerating just as Morocco’s sardine stock has collapsed, causing shortages on supermarket shelves. The Guardian travels to Matosinhos in Portugal, the cultural home of the sardine industry, to investigate who is profiting from the boom, how it has affected local fishermen, and what sustainability looks like in an era of global heating