A Chef Bold Enough to Serve Freshwater Fish in Portugal

In eight days of dining in Portugal, I ate grilled sardines and squid, tiny roe-filled shrimp, stuffed brown crabs, razor clams, barnacles, red mullet, and giant carabineros—all caught in the Mediterranean and Atlantic. I also ate a single freshwater fish: pikeperch, an invasive species harvested from a massive reservoir in Portugal’s Alentejo region.

In addition to shelf-stable salt cod, freshwater fish have long appeared on tables in this rural, interior part...