10. Orange roughy
Because the orange roughy has such a long lifespan (forget parrots; these fish can live to be 100 years old) and a slow rate of maturation, it takes literally dozens of years to replenish decimated orange roughy populations. And decimated they are; the fish became popular in the late 1970s, peaking in 1990, when overfishing led to government-imposed quotas for the fish. Although they're no longer technically critically endangered, many organizations recommend that orange roughy be avoided at all costs to keep it this way.
More specifically, the European freshwater eel. Even farm-raised eel, however, are poor stand-ins for wild-caught eel. A farm-raised eel must be fed three times its own body weight in wild-caught fish, a process that makes eel farming as unsustainable as over-fishing wild eel.
Recently, the ICUN Red List reclassified haddock as merely "vulnerable," albeit still endangered. It's because of this that the Greenpeace International Seafood Red List has listed haddock as one of the 20 species of fish to avoid at all costs. It also notes that while haddock is no longer overfished in U.S. waters, Scottish haddock fisheries should be closed to prevent the same thing from happening across the pond.
Although there's been some question as to whether or not Atlantic halibut should still be listed as endangered, there's no question that the fishery itself is still in terrible shape after years of overfishing. Although there have been conservation measures put in place since the fishery threatened to collapse, the fish are still in danger: Bottom trawlers catch the sea floor-dwelling halibut in their nets, destroying the young stock that are supposed to be replenishing the Atlantic halibut population.
6. Atlantic cod
The bad news is that the Atlantic cod has been fished nearly to extinction. The good news is that cod from Iceland and and the Barents Sea has not. According to the Seafood Watch app, "For centuries, north Atlantic cod was one of the world's largest and most reliable fisheries. However, decades of overfishing have resulted in dramatic population declines." Pacific cod from Japan and Russia is said to be just as bad, but opinions on that are currently divided.