Some fish actually swim into Tamsui River. This video, courtesy of Teng-Feng Fishball Museum, shows the easy catch with an umbrella fishnet:
Notice the inverted V-shaped mouth, a distinct feature of the Black Mullet
Black Mullet fishing has a long history. Each winter, ancient Chinese fishermen would follow the fish to Taiwan. The Dutch East India Co (1624-1662) had levied a 10% tax on the harvests. And during the Ming-Cheng rule (1662-1683), fishermen were required to pay for permits - in the form of a white flag printed with the fisherman's name stamped with gov'tal seals, to be displayed on the boat. One document from that era recorded 94 such permits in Feng-Shan District, "... 給烏魚旗九十四支，旗用白布一幅，刊刷烏魚旗子樣，填寫漁戶姓名，縣印鈴記，插於船頭，帶綑採捕". Taxation, of course, continues to this day.
Unfortunately, Black Mullet fishing in Taiwan may soon become a thing of the past. Out of either greed or ignorance, or both, Mainland Chinese fishermen have intercepted the migrating fish before they even reached Taiwan Strait. These pirate-like fishermen use giant nets to corral the fish, then catch the fish with smaller nets with hooks, a barbaric practice frowned upon by the Taiwanese. As a result of this pre-emptive over-fishing, Black Mullet are deprived of the opportunity to spawn and the stock is now much depleted.
Some have argued that the water temperature in the Taiwan Strait for some reason is no longer high enough to attract and guide the migration; although this theory seems to go against the natural geographical instincts of all migratory species.
In any case, the counter-measure will have to be through large-scale fish farming. This remains a work-in-progress, however.