The judges action defies common sense. If the stocks are depleted, then the fishermen can't catch enough and go out of buusiness. The arguments the environmentalists make must probably follow traditional communist econmomic logic: unless unless the state owns eveything, collectivism won't work. Therefore, testing communism on small collectives won't prove their effectiveness because everybody is not playing by the same rules. Carry this logic into the fishing business. The capitalist fishermen will make big hauls until there are no more fish. The environmental communists fail to explain how capitalist fishermen can make a big haul in a depleted resource. But, who expects logic to flow from the mouths of environmental communists.
If you want to know why the cod fishery collapsed, and why a lot of other fisheries (swordfish, for instance) are in trouble, I invite you to read a long and extensive analysis:
Collapse of the Northwest Atlantic Cod Stocks
Below are a few selected excerpts that provide information that addresses your question.
1. "The Canadian Department of Fisheries and Oceans (DFO) attempted to regulate harvest over the Grand Banks, but made many mistakes and miscalculations which led to overfishing and the eventual closure of the fishery in July 1992. The mathematical models used for setting the Total Allowable Catch required scientists to make several assumptions that were untrue, and thus the "safe" catches were overestimated. Consequently, the cod breeding stocks were overfished until too few mature fish (5-7 years old) existed to lay eggs and thus recruitment fell below an acceptable level."
2. "Many management related factors led to inappropriate Total Allowable Catch (TAC) limits set by the DFO. Among these factors are under-reporting of catch by fishermen, improved technology on vessels making it possible to catch more fish per unit effort, and foreign fishing."
3. "One of the main errors was relying too heavily on commercial catch data to estimate recruitment and population size. Catch data may easily be forged and fishermen can simply cheat on the catch quotas."
4. "The effects of the previously discussed environmental conditions on cod are not clearly proven, but anomalous environmental factors may increase the likelihood of cod distribution patterns and ecology being altered. For example, cold, less saline conditions currently being observed in the Grand Banks have increased the likelihood a mismatch exists between spring blooms of phytoplankton and hatching of cod eggs. The oceanic changes also have apparently pushed the polar front further east, placing the spawning grounds of some Grand Banks cod into arctic rather than boreal waters and driving older cod to depths more than 400m (Conover et al 1995)."
To address your original question, there are two main reasons that fishermen can (for awhile) continue to make large hauls in a depleted resource. The first is improved technology making it easier to find and harvest the last remaining large stocks. The second is that these endgame efforts deplete the "feeder" stock which still has to mature to be the main breeding stock, but the larger fish (the normal main breeding stock) have already been taken.