Once humans were able to navigate the open waters of the entire planet and developed techniques for catching and processing massive harvests at sea, they soon had the means to decimate entire populations of marine organisms.

Humans have been hunting whales for more than 3,000 years—a fact we know from scenes carved on whalebones. Early whale hunters likely had little impact on whale numbers, since they worked to supply food to relatively small local populations. But that changed with the appearance of commercial whaling when the demand for whale products, particularly oil, soared in the 19th century. The main targets of whalers in the North Atlantic Ocean, from the 16th through 19th centuries, were North Atlantic right whales, Eubalaena glacialis, and bowhead whales, Balaena mysticetus (Figure 8.10), which are slow swimmers that float when killed. Using open rowing skiffs and hand-thrown harpoons, whale hunters slaughtered an estimated 120,000 right whales and bowhead whales, jeopardizing their very existence.

For many years, the blue whale, Balaenoptera musculus, and the fin whale, B. physalus, remained beyond the reach of early whaling technology because they were too fast, too strong, and sank when killed. This changed with the invention of the harpoon gun, explosive harpoons, and steam-powered winches and catcher boats, which introduced the age of modern whaling. The populations of blue and fin whales in the Southern Hemisphere declined from about 400,000 in 1920 to a few thousand in 1960.

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The history of commercial whaling is a good example of how the Tragedy of the Commons leads to overexploitation of resources (see Chapter 2, page 49). During the time of peak commercial whaling, no international agreements or regulations limited harvest. Because whales mostly live in areas away from international borders, whalers were free to harvest as many whales as they could sell to meet commercial demand. Ultimately, the collapse of many whale populations and rising popular awareness of the overharvest of whales sparked international agreements to ban whaling in 1982. Those bans are still in place today, with a few notable and controversial exceptions.

As the cod fishery in the northwest Atlantic collapsed (Figure 8.11), marine scientists discovered that other species, such as hake, haddock, and pollock, were also in trouble. All these fish were historically dominant predators, and their absence due to overharvest transformed the ocean ecosystem in many ways (Figure 8.12). Bottom-dwelling invertebrates, such as snow crab, that were common prey for cod doubled in number. Similarly, small forage fish, such as herring and capelin, no longer faced heavy predation pressures by cod and other predators, so their population numbers increased by a factor of 9. All these small fish preyed heavily on zooplankton, causing that population to decline. Because zooplankton feed on phytoplankton, fewer zooplankton meant that phytoplankton populations increased, and their greater abundance ultimately reduced the nutrient content of surface waters. The effect of the collapsed cod fishery rippled throughout the food web, much like the extensive effects of wolves on the Yellowstone food web (see Chapter 4, page 103). These dramatic changes led to speculation that recovery of the apex predators like cod might not be possible for a very long time.

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Similar to the history of whaling, the collapse of the cod fishery off Canada and New England is a sobering example of a Tragedy of the Commons (see page 49). But many other exploited fish populations have also collapsed, including the sardine fishery off California and the blue tuna fishery in the Atlantic. Recent estimates indicate that more than 25% of commercially important fish stocks have suffered declines in numbers sufficient to be classified as a “collapse” in the fishery. Figure 8.13 shows the patterns of population decline under exploitation for two of these fish stocks: the South Atlantic snowy grouper and the South Atlantic black sea bass.

One factor in these declines has been the over-expansion of fishing fleets and a system that creates a competitive, “race-to-fish” approach to harvest. Rather than regulating the amount of fish being caught, regulators in the United States traditionally limit the type of gear that can be used and the number of days a fishing vessel can spend at sea. Regulators also monitor the number of fish being caught and shut down fishing for the season when too many fish are caught. Naturally, fishers scramble to harvest as many fish as possible before regulators cut off fishing, and this system unintentionally incentivizes illegal harvest (i.e., poaching) by unregistered vessels. Under such conditions, it is difficult to avoid overfishing.

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While our understanding of the status and biology of commercially important fish populations grows rapidly, significant gaps in our understanding remain. In 2013, of the 230 stocks of commercially important marine fish under U.S. jurisdiction, the status of 23% was uncertain or undetermined. Meanwhile, significant percentages of commercially important fish stocks off northwest Europe and New Zealand also had uncertain or undetermined status. Clearly, increased information on the status of these stocks would help with their management, especially in regard to regulating fishing pressure.