gif "...Our own interest lay in relationships of animal to
animal. If one observes in this relational sense, it seems
apparent that species are only commas in a sentence, that
each species is at once the point and the base of a pyramid,
that all life is relational to the point where an Einsteinian
relativity seems to emerge. And then not only the meaning
but the feeling about species grows misty. One merges into
another, groups melt into ecological groups until the time
when what we know as life meets and enters what we think of
as non-life: barnacle and rock, rock and earth, earth and
tree, tree and rain and air. An the units nestle into the
whole and are inseparable from it."
John Steinbeck:
The Log from the Sea of Cortez"
In May 1993, the United Nations Food and Agriculture Organization (FAO) warned that "one-quarter of the planet's biodiversity is in danger of extinction in the next 30 years". FAO used strong words to describe the threats posed by commercial fisheries to the planet's valuable pool of marine biological diversity:
"...the over-intensive use of modern technology and search for short-term benefits, coupled with continuous government support for otherwise uneconomic production, has had a devastating impact on many important fish stocks. Commercial fleets have driven some species to commercial extinction and sometimes close to biological extinction, destroying rural ecosystems, such as coral reefs and seagrass beds."
FAO says that commercial fish catches in thirteen out of fifteen of the world's major marine fishing areas have suffered serious declines. Indeed, declines in catches of some commercially valuable fish species have been catastrophic. The annual catch of Atlantic cod, for instance, fell from a peak of around 4 million tons in the late 1960s to just a little over a million tons by 1993.
In another case from the north Atlantic, the size of the spawning stock of Atlanto-scandian herring collapsed from an estimated adult stock biomass of between seven to ten million tons in the 1950s -- when it was considered to be perhaps the single largest population of fish on the planet -- to a mere 100,000 tons by 1970 following massive over-exploitation when herring was fished out in the late 1960s. It wasn't until 1986 that herring was found again in the Norwegian Sea, but not at levels high enough to allow fishing to resume. It can take many years, even decades for a collapsed fish stock to recover, if it can at all.
There is great difficulty nowadays in sustaining global fisheries production at around 82 million tons. In response to declines of commercially valuable stocks of bigger, slower growing species, commercial fishing fleets have turned to "fishing down the food chain", targeting increasingly large quantities of smaller species of fish with less commercial value. Such fish, called small-pelagic fish, are highly mobile shoaling fish that dwell near the ocean's surface, and they play a critical role in the marine foodweb.
gifThis shift typifies the "boom-and-bust" cycle of fisheries exploitation of the world's heavily-exploited, commercially valuable fisheries -- that is, rapid increases in catches of a commercially desirable species, followed by massive declines as its stocks are depleted. As the costs and difficulties of maintaining catch levels increase, highly mobile fleets move to other species, sometimes in the same region, but often thousands of miles away in another part of the world.
Since all components in marine ecosystems are interlinked, you can't change one thing without affecting everything else. If a particular species is heavily fished, it is not simply the targeted population that is affected, but the entire ecosystem in which it occupies a niche. Overfishing results in a form of ecological degradation or debasement, in which the naturally derived community of fish species changes.
In extreme cases, a competing species may take advantage of vacant space or food resources and prevent the over-exploited species from being restored to its former abundance. In the Eastern Bering Sea between Russia and Alaska, for instance, heavily fished pollock and Greenland turbot have declined significantly since 1979 (40% and 35% respectively), while flatfish and skates have increased notably (450% and 600%)
The scale of modern fishing also raises critical evolutionary questions. Intensely exploited fish populations can undergo critical changes. In many instances, exploitation shifts the age composition of fish populations resulting in faster growing fish that mature early, have a shorter life span, and smaller adult size. In heavily exploited North Sea fish stocks, such as plaice, haddock and cod, far more adult fish are killed by fishing than by natural causes and hence fishing becomes the dominant factor in the evolutionary process of natural selection. Fishing can also reduce genetic diversity within a species when a stock size is greatly reduced from natural levels.
Very little is understood about the long term consequences of these dynamics. Some scientists, though, have speculated on the potential of what might be called "evolutionary management": that is, managing fisheries in such a way that fish are harvested selectively at different stages of development so that a targeted stock evolves the properties most appealing to fish markets. This means the market forces are increasingly setting the criteria for evolution rather than nature.