Suppose that your livelihood and the survival of your family depends on fishing. The best return for your time and effort is to catch the largest amount and size of fish possible to sell at the market. So, you set the level of your trolling and the mesh size of your trawl nets to target the largest (and, usually, oldest) predatory fish in the food web.
Because the predators near the top of food chains tend to be the biggest in size and the smallest in number, they are the first species to be harvested. As these large species teeter on the brink of extinction, you must adapt to harvest a broader range of slightly smaller fish in order to stay economically viable. This probably means adjusting your fishing strategy and decreasing the meshing size of your nets: the next species down the food chain are smaller, faster, and shorter-lived.
Over the course of a few generations, the marine ecosystem undergoes the successive loss of the largest species. Fishers like you predictably adjust to the changes in fishery stocks. Over and over again, the economy and marine market ‘corrects’ to favour the next largest species with the highest biomass and yield. The increasing rarity of the old, large species continues to drives up their value, thus making them more prized, more rare, and less viable for people like you who need a steady income.
This happens gradually: with no long-term datasets at your disposal, it is virtually impossible to detect. And if the well-being of your family is on the line — as it is for a vast number of people around the world — sitting down and comparing a bunch of graphs does not score very high on your priority list.
Even more seriously, this phenomenon presents the persistent illusion that the fauna situation is stable. Why? By volume, the amount of fish you haul to the market every day stays roughly the same in terms of weight and biomass. Even though the fish you catch are getting smaller and smaller over time, you keep adapting in order to catch more and more of them.
Imagine that you fish in a large lake. Fifty years ago, your parents harvested a few large fish a week, equalling, for example, 50 kilograms. Today you still catch about 50 kilograms of fish weekly, but they are smaller in size and more numerous. But whereas your parents mainly harvested one or two species, your weekly catch now includes a far more numerous number of different species. This means that every time a strata in the fish size is depleted, the loss in biodiversity is many times greater. (Welcomme 1999)
This phenomenon, known as ‘fishing down the food web’, was first demonstrated by Daniel Pauly (b. 1946) and others in 1998 (Pauly 1998). It is a cautionary and sober reminder that the active and ongoing depletion of a system can be virtually invisible in the immediate, short-term view. Even though you can still drag 50 kilograms of fish out of the water this week — like your parents did fifty years ago — the fishery itself may be on the brink of collapse. Today, remember that it is impossible to reckon the future sustainability of a system without taking historical baseline data seriously.