Overfishing trickles down

There are a lot of reasons that overfishing is a detrimental thing for the overall health of the oceans.  It's not just about the species that's overfished -- it's about all the other species in the food web.   Back when whales were massively depleted down Antarctica way, penguins filled in for the big guys with a massive population increase.  And we can contemplate when top predators (like wolves) disappear in a terrestrial ecosystem, then changes follow all the way down the line, particularly among the herbivores and the plants they eat. 

So while it isn't overly thrilling, it's a good thing to study such things for further insight into the humanly-altered ecosystems of Earth.  In this case, the researchers looked at overfishing effects:

"For example, in the Northern Benguela ecosystem off Namibia, stocks of sardine and anchovy collapsed in the 1970s from overfishing and were replaced by bearded goby and jellyfish. But the bearded goby and jellyfish are far less energy-rich than a sardine or anchovy, which meant that their populations were not an adequate food source for other sea animals in the region such as penguins, gannets and hake, which had fed on the sardines and anchovies. African penguins and Cape gannets have declined by 77 percent and 94 percent respectively. Cape hake and deep-water hake production plummeted from 725,000 metric tons in 1972, to 110,000 metric tons in 1990. And the population of Cape fur seals has fluctuated dramatically."

 And as can be gleaned from the above, the changes due to overfishing are rarely in the direction of good.

The paper describing this research was published in the PNAS.