Rewilding the Sea - Feral: Rewilding the Land, the Sea, and Human Life - page 238

natural spectacles - the vast spawning aggregations on the Grand
Banks and other shallow seas off the Atlantic coast, and the frenzy of
tuna, sharks, dolphins and whales attendant upon them - is a
tragedy.
In some places where cod were abundant they have failed to return
even when fishing for them has ceased. This could be because cod
appear to engineer their environment, creating the conditions neces-
sary for their survival. On the Grand Banks they preyed heavily on
mackerel and herring. When most of the cod disappeared, the popu-
lation of mackerel and herring boomed, with the result that the
relationship was reversed. The smaller fish became major predators of
cod, eating their eggs and fry before they could mature. 25 The same
thing has happened in the Baltic Sea, where cod eggs are eaten by her-
ring and sprats. 26
Turtles also appear to have changed the world to suit themselves.
When Columbus arrived in the Caribbean, according to one study,
that sea contained 33 million green turtles. 27 There were similar con-
centrations off the east coast of Australia and in other tropical and
subtropical seas: turtles all the way down. Today there are 2 million
green turtles, worldwide. They largely subsisted on turtle grass, a
weed which once grew on the beds of great tracts of shallow water.
These were the savannahs of the sea, supporting vast herds of grazing
animals: dugongs, manatees and herbivorous fish as well as unimagin-
able numbers of green turtles (which were, permitted to live into old
age, much larger than the average size of those of today). The grazers,
in turn, supported marine lions, hyenas and cheetahs: big predatory
fish, mammals and in some places reptiles, namely giant saltwater
crocodiles.
When the turtles were slaughtered, mostly before the nineteenth
century, the remaining population could no longer keep the turtle
grass cropped. As the blades grew longer, they shaded the seabed and
shielded the sediments from the current. The weed, uneaten, started to
age and rot, and detritus built up in the still water beneath the beds.
This became a food source for parasites which then began to destroy
the living grass (a process biologists call 'turtle grass wasting disease').
Across much of the range that green turtles once occupied, the turtle
grass has died off. 28 This, in other words, is a similar story to that of