From increased carbon dioxide
By the end of this century, carbon dioxide levels in the ocean are likely reach levels that will interfere with fishes’ ability to escape from predators, hear, and smell. Carbon dioxide is already affecting fish brains and central nervous systems, a serious threat to their survival. Baby fish are especially susceptible to these effects, which makes it harder for them to find a reef at night or avoid during the day to avoid predators. Excessive CO2 also affects schooling behavior, a key way to evade predation since single fish are singled out. Their natural instincts to turn left or right is weakened by higher levels of carbon dioxide. Some crustaceans are likely to be affected as well.
“We’ve now established it isn’t simply the acidification of the oceans that is causing disruption — as is the case with shellfish and plankton with chalky skeletons — but the actual dissolved CO2 itself is damaging the fishes’ nervous systems.”
Göran E. Nilsson, et al. 2012. Near-future carbon dioxide levels alter fish behaviour by interfering with neurotransmitter function. Nature Climate Change; DOI: 10.1038/nclimate1352
According to a 2010 report, the UK’s fishing fleet works 17 times harder than it did in the 1880s to net the same amount of fish. The FAO estimates that more than half of world’s coastal fisheries are over-exploited.
Whaling has also changed the oceans beyond recognition. During the 20th century, several species were hunted to the brink of extinction, and populations have still not recovered. A controversial study published in Science claimed that pre-whaling populations were dramatically higher than previously thought. By this estimate there were once 1.5 million humpback whales, rather than the 100,000 estimated by the International Whaling Commission. It is a similar story for minke, bowhead and sperm whales.
Primeval planet: What if humans had never existed? 14 November 2013 by Christopher Kemp. NewScientist