A recent report from the WWF has just found that in the last 40 years fish populations have fallen by over half. For major commercial species such as tuna, mackerel and bonitos the decline has been as much as 74%. These declines are from a combination of over exploitation of fish stocks and habitat loss. Habitats such as mangroves are crucial breeding grounds for many marine species, but they're the types of areas that are built on or destroyed. Without mangroves and seagrass beds fish stocks will be slow to recover.
This report comes on the back of a recent paper published in Science that found that humans kill adult prey at a rate 14 times higher than any other species. Our impact is especially felt on terrestrial predators and fish. Human impacts are felt even stronger because of the age group of animals we impact on: adult individuals of reproductive age. Most predators kill juvenile or sick members of their prey species but by killing the reproductive adults we have a disproportionate impact on the ability of the species to replenish its numbers.
The oceans are not an environment that we can easily see or come in contact with. They are remote and difficult for us to understand. This may be one of the reasons why so many people seem not to understand how endangered life in the seas is. They are communal resources that any country with a coast line can access and their vastness prevents them being policed. With everyone being able to access them the tragedy of the commons takes hold. This means that to solve the problems of the worlds oceans we are going to need major international efforts to police the the seas and to tackle climate change, as one factor in the decline of the worlds fish stocks is ocean acidification (due to increased CO2 levels) reducing negatively impacting coral reefs.