One obvious response to the disappearance of wild fish from our seas is "Why don’t we replace them with farmed varieties. We do it with land animals, so why not fish?" Of course this is already happening – over 40 per cent of the world's fish production already comes from aquaculture, and 50 per cent of fish sold in UK supermarkets comes from farms. These pen-reared fish grow at a phenomenal rate. For example, wild cod double in size every year, but hatchery cod quadruple in the same period. Given that size determines reproductive rate, at first glance this could be a solution to repopulating wild stocks.
Problem is, the methods used in most fish farms are not sustainable and put enormous pressure on the surrounding wildlife and environment. How? Well, the biggest growth area for fish farming is in high-value carnivorous species like salmon, shrimp, and in recent years cod, which are fed on fishmeal and fish oil from wild-caught small fish. On average you need around 3 tonnes of these wild fish to produce a tonne of farmed salmon, and the ratio's even higher for shrimp. This means that farms need to take such vast amounts of wild small fry from the sea that repopulating wild stocks with farmed fish will never be possible. Taking so much wild fish to feed farmed stocks is depriving whales, dolphins, birds, and other fish of a key part of their diet. It can also have devasting effects on coastal communities by depriving them of a much needed food source. This is particularly true in developing countries, where the fish are farmed mainly for export.
There's more - the high levels of antibiotics and pesticides used to control diseases in farmed fish have a polluting effect on surrounding habitats, while the diseases themselves often spread to wild populations. Excess food and untreated wastes from farms also find their way into the wider environment, raising nutrient levels and causing algal blooms which can de-oxygenate large areas of ocean.
Finally there's genetic pollution. Wild fish survive because they are resistant to disease, skilled at hunting and avoiding predators, and responsive to changes in water temperature which tell them when to migrate at spawning time. Farmed fish lose these characteristics within a few generations – this doesn't matter when they spend their lives in a pen, but if they escape (as some inevitably will) and breed with wildfish they will pass on "bad genes" to any offspring, and these defective fish may flourish or even come to dominate in the wild.
So aquaculture can't be a solution to repopulating our oceans. For that, we're going to need marine reserves - and a lot of time.