Fish feel man's deep impact
It's often been said - I've no idea who started it, but it was good of him or her - that we know less about the Earth's oceans than we do about the surfaces of other planets.
Well, this week we've found out a little more; fishing close to the surface can have a significant impact nearer the bottom of the sea.
The Royal Society's publishes research from the northeast Atlantic - off the southwestern coast of Ireland, to be precise - showing that at depths of about 2.5km, populations of many fish species are significantly lower than they were 30 years ago.
Eel numbers at these depths have halved.
There could be a number of factors involved; but the most likely, according to the researchers, is the commercial fishing operations that have started in the last 30 years for deep-water species such as and .
Yet most commercial trawls don't go down anywhere near that deep - about 1.5km, in this location, is the maximum - so what's going on?
Many deep-water species spend their early years at shallower depths, moving down along the continental slope into deeper waters as they mature; if they're killed when they're relatively young, they'll never make it down the continental slope into deeper waters.
So far, so logical - and perhaps so inescapable, assuming that we want to continue to eat fish from the sea. But what's perhaps unexpected is that the effect is occurring even with species that we don't eat.
s explanation - he's the Glasgow University scientist leading the study - is that trawls dragged across the ocean floor in search of orange roughy and other desirable fish catch at least 13 species that we don't want.
Either these are crushed by the trawling gear, or taken up to the surface in the nets and discarded.
If they were all discarded alive and their chances of survival were good, then presumably they'd just swim back down again and we wouldn't be seeing their numbers fall. The fact that we are suggests that at least a significant proportion of the discarded fish are either dead when they're thrown away or die shortly afterwards.
It would be nice to have more certainty about the scale of the issue. But, David Bailey tells me, there are only three places in the world's oceans where there has been long-term monitoring of deep sea fish.
In parts of the Pacific, scientists are also apparently finding that where fish are heavily caught near the ocean's surface, deeper species decline - here, perhaps, because less detritus falls to the seafloor for scavenging.
What does it all mean for people trying to manage fisheries? Well, if this is a widespread trend, then clearly the impacts of fisheries may be felt a long way from where the fishing actually happens; moving one or two kilometres in depth could mean moving tens of kilometres horizontally, if the continental slope is gradual.
Perhaps the finding should inform the design of Marine Protected Areas, making them bigger than they might have been otherwise, although that wouldn't meet with universal support - there's some suspicion in fishing circles that left to themselves, the tree-huggers of the environment community would protect just about everything and ban fishing completely. It's not true, but it's a politically significant suspicion.
More research, and more regular monitoring, would be an idea. But it costs - days at sea with deep-water research trawls do not come cheap - and with various life-forms in the oceans also affected by shipping, climate change, acidification, pollution, and so on, how can you ensure researchers are not simply chasing a moving target?
Some are urging .
But is this justified? Heavy bottom trawls , there's no doubt about that; but most tracts of sea bed are not rich nurseries for life but relatively barren expanses of mud or silt. That's industry's argument; conservationists counter that fishing fleets target the most ecologically vulnerable areas because that's where the most fish will be found.
Another approach to the problem might be to eliminate discards, as in other fisheries - at least that would mean everything caught is put to use.
But many of these accidentally-caught deep-sea species have little commercial worth, with watery flesh that yields poor taste and nutritional value - bringing them home for fish meal, even, might not be profitable.
I think we are going to have to live with the fact that if we want to harvest seafood, we can certainly be smarter about how we do it, but there will never be an impact-free fish lunch.
I'm Richard Black, environment correspondent for the ³ÉÈËÂÛ̳ News website. This is my take on what's happening to our shared environment as the human population grows and our use of nature's resources increases.
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