How deep is too deep for commercial fishing?

Our ever-improving technology has allowed us to fish longer, catch more, and move further from land.  It has also allowed us to fish deeper.  EU statistics indicate that between 1950 and 2006 fishing depths increased from an average depth of 407 metres, to 535 metres.

Life in the deep is slow-paced.  Food is scarcer than in the sunlit surface waters.  Species grow slower and live longer.  Some deep-sea corals, like the one in the image, are thought to be over 4,000 years old. Traits like these are why organisations like Marine Conservation Institute that ” The deep-sea is the world’s worst place to catch fish” .  It’s not just the sustainability of targeted species that is causing concern, but of those caught as bycatch, as well as damage to the seabed and the flora and fauna living in and on it – like the coral in the photo.  So can deep-sea fishing ever be managed sustainably?    A recently published study from Joanne Clarke, a PhD student at the University of Glasgow, and colleagues suggests that there might be a way to make the practice less damaging.

It’s all about depth
The team took a look at trawl survey data undertaken between 1978 and 2013 in the northeast Atlantic.  The surveys used a range of different gear types, were taken at a number of locations which varied in depth – between 240 and 1,500 metres.  The data provided the researchers with four ‘indices’ to calculate – Simpson’s diversity index (a measure of diversity), the ratio of “discarded” (bycatch species) to commercial biomass (sellable species), the ratio of Elasmobranchii (sharks and rays) to commercial biomass, and finally the value per square kilometre of each trawl.

Running this data through some fancy analysis, the researchers found some clear patterns.  First, fish diversity increases with depth – an average of 18 new species are encountered for every 100 metres depth you go down.  The ratio of discard biomass to commercial biomass also increased with depth.  At 600 metres the ratio was 0.3:1 but by 1,300 metres, the ratio reached 1.6:1.  The ratio of Elasmobranchii biomass to commercial biomass significantly decreased between depths of 500-600 metres, but then steadily increased again.   The value per unit effort was more variable over depth.  The team found significant decreases in value in trawls between depths of 400–700 metres, but rose again when trawls were around 1,300 metres deep.  The team point to the dominance of rock grenadier (Coryphaenoides rupestris) – a commercially valuable species that lives at these depths as the primary reason for the rise in trawl value at 1,300 metres.

Based on the results, the researchers recommended that a ban on deep-sea trawling would be good for biodiversity, meet a number of conservation targets and EU legislation – including those for deep-sea sharks which are believed to be highly vulnerable to deep-sea fishing, without much cost to the industry (the obvious exception being any that were to target rock grenadier at depths of 1,300 metres.  If you are wondering if fisheries do go that deep… orange roughy is the most deeply fished species – 1,500 metres below the sea surface).

Politically timely
With deep-sea fishing causing widespread concern among academics, NGOs, and those concerned with the environment, fishery managers around the world are being pressed to take action to minimise any impacts this industry can have.  Last year the European Union parliament rejected proposals to completely phase out deep-sea bottom trawling and gill-netting – both considered to be highly damaging activities.  The EU are meeting again this month to discuss how deep-sea fishing could be more sustainable – including introducing depth limits to bottom trawling.  The authors hope that the politicians will take note of their findings.

Are depth limits controversial?
When it comes to fishing, there isn’t much that is not controversial.   Bertie Armstrong, chief executive of the Scottish Fishermen’s Federation responded to this study in a press release.  He is not happy with the idea of banning fishing at depths below 600 metres, and does not see the conservation benefit.  ”Using scientific survey data to draw conclusions upon commercial fishing activity is fraught with danger” Bertie said. ”This is because such information is normally collected via random trawl sampling, whereas commercial fishing tows are very targeted and aimed at specific species or groups of species” .   Bertie argues that commercial boats use much more selective nets that have larger meshes compared to those used in scientific sampling.  He also believes that trawls will become more selective in the future.

The original paper
The paper was published in the journal Current Biology.  The authors have paid for the paper to be open access, so why not have a read of it yourself http://dx.doi.org/10.1016/j.cub.2015.07.070

The Fishermen’s perspective
Bertie Armstrong’s comments can be viewed on the Scottish Fishermen’s Federation website.

The Image
Like many commercial fishing techniques, bottom trawling produces bycatch of both plants and animals.  Here, a bottom trawler dumps ancient deep-sea coral overboard.  Credit © Greenpeace International/Malcolm Pullman/Marine Photobank

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