Bottom of the barrel - Expansion of fishing to the deep sea is not sustainable

Over the last 50-60 years many shallow water fish stocks have become depleted and others have outright collapsed. As a result fishers have targeted fish stocks in deeper and deeper waters. Studies on global catch have shown that the average depth of catch has increased in recent years and fishing now regularly occurs over 1000 meters and in some cases to 1800 meters. Fishers are literally reaching into the bottom of the barrel for fish. 

The average depth of the oceans is about 4000 meters. Waters greater than 200m cover half the planet making the deep-sea the single largest living space on earth. Despite all this space some of species that live in the deep-sea may not be there long if harvest continues at the current rate.                                                                                                  

                                                                                                 

Many deep water species seem foreign and rather ugly compared to the bright beauties on the coral reef or kelp bed. Have you eaten one? You may not even know it. As deep-sea fisheries developed, starting in the 1970’s, many deep-sea species were renamed and most are processed or sold as fillets. For instance fishes in the family Macrouridae were called rattails because of their long tapering tails but who wants to eat a rattail? They were renamed grenadiers by seafood savvy marketers. Chilean seabass, which are commonly on the menu, used to be called Patagonian toothfish and Orange roughy used to be called slimeheads. These fish all live at depths of 500 to as much as 2000 meters. 

Pictures of some deep-sea fish.  Some have been renamed to make them more marketable to fish consumers

In the early years of deep-sea fishing little was known of the biology of the species being fished. In the absence of data, fisheries managers would often make assumptions about the status of stocks based on catch trends and what we know of the biology of shallow living stocks. Unfortunately it turns out these assumptions were wrong. The deep ocean is a very different environment than the continental shelves. With increases in depth, pressure increases, light and temperature decline, and food availability drops. 

Vertical ecological zones in the deep ocean as organized by ocean depth below the surface. Seawater temperature, light and biomass - aproxy for food available to fishes are also indicated.  Source:http://www.soest.hawaii.edu/oceanography/faculty/drazen/Oceanography%20website/

Deep living animals mostly rely on sinking detritus or migrating animals to supply the deep food web. My own research has shown that growth rates and fecundity (the number of eggs produced by a female) of deep-sea fishes decline with depth, while age at maturity increases. This means that deeper living species are less productive. Some deep living fishes like the orange roughy don’t reach maturity until nearly 30 and can live to 125 years. This is quite a contrast to many shallow water species, which can reach maturity at age 3 and live to 15 or 20. So, while the bottom of the oceans are large, the fishes that live there are very slow to grow and reproduce. 

Given the low productivity of deep-sea fishes and the current demand of many deep water fisheries, exploitation of these species has not proven sustainable. For example, fishing for orange roughy in Tasmania and New Zealand is a ghost of its former self. In fact, many of the seamounts where the fishes were taken now have names like Graveyard or Morgue. Even more, Red Lobster has a franchise-wide ban on serving orange roughy and Chilean seabass because these fish are in such dire straits. The trawling activities used to capture the fishes also destroy their habitats making recovery more unlikely.

A full trawl net of orange roughy. Source: http://www.southernfriedscience.com/?p=6979

In the western North Atlantic some grenadier stocks and other species have declined so much that some scientists have argued they qualify as endangered. On the other side of the Atlantic data has shown declines in some fishes even below the depth of fishing activity. Scientists speculate that this decline is because juvenile fishes reside at shallower depths (where fishing occurs) and descend deeper as they age, a trend seen for other fish that reside on the continental slope. Consequently, fishing activities may have depleted juveniles of some species, leaving fewer fish to descend to adult depths. As a result, the effects of fishing now extend to at least 2500 meters. 

How most see deepwater species. Here rattail aka grenadier, as fillets. Source: http://www.alibaba.com/product-gs/356482658/GRENADIER_FISH.html

Deep water fisheries represent the last frontier in industrialized fishing; however, they are not likely to continue for long. Given the low productivities of deep-sea fish stocks and the poor track record of sustainable fishing, many are calling for a stop. You might ask, why not fish even deeper? The average depth of the ocean is ~4000m and fishing is only occurring to half that depth. The biomass of fishes declines exponentially with depth so that below about 2000m fish abundance is not great enough to harvest economically. It seems we really have reached the bottom of the barrel.


Dr. Jeff Drazen
Department of Oceanography
University of Hawaii
In addition, read here about how a new study revealed severe mismanagement of European deep-water stocks.

References:

Bailey, D.M., Collins, M.A., Gordon, J.D.M., Zuur, A.F., Priede, I.G., 2009. Long-term changes in deep-water fish populations in the northeast Atlantic: a deeper reaching effect of fisheries? Proceedings of the Royal Society B: Biological Sciences 276 (1664), 1965-1969.

Devine, J.A., Baker, K.D., Haedrich, R.L., 2006. Deep-sea fishes qualify as endangered Nature 439, 29.

Drazen, J.C., Haedrich, R.L., 2012. A continuum of life histories in deep-sea demersal fishes. Deep Sea Research I 61, 34-42.

Morato, T., Watson, R., Pitcher, T.J., Pauly, D., 2006. Fishing down the deep. Fish and Fisheries 7, 24-34.

http://www.smartplanet.com/blog/pure-genius/red-lobster-sustainability-for-the-seafood-lover-of-the-future/4454

http://www.southernfriedscience.com/?p=6979


Links for further reading:

Can Ecosystem-Based Deep-Sea Fishing Be Sustained? http://digitalcommons.library.umaine.edu/sms_facpub/145/

Koslow, J. A. 2007. The Silent Deep: the Discovery, Ecology and Conservation of the Deep Sea. The University of Chicago Press.