Levelling out, trawling the sea

Traces of bottom trawling
from the terramare projet

Bottom trawling is a commercial fishing technique much in vogue since engines powered fishing vessels, although it dates back to the 14th century (in English waters).

Bottom trawling (*)   is a non-selective fishing technique, taking anything into the net. Heavy nets and gear are pulled along the sea floor sweeping-up sediments and anything living there. Currently bottom trawling can be done down to more than 800m depth.

The impact of bottom trawling on fish populations and bottom-dwelling (benthic) animals and plants has received much attention.

However impact of bottom trawling goes beyond  direct effect on biology of the sea bottom. High-energy natural processes, such as tidal currents and bottom morphology interact. They drive sediment erosion, transport and deposition processes over wide parts of coastal shelf and continental margin. Bottom trawling links into these processes.

Satellite image of trawler mud trails
off the Louisiana coast (Wikipedia)

Bottom trawling puts fine sediments back in suspension. Then currents carry the re-suspended sediments further. Consequently, physical, morphological and chemical properties of seabed are altered. Sediment composition is modified, so that chemical exchanges and fluxes at the sediment water interface are altered too. The sea bottom of shelf seas is modified and benthic ecosystems are damaged deeply. Down to the continental slopes, the reworking of the sea floor by trawling gradually modifies the shape of the submarine landscape over large areas, finally altering its physics and chemistry.  

How trawl gear modifies the seabed in coastal seas over wide area has been presented recently [1]. Trawling-induced sediment displacement and sediment removal from fishing grounds thus causes the morphology of the deep sea floor to to change over time. The original complex bottom gets smoother. This is shown by high-resolution maps of the sea-floor relief.  "...marine geologist Pere Puig and colleagues examined a shrimp fishing region off Spain’s Mediterranean coast. Puig, of Barcelona’s Institute of Marine Sciences, used remote-controlled submarines to document gouged and flattened areas along trawling routes. While undersea erosion and other natural processes cut deep valleys into the continental slope, Puig and his team noted that silt had been dumped into one of these canyons by trawling gear" [2]

Testing a steam plough in the 1860s 

During recent decades of commercial fishing on global scale by industrialized fishing fleets, bottom trawling drives evolution of the "seascape", the bottom morphology of coastal and continental seas is influenced over wide areas.

Given the global dimension of bottom trawling, the morphology of the upper continental slope in many parts of the world’s oceans likely has  been altered by intensive bottom trawling. Effects on the deep sea floor are comparable to those generated by agricultural ploughing on land, as Pere Puig concludes [1].

Martin.Mundusmaris@gmail.com

info@mundusmaris.org

(*) Bottom trawling is trawling (towing a trawl, which is a fishing net) along the sea floor.

[1] Ploughing the deep sea floor; Pere Puig et.al. Nature (2012) doi:10.1038/nature11410

[2] from Los Angeles Times

Poseidon's health check ?

The health of coastal seas and global ocean is critical for human well-being and sustainable economies. The sea provide food, livelihoods and recreational opportunities and regulates the regional and global climates of the globe. An aggregated  indicator of the "health of the ocean" that describes our interaction with the seas should integrate different sources of information. Then it could be a useful to shape policies, rise public awareness and guide further research catching the wider context of human uses of the sea. Such an indicator was proposed recently [1].

Halpern et al./Nature 2012 (from):
The ocean health score for an aggregate of all countries.
The outer ring is the maximum possible score for each goal.
The petal’s length represents the score for that goal,
and its width indicates how the goal was weighted.

Sustainable management of coastal seas generates a flow of benefits. To do this well requires comprehensive and quantitative methods to monitor the coupled human–coastal sea systems. May be inspired  by the use of the industrial composite indicators, such as Dow Jones tracking economic "health", marine researchers have created now an index that assesses health of coastal seas. From that extrapolation to health of the human-ocean system may be possible, such the claim.

The proposed index aggregates ten goals into a single score of how well a coastal seas are  doing. These goals include food provision, carbon storage, tourism value, etc. and biodiversity [2] and were chosen to reflect both the needs of human societies and ecosystem sustainability. Different from valuing pristine seas the index combine various public goals for a healthy coupled human–ocean system. A value of the composite index was calculated for the exclusive economic zone of every coastal country.

The index is a composite and its average is calculated in a region depending manner; avoiding that one size fits all. That has the consequence that the relative weight of the different goals determine very much the outcome; likewise the averaging method. This method is the strength and weakness of the method; it gives choice to settle on the regionally best mix of indicators but my rise too biases.

Each goal is assessed comparing the situation today with a value for where one would like to be or how likely it will be in the near future; a kind of "regional optimal value". Achieving each goal is expressed as a percentage of its optimal value. Each country's overall score is then the average of its 10 goal scores. The score rewards sustainable behaviour now and in the future. This approach is practical, but whether it makes sense depends on the policy choices for the "optimal value".

"Globally, the overall index score was 60 out of 100 (range 36–86), with developed countries generally performing better than developing countries, but with notable exceptions. Posting a global score of 60 out of 100, the index offers a seemingly gloomy outlook. Almost one-third of the world's countries scored below 50. But the study authors say that the range of scores for individual countries — from 36 to 86, with 5% of nations scoring higher than 70 — implies that there are successes amid the areas of concern. Only 5% of countries scored higher than 70, whereas 32% scored lower than 50." [3]

The Ocean Health Index is a composite index and therefore relative weight of its different elements reflecting policy choices determine much the final score.  Germany's coastal area in the North Sea scores 73 (Belgium 64); Germany ranks fifth in global ranking shortly behind non-exploited waters in the Pacific because the index rewards “sustainable use” and “conservation.” compared to "sustainable fishing".

Ancient Greek God Poseidon and some of its children

"The index provides a powerful tool to raise public awareness, direct resource management, improve policy and prioritize scientific research.... This should not be considered a failing grade for the oceans, The real value of the index will be the ability to track progress related to management policies over time,” (Co-author Karen McLeod; [ quote by 3])

The  Ocean Health Index gets exposure and debate is opened on the Web. "In October, it will probably be among the metrics considered by the Conference of the Parties to the Convention on Biological Diversity in Hyderabad, India. It may also prove useful in the UN General Assembly's first global integrated marine assessment this autumn", so Virginia Gewin [3]

Martin.Mundusmaris@gmail.com
info@mundusmaris.org

[1] An index to assess the health and benefits of the global ocean, Nature 488, 615–620 (30 August 2012) by  Benjamin S. Halpern et al. and comment by [3] Virginia Gewin 15 August 2012

[2] for full list and further details see Table S.1 in "Supplementary Information" attached to the research paper or in related paper published by Scientific American