Ocean Health Data May Be Flawed – New Analysis Sparks Debate

A recent analysis of catch data calls into question the accuracy of previous surveys of marine ecosystem health. Without accurate data, environmental policy makers may be unable to determine if current reforms to fisheries management are working, and further, if their picture of our oceans’ health is even roughly accurate.

The new analysis was conducted by a team lead by Trevor Branch of the University of Washington (Seattle) and published in an early November, 2010 edition of Nature. Since it’s publication, renewed debate has erupted over how we should measure the vitality of our oceanic ecosystem.

The key metric used in such surveys is known as the mean trophic level (MTL) which is an indicator based upon rank (of a given fish) in the food web, that is, how many other members of the food web are dependent upon it for their survival. This metric data is collected through surveys of fish catches. It is widely held that the total MTL is decreasing (indicating a decrease in ocean biodiversity), due primarily to industrialized fishing practices.

Over population of jelly-fish species and massive phytoplankton blooms are inferred to be the end results of these excessive, over-fishing practices.

According to previous studies, the mean trophic level of the world fisheries catch has steadily declined because many high trophic level fish, such as this tuna, have been over-fished. The new analysis challenges this claim.

But according to this newest analysis paper, the underlying data used in previous surveys (reported catches of seafood) does not accurately reflect marine ecosystem health.  The paper also challenges 1998 results by marine biologist Daniel Pauly (Univ. of B.C., Vancouver) whose survey/analysis identified a phenomenon called “fishing down the food web.”

The team conducted a comparison of catch data using using two additional data sources: trawl surveys and stock assessments, and calculated trends from each data source. Only the traditional catch data yielded a different trend. Branch believes that this data (from reported catches alone) is an unreliable gauge of the state of marine ecosystems across the globe.

The team’s analysis also seems to refute the idea that we are “fishing down the food web”, revealing that all trophic levels (from oysters to tuna) are being caught in greater quantities–suggesting that conditions might not be so bad after all. Branch speculates that top-level predators may not be wiped out completely, though they may become scarcer.

Nets for trawling in surface waters and for trawling in deep water and over the bottom. Note the "tangles" with ensnared marine life

Other marine scientists (such as Daniel Pauly) disagree, citing that Branch’s analysis does not accurately reflect the increase in the sizes of areas being fished — areas which grew dramatically from the 195o’s to 1980’s, as commercial fishing fleets expanding into the high seas and Southern Ocean.

Other studies have shown that top-level predators are particularly susceptible to over-fishing.

Thus, a two-fold question is emerging from all this data analysis and disagreement: what is the current condition of our marine ecosystem and what is the likely trend for the future?

Disagreement and debate in a given scientific community is a sign that the science is robust. Further, it will drive continued research which will benefit the science, and more importantly, benefit humans, as we may better manage our natural resources with better data.

About trophic levels:

An energy pyramid illustrates how much energy is needed
as it flows upwards to support the next trophic level. This diagram is not to scale, because only about 10 % of the energy transferred between each trophic level is converted to biomass.

The term “trophic” means nutrient-related. There are three activities utilized by living things to derive nutrients: producing, consuming, and decomposing. The latter activity is performed by bacteria and fungi.

Trophic levels can be represented by numbers, starting at level 1 with plants; “Higher” trophic levels are numbered according to the organism’s “rank” in  the food chain.

  • Level 1: Plants and algae make their own food and are called primary producers.
  • Level 2: Herbivores eat plants and are called primary consumers.
  • Level 3: Carnivores which eat herbivores are called secondary consumers.
  • Level 4: Carnivores which eat other carnivores are called tertiary consumers.
  • Level 5: Apex predators,  which have no predators, are at the top of the food chain.

A classic example of a trophic chain (or web) is:

clover => rabbit => fox =>  gold eagle => (coming “full circle”)  fungus and bacteria, which decompose the dead apex predator, returning many nutrients to the soil, promoting plant growth (note: level 4 carnivores/consumers are not always present in every food web).

Note: Some of the material for this post came from the news report ‘Key Indicator of Ocean Health May be Flawed’ (Science, 3 November 2010) by Erik Stokstad.

Diagram – Ecological Pyramid by Saric ; CC -by – sa 3.0

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