Effect of Trawling on Benthic Ecosystems

Neelakantan Prakash

I. Qualities of Benthic Systems

1. Spatially small

2. Physical bottom (terra) highly variant composition - rocky, sandy, silty, pebbled, mixture. Can result in different oxygen concentrations, ease of resuspension.

3. Burrows, reefs, vegetation mats - enlarge oxygenated area, prevents creation of anoxic (essentially dead) areas. Habitat for fish, hiding from predators, prevent erosion.

II. Effects of Trawling and Nets

1. Pelagic layer - goal layer of harvesting. Many small and large fish stay in this area right above the benthic layer (~4 in. to a few feet above sea floor). Not very severe effects here; fish populations can recover.

2. Benthic layer - removal of habitat. Extremely harmful, long recovery period.

a. Resuspension - sediments from sea floor rise into the water causing some fish to suffocate, organisms from sea floor to become immobile and easily preyed upon. Reveals anoxic layer of mud below which is hard to repopulate.

b. Disruption of benthic structures - anoxia (layer of mud w/ no oxygen - harsh environment for zooplankton and sea plants to live in), removes hiding area from predators (namely reefs and vegetation mats) which creates imbalance in fish populations causing fluctuations, and destroys area of interaction between two species of symbionts such as sea anemone and clownfish.

3. Compounding - high frequency of trawling is extremely damaging to ecosystem. Ecosystem requires at least one month and in some cases over 14 months to recover minimum amount of original species. Some areas are trawled 400 times a year and never recover.

4. Patch types - type I is a patch where surrounding communities are undisturbed - can recover quickly due to high incidence of species reintroduction to area; type II is where surrounding communities are disturbed - low incidence of species repopulation leads to low recovery. High intensity trawling with high frequency wouldn't be so bad if it created type I patches. Combination of high frequency (400x/annum) high disturbance (from ~0-50% of original fauna/flora remain) and type II patch creates a low probability for repopulation of patches.

5. Natural disturbances - while natural disturbances are present and can alter an ecosystem they are generally either low frequency or low intensity, or if not, the organisms in the area are adapted to the disturbances. The quick introduction of trawling to an area prevents the possibility for species to adapt to the disturbance.

6. Results

a. Desertification - total lack of life in the disturbed patch except for what macrofauna swim by - even this statistic decreases over time as other areas are better hunting grounds as the disturbed patches of remaining fauna are quickly overfished due to lack of cover from predators.

b. Eutrophication - one type of organism, usually a primary producer (ie. algae) has no competition for resources and takes over an entire patch creating anoxic conditions in not only the soil but the pelagic layer above it.

III. Policy Suggestions

1. Place legislative power not only into the hands of those doing the trawling but the local fishing companies whose interests are directly vested in the health of the ecosystem.

2. Use fishing methods that create a disturbance similar to that normally experienced by a certain patch.

3. Establish limits on trawling frequency and nearness to other trawled areas to allow for repopulation.

4. Do not dismiss small fishers' qualms about trawling as a gear conflict - educate public and legislation about the actual effects of trawling versus other methods of fishing.

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last updated 4/23/06