Effects of Trawling and Dredging on Seafloor Habitat (2002) / Chapter Skim
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3 Effects of Trawling and Dredging
3 Effects of Trawling and Dredging
Pages 18-29
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From page 18... ...
Evaluating the effects of bottom trawling on benthic communities is complicated by the sparseness of data on species abundance and composition before intensive bottom fishing began. This is important because recent analyses of the few existing historical data sets suggest that larger bodied organisms (fish and benthos)
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From page 19... ...
Reductions in the total biomass of target fish, along with the direct effects noted above, could be expected to affect predators, prey, competitors of a target species, and overall seafloor community structure. Indirect effects also encompass potential changes in the flow of materials and energy through ecosystems and shifts in the balance among the processes of primary production, primary consumption, and secondary production.
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From page 20... ...
Most research bears out the paradigm of variable environments inhabited by short-lived species recovering more rapidly than stable TABLE 3.1 Examples of Mobile Fishing Gear Effects on Habitat (Based on Reviews by Auster and Langton, 1999, and Barnette, 1999) Hardbottom/ Gear SAV Sand Biogenic Muddy Sand Gravel Scallop Increased dredging Smoothed bedforms; · Single passage can A gradient of · Undredged sites had dredge resulted in reduction of epifaunal kill 70% of the living increasing large higher numbers of significant reductions coverage; shell maerlin the dredge epifaunal cover organisms, biomass, in biomass and aggregate dispersal path.
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From page 21... ...
gear caused damage or loss of epifaunal coverage (11, 13) · Well buried boulders removed and displaced from sediment; trawl doors smoothed sand waves; penetrated seabed 0-40 mm (14)
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From page 22... ...
After five months of trawling, they observed changes only in the relative abundances of different species, but after 16 months total species richness began to decline in the trawled sites. Unfortunately, most research has focused on acute effects, quantifying changes to benthic habitat after only a limited number of trawl passes over a short period.
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From page 23... ...
. The frequency of natural disturbance corresponds roughly with sediment type, but not directly with particle size.
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From page 24... ...
One obvious but important implication of this exponential model is that repeated trawls at the same location kill diminishing numbers of organisms. Hence, if the distribution of the nontarget species is not positively correlated with that of the target species, a more aggregated fishery will inflict a lower mortality rate.
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From page 25... ...
Identifying and quantifying linkages is difficult, especially with data collected during routine population surveys. Figure 3.3 illustrates how the proportion of overall mortality mediated by habitat attributes could change based on life stage and movement rate (as a proxy for Early Benthic Phase Juvenile Late Juvenile Adult _ _ Growth Low Medium High FIGURE 3.3 Conceptual model of the link between habitat attributes and mortality of demersal fishes based on general life history stages.
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From page 26... ...
For example, the early life stages of some pelagic species reside in or depend on benthic communities, for food and shelter. It is difficult to separate the indirect effects of trawling and dredging on benthic and pelagic com
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From page 27... ...
Recovery from trawling will depend on the type and extent of the habitat alteration, the frequency of the disturbance compared with natural changes, habitat characteristics, and species and life history characteristics. Recovery times vary according to the intensity and frequency of the disturbance, the spatial scale of the disturbance, and the physical characteristics of the habitat (sediment type, hydrodynamics)
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From page 28... ...
These regime shifts can affect valuable ecosystem services, including fisheries yield (Collie and Spencer, 1994; Knowlton, 1992~. EFFECTS OF TRAWLING AND DREDGING ON SEAFLOOR HABITAT Human or natural modification of the marine environment might result in the shift of a community from one stable state that provides economically valuable fish to another stable state dominated by fish of higher or lesser value.
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From page 29... ...
Of direct concern to commercial and recreational fisheries is the possibility that losses of benthic structural complexity and shifts in community composition will compromise the survival of economically important demersal fishes. Mobile gear also can change surficial sediments and sediment organic matter, thereby affecting the availability of organic matter for microbial food webs.
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