Aquatic Environment and Biodiversity Annual Review 2012

AEBAR 2012: Benthic impacts
• Recovery time from trawl-induced disturbance can take from days to centuries, and depends
on the same factors as listed above. (All reviews, strong evidence or support).
• Given the above considerations, the effect of mobile bottom gears has a monotonic
relationship with fishing effort, and the greatest effects are caused by the first few fishing
events (All reviews, moderate to strong evidence or support).
• Application of mitigation measures requires case specific analyses and planning; there are no
universally appropriate fixes (Three reviews, moderate to strong evidence or support. The
issue of implementing mitigation was not addressed in the FAO review. It was also stressed in
the US National Academy of Sciences review and discussed in the ICES review that extensive
local data are not necessary for such case-specific planning. The effects of mobile bottom
gears on seafloor habitats and communities are consistent enough with well-established
ecological theory, and across studies, that cautious extrapolation of information across sites
is legitimate).
Rice (2006) concluded “These overall conclusions on impacts and mitigation measures, and
recommendations for management action form a coherent and consistent whole. They are relevant to
the general circumstances likely to be encountered in temperate, sub-boreal, and boreal seas on
coastal shelves and slopes, and probably areas … beyond the continental shelves. They allow use of
all relevant information that can be made available on a case by case basis, but also guide
approaches to management in areas where there is little site-specific information.”
Since Rice’s (2006) paper, Kaiser et al. (2006) published a meta-analysis of 101 separate
manipulative experiments that confirms many of Rice’s findings. Shellfish dredges have the greatest
effect of the various mobile bottom fishing gears, biogenic habitats are the most sensitive to such
disturbance (especially for attached fauna on hard substrates) and unconsolidated, coarse sediments
(e.g., sands) are the least sensitive. Kaiser et al. (2006) concluded that recovery from disturbance
events can take months to years, depending on the combination of fishing method and benthic habitat
type. This meta-analysis of manipulative experiments was an important development, reinforcing the
inferences drawn from multiple mensurative observations at much larger scale (“fisheries scale”) in
New Zealand (e.g., Thrush et al. 1998, Cryer et al. 2002) and overseas (e.g., Craeymeersch et al.
2000, McConnaughey et al. 2000, Bradshaw et al. 2002, Blyth et al. 2004, Tillin et al. 2006, Hiddink
et al. 2006). This is a powerful combination that implies substantial generality of the findings.
The international literature is, therefore, clear that bottom (demersal) trawling and shellfish dredging
are likely to have largely predictable and sometimes substantial effects on benthic community
structure and function. However, the positive or negative consequences for ecosystem processes such
as production had not been addressed until more recently (e.g., Jennings et al. 2001, Reiss et al. 2009,
Hiddink et al. 2011). It has been mooted that frequent disturbance should lead to the dominance of
smaller species with faster life histories and that, because smaller species are more productive than
larger ones, system productivity and production should increase under trawling disturbance. However,
when this proposition has been tested, it has not been supported by data in real fishing situations (e.g.,
Jennings 2002, Hermsen et al. 2003, Reiss et al.2009) and where overall productivity has been
assessed, it decreases with increasing trawling disturbance.
For example, Veale et al. (2000) examined spatial patterns in the scallop fishing grounds in the Irish
Sea and found that total abundance, biomass, and secondary production (including that of most
individual taxa examined) decreased significantly with increasing fishing effort. Echinoids,
cnidarians, prosobranch molluscs, and crustaceans contributed most to the differences. Jennings et al.
(2001) showed that, in the North Sea, trawling led to significant decreases in infaunal biomass and
production in some areas even though production per unit biomass rose with increased trawling
disturbance. The expected increase in relative production did not compensate for the loss of total
production that resulted from the depletion of large-bodied species and individuals. Hermsen et al.
(2003) found that mobile fishing gear disturbance had a conspicuous effect on benthic megafaunal
production on Georges Bank, and cessation of such fishing led to a marked increase in benthic
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