Aquatic Environment and Biodiversity Annual Review 2012

AEBAR 2012: Benthic impacts
changes in the benthic system caused by over 130 years of oyster dredging, but Michael et al. (2006)
did not support such conclusions in the same system. Subsequent review of these studies found much
common ground but no overall consensus on the long-term effects of dredging on the benthic
community of the strait.
These studies have focussed predominantly on changes in patterns in biodiversity associated with
trawling and/or dredging and less work has been done to assess changes in ecological process or to
estimate the rate of recovery from fishing. Projects that have started on recovery rates are focussed on
relatively few habitats and primarily those that are known to be sensitive to physical disturbance,
including by trawling or dredging (e.g., seamounts, project ENV2005/16, and areas of high current
and natural biogenic structure, projects ENV9805, ENV2005/23 and BEN2009/02). Thus, the
understanding of the consequences of fishing (or ceasing fishing) for sustainability, biodiversity,
ecological integrity and resilience, and fish stock productivity in the wide variety of New Zealand’s
benthic habitats remains incomplete. Reducing this uncertainty would allow the testing of the utility
and likely long-term productivity of a variety of management strategies, and enable a move towards a
regime that maximises value to the nation consistent with Fisheries 2030.
Table 7.4: Summary of studies of the effects of bottom trawling and dredging in New Zealand waters.
Location Approach Key findings References
Mercury
Islands sandy
sediments.
Scallop dredge
Hauraki Gulf
various soft
sediments.
Bottom trawl &
scallop dredge.
Bay of Plenty
continental
slope. Scampi
and other
bottom trawls.
Foveaux Strait,
sedimentary &
biogenic reef.
Oyster dredge.
Spirits Bay,
sedimentary &
Experimental Density of common macrofauna at both sites decreased as a result
of dredging at two contrasting sites; some populations were still
significantly different from reference plots after 3 months.
Observational,
gradient
analysis
Observational,
multiple
gradient
analyses
Observational,
various
Observational,
gradient
Decreases in the density of echinoderms, longlived taxa, epifauna,
especially large species, the total number of species and
individuals, and the Shannon-Weiner diversity index with
increasing fishing pressure (including trawl and scallop dredge).
Increases in the density of deposit feeders, small opportunists, and
the ratio of small to large heart urchins.
Depth and historical fishing activity (especially for scampi) at a site
were the key drivers of community structure for large epifauna.
The Shannon-Weiner diversity index generally decreased with
increasing fishing activity and increased with depth. Many species
were negatively correlated with fishing activity; fewer were
positively correlated (including the target species, scampi).
Interpretations of the authors differ. Cranfield et al’s papers
concluded that dredging biogenic reefs for their oysters damages
their structure, removes epifauna, and exposes associated sediments
to resuspension such that, by 1998, none of the original bryozoan
reefs remained.
Michael et al. concluded that there are no experimental estimates of
the effect of dredging in the strait or on the cumulative effects of
fishing or regeneration, that environmental drivers should be
included in any assessment, and that the previous conclusions
cannot be supported.
The authors agree that biogenic bycatch in the fishery has declined
over time in regularly-fished areas, that there may have been a
reduction in biogenic reefs in the strait since the 1970s, and that
simple biogenic reefs appear able to regenerate in areas that are no
longer fished (dominated by byssally attached mussels or reefbuilding
bryozoans). There is no consensus that reefs in Foveaux
Strait were (or were not) extensive or dominated by the bryozoan
Cinctopora.
In 1999, depth was found to be the most important explanatory
variable for benthic community composition but a coarse index of
180
Thrush et al.
1995
Thrush et al.
1998
Cryer et al.
1999
Cryer et al.
2002
Cranfield et al.
1999, 2001,
2003
Michael et al.
2006
Cryer et al.
2000