Application 124771 - Ministry of Fisheries

CAWTHRON INSTITUTE | REPORT NO. 2134 MAY 2012
usually difficult to detect within 20-50 m away. A similar conclusion was reached in a
recent study under mussel farms in the Hauraki Gulf (Wong & O’Shea 2011), who
suggested that the extent of mussel clumps on the seabed was the best indicator of
the extent of benthic effects. The most important factors influencing the area and
magnitude of effects surrounding mussel farms are water depth and current speeds
(Hartstein & Stevens 2005); hence severity of effects is very much site-specific and
effects are minimised by locating farms in well-flushed areas, where species and
habitats of special value are not present.
Enrichment from farm-derived biodeposits is the primary cause of seabed effects in
soft-sediment habitats and the type of effects are reasonably well described (Keeley
et al. 2010), as such they can be placed on an enrichment gradient ranging from
natural to azoic (Figure 3). Using biological and physico-chemical data from benthic
enrichment studies under finfish and mussel farms throughout New Zealand, the
gradient has been described numerically with enrichment stages (ES) 1 to 7
(Figure 3). This method can now be assigned to benthic grab samples to allow
comparison of enrichment effects from sites around New Zealand (Keeley et al. 2012,
Keeley et al. In press).
Figure 3. Stylised depiction of a typical enrichment gradient experienced at low flow sites, showing
generally understood responses in commonly measured environmental variables
(species richness, infauna abundance, sediment organic content and sulfides and
Redox). Apparent Redox Potential Discontinuity depth (aRPD) and prevalence of bacteria
(Beggiatoa spp.) mats and methane/H2S out-gassing also indicated. The gradient spans
from natural or pristine conditions on the right (ES = 1) to highly enriched azoic conditions
on the left (ES = 7).
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