Application 124771 - Ministry of Fisheries

CAWTHRON INSTITUTE | REPORT NO. 2134 MAY 2012
EXECUTIVE SUMMARY
In August 2011, a collective of inshore mussel farmers from Coromandel approached
Cawthron Institute (Cawthron) to provide baseline benthic assessments of effects as a part of
their applications for 1 ha extensions to 32 mussel farms. These ‘baseline’ assessments and
their structure are detailed in the Waikato Regional Council (WRC) Draft Coastal Plan.
To fulfil the requirements of the Coastal Plan, a wider-scale assessment programme was
agreed upon by WRC, Cawthron and the collective of farmers, whereby eight representative
‘reference’ farms were selected for full benthic and water column assessments to provide an
indication of the effects of mussel farms, and six ‘control’ sites were selected to provide a
‘baseline’ or ‘background’ assessment of the unfarmed benthic and water column
environments. Fieldwork was carried out between 12-16 December 2011.
The seabed beneath and adjacent to the six control and eight reference farm sites was
characterised and mapped using a range of sampling techniques including depth profiling,
sediment grab sampling and video transects. Sediments were tested for total nitrogen (TN),
total organic carbon (TOC), particle grain-size and infauna (richness, abundance and
diversity). At all 32 mussel farm sites, sufficient sampling was undertaken to delineate the
extent of mussel clumps around farm areas (see Appendix 7-39).
No clear patterns in sediment particle grain-size components relating to area (north or south
of Coromandel Harbour) or site type (’control’ versus ‘farm’) were evident (see Section 5.1).
At the majority of sites sediments were dominated (> 50%) by fine muddy silts (< 64 μm).
Sediment cores at the majority of sites were characterised by a fairly uniform light grey/brown
colour and appeared well oxygenated, with no evidence of an apparent Redox Potential
Discontinuity (aRPD) layer or sulphide odours.
Total nitrogen was significantly higher in sediments collected from under the reference farms
than at control sites (see Section 5.1). In general, lower average TN was found at sites
where sediments had a greater sand component.
Sediment organic content (total organic carbon, TOC) at the reference farms and control
sites was different in the two different areas (north or south of Coromandel Harbour) (see
Section 5.1). Farm sites to the north, generally had greater TOC than those to the south of
Coromandel Harbour, while control sites had similar TOC regardless of which area they were
in. Overall, TOC was greatest in sediments that contained the most gravel - sized particles,
and lowest at sites with sediments that had a greater sand component.
Dominant infauna taxa across all sites were the window shell (Theora lubrica), various
species of polychaetes (e.g. Heteromastus filiformis, Prionospio multicristata, Prionospio
yuriel and Sphaerosyllis sp.), brittle stars, and oligochaete worms (Section 5.2). The
dominant epifauna taxa by abundance were Phoxocephalidae amphipods, Decapoda larvae
(unidentified) and Melitidae amphipods.
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