Application 124771 - Ministry of Fisheries

CAWTHRON INSTITUTE | REPORT NO. 2167 MAY 2012
Mussel shell drop-off is commonly used an indicator of the extent of benthic effects
beneath mussel farms (Wong & O’Shea 2011), and video transects were used to
determine the limits of mussel shell drop-off and/or mussel clumps on the seabed
around the block of farms. Li 373 is situated adjacent to Li 396 and Li 361 and the
extent of mussel shell drop-off was determined around the entire block of farms. An
underwater video camera and light was attached to a sled and tethered via cables to a
VCR and television on the boat. Six transects (Figure 2) were undertaken by lowering
the sled and camera to the seabed and towing it in the required direction. GPS
positions were recorded for each transect with observations of conspicuous epifauna
and substratum type.
A single grab sample was collected from t outside the southwest corner of the farm
using a 0.01 m 2 van Veen grab sampler. A 63 mm diameter core sub-sample was
photographed and the top 25 mm was collected for analyses of sediment grain size.
Grain size was determined gravimetrically after separation of fractions by wet sieving
and drying at 105°C, for gravel (≥ 2 mm), sand (≥ 63 μm - < 2 mm) and silt/clay (< 63
μm) size classes.
Depth profiling was undertaken to assist in characterising the seabed. Continuous
depth readings from a Garmin F100 depth sounder within and adjacent to the farm
areas, and were sent to a PC via a RS232 serial output. The PC simultaneously
collected separate RS232 serial output of latitude and longitude from a GPS, and both
data streams were incorporated using communications software. Depths were
standardised to chart datum and plotted as depth contours in ArcMap.
1.3. Benthic assessment results
Video transects along the perimeter of the farm revealed mussel clumps, mussel shell
drop-off and a fine covering of shell hash overlying mud (e.g. Appendix photographs 1
and 7). A patchy film of orange/brown benthic diatoms was present in some mud
areas (Appendix photograph 2). Conspicuous epifauna included cushion stars
(Patiriella sp.), ascidians (Styela clava), eleven-armed sea stars (Coscinasterias
muricata), sea cucumbers and small feather hydroids. Two horse mussels (Atrina
zelandica) were observed along the inshore transect. Burrows in the mud were
common, suggesting that regular bioturbation occurs over the site.
Other transects around this block of farms have shown a similar environment of
mussel clumps and shell drop-off on mud (e.g. Appendix photographs 9 and 11);
therefore, it is likely that this habitat is widespread beneath the farm. Mussel clumps
and shell drop-off became less dense with distance from the farm. Beyond the mussel
clump/shell drop-off areas (> 50-115 m north and south of the farm; > 20 m inshore of
the farm) the substratum was mud, often covered with shell hash, with few epifauna
present (Appendix photographs 2 and 6).
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