Application 124771 - Ministry of Fisheries

CAWTHRON INSTITUTE | REPORT NO. 2134 MAY 2012
4. METHODS
4.1. Characterisation and mapping of the seabed
The seabed beneath and adjacent to the reference farms and control sites was
characterised and mapped using a range of sampling techniques including depth
profiling, sediment grab sampling and video transects (see Appendix 1 for sampling
coordinates). Sufficient sampling was undertaken to delineate the extent of mussel
clumps around farm areas. The fieldwork was carried out over four days from 12-16
December 2011.
4.1.1. Site bathymetry
Depth profiling at the reference farm sites was calculated from digitised bathymetric
charts and in-situ measurements to assist in the characterisation the seabed. In-situ
measurements were taken using continuous depth readings from a Garmin F100
depth sounder within and adjacent to the reference 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. In-situ depth measurements were standardised to
chart datum and plotted using Surfer v7 surface mapping software. The 2-D
graduated colour contour map was gridded using the natural neighbour method
(Sibson 1981).
4.1.2. Sediment physical, chemical and biological properties
Sediment grab samples were collected using a 0.01 m 2 van Veen grab sampler from
three sampling stations within the eight reference farm sites and six control sites. The
following sub-samples were collected from each grab sample to characterise the
physical, chemical and biological properties of the sediments:
Sediment core samples: A 63 mm diameter core was photographed and the
top 25 mm was collected for analyses of sediment grain-size and total organic
carbon (TOC) and total nitrogen (TN). Grain-size was determined gravimetrically
after separation of fractions by wet sieving and drying at 105 ºC, for gravel
(≥2 mm), sand (≥63 μm -