Application 124771 - Ministry of Fisheries

MAY 2012 REPORT NO. 2134 | CAWTHRON INSTITUTE
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enrichment, together with the accumulation of debris beneath coastal mussel farms,
increases both the food available for scavengers and habitat heterogeneity. This in
turn can lead to significant increases in the diversity of infauna and epibiota (Inglis et
al. 2000, Wong & O’Shea 2011). However, in shell- and sand-dominated benthic
habitats, shifts in epibiota feeding and living on mussels and changes to enrichment
tolerant infauna taxa can occur (Keeley et al. 2010).
Clumps of mussels beneath the reference farm sites had been colonised by a range
of organisms and provided a reef-like habitat for a variety of small fishes and mobile
fauna. Elsewhere, biodeposits beneath mussel farms have been are known to attract
large numbers of predatory fish, seastars, crabs, sea urchins and other echinoderms
(Mattsson & Lindén 1983, Kaspar et al. 1985, Cole & Grange 1996). Furthermore,
exclusion of trawling by the farm infrastructure also means that species which are
regularly harvested or damaged by trawling in open areas of seabed, such as scallops
(Pecten novaezelandiae) and horse mussels (Atrina zealandica), are occasionally
abundant beneath the farms. The extent to which protection for these species is
occurring beneath Coromandel mussel farms however is unknown; scallops were rare
at all sites and occasional beds of horse mussels were observed beneath three
reference farm sites and at four control sites.
In general, mussel farming is unlikely to result in irreversible ecological effects
(Forrest 1995, Keeley et al. 2010). Farms placed over mud / silt habitat, as were the
majority of farm sites in this study, would likely return to pre-farm conditions within one
to two years of farm removal. However, this would depend on the level of shell cover
at each site. In extreme cases, banks of sediment and shell material up to 2 m high
have been observed beneath Coromandel mussel farms (e.g. Coffey 2001), but the
accumulation and extent of shell material depends on processes such as water
movement and bioturbation which tends to disperse and modify deposited materials
(Morrisey et al. 2000). Overall, the enrichment effects of inshore mussel farms on the
benthic environment in the Coromandel were minor; while shell deposition and related
effects on diversity and abundance of epibiota, while considerable, were not
necessarily negative (Keeley et al. 2010).
6.2. Water column effects
The water column properties measured at reference farm sites and control sites were
broadly similar and no clear patterns in differences were observed. Water column
turbidity in the top 1-2 m of the water column was similar between farm and control
sites, but farm sites were less turbid between 3-5 m and below 7 m depth. This lower
turbidity may be attributed to the filtering effect of the mussels on the farm droppers,
but this effect will depend greatly on the state of the tide and the water velocities at
each site, which are known to influence the residence time of water passing through
mussel farms (Plew 2011).