Application 124771 - Ministry of Fisheries

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CAWTHRON INSTITUTE | REPORT NO. 2134 MAY 2012 6. DISCUSSION 6.1. Benthic effects The effects of the reference farm sites on the benthic environment were consistent with those observed beneath mussel farming throughout New Zealand (e.g. Kaspar et al. 1985, Hartstein & Stevens 2005, Giles et al. 2006, Keeley et al. 2010, Wong & O’Shea 2011). The physico-chemical effects beneath reference mussel farms included slight increases in TOC and TN compared to control sites, but the composition of sediment was similar. Increases in TOC were not always evident, however, and were only observed between reference farms and control sites to the north of Coromandel Harbour. Interestingly, the average TOC recorded across the reference farms (2.1%, s.e. = 0.3) was considerably lower than levels observed under similar farms in the Marlborough Sounds, which regularly exceed 10% in sheltered areas (Hartstein & Stevens 2005). Average levels of TN (0.2%) were similar to those observed elsewhere in the Firth of Thames (Giles & Pilditch 2006), but considerably lower than those observed beneath mussel farms elsewhere in New Zealand, which can exceed 1% (Hatcher et al. 1994). Therefore, either the majority of reference farm sites in this study were in relatively high flow areas, where organic wastes are dispersed rather than accumulating beneath the farms, or the inshore mussel farming in the Coromandel is less intensive than in the other areas (e.g. Marlborough Sounds). Infaunal community compositions at reference farms and control sites were broadly similar but farm sites generally had greater abundances of each taxa type. Most of the infauna communities were characteristic of those found in mildly enriched sediments observed elsewhere in the Hauraki Gulf (Wong & O’Shea 2011), possibly due to wider scale enrichment occurring in the environment. This conclusion was supported by similar enrichment stages (ES) of grab samples from control and reference farm sites, with samples ranging from natural to mild enrichment (ES 1.7 – 2.8 in Figure 3). The epibenthic communities and the extent of mussel shell drop-off at the reference farm sites were typical of those observed under mussel farms elsewhere in the Hauraki Gulf (Wong & O’Shea 2011) and in the Marlborough Sounds (Kaspar et al. 1985, Keeley et al. 2010). Shell material extended no more than 50 m from farms and formed a layer approximately 10 cm deep over most of the reference farm sites. This shell-covered substratum was characterised by a greater diversity and abundance of epibiota than observed at control sites. The ecological effects of biodeposition from mussel farms depend on the level of shell deposition and enrichment from biodeposits, which will vary according to site-specific characteristics such as water currents, water depth and farm management practices (Forrest 1995). The existing substratum type also plays a major role in determining the level of benthic effects (Giles et al. 2006). In mud/silt dominated areas, organic 25
MAY 2012 REPORT NO. 2134 | CAWTHRON INSTITUTE 26 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).
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REPORT NO. 2167 COROMANDEL MUSSEL F
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Application 124771 - Ministry of Fisheries

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