Fishing

Oil and Gas Exploration and Production

Cable and Pipelaying

Spoil and Waste Disposal

Aquaculture

Knowledge of sensitivity to human impacts is limited to very few direct studies. However it can be assumed that recovery from physical impacts will depend on the spatial scale of the impacts. Recruitment is slow and sporadic. Spat survival to adulthood occurs best where the spat shelter amongst the mass of adults. Thus, where impacts are so severe that extensive areas are cleared of horse mussels, recovery is unlikely even in the medium term. The time taken for small breaks in a bed to close up by the growth of surrounding clumps is not known, nor is the survival of clumps torn away from the main bed.

At present there seems to be no large scale Modiolus fishery in the UK, but there have been small Modiolus fisheries in Scotland (Rohan Holt, pers. comm.) where it appears to be widely eaten, and also used for fishing bait, on a local scale (McKay, pers. comm.). Occasional local use in the west of Scotland was also mentioned by Comely (1978) and Modiolus may also have been taken on a very small scale from Loch Creran recently (D. Donnan, pers. comm.). Modiolus has occasionally been seen on markets in Lancashire (Bill Cook, pers. comm.). In Norway Modiolus is rather more important as a fishery in some areas, although no information on fishing methods or their effects has been found. Given the sporadic and low recruitment of Modiolus in UK populations, direct fishery activity obviously has potential to be very damaging, and should be prohibited from important biogenic reef areas unless it can be conclusively demonstrated that it is sustainable.

Scallop and queen scallop dredging has been implicated in the dramatic reduction in density and extent of the widespread and often dense areas of Modiolus bed, (probably representing true biogenic reef) which was described by Jones (1951) off the south east of the Isle of Man. The scallops and queens are fished using heavy metal dredges, usually with large prominent metal teeth along the leading edge. The beds of Modiolus have become progressively much more scattered and less dense over the years (skipper of R V Roagan, Port Erin Marine Laboratory, pers. comm.). Unfortunately no surveys of these beds have been done since Jones’s initial descriptions. The effect on associated communities has also not been studied, although it is known that the very large barnacle Balanus hameri, which used to be abundant on this particular community, has not been found there recently (Rees, pers. obs.; skipper of R V Roagan, Port Erin Marine Laboratory, pers. comm.). It is unlikely that scallop or queen fishing would be very viable over very dense reef areas, and it has therefore been assumed that many years of fishing on adjacent areas have to some extent ‘nibbled away at the edges’ of the denser beds. Scallop and queen dredging is known to be damaging to a variety of epibenthic organisms, including many found in association with Modiolus, such as Alcyonium digitatum, spider crabs such as Hyas and Inachus, Cancer, Echinus esculentus, Psammechinus miliaris and to a lesser extent Buccinum undatum (Hill et al., 1997) and probably others including particularly sponges (Veale, pers. comm.).

Scallop dredging on the very rough Modiolus reef areas to the north of the Isle of Man has apparently never taken place, presumably because the ground is too rough. In this area the Modiolus create numerous banks and reefs up to 1 m high (see earlier chapters).

Obvious damage including severe damage to Modiolus (ie the majority broken), flattening of emergent Modiolus clumps, and loss of the majority of epifauna, especially emergent species such as Alcyonium, was observed as a result of queen trawling in Strangford Lough (Magorrian et al., 1995) where the Modiolus seem frequently to occur as clumps on a muddy substratum.

It is suspected that fishing for queen scallops has taken place over Modiolus reef areas in upper Loch Creran recently but no details are yet available (D. Donnan, pers. comm.). Video footage of these areas taken during 1996, and showing the reefs to be in apparently good condition, is held by SNH. Queenie dredging was widespread in the Shetland Voes in the 1970’s and 1980’s and may conceivably be responsible for the apparent existence of Modiolus there mainly in clumps rather than dense beds or reefs (McKay, pers. comm.).

Whelk fishing using pots is frequently carried out over rough ground, so might be assumed to be relatively common on Modiolus dominated areas. Strings of several hundred pots can be used on Modiolus beds to the north of the Isle of Man and off the Lleyn Peninsula. Damage is not likely to be severe, and it is suspected that the emergent epifaunal bioherms of the Lleyn are probably more sensitive than the more infaunal reef / bank areas off the north of the Isle of Man.

On the ‘infaunal’ Modiolus reef areas to the north east of the Isle of Man drilling of a single oil/gas exploration well from a jack up rig using water based drilling muds was carried out recently. Drill cuttings disposal was carried out close to the seabed in an attempt to minimise the cuttings pile. Short-term studies using mainly video surveys, with limited anchor dredge sampling, revealed no very obvious impacts to Modiolus reefs, including cover of Alcyonium, hydroids and sponges, even within 50 m of the wellhead, although the survey design was such that only very obvious effects would have been detected (Holt & Shalla, unpublished). Contamination by barium from the drilling muds was detected in all Modiolus sampled (up to 250 m away) but there was no apparent contamination by any other metals or hydrocarbons even at the wellhead location. No long-term studies were carried out. Direct effects of the rig legs were assumed to be severe but no damage was detected by towed video. No long-term studies were carried out. No other studies on Modiolus communities in relation to oil and gas exploration are known.

Information on the effects of laying or trenching in cables or pipelines through Modiolus beds was not found. Intuitively it is assumed that cables laid on the surface will be soon be covered over, but scars caused by trenching are more likely to remain. Associated with pipelaying there would probably also be disturbance caused by lay-barge anchors and mooring wires to consider.

Effects of offshore disposal of dredge spoil and other solid wastes are little known. In a bed off the Humber long-term changes in contaminant loads associated with spoil disposal were detectable in the shells of these very long-lived animals. While this indicates survival of the mussels within a dispersal zone around a disposal ground, information on loss of condition, as occurs when Mytilus are subjected to excessive sediment loads, is not available. Deposition of capital dredgings such as barge loads of boulder clay which will initially settle as a mass will almost certainly smother the patch it lands on. From such spoil mounds the material usually disperses, but there are no case histories to indicate rates of sediment accretion that Modiolus clumps can keep up with. Exploratory benthos sampling off North Wales in the 1960s showed that there were Modiolus beds in or near the ground for which FEPA licences are presently issued for disposals from Holyhead. No monitoring is known to have been done.

Although no studies relating to the effects of aquaculture on Modiolus have been found, intuitively there must be some potential for damage in enclosed sea lochs and Voes where both occur together. In terms of SACs the place most likely for this to be an issue would be the Lochs Duich, Long and Alsh cSAC, where intensive salmon and mussel farming are carried out, though in fact there are few sea loch systems now unaffected by salmon farming (Black, 1996). Salmon farming in particular is known to produce large quantities of detritus, with localised deoxygenation leading to death of much of the benthos (Brown et al., 1987; Gowen & Bradbury, 1987). The majority on studies have been carried out on macrobenthic infauna, and effects in some instances have been reported to be detectable up to 45 m (Brown et al., 1987) or even 150 m or more (Weston, 1990). It is worth mentioning that, despite currents running for part of the tidal cycle from salmon farm cages to a Mytilus edulis bed, Taylor et al. (1992) were unable to demonstrate an influence of waste salmon food on mussel growth. Until studies specific to Modiolus have been carried out, it is impossible to predict with any certainty the likely effects of salmon cage aquaculture, but there is nothing presently to indicate that any wider effects than those reported above on macrobenthic infauna would occur.

Next Section                     References