Trawling and creeling

Organic enrichment

Nephrops norvegicus is the only species within this biotope complex to be the target of a large commercial fishery (a small-scale local fishery for Munida rugosa exists in the Clyde). The Nephrops fishery is of major economic importance and is pursued throughout most of the geographic extent of the biotopes in which the species occurs. This includes both shallow, semi-enclosed sea loch areas, and open-coast grounds in deeper water (eg. the Irish and North Seas). In British waters the Nephrops fishery has grown rapidly since its inception in the 1950s (Howard, 1989), and the species is now one of the most valuable shellfish resources in the north-eastern Atlantic. Because of its intensity and wide geographic coverage, the Nephrops fishery has the potential to affect the biotopes in question throughout their range. There are potential consequences both for the Nephrops populations themselves, and for the associated fauna of sea pens and megafaunal burrowers.

Impacts on specific elements of the biotope complex

The release of large quantities of nutrient-rich organic matter into the sea is one of the most widespread and important human impacts on the marine environment. Circumstances in which this occur include:

• Discharge of sewage from coastal outfalls
• Larger-scale dumping of treated sewage sludge at offshore sites
• Sedimentation of faeces and uneaten food around marine fish farms
• Release of organic effluent from industrial sites (eg. pulp mills, alginate factories)

These various forms of organic enrichment can all have profound effects on benthic communities, and these have been widely-studied in the context of pollution monitoring (Pearson & Rosenberg, 1978). Typically, a moderate input of organic matter can enhance the abundance and diversity of the benthic fauna by increasing the supply of food, but as the organic load rises the faunal diversity declines and the benthos becomes increasingly dominated by a small number of hardy, opportunistic species (usually polychaetes of the genus Capitella), which may be numerically extremely abundant. In grossly polluted situations, even these animals are excluded and the sediment surface becomes covered by a whitish blanket of bacteria (Beggiatoa spp.) which obtain energy by oxidising sulphide diffusing out of the anoxic sediments. The threshold levels of organic enrichment at which these successive stages occur will be determined by factors such as temperature, sediment type and local hydrography. Where organic matter is released into the sea from a point source (eg. a salmon cage or sewage outfall), a gradient of faunal and sedimentary change will be produced, with progressively more impacted conditions encountered with increasing proximity to the source.

The burrowing megafauna has been regarded as belonging to the ‘normal’ (ie. unimpacted) sediment community, and to be excluded from areas of high organic enrichment (Pearson & Rosenberg, 1976). The critical factor in causing this exclusion may be the oxygen depletion (hypoxia) that is often associated with organic pollution. Hypoxia results from the elevated biological oxygen demand of degradative microbial processes, stimulated by the input of organic matter. Large, active animals with high respiratory demands will be the most affected by oxygen depletion. Bagge & Munch-Petersen (1979) reported that catches of Nephrops novegicus in the Kattegat (Denmark) were greatest in September when the concentration of dissolved oxygen in the bottom water was at its lowest. Low levels of oxygen within their burrows probably forced the Nephrops onto the sediment surface where they were more vulnerable to capture by fishing gear.

Several studies have found that thalassinidean mud-shrimp burrows are often very depleted in oxygen and enriched in sulphide, and that the shrimps themselves have physiological mechanisms that allow them to tolerate these conditions (Anderson et al., 1991; Astall et al., 1997a, b; Johns et al., 1997). These animals are therefore highly resistant to environmental oxygen depletion, and some species can withstand total anoxia for several days (R.J.A. Atkinson, personal communication). However, even mud-shrimps have their limits of tolerance. Stachowitsch (1984) recorded individuals of Upogebia tipica, Jaxea nocturna and Axius stirhynchus abandoning their burrows during a severe episode of oxygen depletion in the Adriatic. Christiansen & Stene (1998) observed specimens of Callianassa subterranea killed by upwelling of anoxic, sulphide-rich water in a Norwegian fiord. Organic enrichment will have other adverse effects besides hypoxia. These can include burial by large volumes of material (for example, in sludge dumping grounds), excess turbidity, changes in sediment characteristics that may inhibit the construction of burrows, and the presence of associated toxins (present in sewage sludge). All these factors may contribute to the exclusion of megafaunal burrowers from the most polluted situations.

Very little has been recorded concerning the sensitivity of sea pens to organic pollution, but it is reasonable to suppose that they will be susceptible to the same adverse effects as the other components of the benthic fauna. Hoare & Wilson (1977) noted that Virgularia mirabilis was absent from part of Holyhead Harbour heavily affected by sewage pollution. Both Virgularia mirabilis and Pennatula phosphorea were found to be abundant near the head of Loch Harport, Skye, close to a distillery outfall discharging water enriched in malt and yeast residues and other soluble organic compounds (Nickell & Anderson, 1997). Sediment organic carbon content in the study area was < 5%, and macrofaunal analysis indicated that the distillery effluent had very little effect on the benthic fauna.

Although the general observation that burrowing megafauna are absent from highly polluted areas appears to hold true, these animals do nevertheless flourish in many localities where the sediments are naturally rich in organic matter (for example, in many Scottish sea lochs). There is little information on the critical environmental thresholds causing changes in these communities. The only study to examine the distribution and abundance of megafaunal burrowers along a gradient of organic enrichment is that of Smith (1988), who carried out a towed video survey around the Garroch Head sludge dumping ground south of the Isle of Bute in the Firth of Clyde. Water depth was generally 70 - 80 m, but reached 100 m in places. The burrowing megafauna observed or identified from their burrow openings were Nephrops novegicus, Calocaris macandreae, Callianassa subterranea, Lumpenus lampraetiformis and Cepola rubescens. As predicted, megafaunal abundance declined as the centre of the dumping ground was approached. At the centre, the sediment contained about 10% organic carbon. Burrowing megafauna were abundant in areas of < 4% organic carbon, and absent where this exceeded 6%. The snake-blenny Lumpenus lampraetiformis seemed to have the highest tolerance, and extended furthest into the dumping ground. Calocaris macandreae did not extend as far along the enrichment gradient as Lumpenus or Nephrops. Epifauna such as whelks and hermit crabs also did not extend into areas of highest enrichment. The burrowing anemone Cerianthus lloydii was found at the sample site nearest to the dump centre, but was most abundant at an intermediate level of enrichment.

Cage aquaculture of Atlantic salmon has become a major industry along the west coast of Scotland, and few sea lochs are now untouched by its effects (Black, 1996). The rain of fish faeces and uneaten food from moored cages to the sea bed can cause local enrichment, progressing in severe cases to complete faunal exclusion and the development of bacterial mats on the sediment surface (Dixon, 1986; Brown et al., 1987; Gowen & Bradbury, 1987). The radius over which a salmon farm affects the bottom sediments will depend on local factors such as the size of the farm, depth of water and current speed. In some circumstances effects on sediment chemistry and faunal composition are limited to the sea bed directly underlying the fish cages, and out to a distance of only ~15 m from them (Brown et al., 1987). In contrast, Weston (1990) found detectable effects on sediment chemistry out to 45 m from a fish farm, while benthic community effects were found to at least 150 m distance.

Megafaunal burrowers are certainly absent from heavily-impacted sea beds below salmon cages, but as for other forms of organic pollution there has been no systematic study of this. The threshold levels of enrichment causing changes in megafaunal communities around sea loch salmon farms have not been determined, and information is largely anecdotal at present.

Atkinson (1989) recorded changes in the south basin of Caol Scotnish, a narrow upper arm of Loch Sween, apparently associated with the installation of salmon cages there. In 1987, most of the fine mud bottom of the basin was clear of Beggiatoa, with bacterial mats restricted to the immediate vicinity of the salmon cages. The density of megafaunal burrows (Maxmuelleria lankesteri, Callianassa subterranea and Jaxea nocturna) was relatively high in the unimpacted areas, ranging from 1 - 5 m^-2. In 1988 a carpet of Beggiatoa covered the sea bed throughout the south basin and the sediment was close to becoming totally anoxic. Burrows were much fewer than in 1987 (0.46 m^-2) and confined to small patches of Beggiatoa-free sediment. In the year separating these observations salmon cages had been positioned at both ends of the south basin (they were previously near one shore in the centre of the basin) and production had been increased. Although the salmon cages were removed from Caol Scotnish in 1989, sediment conditions continued to deteriorate, with Beggiatoa cover increasing further (Dr L.A. Nickell, personal communication). Some recovery was apparent by 1990, with megafaunal burrow openings reappearing. The large size of individuals of Callianassa subterranea and Maxmuelleria lankesteri collected at this time suggested that these were not recent recruits, and that the megafaunal burrowers had persisted in this area of the loch during the period of peak enrichment.

Such observations suggest that the interaction between sedimentary organic content and megafaunal burrowers may be more complex than previously believed, and that some species may persist in highly-enriched conditions even when burrow openings are rare or absent.

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