Impacts on Nephrops populations

Impacts on other megafaunal burrowers

Impacts on sea pen populations

The waters around Scotland yield approximately one-third of a total world Nephrops catch of about 64000 tonnes (1995 figures, Marine Laboratory, Aberdeen). In terms of Nephrops landings per unit area fished, the Firth of Forth is the most heavily fished area, whereas the Fladen Ground in the North Sea is relatively lightly exploited (SOAEFD, 1997). With the exception of the Fladen, most Scottish stocks appear to be fully or over-exploited. Considerable efforts are currently being made to develop reliable methods for stock assessment (Tuck et al., 1997b) as a necessary step towards determining the effects of the fishery on Nephrops populations.

Recent findings from the western Irish Sea suggests that the structure of some Nephrops populations may render them vulnerable to over-exploitation. A large (approximately 3400 km²), discrete area of muddy sediments between the Isle of Man and the Irish coast supports a large Nephrops fishery (average yield 8252 tonnes year^-1 between 1989 - 1993). In spring and summer a large near-surface gyre (a circulating water mass) forms over a static dome of colder bottom water left over from the previous winter (Hill et al., 1997). The existence of this circulation system coincides with the period when Nephrops larvae are present in the plankton. Larvae appear to be retained within the gyre and eventually settle back onto the underlying muddy sea bed rather than being carried by currents into areas of unsuitable substratum (Hill et al., 1996). The retention of larvae by the gyre may be essential for the maintenance of the local Nephrops population, which acts as a largely self-perpetuating unit. The situation may be complicated by the fact that some larval production occurs in early spring, prior to the establishment of the gyre, and the population may therefore not be entirely ‘closed’, but it is possible that over-exploitation of Nephrops in this area could lead to a self-perpetuating population decline due to a reduction in recruitment.

On a smaller spatial scale, observations using towed video have given some insight into the localised effects of trawling for Nephrops in sea lochs. In Loch Fyne, Howson & Davies (1991) observed the highest densities of Nephrops in areas of muddy substratum close to, or surrounded by, submarine rock outcrops or boulders. These features presumably give some protection from trawling, as boats will tend to avoid areas where there is a high risk of damage to the fishing gear. In the intensively-trawled areas of lower Loch Fyne Howson & Davies considered it likely that trawling had reduced the density of the Nephrops population. Atkinson (1989) came to similar conclusions in his survey of Loch Sween. Variations in the density of Nephrops burrows in different regions of the loch seemed likely to reflect differences in trawling pressure. Highest burrow densities were found where submerged rock pinnacles or arrays of anchored buoys limited access by trawlers. However, it is possible that other factors such as differences in sediment type around submarine rock outcrops might also contribute to this pattern, and identifying a direct cause-and-effect relationship is difficult in cases such as this (Hall et al., 1993).

These limited observations lead to the general conclusion that trawling can possibly reduce the density of Nephrops in the confined situations of sea lochs. It is also important to note that even where the local topography is unfavourable to trawling, sea loch populations are still usually subject to exploitation by creel fishing. However, the resilience of Nephrops populations to fishing pressure may be enhanced by the fact that juveniles and egg-carrying

females remain within their burrows and are not usually caught in trawls (R.J.A. Atkinson, personal communication). Self-seeding populations such as that in the north-western Irish Sea are probably the exception rather than the rule, and most stocks have the potential to ‘bounce back’, even after heavy fishing pressure.

Atkinson (1989) concluded that trawling was unlikely to affect other megafaunal burrowers to any great extent. The deep-burrowing species (mud-shrimps and Maxmuelleria lankesteri) will usually be too far below the sediment surface to be displaced by towed fishing gear. The uppermost parts of burrows will be disrupted by trawling, but observations in Loch Sween have shown that surface openings are soon re-established following experimental disturbance (personal observations). However, in Loch Fyne, Howson & Davies (1991) found that the density of all burrow types was lower in frequently-trawled areas than in sites protected by submarine obstructions. The impact of bottom trawling on benthic communities has been the subject of intensive study in recent years (Auster et al., 1996; Kaiser & Spencer, 1996; Tuck et al., 1998), and research on the cumulative effects of trawling on deep-burrowing megafauna is currently under way in the Mediterranean (M.J. Kaiser, personal communication).

The indirect effects on the burrowing community arising from the selective removal of Nephrops are unknown.

Sessile animals such as sea pens which project above the sediment surface are clearly likely to be damaged or uprooted by the passage of a trawl. In Loch Sween, Virgularia mirabilis was more abundant in the vicinity of rock pinnacles than on open mud plains (Atkinson, 1989). Protection from trawling is one obvious explanation for this, but it must also be remembered that as suspension-feeders, sea pens may require a certain degree of water movement, and that more favourable conditions for growth may exist where local hydrography is modified by irregularities in the sea floor. In Loch Fyne, Virgularia was scarce on the deeper muds irrespective of whether or not these were trawled (Howson & Davies, 1991). At shallower depths where the species was more abundant, densities were similar at untrawled (3 - 4 individuals m^-2) and trawled (2 - 7 m^-2) sites. Howson & Davies concluded that there was no clear evidence that trawling had affected Virgularia densities in Loch Fyne. The resilience of Virgularia to trawling is supported by the findings of Tuck et al. (1998), who found no changes in density in a sea loch following experimental trawling carried out repeatedly over an 18-month period.

Hoare & Wilson (1977) observed that Virgularia was absent from areas of Holyhead Harbour subject to disturbance by dredging or boat moorings, although a direct cause-and -effect relationship was not demonstrated.

Virgularia mirabilis is able to withdraw rapidly into the sediment when disturbed, an ability which should provide some protection from dislodgement by trawls. Pennatula phosphorea is also able to withdraw, but the taller Funiculina quadrangularis cannot do so. This species may therefore be more vulnerable to human-induced disturbance of the sea floor, particularly by mobile fishing gear. It is possible that the apparent absence of Funiculina from open-coast Nephrops grounds may be a consequence of its susceptibility to trawl damage (D.W. Connor, personal communication).

Creeling for Nephrops is pursued in many coastal areas, including those in which trawling does not take place. This is consequently another possible source of damage to sea pens. However, an experimental study in Loch Broom found that sea pens were quite resilient to being smothered, dragged or uprooted by creels (Kinnear et al., 1996). All three species proved able to re-anchor themselves provided the basal peduncle remained in contact with the sediment surface, and mortality rates following experimental creel disturbance were very low.

The overall conclusion arising from these studies is that Funiculina is likely to be the sea pen most susceptible to damage by fishing gear, but that Virgularia and Pennatula will be much less affected.

References