Lugworms

Ragworms, Neridae

White ragworms Nephtys species

Other Species

The population biology of lugworms Arenicola spp. (see Appendix) is thought to make them a particularly resilient and reliable bait species, although the life history of only one species, Arenicola marina, has been described in literature.

Blow lug Arenicola marina is widely distributed around the British coasts in suitable sediment habitats. It seems likely that most studies of lugworm collection have targeted this common species. Blow lug are particularly abundant and very resistant to heavy exploitation because harvesting adult worms for bait usually does not affect the supply of juveniles from the nursery beds elsewhere on the shore, if these areas are left untouched. They are able to recolonise dug beds by recruitment of young worms from separate nursery beds on the upper shore or by migration of adults from unexploited populations in adjacent areas (possibly including subtidal beds), provided these are not also exploited (Olive 1993).

Bait diggers usually remove only about 50% (Heiligenberg 1987) or 70% (Blake 1979a) of blow lug A. marina present in each area dug. Some professional and experienced bait diggers may remove more, and inexperienced diggers could be much less efficient. Studies of the recovery of lugworm beds after bait digging have indicated that complete recolonisation occurs quickly (one month after areas had been experimentally dug out at Whitley Bay: Blake 1979a). Cryer et al. (1987) found no significant increase in the density of worms in depopulated areas on South Wales beaches after six months during the autumn and winter. However, the initial densities at these sites were very low (9 and 16 worms/m²), and population growth would not be expected until spring and summer.

There is only one well-documented example of a blow lug A. marina bed being dug out by bait collectors. This occurred in exceptional circumstances when a formerly protected area of Budle Bay, in the Lindisfarne National Nature Reserve, was opened to bait diggers during a period of unusually heavy commercial exploitation in the winter of 1984 (Olive 1985a, 1993). Density of worms in the most heavily dug area of 200 m x 1 km fell from 40 m-² to <1 m-² within a period of about six weeks. In total, about four million worms were removed. Recovery took place within a few months with immigration of worms from neighbouring areas when the bait diggers ceased to use the site, even though this was during the winter period when lugworm populations are at their lowest levels. This site was, however, a relatively small area within a large expanse of intertidal sand flats, with ample capacity for recolonisation from nearby populations. Recovery after over-digging may not be so rapid where lugworms are present on a small pocket beach, with limited opportunity for recolonisation of the dug beds by adult worms from elsewhere. If the nursery beds of small worms at the top of the beach have also been affected by digging, recruitment to the adult population will be reduced. This may have a serious long-term effect upon the worm stocks.

Black lug, A. defodiens, is a relatively recently described species, whose distribution has not been as well studied. Its populations appear to be confined to the lowest part of the shore on more exposed coasts and it presumably also occurs in adjacent subtidal areas, suggesting that the species is likely to be widespread in suitable habitats. If so, only part of the population will be affected by bait collection at any time. However, because black lug casts are not permanent, it is not easy to calculate population densities and depletion rates caused by collection. Additionally, worm populations at the bottom of the shore and in the shallow sublittoral are difficult to study. Possibly for this reason, no studies are known to have investigated the effects of bait digging on black lug, but it is unlikely that this species has been included in former blow lug A. marina studies – these have concentrated on worm populations on less exposed shores. As a result there are no published data on the impacts of collection on black lug, and its ability to recolonise from subtidal beds or nursery grounds (if any) is unknown. This species is now commonly collected by bait pumping, which is covered in a later section.

Ragworms are quite widespread in more sheltered sediment areas. Their life cycle provides a naturally high population turnover, with the death after breeding of at least one third of the population each year followed by swift recruitment from the larvae (Brafield and Chapman 1967, Olive 1993). This enables a population to recover quickly from baitdigging, provided that some adults remain to breed. Refuge populations will usually be present in adjacent subtidal areas and will act as a source of juveniles. These species therefore have a resilient population ecology and are not considered to be threatened (with rare local exceptions) by bait digging activities.

Exploited and unexploited populations of king ragworms were studied for one year on the north-east coast of England by Blake (1979b). The densities of these populations were not significantly different, at about 15 m-² in summer and 3 m-² in winter, indicating that the dug population (which was most heavily exploited in the summer) was probably not threatened by bait digging.

The unusual population of king rag Nereis virens in boulder clay in the Menai Strait, however, exhibits delayed maturation, which has the potential to make over-digging a serious problem for this species (Olive 1993). Because only a small proportion of the worms in the population breeds each year, the impact of baitdigging may be much more severe. This is because a significant proportion of large worms are likely to be taken by bait diggers before they mature, and the small number which do mature produce a relatively small number of eggs compared with other populations (despite the millions of eggs produced by each large spawning female). The king rag population density in the Menai Strait may also be smaller than normal because of predation pressures on small worms by the largest individuals and territorial behaviour by the adults; it is certainly depressed below its carrying capacity by heavy bait digging. Olive (1987) recorded densities of 5-15 king rag per 25 m². Bait diggers selectively search for individual large adults and may be very efficient in taking a high proportion of the sparsely distributed worms present. In this situation, intensive baitdigging can cause a significant reduction in the worm population, particularly if there is little opportunity for recolonisation from adjacent areas on the shore or in the intertidal. Suitable habitats below the low water mark in the Strait also appear to be scarce, so a refuge population is not available to act as a source of recruitment.

Reports from anglers of massive ragworms in similar habitats elsewhere (e.g. boulder clay underlying sediment in Milford Haven) suggests that Nereis virens populations with these characteristics may occur elsewhere.

White rag or catworms are quite local in distribution compared with other polychaete bait species and are hard to find. Locations of worm beds are often guarded closely by diggers who recognise that their populations may easily be severely depleted by baitdigging, as a result of their slow growth, longevity, very infrequent reproduction and low recruitment rates (Olive 1985b, Caron et al. 1995). White ragworms are also much in demand by some anglers as bait (Olive 1993, Dyrynda and Lewis 1994). Several sea anglers and commercial bait collectors consulted have expressed concern over the over-exploitation of some populations, particularly by match fishermen and commercial collectors visiting beds from other parts of the country, and suggested that temporary closure of depleted beds would be a useful management option. This might be the case, but possibly only if successful recruitment occurred during closure.

Digging is also used for the collection of burrowing bivalves, including razor shells Ensis spp., soft shell or gaper clams Mya spp., quahogs Mercenaria mercenaria, and other carpet shells (see section 4 for descriptions of examples of these species). These may be taken for personal consumption or for fishing bait. Collection could potentially impact the target population where the species is particularly long-lived and slow to recruit, but most large species of bivalve are more commonly found below the low water mark, with only a small proportion of the total population being vulnerable to hand collection on the shore. In a few locations, however, populations of large long-lived bivalves occur in the intertidal. These may form part of an infaunal community of high nature conservation interest purely because of their unusual distribution and accessibility to researchers for scientific study and monitoring purposes. Under these circumstances, removal of this intertidal population by collectors may be of nature conservation concern, even if only a small proportion of the overall population in the area is affected. Additionally, it is possible for the entire population of a large burrowing species to be found in the intertidal, where it is vulnerable to exploitation (for example, where suitable habitats are not present in adjacent subtidal areas). Collection of such species may therefore be of nature conservation concern.

Next section                     References