Scottish Marine and Freshwater Science Volume 5 Number 14: Electrofishing for Razor Clams (Ensis siliqua and E. arquatus): Effects on Survival and Recovery of Target and Non-Target Species

Trawling and tank based trials were conducted to assess whether electrofishing (which is currently prohibited under EU regulations) for razor clams Ensis siliqua and E. arquatus affects survival and behaviour patterns in Ensis spp. and non-target species.


Razor clams ( Ensis spp. also known as razorfish or, more colloquially, "spoots" in Scotland) are common burrowing bivalve molluscs found in sandy intertidal and subtidal areas throughout Europe (Muir 2003). They burrow in sandy sediments and position themselves perpendicular or diagonal to the seabed with their valves and siphons extended into the water column to suspension feed. When threatened they are able to rapidly withdraw deep into the sediment using a strong muscular foot (Muir 2003). In Scottish waters there are two commercially important species: Ensis arcuatus, colloquially known as bendies; and the larger and more valuable pod razor Ensis siliqua (Breen et al. 2011) which have different habitat preferences but can occur in the same areas. E. arcuatus inhabits coarse sandy areas which are partially sheltered (Breen et al. 2011). It can reach 180 mm in length and reaches sexual maturity between 73 and 130 mm (Muir and Moore 2003). E. siliqua generally prefers more sheltered areas with finer sand or muddy sand (Breen et al. 2011). In Scottish waters they are slow growing, taking 4-5 years to reach 100 mm, in comparison to 3-4 years in Wales and 1 year in Portugal. They also mature at larger sizes: between 118 - 140 mm in Scotland compared to between 60 - 100 mm in Portugal (Muir and Moore 2003). Both species require oxygenated sands and have a low tolerance of reduction, where low oxygen levels cause anoxic conditions within the sand, indicated by a characteristic black colouration below the surface (Holme 1954). As such, populations may be vulnerable to organic enrichment and increased freshwater runoff (Muir 2003) which can lead to a reduction in sediment oxygen levels. Razor clams are highly mobile and the ability of individuals that live on the edge of a fished ground to rapidly move into a depleted bed following fishing activity can lead to an overestimate of the abundance of the species and its ability to recover. This can lead to intense and sustained fishing effort of known fishing grounds as has been observed in Spain (1980s), Portugal (1990s) and Ireland (2000s) where profitable razor clam fisheries have been depleted (Fahy 2011). Once depleted, studies on Irish populations have shown that razor clam stocks are very slow to recover (Fahy 2011).

Ensis Fishery in Scotland

At present the market preference is for the larger and more valuable E. siliqua (Muir and Moore 2003) which are priced by size (small, medium and large) for live animals (N. Grieve pers. comm.). E arcuatus are all sold at the same price and generally at a lower value per kilo than E. siliqua. Ensis spp. caught in Scottish waters supply a limited market in the UK (mostly restaurants), shrinking Spanish and Portuguese markets, and, for E. siliqua, a rapidly growing market in South East Asia, predominantly in China (J. Grieve and A. Forbes pers. comm.). There has been a small fishery in Scotland since at least 1990 (Hauton et al. 2011), which has steadily grown (Figure 1). Razor clams were first mentioned in the Scottish Sea Fisheries Statistics in 1994, when around 43 tonnes were landed in Scottish ports valued at £60,000 (Muir 2003). By 1997 landings were up to 200 tonnes worth £500,000 (Scottish Government 2013a), and in recent years reported landings of Ensis spp . have increased to 526 tonnes in 2008 and to 900 tonnes in 2012 (Scottish Government 2013b). The value of the 2012 landings in Scotland was £2,559,000. Ensis spp . are the main species (defined as where more than 20 tonnes of a given species were landed in 2010) at Anstruther, Mallaig, Oban, Ayr and Campbelltown (Scottish Government 2013b).

Figure 1: Scottish landings of Ensis spp. 1997-2012 by tonnage (a) and value (b). Source: Scottish Sea Fisheries Statistics 2012 (Scottish Government 2013b).

Figure 1: Scottish landings of Ensis spp. 1997-2012 by tonnage (a) and value (b). Source: Scottish Sea Fisheries Statistics 2012 (Scottish Government 2013b).

Available information on the size and extent of Scottish razor clam stocks is limited. There are no stock estimates and population dynamics are poorly understood. At present there are few measures limiting this fishery as there are no quota systems and no total allowable catch ( TAC) figures (Hauton 2011) however, from Autumn 2014 vessels will require a licence to land razor clams. Currently the only restriction on landings is a minimum landing size ( MLS) of 100 mm ( EU Regulation 850/98, Annex XII), which is set for the genus and applied to all European stocks. This is considered to be too low to ensure that all individuals in Scottish waters have reached sexual maturity prior to capture (Muir 2003). Under management proposals for Inshore Fisheries Groups (Scottish Government 2013c) there is a suggestion that the MLS should be increased for Scottish landings.

Fishing Methods

Salting and Hand-Pulling

Historically, razor clams were collected from Scottish beaches during low spring tides, often as an additional source of income for crofters. This was usually done by "salting" whereby salt is poured onto indentations in the sand which indicate the presence of razor clams. Salting irritates the clams into emerging from their burrows and they can then be collected. This method has been adapted for divers working in sub-tidal waters. Salt is dissolved into hot seawater to create a hyper-saline solution that divers take down in small watering cans. The solution is poured onto the indentations in the seabed and the clams will usually emerge within 15 minutes. Salting in the intertidal zone causes a localised increase in salinity which dissipates following a flood tide and has not been shown to have any effect on benthic communities (Constantino et al. 2009). When used by divers, however, localised increased salinity at the seabed can be persistent (Muir 2003). Skilled divers are also able to pick razor clams directly out of the sediment by gripping the valves and pulling the clam up with a twisting motion. This hand-pulling technique requires a great deal of skill and practice to perfect but can be very effective. It is much more difficult in intertidal areas as the clams can detect footsteps approaching and withdraw into the sand (Muir 2003). Both intertidal salting and hand-pulling by divers are considered environmentally friendly (Constantino et al. 2009). Neither has associated bycatch nor impacts on the benthic habitat, however, both methods are low yield and are unlikely to be widely adopted by the commercial fishery (J. Grieve pers. comm.).

Hydraulic and Suction Dredging

Two dredge types have been developed for fishing Ensis species: the suction dredge and the hydraulic dredge. Suction dredges operate by using a suction pump to remove razor clams and any other animals living in the seabed, which are then pumped through a pipe onto the boat. Their use in the Scottish razor clam fishery has been reported in Loch Gairloch and Orkney (Hall et al. 1990). Hydraulic dredging is thought to be the favoured dredge method in Scotland and its impacts are the best studied (Hall et al. 1990, Tuck et al. 2000, Hauton et al. 2003, Hauton et al. 2007). It involves pumping water into the seabed to fluidise the sand (Tuck et al. 2000) in the first instance, and then dragging a metal frame box dredge through the fluidised sand to harvest any animals living in the seabed. The seabed itself can remain fluidised for some time after dredging (over 11 weeks, Tuck et al. 2000, Figure 2), and traces of the dredged trench can remain visible for three years (Gilkinson et al. 2003). Repetitive fluidising of sediment has the potential to make the substratum inhospitable to species which prefer finer particulates, including E. siliqua, as the smallest particles may be lost, changing the properties of the seabed (Fahy 2011). Hydraulic dredging can remove up to 90 % of the target species from the seabed (Hauton et al. 2003), however, there are high levels of bycatch associated with this method. Whilst in some parts of Scotland over 70 % of the catch are Ensis species (Tuck et al 2000), in the Clyde it can be far lower (25 %, Hauton et al. 2003) due to the high abundance of the burrowing urchin Echinocardium cordatum. Dredging is restricted in sheltered areas to protect sensitive habitats under the Inshore Fishing (Scotland) Act 1984 (Tuck et al. 2000).

Figure 2: Photograph of cross section of water jet dredge track with depth marker in place. Banding on vertical rod = 5 cm. (Figure 4 in Tuck et al. 2000).

Figure 2: Photograph of cross section of water jet dredge track with depth marker in place. Banding on vertical rod = 5 cm. (Figure 4 in Tuck et al. 2000).

Dredging has advantages over diver caught clams (by salting or hand-pulling) as it results in a larger catch. This catch, however, is generally of a poorer quality as valves can be chipped or broken in the dredge and the clams tend to accumulate grit in their valves during dredging, making them more difficult to sell on the more profitable live market. Dredging for razor clams is not currently believed to be widespread in Scottish waters. There have been several studies conducted on the immediate and short term impacts of dredging in Scotland (Hall et al. 1990, Tuck et al. 2000, Hauton et al. 2003), but the widespread long term effects have been more comprehensively studied in Ireland. As commercial SCUBA diving for shellfish is illegal in Ireland, Irish razor clam stocks have been exploited almost exclusively using dredges. Gormanstown bed in Co. Meath was heavily exploited using hydraulic dredges between 1997 and 2005 which impacted on biodiversity. In the seven years following, the number of species present in the seabed did not recover to pre-1997 levels. Furthermore, the species composition changed. Scavengers and opportunistic deposit feeders are now more abundant and E. siliqua stocks have never recovered. E. siliqua has been replaced with another suspension feeding bivalve Lutraria lutraria (Fahy 2011).


Since 2004 divers have been using electricity to stimulate razor clams to emerge from the seabed (Breen et al. 2011). Up to three pairs of electrodes are slowly dragged across the seabed, followed by divers who collect emerging razor clams (Video A1). Electrofishing is currently illegal in European waters ( EU Regulation 850/98, Article 31) and at present any vessel caught using electrical fishing gear in Scotland will have their equipment confiscated and face a fine of up to £2,000. It is proposed that this penalty will rise to a maximum fixed penalty of £10,000 in late 2014 (Scottish Government 2014) to address potential financial gain. Despite these measures, electrofishing is likely to be a widespread method of collecting razor clams in Scotland as alternative methods available are either far less efficient (hand pulling and salting) or yield a poorer quality, less valuable product (broken shells and excessive grit in dredged razor clams). Marine Scotland estimated that there were 14 - 27 vessels actively fishing for Ensis spp . in Scotland throughout the year in 2011 (Breen et al. 2011). Under new 2014 legislation vessels will require a licence to land razor clams and licences will be issued this year (Marine Scotland Licencing pers. comm.). Within the fishing community it is believed that all boats currently harvesting razor clams in Scotland are electrofishing to some degree (R. Grieve pers. comm.), and that this is the only method currently available that is economically viable (J. Grieve and A. Forbes pers. comm.).

Although electrofishing is not a new concept (Stewart 1967) there has been a recent resurgence in research into electrical fishing techniques in Europe. Where there is evidence to support the assertion that electrofishing can be less damaging to the marine habitat or reduce bycatch in comparison to conventional fishing methods, derogations have been issued to allow electrofishing for specified target species in European waters. Recent research has focused on the effects of electric trawling for flatfish by the Dutch fishery (van Marlen et al. 2014) and the brown shrimp Crangon crangon by the Belgian fishery (Polet et al. 2005a). Studies on the effects of electric trawling on benthic invertebrates have concluded that effects are low, but that subsequent survival and food intake rates are affected in some species (van Marlen et al. 2009). Research on the effects of electrofishing for Ensis spp. is limited. Organisms are likely to be exposed to an electric field for far longer in the razor clam fishery than the 4 s exposure used in the van Marlen et al. (2009) study. Whilst a fishing derogation was issued in Ireland in 2010 to develop an electric dredge for razor clams (Breen et al. 2011), to date no research has been published on this technology. The only experimental study on electrofishing effects in UK waters so far was conducted in Wales (Woolmer et al. 2011) and focused on short term effects on behaviour and medium term effects on biodiversity. They found organisms to be stupefied and disorientated in the minutes following electrofishing activity but found no change in species composition or abundances in the short (24 hour) and medium (28 day) terms.

Objectives of this Study

This study aimed to investigate the immediate behavioural and short term survival effects of electrofishing on Ensis spp.; identify the main non-target species likely to be affected by electrofishing; determine whether exposure to electrofishing affects invertebrate survival in the short term; and to define the field properties of the electrodes used through a combination of in situ boat observations and tank based experiments.

The specific objectives for the boat trials were:

  • To monitor and record recovery rates of Ensis spp. following emergence from the sediment stimulated by the electrodes
  • To identify non-target species which may be affected by Ensis electrofishing
  • To record the recovery of non-target species following exposure to electrofishing
  • To determine if electrofishing may cause mortalities in sandeels
  • To make video observations of the impact of the electric rig on the seabed.

The objectives for the tank experiments were:

  • To determine the properties of the electric field generated by electrofishing
  • To record the behavioural response to, and recovery from exposure to, an electric field in E. siliqua
  • To monitor the short term survival of E. siliqua (for five days) following exposure to an electric field
  • To record the behavioural response to, and recovery from exposure to, an electric field in three non-target invertebrate species
  • To monitor the short term survival of three non-target invertebrate species (for five days) following exposure to an electric field.

This study did not make any attempt to assess the state of the numerous razor-clam populations around Scotland or to offer advice on the likely scale of fisheries in any of the areas. Regardless of the method of fishing, it is clear that the level of fishing activity and removal rate of razor clams needs to be carefully aligned with the available resource in order to maintain a sustainable fishery. The authors recognise that appropriate stock assessments will necessarily form the main objective of any follow up work conducted in future studies.


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