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

Discussion

The tank tests of the properties of the generated electric field show that between the electrodes animals are exposed to field strengths to a maximum of approximately 50 v m ^-1 close to the electrodes. The field extends outside of the pairs of electrodes and downwards into the seabed. Videos from the boat trials suggest many razor clams respond to the electric field by emerging from the seabed, although it cannot be ascertained from this study if all razor clams will emerge. This is supported by data from the tank trials where all individual razor clams in the stimulus tanks emerged from the sediment within seconds of exposure to the electric field. The size of the razor clams was not an important variable for emergence or recovery times in either the boat or the tank trials, which is consistent with other studies (Henderson and Richardson 1994, Muir 2003), and there were no differences in recovery times between Ensis species. Whilst the overall recovery time was not found to be influenced by any of the variables studied, razor clams in Loch Nevis were quicker to start recovering than those in East Fife. The density of razor clams also affected recovery start times, as razor clams in more densely populated areas started to recover more quickly. It may be that competition for space within the population drives razor clams to rebury faster. Razor clams are known to be highly mobile (Fahy 2011) and fished ground is quickly recolonized (Fahy 2011), so it may be that competition for a good space is high.

Of the 133 razor clams observed to emerge from the seabed during the boat trials, four were unable to rebury within 30 minutes. Of these, one was trying to bury into the metal frame of the quadrat, one had kicked its foot out of its shell and seemed unable to recover (Figure 15b), one was predated by a crab and fish before it could recover and the fourth was moving its foot but had not yet reburied after 30 minutes. This indicates that whilst most of the razor clams to emerge in response to the electric field recover and rebury quite quickly, those that are slower are left vulnerable to predation (Figure 15b and c). Whilst most of the razor clams caught during commercial fishing will be removed by divers, smaller undersized clams are left behind and their recovery is important to the sustainability of the fishery. Recovery times recorded during the boat trials were much shorter than those observed in the tanks trials. This is most likely a result of the colder water temperatures in the tanks (12 °C during the sea trials compared to 8 °C in the tanks,) and because razor clams in the tanks did not have the opportunity to establish semi-permanent burrows as they would in their natural habitat (Muir 2003). Recovery into a pre-established burrow is more rapid than creating a new one (Muir 2003). In addition, the electric stimulus used was longer and other cues for recovery that may be important such as water movement and the presence of predators were absent. Longer exposure to an electric field may also increase the likelihood that a razor clam has "kicked" itself out of reach of its own burrow, increasing recovery time. The repeated kicking observed in both boat and tank trials appears to be an involuntary response to the electric field, emphasised where the foot kicks out of the shell and the clam seems incapable of pulling it back in.

Figure 15: Observations from the boat trials. (a) Three non-target species with an emerged razor clam. The top left circled animal is a shrimp, the top right an ophiuroid and the centre is a starfish A. rubens. (b) Razor clams in different stages of recovery within a single quadrat from almost completely buried in the top right (circled) to one which has kicked its foot out of its shell and is unlikely to recover. (c) A crab eating an emerged razor clam.

Ensis spp. were the only invertebrates observed emerging from the seabed. Burrowing urchins such as Echinocardium cordatum, which are known to inhabit Ensis grounds in the Clyde (Hauton et al. 2003), are related to A. rubens and so may be similarly unaffected. Other species may be stunned, but not stimulated to emerge from the sediment. This may increase their vulnerability to burrowing predators, but will not expose them to fish or crustaceans. Further research is required to establish the effects of electrofishing on burrowing species. The non-target species observed on the seabed (Figure 15a), and the sandeels recorded separately, recovered much more quickly than Ensis spp., as did the non-target species studied in the tank trials. The physical impact of the fishing gear on the seabed was minimal and comparable to that caused by bad weather. As razor clam beds occur in moderately exposed areas, the habitat and benthic communities are adapted to recover quickly from physical effects of the extent caused by electrical fishing equipment. There was no evidence of chemicals being released into the seawater, as chloride compounds were not found to evolve from the electrodes during the tank trials, nor was there any indication of erosion of the electrodes as has been reported in DC systems (Woolmer et al. 2011). This fishing method appears, therefore, to have a low effect on non-target species and the marine environment.

These results are consistent with other studies on electrofishing. Survival and behaviour of adult and juvenile freshwater mussels have been found to be unaffected by exposure to electrofishing in freshwater (Holliman et al. 2007), and high survival rates with no impacts on feeding and behaviour has been reported for Crangon spp. and non-target invertebrate species likely to be affected by the Crangon fishery (Polet et al. 2005a). In both the Dutch flatfish fishery ( ICES 2006) and the Belgian Crangon fishery (Polet et al 2005b) the use of electric trawling instead of tickler chains has reduced bycatch and the physical impact of trawling on the seabed. Similarly, where electrofishing for razor clams is used as an alternative to dredging both bycatch and habitat destruction are greatly reduced. Whilst electrofishing is not a zero impact fishing method (a feature shared by all methods), the low effects on non-target species and the benthic habitat make it a far more environmentally friendly method than dredging, as has been acknowledged by Scottish Natural Heritage ( SNH 2014).

The razor clam fishery in Scotland has grown rapidly. In an effort to provide some restraint on further uncontrolled growth, new national legislation has recently been introduced and vessels now require a specific licence to land razor clams (Scottish Government 2014), as electrofishing remains illegal. In order to achieve better compliance with the EU regulation banning electrofishing, the new legislation also increases the penalty for using electrofishing to catch razor clams to a maximum of £10,000. If this legislation pushes more vessels towards hydraulic or suction dredging for Ensis spp. there is a risk that razor clam populations would be more rapidly impacted. Widespread habitat destruction could occur, similar to that reported in Ireland in the 2000s (Fahy 2011), where the Ensis populations and the seabed habitats are yet to fully recover.

Such a development would be unfortunate given the observations in this study pointing to the potential environmental benefits of electrofishing for razor clams. Other advantages conferred by the method include the highly selective nature of the fishery, where clams are selected at the seabed avoiding the need to bring young recruits or undersized individuals onto the deck of a boat. Waste of the type associated with mechanical dredging, where shell damage and breakage reduces the potential value of a catch, is also highly reduced.

It is also true however, that electrofishing is very efficient with little opportunity for marketable razor clams to escape capture once the track of a pair of electrodes passes them. This, in combination with a relatively slow growth rate and late maturity, makes them potentially vulnerable to overexploitation. As with all exploited fish and shellfish species, the rate of harvesting should be regulated to safeguard sustainability. Given the patchy distribution of this species, occurring as it does in discrete populations, it is important that regulatory measures tailored to the different areas are developed. Ahead of this, there is an urgent need to determine the size of the different populations and follow up work should include stock assessments using appropriate survey techniques.

This study has made important observations on the immediate and short term effects of electrofishing on individual species. Further research may be required to establish the medium to long term implications and if there are any effects of electrofishing on fertility and fecundity of both razor clams and non-target species.