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

Results

Boat Trials

Observations

Razor clams were the dominant species at all sites, making up 73 % of individual organisms observed (145 of 200). Razor clam species were the only organisms evidenced in the quadrat videos to emerge from the sediment and, as such, all non-target species observed were epifaunal (species which live on, rather than in, the seabed, Figures 7 and 9). Species assemblages varied with site: in Fife E. siliqua was the only razor clam species observed, and epifaunal species observed were the sea mouse Aphrodita aculeata, the sea star Asterias rubens, crab species (predominantly hermit crabs), ophiuroids (brittle stars) and shrimp similar to Crangon spp. ( Video A2 shows a sequence of video clips of E. siliqua from one quadrat placement). None of the ophiuroids or crustaceans could be identified to species level from the videos. In Loch Nevis there were two razor clam species, E. siliqua and E. arcuatus, and hermit crabs and gobies which could not be identified to species level ( Video A3 shows a series of clips taken from sampling in Loch Nevis). In the Clyde E. siliqua, A. rubens and hermit crabs were observed. Several mobile epifaunal organisms were observed entering quadrats in Fife and Loch Nevis following placement: Crangon like shrimp, gobies, flatfish, A. rubens and crab species.

Figure 7: Number of each species observed in East Fife ( FV Ensis) and Loch Nevis ( FV Nicola Jane).

Tracks made by the electric rods were visible in the sediment but the physical impact on the seabed was minimal. The brass rods leave shallow indentations, less than 1 cm deep on the seabed which can be seen in the video footage ( Video A1, Figure 8a). Occasionally debris on the seabed such as strands of kelp may get caught on the electric rig and dragged with it, increasing the physical impressions left on the seabed (Figure 8b). The divers also leave tracks in the sediment by dragging the bag that the razor clams are collected in (Figures 8c and d). Such disturbances are comparable to those left by the natural movement of debris in strong currents and would be expected to have a lower impact than the effects of bad weather.

Figure 8: Freeze frame stills of impact on seabed. a) lines from the electrodes; (b) kelp fronds caught on electric rig; (c) diver dragging bag of razor clams; (d) track from the dragged bag of razor clams.

Ensis Species

Divers reported that some razor clams had re-buried prior to placing the quadrats. Characteristic indentations in the sediment were evident in the video footage. However, it could not be ascertained if these represented buried clams or recently vacated burrows, so these were not included in the analysis. Eight clams were observed to have re-buried to the extent that 1 cm or less was showing (these clams were all still visible). These eight individuals were included in calculating the density of razor clams in the quadrat but excluded from analysis of recovery times to provide conservative estimates. All razor clams observed in Loch Nevis and 93 % of those observed in East Fife (54/58) recovered within 30 minutes following exposure to the electric field (Figure 9). The mean length of time between the electricity turning off and re-burial was 7 m 28 s ( SD = ± 5 m 10 s).

Figure 9: Frequency at which razor clams recovered over time. Each bar represents the number of individual razor clams that had recovered by the time shown after the electrodes had been switched off. Recovery start times are the first sign of movement from the razor clams. Recovery end time is the time of reburial to less than 1 cm showing above the sediment.

Razor clams in Mallaig began to recover, i.e. show first signs of movement following exposure to the electric field, significantly more quickly than those in East Fife (F _99,96, p = <0.001, Figure 10), and razor clams were more likely to start to move earlier in more densely populated ground (F _99,96, p = 0.004, Figure 10). Sea water temperature recorded was 12 °C at both sites and was therefore not a factor in the differences in recovery times. There was no effect detected for the size of the razor clams, nor were differences detected between species. None of the variables examined (species, density of razor clams, shell length or location) were found to have a significant effect on the end recovery time, i.e. re-burial to 1 cm shell left showing.

Figure 10: The effects of population density on the recovery start times of Ensis spp. The sampling vessel is indicated, which also corresponds to the location of the sampling: Ensis: East Fife, Nicola Jane: Loch Nevis.

Non-Target Species

Over half of the non-target individuals observed (24 of 42) had either recovered before the quadrat was positioned or did not react to the electric field. Most of the individuals observed were crustaceans (27, 23 of which were hermit crabs) or echinoderms (9). Three organisms observed in the quadrats could not be identified from the videos. All of the non-target epifaunal organisms observed recovered within 8 minutes of the electricity being switched off (mean = 3 m 20 s, sd = 1 m 22 s). There were significant differences between recovery times in different species observed (F ₃₄ = 2.375, p = 0.043, Figure 11). The fish and the starfish Asterias rubens were either unaffected by the electrofishing or had recovered before the quadrats were positioned. The crustacean, ophiuroid and polychaete species observed took several minutes to recover. There were no differences in the recovery times observed for non-target species between the locations studied.

Figure 11: The times of the first sign of movement following exposure to the electric field for non-target species.

Sandeels

Divers reported that some of the sandeels emerging following exposure to the electric field were not stunned and could not therefore be caught. However, reduced visibility and a limited field of view prevented the number of sandeels present being estimated from video footage. Of the 31 sandeels that were stunned and collected by divers, 20 had recovered within minutes of collection and escaped the net the diver used to collect them. 11 sandeels were landed and all recovered within five minutes of being placed in a bucket of seawater ( Video A4).

Tank Trials

Electrical Tests

During the 20 hour period in which the electric trials were run, the tank water was not recycled or replaced. Salinity remained constant at 35 ppm, and water temperature rose by 1 °C from 10.6 °C to 11.6 °C. Dissolved oxygen varied from 84 - 92.5 % saturation. Observations of the system in use and testing of water samples suggest that there was no appreciable liberation of any gaseous products. Further, none of the gas collection receptacles used for the duration of the tank trials showed any evidence of gas being evolved.

Configuration 1

Potential differences were measured at three points along the electrode and values of 22.6 - 22.9 v were recorded. These values were lower than the no-load measurement of 25 v due to the resistance of the cables, connectors and electrodes. This suggests that there was no significant voltage loss along the electrodes. The input current was 48.5 A. The power output from the system was calculated at 1.1 KW for the 1 m electrode separation (Power = Volts * Amps). An attempt was made to estimate field strength or voltage gradient. Further investigation of this may be required as the test point spacing was relatively coarse between the electrodes. Preliminary results suggest that field strengths in the order of 50 v m ^-1 were present at distances within 10 cm of the reference electrode. These reduced to less than 10 v m ^-1 at distances greater than 20 cm from the electrode. The vertical voltage profile showed no significant differences in measured voltages between the water column and the sediment. Within the limits of the measurement system this suggests that the field is symmetrical around the reference electrode (Figure 12).

Figure 12: Voltage contour plot around the reference electrode (voltage = 0) for Configuration 1. Voltage profiles are displayed for the vertical section from water surface to tank base on the right hand side; and for horizontal section between electrodes below each contour plot (distances in the voltage profiles are referenced to the bottom corner of the measurement rig).

Configuration 2

After reducing the separation distance between electrodes, a comparison of the voltage contour plots from Configurations 1 and 2 showed no significant differences. This suggests that, despite the limited space in the tank, the electric field properties could be determined in electrodes spaced 1 m apart as used in the fishing industry (Figure 13).

Figure 13: Voltage contour plot around the reference electrode (voltage = 0) for Configuration 2. Voltage profiles are displayed for the vertical section from water surface to tank base on the right hand side; and for horizontal section between electrodes below each contour plot (distances in the voltage profiles are referenced to the bottom corner of the measurement rig).

Configuration 3

Study of the voltage contour plot suggests that the voltage gradient outside the electrode will be significantly less than those between the electrodes (Figure 14).

Figure 14: Voltage contour plot around the reference electrode (voltage = 0) for Configuration 3. Voltage profiles are displayed for the vertical section from water surface to tank base on the right hand side; and for horizontal section between electrodes below each contour plot (distances in the voltage profiles are referenced to the bottom corner of the measurement rig).

Ensis Siliqua

Initial experiments without electrical stimuli showed 91 % survival over three weeks. One individual died after three days, a further three died in week three and 40 survived for the full length of the trial. From this it was determined that the conditions in the tanks were suitable for addressing the aims of this study, but that survival should not be monitored for more than one week to avoid mortalities due to husbandry difficulties, in particular those associated with reduced oxygen tensions within the sand. There was 100 % survival in both the control and stimulus tanks in all four experimental runs. All razor clams responded to the electrical stimulus and emerged from the sand within 37 seconds of exposure to the electric field (mean ± 1 SD = 13 ± 8.5 s, Video A5). Once emerged from the sediment the razor clams were observed to repeatedly "kick" their muscular foot whilst the electricity was switched on. 65 % of the razor clams reburied within 1 h and the remainder within 12 h following the stimulus. All remained in the sand until they were removed at the end of the experiment. The recovery times observed in the tank trials were slower than those observed in the boat trials (mean of 14 min not including those that took over 1 h c.f. 7 min 28 s in the boat trials). Shell length was not a significant explanatory variable for emergence or recovery times.

Non Target Species

There was no significant difference in survival between species or treatment groups (F _5,24 = 1, p = 0.439). Of the 60 individuals used two individuals died. Both were P. bernhardus and were in the same stimulus trial (dying after 2 and 3 days). There were observed behavioural responses to the electrical stimulus that were highly consistent within a species and varied between species.

Asterias rubens (common starfish)

A. rubens did not appear to respond to the electrical field ( Video A6). Individuals that were traversing the sediment at the time the electricity was switched on continued to do so, and did not change direction. Those that were not moving did not start to move in response to the electricity. (Picture: Keith Summerbell, Marine Scotland Science).

Pagurus bernhardus (common hermit crab)

All individuals of P. bernhardus exposed to the electric field retreated into their shells and did not re-emerge until after the electrodes were turned off ( Video A7). (Picture: Keith Summerbell, Marine Scotland Science).

Spisula solida (surf clam)

In response to the electric field some individuals of S. solida were observed to kick their muscular foot, a behaviour that was not observed in the control individuals. Kicking stopped after the electric field was switched off. Most individuals showed no visible reaction to the electric field and remained stationary with their valves closed for the duration of the trial ( Video A8). (Picture: Bob Williams, www.marlin.ac.uk).