|dc.description.abstract||This study investigated the survival, incidence and severity of injuries and physiological reactions to stress caused by boat electrofishing in comparison with standard netting techniques in three New Zealand native fish species. Shortfin eel (Anguilla australis), grey mullet (Mugil cephalus) and common smelt (Retropinna retropinna) were captured from the Waikato River and its surrounding lakes with standardised boat electrofishing techniques (60-Hz pulsed direct current, 45-60% of range with the 0-500-V peak voltage machine setting, 3-4 Amps root mean square output). Fish were captured in water of 133-154 μScm─1 ambient conductivity at 18-21oC. Capture techniques used for comparison were fyke netting for eels, gill netting for grey mullet and beach seine netting for smelt.
Survival was assessed in eels that held for 30 days after capture and smelt that were held for 30 min after capture. Several different methods were used to assess the incidence and severity of injuries. External examinations were used to assess branding and abrasions, and internal injuries were assessed by radiographic imaging and bilateral filleting to reveal haemorrhaging and spinal damage. Finally, physiological stress was determined by an analysis of haematocrit and haemoglobin concentration and an analysis of blood plasma ions (sodium, potassium, magnesium and calcium).
Survival over 30 days of shortfin eels captured by electrofishing (92% for Lake Areare eels and 96% for Lake Rotongaro eels) was similar to that for fyke netting (92% for Lake Areare and 88% for Lake Rotongaro). Two eels appeared to have died from fungal infections as no haemorrhaging or spinal injuries were apparent among the mortalities. External examinations revealed that no abrasions or branding occurred in eels captured by electrofishing; however, abrasions occurred in eels captured by fyke netting (24% for Lake Areare eels and 16% for Lake Rotongaro eels).
Rates of internal haemorrhaging immediately after electrofishing were inconsistent (0% in Lake Areare and 28% in Lake Rotongaro); in Lake Rotongaro, this was significantly greater than those captured by fyke netting, in which only 8% had haemorrhages. No haemorrhaging was observed in eels after 30 days from Lake Areare. Haemorrhages from electrofishing in our study appeared to heal within the 30-day holding period, as the incidence of haemorrhaging in Rotongaro eels reduced from 28% at capture to only 4% after 30 days in captivity. Rates of spinal injury in eels captured by electrofishing was also variable (12% in Lake Areare eels and 8% in Lake Rotongaro eels), compared to the rate of spinal injury in eels captured by fyke netting (0% in Lake Areare eels and 4% in Lake Rotongaro eels). The upper rate of haemorrhaging that we found for electrofishing in shortfin eels (28%) was similar to injury rates for electrofished American eels (25%), but our rates of spinal damage (8-12%) were much lower than for American eels (60%).
In grey mullet, there was no significant difference (P > 0.101) in the occurrence of haemorrhages or spinal injuries between the two capture methods. However, physiological stress from electrofishing caused a significantly (t = 2.37, P = 0.02) reduced sodium ion concentration (139.38 ± 13.24 mM) compared to that of gill netting (149.06 ± 15.58 mM). Electrofishing also caused a significantly (t = 4.61, P < 0.001) lower concentration of haemoglobin (86.55 ± 9.21 g L-1) compared to that of grey mullet captured by gill netting (108.81 ± 23.86 g L-1). However, there were no significant differences between capture methods for the other blood plasma constituents tested (haematocrit and the blood plasma ions sodium, potassium, magnesium and calcium).
Survival was high in smelt captured by both electrofishing (94.8%, n = 1217) and by seine netting (92.9%, n = 1271), with no statistically significant difference between the two fishing methods (t = −1.021, P = 0.320). In a subsample of smelt analysed for injuries (n = 40 for each method), external injuries were observed in 15% (n = 6) of smelt captured by electrofishing and in 20% (n = 8) of smelt captured by seine netting. No haemorrhages were detected in smelt for either of the capture methods and there was no significant difference (G = 0.215, P = 0.642) in the rate of spinal injuries in smelt captured by electrofishing (10%, n = 4) or by seine netting (5%, n = 2).
These results suggest that boat electrofishing as practiced in New Zealand is in the long term generally no more harmful to shortfin eels, grey mullet, and common smelt than other comparable capture sampling techniques. Exceptions to this
conclusion were increased haemorrhaging in electrofished shortfin eels, from which they recovered within 30 days, and reduced sodium and haemoglobin concentrations in electrofished grey mullet, which are signs of physiological stress. All fishing methods examined caused some level of injury, stress, or mortality in the three species studied. Netting techniques (fyke net, gill netting, and seine netting) that are commonly used will also cause harm and these techniques are currently used far more widely than boat electrofishing.||