The Role of Boat Electrofishing in Restoring Fish Populations in Shallow Lakes

We estimated the efficiency of boat electrofishing and its use in estimating fish abundance in New Zealand’s lakes and rivers. One example is Lake Mangahia, a small (10 ha), shallow (maximum depth 1.5 m), hypereutrophic peat lake 12 km southeast of Hamilton City, New Zealand. The lake is situated within a rural catchment dominated by dairy farming but is the subject of an ecological restoration project to restore biodiversity values and water quality. The fish fauna of the lake comprises 4 native fish species (common smelt Retropinna retropinna, common bully Gobiomorphus cotidianus, shortfin eels Anguilla australis, and longfin eels A. dieffenbachii) and 4 exotic fish species (brown bullhead catfish Ameiurus nebulosus, goldfish Carassius auratus, koi carp Cyprinus carpio, and rudd Scardinius erythophthalmus). We estimated fish abundance by mark-recapture. We marked a large proportion of the fish population by boat electrofishing (10.3 h effort) and fyke netting (54 net-nights effort). In March 2010 a total of 2,934 fish longer than 100 mm (1,181 catfish, 1,342 goldfish, 40 rudd, 338 shortfin eels and 33 longfin eels) were marked and released back into the lake, where they were left for 11 days before removal began. No koi carp were captured during preliminary fishing or during the marking process. Biomass removal involved boat electrofishing (21.3 h), fyke netting (231 net-nights), gill netting (360 m monofilament net). Mark-recapture estimates of the resident fish population were 4,878 catfish (46% removal), 24,361 goldfish (13% removal), 822 shortfinned eels (57%) removal and 47 longfinned eels (43% removal). Fishing techniques were highly selective, with boat electrofishing biased towards goldfish and fyke nets capturing mainly eels and catfish. Estimates for koi carp (no fish marked) and rudd (no marked returns) could not be made. Biomass estimates for the most abundant pest fish species were 56 kg/ha for catfish and 308 kg/ha for goldfish, well above thresholds for impaired water quality. Boat electrofishing was a highly effective tool for mark-recapture of goldfish. However, because of the sheer number of individuals, and a combination of highly turbid water and shallow littoral areas where boat access was restricted, fishing alone failed to achieve a reduction to below 100 kg/ha.