Exclusion and Removal of Pest Fish from Lake Ohinewai
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Exclusion and Removal of Pest Fish LERNZ from Lake Ohinewai - Fact Sheet Linking lake restoration with end users for positive environmental outcomes Lake Ecosystem Restoration New Zealand Lake Ohinewai Many of the shallow lakes in the lower Waikato River floodplain have significantly degraded water quality as a res ult of nu trient and sediment enrichment from non-point sources. Pest fish species such as koi carp, goldfish, and catfish have exac erbated lake decline by resuspending lake sediments and uprooting submerged macrophytes. This this resulted in a col lapse of submerged macrophytes and progression from clear-water oligotrophic state to a eutrophic (algal-dominated) state. Lake Ohinewai is a shallow (4.5 m depth}, moderately-sized (16.8 ha) riverine lake, with a 331 ha catchment that is primarily flat and dominated by intensive pastoral farming. A single outlet drain leads to Lake Waikare and passes through a circular road culvert 930 m from the lake outlet. The water quality of Lake Ohinewai has deteriorated significantly and the lake has been devoid of aquatic macrophytes since the 1990s. This decline coincides with increasing populations of koi carp (Cyprinus carpio). Koi Carp Exclusion The presence of a single outlet from Lake Ohinewai provided an opportunity to exploit the behavioural instinct of koi carp to periodically migrate. A one-way barrier was installed across the •-.IIP.!'!'!1!!1'91111!•~----..... outlet allowing fish within the lake to exit, but preventing adult fish from entering (Figure 1). The barrier did not prevent juve nile fish from entering the lake, but it was hoped that exclusion of adult koi carp would reduce disturbance of the lake bed and as sist recovery of the lake. Carp Biomass Reduction Installation of the one-way barrier and associated fish removal efforts from Lake Ohinewai have reduced the koi carp population from an estimated 373.6 kg/ha in January 2011 to 10.4 kg/ ha in December 2014, a 97% reduction installed on the Lake Ohinewai outlet. in koi carp biomass. The barrier is designed to allow one way passage of pest fish species out of the lake. Photos: Adam Daniel For more information visit www.lernz.co.nz or contact [email protected] \~J wA"IKATO Version : April 2015 I MBIE Contract uowxosos ~ "l, ll h,,dla,i,•,tao llil,kuro Exclusion and Removal of Pest Fish from Lake LERNZ Uke Ecosystem Restoration Ohinewai - Fact Sheet Continued New Zealand Pest Fish Removals Effects on Lake Ecology A total of 3494 kg of pest fish was removed from Lake Prior to the reduction in pest fish biomass, Lake Ohinewai was Ohinewai, including 3003 kg of koi carp, 233 kg of catfish devoid of submerged aquatic vegetation. Following a 95% re and 257 kg of goldfish. Pest fish removals were undertak duction in pest fish (koi carp, catfish, goldfish) biomass there en on six occasions between December 2010 and Decem has been some limited establishment of exotic marginal ber 2014. In addition, mark-recapture programmes were aquatic species such as parrot's feather (Myriophyl/um conducted on three occasions in order to estimate the re aquaticum) and Ludwigia sp. Increases in shortfin eel condition maining biomass (Figure 2). Removal methods used includ have also been observed (F igure 3), inferri ng that competition ed boat electrofishing, fyke netting, automated feeder for resources has declined. traps and beach seining. 0.2 The single largest factor responsible for the decline in the x -g"' 0.19 koi carp population appears to be the installation of the c: 0 ',P one-barrier on the Lake Ohinewai outlet. An estimated 'ti 0.18 c: 0 208.6 kg/ha (55.8%) of koi carp were removed by this pas u c: sive method in a six month period. ~ 0.17 I ~ I .... I I 0.16 -I------.!,•---.---..--- --,---.-- -~-~ Sep 10 Jun 11 Feb 12 Oct 12 Jun 13 Mar 14 Nov 14 Jul 15 Figure 3. Fulton condition index of La ke Ohinewai shortfin eels 1000 120 (± SE). Arrow indicates installation of one-way fish barrier. 900 Improvements in water clarity have not occurred following 100 800 the biomass removals (Figu re 4). However, ecological model ling indicates that biomass removals need to be supported by 700 so m :E. catchment remediation measures such as sediment detain .......r:. "' 600 ~ 'O ment structures and riparian planting to reduce nutrient and ~ QJ ., :, 1o 0 sediment loading. Significant improvements in water quality 500 60 E -~ ~ u.,.,. ::l are most likely to occur t hrough management approaches t hat "' !a 400 8 address both internal and externa l nutrient sources. s ;;; "' 40 300 1.2 200 0 20 e-o.s .r:. 100 0. ~ 0.6 1: •• • • 0 0 .,8 0.4 • ~ u u u •u V) 0 ~ • G G a !f G a !f ~ a !f a !f <( • 0 <( <( 0 < <( 0 < <( 0 <( 0 0.2 • ... ... N N m m ... ... • ~ ... "' ... "' ... s i;a ~ ~ ~ ~ ~ ~ ij ~ ~ ~ ~ ~ 0 Jan04 Oct 06 Jul 09 Apr 12 Dec 14 Sep 17 Figure 2. Pest fish (koi carp, catfish, goldfish) biomass estimates (red squares± 95% Cl) and removals (blue bars) from Lake Figure 4. Lake Ohinewai Secchi disk measu rements of water clarity Ohinewai. The one-way barrier was installed on the lake outlet in prior to (black circles), and following (open sq uares) installation of mid-2011. the one-way barrier. 11 1 For more information visit www.lernz.co.nz or contact [email protected] r.·:1 r r i:si, · R<irY ,)r r l WAIKATO Version: April 2015 I MBIE Contract uowxosos ~('¥ Tdl h,,rlli,s, •1,:.,011;,,i;...,u .