Responses of the fish community and biomass in Lake Ohinewai to fish removal and a carp exclusion barrier

Transcription

1 Responses of the fish community and biomass in Lake Ohinewai to fish removal and a carp exclusion barrier New Zealand Freshwater Sciences Society annual conference, Hamilton Nov 2017 Brendan J Hicks 1, Grant W Tempero 1 Nicholas Ling 1, Dai KJ Morgan 2 1 University of Waikato, Hamilton, NZ 2 NorthTec, Whangarei, Northland, NZ b.hicks@waikato.ac.nz

2 Lake Ohinewai Shallow, riverine lake on the Waikato River floodplain Maximum depth 4.5 m Surface area 16.8 ha 331 ha catchment primarily flat Dominated by intensive pastoral farming 45 2

3 Common carp (koi) introduced in late 60s Widespread in the North Island Abundant in shallow Waikato lakes Comprises 75% of fish biomass these lakes 300 kg/ha in Lake Ohinewai 3

4 Decline of Lake Ohinewai In 1981, 80% of the lake was covered by aquatic macrophytes Deteriorated during the 1980s; by 1991 no macrophytes remained Now hypertrophic, TLI 6-7 Dominated by cyanobacterium Dolicospermum circinalis (previously called Anabaena) potentially toxic algal blooms lack of macrophytes Hypothesis invasive fish cause blooms in Lake Ohinewai 4

5 Lake trophic status Trophic level index (TLI) developed for NZ lakes Values of chl-a conc., SD, TP, TN Burns et al. (1999) Lake and Reservoir Management 15: THE UNIVERSITY OF WAIKATO TE WHARE WANANGA O WAIKATO 5

6 Carp migrations 74% leave a lake each year Daniel et al. (2011) North Am J Fish Man 31: Waikato River One-way gate 6

7 One-way barrier installation

8 Hinged front for easy cleaning every 2 months Based on finger trap/salmon trap bars Thwaites et al. (2007) Barrier designed to be low maintenance Stops adult carp and allows natives to pass Does not stop small goldfish or catfish Designed built and installed for under $NZ 5,000 8

9 Adult carp massing downstream of the barrier

10 Lake Ohinewai removal phases Mass removal (Jan-May 2011) Boat electrofishing Baited traps Fyke netting Seine netting One way gate (May 2011) Further invasive fish removals Nov-Feb 2012, Nov-Dec 2014, Nov-Dec 2016 All fish removals incorporated mark-recapture Water quality monitoring and nutrient dynamics model 10

11 Population estimates R, FSA package (Ogle 2016)?? 11

12 Fish biomass estimates Original fish biomass estimated as 6 tonnes, 97% carp We removed 3 tonnes of fish By subtraction, 3 tonnes of carp left through the oneway gate Total Invasive Eels Sampling period Biomass (kg) Areal biomass (kg/ha) Biomass (kg) Proportion of total biomass Areal biomass (kg/ha) Biomass (kg) Proportion of total biomass Jan Nov Jan Nov-4 Dec Nov-8 Dec Areal biomass (kg/ha) 12

13 Changes in size structure - invasives 100% 75% 50% 25% Catfish Goldfish Koi carp n = 89 n = 428 n = % 100% 75% n = 35 n = 149 n = 138 Frequency 50% 25% 0% % 75% n = 40 n = 233 n = % % 0% 100% 75% n = 67 n = 341 n = % % 0% Length (mm) Recruitment pulses 13

14 Changes in water quality Mean TLI3 (N, P, chl-a, excluding Secchi depth, Burns et al. 1999) hypertrophic range TLI 6-7 Ohinewai TLI 6.1 to 6.6, lake remained hypertrophic Invasive fish kg/ha Invasive fish kg/ha

15 Nutrient availability Years of N and P additions from pastoral farming Nutrient inputs by carp 10% of TN and 26% of TP (Morgan and Hicks 2013) Internal nutrient recycling processes dominated nutrient inputs (Allan 2016) Invasive fish kg/ha Invasive fish kg/ha

16 Conclusions 80% reduction of invasive fish biomass insufficient to push the lake below hypertrophic status The one-way gate alone was not sufficient to maintain low carp numbers carp bred in the lake No spontaneous regeneration of macrophytes Carp removal allowed strong recovery of native benthivorous eels Invasive benthivorous catfish also responded strongly 16

17 Acknowledgements Adam Daniel for coordinating the early work and for design, construction, and installation of the one-way gate Dudley Bell and Warrick Powrie operated boats Many students and staff assisted through the project Waikato Regional Council provided monitoring data for lake water quality, funded 2016 fish removal Funded by Ministry of Business, Employment and Innnovation contract UOW

18 References Allan M (2016). A coupled hydrodynamic-ecological model of management options for restoration of Lake Ohinewai. ERI Report No. 68. Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato. Hamilton, New Zealand. Burns NM, Rutherford JC, Clayton JS (1999). monitoring and classification system for New Zealand lakes and reservoirs. Lake and Reservoir Management 15: Hicks BJ, Brijs J, Daniel AJ, Morgan D, Ling N Biomass estimation of invasive fish. Section 6.2 in Collier KJ & Grainger NPJ eds. New Zealand Invasive Fish Management Handbook. Lake Ecosystem Restoration New Zealand (LERNZ; The University of Waikato) and Department of Conservation, Hamilton, New Zealand. Pp Ogle, D. H FSA: Fisheries Stock Analysis. R package version Tempero GW, Hicks BJ Responses of the fish community and biomass in Lake Ohinewai to fish removal and the koi carp exclusion barrier. Waikato Regional Council Technical Report 2017/10, Waikato Regional Council, Hamilton. 18