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Complete Versus Partial Macrophyte Removal: the Impacts of Two Drain Management Strategies on Freshwater fish in Lowland New Zealand Streams

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Complete Versus Partial Macrophyte Removal: the Impacts of Two Drain Management Strategies on Freshwater fish in Lowland New Zealand Streams Ecology of Freshwater Fish 2012: 21: 510–520 Ó 2012 John Wiley & Sons A/S Printed in Malaysia Á All rights reserved ECOLOGY OF FRESHWATER FISH Complete versus partial macrophyte removal: the impacts of two drain management strategies on freshwater fish in lowland New Zealand streams Michael J. C. Greer1,2, Gerard P. Closs1, Shannan K. Crow2, Andy S. Hicks3 1Department of Zoology, University of Otago, Dunedin, New Zealand 2National Institute of Water and Atmospheric Research, Christchurch, New Zealand 3Environment Southland, Invercargill, New Zealand Accepted for publication March 23, 2012 Abstract – Complete macrophyte removal to maintain drainage performance in lowland streams can have a negative effect on resident fish communities, but few studies have quantified this impact. Moreover, limited research has been carried out exploring alternative approaches for macrophyte removal that minimise the impact on the resident fish community. The aims of this study were (i) to determine how the current practice of removing almost 100% of available macrophyte cover affects native fish populations in lowland New Zealand streams and (ii) to see whether this impact can be reduced by limiting macrophyte removal to alternating 50-m sections of the waterway. Native fish populations were surveyed before and after experimental macrophyte removal for the following three treatments: (i) complete macrophyte removal, (ii) macrophyte removal from alternating 50-m reaches and (iii) control with no macrophyte removal. Radiotelemetry was used to monitor the behavioural response of individual giant kokopu (Galaxias argenteus) to the different treatments. The results of this study suggest that current drain management practices reduce CPUE of fish by 60%. Although limiting macrophyte removal to alternating 50-m sections did not minimise the community impacts of drain clearing, large giant kokopu did benefit from this strategy. All tagged giant kokopu remained in stream reaches partially cleared of macrophytes, while in completely cleared reaches all individuals were displaced. These results demonstrate the threat current drain management practices pose to New Zealand native fish and highlight the value of trialling alternative methods of macrophyte removal. Key words: catchment management; common bully; plant management; aquatic weeds; anthropogenic disturbance; giant kokopu communities (Maitland 1995). Thus, changes in fish Introduction abundance can have a disproportionately large effect Anthropogenic disturbance of streams draining agri- on community structure (Maitland 1995). cultural and industrial land has reduced both the A little understood source of disturbance in low- abundance and the range of native fish species glob- altitude rivers draining pastoral land is the regular ally (Maitland 1995). The negative effects of dams removal of aquatic macrophytes (Swales 1982; and chemical pollution are well documented (e.g., Young et al. 2004). For successful drainage of agri- Alabaster & Lloyd 1982; Maitland 1995; Santos cultural run-off, streams must remove water from the et al. 2006; Zhai et al. 2010), but relatively little is pasture quickly and efficiently while promoting phys- known about the effects of many other human distur- ical and chemical conditions in the soil that are bances on freshwater ecosystems. For effective con- favourable for agricultural production (Hudson & servation management of lotic ecosystems it is Harding 2004). Accelerated macrophyte growth asso- essential that anthropogenic threats to native fish are ciated with increased nutrient input can limit drainage understood, as they often play key roles in aquatic outfall (Armitage et al. 1994; Kaenel et al. 1998). Correspondence: M. J. C. Greer, Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand. E-mail: [email protected] 510 doi: 10.1111/j.1600-0633.2012.00569.x Impacts of macrophyte removal High densities of these plants can increase sediment starwort (Callitriche sp.), swamp willowweed (Polyg- deposition, reduce flows and potentially flood the sur- onum sp.) and pondweed (Potamogeton spp.) affects rounding pastoral land (Kaenel & Uehlinger 1988; New Zealand’snativefish (Hudson & Harding 2004). Hearne & Armitage 1993). To prevent this, it is nec- One species of fish that may be particularly suscep- essary to regularly clear macrophytes from the tible to macrophyte removal is the drift-feeding giant streams that drain agricultural land, using herbicides, kokopu (Galaxias argenteus). Macrophyte removal mechanical or manual extraction of plant material not only reduces the availability of diurnal cover for and, occasionally, plant-eating fish (Pieterse & Mur- this fish, but also results in the removal of a large phy 1990; Wells et al. 2003; Hudson & Harding quantity of invertebrate prey from the water column 2004). (Hudson & Harding 2004). Therefore, we expect Although macrophytes limit the drainage efficiency reduced food availability to increase the size of giant of pastoral land, they play a key role in maintaining kokopu home ranges, increase levels of aggression desirable physical and chemical conditions in fresh- and alter diel activity patterns (David & Closs 2003; water systems (Fox 1992). It is therefore likely that Hansen & Closs 2005, 2009). Understanding how the absence of macrophytes following drain clearing macrophyte removal impacts this species is particu- reduces the functioning of aquatic ecosystems. In larly crucial given its ‘declining’ status (Allibone addition, the physical process of removing macro- et al. 2010). phytes can be a source of disturbance in itself. Herbi- The present study aimed to examine the commu- cides may adversely affect non-target organisms and nity level impacts of mechanical drain clearing on alter the chemical properties of the water column via native fishes and the possibility of minimising these the decay of aquatic plant material (Murphy & effects with an alternative macrophyte clearing Barrett 1990). Mechanical excavation is the most approach. Specifically, we aimed to quantify the disruptive method of macrophyte control and can impacts of complete drain clearing on native fish result in the immediate loss of a high proportion of communities and contrast these changes with those the available plant cover (Kaenel & Uehlinger 1988). from areas where macrophyte removal was limited to This can reduce the heterogeneity of the stream bed, alternating 50-m sections. We predict that fish abun- thereby limiting the number of species it can support dance will be reduced by macrophyte clearing and (Hicks & Reeves 1994). Despite a widespread that community composition will change. We also understanding that macrophyte removal probably has predict that these changes will be less severe when a detrimental impact on community structure in macrophyte removal is limited to alternating sections streams (Swales 1982), its impact on freshwater fish and that large giant kokopu will be less likely to be populations has been the focus of relatively few stud- displaced from streams cleared in this manner. ies. The limited research, however, has suggested that mechanical removal of macrophyte cover reduces Study area growth rates and increases the predation of juvenile fish (Mortensen 1977; Garner et al. 1996), leads to Waituna lagoon is a largely unmodified coastal lake the removal of fish during mechanical clearing (Daw- located approximately 40 km south-east of Inver- son et al. 1991; Serafy et al. 1994), and alters the cargill in the South Island of New Zealand (Fig. 1). behavioural patterns of many freshwater fish species The lagoon’s catchment consists of about 20,000 (Swales 1982). hectares of farmland, native forest and the interna- To date, research on the effects of drain manage- tionally important Awarua wetlands. Drainage input ment on New Zealand’s native fish populations has for the catchment is provided by three major produced conflicting results. Goldsmith (2000) streams that flow directly into the lagoon. Of these reported that there is anecdotal evidence that macro- streams, the Waituna Creek drains the largest area phyte removal increases the abundance of some fish (12,500 ha), followed by Carrans Creek (5700 ha) species like inanga (Galaxias maculatus) and smelt and Moffat Creek (1700 ha). The majority of water- (Retropinna spp.), but leads to decreases in the abun- ways in the region are extensively modified as a dance of other species like common bully (Gobio- result of agricultural development, and regular morphus cotidianus) and eels (Anguilla spp.). mechanical excavation is needed to maintain ade- Unfortunately, this information is not available in the quate drainage of the surrounding low-lying pasto- primary literature, and these unpublished findings ral land (Riddell et al. 1988). Despite regular have since been questioned because of a lack of ana- macrophyte removal, the fish community in the area lytical power (Young et al. 2004). As a result of the is relatively diverse, and the catchment has a large limited research on this topic, it is unclear how the population of the declining giant kokopu (Riddell widespread removal of frequently abundant macro- et al. 1988; Thompson & Ryder 2002; Allibone phyte species like oxygen weed (Egeria spp.), et al. 2010). 511 Greer et al. Fig. 1. Study sites located in the reaches and tributaries of Armstrong Creek, Smit Creek, Moffat Creek and Carrans Creek in the catch- ment of Waituna Lagoon in the South Island of New Zealand. and three in Carrans Creek). Unfortunately, the differ-
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