Use of fine mesh monofilament gill nets for the removal of rudd (Scardinius erythrophthalmus) from a small lake complex in Waikato, New Zealand

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1 New Zealand Journal of Marine and Freshwater Research ISSN: (Print) (Online) Journal homepage: Use of fine mesh monofilament gill nets for the removal of rudd (Scardinius erythrophthalmus) from a small lake complex in Waikato, New Zealand Keri Neilson, Rachel Kelleher, Grant Barnes, David Speirs & Johlene Kelly To cite this article: Keri Neilson, Rachel Kelleher, Grant Barnes, David Speirs & Johlene Kelly (24) Use of fine mesh monofilament gill nets for the removal of rudd (Scardinius erythrophthalmus) from a small lake complex in Waikato, New Zealand, New Zealand Journal of Marine and Freshwater Research, 38:3, , DOI: To link to this article: Published online: 3 Mar 2. Submit your article to this journal Article views: 458 Citing articles: 7 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Marine and Freshwater Research, 24, Vol. 38: The Royal Society of New Zealand Use of fine-mesh monofilament gill nets for the removal of rudd (Scardinius erythrophthalmus) from a small lake complex in Waikato, New Zealand KERI NEILSON Department of Conservation Science and Research Unit P.O.Box 2 Hamilton, New Zealand kneilson@doc.govt.nz RACHEL KELLEHER Department of Conservation Waikato Area Office P.O. Box 225 Hamilton, New Zealand GRANT BARNES* DAVID SPEIRS JOHLENE KELLY Waikato Regional Council P.O. Box 4 Hamilton East, New Zealand *Present address: Auckland Regional Council, P.O. Box 92 2, Auckland, New Zealand. Abstract Fine-mesh monofilament gill nets were deployed within the three shallow lakes of the Rotopiko complex, Waikato, New Zealand to assess their potential as a tool for controlling or eradicating rudd (Scardinius erythrophthalmus). Nets of different mesh sizes were placed at different spacings and orientations throughout the lakes for two fishing periods, to determine methodology to be used for intensive removal. Rudd were intensively netted for a further two periods and then the success of the operations was assessed. Gill nets of a 3-mm mesh were more effective at capturing rudd when set perpendicular rather than parallel to the shore, whereas there was no significant effect of orientation in 25 mm and 38 mm nets. Comparisons of catch per unit effort (CPUE) on the first night of fishing for M387; Online publication date 3 August 24 Received 2 December 23; accepted 28 April 24 each fishing period showed a significant reduction in the initial CPUE as fishing proceeded. A reduction in the numbers of rudd captured was most marked in the 38 mm nets. 8% of rudd captured over a 7-night period were caught in the first 3 nights of fishing. Post-removal sampling using gill, fyke, and trammel nets, and an electric fishing boat, showed that rudd remained in all of the lakes following intensive removal efforts. However, relative to other methods currently available in New Zealand for control or eradication of unwanted fish, monofilament gill nets appear to be a potentially viable and cost-effective option where ongoing control may provide sustained conservation outcomes. Keywords rudd; Scardinius erythrophthalmus; gill nets; pest fish control; peat lakes INTRODUCTION Ecological values of shallow lakes have typically been compromised by factors such as eutrophication, high turbidity, and biotic disturbance (Scheffer 989; Scheffer et al. 993). Shallow lakes of the Waikato region of New Zealand are degraded to varying extents by these same factors (Boswell et al. 985; Barnes 22). This has led to a shift from clear water systems typically dominated by submerged macrophytes supporting diverse faunal communities, to turbid, algal-dominated water bodies (Perrow et al. 999). Evidence from studies outside New Zealand has implicated freshwater fish species in the promotion of phytoplankton through removal of submerged macrophytes (Van Donk & Otte 996). Rudd (Scardinius erythrophthalmus L.), one of many introduced fish species in New Zealand, has specifically been identified as a causative factor in decreased water quality as a consequence of grazing pressure on aquatic vegetation (Van Donk & Gulati 995). The diet of rudd in its native range varies considerably with body size and food availability,

3 526 New Zealand Journal of Marine and Freshwater Research, 24, Vol. 38 and varies from almost entirely planktonic crustaceans (Maitland & Campbell 992) to predominately submerged plants (e.g., Kennedy & Fitzmaurice 974; Klimczyk-Janikowska 975). In New Zealand rudd are primarily herbivorous feeders, undergoing a dietary shift from benthic invertebrates to aquatic macrophytes at c. 5 mm in length (Lane 983; Kane 995), and fill a niche unoccupied by native freshwater fish species (Cadwallader et al. 977; Cadwallader 978). Grazing trials undertaken in New Zealand have indicated that rudd selectively feed on aquatic plants with a preference for native species, in particular Nitella and Potamogeton spp. During captive grazing trials, rudd consumed 22% of their body weight per day of Nitella spp., suggesting that they are having a profound impact on vulnerable native aquatic plant communities (Lake et al. 22). Rudd were illegally introduced to New Zealand as part of an aquarium consignment imported from England in the late 96s (McDowall 99), and have since been spread around ponds, lakes, and rivers throughout New Zealand, becoming well established in the Waikato region. In 983, rudd were declared to be noxious in all parts of New Zealand under the Freshwater Fisheries Regulations, but in 986 the status of rudd as a noxious fish was lifted in the AucklandWaikato Fish & Game Region because of their well established populations and popularity as a coarse sports fish. Of c. lakes in the Waikato region, only three are known to contain solely native aquatic plant assemblages (John Clayton, National Institute of Water and Atmospheric Research Limited, Hamilton pers. comm.); the Rotopiko lake complex south-east of Hamilton is one of these lakes. During summer 2, a fisheries survey of the Rotopiko lakes confirmed the presence of rudd which had not previously been recorded there. Given the findings of recent studies on rudd impacts on aquatic vegetation, this discovery prompted the agencies responsible for biodiversity and water quality management (Department of Conservation and Waikato Regional Council) to undertake management actions to protect the unique values of the lakes. This provided an opportunity to investigate how best this could be achieved, with implications for future management at this and other sites nationally. Fine-mesh monofilament gill nets were chosen for rudd removal for their potential to catch large numbers of scaly fish (Hamley 975; Henderson & Nepszy 992) whilst keeping nontarget by-catch to a minimum (Closs et al. 23). The objective of the research component was twofold. First we wanted to determine the best ways to set fine-mesh monofilament gill nets to maximise rudd catch. Although few removal studies have examined this, it has been found that netting strategy can significantly increase capture success of a target species (Jester 973, 977). For our research we focused on net orientation and spacing. The second aim was to evaluate the potential of monofilament gill nets as a tool for controlling or eradicating rudd from small shallow lake systems. MATERIALS AND METHODS Study sites The Rotopiko complex comprises three shallow eutrophic peat lakes (North, South, and East) located c. 2 km south-east of Hamilton in the North Island of New Zealand (37 57'S; Ti). The complex is one of 29 peat lakes in the Waipa District formed some 7 years ago following the last glaciation (Green & Lowe 985). All of these lakes have become modified by agricultural practices in their surrounding catchments. However, the Rotopiko complex still retains peaty characteristics and represents one of the best assemblages of such habitats in New Zealand (Cromarty & Scott 995). The three lakes making up the Rotopiko complex were once part of a much larger water body, Lake Rotopiko, before the drainage of the extensive Moanatuatua peat bog during early European occupation (Champion et al. 993). They are now gazetted as Lake Serpentine Government Purpose Reserve (Wildlife Management) administered by the Department of Conservation. All three lakes are surrounded by pastoral land and are fringed by marginal vegetation dominated to varying degrees by introduced willow (Salix cinerea) and native manuka (Leptospermum scoparium). The aquatic plant assemblages vary substantially between the three lakes. Rotopiko North (5.3 ha, max. depth: 4. m) is characterised by a dense margin of Eleocharis sphacelata up to 2 m in width. Submerged aquatic plants are present to depths of 3 m with Potamogeton ochreatus the dominant species at depths of -2 m, P. cheesemani and Nitella cristata common to depths of 2-3 m, and occasional Chara corallina plants present. Rotopiko East (.6 ha, max. depth: 4.4 m) has a Potamogeton sp. and charophyte assemblage similar to that of Rotopiko North but plant abundance is relatively depauperate. Rotopiko

4 Neilson et al. Use of gill nets to remove rudd from lakes 527 South, the largest of the three lakes (8.3 ha, max. depth: 3.6 m) has no established submerged plants with only sparse Potomogeton seedlings present. The margin is dominated by raupo (Typha orientalis) with emergent E. sphacelata (T. Dugdale, NIWA, Hamilton unpubl. data). The three lakes will hereafter be referred to as North, East, and South only. The indigenous fish fauna of the lakes consists of shortfin eel (Anguilla australis (Richardson)), longfin eel (Anguilla dieffenbachii (Gray)), lacustrine smelt (Retropinna retropinna (Richardson)), and common bully (Gobiomorphus cotidianus (McDowall)). Exotic species goldfish (Carassius auratus (Linnaeus)), brown bullhead catfish (Ameiurus nebulosus (Lesueur)), and rudd have all been introduced to the complex at unknown times (D. Speirs, Waikato Regional Council, Hamilton unpubl. data). Field equipment The gill nets used during the testing and removal phases of this project were fine-mesh monofilament gill nets from the Lindeman Company in Finland. Nets were 5 m long and.8 m deep with a twine diameter of.5 mm and a hanging co-efficient of 2. Three mesh sizes were used: 3, 25, and 38 mm (knot to knot). In addition, -mm mesh nets were used during the final removal period (March 23) and 3, 23, 27, and 38 mm nets, 3. m deep, were used for the post-removal sampling (September 23). Nets were set so that the float line was at or near the lake surface. When set perpendicular to the shoreline, nets were anchored at the shore by a bamboo pole and at the other end by a weight and buoy. The open-lake nets were anchored by weights and buoys at both ends. Boat electro-fishing was carried out using a specially adapted 4.5 m aluminium hull with a 6 kw custom-wound generator, a 5 kw gas-powered pulsator, and anode poles and droppers that create the fishing field at the bow. The boat was lifted on to the lakes using a Squirrel helicopter. Fyke nets (7 mm and 3 mm mesh) with a single 2-m leader, and trammel nets (45 and 6 mm x.8 m x 3 m with a 3 mm trammel) were used during post-removal sampling. Fishing periods and lake zones Tests of net orientation and spacing were carried out during the periods September-5 October 2 and 5-22 March 22. As a result of this work, a best practice methodology was developed for the intensive removal of rudd from the lakes using gill nets. This was attempted during the periods 3-3 September 22 and 3 March-3 April 23. A postremoval sampling of fish populations in the lakes, using a variety of different methods, was carried out from to 26 September 23. To ensure full coverage by nets of the different aquatic habitats, the lakes were divided into zones. Zone encompassed the shallow marginal areas extending from the shore and 5 m out into the lake. In South this zone was without submerged aquatic vegetation, but in North and East was vegetated. Zone 2 represented the area dominated by Potamogeton spp. and charophytes from the end of zone to the unvegetated areas. This was typically between 5 and 3 m from shore. Zone 3 covered the deeper water in the centre of the lakes where aquatic plants were absent. All three zones occurred in North lake, South had only zones and 3, and East had only zones and 2. Net orientation tests Net orientation tests were carried out in South only during September 2 and March 22. Nets of alternating mesh sizes were set around the edge of the lake (zone ) alternating between perpendicular and parallel to the shore (Fig. ). Perpendicular nets were set with one end at the shore edge and the other end 5 m from this point. This meant that at times up to half the net was set within the marginal aquatic vegetation. Parallel nets were set 5 m from the shoreline. A net was set every 6 m around the lake circumference with c. 8 m between nets of the same mesh size. This meant a total of 6 nets were set (Fig. ). Nets were set at 4 h and cleared at 9 h for 4 consecutive days in both September 2 and March 22. These pre-dawn sets were carried out to minimise scavenging by eels, decreasing net clearance time, and allowing more comprehensive fish capture data to be gathered. Net spacing tests Following assessment of the most successful net orientation, nets of all three mesh sizes were set perpendicular to the shore in North for the net spacing tests. Thirty nets of each mesh size were set around the margin of the lake with alternating mesh sizes (Fig. ). The distance between nets of the same mesh size was 96 m. Nets were set overnight and cleared each day. This regime was followed for 6 consecutive nights in September 2 and March

5 528 New Zealand Journal of Marine and Freshwater Research, 24, Vol. 38 Rotopiko North Fig. South, North, and East Rotopiko lakes, Waikato, New Zealand indicating initial net positions during tests of net orientation and spacing in September 2. Positioning of the lakes relative to each other is not to scale. 3m 22. After this in September 2, ten 3 mm and ten 38 mm nets were added to North and placed so that each was 48 m from another net of the same mesh size and these new nets were set in previously unfished sites. During March 22, following the first 6 days of fishing, ten 25 mm nets were added to the lake resulting in a distance of 48 m between nets of this mesh size too. This regime was carried out for a further 7 days. The purpose of adding 3, 25, and 38 mm nets was to determine whether the original distance (96 m) between nets of the same mesh type targeted all fish in the lake, or whether there were fish in between nets that were not coming into contact with an appropriate sized mesh. Because of low rudd capture numbers in East, net effectiveness tests were not carried out in this lake. Eighteen nets of alternating mesh size were set in zone of the lake, with 9 m between nets of the same mesh size. This regime was carried out for 6 nights in September 2 and nights in March 22. In North during September 2 and March 22, six 3 m nets (two of each mesh size) were set perpendicular to the shore in zone 2. An additional six 5 m nets (two of each mesh size) were set in zone 3. Similarly in East, three 5 m nets (one of each mesh size) were set in zone 2 (Fig. ). The purpose of these nets was to determine whether rudd were present in all zones of the lake. In addition to testing the nets, catch per unit effort (CPUE) (number of rudd5 m net per night) was recorded from each lake for each mesh size for comparisons over time.

6 Neilson et al. Use of gill nets to remove rudd from lakes 529 Intensive removal efforts Based on the results of the net tests, a regime for intensive removal of rudd from North and East was developed. Because of restricted resources, fishing effort was concentrated in these two high habitat quality lakes and a lower intensity removal was continued in South. Nets were set c. 3 m apart around the perimeter of the lake (zone ) in North and East. Nets were also set above zone 2 in both lakes, and then in North above zone 3. These open water sets were 48 m apart and only involved 25 mm and 38 mm nets. Nets were only set in zone in South and these were all 48 m apart. During both removal periods, only one mesh size was set at a time in all three lakes and nets remained set for 5-7 days before they were replaced with a different mesh size (Table ). The exception to this was in East during March 23 where we set a goal of leaving the nets in the lake until we had five consecutive nights without catching any rudd per mesh size. Fish data collection All whole rudd captured during the net testing and intensive removal were weighed, measured, sexed, and their reproductive status assessed. Where fish had been badly scavenged by eels, every effort was made to identify the species, and the mesh size in which it was captured was recorded. Post-removal sampling Sampling of Rotopiko fish populations after rudd control was carried out over a 3-week period in September 23. The purpose of this sampling was to assess the rudd populations in the lakes post intensive removal. To do this effectively a range of methods were used to test whether the gill nets had been targeting all rudd in the lakes. Information on all fish species in the lakes was collected for future reference. During the first week, gill nets (5 m long x.8 m deep;,3,25, and 38 mm mesh) were set in groups of four (one net per mesh size) around the perimeter of the lake (zone ). The order nets were set in was randomised within each group. Nets within groups were placed 3 m apart, with 65 m between groups. In South there were six groups, in North there were five, and in East there were three groups. In zones 2 and 3, gill nets (5 m long x 3 m deep; 3,23,27, and 38 mm mesh) were placed 48 m apart in a random order. Nets were set for 4 nights and cleared daily. During the second week of sampling, boat electrofishing was carried out on North and East only because of budget limitations. We chose North and East so that we could compare results in a lake where rudd density was at or near zero (East), with a lake of comparable habitat quality where rudd were known to still be present in comparatively high numbers (North). Fishing was undertaken for 6 min on North and 57.5 min on East. In both lakes a high voltage range (5- V) at 4% of range and 6 pulses per s (pps) was used in zones and 2. In addition, in North a low voltage range (5-5 V) at 8% of range and 6 pps was used over zone 3. For the final week of sampling, fyke nets were set around the perimeter of the lakes with one end attached to a bamboo pole and the other end anchored by a weight and float. Nets were set 9 m apart and alternated in mesh size. They were baited with sheep heart for 2 nights (to specifically target eels) and left unbaited for 2 nights. Trammel nets were set in zones 2 and 3 of the lakes. They were placed 96 m apart and alternated in mesh size across the lake. Fyke and trammel nets were set for 4 nights and cleared daily. Table Total net nights (no. of netsno. of nights) fished per mesh size per lake during intensive rudd (Scardinius erythrophthalmus) removal efforts from South, North, and East Rotopiko lakes, Waikato, New Zealand in September 22 and March 23. Period Lake Mesh 3 size (mm) Sep 22 Mar 23 South North East South North East

7 53 New Zealand Journal of Marine and Freshwater Research, 24, Vol. 38 Data analysis Analysis of variance (ANOVA) with post hoc Sheffe's test was used to analyse net orientation data and unpaired t-tests were used to analyse net spacing data and CPUE by fishing period. For the orientation tests, data from 4 days of fishing per net was pooled for each net and the totals square-root transformed to meet the equal variance assumption of the ANOVA. All analyses were carried out using the program SPSS.. RESULTS Net orientation Data from South was used to determine whether there was any difference in the effectiveness of nets set at different orientations to the shore. Data for September and March were analysed separately. In September, there was a significant effect of orientation by mesh size on number of rudd captured in the nets (F = 24.66, P <.5; Fig. 2). A post hoc Sheffe's test indicated that the 3 mm mesh size caught significantly more rudd when set perpendicular to the shore than parallel and 5 m from the shore (t = 3.5, P =.3). However, there was no significant difference in the capture rate of rudd in the 25 mm (t =.8, P =.99) and 38 mm nets (t =.39, P =.99) between the different orientations. In March there was no detectable difference in numbers of rudd captured in each orientation for any of the mesh sizes, although there appeared to be a tendency for perpendicular nets to be more successful for catching rudd in the 3 mm nets (Fig. 2). Net spacing After 6 days of fishing in North in September 2, catch per net in all mesh sizes was greatly reduced. Following this, a comparison of capture rate between 3 mm nets set at previously fished sites (96 m apart), and nets set at unfished sites (48 apart), indicated that the unfished sites caught significantly more rudd over 7 nights of fishing (mean = 5.3 ± 4.3 SD) than the fished sites (5.8 ± 3.9; t = -4.63, P <.). Only four rudd were captured in 38 mm nets over this period two at fished sites and two at unfished sites. Therefore there was no evidence of a net spacing effect on fish captured in 38 mm nets. After week of fishing in North during March 22 we added ten 25 mm nets to reduce spacing from 96 to 48 m. There was no significant difference o J A A A A A A a Mesh size (mm) Parallel A Perpendicular Parallel A Perpendicular Fig. 2 Rudd (Scardinius erythrophthalmus) captures in monofilament gill nets by orientation of nets to the shore, during A, September 2 and B, March 22. Captures were pooled for each net over a period of 4 nights in South Rotopiko. in the number of rudd captured in four nights fishing at fished (.8 ±.) and unfished sites (.7 ±.9; t =.23, P =.82). Small numbers of rudd were captured in 25 mm and 38 mm nets set above zone 2 and zone 3. No rudd were captured in the 3 mm nets in either of these zones. No rudd were captured in zone 2 in East. Intensive removal The intensive netting periods during September 22 and March 23 resulted in the removal of 6 rudd from the Rotopiko lakes. When combined with the two experimental periods, a total of 74 rudd were removed from the lakes between September 2

8 Neilson et al. Use of gill nets to remove rudd from lakes 53 Fig. 3 Mean catch per unit effort (CPUE) of rudd (Scardinius erythrophthalmus) on the first night of netting in South, North, and East Rotopiko lakes between September 2 and March 23. Error bars represent SEM. u i K < - A-- Rotopiko South Rotopiko North - Rotopiko East LU z> Q_ O J - Sep 2 Mar 22 Sep 22 Fishing period Mar 23 Table 2 Total rudd (Scardinius erythrophthalmus) captured in each lake by fishing period and mesh size in South, North, and East Rotopiko lakes, Waikato, New Zealand. Lake Period 3 Mesh 25 size (mm) 38 Total South North East Sep 2 Mar 22 Sep 22 Mar 23 Sep 2 Mar 22 Sep 22 Mar 23 Sep 2 Mar 22 Sep 22 Mar and March 23 (Table 2). Although differing numbers of net nights were used for each lake and period, there was a general trend for the total number of rudd caught per lake to decline with each fishing period the exception being in North where capture numbers increased over the last two intensive fishing periods (September 22 and March 23) with the increased fishing effort. Reductions in the numbers of rudd captured in 38 mm nets were the most marked, with only 48 rudd captured in these nets

9 532 New Zealand Journal of Marine and Freshwater Research, 24, Vol. 38 total cater r. Fig. 4 Daily cumulative percentage of rudd (Scardinius erythrophthalmus) captured in South, North, and East Rotopiko lakes over seven nights of netting. Results from September 22 and March 23 only are pooled for each lake. 4 o Percentage Rotopiko South (n = 4) Rotopiko North (n = 423) Day of net set over the two intensive removal periods. Comparatively low numbers of rudd (9-3) were captured in South in 3 mm nets since the initial fishing period (September 2), despite increased fishing effort in September 22 and March 23. This also appeared to be so in North until the March 23 period when 6 rudd were captured in the 3 mm mesh nets. A comparison of the CPUE on the first night of fishing in each lake over fishing periods indicated a substantial reduction in initial CPUE as rudd removal proceeded (Fig. 3). During the first fishing period in South, mean CPUE on the first night of fishing was 7.4 ± 6.9 rudd per net. By the fourth fishing period this had been significantly reduced to.5 ±.6 rudd per net (t = 4., P =.). Similarly in North, CPUE went from 4.9 ± 6.2 in September 2 to. ±. in March 23 (t = 3.5, P =.2). In East, CPUE was low even during September 2 with a mean CPUE of.9 ±.2 rudd per net. This decreased to. ±.3 rudd per night by March 23 a change which approached significance (t = 2., P =.74). During the final intensive removal period in East during March 23, the goal of 5 consecutive nights with no rudd captured was achieved for all four mesh sizes, with 6 and 7 rudd-free nights achieved for the 38 and mm nets respectively. Although we didn't have this aim for South we still achieved 4,5, and 7 consecutive rudd-free nights for the 3, 25, and 38 mm nets, respectively. Cumulative catch Over a 7-night period of fishing in all three lakes, 8% of the rudd captured were caught during the first 3 nights (Fig. 4). Between 49% and 56% of rudd were captured on the first night of fishing and between 7% and 8% were captured by the second night of fishing. In South, 99% of rudd were captured by the fourth night. Length-frequency of captured rudd The length-frequency distribution of captured rudd changed over time as removal proceeded (Fig. 5). During September 2, two broad size classes were captured in all three lakes. The first class comprised rudd between 77 and 8 mm fork length (FL) and the second comprised rudd between 86 and 289 mm FL. Only two fish over 3 mm FL were captured. The majority of rudd captured in this period were caught in either the 3 mm nets or the 38 mm nets. During March 22 only a small number of fish less than mm FL and greater than 25 mm FL were captured (Fig. 5). However, anew size class was also captured comprising rudd between 5 and 2 mm FL. This was the most commonly captured size class during this period and captures were predominantly in 25 mm nets. Size classes were similar during September 22 and therefore differed from those found the previous September. Rudd of all size classes were found to be fecund during all fishing periods.

10 Neilson et al. Use of gill nets to remove rudd from lakes 533 Fig. 5 Length-frequency of whole rudd (Scardinius erythrophthalmus) captured in mono-filament gill nets (3, 25, 38 mm) from South, North, and East Rotopiko lakes combined. A, September 2; B, March 22; C, September 22. Note differing values on y axes. Bars above graph A indicate the ranges and mean ± SD (mm) of rudd lengths captured in each of the mesh sizes A 3 mm mm 239 ± mm 79 ± J h. I -Jll lllllll.l... There was no overlap in the lengths of rudd captured in 3 and 25 mm nets (Fig. 5), but there was substantial overlap in the size ranges captured in 25 mm and 38 mm nets, with rudd around 2 mm 2 Fork length (mm) 3 4 FL being susceptible to both mesh sizes. However, the mean capture size in the 25 mm mesh was 79 mm FL. Very few rudd this size or smaller were captured in the 38 mm nets. Similarly, the mean

11 534 New Zealand Journal of Marine and Freshwater Research, 24, Vol. 38 capture size in 38 mm nets was 238 mm FL, with very few rudd this size or larger captured in the 25 mm nets. Post-removal sampling During September 23 we returned to the three lakes to assess the remaining fish populations. Sampling indicated that rudd remain in all three lakes following the intensive removal attempts (Table 3). Only four rudd (c. 5 mm FL) were captured in East over 3 weeks using four different sampling methods. These were all captured in the.8 m gill nets set in zone during the first week of sampling. Gill netting indicated a similar post-removal CPUE for rudd and goldfish within each lake (Table 3). North had the highest CPUE of both rudd and goldfish out of the three lakes, with East and South having low capture rates of both these species. Electro-fishing in North led to times the CPUE of goldfish than rudd a result substantially different from that found with the gill nets. Ten rudd were captured by electro-fishing in North and none were captured by this method in East. Subsequently we set four gill nets (23 mm, 27 mm x 3 m) in zone two of North during the final week of sampling, to test whether rudd densities were as low as the electrofishing indicated. In 6 net nights we captured 7 rudd within the size range of those captured by the electro-fishing boat, and 25 goldfish. No rudd were captured in the large mesh (45 and 6 mm) trammel nets set in any of the lakes. The CPUE of common bullies and shortfin eels in fyke nets was highest in South and lowest in East. Five species of fish (three native, two exotic) were captured during the post-removal sampling in North and six in South and East. Catfish were captured in South and East but not North. Numbers of smelt were not quantified during electro-fishing, however numerous large schools were seen in both North and East. DISCUSSION Net effectiveness Attempts at intensive control or eradication of rudd in New Zealand have been few. In 98 Rowe & Champion (994) successfully eradicated rudd (among other species) from Lake Parkinson a small.9 ha dune lake using a liquid formulation of rotenone. The eradication was carried out following the removal of exotic weed beds using grass carp (Ctenopharyngodon idella). Rotenone (applied as a powder) was also successfully used for the eradication of gambusia (Gambusia affinis) and koi carp (Cyprinis carpio) from small, largely artificial, waterways ( ha) in the South Island of New Zealand in 2 (Chadderton et al. 23). This work was carried out under an experimental use permit issued by the Pesticides Board to test the efficacy of rotenone as a piscicide in New Zealand. Using rotenone in the Rotopiko complex Table 3 Catch per unit effort for all fish species captured in the Rotopiko lakes post-intensive rudd (Scardinius erythrophthalmus) removal, using four different net types and an electro-fishing boat during September 23. (GF, goldfish (Carassius auratus); CF, catfish (Ameiurus nebulosus); CB, common bully (Gobiomorphus cotidianus); eel, shortfin eel (Anguilla australis);, detected. Results for the nets are reported as fishnet per night, and results for the electro-fishing boat are reported as fishh.) Method Lake Rudd GF CF CB Eel Smelt Gill nets (.8m deep) Gill nets (3. m deep) Boat electro-fishing Trammel nets Fyke nets South North East South North East South North East South North East South North East Not carried out

12 Neilson et al. Use of gill nets to remove rudd from lakes 535 was considered inappropriate for several reasons. Most importantly, rotenone was not registered for use as a piscicide in New Zealand at the time this work was undertaken. There were also concerns about the effects on the large numbers of non-target species, in particular shortfin eels. In addition, the presence of aquatic macrophytes in North and East provides potential refugia from the toxin for rudd. Note that success of the Lake Parkinson rudd eradication was attributed to the removal of all aquatic plants before the programme began (Rowe & Champion 994). Previous attempts using standard nylon monofilament panel nets (2, 32, and 6 mm knot to knot) to catch rudd in the Rotopiko lakes were relatively ineffectual (D. Speirs unpubl. data). In our study fine-mesh monofilament gill nets proved very effective at catching large numbers of rudd. This is consistent with work outside New Zealand where nets of thinner twine have been found to catch many times more fish as they are less visible, easier to stretch, and more flexible (Hamley 975; Henderson & Nepszy 992). Although numerous studies have used physical removal methods for pest fish (e.g., Wise 99; Rowe & Champion 994; Kane 995; Knapp & Matthews 998; Tyus & Saunders 2) few have examined the most efficient way of using nets to target individual species. We were fortunate in being able to do this using a method that targeted exotic, but not native species. Our initial investigation into the effectiveness of different net orientations and spacings provided valuable insight into the catchability of rudd in these lakes and the most efficient way to use the nets. Small rudd (77-8 mm) were more likely to be captured in nets set perpendicular to the shore and less than 96 m apart, whereas larger fish were not significantly influenced by these factors. We therefore recommended having net placements no further than 48 m apart for 3 mm nets for intensive rudd removal. This meant that the same net placements could be set and used for all mesh types, maximising captures in 3 mm nets while not limiting captures in 25 and 38 mm nets. We also recommended that 25 and 38 mm nets be set in zones 2 and 3 of the lakes as the rudd were not restricted to the vegetated littoral margins. Studies outside New Zealand have found that rudd can congregate in small groups which stay near a particular area, usually the previous spawning site, for most of the year (Holcík 967), and that they are rarely found far from cover, into which they frequently retire between spells of feeding (Kennedy & Fitzmaurice 974). Although these statements are consistent with captures of small rudd in the Rotopiko lakes, our results indicate that rudd in the larger size classes were relatively mobile, often being captured in the central, un-vegetated area of North. Furthermore, the lack of any significant net spacing effect (at least up to 96 m) indicates that these fish are mobile: not only to and from shore but also parallel to the shore. Given the well-developed aquatic plant communities present in both North and East it seems unlikely that this movement is driven by resource limitations. The catchability of rudd smaller than 77 mm was very limited using fine-mesh monofilament nets. We observed rudd in this size class shoaling amongst littoral vegetation, but were for the most part unable to catch them even with the mm mesh nets used during our post-removal sampling. This was considered to be principally because of the lack of mobility of these small fish and the restricted catch efficiency of the nets in the dense vegetation of the littoral margin favoured by fish in this size class (Berst & McCombie 963; Barnett & Schneider 974; Kennedy & Fitzmaurice 974; Hamley 975; Bayley & Peterson 2). The low catchability of small rudd is of concern, particularly as small fish (< mm FL) often had well developed gonads (both male and female fish) and were presumably able to breed at this size. In New Zealand male rudd mature at age + (>6 mm) and females generally at age 2+ (McDowall 99; Hicks 23). This presents problems for any attempts at rudd control or eradication using monofilament gill nets, as these small potentially fecund fish are relatively difficult to catch, particularly in complex habitats such as found in North. This problem is evidenced by the large numbers of c. 5 mm rudd continuing to be captured in this lake fish missed by 3 mm nets during previous fishing periods. It is possible that increasing the number of mesh sizes used may have targeted a greater number of rudd in the lakes at any one time. Although we are not aware of a gill net selectivity curve for rudd, Baranov (cited in Hamley 975) suggests that fish more than 2% longer or shorter than the optimum length for a given mesh size, are seldom caught. If this was applied to the mean capture sizes of rudd in our 3 and 25 mm nets it would predict that we would not expect to catch many fish between the lengths of 2 and 43 mm. Examination of the length-frequency data in Fig. 5 indicates that there were indeed few captures offish within this size range. Therefore it is recommended that an additional mesh of c. 8 mm be tested in

13 536 New Zealand Journal of Marine and Freshwater Research, 24, Vol. 38 future control efforts to determine whether a size class was missed during our study. It is also possible that a mesh smaller than mm may have been required to target the young-of-year rudd. However nets of this size would likely raise the non-target native by-catch of common bullies and smelt and are therefore not recommended in the Rotopiko lakes. For future work using gill nets for removal of rudd we would recommend a pre-removal evaluation of the population using a wide range of mesh sizes. This would better allow the determination of the sizefrequency distribution of the fish present, and enable selection of the most appropriate mesh sizes for intensive removal. Notwithstanding our relatively limited ability to catch small rudd, declining trends in numbers of rudd captured in 3 mm nets over the intensive removal period suggest that the removal of the larger rudd has reduced breeding success in all lakes. The rudd population structure has been substantially altered as a result of the removal efforts. It appears likely that further efforts using the same methods would result in the continuation of relatively large catches in the 25 mm nets of fish that had previously evaded capture in the 3 mm nets, unless a between sized mesh is found to be effective. Although the changes in capture rates reflect the fishing effort expended on East and North, the results from South are surprising. In terms of net density and net nights, relatively little effort was expended in South. Aside from area, the most significant difference between South and the other lakes is the absence of submerged aquatic vegetation. The decline of rudd in this lake may be the result of greater mobility of all size classes seeking habitat and food resources: hence an increase in the frequency of net encounters, along with an inability to seek refuge in aquatic vegetation. Preferred rudd habitat, food, cover, and spawning substrate (Kennedy & Fitzmaurice 974; Cadwallader et al. 977; Lake et al. 22) are likely to be limited by the absence of submerged aquatic macrophytes in South. It seems unlikely that the decline of rudd in South is a reflection of the highly variable levels of recruitment common to Cyprinid populations (Mann 99) given the magnitude of the decline, and the continued recruitment in the adjacent North lake. Management implications Eradication of rudd did not occur in any of the Rotopiko lakes during intensive removals, despite the substantial effort. Although eradication of some species in fresh waters has apparently been achieved using physical removal (Knapp & Matthews 998) other attempts have proved costly and have used new technology such as radio-tagged Judas fish (J. Day, CSIRO Marine Research, Hobart, Australia pers. comm.). Knapp & Mathews (998) eradicated trout from a lake in California using gill nets, fishing for 2 years without catching further trout before declaring the eradication successful. Clearly our 5 consecutive nights per mesh size without catching rudd in East was an insufficient test of eradication, even at the high net densities used. Ideally for an eradication attempt an approach using continuous netting without breaks which allow recruitment, and a substantially longer period without catching rudd would be used. Although electro-fishing failed to catch further rudd in East, it is recommended that high-density netting be continued in this lake to either maintain low rudd densities, or achieve and confirm eradication. A total of 64 and 57 person hours were spent carrying out intensive removal over the three lakes in September 22 and March 23 respectively. The most labour-intensive part of the fishing effort was undoing knots in nets that had been created by eels scavenging captured fish. This was particularly so in North where large numbers of goldfish were captured in addition to rudd. Using the Department of Conservation standard operating procedure charge-out rate for field staff of NZ$6 per hour, the labour component of the two intensive removal periods came to NZ$72,6. In comparison the cost of the nets was just NZ$472, or 6.% of the total cost. In all three Rotopiko lakes, over 8% of the total catch during a 7-night period was taken in the first 3 nights. Given that the cost of nets was such a small proportion of the total cost of removal, we suggest in the absence of appropriate or proven eradication methods, an ongoing control programme based on a minimum of 3-4 high net density nights per year. Nets could be set for or 2 nights before spring spawning (September), checked, and if filled with large numbers of scavenged fish, discarded and replaced by fresh nets for a further 2 nights. It is likely to be important that fresh nets be provided for high capture efficiency. Studies have indicated that capture efficiency decreases with fish or algal accumulation (Hamley 975; Henderson & Nepszy 992). This practice would save considerably on the labour cost of net clearing and may be a viable and cost-effective option for lake managers. In situations where rudd are the target species and the management goal is the protection of aquatic vegetation, further economies could be achieved by

14 Neilson et al. Use of gill nets to remove rudd from lakes 537 specifically targeting fish which are already herbivorous (>5 mm) (Lane 983; Lake et al. 22). This would avoid the difficulties we encountered in targeting small, planktivorous rudd in physically complex littoral habitats, and target the larger, more easily captured sizes. Closs et al. (23) recommended a similar regime for carnivorous adult perch when the goal was enhancement of common bully (Gobiomorphus cotidianus) populations. Although unsuccessful in achieving rudd eradication, annual removal of rudd using monofilament gill nets is likely to be the most cost-effective option, and to have the least detrimental environmental impact of currently available control methods. This is particularly so in lakes such as Rotopiko where there are no desirable non-target fish species present that are susceptible to the nets. During the entire period of our work only two eels were found dead in gillnets and smelt and bullies were not captured in the 3 mm or larger meshes. The electro-fishing boat appeared to be very efficient at catching goldfish in North, but relatively ineffective at catching rudd compared to the gill nets. The presence of dense macrophyte beds may explain this (Bayley & Austen 22). In addition no catfish, known to be present in East, were caught using the boat. The cost of the electro-fishing boat is also reasonably prohibitive when access to the lakes is restricted to helicopter. The boat may be more cost-effective in devegetated lakes with road access. Rotenone may be a future option for the Rotopiko lakes given that it is now registered in New Zealand for use as a piscicide. Although the cost of the toxin itself is relatively low, the costs of the approval and consultation process, and of post-treatment monitoring remain high. The total cost of the gambusia rotenone operations on four small waterways in Nelson (average.6 ha) was very similar to the cost of our two intensive removal periods on the Rotopiko lakes (average 5. ha) (P. Elkington, Department of Conservation, Motueka pers. comm.). The clear benefit of the rotenone treatment was that eradication occurred. We suggest that work is carried out in natural water bodies of comparable size before attempting this at the Rotopiko lakes. We therefore intend to continue with annual control, as described above and including an 8 mm mesh, on all three Rotopiko lakes. This, in conjunction with ongoing macrophyte monitoring currently being carried out by the National Institute of Water and Atmospheric Research Limited, will determine whether long-term control of rudd has measurable benefits for the macrophyte communities within the lakes. ACKNOWLEDGMENTS We are grateful for the field assistance of R. Allibone, C. Annandale, D. Bell, N. Carter, L. Chadderton, J. Crozier, S. Fergie, B. Hicks, K. Hutchison, B. Jenkins, H. Kendall, A. Murray, D. Purdey, P. Shilov, and D. West. I. Westbrooke provided statistical advice. Thank you to the private landowners and Waipa District Council who allowed access through their properties, and to NIWA for ongoing macrophyte monitoring. We also thank A. Holzapfel, N. Grainger, and three anonymous referees for helpful comments on drafts of this paper. B. Taylor produced Fig.. This research was funded as part of Department of Conservation Science and Research Investigation 3349, Biodiversity Strategy Funding, and Environment Waikato State of Environment monitoring. REFERENCES Barnes, G. E. 22: Water quality trends in selected shallow lakes in the Waikato Region, Environment Waikato Technical report 22. Hamilton, New Zealand, Waikato Regional Council. 23 p. Barnett, B. S.; Schneider, R. W. 974: Fish populations in dense submersed plant communities. Hyacinth Control Journal 2: 2-4. Bayley, P. B.; Austen, D. J. 22: Capture efficiency of a boat electrofisher. Transactions of the American Fisheries Society 3: Bayley, P. B.; Peterson, J. T. 2: An approach to estimate probability of presence and richness of fish species. Transactions of the American Fisheries Society 3: Berst, A. H.; McCombie, A. M. 963: The spatial distributions of fish in gill nets. Journal of the Fisheries Research Board of Canada 2(3): Boswell, J.; Russ, M.; Simons, M. 985: Waikato small lakes: resource statement. Hamilton, New Zealand, Waikato Valley Authority. 3 p. Cadwallader, P. L. 978: Acclimatisation of rudd Scardinius erythrophthalmus (Piscies : Cyprinidae) in the North Island of New Zealand. New Zealand Journal of Marine and Freshwater Research 2: Cadwallader, P. L.; Coates, G. D.; Turner, A. S. 977: Introduction of rudd Scardinius erythrophthalmus into New Zealand. Fisheries Technical Report No. 47. Wellington, New Zealand Ministry of Agriculture and Fisheries. 24 p.

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