Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand

Transcription

1 Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand NIWA Client Report: CHC July 2009 NIWA Project: DOC09503/0809

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3 Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand Cathy Kilroy Martin Unwin Prepared for Department of Conservation NIWA Client Report: CHC July 2009 NIWA Project: DOC09503/0809 National Institute of Water & Atmospheric Research Ltd 10 Kyle Street, Riccarton, Christchurch 8011 P O Box 8602, Christchurch 8440, New Zealand Phone , Fax All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

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5 Contents Summary i 1. Introduction 1 2. The spread of didymo in the South Island, Methods Surveys Field sampling procedures Data Mapping Results and discussion 5 3. Potential pathways and vectors for the spread of didymo Methods Pathways Potential vectors Results and discussion 9 4. Distribution of didymo in the southern Waitaki catchment: a case study Methods Results and discussion Angler usage analysis Methods Results and discussion The role of kayaking/rafting Results and discussion General discussion Pattern of spread Vectors The biology of establishment and spread of didymo Rivers at risk Conclusions Acknowledgements References 30

6 Appendix 1. Appendix 2. Appendix 3. Sites which have tested positive for D. geminata, as used in this report. Rivers listed in the New Zealand Recreational Canoeing Association river guide. Assessment of risk of invasion by didymo for >500 river sites in North and South Islands Reviewed by: Approved for release by: JoAnna Lessard Ned Norton

7 Summary As part of ongoing investigations into the biology and effects of the non-indigenous bloomforming diatom, Didymosphenia geminata (didymo) the Department of Conservation (DoC) has requested an evaluation of the different possible vectors of didymo dispersal into and around the South Island, and likely pathways for its spread. In this report, we used survey data and both published and local knowledge about river usage to examine the spread of didymo in the South Island and possible vectors. The report is based on the assumption that most spread has been caused by human activities, and we stress that the evidence for this is circumstantial. We also emphasise that our ability to demonstrate links between human activity and the spread of didymo is severely curtailed by the lack of available data on recreational usage of New Zealand waterways. The exception is a database of angler usage of rivers supplied by Fish & Game New Zealand (FGNZ). The spread of didymo in New Zealand has been tracked in eight delimiting surveys funded by MAF Biosecurity New Zealand, and subsequent surveys coordinated locally. After its discovery in Southland in October 2004, didymo was not detected elsewhere until September 2005, when visible growth was reported almost simultaneously in the Clutha and Buller catchments. The annual reporting rate for new positive sites increased steadily from 2005 to The temporal spread suggests the most rapid range expansion in 2007, but the data are partially confounded by variation in sampling effort. Possible vectors for the spread of didymo were assessed using information from local agencies on sites at which didymo had been detected. Anglers were considered the most likely vector associated with incursions into new catchments or areas, followed by kayaking. Other vectors implicated included power boats; hydroelectric or irrigation canals; 4WD all-terrain vehicles; general recreation based (e.g., picnicking, swimming); field staff; gold mining. Upstream incursions from known positive sites were attributed to a more diverse range of vectors, including domestic livestock and non-domestic wildlife. A more detailed study of didymo distribution in parts of the Waitaki catchment showed that absences of didymo from some tributaries were not necessarily easily explained. However, the distribution pattern in a small subcatchment (Avon Burn) was clearly attributable to the presence of a mountain biking / tramping track which forded the stream. The FGNZ angler usage data provided indirect but strong evidence that the spread of didymo into many South Island rivers has been mediated at least partly by anglers. Rivers where didymo was present tended to be more heavily fished than those where it was absent. The incidence of didymo in rivers fished by anglers was also weakly related to the mean travel distance associated with each fishery, with the diatom less likely to be present in rivers fished

8 by locals. Rivers where didymo was present tended to receive a significant component (at least 10%) of their usage either from anglers visiting from other New Zealand regions, or from overseas visitors. We stress that our conclusions regarding the role of anglers in mediating the spread of didymo should not be interpreted as implying that angling is the most important vector. It is clear that many other vectors have contributed to didymo s dispersal around the South Island. Limited knowledge of the colonisation process in didymo, and of the diatom s environmental requirements, means that we cannot yet explain why it has not yet been detected in the North Island. Until this information is available, we suggest that it is prudent to assume that the reason is success of measures to prevent transport by human means. A risk assessment was carried out in 2006 for likelihood of arrival and establishment of didymo at >500 rivers sites in both islands. We consider that this effort is still likely to be useful for highlighting high-risk sites in the North Island.

9 1. Introduction Since the initial discovery of the non-indigenous diatom Didymosphenia geminata (didymo) in the Lower Waiau River, Southland, in 2004, the species has spread to at least 30 catchments in the South Island. In at least 12 catchments didymo has grown to bloom proportions, attaining biomass up to 10 times that recommended as the maximum for maintenance of biodiversity values (e.g., Kilroy et al. in press). Because of its capacity to bloom, MAF Biosecurity New Zealand (MAF BNZ) declared didymo to be an unwanted organism soon after its discovery. It is generally assumed that human activities have been responsible for the rapid spread of didymo around the South Island. This was the assumption made in a preliminary impact assessment undertaken soon after the 2004 discovery (Kilroy 2004a). The assumption has been generally supported since then, based on many known first entry points at locations where recreational or other visits are high. The felt-soled waders commonly used by anglers and other river users may be especially efficient at transporting cells because they retain moisture for a long time, and were suggested as potential vectors almost immediately. Other river users and their equipment (e.g., fishing lures, kayaks, four-wheel drive vehicles) have also been implicated. In addition, it has been acknowledged that didymo could spread within catchments by other means, for example, transport upstream by invertebrates, fish, birds or livestock. As part of ongoing investigations into the biology and effects of didymo, the Department of Conservation (DoC) has requested an evaluation of the different possible vectors of didymo dispersal into and around the South Island, and likely pathways for its spread. In this report, we address this request by examining the arrival and subsequent spread of didymo in New Zealand in conjunction with information about potential vectors for individual sites and for rivers in general. We used survey data and both published and local knowledge about river usage to examine the following: (a) (b) (c) the spread of didymo in the South Island; possible vector(s) for individual sites; relationships between angler usage of rivers and the occurrence of didymo. The report is divided into sections, corresponding to these three topics. In Section (b) we include a case study of the distribution of didymo in a single catchment. We then discuss use of vector information to identify rivers where the risk of invasion was/is high, relative risk in other rivers, and the potential for further spread throughout New Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 1

10 Zealand. For this we refer to an exercise carried out in 2006 for site selection in MAF BNZ delimiting surveys. Throughout this report, we stress that the information used to infer potential vectors is necessarily circumstantial because we do not have, and are unlikely to ever have, direct evidence that cells of didymo arrived in a certain location by a particular means. Authors of descriptions of the spread of other invasive species have faced the same problem (e.g., Aldridge et al. 2004, Johnson & Padilla 1996). We also emphasise that our ability to demonstrate links between human activity and the spread of didymo is severely curtailed by the lack of available data on recreational usage of New Zealand waterways. We draw heavily on data on angler usage of rivers supplied by Fish & Game New Zealand (FGNZ), who are virtually the only agency which regularly collects such data at a national level. As described later, these data establish a clear link between angling activity and the spread of didymo but should not be interpreted as evidence that other vectors are not important. Data sets of similar quality were not available for other recreational activities such as kayaking, jet boating, and family outings (e.g., swimming, picnicking); however, these vectors are also likely to be important for the past and future spread of didymo. 2. The spread of didymo in the South Island, After didymo had been declared an unwanted organism by MAF BNZ, one of their first actions was to carry out surveys in other Southland rivers to determine whether the diatom had already spread from the Mararoa and Waiau Rivers. A combination of benthic and drift net sampling at 62 sites was followed by microscope analysis of over 500 samples, all of which were negative for didymo (Kilroy 2004). Because of the difficulties involved in demonstrating absence of a microorganism from an area, the sampling methods were subsequently refined based on trials in recently invaded river systems (Kilroy & Dale 2006). As a result of these trials, a detailed field sampling protocol was developed aimed at maximising the chances of detecting didymo cells, if they were present, and also minimising the chances of false positives from crosscontamination (Duncan et al. 2007). Most reports of the presence or absence of didymo at sites in New Zealand are now based on these sampling procedures. In the following we summarise the methods and data used to track the spread of didymo in the South Island, and then describe the course of the spread. At the time this report was written, didymo had not been detected in the North Island. Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 2

11 2.1. Methods Surveys Eight nationwide surveys to document the spread of didymo in New Zealand were carried out by a range of agencies, funded by MAF BNZ, in October 2005, January 2006, April/May 2006, August 2006, November 2006, January/February 2007, May 2007, and November/December Surveillance has been continued by local agencies since then. In addition to organised surveys, samples suspected to be didymo have been submitted by members of the public Field sampling procedures The 2006 sampling methods trials showed that material filtered from the water column of rivers known to be affected by didymo yielded clean samples that were always positive for didymo (microscope examination) even at visually unaffected sites up to 60 km downstream from visible colonies (Kilroy & Dale 2006). Cells were usually also detected in benthic samples at downstream sites, but at the cost of greater sample analysis effort. To increase the chances of detecting didymo if it was present at low densities, from May 2006 the standard survey included both net and benthic samples. The October 2005 and January 2006 surveys included benthic sampling only. For net samples, a 40 µm-mesh plankton net was deployed for ~10 minutes in fastflowing water. The samples were collected onto a piece of new 40-µm mesh secured at the collecting end of the net. A 250 µm-mesh pre-net eliminated large particles, while allowing most didymo cells (up to 150 µm) to pass through. Benthic samples comprised the combined algae scraped from 20 rocks collected along four transects or partial transects up to 10 m apart, to depths of 0.7 m. To minimise the chances of false positives at subsequent sampling sites, a strict protocol of cleaning and disinfecting all equipment was adhered to following sample collection (Duncan et al. 2007). In the laboratory, three 1 2 ml subsamples from each net and benthic sample were scanned under an inverted microscope at 100 x magnification, for a total standard scanning time of 15 minutes. The presence of live didymo cells (i.e., containing chloroplasts) signified a positive result. Where only empty frustules are found, the sample was deemed suspect positive and the site was re-sampled at the earliest opportunity. Stringent precautions were taken to avoid cross-contamination of samples. Since 2007, surveys in the North Island have used a rapid-throughput, highly sensitive genomic detection method (Cary et al. 2008). This method has enabled highfrequency surveillance over a wide area, as well as high confidence in negative Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 3

12 prediction. Plankton net samples only are required for this analysis, using a modified net design Data MAF BNZ maintains a password-controlled internet based database intended to provide a single source registry of all didymo sampling results taken throughout New Zealand 1. All samples were collected and analysed using the techniques described above, with the exception of positive samples detected as visible growth. The database includes both negative and positive results, allowing users to generate data summaries and distribution maps, and to download selected data for export as appropriate. On 15 May 2009 we downloaded the full database, comprising 3357 records covering the period from 25 January 2005 to 8 May After deleting 73 records with invalid map references or sampling dates the remaining 3284 records represented 1307 sites in 14 of 15 territorial authority regions in New Zealand, covering all of the country except Northland. Most sites were sampled at most once per year, but in some regions sites of particular interest were sampled monthly, with a maximum of 27 records for one site. Fields retained for subsequent analysis included sample and site identifiers; territorial authority region; the name of the waterway and a brief description of the sampling site; an NZMS260 Topomap reference; sampling date; and the presence or absence of didymo. All sites also included an identifier linking the site to the corresponding segment of the River Environment Classification (REC) scheme (Snelder & Biggs 2002), a spatial network of linked segments corresponding to all New Zealand lake and river systems, thereby allowing each site to be mapped in relation to upstream and downstream river segments within the same catchment. The BNZ database is comprehensive but not exhaustive, reflecting gaps in coverage where recent data have yet to be forwarded or entered, or where didymo has been positively identified during field surveys by experienced staff but the records have not been reported to BNZ. We therefore compiled a secondary database of positive sites based on our own knowledge, personal contact with key regional field staff, and various unpublished reports. After merging this with the BNZ data and eliminating duplicates, we obtained a final working data set of 212 positive sites including the first known New Zealand incursion in the Waiau River on 20 October 2004 (Appendix 1). All subsequent analyses were based on this set of positive records, together with all negative records from the BNZ database (2869 records from 1101 sites). 1 Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 4

13 To identify and classify rivers known to be positive for didymo at the finest available spatial scale, we used the REC to trace downstream from all positive sites, and hence to identify all river segments where didymo could be assumed to be present. Data extracted for each segment included segment length (typically < 1 km), and stream order (which indexes stream network complexity) Mapping We used the locations of both positive and negative sites on the MAF BNZ database to map the spread of didymo in New Zealand in yearly steps from 2004 to Positive sites were mapped as river lines, on the assumption that, once present at a site upstream, downstream spread was inevitable. Negative sites were mapped as points, as there was no way of knowing whether didymo was already present downstream Results and discussion After its discovery in the Lower Waiau and Mararoa Rivers in October 2004, didymo was not detected in any other location until September 2005, when visible growth was reported almost simultaneously in the Hawea (Clutha), Buller, Oreti, and Upper Waiau Rivers (Figure 1). A delimiting survey in October 2005 covered 450 sites in both North and South Islands. Only one additional river was positive the Von River, which is connected by a road link of only 38 km from the Mararoa River site where the first incursion of didymo was assumed to have occurred. The annual reporting rate for new positive sites increased steadily from 2005 to 2007 after which it appears to have stabilised and possibly decreased (Table 1), although interpretation of these data is partly confounded by temporal and regional variation in the number of sites sampled (Figure 1). Downstream dispersal following first incursion in a new catchment was sometimes detected within a few days of the first positive (e.g., Waiau River, Buller River, upper Clutha River, Waitaki River), almost certainly reflecting a well-established incursion which had escaped notice until it was detected essentially simultaneously at multiple sites. Upstream dispersal has been more gradual, but has been the dominant source of new positives since The temporal spread of didymo by year and river is also consistent with a rapid range expansion in 2007, but again these data are partially confounded by annual variation in sampling effort. Nevertheless, it seems clear that new incursions were relatively limited during 2005 and 2006, most strikingly in the northern West Coast and Tasman regions (Figure 1) where 15 months elapsed between first incursion in the upper Buller River (24 September 2005) and the first positive in another catchment (Takaka River, 25 January 2007). By the end of 2007 the diatom was established over most of Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 5

14 Figure 1: Maps showing the cumulative river reaches found to be positive for didymo in yearly blocks from 2004 to Green symbols indicate sites with samples collected in that year, in which no didymo was detected. Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 6

15 Table 1: Numbers of sites in South Island rivers testing positive for didymo by year and pathway category (as defined in Section 3: new, downstream, upstream), Note that data for 2009 represent only the four months from January to April. Year new downstream upstream Total D R A F T 21/08/ Total the South Island, although its range continued to expand in 2008 (with new positives in the Wairau and Grey Rivers, for example) and 2009 (e.g., the Paringa River in south Westland, and the Tekapo Canal, Canterbury). Analysis of positive sites by stream order, based on the assumption that all locations downstream of a known positive site for didymo are also positive, provides further insight into the nature of its range expansion (Table 2). New Zealand s two order 8 mainstem rivers (the Clutha and Waitaki) were uniformly positive by 2006, since when there is evidence that the diatom has become increasingly prevalent in lower order streams. At the time of writing, an estimated 4437 km of waterway were positive for didymo, including all 337 km of 8 th order streams, 724 km (76.5%) of 7 th order streams, and 1868 km (54.1%) of 6 th order streams. Relatively few streams of 3 rd or lower order have been surveyed, but the presence of the diatom in some 1 st order streams confirms that given time it has the potential to spread upstream well into the headwaters of catchments in which it is present. The South Island distribution of didymo in 2009 shows that there are still major gaps (Figure 1). The most obvious are the Taieri catchment in Otago, and the Waimakariri / Ashley /Okuku catchments in Canterbury. Because representation of negative samples in 2009 has been sparse, we cannot rule out the possibility that these rivers may now be positive for didymo. Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 7

16 Table 2: Chronology of didymo incursions in South Island rivers by year and stream order, based on downstream traces using NIWA s River Environment Classification (REC) scheme. In the upper portion of the table successive columns for each order show the total number of segments mapped in the REC; the number of segments downstream of a known positive segment by year; the total for all years; and the percentage of positive or assumed positive segments. The lower portion shows the same statistics expressed in terms of total segment length in km. Stream order Number of segments Total, all years % of positive segments 1 166, % D R A F T 21/08/ , % 3 42, % 4 23, % 5 11, % % % % Total 327, % Stream order Total length (km) Total, all years % of total length 1 132, % 2 56, % 3 28, % 4 14, % % % % % Total 244, % 3. Potential pathways and vectors for the spread of didymo 3.1. Methods Pathways For the purposes of this study, we use the term pathways to describe different types of movement (as defined by Wilson et al. 2009) observed during the progression of spread of didymo around the South Island. All 212 positive records were examined, Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 8

17 with reference to the timing of the record and its location compared to other known positive sites. All findings were placed into one of three pathway categories: 1. new: a positive site in a new catchment or sub-catchment with no previous positive records; 2. downstream: a positive site in a catchment which already has positive sites upstream; 3. upstream: a positive site in a catchment which already has positive sites downstream. D R A F T 21/08/09 In allocating sites to category 3, we wished to identify sites at which didymo was most likely to have originated from downstream. Therefore we assigned category 1 to some sites upstream of previous records in large catchments where configuration of valleys, physical barriers such as dams or large lakes, or the great distances involved suggested that a separate source was more likely. By contrast, category 3 sites were limited to those at most km upstream from a known positive site, in situations where natural vectors (e.g., birds, fish, wildlife) or short-range human dispersal could plausibly be implicated Potential vectors To determine potential vectors of didymo for each of the positive sites, we approached local agencies and interviewed staff members who were familiar with rivers and river sites in their region. Agencies included DoC, Regional Councils, and FGNZ. We asked about usage of each site at which didymo had been detected, and its significance for recreation, the upstream limit of didymo in relation to that site (if known), as well as the specific question about the most likely vector(s) of didymo into that site. We requested consideration of all possible vectors, including natural means (for example: birds, wild animals). As far as possible we obtained information from at least two organisations for each site Results and discussion We identified twelve distinct vectors which could potentially have contributed to the spread of didymo in South Island rivers (Table 3). Of the 212 positive sites, 132 were associated with one vector, 67 with two, and 13 with three. Excluding natural downstream dispersal, all but two vectors (livestock and wildlife) were associated with human activity. Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 9

18 Table 3: Vectors likely to have contributed to the spread of didymo in South Island rivers versus pathway category (new, downstream, upstream), Figures shown in each cell are the number of sites for which the corresponding vector is likely to be implicated, as a count and (in parentheses) as a percentage of the total sites in that category. Vector New (44 sites) Downstream (62 sites) Upstream (106 sites) Total (212 sites) anglers 25 (57%) 4 ( 6%) 61 (58%) 90 (42%) kayaks 13 (30%) 3 ( 5%) 17 (16%) 33 (16%) power boats 5 (11%) 15 (24%) 13 (12%) 33 (16%) livestock 19 (18%) 19 ( 9%) vehicles 4 ( 9%) 14 (13%) 18 ( 8%) general recreation 2 ( 5%) 4 ( 6%) 9 ( 8%) 15 ( 7%) D R A F T 21/08/09 trampers 12 (11%) 12 ( 6%) diversion canal 5 (11%) 1 ( 2%) 3 ( 3%) 9 ( 4%) wildlife 7 ( 7%) 7 ( 3%) field staff 1 ( 2%) 3 ( 3%) 4 ( 2%) gold mining 1 ( 2%) 2 ( 2%) 3 ( 1%) downstream dispersal 62 (100%) 62 (29%) Anglers were considered the most likely vector associated with new incursions (25 of 44 sites), with kayaking implicated in one third of new incursions (14 of 44 sites). Six other vectors were considered potential vectors for new incursions at 18 out of 44 sites (41%). These included power boats (Waitaki River, Makarora River, Wairaurahiri River, Rangitata River, Paringa River); hydroelectric or irrigation canals (Waihao River, Deep Cove, Ohau canal, Rakaia River, Ashburton River); and 4WD all-terrain vehicles (North Opuha River, Tasman River, Cass River, Orari River). General recreation based on family activities such as picnicking and swimming were potential vectors for new incursions on three rivers (Orari, Takaka and Waimea Rivers), while field staff responsible for tasks such as river gauging, and gold mining, were each potentially linked to one new incursion (Aparima River and Nevis River, respectively). Animal vectors (domestic livestock and wildlife) were not considered likely vectors for any new incursions. Upstream incursion from known positive sites appeared to reflect a more diverse range of vectors than did new incursions (Table 3). Angling (61 of 106 sites) was by far the most commonly implicated vector, but at least one of eight other human mediated vectors was a potential vector at over two thirds (69%) of the 106 sites in this category. In addition, non-human vectors (domestic livestock and non-domestic wildlife) were potential vectors at 21 sites on 19 rivers. Most of these were small streams in areas where land use was dominated by pastoral farming, and involved small tributary streams no more than 5 km upstream from a primary source. In Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 10

19 particular, upstream incursions in numerous small streams in rural Southland (e.g., tributaries of the Mararoa and Oreti), and some lower Waitaki tributaries (e.g., the Awakino and Otiake) were potentially mediated by domestic livestock. Obvious associations with human-induced vectors were only rarely associated with downstream dispersal of didymo. Power boats were considered potential vectors in 15 of 62 such cases, most of which were shoreline or river mouth sites in Lake Te Anau (10 sites), or shoreline sites in Lake Dunstan (4 sites). The Kawarau was the only river where power boats were considered likely to have been responsible, following previous incursions in three Lake Wakatipu tributaries. The different proportions of named possible vectors are clearly seen when mapped according to dispersal pathway (Figure 2). D R A F T 21/08/09 As emphasised earlier, the identity of the vector human-assisted or natural which transported cells of didymo to any particular site cannot be proven. However, the circumstantial evidence for the mainly human-assisted spread of didymo is compelling. First, didymo has appeared as a new incursion in many catchments at locations known to be heavily used for recreation of various types. Second, surveys at remote, rarely visited sites, have failed to detect didymo even though these sites are much closer to heavily affected rivers than were some of the new positive finds in the surveys. For example, DoC surveys of Fiordland rivers draining into Lake Te Anau have detected didymo only in those which are regularly visited. 4. Distribution of didymo in the southern Waitaki catchment: a case study The delimiting surveys up to December 2007 focused on sampling high order rivers and streams, on the assumption that if didymo was present upstream, the probability of detecting cells at a downstream site was high, based on the sampling method trials (Kilroy & Dale 2006). Although efficient in terms of coverage of rivers, this strategy fails to account for the smaller-scale distribution of didymo in low-order tributaries. The progression of spread into tributaries could provide further clues about vectors. Because smaller streams are less likely to have multiple recreational and other uses, patterns of presence and absence within a larger catchment might be easier to explain. We therefore carried out a survey of tributaries draining from southern and western areas of the Waitaki catchment Methods From mid-march April 2009, we checked 21 sites in 12 tributaries in the southern and western part of the Waitaki catchment, using the methods described in Section 2. Tributaries already known to be positive for didymo were included to provide a check Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 11

20 Figure 2: Vectors likely to have contributed to the spread of didymo in South Island rivers versus pathway category (new, downstream, upstream), Vectors were identified through interviews with agencies familiar with the rivers particular regions. Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 12

21 on the ability of the sampling method to detect didymo even if there was no sign of growth. Some of the eastern tributaries had recently experienced very high flows, which had removed all visible biomass growth. In tributaries not already found to be positive for didymo, we first checked the most upstream accessible site. Within one tributary (Avon Burn), we checked seven sites including representatives of stream orders 1, 2 and 3. In all cases, where visible didymo was suspected, we took samples for microscopic confirmation. Where no didymo was visible, both net and benthic samples were collected as described in Section 2, then checked microscopically for didymo. The aim was to determine whether any tributaries were still free of didymo, then to explain the patterns with reference to the characteristics and usage of each subcatchment. D R A F T 21/08/ Results and discussion Access constraints meant that the most upstream site in some streams was quite far downstream. No sample could be taken from three further planned sites because the streambed was dry. We identified no tributaries which had previously tested positive at a downstream site, but were negative farther upstream. Of the 21 sites checked, seven were negative for didymo (Table 4). While the presence of didymo could be explained by human activities in most of the positive sites (Table 4), no consistent feature explained why those particular seven sites were negative. A closer look at the characteristics and usage of each stream provided some clues, in some cases, as discussed below. Fraser Stream (sites 2 and 3). Fraser Stream joins the Twizel River close to Twizel township. Despite being very accessible for recreation (including angling) no didymo has yet been detected in the river in a series of DoC surveys. The stream arises from relatively low-altitude wetlands and is slightly tannin-stained, with slightly lower ph (ph = ~6.4) compared to neighbouring affected rivers (e.g., Twizel, Long Slip Creek, ph ) (NIWA data, collected at the time of the surveys). Although this may yet be disproven, it is possible that the Fraser Stream environment (i.e., low ph) is unsuitable for didymo. Birch Creek (site 5). This small stream is close to the known upstream limit of didymo in the Ahururi River. There is a marked DoC access to the Ahuriri hunting block on the true left of the stream. 2 A 4WD track from Ben Avon station is marked on the NZMS260 map, on the true right. It does not cross the stream. From this we infer that 2 See: canterbury/twizel/hunting/ahuriri-hunting.pdf Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 13

22 Birch Creek itself is not regularly accessed by recreationalists and is not forded by vehicle or foot traffic in its lower reaches. Table 4: Southern and western tributaries of the Waitaki River, checked for didymo presence / absence in March April Ref. refers to the number marked on the map (Figure 3). Asterisks indicate tributaries already known to be positive from previous surveys. River Ref. E N Didymo Extent Stream usage Twizel* yes not visible mountain bike crossing upstream Fraser no absent angling Fraser no absent next to MTB and walking track Twizel* yes visible public access / ford upstream D R A F T 21/08/09 Birch Creek no absent hunting access / no ford Avon Burn yes visible MTB, tramping / ford Avon Burn yes not visible access to hunting block Avon Burn yes visible MTB, tramping / ford Avon Burn yes visible MTB, tramping / ford Avon Burn no absent runs from grazed hillside Avon Burn yes visible track upstream, livestock / ford Avon Burn yes not visible MTB, tramping / ford Long Slip Ck yes visible runs beside road / public access Long Slip Ck yes visible runs beside road / public access Otamatapaio no absent no public access Otematata* yes not visible angling, no easy access upstream Deep Stream no absent angling access 20 mins on foot Awakino yes not visible ford next to bridge Otiake River yes visible limited access Otekaieke no absent no public access Maerewhenua* yes not visible motorcamp upstream, easy access Otamatapaio (site 15). The Otamatapaio riverbed is often dry where it crosses SH83 (G. Hughes, FGNZ, pers. comm.). The site sampled was approximately 3 km upstream in a shallow gravel-bedded area. Access was via a private road (permission required). According to FGNZ, the river is fished. However, the private access may limit this. Assuming the river is environmentally suitable for didymo, limited access may have lowered the risk of didymo becoming established. Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 14

23 Birch Ck Avon Burn D R A F T 21/08/ Figure 3: 21 Locations of 21 sites in the Waitaki catchment checked or sampled for didymo during March April Site are numbered 1 to 21, as referred to in Table 4. Sites positive for didymo are indicated by red dots, negative sites by green. Red triangles are other sites in the catchment where didymo is known to be present from previous surveys. Deep Stream (site 17). This was the only site sampled from the north side of the Waitaki River (Lake Aviemore). It was included as an example of a stream accessible to the public, and advertised as an angling destination, but requires some effort to reach (a 20 minute walk along a narrow, steep track). Therefore, while accessible to the public, site visits are limited to those willing to make the walk in and out. Otekaieke (site 20). The Otekaieke is not advertised as an angling river and does not have easy public access. However, its catchment appears similar to that of the neighbouring Otiake River, which is also unfished and not easily accessed by the public, yet is heavily affected by didymo. One difference noted was that in the Otiake there is a vehicle ford across the river about 3 km upstream from the Waitaki River confluence; there are no such accessible fords on the Otekaieke. Avon Burn. Of the seven sites checked in the Avon Burn, only one (site 10) was negative for didymo. This was a first-order streamlet draining a hillside with stock grazing, which appeared to provide a good habitat for didymo (full light, fast-flowing, rock substrate colonised by algae frequently seen with didymo). Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 15

24 A marked DoC track gives access to the Avon Burn from Birchwood Road, which runs along the true left of the Ahuriri River. This track is part of the Te Araroa walking / mountain bike track, and connects to the Timaru River catchment and Lake Hawea (off the map to the left on Figure 3). Part of the route is the 4WD vehicle track continuation of a farm access road, which is clearly in use. The track also connects to mountain bike trails which lead over to the headwaters of the Lindis River. The track fords the Avon Burn within 4 km of the road (site 8), crossing a first-order tributary after about 2 km (site 6), and a second-order tributary (site 11) after another 2 km. All three sites had prolific didymo growth when checked in mid-march. There was no sign of didymo in a third-order tributary (site 7) a further 2 km upstream, but we found a few live cells in the samples. The main stem was still visibly infested at this point (site 9). A sample collected farther upstream in the main stem at a later date (site 12), and following a flood, was also positive. D R A F T 21/08/09 The pattern of infestation in the Avon Burn suggests that human traffic is mainly responsible for spreading didymo through the catchment, for the following reasons: a. the track crosses the Avon Burn main stem twice, within a distance of 6 km; b. the track also crosses two obvious tributaries; c. the main stem and both tributaries had obvious didymo growth when conditions were suitable; d. a large tributary not forded by the track (site 7) had only microscopic amounts of didymo when the above sites were clearly infested, suggesting that didymo had only recently been introduced there; e. the only negative site was a small stream inaccessible to vehicle and foot traffic; f. entries in a hut book in a hut close to the most upstream ford suggested frequent mountain bike and foot traffic. g. Furthermore, the neighbouring catchment, Birch Creek, which is not forded by a well-used recreational track, was negative for didymo. Given its wide distribution in this small catchment, it is possible that didymo could have been carried over into the Lindis and / or Timaru Rivers from this source, assuming that didymo spread to the Avon Burn first, from the Ahuriri. Didymo was found in the Ahuriri in January 2006 well before positive finds downstream in the Lindis (January 2007) or Timaru Rivers (November 2007). This could be checked by Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 16

25 sampling the headwaters of both the Lindis and Timaru Rivers in the vicinity of the tramping / mountain bike tracks, though it would be impossible to be sure of the direction of the spread. In summary, a closer look didymo distribution within the Waitaki catchment generally supports the view that most of the spread of didymo can be linked to human activities. Its distribution in the Avon Burn catchment, in particular, would be difficult to explain unless it had been transported by humans. At the same time not all cases of presence and absence have an obvious explanation, as shown by the neighbouring Otiake and Otekaieke Rivers. The initial arrival of didymo in the Otiake has been attributed possibly to transport by paradise ducks, which are known to nest in the catchment. Subsequent upstream spread could have been by livestock (sheep). We have been unable to ascertain whether paradise ducks also nest in the Otekaieke catchment. D R A F T 21/08/09 5. Angler usage analysis Recreational angling for acclimatised freshwater fish in New Zealand is jointly managed by DoC in Lake Taupo and its inflowing tributaries, and by FGNZ in all other regions. All persons wishing to fish for acclimatised species must purchase a freshwater fishing licence at least annually. Licences purchased from FGNZ are freely interchangeable between regions and are priced without regard to angler origin: overseas anglers pay the same as New Zealand residents, and residents of each region pay the same as non-residents. It is possible, therefore, for anglers to live in one region, purchase a licence from a second region, and fish in a third. The Taupo Conservancy is the sole exception: FGNZ licences are not valid within the Conservancy, and DoC Taupo licences are not valid elsewhere in New Zealand. FGNZ has used national surveys of its licence holders to estimate annual effort for all recognised river and lake fisheries every 6-7 years since 1994/95 (Unwin & Brown 1998, Unwin & Image 2003), most recently in 2007/08 (Unwin 2009). These surveys treat most fisheries as a single entity and do not generate usage estimates for specific locations, but provide a rich data set on the geographic origin of anglers fishing each river, and the proportion of the effort on each river associated with visitors from other FGNZ regions (Unwin & Deans 2003). The 2007/08 survey included overseas visitors to New Zealand as a separate stratum, allowing their fishing patterns to be contrasted with those of New Zealand residents Methods For the present study, we used the 2007/08 FGNZ data to compile a list of all river and lake fisheries which were fished during the 2007/08 angling season (1 October 2007 to Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 17

26 30 September 2008), and to estimate total usage for each fishery over this period. This list comprised 848 waters (204 lakes and 644 rivers), of which 236 waters were fished for at least 500 angler-days per year and were used for more detailed analysis. These fisheries accounted for 93.7% of the total estimated effort for the 2007/08 season (1 271,000 ± 20,000 angler-days). To characterise the relative vulnerability of each fishery to angler mediated incursion of didymo we derived four metrics of angler origin based on each respondent s home address and FGNZ region. These were: mean travel distance (in km) from the angler s home to a nominal centroid for each fishery (cf. Unwin & Deans 2003); the proportion of the total annual usage contributed by New Zealand visitors from outside the local region; total annual usage (angler-days) by overseas visitors; and the proportion of the total annual usage contributed by overseas visitors. D R A F T 21/08/ Results and discussion Analysis of the presence or absence of didymo in relation to angler usage estimates and associated metrics of angler origin suggested a clear relationship between angler activity and the spread of didymo in the South Island. Rivers where didymo was present tended to be more heavily fished than those where it was absent (Figure 4a). Of sixteen South Island rivers which received more than 5000 angler visits per year, all but three (the Waimakariri, Taieri, and Ashley) have recorded positive for didymo. Median annual angler usage for rivers where didymo was present was 1730 days, compared to 911 days for rivers where it was absent. Figure 4: Presence or absence of didymo in 107 South Island rivers with an estimated 2007/08 angling usage of at least 500 angler-days. Vertical arrows denote medians for each histogram. Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 18

27 Analysis of fishing activity by angler origin provided further evidence of an association between angler mobility and the presence of didymo (Figure 5). In the South Island didymo was rarely found in rivers for which local anglers accounted for 90% of more of total usage, irrespective of their level of usage. Rivers where didymo was present showed considerable variability with respect to angler origin, but tended to receive a significant component (at least 10%) of their usage either from anglers visiting from other New Zealand regions, or from overseas visitors. D R A F T 21/08/09 Figure 5: 100 D. geminata status absent present Usage by overseas visitors (%) Usage by NZ visitors (%) Usage by NZ locals (%) 80 Days per year Presence or absence of didymo in 107 South Island rivers with an estimated 2007/08 angling usage of at least 500 angler-days vs. angler origin. The radius of each plotting symbol is scaled as the cube root of total annual usage We interpret the above results as indirect but strong evidence that the spread of didymo into many of the currently known South Island rivers which have tested positive for the diatom has been mediated at least partly by anglers. Both total levels of usage and diversity of angler origin appear to be implicated, with most positive sites being recorded either on rivers which sustain high levels of usage (Figure 4a), or rivers which are fished by anglers from other New Zealand regions or from overseas (Figure 5). Didymo is present in virtually all large rivers draining to the east and south coasts, most of which sustain mean angler densities of 1-2 anglers per km per day (Figure 6). For these rivers, sheer force of numbers is likely to been sufficient to ensure the spread of didymo, even if visitors from outside the region account for only a small proportion of the total effort. However, for rivers in other regions, notably Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 19

28 D R A F T 21/08/09 Figure 6: Angler usage in New Zealand rivers, where days refers to the number of anglerdays per year. The more detailed analyses used only rivers with >500 angler-days per year. Therefore in this map, rivers coloured blue to blue-green are unfished or have minimal angling activity Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 20

29 Nelson, Buller and inland Southland, angler mobility rather than angler density appears to have been an important contributing factor. It is particularly striking that the two regions which show up most strongly in terms of overseas visitor presence, the Buller River and its catchment, and the headwaters of the Waiau, Mararoa, and Oreti Rivers in Southland, include the two sites which now appear to be the source of all subsequent incursions (Figure 7). Indeed, we cannot rule out the possibility that the original Waiau and Buller positives were independent incursions, particularly since the time interval between incursion and first detection remains unknown. D R A F T 21/08/09 Angler mobility patterns for North Island rivers tend to be more muted than for their South Island counterparts. Few rivers derive more than 10% of their total annual effort from overseas visitors (Figure 7), and movement of anglers between New Zealand regions is also comparatively restricted (Figure 8). The main exceptions tend to lie within the central North Island, and include the headwaters of the Mohaka, Tukituki, Rangitaiki, Whakapapa, Ruakituri, and Whirinaki (Figure 8). The Motu was the only North Island river for which overseas visitors accounted for more than 50% of total 2007/08 usage, but with a total estimated usage of 510 ± 240 angler-days it was only just above the 500 day per year threshold used to compile our data set. The relatively low level of angler interchange among North Island rivers is consistent with the absence of didymo in all surveys to date. However, should the diatom reach the North Island we have little doubt that mobility patterns for New Zealand resident anglers are more than sufficient to ensure that its spread to other rivers is inevitable over time scales of at most one or two years from first incursion 6. The role of kayaking/rafting The second most common possible vector named in the interviews with local agencies (Section 2) was kayaking (including rafting). No database on kayaking and rafting usage exists which is comparable to the angler usage database (Section 5), although at least one Regional Council (Otago Regional Council) has made observations on kayaking usage of selected rivers as part of their summer aquatic pest programme (data available on The most comprehensive information located was on the website of the New Zealand Recreational Canoeing Association (NZCRA) website (rivers.org.nz). The site includes the NZRCA New Zealand River Guide, which is a list of kayaking / rafting runs in New Zealand, by region, with rafting class, and descriptions in many cases. We combined this list with our list of South Island sites positive for didymo, via new or upstream pathways. Two percentages were determined for each region: proportions of rivers listed in the river guide, which are positive for didymo, and proportions of didymo-positive rivers listed in the river guide. North Island and South Island were compared using numbers of kayak / rafting rivers and runs in each region Vectors and pathways influencing the spread of Didymosphenia geminata (didymo) in New Zealand 21