Biological Conservation 212 (2017) 256–264

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Life-histories explain the conservation status of two estuary-associated pipefishes

MARK

Alan K. Whitfielda,⁎, Thomas K. Mkareb,1, Peter R. Teskeb, Nicola C. Jamesa, Paul D. Cowleya a b

South African Institute for Aquatic Biodiversity (SAIAB), Private Bag 1015, Grahamstown 6140, South Africa Molecular Zoology Laboratory, Department of Zoology, University of Johannesburg, Aukland Park 2006, South Africa

A B S T R A C T Two endemic southern African pipefish species (Teleostei: Syngnathidae) co-occur in estuaries on the southeast coast of South Africa. The larger longsnout pipefish, Syngnathus temminckii, is abundant and has a wide range that comprises coastal and estuarine habitats in all three of the region's marine biogeographic provinces. In contrast, the smaller estuarine pipefish S. watermeyeri is critically endangered, and confined to a few warmtemperate estuaries. Here, we explore reasons for these considerable differences in conservation status. Fecundity is related to fish size, with large live-bearing S. temminckii males carrying up to 486 developing eggs/ embryos, compared to a maximum of only 44 recorded for S. watermeyeri. Loss of submerged seagrass habitats due to episodic river flooding appears to be correlated with the temporary absence of both species from such systems. Prolonged cessation in river flow to estuaries can cause a collapse in estuarine zooplankton stocks, a food resource that is important to pipefish species. The greater success of S. temminckii when compared to S. watermeyeri can be attributed to the former species' wider geographic distribution, fecundity, habitat selection and ability to use both estuaries and the marine environment as nursery areas. Genetic data indicate that this has resulted in a much smaller long-term effective population size of S. watermeyeri, a situation that has persisted since the beginning of the present interglacial period. Syngnathus watermeyeri is thus naturally more susceptible to anthropogenic disturbances, which have resulted in an alarming reduction in its contemporary population size. Possible measures to promote the conservation of S. watermeyeri are presented.

1. Introduction Fishes have over 30 reproductive guilds that can essentially be divided into three main categories, namely non-guarders, guarders and bearers (Balon, 1975). Pipefishes belong to the bearer category and more specifically the external bearers. Typically they exhibit parental care, have a low fecundity but invest a large amount of energy in each of a small number of well-developed precocial young. The adults of such species are often specialists, have a narrow trophic niche and usually live in a stable and predictable environment (Bruton, 1989). Although the longsnout pipefish Syngnathus temminckii Kaup, 1856 and the estuarine pipefish S. watermeyeri Smith, 1963 fulfil many of the criteria outlined above, both species occur in estuaries that are generally unstable and unpredictable environments (Whitfield, 1990). Fortunately for S. temminckii, it also occurs in the marine environment that is much more stable and predictable, thus conferring this species with a distinct advantage over S. watermeyeri. This study will show that this is but one of the many life-history traits that places the former ⁎

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species at an advantage over the latter. Syngnathus temminckii and S. watermeyeri have a conservation status that also differs considerably between the two species. The former is common within its South African distributional range, from the cool temperate west coast, through the estuaries and marine environment of the warm temperate southern and south-eastern coasts, reaching into the subtropical zone on the east coast (Mwale et al., 2014). In contrast, S. watermeyeri has been recorded in only a limited number of estuaries on the warm temperate south-east coast and, even in those estuaries, the numbers are generally very low (Whitfield, 1995). More recently, S. watermeyeri has been listed as Critically Endangered (CR) in the IUCN Red List (www.iucnredlist.org). The main threats to its existence are habitat loss, river degradation and loss of freshwater inputs to estuaries (Vorwerk et al., 2007). River inflow provides the nutrients required to stimulate planktonic productivity in estuaries (Grange et al., 2000), the food chain upon which this species depends for its survival (Whitfield, 1995). Loss of river pulses due to excessive freshwater abstraction in the catchments leads to a reduction

Corresponding author. E-mail address: a.whitfi[email protected] (A.K. Whitfield). Present address: Kenya Marine and Fisheries Research Institute, P.O. Box 81651, Mombasa 80100, Kenya.

http://dx.doi.org/10.1016/j.biocon.2017.06.024 Received 3 October 2016; Received in revised form 2 June 2017; Accepted 15 June 2017 0006-3207/ © 2017 Elsevier Ltd. All rights reserved.

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Bushmans and the East Kleinemonde estuaries in the Eastern Cape Province (Whitfield, 1995; Fig. 1), spanning a coastal distance of only 60 km. The type specimens of S. watermeyeri were collected in the Kariega Estuary during the early 1960s, but by the 1980s and 1990s this species had disappeared from that system (Whitfield and Bruton, 1996), only to reappear in 2006 following river flushing of the estuary (Vorwerk et al., 2007). It is important to note that S. temminckii remained present throughout and was sometimes abundant in the Kariega Estuary during the decades when S. watermeyeri was locally extinct in that system (Ter Morshuizen and Whitfield, 1994; Whitfield and Bruton, 1996). It should also be noted that S. temminckii has also been recorded from estuaries where submerged aquatic macrophyte beds are absent (Mwale et al., 2014). Both S. temminckii and S. watermeyeri have been recorded in Zostera capensis and Ruppia cirrhosa plant beds within the Kariega and East Kleinemonde estuaries, respectively (Fig. 1). Indeed, the type specimens of S. watermeyeri were captured together with S. temminckii individuals from the same eelgrass bed in 1963 (D. Galpin, pers. comm.). In estuarine systems elsewhere in South Africa where submerged plant beds are present, S. temminckii usually occurs on its own, although other pipefish species (e.g. Hippichthys spicifer) are sometimes also recorded from these habitats (Harrison and Whitfield, 2006). The juveniles and adults of both Syngnathus species are normally associated with submerged seagrass beds in certain Eastern Cape | Province estuaries, with S. temminckii also being associated with seaweed and reef habitats in the coastal marine environment. Syngnathus temminckii in southern African waters was previously

in estuarine zooplanktonic resources (Froneman and Vorwerk, 2013), with dire consequences for an endangered species such as S. watermeyeri. The main aim of this paper is to examine the life-history styles and population dynamics of S. temminckii and S. watermeyeri in the East Kleinemonde and Kariega estuaries and to equate these findings with the current conservation status of the two species. We use modern molecular techniques in our analysis of the population genetics of these syngnathids (Mobley et al., 2011) and hypothesise that the long-term effective population sizes of these two species should reflect their present IUCN categorization. This would imply that S. watermeyeri, which is of major conservation concern, already had a smaller population size under natural conditions that made it particularly vulnerable to contemporary habitat disturbances. Finally, we attempt to link the lifehistory cycles of the two pipefish species to their current status in the wild, with S. temminckii showing all the attributes of a successful coastal species, whereas the closely related S. watermeyeri is at considerable risk of extinction (Whitfield and Bruton, 1996). 1.1. Study species Syngnathus temminckii and S. watermeyeri are two southern African representatives of the pipefish genus Syngnathus. The former species is the more abundant and widely distributed of the two, occurring in coastal waters (including estuaries) from cool-temperate Walvis Bay in Namibia to subtropical waters along the east coast of South Africa (Heemstra and Heemstra, 2004). In contrast, the latter species is rare and restricted to a few estuaries on the subcontinent between the

Fig. 1. Published sites of occurrence for Syngnathus temminckii (•) and S. watermeyeri (*) in southern Africa (map modified from Mwale et al., 2014). The boundaries of the three estuarine biogeographic zones along the South African coast are also shown.

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(December, January or February) and winter (June, July or August) from December 1994 to July 2014 (sampling methods are described in detail by James et al., 2008). The estuary was divided longitudinally into lower, middle and upper reaches, with up to 18 sites sampled on each occasion. A small mesh (5 mm bar) seine net (30 m × 2 m) was used for small estuarine spawning species including pipefish. Captured pipefish were identified, measured (mm standard length) and released alive at the sampling site. As a follow-on from littoral fish sampling in spring undertaken by Ter Morshuizen and Whitfield (1994) and Vorwerk et al. (2007), an intensive survey of the ichthyofauna in the littoral zone of the Kariega Estuary was undertaken annually in spring (October or November) from November 2012 to November 2015. As was the case in the previous two studies, 60 sites were sampled from the upper to the lower reaches on each occasion using a fine mesh 5 m seine net. Syngnathus species caught were identified to species level, measured (mm standard length) and then released back into the estuary. At each site the development of Z. capensis was recorded as absent, sparse, medium or dense. An estimate of the maximum contemporary population of S. watermeyeri was calculated on the basis of a population estimate for this species in the East Kleinemonde Estuary under optimum aquatic macrophyte conditions. According to Cowley and Whitfield (2002) there were 3584 S. watermeyeri present in 14.5 ha of macrophytes during 1995. Assuming similar maximum S. watermeyeri densities in the other estuaries where this species has been recorded, and a maximum total macrophyte area in all these systems of 77.4 ha, we therefore an estimate a maximum estuarine pipefish population of 19,131 individuals.

misidentified as Syngnathus acus Linnaeus, 1758, but a detailed morphological and genetic study of the species (Mwale et al., 2013) confirmed that it was in fact S. temminckii. The snout of this species is a distinguishing feature when compared to S. watermeyeri, with the former having a snout longer than the rest of the head whereas the latter species has a snout one third of its corresponding head length (Heemstra and Heemstra, 2004). Syngnathus watermeyeri reaches a maximum recorded length of approximately 15 cm SL whereas S. temminckii can attain 30 cm SL (Mwale et al., 2014). However, the larger adult cohorts of S. temminckii appear to be located in the marine environment and not in estuaries (Dawson, 1986). Spring and early summer reproductive activity in S. watermeyeri commences at approximately 10–12 cm SL whilst that of S. temminckii commences at approximately 12–13 cm SL (Mwale et al., 2014). All male and female S. watermeyeri are mature by 13 cm SL and all S. temminckii by 16 cm SL. Females of both species are reproductively active before the males in terms of size, and the sex ratio of both species is biased in favour of females (Mwale et al., 2014). Breeding of S. watermeyeri appears to be limited to estuaries but S. temminckii can breed in both the marine and estuarine environments. 2. Methods 2.1. Main study sites The 4 km long East Kleinemonde Estuary (Fig. 1) is an intermittently open system that closes off to the sea for much of the year due to a sand bar that develops at the mouth. This estuary usually opens following heavy rains and river flooding that breaches the sand bar at the mouth (Whitfield et al., 2008). The dominant submerged macrophyte in the East Kleinemonde in the 1990s was R. cirrhosa, which occurred in a continuous band of varying width along both banks above the road bridge (Fig. 2). A major flash flood in May 2003, and the subsequent prolonged exposure of R. cirrhosa and Potamogeton pectinatus plants resulted in an almost complete loss of most of these macrophyte beds (Riddin and Adams, 2012). Recovery of the aquatic macrophytes from seed banks was slow, with R. cirrhosa only found in small patches along both banks by the end of 2008 (Riddin and Adams, 2012). By 2010 the macrophyte beds had expanded but never attained their pre-May 2003 abundance. The 18 km long Kariega Estuary (Fig. 1) is a permanently open system that is dependent on the tidal prism to prevent its mouth from closing. In recent decades the estuary has received a reduced freshwater input due to increased freshwater demand from a relatively small catchment. Average monthly flow in the Kariega Estuary is usually negligible ranging from zero flow to < 1 m3 s− 1 in most months, resulting in high salinities throughout the system and often hypersaline conditions in the upper reaches (Grange et al., 2000). River flooding can occur during heavy rainfall events when catchment dams are full and monthly flows into the estuary are > 27 m3 s− 1 (P. Nodo pers. comm.). Water temperatures in the Kariega Estuary do not deviate significantly from the natural state since considerable tidal exchange of marine water in the lower and middle reaches prevents adverse warm conditions from developing in these regions (Allanson and Read, 1995). Zostera capensis, occurs mostly as a littoral band in the intertidal zone, mainly in the lower and middle reaches of the Kariega system but extending into the upper reaches during prolonged droughts (Hodgson, 1987). The width of the Z. capensis band is variable, usually between 1 and 5 m but can exceed 20 m in the lower reaches where large intertidal areas are present. The density of Z. capensis increases during spring and summer and is lowest during winter.

2.3. Genetic analyses Genetic data from the mitochondrial genome were generated for both pipefish species collected from three and four different estuaries for S. watermeyeri and S. temminckii, respectively (Table S1). Pipefish samples used for genetic analyses included either freshly acquired samples or previously extracted DNA (Mwale et al., 2013). A total of 38 samples, 15 from S. watermeyeri and 23 from S. temminckii, were of sufficiently good quality for genetic analyses (Table S1). Genomic DNA was extracted using the cetyltrimethyl ammonium bromide (CTAB) procedure (Doyle and Doyle, 1987, 1990), and DNA sequence data from two partial mitochondrial DNA (mtDNA) fragments, cytochrome b gene (cytb) and control region (CR) were sequenced. The two species were compared on the basis of genetic diversity indices (haplotype diversity, nucleotide diversity and allelic richness), and haplotype networks were constructed to assess genealogical relationships among sequences. In addition, historical effective female population sizes during the past 10,000 years (i.e., the present interglacial period) were inferred to determine whether small contemporary population sizes based on survey results are a recent, potentially anthropogenic, development. Details of laboratory procedures and data analyses are provided in the Supplementary Materials section. 3. Results 3.1. East Kleinemonde estuary Synganthus watermeyeri was recorded consistently in catches between 1996 and 2001, when macrophyte beds were present in a mostly continuous band along the length of the estuary (Fig. 2). No individuals were captured in 2002 and only one individual was sampled in June 2003 (Fig. 3) after loss of the macrophyte beds from the system. The highest catch was recorded in 1998, with 26 individuals sampled in the estuary, and coincided with the development of dense R. cirrhosa beds such that seine netting at these sites became very difficult. The length of S. watermeyeri caught between 1996 and 2003 ranged from 58 mm to 147 mm, with mean length ranging from 91 mm in 2001 (single individual caught) to 129 mm ( ± 3.2 S.E.) in 2000 (Fig. 3). The

2.2. Pipefish population dynamics As part of a long-term monitoring project, the ichthyofauna of the East Kleinemonde Estuary was sampled biannually in summer 258

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Fig. 2. Bubble plots showing the occurrence and abundance of Syngnathus watermeyeri and associated Ruppia cirrhosa habitat in the East Kleinemonde Estuary from 1996 to 2001.

to 253 mm, with the mean length ranging from 116 mm ( ± 30.0 mm S.E.) in 2012, with catches dominated by juveniles to 168 mm ( ± 9.0 mm S.E.) in 2013, with only mature individuals caught (Fig. 3). The highest catch was recorded in 2014 (n = 54), with both juvenile and mature individuals present (mean length ± S.E.; 117 ± 4.4 mm). Although S. temminckii catches were highest in the middle and lower reaches of the estuary, where Z. capensis beds were most extensive, some pipefish were also captured at sites where Z. capensis was absent or sparse (Fig. 4).

highest catches were recorded in the middle reaches of the estuary, with S. watermeyeri only sampled at sites where submerged macrophytes were present (Fig. 2). Since June 2003 no S. watermeyeri were captured in the estuary but one mature (132 mm) S. temminckii was recorded in February 2006 in the lower reaches (Fig. 3).

3.2. Kariega estuary During the current sampling period an episodic flood event was recorded in October 2012 (436.6 m3 s− 1) and a smaller flood in October 2013 (30.6 m3 s− 1). The 2012 flood was the largest recorded in the system for > 50 years and resulted in the loss of most of the Z. capensis leaf material from the estuary. Recovery of the eelgrass beds from root stock in the post flood period was rapid, with some stands of Z. capensis present by November 2014 (Fig. 4). Only one immature (106 mm) S. watermeyeri individual was caught in the post flood period (November 2013) in the Kariega Estuary. Syngnathus temminckii were recorded every year from November 2012 to November 2015. The length of specimens caught ranged from 58 mm

3.3. Genetic analyses A total of 11 haplotypes (Syngnathus temminckii = 9; S. watermeyeri = 2) were recovered for the cytb gene (Table S3) and 11 (S. temminckii = 9; S. watermeyeri = 2) for CR (Table S4). When the two fragments from the same species were concatenated, a total of 12 haplotypes for S. temminckii and three for S. watermeyeri were recovered (Table S5). All unique haplotypes of each mitochondrial marker were deposited in GenBank (Table S3 and S4). Genetic diversity indices of S. 259

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S. watermeyeri from many Eastern Cape estuaries with similar habitat and physico-chemical conditions to the Kariega and East Kleinemonde systems (e.g. Beckley, 1983, 1984; Vorwerk et al., 2001), suggests that S. watermeyeri has a narrow range and is less environmentally tolerant than S. temminckii. Similarly, S. watermeyeri has never been recorded in the high wave action marine environment off the Eastern Cape coast, despite numerous studies in various littoral marine habitats having yielded catches of only S. temmminckii from this environment (Whitfield and Pattrick, 2015). There is a possibility that S. watermeyeri is more dependent on aquatic macrophyte beds than S. temminckii, a species that is also known to occur in relatively large numbers in estuaries without submerged plants, e.g. the Great Fish Estuary (Mwale et al., 2014). Longterm monitoring of fish populations in the East Kleinemonde Estuary showed that a breeding population of S. watermeyeri was present during a macrophyte-dominated phases between 1998 and 2003, but absent from catches during a macrophyte-senescent period (post major flood event) from 2004 to 2009. Recovery of the macrophytes in this estuary did not lead to re-colonization of the system, suggesting that S. watermeyeri were not present in the estuary during the macrophyte-senescent phase. In addition, S. watermeyeri were only recorded at East Kleinemonde sites where submerged macrophytes were present. Similarly, in the Kariega Estuary Vorwerk et al. (2007) found that peaks in the abundance of S. watermeyeri were recorded at stations with dense stands of macrophytes. Sampling during this study in the Kariega Estuary during a post-episodic flood event in 2012 resulted in the capture of only a single individual of S. watermeyeri. In contrast, S. temminckii was recorded throughout the study period at sites where submerged macrophytes were abundant, as well as some sites where macrophytes were sparse or absent. The dispersal capabilities of the larger S. temminckii are considerably greater than that of S. watermeyeri. Not only does the former species have a much wider geographic distribution, it is also able to breed and disperse via both the estuarine and marine environments. In contrast, the smaller S. watermeyeri does not appear to breed within the marine environment but it may sporadically disperse from one suitable estuary to another following river flood events in the Eastern Cape Province. The apparent absence of S. watermeyeri from the Kowie Estuary, which is in the middle of its distributional range (Fig. 1) is puzzling, especially as the larvae, juveniles and adults of S. temmminckii are well represented in the same system (Whitfield et al., 1994). Fecundity is related to fish size, with large S. temminckii males carrying up to 486 developing eggs/embryos (Mwale et al., 2014), compared to a maximum of only 44 recorded for the much smaller S. watermeyeri males (Whitfield, 1995). There is a significantly positive relationship between the size of S. temminckii and the number of oocytes in the female and eggs/embryos carried by the male (Mwale et al., 2014), thus implying that reproductive efficiency is higher in this species when compared to the smaller S. watermeyeri. In the absence of any evidence to suggest that survivorship of S. watermeyeri larvae is higher than that of S. temminckii larvae, we must surmise that the latter species is at a considerable advantage in terms of populating new or existing habitats. Another possibility for the differential success of the two syngnathids is that there is competition for food between adults of S. temminckii and S. watermeyeri, especially under conditions of little or no river flow which result in poor zooplankton stocks (Grange et al., 2000). Syngnathids feed on small invertebrates associated with submerged plant beds or in the water column (Garcia et al., 2005), so there is a distinct possibility that direct dietary competition exists, but this assumption needs to be tested. Given the larger mouth size and longer snout of S. temminckii when compared to S. watermeyeri, it is probable that adults of the former species can suck a wider range of small invertebrates into its buccal cavity than the latter (Bergert and Wainwright, 1997). Thus, in a highly competitive situation, S. watermeyeri would be less effective at securing prey than S. temminckii, which

Fig. 3. The mean length (mm SL ± S.E.) of Syngnathus watermeyeri and Syngnathus temminckii caught in a) the East Kleinemonde Estuary between 1995 and 2014 and b) the Kariega Estuary between 2012 and 2015. Numbers of individuals caught are shown per year.

watermeyeri were low compared to those of S. temminckii (Table S5). This is also evident on the basis of the haplotype networks, which indicate that the haplotypes of S. watermeyeri are separated by comparatively few mutational steps (Fig. 5). Allelic richness (Ar) was always higher for S. temminckii (Table S5). Trends in effective female population sizes estimated using BSP indicate that both species had relatively stable population sizes since the onset of the present interglacial period (~ 10,000 years ago), with that of S. temminckii being on average ~5 times as high as that of S. watermeyeri (Fig. 6). This result was confirmed by the LAMARC analyses, which identified a significantly larger long-term population size for S. temminckii (~ 10 times higher than that for S. watermeyeri), with 95% confidence intervals that did not overlap for the two species, thus indicating that these differences are significant. The 95% credibility intervals for the growth parameter g encompass zero (Table S6), which suggests long-term stability in population size for both species. It is possible that individual populations of both species experienced genetic bottlenecks that reduced their genetic diversity, but it seems unlikely that these occurred range-wide. The inclusion of individuals from different populations may mitigate localised demographic stochasticity on overall genetic diversity to some extent (Grant, 2015), and particularly when populations recovered quickly after a bottleneck, such subtle demographic changes would not be detectable using mitochondrial DNA sequences (Mourier et al., 2012).

4. Discussion The distribution and abundance of S. temminckii and S. watermeyeri in Eastern Cape estuaries provides strong evidence that the former species is more widespread and successful than the latter. Clearly the broad biogeographical distribution of S. temminckii, together with its occurrence in both estuaries and the coastal marine environment (Mwale et al., 2014), implies that this fish is physiologically tolerant of a wide range of physico-chemical conditions. Direct and indirect evidence suggests that this species can tolerate a range of salinities from 8 to at least 35 (Whitfield et al., 1981) and water temperatures from approximately 9 to 30 °C (Russell, 1994). Conversely, the limited geographical distribution and absence of the 260

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Fig. 4. Bubble plots showing the occurrence and abundance of Syngnathus temminckii and associated Zostera capensis habitat in the Kariega Estuary. A single specimen of Syngnathus watermeyeri was captured in 2013, the locality of which is shown on the map by a star.

might then be forced to find an alternative habitat or perish. Both pipefish species breed mainly in spring and early summer (September – November) (Mwale et al., 2014), which enables the juveniles to benefit from increased planktonic food resources during the productive summer months (Jerling and Wooldridge, 1991). Competition for zooplanktonic prey by juveniles occupying the same habitat is likely, but studies have not been conducted on this aspect. The lower recorded densities of both fish species during the winter months (Mwale et al., 2014), when food resources are likely to be more limiting (Jerling and Wooldridge, 1991), may be real or an artefact of reduced sampling frequencies during this time of the year. It is perhaps also significant that S. temminckii has been reported to also breed during the remainder of the year, although not as frequently as in summer, whereas that for S. watermeyeri is restricted to the spring and summer months (Mwale et al., 2014). Syngnathus watermeyeri can occur in both permanently open and temporarily open/closed estuaries, sometimes in moderate numbers. In

contrast, S. temminckii is most abundant in permanently open estuaries and, when present in small temporarily open/closed estuaries, is usually recorded as isolated individuals (Harrison and Whitfield, 2006). Indeed, during this study only a single S. temminckii was recorded in the East Kleinemonde Estuary over more than a decade. In contrast, for a period of five years prior to loss of the submerged macrophyte beds, S. watermeyeri was regularly recorded in this system. The reason why S. watermeyeri does not occupy all estuaries within its limited distributional range that have submerged macrophyte beds (Vorwerk et al., 2001) is unknown. The consistent abundance of S. watermeyeri at selected submerged macrophyte sites in the East Kleinemonde Estuary over many years suggests that this species is well adapted to both the oligohaline and mesohaline conditions that prevailed at different times within this system (Whitfield et al., 2008). The healthy population between 1998 and 2003, with both juvenile and mature individuals present, suggests that breeding and the rearing of the juveniles was also successful during the predominantly closed phase 261

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macrophyte beds had recovered and the S. temminckii population then consisted of both juveniles and mature individuals, many of which may have recruited from the adjacent marine environment. In contrast, although S. watermeyeri may recolonise estuaries via the sea, there is no evidence to suggest that there is a constant marine population of this species ready to colonise vacant estuaries within its distributional range. Hence, when this fish species becomes locally extinct (Whitfield and Bruton, 1996), there is less probability for marine populations to readily replenish an estuary, even though conditions are once again favourable for settlement. Recolonization of estuaries that have lost their S. watermeyeri population are dependent on river flood events that wash individuals into the sea from nearby systems, a process that is not guaranteed to be successful given the vagaries of coastal ocean currents and the poor swimming ability of syngnathids. The recolonization of the Kariega Estuary by S. watermeyeri following river flooding in 2006 (Vorwerk et al., 2007) was almost certainly facilitated by individuals that had been washed out of nearby estuarine systems such as the East and West Kleinemonde. The re-establishment of healthy zooplankton stocks in the Kariega Estuary following this river pulse event (Froneman and Vorwerk, 2013) was probably a major reason why S. watermeyeri was recorded in subsequent years within this system. Their depletion and eventual loss from the Kariega Estuary followed a major episodic river flood in 2012, an event that destroyed almost all the submerged macrophytes within this system. The loss of the eelgrass beds also had an adverse impact on S. temminckii but, in contrast to S. watermeyeri, the populations of S. temminckii could be easily replenished from the sea once the submerged macrophyte habitat in the estuary became re-established. Both pipefish are endemic to southern Africa and belong to a common clade within the genus Syngnathus (Mwale et al., 2013). However, whilst S. temminckii is a widespread southern African endemic in the mould of the bay pipefish Syngnathus leptorhynchus from northwest America (Wilson, 2006), S. watermeyeri is a much more restricted endemic. It is perhaps significant that the estuarine pipefish occurs in a region where endemic fish species are most common (Turpie et al., 2000) and this may be attributed to the localised current and weather patterns that create the isolation needed for speciation in this area (Teske et al., 2005). In terms of life-history strategies, Mwale et al. (2014) suggested that S. watermeyeri and S. temminckii have evolved a reproductive strategy that falls within the r-range of the r–K continuum. They highlighted that both species exhibit some K-selection traits, such as being bearers, allocating time and resources to ensure the survival of embryos during gestation, and producing young that are fully developed and independent. However, both species provide no parental care after birth, which contrasts to most K-selected organisms but is consistent with many fish species that show diverse attributes within the r–K

B

A

Fig. 5. Statistical parsimony networks showing the evolutionary relationships of the haplotypes of Syngnathus temminckii (A) and S. watermeyeri (B). Circles represent haplotypes, and their sizes are proportional to the number of sequences collapsing into that haplotype. Colour represents sampling localities, except for black, which represents unsampled or extinct interior node haplotypes. Vertical cross bars represent mutational steps. Both networks were reconstructed using concatenated mitochondrial DNA cytochrome b and control region data.

Fig. 6. Bayesian Skyline plots showing trends in effective female population size (Nef, median estimate) of the two endemic southern African species of Syngnathus from the onset of the present interglacial period (~ 10,000 years ago) to the present. Estimates were based on a combined dataset of mitochondrial DNA cytochrome b and control region fragments.

of this estuary. In the Kariega Estuary, no S. temminckii juveniles were recorded in 2013 following major flood events in 2012 and 2013. By 2014, the

Table 1 Selected life-history traits that, on balance, render an advantage to either the longsnout pipefish Syngnathus temmincki or the estuarine pipefish S. watermeyeri. Traits where both species have similar attributes are not included in this summary. Life-history trait

Syngnathus temmincki

Syngnathus watermeyeri

Advantage

Biogeographical distribution Ecosystem occupation Estuary type occupation Habitat occupation

Cool temperate, warm temperate and subtropical Estuaries and coasts Mainly large permanently open estuaries Submerged macrophyte beds, turbid estuarine littoral and marine reefs Estuarine and marine spawning High fecundity and large brood size Mainly spring and summer but breeding can occur in any month Tolerance of a wide range of both salinities and water temperatures More mobile due to large adult size High

Warm temperate Estuaries Permanently open and temporarily open/closed estuaries Submerged macrophyte beds

S. S. S. S.

Estuarine spawning Low fecundity and small brood size Spring and summer

S. temmincki S. temmincki S. temmincki

Tolerance of a wide range of salinities but a moderate range of water temperatures Less mobile due to small adult size Low

S. temmincki

Reproductive strategy Reproductive output Reproductive timing Physiological tolerances Mobility and connectivity Genetic diversity

262

temmincki temmincki watermeyeri temmincki

S. temmincki S. temmincki

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References

continuum (Pianka, 1970). Table 1 summarizes some of the major reasons why S. temminckii has been so successful, whilst S. watermeyeri is possibly threatened with extinction. The extent to which human-induced change of the coastal environment in the Eastern Cape Province has exacerbated this trajectory towards extinction is not fully understood. However, given the relative disadvantages of some of the life-history traits of S. watermeyeri (Table 1), it is likely that these differential trajectories were already in place since the evolution of both species. This idea is supported by fact that the demographic trends reconstructed using genetic data indicate a long-term effective female population size of S. watermeyeri which has been relatively stable over the past 10,000 years, and which has remained much lower than that of S. temminckii. This indicates that S. watermeyeri was already at a greater risk of extinction prior to any human impacts. Current census data for S. watermeyeri suggests a maximum total population size of approximately ~10% of the species' long-term effective population size of about 190,000. In reality, considerable loss of submerged macrophyte areas has occurred in most of these systems over the past two decades (e.g. Riddin and Adams, 2012) which would have resulted in a much smaller actual S. watermeyeri population than under the optimal conditions used to determine the maximum abundance of this species per estuary. It is thus likely that excessive river water abstraction and other anthropogenetic effects have merely exacerbated an existing historical trend of increasing vulnerability to natural and, more recently, human driven environmental change in these estuaries.

Allanson, B.R., Read, G.H.L., 1995. Further comment on the response of Eastern Cape Province estuaries to variable freshwater flows. S. Afr. J. Aq. Sci. 21, 56–70. Balon, E.K., 1975. Reproductive guilds of fishes: a proposal and definition. J. Fish. Res. Bd. Can. 32, 821–864. Beckley, L.E., 1983. The ichthyofauna associated with Zostera capensis Setchell in the Swartkops estuary, South Africa. S. Afr. J. Zool. 18, 15–24. Beckley, L.E., 1984. The ichthyofauna of the Sundays estuary, South Africa, with particular reference to the juvenile marine component. Estuaries 7, 248–258. Bergert, B.A., Wainwright, P.C., 1997. Morphology and kinetics of prey capture in the syngnathid fishes Hippocampus erectus and Syngnathus floridae. Mar. Biol. 127, 563–570. Bruton, M.N., 1989. The ecological significance of alternative life-history styles. In: Bruton, M.N. (Ed.), Alternative Life-History Styles of Animals. Kluwer Academic Publishers, Dordrecht, pp. 503–553. Cowley, P.D., Whitfield, A.K., 2002. Biomass and production estimates of a fish community in a small South African estuary. J. Fish Biol. 61 (Suppl. A), 74–89. Dawson, C.E., 1986. Family no. 145: syngnathidae. In: Smith, M.M., Heemstra, P.C. (Eds.), Smiths' Sea Fishes. Macmillan, Johannesburg, pp. 445–458. Doyle, J.J., Doyle, J.L., 1987. A rapid DNA isolation procedure for small quantities of fresh leaf tissue. Phytochem. Bull. 19, 11–15. Doyle, J.J., Doyle, J.L., 1990. Isolation of plant DNA from fresh tissue. Focus 12, 13–15. Froneman, P.W., Vorwerk, P.D., 2013. Response of the plankton to a freshwater pulse in a freshwater deprived, permanently open South African estuary. J. Water Resource Prot. 5, 405–413. Garcia, A.M., Geraldi, R.M., Vieira, J.P., 2005. Diet composition and feeding strategy of the southern pipefish Syngnathus folletti in a Widgeon grass bed of the Patos Lagoon Estuary, RS, Brazil. Neotrop. Ichthyol. 3, 427–432. Grange, N., Whitfield, A.K., de Villiers, C.J., Allanson, B.R., 2000. The response of two South African east coast estuaries to altered river flow regimes. Aquat. Conserv. 10, 155–177. Grant, W.S., 2015. Problems and cautions with sequence mismatch analysis and Bayesian skyline plots to infer historical demography. J. Hered. 106, 333–346. Harrison, T.D., Whitfield, A.K., 2006. Estuarine typology and the structuring of fish communities in South Africa. Environ. Biol. Fish 75, 269–293. Heemstra, P.C., Heemstra, E. (Eds.), 2004. Coastal Fishes of Southern Africa. National Inquiry Services Centre and South African Institute for Aquatic Biodiversity, Grahamstown. Hodgson, A.N., 1987. Distribution and abundance of the macrobenthic fauna of the Kariega estuary. S. Afr. J. Zool. 22, 153–162. James, N.C., Whitfield, A.K., Cowley, P.D., 2008. Long-term stability of the fish assemblages in a warm-temperate South African estuary. Estuar. Coast. Shelf Sci. 76, 723–738. Jerling, H.L., Wooldridge, T.H., 1991. Population dynamics and estimates of production for the calanoid copepod Pseudodiaptomus hessei in a warm temperate estuary. Estuar. Coast. Shelf Sci. 33, 121–135. Mobley, K.B., Small, C.M., Jones, A.G., 2011. The genetics and genomics of Syngnathidae: pipefishes, seahorses and seadragons. J. Fish Biol. 78, 1624–1646. Mourier, T., Ho, S.Y.W., Gilbert, M.T.P., Willerslev, E., Orlando, L., 2012. Statistical guidelines for detecting past population shifts using ancient DNA. Mol. Biol. Evol. 29, 2241–2251. Mwale, M., Kaiser, H., Barker, N.P., Wilson, A.B., Teske, P.R., 2013. Identification of a uniquely southern African clade of coastal pipefishes, Syngnathus spp. J. Fish Biol. 82, 2045–2062. Mwale, M., Kaiser, H., Heemstra, P.C., 2014. Reproductive biology and distribution of Syngnathus temminckii and S. watermeyeri (Pisces: Syngnathidae) in southern Africa. Afr. J. Mar. Sci. 36, 175–184. Pianka, E.R., 1970. On r- and K-selection. Am. Nat. 104, 592–597. Riddin, T., Adams, J.B., 2012. Predicting macrophyte states in a small temporarily open/ closed estuary. Mar. Freshwat. Res. 63, 616–623. Russell, I.A., 1994. Mass mortality of marine and estuarine fish in the Swartvlei and Wilderness lake systems, southern Cape. S. Afr. J. Aq. Sci. 20, 93–96. Schlacher, T.A., Wooldridge, T.H., 1996. Ecological responses to reductions in freshwater supply and quality in South Africa's estuaries: lessons for management and conservation. J. Coast. Conserv. 2, 115–130. Ter Morshuizen, L.D., Whitfield, A.K., 1994. The distribution of littoral fish associated with eelgrass Zostera capensis in the Kariega estuary, a southern African system with a reversed salinity gradient. S. Afr. J. Mar. Sci. 14, 95–105. Teske, P.R., Hamilton, H., Palsboll, P.J., Choo, C.K., Gabr, H., Lourie, S.A., Santos, M., Sreepada, A., Cherry, M.I., Matthee, C.A., 2005. Molecular evidence for long-distance colonization in an Indo-Pacific seahorse lineage. Mar. Ecol. Prog. Ser. 286, 249–260. Turpie, J.K., Beckley, L.E., Katua, S.M., 2000. Biogeography and the selection of priority areas for conservation of South African coastal fishes. Biol. Conserv. 92, 59–72. Vorwerk, P.D., Whitfield, A.K., Cowley, P.D., Paterson, A.W., 2001. A survey of selected Eastern Cape estuaries with particular reference to the ichthyofauna. Ichthyol. Bull. J.L.B. Smith Inst. Ichthyol. 72, 1–52. Vorwerk, P.D., Froneman, P.W., Paterson, A.W., 2007. Recovery of the critically endangered river pipefish, Syngnathus watermeyeri, in the Kariega Estuary, Eastern Cape Province. S. Afr. J. Sci. 103, 199–201. Whitfield, A.K., 1990. Life-history styles of fishes in South African estuaries. Environ. Biol. Fish 28, 295–308. Whitfield, A.K., 1995. Threatened fishes of the world: Syngnathus watermeyeri Smith, 1963 (Syngnathidae). Environ. Biol. Fish 43, 152. Whitfield, A.K., Bruton, M.N., 1996. Extinction of the river pipefish Syngnathus

5. Conservation recommendations Evidence suggests that S. watermeyeri is vulnerable to localised extinction when adverse environmental perturbations occur in a particular estuary (Whitfield and Bruton, 1996). In addition, it is likely that river pulse events are becoming less frequent due to increased river impoundment and freshwater abstraction for both urban and agricultural users in the Eastern Cape Province (Schlacher and Wooldridge, 1996). If the above pipefish attributes and river flow scenarios are true, this means that S. watermeyeri populations are unlikely to naturally repopulate all systems where they previously occurred due to a reduction in water volume entering the marine environment. Under such circumstances it can be argued that translocations of individuals into previously occupied estuaries is an acceptable conservation strategy, particularly as the current known population of S. watermeyeri is limited to the Bushmans Estuary and no captive individuals of the species exist. There is also strong evidence to suggest that excessive freshwater abstraction has a negative influence on the zooplanktonic food resources of S. watermeyeri (Wooldridge, 2007). Therefore dams in the Eastern Cape Province need to have a freshwater release policy to ensure the ecological sustainability of downstream estuaries, thereby promoting the long-term conservation of the critically endangered S. watermeyeri. Acknowledgements Financial support was provided by the National Research Foundation of South Africa (Grant No. 85373). The genetic research was partly funded by the Rufford Foundation (Small Grant 14490-1). We are grateful for accommodation provided by Sibuya Game Reserve during the pipefish surveys of the Kariega Estuary. We also thank two anonymous referees whose reports were of great assistance in improving an earlier draft of this manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.biocon.2017.06.024. 263

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Froneman, P.W., Gama, P.T., James, N.C., Mackenzie, B., Riddin, T., Snow, G.C., Strydom, N.A., Taljaard, S., Terörde, A., Theron, A.K., Turpie, J.K., van Niekerk, L., Vorwerk, P.D., Wooldridge, T.H., 2008. A multidisciplinary study of a small, temporarily open/closed South African estuary, with particular emphasis on the influence of mouth state on the ecology of the system. Afr. J. Mar. Sci. 30, 453–473. Wilson, A.B., 2006. Genetic signature of recent glaciation on populations of a near-shore marine fish species (Syngnathus leptorhynchus). Mol. Ecol. 15, 1857–1871. Wooldridge, T.H., 2007. Biotic response to altered freshwater inflow patterns to the Kromme River Estuary, South Africa. Trans. Ecol. Env. 103, 687–696.

watermeyeri in the eastern Cape Province, South Africa. S. Afr. J. Sci. 92, 59–60. Whitfield, A.K., Pattrick, P., 2015. Habitat type and nursery function for coastal marine fish species, with emphasis on the Eastern Cape region, South Africa. Estuar. Coast. Shelf Sci. 160, 49–59. Whitfield, A.K., Blaber, S.J.M., Cyrus, D.P., 1981. Salinity ranges of some southern African fish species occurring in estuaries. S. Afr. J. Zool. 16, 151–155. Whitfield, A.K., Paterson, A.W., Bok, A.H., Kok, H.M., 1994. A comparison of the ichthyofaunas in two permanently open eastern Cape estuaries. S. Afr. J. Zool. 29, 175–185. Whitfield, A.K., Adams, J.B., Bate, G.C., Bezuidenhout, K., Bornman, T.G., Cowley, P.D.,

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