Carcinus maenas in South Africa : status, impacts and management

Mabin, Clova Airth (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.


ENGLISH ABSTRACT: The European shore crab, Carcinus maenas, has established invasive populations on four continents, with its congener, the Mediterranean shore crab, C. aestuarii, invading another. Carcinus maenas, in particular, has caused ecological and economic impacts in several regions which led to targeted management. This crab was first detected in Table Bay harbour, Cape Town, South Africa in 1983, with little research undertaken in this region until the current study. Research conducted two decades ago suggested the presence of interspecific Carcinus hybrids in South Africa and accordingly, unless specified otherwise, the crab will be referred to as Carcinus only from hereon. This thesis firstly conducted a detailed assessment of the current population status in South Africa, followed by the undertaking of an intensive management programme. This intervention offered a rare opportunity to experimentally assess the trophic impacts of this crab in the field and apply genetic techniques to reconstruct its invasion history in the South African context. To establish the current status of the species in South Africa, Chapter 2 assessed the distribution and abundance of Carcinus. This involved the first account of seasonal patterns in abundance and demographics for the crab. Six intertidal sites were visually surveyed during shore walks on a monthly basis for a year, with surveys extended quarterly at one site for another year. An additional four sites outside the known range of the species were surveyed quarterly, resulting in a total of 92 intertidal surveys through time over the ten sites. Subtidal surveys were conducted at three sites along the open coast. Baited traps were used to confirm the presence of the crab in two harbours (Hout Bay and Table Bay). Following this, Table Bay harbour was surveyed monthly for one year while targeting three life stages (postlarvae, juveniles and adults). The crab was only found at one intertidal site and no crabs were found during open coast subtidal surveys. Gravid females were recorded in all months except for February - May. Unexpectedly, no postlarvae were detected, suggesting that settlement collectors are ineffective in the South African context and should not be used for future monitoring. These results highlight a decline in open coast populations and substantial harbour populations. This suggests that management of harbour populations of Carcinus only, would be sufficient to remove the species from South Africa. Due to the ecological impacts of this genus elsewhere, the Carcinus population in Hout Bay was targeted for management, with the aim of assessing the feasibility of removing the crab from the two invaded South African harbours. This first attempt at managing a marine invasive species in Africa is documented in Chapter 3. Prior to management, four potential control methods (baited traps, crab condos, diver collections and sediment dredging) were trialled to establish their efficacy and suitability for use on a large scale in the South African context. Baited traps were selected as the most appropriate and cost-effective method. Management was conducted from June 2014 to June 2015. During this period, approximately 21,800 traps were deployed and a total of 18,450 person hours were spent in the field. This resulted in the removal of 36,244 crabs from the harbour, approximately six times the most recent population estimate. Despite this, extirpation was not achieved. The recovery of the population was then monitored for six months. A Bayesian depletion model was developed to simulate the response of the population under management scenarios of varying effort. This approach suggested that even if control effort had been doubled (i.e. to 8,000 hours per month) there would only have been an 86% probability of extirpation. If this effort were to be applied in both Hout Bay and Table Bay harbours, it would cost an estimated ZAR 60 million. This cost would need to be considered against the threat posed by the invasion and the likelihood of reintroduction. Despite the impacts reported from invasive populations elsewhere in the world, the trophic impacts of Carcinus remain unknown in South Africa, therefore, this was addressed in Chapter 4. The management of Carcinus provided an opportunity to experimentally assess the impact of the crab by comparing community structure in Hout Bay and Table Bay using benthic cores and vertical fouling scrapes collected prior to (2014), during (2015) and after (2016) management. A total of 78 tethering experiments were conducted to identify potential predators of the crab in Hout Bay harbour and a large public aquarium setting (as a proxy for natural habitats in the absence of Carcinus in such systems), resulting in 128 hours of underwater camera footage. Benthic communities were similar between harbours prior to management, but contrary to expectations, the community remained similar through time in the managed harbour, while changing in the control harbour. In contrast, fouling communities differed between harbours at the onset and changed at both locations through time. Thus, the crab appears to have no detectable impact on benthic communities while its impact on fouling communities remains uncertain. While, no tethered crabs were attacked in Hout Bay, under aquarium conditions all crabs were preyed upon by native fish, suggesting that predator-driven biotic resistance might constrain the spread of Carcinus in natural habitats. Thus, despite notable impacts elsewhere, impacts of Carcinus were not detected in this study, highlighting that alien species impacts can vary among regions and context-dependent risk analyses are needed to inform evidence-based management. In order to elucidate the historical and contemporary gene flow between selected international populations and South Africa, microsatellite DNA markers were used to assess the genetic diversity and structure of Carcinus populations across the native and invasive ranges in Chapter 5. To determine if the management action in Hout Bay had a detectable genetic impact on the demographics of the crab, Hout Bay and Table Bay were sampled prior to, during and after management. DNA was extracted from 898 samples, collected from eight international and the two South African populations (across three years) and amplified across eight microsatellite loci. Clustering analyses and approximate Bayesian computations (ABC) were performed to reconstruct the invasion history of the South African populations. South Africa had significantly higher genetic diversity than the other alien Carcinus populations, suggesting a lack of a founder effect. In the most supported ABC invasion scenario, the South African populations established following admixture between native and invasive C. maenas populations approximately 150 years ago. Evidence of interspecific hybridization between C. maenas and C. aestuarii was detected, with the presence of several hybrid genotypes (5% of all individuals sampled). The harbours had a similar genetic diversity and no structure, suggesting migration between the populations or the same founding populations. There was no effect of management on genetic diversity, its structure, or effective population size of Hout Bay. Migration from Table Bay is, however, unlikely to be the dominant explanation of such observations. Rather, the most likely explanations are that a cohort of crabs produced prior to management was detected during sampling and reflected the gene frequencies of the previous generation or management interventions did not reduce crab populations below levels required to observe changes in genetic diversity. Given the probability of migration between the South African populations and the potential for re-introduction from international populations, the probability of successful management is considered low. This thesis utilised approaches from several different disciplines, including ecology, population genetics and environmental management, to better understand Carcinus invasions in South Africa and to undertake the first management of a marine alien species on the continent. Despite the recent listing in national legislation that mandates the management of this crab, the results of this project suggest that (1) there is a lack of evidence of trophic impacts, (2) migration occurs between the harbours, (3) there is evidence for a complex introduction history to South Africa and appreciable levels hybridization between different crab species, and (4) there are administrative challenges to be addressed prior to management, including insufficient financing. It is thus recommended that a national eradication programme not currently be implemented. Importantly, while this project has provided responses to locally relevant questions, the scientific and administrative insights gained have broad applicability within the field of invasive alien species management. The scientific insights include the value of trialling methods used elsewhere, as well as the use of depletion models to estimate the effort required to increase the probability of management success. Often managers of invasive species rely on dated population estimates and impacts assessed elsewhere to inform management decisions. However, based on the outcomes of this study, management undertaken prior to the assessment of such factors can be unsuccessful and result in a sub-optimal use of limited resources. Overall, this work highlights the importance of context-specific field studies and molecular approaches to establish the need for and determine the feasibility of management.

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