Browsing by Author "Blamey, Laura K"
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- ItemRestrictedAssessing the adequacy of current fisheries management under changing climate: a southern synopsis(Oxford University Press, 2011) Plagányi, Éva E; Weeks, Scala J; Skewes, Tim D; Gibbs, Mark T; Blamey, Laura K; Soares, Muri; Robinson, William M L; Poloczanska, Elvira S; Norman-Lopez, AnnaClimate change is likely to have a significant impact on both target and non-target marine stocks worldwide, with the concomitant need for management strategies capable of sustaining fishing in future. We use several southern hemisphere fisheries to highlight the likely impacts of climate change at a range of levels, from individual to population responses, as well as ecosystem ramifications. Examples span polar (Antarctic krill fishery), temperate (west coast pelagic fishery, abalone and rock lobster), and tropical (Torres Strait rock lobster) commercially important fisheries. Responses of these fisheries to either past observed environmental changes or projected future changes are used to deduce some anticipated implications of climate change for fisheries management, including economic impacts and governance considerations. We evaluate the effectiveness of current single-species assessment models, management strategy evaluation approaches and multispecies assessment models as future management tools to cope with likely climaterelated changes. Non-spatial stock assessment models will have limited ability to separate fishery effects from the impacts of climate change. Anthropogenic climate change is occurring at a time-scale relevant to current fisheries management strategic planning and testing. Adaptive management frameworks (with their feedback loops) are ideal for detecting and adapting to changes in target stocks
- ItemMetadata onlyAssessing the adequacy of current fisheries management under changing climate:a southern synopsis(ICES Journal of Marine Science, 2012) Plagányi, Éva E; Weeks, Scala J; Skewes, Tim D; Gibbs, Mark T; Blamey, Laura K; Soares, Muri; Robinson, William M L; Norman-Lopez, AnnaClimate change is likely to have a significant impact on both target and non-target marine stocks worldwide, with the concomitant need for management strategies capable of sustaining fishing in future. We use several southern hemisphere fisheries to highlight the likely impacts of climate change at a range of levels, from individual to population responses, as well as ecosystem ramifications. Examples span polar (Antarctic krill fishery), temperate (west coast pelagic fishery, abalone and rock lobster), and tropical (Torres Strait rock lobster) commercially important fisheries. Responses of these fisheries to either past observed environmental changes or projected future changes are used to deduce some anticipated implications of climate change for fisheries management, including economic impacts and governance considerations. We evaluate the effectiveness of current single-species assessment models, management strategy evaluation approaches and multispecies assessment models as future management tools to cope with likely climaterelated changes. Non-spatial stock assessment models will have limited ability to separate fishery effects from the impacts of climate change. Anthropogenic climate change is occurring at a time-scale relevant to current fisheries management strategic planning and testing. Adaptive management frameworks (with their feedback loops) are ideal for detecting and adapting to changes in target stocks
- ItemOpen AccessDiet of the Tristan rock lobster Jasus tristani following the 2011 soya spill at Nightingale Island(2017) Jones, Luke; Blamey, Laura K; Branch, George M; De Lecea, Ander MThe spiny lobster Jasus tristani inhabits the Tristan da Cunha Island archipelago in the South Atlantic Ocean 2,400km from the West coast of South Africa. It is commercially exploited across the archipelago and is the main economic source of income for the local government as it accounts for nearly 80% of the local gross domestic product. The commercial rock lobster industry was established in 1949, and the fishery is currently recognised as sustainable, and as a result was awarded a Marine Stewardship Council Certification in 2011. That same year however, the sinking of the OLIVA at Nightingale Island spilt 60,000 tonnes of soya beans (Glycine max), greatly affecting the local benthic environment, and with probable consequences for the local food web, including the diet of J. tristani. It is still unclear whether the soya beans are still on the seafloor at Nightingale Island as there have been no scientific surveys conducted since the spill. Using samples from 2015, I assessed whether the diet of the lobsters from Nightingale Island differs from that of lobsters from the unaffected Tristan and Inaccessible Islands, and whether there is any evidence of soya in the diet of the lobsters from Nightingale Island. In addition, I examined whether diet differed between lobsters of small or large size, and between shallow and deep depths. In total, 540 lobsters were sampled across the three islands, and a combination of gut content and stable isotope (SI) analysis was conducted to assess the dietary components of the lobsters. Gut fullness was significantly less at Nightingale Island suggesting there may be less food available on the reef. Statistical analyses showed that diet differed between all islands, depths and sizes, although this was difficult to ascertain from visual multidimensional scaling plots as diet showed considerable overlap and variability both within and between islands, depths and sizes. The stable isotope analysis showed significant differences in nitrogen levels among the three
- ItemRestrictedEcosystem modelling provides clues to understanding ecological tipping points(Inter Research, 2014) Plagányi, Éva E; Ellis, Nick; Blamey, Laura K; Morello, Elisabetta B; Norman-Lopez, Anna; Robinson, William M L; Sporic, Miriana; Sweatman, HughEcological thresholds, associated with abrupt changes in the state and organisation of ecosystems, challenge both scientists and managers. Adaptive response to such changes, and planning for their occurrence, requires an understanding of the underlying drivers and system responses as well as appropriate monitoring. In addition to field studies, modelling can advance our ability to anticipate or deal with such major ecosystem shifts. Here, we used an existing multispecies model with smooth continuous functions that were modified to include thresholds representing 3 alternative scenarios of predator responses when prey numbers drop below a critical threshold: (I) no threshold-like response; (II) an abrupt decrease in breeding success by 90%, and (III) an abrupt halving of adult survival. Second, we analysed field observations from 3 independent marine case studies (abalone, starfish, penguins) for evidence of abrupt non-linear responses of predators to changes in abundance of principal prey. Third, we compared the model output with empirical results and tested (using both a statistical method and by fitting multispecies models) the 3 alternative response scenarios. With this approach, we found evidence for nonlinear changes in population parameters (such as survival rate) of predators as prey numbers declined below critical thresholds. As an example of the potential for this approach to inform management, we found that abundances of a range of marine predators become more variable as prey numbers decline, which may be a useful indicator that a system is approaching a tipping point.
- ItemOpen AccessThe environmental factors determining temporal distributions of cetaceans in Mossel Bay, South Africa(2017) Levy, David; Blamey, Laura K; Elwen, Simon; James, BridgetA variety of cetacean species make use of the bays along the South African coast for different purposes, including feeding, mating and calving. Sightings of five species of cetaceans were recorded from shore based locations between February 2010 and August 2014 in Mossel Bay, Western Cape. In this study, we aimed to examine the underlying environmental variables: sea surface temperature (°C), chlorophyll a concentration (mg.m-3) and moon brightness (as % of full) that potentially affect presence patterns within the bay, and several temporal scales (diurnal, monthly, seasonal, annual). The study focused on three whale species: the southern right whale (Eubalaena australis), humpback whale (Megaptera novaeangliae) and Bryde's whale (Balaenoptera brydei); and two dolphin species: the Indo- Pacific bottlenose dolphin (Tursiops aduncus), and the Indian Ocean humpback dolphin (Sousa plumbea). Generalized additive models (GAM) were used to model the sighting rate of the common cetacean species in the area, by relating sighting rate to the environmental variables. Cow-calf groups and adults-only groups were modelled separately for humpback and southern right whales. Chlorophyll a concentration is commonly indicative of high trophic productivity, and sea surface temperature is indicative of biophysical processes that influence cetacean distribution, as well as cetacean migration preferences. Change in sea surface temperature (over the study period) was a significant contributing factor to the sighting rate of cow-calf paired groups and adults-only groups of both right whales and humpback whales, underlying their greater sighting rate during the winter and spring months annually; thus, conforming to the seasonal migration from Antarctica for breeding and calving. Chlorophyll a concentration was a significant factor contributing to Bryde's whale, bottlenose and humpback dolphin distribution. These species reside along the South African coast all year round, but are more frequently seen when trophic productivity is high. During 2011, sea surface temperature values were lower and chlorophyll a concentrations greater than average in Mossel Bay, due to the La Niña effect. This was found to positively correlate with the use of Mossel Bay by the cetacean species in the area. This paper highlights the importance of biophysical processes on cetacean distribution at various temporal scales (diurnal, monthly, seasonal and annual), and the information it provides may be used for conservation planning management.
- ItemOpen AccessKelp forests in False Bay: urchins vs. macroalgae in South Africa's south-west coast biogeographical transition zone(2017) Morris, Kathryn; Blamey, Laura KThere is ongoing global concern over unwanted regime shifts in marine systems. Shifts from diverse and productive algal-dominated ecosystems to less productive urchin and coralline-dominated temperate reefs are becoming increasingly common. Kelp forests found along South Africa's south-west coast between Cape Point and Cape Agulhas occur in a region of biogeographical overlap. They are commonly referred to as transition zone kelp forests and are dynamic ecosystems that are particularly susceptible to grazing influence from species such as sea urchins. This study (1) explores the uniformity of these transition zone kelp forests along the western side of False Bay, with a focus on macroalgae and urchins, (2) identifies a threshold in urchin density above which algal abundance declines and (3) seeks to identify relationships between attached and drift algal abundance. Twenty replicate quadrats were sampled in six kelp forests along the western side of False Bay. Within each quadrat, urchins (Parechinus angulosus) and kelps (Ecklonia maxima) were counted, percentage covers of various understorey algal species were recorded and drift algae were collected. Although there was significant variability in algal and urchin cover across the six sites, kelps generally increased from north to south, while urchins did the opposite. Urchins were negatively correlated with algal communities, and a localised threshold of 1.43kg/m² (50 urchins/m²) was identified, above which attached kelp density failed to increase above 10/m² and percentage cover of understorey algae usually remained below 20%. Surprisingly, no relationship was discovered between abundance of drift kelp and attached kelp, understorey algae or urchin density. This result was likely distorted by the naturally turbulent conditions of South African waters. Results highlight the complexity of these cool-water environments. To better understand the role of urchins in this system, experimental research into the feeding behaviour and effect of P. angulosus on kelps and understorey seaweeds in the presence/absence of drift algae is advised.
- ItemOpen AccessModeling a regime shift in a kelp-forest ecosystem caused by a lobster range expansion(University of Miami, Rosenstiel School of Marine and Atmospheric Science, 2013) Blamey, Laura K; Plagányi, Éva E; Branch, George MThe South African West Coast rock lobster, Jasus lalandii (H. Milne-Edwards, 1837), has expanded its range to the southeast, where its abundance has increased radically. The ecological consequences of this “invasion” are likely to be considerable. We employed a minimally realistic model to simulate the “invasion” and to explore interactions of J. lalandii with the sea urchin, Parechinus angulosus (Leske, 1778), and the abalone, Haliotis midae Linnaeus, 1758, juveniles of which shelter beneath this urchin. Model fits to empirical data were good, although species-interaction terms were difficult to estimate. Base-case trajectories indicated: (1) Lobster biomass peaked at about 1000 t in 1994 and was then reduced by fishing to a stable value approximately 50% lower by 2008. (2) Urchins remained close to carrying capacity in “noninvaded” areas but collapsed to local extinction by 1997 in the invaded area. (3) Abalone declined over 2000–2008 in noninvaded areas because of illegal fishing and collapsed to near zero in the “invaded” area because of illegal fishing combined with increased lobster abundance. Sensitivity analyses favored the hypothesis that the invasion was due to adult immigration rather than larval recruitment. Modeled 50-yr projections indicated that urchins will remain locally extinct in the invaded area, even 50 yrs into the future. The abalone collapse in the invaded area would persist >50 yrs, even if lobsters were absent. We argue that the lobster “invasion” triggered an alternative stable state, making a return to pre-invasion conditions unlikely.
- ItemRestrictedWas overfishing of predatory fish responsible for a lobster-induced regime shift in the Benguela?(Elsevier, 2014) Blamey, Laura K; Plagányi, Éva E; Branch, George MThe top-down effects of predators have been demonstrated for terrestrial, freshwater and marine systems and their removal can cause a shift in ecosystem state. In many cases, the depletion of top-predators occurred long before humans began monitoring these systems, but models can elucidate likely ecosystem changes. In this paper we use a multispecies model to demonstrate that the abundance of predators can induce different ecosystem states, emphasising the importance of sustainable harvesting. Our model is founded on empirical data documenting a regime-shift that took place in the kelp-forest ecosystem along the south-west coast of South Africa following an ‘invasion’ by the West Coast rock lobster Jasus lalandii in the early 1990s. Formerly dominated by herbivores and encrusting corallines, the ecosystem became dominated by lobsters and macroalgae. A combination of illegal fishing of abalone Haliotis midae and the lobster-induced extermination of the urchin Parechinus angulosus (which shelters juvenile abalone) depleted abalone stocks severely. In this currently over-fished ecosystem where predators of lobsters are scarce, the regime-shift appears irreversible. Using an intermediate complexity model of lobster, urchin and abalone relationships, we simulated the effects of predatory fish at various population levels on the course of development of the currently lobster-dominated system. Our results indicate that current levels of fish biomass (<10% of the pristine population) would have been too small to significantly affect the lobster population. However, at pristine levels of fish abundance (or even 50% less), the invasion of lobsters would have been controlled by linefish, and urchin and abalone populations would not have collapsed. The effect of the lobster invasion had a greater influence on abalone abundance than illegal fishing, but together they severely depleted the abalone. Ecosystem effects of historical exploitation of top predators are of central concern, and our ecosystem model indicates that depletion of linefish had severe consequences for lobsters, urchins and abalone, illustrating how overfishing of top predators may precipitate regime shifts, with profound socio-economic consequences.