Browsing by Subject "rock lobster"

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    Effect of the 2011 oil and soya spill events on rock lobster yields at Inaccessible and Nightingale islands
    (2011) Johnston, Susan J; Butterworth, Doug S
    Assessing the effects of the oil and soya spills resulting from the OLIVA incident at Nightingale on the lobster fisheries at the Tristan group of islands is difficult for a number of reasons: 1. the quantification of the impact of the oil spill on settlement and juvenile mortality is not straightforward, both at Nightingale and at Inaccessible islands to which the oil spread; 2. the soya spill, which is restricted to Nightingale island, has certainly had an impact there as evidenced by the poor catch rates experienced for recent experimental catches; however this would have been caused by either or both of a short term migration of the lobsters from the fishing area1 and an immediate additional mortality on adult lobsters; the quantification of these effects is again difficult, and importantly the consequences of the two possibilities are rather different; 3. there are two alternative models for the growth rate of lobsters at Nightingale island, and results do change appreciably depending upon which of these better approximates reality.
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    Further results for the Sex- and Area-specific Age-Structured Production Model for the South Coast rock lobster resource
    (2008) Johnston, Susan J; Butterworth, Doug S
    This document provides an update on the development of sex- and area-specific operating models for testing candidate OMPs for the South Coast rock lobster resource. Models are needed which can provide reasonable fits to both the CPUE and catch-at-length data available. Generally this has required the introduction of either time-varying selectivity or effort saturation effects. For the former, two approaches have been suggested: “MARAM” and “OLRAC”. This document presents improved results for the MARAM approach through use of a more complex selectivity function for Area 3 (Model 2), and also implements a version of the “OLRAC” approach (Model 3e). Implementations involving effort saturation are underway
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    OMP 2011 to be used for setting TACs for the west coast rock lobster fishery for the 2011+ seasons
    (2012) Johnston, Susan J; Butterworth, Doug S; Glazer, Jean Patricia; Brandão, Anabela
    This document provides full specifications of OMP 2011 to be used to set allocations for various sectors and super-areas for the 2011+ seasons for the West Coast rock lobster fishery. This OMP is implemented slightly differently for the first season (2011), than for 2012+. The management objective is to increase the male biomass above 75mm CL by 35% by 2021 relative to the 2006 level in median terms. This document provides details of the three main components of OMP 2011: 1) how data are combined across the five super-areas (Area 1-2, Area 3-4, Area 5-6, Area 7 and Area 8+) for input into the OMP; 2) the OMP formulae which provide the global TAC recommendation; and 3) the manner in which the global TAC is split amongst the super-areas and different sectors.
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    The potential for using acoustic tracking to monitor the movement of the West Coast rock lobster Jasus lalandii
    (2005) Atkinson, L J; Mayfield, S; Cockcroft, A C
    Although acoustic tracking has been used to study the movement of several species of clawed and spiny lobsters, only recent technological advances have provided sufficiently small transmitters to examine the utility of using acoustic tracking as a means to analyse the movement of relatively small spiny lobsters, such as Jasus lalandii. The effect of the transmitter on the mobility of J. lalandii was tested in aquarium experiments and was shown to have no influence on movement in three separate experiments. Thereafter, adult male rock lobsters (86–98mm carapace length) were tracked in field trials for up to 32 days at Betty's Bay (n = 4) and Hermanus (n = 5) off the Western Cape, South Africa. Tracking J. lalandii in the field using acoustic tags was successful, even in areas with dense kelp beds and rocky outcrops. The signal from the transmitters was readily detectable from the surface and subsequent use of underwater tracking equipment enabled visual confirmation of the position of the rock lobsters. Lobsters moved significantly longer distances (>45m day−1) in the first two days following tagging than during any subsequent time period (<10m day−1). This suggests that transmitter attachment and/or handling altered the movement pattern for the first 72 hours after tagging. During the period of observation, J. lalandii displayed classical nomadic behaviour.
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    Updated West Coast rock lobster operating models
    (2011) Johnston, Susan J; Butterworth, Doug S
    A number of recommendations pertaining to the assessment of West Coast rock lobster were made by the panel at the international workshop held in Dec 2010. The authors have taken these into account and updated all five operating models for the West Coast rock lobster resource. The following are items that have been checked and modified in the operating models and associated input data.
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    Was overfishing of predatory fish responsible for a lobster-induced regime shift in the Benguela?
    (Elsevier, 2014) Blamey, Laura K; Plagányi, Éva E; Branch, George M
    The 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.