Browsing by Author "Attwood, Colin G"

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    Open Access
    A change of the seaward boundary of Goukamma Marine Protected Area could increase conservation and fishery benefits
    (2009) Götz, Albrecht; Kerwath, Sven E; Attwood, Colin G; Sauer, Warwick H H
    Goukamma Marine Protected Area (MPA) on the South African temperate South Coast has been shown to be effective in maintaining a spawning stock of roman, Chrysoblephus laticeps (Sparidae). The larval ecology and the oceanographic conditions in the area suggest a good potential for the enhancement of roman stocks outside the reserve through larval dispersal. A high rate of illegal fishing just inside the seaward boundary of the MPA could severely compromise its function. We suggest that a change of the seaward boundary of the reserve to coincide with a latitudinal line could increase its function as a harvest refuge for resident reef fishes such as roman, facilitate voluntary compliance and monitoring and prosecution of illegal fishing without a significant negative impact on the commercial linefishing fleet in the area. Simple adjustments such as the one proposed here could be attempted at a number of South African MPAs as they would be beneficial to achieve fishery and conservation goals alike.
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    A step-by-step framework to assess benefits of established temperate marine protected areas
    (2013) Götz, Albrecht; Kerwath, Sven E; Attwood, Colin G
    Marine protected areas (MPAs) have been advocated as a solution to the challenges of both conservation and modern fishery management, but their application remains controversial, partly because there are only general guidelines for evaluating their effectiveness. We propose a framework to specifically evaluate established MPAs in six steps. We tested the approach by reviewing published research and unpublished information on the Goukamma MPA in the centre of the South African temperate south coast. Information reviewed included effects on the structure of fish populations, catch and abundance indices of fish species, and ecosystem effects. We investigated factors that determine the usefulness of a MPA in fisheries management, including the movement behaviour of adult fishes, larval dispersal and fisher-displacement patterns. We found that differences in the rates of exploitation across the MPA border resulted in differences in abundance, size and condition of the main target species, roman (Chrysoblephus laticeps). The diversity and abundance of non-target fish species, and the composition of the benthic invertebrate community, were affected by the cessation of fishing. The potential for ‘spillover’ of adult roman might be limited to the vicinity of the MPA by their small home range, but there is potential for self-seeding and dispersal of roman eggs and larvae over wider areas. These theoretical considerations were confirmed by an analysis of catch data from before and after MPA implementation. The framework presented here may help to identify and fill gaps in the knowledge of established MPAs along South Africa’s temperate south coast.
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    Automated detection and classification of red roman in unconstrained underwater environments using Mask R-CNN
    (2021) Conrady, Christopher; Er, Sebnem; Attwood, Colin G
    The availability of relatively cheap, high-resolution digital cameras has led to an exponential increase in the capture of natural environments and their inhabitants. Videobased surveys are particularly useful in the underwater domain where observation by humans can be expensive, dangerous, inaccessible, or destructive to the natural environment. Moreover, video-based surveys offer an unedited record of biodiversity at a given point in time – one that is not reliant on human recall or susceptible to observer bias. In addition, secondary data that is useful in scientific study (date, time, location, etc.) are by default stored in almost all digital formats as metadata. When analysed effectively, this growing body of digital data offers the opportunity for robust and independently reproducible scientific study of marine biodiversity (and how this might change over time, for example). However, the manual review of image and video data by humans is slow, expensive, and not scalable. A large majority of marine data has never gone through analysis by human experts. This necessitates computer-based (or automated) methods of analysis that can be deployed at a fraction of the time and cost, at a comparable accuracy. Mask R-CNN, a deep learning object recognition framework, has outperformed all previous state-of-the-art results on competitive benchmarking tasks. Despite this success, Mask R-CNN and other state-of-the-art object recognition techniques have not been widely applied in the underwater domain, and not at all within the context of South Africa. To address this gap in the literature, this thesis contributes (i) a novel image dataset of red roman (Chrysoblephus laticeps), a fish species endemic to Southern Africa, and (ii) a Mask R-CNN framework for the automated localisation, classification, counting, and tracking of red roman in unconstrained underwater environments. The model, trained on an 80:10:10 split, accurately detected and classified red roman on the training dataset (mAP50 = 80.29%), validation dataset (mAP50 = 80.35%), as well as on previously unseen footage (test dataset) (mAP50 = 81.45%). The fact that the model performs equally well on unseen footage suggests that it is capable of generalising to new streams of data not used in this research – this is critical for the utility of any statistical model outside of “laboratory conditions”. This research serves as a proof-of-concept that machine learning based methods of video analysis of marine data can replace or at least supplement human analysis.
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    Biology, movement behaviour and spatial dynamics of an exploited population of smooth hound shark mustelus mustelus around a coastal marine protected area in South Africa
    (2018) da Silva, Charlene; Attwood, Colin G; Kerwath, Sven E
    Aspects of the life-history, movement in relation to a Marine Protected Area (MPA) boundary, and short and long-term spatial behaviour in relation to environmental preferences of Mustelus mustelus were studied in the Saldanha Bay region on the west coast of South Africa. The overarching aim of this thesis was to examine the biological and ecological circumstances under which a MPA could provide effective protection to a commercially caught coastal shark from fishing activity. The pigmentation, reproductive biology, diet, growth and maturity of Mustelus mustelus was examined from 217 ranging from 381 to 1734 mm TL and 467 to 1267 mm TL for females and males, respectively. Sharks in the bay represented the largest females and males recorded worldwide. The seasonal changes in oocytes and testes development, embryo length and the occurrence of near-term and postpartum females indicated that female parturition and ovulation occurs between November and December after a gestation period of 10-11 months. The presence of juveniles, neonates and pregnant females inside the Langebaan Marine Protected Area indicates it to be a nursery ground for this species. The largest part of the diet of M. mustelus consisted of three species of crustaceans: Hymenosoma orbiculare, Upogebia africana, Callichirus kraussi. No ontogenetic shift in diet was found for M. mustelus from Langebaan Lagoon. M. mustelus grow relatively rapidly, matured early (between 3 and 6 years) and attained a maximum observed age of 13 years. The movements of individual Mustelus mustelus in and adjacent to a small closed area (Langebaan Lagoon MPA, 34 km2 ) situated on the West Coast of South Africa were investigated over two years using acoustic telemetry. Sharks spent the majority of the time (in hours, average 79%) inside the Langebaan Lagoon MPA, and some sharks (n = 2 of 15 recorded during a full year) did not leave the reserve during the observation period. Time spent inside the closed area and the number of crossings of its boundary was strongly influenced by season. Sharks concentrated inside the closed area during summer, while they were widely distributed throughout the study area during winter months. A combination of shallow and sheltered waters in close proximity to the Saldanha Bay port and other boataccess points would normally make this summer aggregation highly vulnerable to fishing activity. The residency of M. mustelus within the closed area suggests that spatial protection may be effective for this species. Acoustic telemetry and in situ environmental data were used to investigate movement of M. mustelus in relation to changing environmental conditions over long (seasonal) and short (20 min) time scales. Results of Generalised Additive Mixed Modeling (GAMMs) indicated no significant influence of tide or moon phase and only a weak influence of diel period on movement and direction of movement. The thermal preference for M. mustelus was between 18 and 22 C as determined by GAMMS. Absolute temperature and the relative change in temperature at the shark’s position were the best predictors for shark movement and its direction in summer, explained 4.4 and 42.7 % of the deviance, respectively. This study provided evidence that M. mustelus inside the embayment decide their position within their area of residency according to their thermal preference and that temperature change constitutes the trigger that determines movement direction. This study confirms that M. mustelus are resident within the Saldanha embayment and distinct by diet, life-history parameters and colouration from stocks elsewhere. M. mustelus from this group are more fecund and larger in body size than those from all other populations globally, possibly due to the favourable temperature conditions in the warm sheltered lagoon and the existence of a MPA closed to fishing which includes preferred habitat for all life- IX history stages of this species. In the absence of a comprehensive stock assessment and species-specific management, well-positioned closed areas that include preferred habitat can aid the sustainability of coastal shark fisheries.
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    Dedicated dads: a study on the nesting behaviour of Spondyliosoma emarginatum (Telostei: Sparidae)
    (2021) Faure, Beaulieu Nina; Attwood, Colin G
    Fish display the most diverse parental care behaviours within the animal kingdom. These behaviours are important from evolutionary and conservation perspectives as parental care is critical for the development and survival of the young. This study used video monitoring to uncover the nesting behaviour of an endemic southern African species of the Sparidae family, Spondyliosoma emarginatum (steentjie). S. emarginatum has evolved a nesting strategy by which males create nests on the seafloor for females to lay their eggs in. The eggs are fertilised and guarded by the male until they hatch. This species is particularly interesting as it has evolved a life-history strategy unique to the Sparidae, a combination of protogyny (female to male sex change) and male parental care. A compound nesting site with over 50 nests was discovered in 9 to 14 m depth in False Bay, South Africa. A large diversity in nest size and habitat was uncovered. The first nest with eggs appeared on the 3rd of September and this number gradually increased to a maximum of 26 nests on the 3rd of October. Eggs took from seven to nine days to hatch. During nesting, males were affected by stochastic weather events in the form of south-easterly gales. Nesting sites are likely limited to sheltered bays along South Africa's mostly exposed coast, and the optimal depth is probably a tradeoff between storm exposure and temperature. Deeper nests are expected in the east where the water is warmer. Nests were filmed daily to reveal how male behaviour changes before egg deposition and during egg development. After egg deposition, males increased their time on the nest from 30 to 52 minutes per hour. Nest defence included the regular clearing of invertebrate invaders (brittle stars, hermit crabs, sea cucumbers, and sea stars), and chasing away other fish species (sand gobies, Roman, and hottentot) and neighbouring male steentjies. Energy intensive behaviours such as clearing the nest and fanning the eggs remained constant irrespective of egg presence. In addition, males do not feed when guarding eggs, which explains the drop in male condition during spring. The revelation of this nesting site is useful for conservation and fishery management as the nests and nesting males are vulnerable to both fishing and seabed disturbances. Spondyliosoma spp. fulfil the requirements of the size-advantage model of protogyny. Their short life-span, in particular, their even shorter egg-laying life-span, classifies this species as an opportunist. This strategy may explain its success and numerical dominance in a wide range of biogeographic zones. The nesting behaviours shares much in common with freshwater opportunistic fish species and set it apart from the bulk of the Sparidae.
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    The biology, ecology and population trends of jacopever Helicolenus dactylopterus in South Africa
    (2021) Weston, Laura Frances; Attwood, Colin G; Sink, Kerry J
    Helicolenus dactylopterus is a deep-water benthic fish in the subfamily Sebastinae. Sebastinae species are slow-growing, long-lived and potentially vulnerable to overexploitation. Helicolenus dactylopterus is exploited throughout its Atlantic distribution. In South Africa, it is the eighth-most landed species in the demersal trawl fishery. Because little research has been conducted on H. dactylopterus locally, it is difficult to understand its response to fishing pressure. The life history, habitat preference and population trends of the species in South Africa were thus investigated. The age, growth, reproductive characteristics and diet were examined from 719 specimens collected from west and south coast trawls. The maximum age estimated was 32 years and the growth rate was low (k=0.13 year-1 ). Gamete release in both sexes was asynchronous, suggesting an unusual delay between insemination and fertilisation. Sperm storage was found in histologically analysed ovaries. The diet investigation indicated a selective benthic predator with an ontogenetic shift in diet from crustaceans to teleosts. To assess habitat preference, generalised linear models were fitted to broad and fine resolution abundance data from research trawls and video footage respectively. Trawl data indicated that H. dactylopterus primarily occurs in the Atlantic, decreasing in abundance in the Indian Ocean. Depths from 100 to 600 m were preferred, corresponding with the offshore demersal trawl footprint. Video data highlighted that H. dactylopterus, especially individuals >200 mm TL, preferred complex benthic habitat, typical of deep-water reef systems on the South African outer shelf and shelf edge. Standardised catch-per-unit-effort from trawl data indicated an increase in the abundance of H. dactylopterus from 1987 to 2001 in response to management reforms. Subsequently, a reduction in abundance occurred, but sustained population growth of 1.42% per year since 2005 suggests a national classification of the species as Least Concern against the IUCN Red List criteria. This study provides a thorough region-specific account of a previously data-limited species. Helicolenus dactylopterus shows life history traits typical of the Sebastinae, yet, in South Africa, the species has not succumbed to fishing pressure to the expected extent.