Browsing by Author "Marsac, Francis"

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    Biology of Albacore tuna (Thunnus alalunga, Bonnaterre 1788) off the South West Coast of South Africa
    (2013) Norman, Stewart James; Attwood, Colin; Marsac, Francis
    Albacore tuna (Thunnus alalunga) is a highly migratory species found in all of the worlds oceans. The origin of albacore south of Africa is in question. This species constituted 85% of catches of the South African commercial tuna fishing fleet from 2000-2009 and is an important species in supporting a large boat-based recreational fishery. Albacore were sampled at angling competitions, which offer a repeatable and cheap source of tuna, in the Western Cape of South Africa during 2012 and 2013. 119 Samples were used to determine a length-weight relationship and to provide conversion ratios of various body measurements to fork length when total length was not available. Visual examination of testes and ovaries indicated that albacore are not spawning off the coast of South Africa.A comparison between the ease of using sectioned sagittal otoliths and first dorsal spines indicated that otoliths were more precise for estimating the age of albacore. Von Bertalanffy growth parameters were estimated from 51 fish, ranging from 2-9 years old (L∞=1100.07 mm; K=0.238 y-1; t₀=-2.14). Stomach content analysis indicated that the mesopelagic squid Lycoteuthis lorigera is the most important prey item for South African albacore. δ13C and δ15N stable isotope analysis of albacore and yellowfin tuna (T. albacares) muscle tissue showed that they feed on prey that may depend on different primary producers but that the two species of tuna share the same niche in the southern Benguela food web. Trophic levels of 3.8 and 3.76 were assigned to albacore and yellowfin tuna respectively.
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    Environmental influences on banana shrimps of the Sofala Bank, Mozambique Channel
    (2015) Malauene, Bernardino Sergio; Moloney, Coleen L; Roberts, Michael J; Marsac, Francis; Penven, Pierrick; Lett, Christophe
    The Sofala Bank in the western Mozambique Channel is an essential habitat for shallow-water penaeid shrimps. It supports an important multi-sector and -species fishery, with Fenneropenaeus indicus and Metapenaeus monoceros (banana shrimp) being the two main target species. Over the past decade this valuable resource has been declining, which has been attributed to environmental changes, but no conclusive evidence has been found. This PhD thesis aims to understand the interactive roles of biophysical processes on recruitment of banana shrimps, par ticularly their larvae on the Sofala Bank. It is hypothesized that shrimp larvae can be advected offshore by passing mesoscale eddies to regions where they are unable to survive and are thus lost. In the absence of both physical and biological observations, a modelling approach is used. A high-resolution, nested, coastal, Regional Ocean Modeling System (ROMS) of the Sofala Bank is developed. In general, the model agrees well with available observations and literature. The ROMS outputs and self-organizing map analysis indicate that the shelf circulation, structure and river plumes are strongly influenced by the highly energetic o_shore eddy activity. A biophysical, individual-based model (IBM) coupled to the ROMS was developed for early life stages of banana shrimps on the Sofala Bank. The IBM uses spawning patterns identified from analyses of both commercial and research survey data. Simulations indicate that shrimp larvae are lost offshore by entrainment in mesoscale eddies at inter- and intra-annual scales and eddies therefore are unlikely to produce a continuous declining in the catch. In contrast, these eddies induce onshore transport of larvae, promoting coastal settlement, compared with periods without eddies. Locations for simulated larval coastal settlement are identified: northern, central and southern. The roles of tides and larval diel vertical migration in influencing simulated larval settlement success was not conclusive; further research considering a selective tidal stream is needed. Simulated larvae were sensitive to low lethal temperature and river plumes. A conceptual model for the Sofala Bank circulation and another for the banana shrimps, early life history dynamics are proposed based on the results of this thesis.
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    Environmental influences on tuna movement patterns in the Indian Ocean
    (2017) Motah, Beenesh Anand; Marsac, Francis; Field, John G; Gaertner, Daniel
    The Indian Ocean Tuna Commission conducted a small-scale tagging programme (2002-2009) and also a large-scale tagging programme: the Regional Tuna Tagging Programme of the Indian Ocean (RTTP-IO, 2005-2009). Both tagging programmes known as the Indian Ocean Tuna Tagging Project (IOTTP), targeted three main species of tuna commercially exploited in the Indian Ocean: bigeye (Thunnus obesus), skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares). The two programmes tagged 219,149 tuna and 34,294 recaptures were reported to the commission. This study focused on tuna behaviour in the Indian Ocean looking at seasonal impacts, inter-annual variability in relation to ocean environment, survival estimates, movement patterns, size-groups and school-type: Free Schools (FS) and Fish Aggregating Devices (FADs). Using a multivariate approach, it was found that the years 2005 to 2007 were most abundant in recoveries of skipjack adults (77.45%) while yellowfin adults were mainly abundant during 2008 to 2011. It also showed that year and zone were significant factors influencing local abundance in tuna. The Kaplan-Meier survival curves enabled estimates on the longevity of the three species to be made. It was estimated that the cohorts (99%) vanished at 12, 5.8 and 10 years for bigeye, skipjack and yellowfin, respectively. The years 2006 (cold-productive phase) and 2007 (warmchlorophyll depleted phase) showed tuna movement patterns changing with an El Ni˜no event and primary productivity. Tuna tagged in the Tanzanian region, showed that those under FADs moved pre-dominantly towards the Somalian and Seychelles waters, while those in FS moved to the Seychelles and Mozambique waters. General Additive Model (GAM) analyses showed that the area bounded by 5⁰N-5⁰S and 45⁰-55⁰E was the main tag recovery regions for tuna under FADs. While in FS, the core recovery region was observed to be from 0⁰N-10⁰S and 50⁰-60⁰E. Recoveries were distributed in the temperature range 25-29 ⁰C. Modelling tuna movement and drift related to ocean surface currents and swimming speed, a closer match between simulated and actual recovery positions were obtained for large tuna (particularly free schools) in comparison to small tuna associated with FADs.
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    Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean
    (2013) Harikishun, Ameil; Moloney, Coleen; Marsac, Francis; Ndjaula, Hilkka O N
    Standard weight (Ws) equations were established for yellowfin tuna (Thunnus albacares) in the Western Indian Ocean and used as a baseline for calculating relative weight (Wr) indices. Length-weight data for yellowfin tuna were obtained from the Indian Ocean Tuna Commission (IOTC) for 1984 to 1991 and 2003 to 2013. Four quadratic standard weight equations were established (based on mean, median, first and third quartile statistics for each length class) using log10-transformed weight (W, in grams) and fork length (FL, in mm) data. The median Ws equation (log10(W) = -3.903 + 2.425(FL) + 0.103(FL)2) was the preferred baseline for the Wr analysis. Relative weight estimates for individual fish ranged from 92 to 110. Annual and monthly Wr estimates ranged from 99 to 101 and 99.9 to 100.5 respectively. A GLM was fitted to separate the effects of year, month and sex on the response variable Wr. Year, month, sex and the interaction terms Year:Sex and Month:Sex all contributed significantly to the variability in Wr explained. Correlations between environmental variability and yellowfin tuna condition were observed. Concentrations of suitable prey in 2003 and 2004 correlated to above average condition, although 2005 and 2006 were below average condition during similar prey availability and environmental conditions. Relative weight followed an increasing trend from 2008 to 2013 despite lower biological enrichment. Mechanisms such as a shallower thermocline and reduced fishing pressure are proposed explanations. This study provided a reference study for body condition studies of yellowfin tuna in the Indian Ocean. It also provided support for the application of the Wr index to other tuna species in the Indian Ocean managed by the IOTC.
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    Patterns among micronekton communities in relation to environmental conditions at two shallow seamounts in the south-western Indian Ocean
    (2020) Annasawmy, Pavanee Angelee; Marsac, Francis; Attwood, Colin
    Seamounts are ubiquitous topographic features across all ocean basins. They rise steeply through the water column from abyssal depths. Depending on their size, shape and summit depths, seamounts reportedly have an influence on the physical flow regimes which may promote the aggregation of zooplankton, micronekton, and top predators above or in the immediate vicinity of their summits. Micronekton form a key trophic link between zooplankton and top marine predators, and are divided into the broad categories: crustaceans, cephalopods and mesopelagic fishes. The vertical and horizontal distributions, assemblages and trophic relationships of micronekton were investigated at two shallow seamounts of the south-western Indian Ocean. La Pérouse is a steep bathymetric feature rising from a deep seabed located at 5000 m and with a summit depth at ~60 m below the sea level. This seamount is located at the north-western periphery of the oligotrophic Indian South Subtropical Gyre province. A seamount to the south of Madagascar, named “MAD-Ridge” in this study, has a summit depth at ~240 m below the sea level and rises from a base at ~1600 m. MAD-Ridge is located within an “eddy corridor” within the productive East African Coastal Province. The micronekton acoustic densities were greater at MAD-Ridge relative to La Pérouse, in accordance with the difference in productivity between the two sites. Physical processes within the cyclonic mesoscale eddy sampled during the MAD-Ridge cruise led to enhanced micronekton acoustic densities in the eddy relative to the MAD-Ridge seamount. While the shallow scattering layer (0-200 m) consisted of common oceanic micronekton species, the summits and flanks of La Pérouse and MAD-Ridge both showed presence of resident or seamount-associated fish species during day and night. Micronekton were also shown to exhibit a range of migration strategies such as diel vertical migration, mid-water migration and no diel migration. However, despite the differing productivity between both pinnacles, crustaceans, smaller-sized squids and mesopelagic fishes exhibited trophic levels ranging from 3 to 4 at both seamounts. This thesis highlights important knowledge gaps on seamount ecosystems and ecological patterns associated to shallow pinnacles. It also underlines the importance of studying seamount ecosystems of the south-western Indian Ocean in order to promote management and conservation measures for a sustainable use of such specific environments.