Academic literature on the topic 'Merluccius capensis – Namibia'

Defining the trophic relationships of marine predators and their dietary preferences is essential in understanding their role and importance in ecosystems. Here we used stable isotope analysis of skin samples (δ15N values reflecting trophic level and δ13C values reflecting foraging habitat) to investigate resource partitioning and spatial differences of the feeding ecology of dusky dolphins Lagenorhynchus obscurus and Heaviside’s dolphins Cephalorhynchus heavisidii from 2 coastal study sites separated by 400 km along the coast of central (Walvis Bay) and southern (Lüderitz) Namibia in the Benguela upwelling ecosystem. Overall, isotopic niches of both predators were significantly different, indicating partitioning of resources and foraging habitats. Despite their smaller body size, Heaviside’s dolphins fed at a significantly higher trophic level than dusky dolphins. Stable isotope mixing models revealed that both species fed on high trophic level prey (i.e. large Merluccius spp., large Sufflogobius bibarbatus, and Trachurus t. capensis) at Walvis Bay. The diet of both dolphin species included smaller pelagic fish and squid at Lüderitz. Spatial differences highlight that Heaviside’s and dusky dolphins may exhibit dietary plasticity driven by prey availability, and that they likely form distinct population segments. Important prey for both dolphin species, specifically Merluccius spp. and T. t. capensis, are the main target of trawl fisheries in the Benguela upwelling ecosystem, highlighting potential resource overlap between dolphins and fisheries.

Management of the Cape hake (Merluccius capensis) stock off the coasts of Angola and Namibia has been based on surplus production modeling, which incorporates an assumption of constant catchability. Results of least-squares catch-at-age analysis and virtual population analysis were used to estimate catchability from 1968 to 1986. Estimated catchability was either relatively constant or gradually increasing from 1968 to 1978, but fluctuated considerably between about 1979 and 1983. The fluctuations may have been due to an inverse relationship between catchability and abundance or to environmental factors affecting fish distribution. The period of highest estimated catchability corresponded to the period of lowest estimated abundance and below-average water temperatures. If management is based on the assumption of constant catchability but catchability is inversely related to abundance, the risk of overfishing will be greatest when the stock is at the lowest levels. Future assessments should place increased reliance on survey data or incorporate an appropriate model of catchability changes.

A new species of myxosporean parasite is described from the gall bladders of the hakes Merluccius capensis Castelnau and M. paradoxus Franca (Pisces: Teleostei) caught off the west and south coasts of South Africa. The new species, Pseudalataspora vanderlingeni, is described morphologically and molecularly and compared with the 15 other species of Pseudalataspora previously described from marine fish. Although a molecular description is available on GenBank for only one of these 15 species, the morphological description supports the status of P. vanderlingeni as a new species. Earlier reports, without detailed descriptions, of Leptotheca sp. and Ceratomyxa sp. from the same hosts caught off Namibia were very likely to have been of P. vanderlingeni. These earlier studies reported high prevalences of infection, similar to those of >60% described in the present study. The effects of fixation and freezing on the dimensions of spores of Pseudalataspora spp. are described, and the status of the genus Pseudalataspora within the family Ceratomyxidae is discussed.

Sewage effluents disposed into the marine environment from De Beers Marine Namibia diamond mining vessels have the potential to cause endocrine disruptive effects in marine organisms. Endocrine disruption refers to the alteration of the normal functioning of the endocrine system and various chemicals have the ability to mimic hormones, effecting endogenous hormone synthesis, transport, receptor interaction and intracellular signaling. The potential endocrine disruptive effects, caused by the release of different types of sewage effluents into the ocean, on fish species is a concern due to the commercial importance of fish species found in the mining area e.g. hake, sole, horse mackerel. Increased awareness of marine environmental degradation due to the presence of chemical contaminants has resulted in research being done on early warning systems, in the form of biomarkers. Cytochrome P450 monooxygenase 1A (CYP1A) and vitellogenin (vtg) are important proteins found in fish liver and blood, that have been used as biomarkers for the detection of pollutants in fish. CYP1A is a subfamily of the P450 superfamily of enzymes and catalyzes the oxidation, hydrolysis and reduction of exogenous and endogenous compounds (phase I reactions) and thus has the capacity to regulate the metabolism of several organic contaminants. CYP1A expression is altered by exposure to planar xenobiotic compounds e.g. polyaromatic hydrocarbons. Vtg is an important precursor for egg yolk proteins and plays a role in the growth and development of an oocyte. Expression of this protein is altered upon exposure to estrogenic compounds. The aim of this project was to isolate CYP1A from fish liver by differential centrifugation and optimize conditions for the CYP1A-mediated ethoxyresorufin-Odeethylase (EROD) assay and western blot analysis (to assess CYP1A expression). Another aim of this study was to evaluate the potential effects of biologically disruptive chemicals from sewage effluents, discharged into the marine environment, on the expression of CYP1A in two species of hake, Merluccius capensis and M. paradoxus (Cape hake). CYP1A in Cape hake is approximately a 60 kDa protein and the highest EROD activity was detected in the microsomal fraction after differential centrifugation. Optimal EROD assay conditions were observed at pH 7.5, a temperature of 25 °C, 10 μl of sample and a reaction time of 30 seconds. Enzyme stability assays indicated a drastic decrease in enzyme activity after 30 seconds. The EROD assay was not NADPH dependent but was limited by NADPH supply, with an increase of 300% in EROD activity being observed with the addition of 0.1 M exogenous NADPH. The addition of dicumarol (40 μM), a phase II enzyme inhibitor, showed a 232% increase in EROD activity. This is because dicumarol inhibited enzymes with the capacity to metabolize the product (resorufin) of the EROD reaction. With regard to western blot analysis, the optimal primary (rabbit antifish CYP1A peptide) and secondary (anti-mouse/rabbit antibody-horseradish peroxidase conjugate (POD)) antibody dilutions were determined to be 1:1000 and 1:5000, respectively. The comparison of CYP1A expression in Cape hake samples from De Beers Marine mining area and reference sites showed higher EROD activity (16.29 ± 0.91 pmol/min) in fish samples from the mining area in comparison to the reference site (10.42 ± 2.65 pmol/min). Western blot analysis was in agreement with the EROD assay results and a higher CYP1A expression was observed in fish from the mining sites. The increased CYP1A expression observed in fish from the mining area is not definitively an indication of a pollutant effect in the environment, as several environmental and biological factors (e.g. photoperiod and age) must also be considered before reaching this conclusion. Another aim of this study was to purify vtg from Cape hake blood samples. Cape hake vtg was purified from fish plasma by selective precipitation with MgCl2 and EDTA. Precipitated sample was subjected to anion exchange chromatography using fast protein liquid chromatography (FPLC). Vtg eluted as two broad peaks and had a molecular weight above 200 kDa. SDS-PAGE analysis also resolved smaller molecular weight proteins below 70 kDa, which were thought to be vitellogenin cleavage proteins, lipovitellin and phosphovitins. Western blot analysis was performed; however, it did not produce any conclusive results. The purification of vtg enables further studies in characterizing this protein and developing assay aimed at detecting estrogenic pollutants in the marine environment

Two species of hake (Merluccius capensis and Merluccius paradoxus) account for most of Namibia’s fisheries catch, and they are important secondary consumers in the Benguela Current ecosystem. Inferences on their trophic relationships have been based mainly on stomach content analyses. However, such data are limited temporally because they represent only snapshots of recent feeding, and are quantitatively biased because of variation in the digestion rates of different prey. The principal aim of the thesis was to understand the trophic relationships of two hake species relative to each other, their known prey and top predators (demersal sharks) in the northern Benguela Current ecosystem (Namibia), using time-integrating trophic biomarkers. By using stable isotope (carbon and nitrogen) and fatty acid signatures of their muscle tissues, my overall objectives were to produce new knowledge about 1) hake ontogenic trophic relationships, 2) the contributions of different prey to hake diets, 3) hake dietary differences, and 4) some aspects of hake’s trophic relationships with demersal sharks. Tissues of hake (n=358), their potential prey (n=455), and demersal sharks (n=42) were collected between 2008 and 2012 during demersal bottom trawl surveys off Namibia, for stable isotope and fatty acid analyses. And more...

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Life history traits and tactics of commercially important Namibian shallow-water hake, Merluccius capensis, were investigated in relation to their environment. A time series of length-frequency distributions (LFDs) from otoliths collected from fur seal scat samples was used to identify cohorts and calculate the approximate hatchdates and growth rates of young M. capensis from 1994 to 2009. Monthly otolith samples of five of these cohorts (1996, 1998, 2002, 2005 and 2006) were used to evaluate the translucent zone periodicity over the first 21 months of their life. Additionally, LFDs from research surveys and commercial samples were used to calculate growth rates for M. capensis up to 65 cm total length (TL), and to further validate the translucent zone formation of three of the five cohorts (1996, 1998 and 2002) on fish up to 3.5 years old.

Fish muscle proteins undergo complex physicochemical changes during frozen storage which result in the toughening of muscle and unpalatability. The aim of this thesis was to examine the aggregation of fish proteins as a result of frozen storage, in particular the effect of formaldehyde on proteins. Atlantic cod (Gadus morhua) and Namibian hake (Merluccius capensis and Merluccius paradoxus) muscle proteins were characterised and differentiated by electrophoretic and immunological approaches. The nature of the chemical bonds involved was elucidated by the use of reagents capable of breaking the non-covalent and covalent bonds. Conformational changes of the protein were investigated by both immunological and chemical techniques. Structural and textural changes on frozen storage in the presence of formaldehyde were examined by microscopic, deformation-compression analysis and dynamic mechanical testing. In addition, a lysosomal enzyme (Trimethylamine oxidase) which is responsible for the toughening of gadoid fish muscle during storage was characterised. Electrophoretic separation of the sarcoplasmic proteins from Merluccius capensis and Merluccius paradoxus confirmed the biological closeness of the two species. A 19kD protein band from M. capensis, excised from the SDS gel, was used for the production of polyclonal antibodies as analysed by an Enzyme Linked Immunosorbent Assay (ELISA). Formaldehyde and frozen storage augmented the extent of protein insolubilisation. The results of this study showed conclusively that formaldehyde is only one of many factors involved in the denaturation of fish proteins during frozen storage. List of objectives of the project. The objectives were: to characterise the fish muscle proteins and differentiate between the Namibian hake species Merluccius capensis and Merluccius paradoxus; to elucidate the chemical aspects related to solubility and foaming of fish muscle proteins in the presence of formaldehyde; to assess structural and textural changes in frozen and formaldehyde treated fish proteins by microscopy and viscoelastic measurements; to examine the changes in sulphydryl, disulphide and hydrophobic groups during frozen storage; to examine the changes in sulphydryl, disulphide and hydrophobic groups in the presence of formaldehyde; to monitor the degradation of trimethylamine oxide, formaldehyde and trimethylamine during frozen, storage of fish; and to characterise the enzyme TMAOase and to assess the effect of inhibitors.

Dissertação mest., Gestão da Água e da Costa, Universidade do Algarve, 2007
Age and growth estimates of Cape hake (Merluccius capensis), in the waters off Namibia were described from 804 specimens collected during January and February 2003. Each specimen was aged twice using whole otoliths. Otolith growth measurements was taken and used in back-calculated length-at-age data of M. capensis. Direct average otolith growth measurements have shown that up to three false rings formed prior to the first annulus. Ogives demonstrated the differences in otolith diameter, location frequency between the annuli and false rings. A power fish length-otolith diameter relationship used for back-calculation is: (L = 1.39 * OD). The average back-calculated lengths for the first four years of life were 19.3, 25.6, 31.8 and 38.2 cm total fish length. Age composition, estimated from an age length key, indicated that juveniles and young fish predominate in survey and commercial catches. The von Bertalanffy growth parameters was (growth coefficient, K = 0.07; asymptotic length, = 123.13; = -1.5). Sexual maturity was reached at age 1.67 years. The high correlation between otolith weight and age permits the use of otolith weight as a substitute for future age assessments. 1717.1∞L0t Key