Department of Fisheries and Ocean Sciences

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Occurrence and control of Vibro spp. as contaminants of processed marine fish
(2010) Shikongo-Nambambi, Martha N.
Natural radioactivity in soils of the Walvis Bay - Henties Bay coastal area, Namibia
(2013) Shimboyo, Simon A.
Naturally occurring radionuclides such as 238U, 232Th and 40K are present in the soil where they disintegrate spontaneously releasing ionizing radiation which could pose health hazards to the inhabitants of a given location especially when the concentrations of the radionuclides are high. In Namibia, the Walvis Bay – Henties Bay coastal area is known to have many mineral resources including uranium and therefore the concentrations of the radionuclides in the soils and the associated radioactivity may be high in some towns in the area. It is therefore important to study the concentrations of radionuclides and the radioactivity in the soils of the major towns in the area. In this study, the concentrations of the radionuclides 238U, 232Th and 40K and their progeny, and their contribution to natural radioactivity in the soils of six major towns in the Walvis Bay – Henties Bay coastal area have been determined. These towns are Usakos, Arandis, Swakopmund, Walvis Bay, Wlotzkasbaken and Henties Bay. A total of 305 soil samples were collected across the six towns and dried under laboratory temperature for 72 hours. These samples were pulverised and passed through a 2 mm mesh screen and sealed in 500 ml polythene bottles. The specific activities of the radionuclides 238U, 232Th and 40K in the samples were subsequently analysed using a well-shielded high-purity germanium (HPGe) detector. The average concentration for 238U varies from a minimum of 14.5 ± 2.5 Bq kg-1 in Walvis Bay to a maximum of 104.1 ± 28.1 Bq kg-1 in Wlotzkasbaken, with a mean of 52.6 ± 24.5 Bq kg-1 for the six towns. Also, the average concentration for 232Th varies from a minimum of 18.0 ± 3.0 Bq kg-1 in Walvis Bay to a maximum of 334.4 ± 86.9 Bq kg-1 in Arandis, with a mean of 104.3 ± 85.9 Bq kg-1 while the average concentration for 40K varies from a minimum of 363.3 ± 41.3 Bq kg-1 in Walvis Bay to a maximum of 1336.5 ± 142.0 Bq kg-1 in Usakos, with a mean of 778.7 ± 204.6 Bq kg-1 for the six towns. These values were used to calculate the mean absorbed dose rate and the annual effective dose. The average annual effective dose varies from a minimum of 0.04 ± 0.01 mSv in Walvis Bay to a maximum of 0.35 ± 0.07 mSv in Arandis, with a mean of 0.15 ± 0.08 mSv for the six towns. These values are lower than the maximum permissible limit of 1.0 mSv recommended by the International Commission on Radiological Protection (ICRP) for the public so that the towns do not have high natural radioactivity.
Trophic relationships of hake (Merluccius Capensis Castelnau, 1851 and M. Paradoxus Franca 1960) from the Nothern Benguela current ecosystem (Namibia): Inferences from stable isotopes and fatty acids
(2014) Iitembu, Johannes A.
Two species of hake (Merluccius capens is 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 offNamibia, for stable isotope and fatty acid analyses. Nitrogen isotopes (8 15N) showed a significant positive relationship with size in both M capensis and M paradoxus; however, the correlation slopes of size and 8 15N in the two species were significantly different, with M paradoxus displaying a steeper trophic shift. A significant increase in carbon isotope signature (8 13C) with size was observed in M capensis but not in M paradoxus. In all size classes (except 20-29 em) M capensis had significantly enriched 13C values, generally matching their respective adult shelf-slope distribution. Smaller hake from both species (20-39 em) were trophically indistinguishable, with trophic levels (TL) of3.3 indicating predominant zooplanktivory. The largest M capensis (60-70 em) had TLs of approximately 3.5- 3.6, whereas M paradoxus of the same size. were slightly higher at 3.7-3.8 TL, indicating greater piscivory in the latter. A Bayesian isotope mixing model indicated that teleosts Synagrops microlepsis and Chlorophthalmus agazizi, the shrimp Plesionika martia, myctophids and euphausiids dominated the diets of both hake species. The greatest contributor to the diet of M paradoxus changed depending on the body size of the hake, while the diet of all size classes of M cap ens is was dominated by myctophids. Cannibalism accounted for less than 6% and less than 4% of the diets of M capensis and M paradoxus respectively. Isotope-based estimates of prey dietary contributions differed from those of published accounts of gut content analyses, an indication of overestimation of the time-integrated diet by the latter. The fatty acid (FA) profiles of the two species (M paradoxus and M cap ens is) and potential prey were similar to those typically abundant in marine fish. Palmitic acid (16:0) was the dominant saturated fatty acid (SFA), oleic acid (18: ln-9) was the dominant monounsaturated fatty acid (MUFA) and docosahexaenoic acid (22:6n-3) was the dominant polyunsaturated fatty acid (PUF A) in both hake species. Fatty acid (FA) profiles in the neutral storage lipids of M paradoxus and M capensis were significantly different, an indication of hake species' dietary resource partitioning, although some overlap was apparent. There was a significant temporal effect on hake FA compositions, reflecting temporal variations in quantity and quality of dietary resources. Feasible contributions of hake to the diets of three demersal sharks indicated that hake are not major prey of Centrophorus squamosus, whose diet was dominated by crustaceans. However, there was an increased contribution of hake to the diets of Deania sharks. All three sharks studied (Deania calcea, D. profundorum and C. squamosus) potentially consumed upwards of 18 % (208 960 tonnes per year) of the hake biomass [2011 hake biomass estimates (1111 196.8 tonnes)]. The two Deania species were isotopically indistinguishable in terms of 815N, and fed on a prey composition that overlapped with that of M paradoxus. 813C values indicated that hake and sharks fed on prey from similar basal resources, with C. squamosus being significantly different from M paradoxus but not from M capensis. The range of 813C and 815N indicated that the two hake species had a higher range of carbon sources as well as enhanced trophic diversity in their feeding patterns than did the three shark species. This study is the first combined usage of stable isotope and fatty acid techniques to analyse trophic relationships of M capensis and M paradoxus in the northern Benguela Current (Namibia). The study contributes towards understanding of trophic interactions in Namibian waters, and can aid the implementation of a science-based ecosystem approach to fisheries management in the Northern Benguela Current region (Namibia). The extension of this analysis throughout hake distribution ranges will provide additional insights into the trophic dynamics of hake species in future, and my study represents a solid foundation from which to develop such extensions.
Assessment of trace metals pollution along the Central Namibian marine coastline: using Choromytilus Meridionalis (Black Mussel) as indicator organisms.
(2014) Vellemu, Emmanuel C.
This study was carried out at four stations along the Central Namibian marine coastline towns (Walvis Bay, Swakopmund, Henties Bay and Cape Cross) to assess trace metals pollution using Choromytilus meridionalis as indicator organism. Samples were collected using randomized sampling techniques during winter and summer months of 2012. EPA 3050B and ICP-OES protocols were used to digest and assimilate the samples. Data were analysed using a 4x2x3 factorial model of a completely randomised design and results showed there was a significant interaction (p < 0.05) of only Pb and Fe levels between stations, seasons and substrates studied. However, there was no significant interaction (p > 0.05) between Cu and Zn levels between stations, substrates and seasons respectively. Metal levels in mussels and sediments ranked in the order: Fe > Zn > Cu > Pb. Walvis Bay recorded increased metal levels (P < 0.05) than other stations probably due to factors like port activities e.g. ship repairs, urban and residential settlements, tourism and recreation as well as run-off water from catchment areas. Sediments exhibited significantly higher mean trace metals than mussels and water (P < 0.05). In addition, clear-cut significant correlation (p < 0.05) of metals was observed between some stations and substrates. Maximum Fe, Zn, Cu, and Pb mean values in mussels were 483.7, 79.3, 6.50, and 2.30 ppm of sample dry weight respectively while maximum Fe, Zn, Pb; and Cu values in sediments were 44, 749.90, 100.0, 79.30 and 50.50 ppm of sample dry weight respectively. All metal values recorded in this study were lower or within those reported elsewhere. Overall, mussels grown in this part of the coastline could be regarded safe for human consumption; however, levels observed in samples from Walvis Bay are of health concerns. Findings from this study have stressed the need for continued monitoring of these metals along the Namibian marine shoreline for regulatory purposes.
Occurrenceand distribution of fish parasites of potential threat to the acquaculture sector along the okavango river, Namibia
(2015) Mumba, Victoria
Cichlids are subject to a wide range of diseases and parasites. Previously studies were conducted on fish parasites on the Okavango Delta, Botswana. This thesis is the first on fish parasites in the Kavango River, Namibia. The study aims to identify parasites occurring on commercially farmed fish i.e.: Oreochromis andersonii, Tilapia rendalli and Serranochromis robustus and to determine if these parasites may have potential threat to aquaculture and humans. For the purpose of identification and calculating prevalence (pr) and abundance (ab) parasites collected were fixed and stained using standard methods for each taxon. Kruskal-Wallis One Way Analysis of Variance was used to determine whether there were any significant differences in the number of fish parasites per zone, fish species and fish size. No significant differences were found in the number of parasites found in each zone, as well as the host fish species and group size of target fish species. A total of 205 specimens (91 T. rendalli, 89 O. andersonii and 25, S. robustus), were collected and examined for endo- and ectoparasites over twelve months. Of these 102 were infested with various parasites (total pr 49.7%), T. rendalli, 45%, O. andersonii63% and S. robustus 13%. Parasites included Trichodina sp. (ab0.18) followed by Dactylogyrus sp. (ab0.17). Some Tripartiella and Epistylis spp. were collected in low levels as compared to other protozoan parasites. Helminths parasites such as Contracecum sp. were found in high abundance, whilst Proteocephalus, Clinostomum and Acanthocephala spp. were found in low abundance and infected mostly S. robustus. Opistolernaea (ab0.02) occurred in high numbers on T. rendalli and O. andersonii in zone 1 (Katwitwi to Kasivi). Dolops ranarum, Lernaea hardingi and Lamproglena monodi were also collected (ab0.02). Most of the infested fish species ranged from 1– 20.9 cm in total length, while the parasite Contracecum sp. was mostly found in fish ranging from 21 – 30.9 cm. Most of the infested fish were found in zones 3 (Mbambi to Popa Falls) and 4 (Popa Falls to Kwetze). This study could be a useful management tool for fisheries researchers in the aquaculture industry. In conclusion some protozoans and parasitic crustaceans, if found in high abundance, may cause a threat to aquaculture. Other parasites such as Contracecum sp. could be a concern to human health when fish is eaten raw, half cooked or inadequately smoked.
Analysis of marine biotoxins: Paralytic and lipophilic shellfish toxins in Mussels (Mytilus Galloprovincialis) along the Namibia coastline
(2015) Dijerenge, Kahe J.
The study was carried out along the Namibian coastline, this includes Henties Bay, Swakopmund, Bird Island, Walvis Bay and Lüderitz to assess presence of shellfish marine biotoxins in mussels, Mytilus galloprovincialis which are filter feeders and feed on some of the algal species that produces phycotoxins, that can negatively affect the mariculture industry and human health. Samples were analysed for Paralytic and lipophilic shellfish toxins; including environmental parameters and phytoplankton species composition. Samples were collected using randomized sampling techniques for the period of May 2012 to April 2013. Mussel toxin content was measured by liquid chromatography-mass spectrometry (LC-MS/MS) for lipophilic toxins and paralytic shellfish poisoning Mouse bioassay (PSP MBA). The diarrhetic shellfish poisoning (DSP) toxin profile was found to be primarily composed of Okadaic acid (OA) and dinophysistoxins-1 (DTX-1), with the highest concentration recorded at Walvis Bay area, each toxin accounted for at least 50% of the total toxin content, this is probably due to the Dinophysis species reported in this area during the sampling period. In addition, Yessotoxin (YTX) and its analogues 1a-Homoyessotoxin (homo-YTX) and 45- hydroxy-yessotoxin (45-OH-YTX) were all detected at concentrations below the regulatory limit of 1 mg YTX equivalents/kg with homo-YTX dominating at Swakopmund and Bird Island stations with the highest toxin content of 0.14 mg YTX equ./kg recorded at both stations. Furthermore, PSP toxin contents in mussels were only detected at Walvis Bay and Lüderitz, with highest toxin concentrations of 80 and 903 μg STX equivalents/100g respectively. This coincided with the time at which Alexandrium species cell concentration at Lüderitz were at maximum, and could be a source of the paralytic shellfish toxins (PST). Although the toxin content in mussels were generally low, it still highlights the importance of a continued monitoring of both shellfish toxicity and their causative phytoplankton species, in order to produce safe shellfish for both local and international consumers.
Effect of pawpaw (Carica Papaya) seed meal on growth performance, feed utilization, survival and masculinization of sexualy undifferentiated three spotted Tilapia (Oreochromis Andersonii) fry
(University of Namibia, 2019) Iipinge, Linda Nuushona
The objective of this study was to determine the effects of dietary Carica papaya seed meal supplementation at different levels on growth performance, feed utilization, survival rate and sex ratio of sexually undifferentiated three spotted tilapia (Oreochromis andersonii) fry. Six treatments (replicated thrice) were designed based on the control (basal diet), 17 α-methyltestosterone (MT) (60 mg) and C. papaya seed meal incorporated into tilapia feed at 5, 15, 25 and 35 g / kg diet and administered to freshly hatched fry (9 days old) for 120 days. The swim up fry (initial body weight 0.04 g / fry) were collected from the aquarium and randomly distributed (𝑛=80) at each of eighteen black bins with a holding capacity of 200 L. The results showed that the growth performance of the fish fed varying inclusion levels of pawpaw seed meal did not differ significantly from those fed basal diet and 60 mg of MT. A decrease in growth performance and poor feed utilization capacity was observed in the treated fish with increasing dietary C. papaya inclusion levels from 15 g / kg to 35 g / kg diet, suggesting that it had inhibited the growth at high dosage. Pawpaw seed meal was able to skew the sex ratio in favour of males, from the expected ratio of 1:1 male: female for all the treatments. The percentage of males increased for the treatments with increasing the dietary inclusion level, with the highest masculinization percentage (82%) (P < 0.05) observed in fish fed with 15 g and 35 g of C. papaya extracts / kg diet. Dietary C. papaya did not significantly change the fish survival rate among the treatments (P > 0.05), signifying that it had no lethal effect. This study showed the possibility of using pawpaw seed meal as a natural agent to induce sex reversal in three spotted tilapia. Due to the safety, ease of biodegradation and local availability of this naturally sourced sex reversal agent, the study recommends that, it could be used as a viable alternative to popular synthetic hormones. In order to improve growth and induce sex reversal at the same time, the study recommends the use of 5 g but not ≥ 15 g C. papaya extracts / kg diet to be economically viable and efficient for fish farms.
Ecto-and endo- parasites of silver kob (Argyrosomus Inodorus) from northern Namibia (21º 24ºS)
(University of Namibia, 2019) Amakali, Annette Megameno
The silver kob (Argyrosomus inodorus) has been identified as a candidate species for finfish culture due to its outstanding qualities. There is little information available on the health of this species, especially parasite infestation that can pose a threat to fish cultivation and therefore to the success of mariculture. This study was thus designed to provide information on parasites that are affecting this species. Silver kob were collected monthly (2017–2018 for 11 months) using conventional fishing gear (n = 55) in Toscanini, Mile 108 and Henties Bay, northern Namibia (21° - 24°S). Fish were examined for ecto- and endo-parasites. Drawings and measurements of parasites were made using a camera lucida and calibrated eyepiece of an Olympus BX50 binocular microscope and/or a Zeiss (Discovery V8) camera calibrated on a Leica dissecting microscope. Parasite organ specificity was determined. Parasite prevalence, mean intensity and mean abundance were analysed by season of capture, fish length and fish sex. Chi-square tests were used to determine differences in mean abundance by season, host sex and host length. Twenty-eight species from 17 parasite genera were found, including monogeneans (five Diplectanum spp., Sinodiplectanotrema sp., four Calceostoma spp., Neocalceosoma sp. and Sciaenacotyle sp.), digeneans (Helicometra sp., three Helicometrina sp., and Stephanostomum sp.), cestode larvae (Callitetrarhynchus sp., a Tetraphyllidean plerocercoid), a nematode (Anisakis sp.), a palaeacanthocephalan (Corynosoma australe), copepods (Caligus sp., Sciaenophilus sp., Lernanthropus sp., two Brachiella spp.) and an unknown parasite. More ecto-parasites were organ specific than endo-parasites. Corynosoma australe and Calceostoma spp. were significantly more abundant during the cold season (June-November) (X2 = 31.56, p <0.001 and X2 = 3.10, p value = 0.048, respectively), and Diplectanum spp. were significantly more abundant during the warm season (December-May) (X2 = 24.44, p <0.001). With the exception of digeneans, larger sized fish (TL > 47.3 cm) showed the highest prevalence and mean abundance of parasites compared to smaller sized fish (TL ≤ 35.8 cm). Calceostoma spp. and Helicometrina spp. showed a significant decrease in mean abundance with increasing host length (X2 = 28.22, p <0.01 and X2 = 5.77, p = 0.03, respectively). Corynosoma australe and Diplectanum spp. showed a significant increase (X2 = 20.2, p <0.01 and X2 = 41.1, p <0.001) in mean abundances with increasing host length. No fish sex preference was observed for parasite infections in silver kob. Macroscopically, most fish showed no visible symptoms of the parasite infestations. In severe intensities however, lesions of the fins and skin, and signs of haemorrhages were observed. This is the first study of ecto- and endo-parasites of Namibian silver kob A. inodorus. Molecular work and studies including Scanning Electron Microscopy (SEM), histopathology, Health Assessment Index (HAI) and descriptive studies should be done to supplement the findings, identify all the parasites to their species level and determine the nature of infections of the parasites on the fish host.
Trophic relationships of shallow water Cape Hake (Merluccius Capensis) and Cape Horse Macherel (Trachurus Capensis) in the northern Benguela ecosystem
(University of Namibia, 2019) Kadila, Hendrina Kapawanwa
Shallow water Cape hake (Merluccius capensis) and Cape horse mackerel (Trachurus capensis) are ecologically and commercially important species in the northern Benguela ecosystem (Namibia). The understanding of their trophic relationships is however still limited. In this study stable isotope measurements [carbon (δ13C) and nitrogen (δ15N)] of their muscles and stomach contents were used to investigate their feeding interactions. Understanding the feeding interactions of these two species is vital, in order to consider trophic dynamics in their fisheries management strategies. Muscle tissues (n = 404) and stomach contents (n = 404), were collected during November 2017 bottom trawl survey in Namibian waters. Results indicated that krill (Euphausiids) was a dominant prey in the diet of T. capensis and M. capensis. The diet compositions of T. capensis remained the same with both total length and latitude but not for M. capensis, an indication of the influence of prey availability. A potential for interspecific feeding competitions between the two species was observed as krill and anchovy were found as their common prey species. The feeding interactions of the two species appear to have not changed much over time. Significant differences were found in both δ15N values and δ13C values of the two species. Although niche overlap was observed, a wider niche for M. capensis than T. capensis was observed; an indication of M. capensis broader trophic diversity and a more specialized niche of T. capensis. This is the first study that has combined stable isotopes and stomach content analysis methodologies, to understand the feeding interaction of M. capensis and T. capensis.
Stock separation of the shallow-water hake Merluccius Capensis in the Benguela using otolith shape analysis and parasite infestation
(University of Namibia, 2020) Shoopala, Ester Ndapewa
The fishing industry is an important sector in Namibia with hake contributing about one third of the total commercial catch. Merluccius capensis, the shallow water hake, forms the bulk of this resource. Studies on the distribution of spawners and juveniles, spawning areas and genetics have proposed three stock structure hypotheses of M. capensis in the Benguela: (1) one stock throughout, (2) one in the northern and one in the southern Benguela or (3) three stocks: one in the northern, one in southern Namibia and the Orange River (border between Namibia and South Africa), and one in the southern Benguela. This study aimed to investigate these hypotheses of M. capensis stock structure within the Benguela using otolith shape analysis and parasite infestation, and to identify parasite species as potential biological tags. M. capensis otolith samples were collected during demersal trawl surveys along the Namibian coast in 1992, 2004 and 2005 and along the South African West coast in 2005. The Benguela ecosystem was divided into three areas, northern (17.00-25.29 S), central (25.30 - 29.05 S) and southern Benguela (South African waters, 28.86 - 35.81 S), for this study. A total of 1628 otolith images were analyzed using the ShapeR package in R. Otolith outlines from Wavelet transformations and ANOVA permutation tests indicated no significant differences between the northern and central Benguela for the years 1992, 2004 and 2005 (p > 0.05 for all) but showed significant differences between the northern and southern Benguela in 2005 (F = 14.81, p < 0.001). Comparing otolith size descriptors and otolith shape indices showed that otoliths from the southern Benguela were significantly more elongated (longer, narrower and less round) than those from the northern and central Benguela. 80 M. capensis were collected off Namibia and examined for parasites and compared with a study done on M. capensis parasites in South Africa in the previous year. A total of 10 parasite species were found on Namibian M. capensis. Prevalence (number of infected fish/number of fish examined)*100 and abundance (total number of parasites/total number of fish examined) were compared between areas. The following species have the potential to be used as biological tags (1) Chondracanthus merlucii indicating significant differences in abundance between the northern and central Benguela, and northern and southern Benguela; (2) Parabrachiella australis, only present in the northern and central Benguela, and (3) Stephanostomum multispinosum, only present in the southern Benguela. Otolith shape analysis and parasite infestation levels therefore revealed one M. capensis stock in the northern Benguela and another in the southern Benguela, with a possible separation further north than the Orange River, as shown by parasites, supporting stock structure hypothesis number (2). Further studies should include sampling for parasites conducted seasonally and adding stock identity methods such as otolith microchemistry along the Benguela.
Cadmium and lead levels in three freshwater fish species from the Zambezi region, Namibia
(2020) Madzingira, Oscar; Lifumbela, L.Z.; Kandiwa, Erick; Kandjengo, Lineekela; Mushonga, Borden
Muscle tissue from three-spotted tilapia (Oreochromis andersonii) (n = 11), sharp tooth catfish (Clarias gariepinus) (n = 14) and tigerfish (Hydrocynus vittatus) (n = 12) were sampled from a market in the Zambezi region, Namibia in June 2018 and analysed for cadmium and lead. Cadmium was not detected in the three fish species. Mean lead levels in all three fish species (0.09–0.19 mg kg−1) were below the recommended safe level of 0.3 mg kg−1. Catfish had the highest mean lead concentrations, followed by tigerfish and tilapia in descending order. However, the differences in mean lead concentrations between the three fish species were not statistically significant (ANOVA, p = 0.18). These results show that the fish tested were safe for human consumption with respect to the metals tested. It is recommended that more metals and other contaminants, such as pesticides, be monitored robustly in water, sediment and fish, using the recommended protocols, in order to generate data that can be reliably used for human health risk assessments in the future.
Dietary aloe and garlic crude polysaccharides: Effects on growth performance, haematological, and body composition parameters of Clarias Gariepinus
(University of Namibia, 2021) Gabriel, Ndakalimwe Naftal
Fish health management in aquaculture is one of the main challenges across the globe (including Namibia), worsened especially by the wide adoption of intensive farming systems. Nowadays, attention is focused on the use of medicinal herbs as alternative to unsustainable pharmaceutical drugs in aquaculture. This study aimed to develop andintroduce phytogenic diets made up of aloe vera (Aloe vera), and garlic (Allium sativum)crude polysaccharide extracts (separately and in mixture), which would promote growth, feed utilization, health, meat quality, and increase resistance against stress in African catfish, Clarias gariepinus reared in intensive aquaculture systems. First, this study evaluated the effects of dietary A. vera crude polysaccharides on growth performance, feed utilization, haemato-biochemical parameters, and resistance against low water pH in African catfish fingerlings. Fish were divided into five triplicate groups before being fed feeds supplemented with control 0%, 0.5%, 1.0%, 2.0% and 4.0% A. vera for 60 d. Fish fed a 1.0% A. vera supplemented diet showed a significant increase in all growth parameters compared to the control (P < 0.05). The protein efficiency ratio (PER) was significantly higher in fish fed 1.0% A. vera supplemented diet (1.31 0.22) compared to unsupplemented fish (0.85 0.10) and those fed 4.0% A. vera supplemented diet (0.85 0.14) (P < 0.05). The optimal dietary A. vera polysaccharide crude extract requirement was estimated to be 1.77% (y = - 0.043x2 + 0.152x + 0.593, P = 0.045) and 1.79 % A. vera (y = -2.778x2 + 9.95x + 29.29, P = 0.037), for growth and feed utilization respectively. Overall, A. vera extracts improved haemato-biochemical indices in A. vera supplemented fish when compared to unsupplemented ones, but decreased some of the indices at the 4.0% A. vera level. After blood sampling, fish were subjected to a low water pH (5.2 - 5.5) challenge and survival probability was measured. Fish fed diets supplemented with 1.0%, and 2.0% A. vera showed higher survival probability (above 70%) throughout the challenge period compared to the control (below 70%) and those fed the 4% A. vera supplemented diet (below 60%).
Assessment of bycatch species in the Namibia Hake directed bottom trawl fishery (1997-2014)
(University of Namibia, 2021) Mbaimbai, Greg Limbo
One of the main problems faced by fisheries management, is how to manage and mitigate bycatch caught during commercial fishing. The Namibian hake-directed bottom trawl fishery has been an important component of the Namibian fishing industry for decades. Due to the low selectivity of the bottom trawling method, bycatch has been a common feature of this hake-directed fishery. This study examines the spatial distribution of bycatch; monthly and inter-annual variations in the bycatch catch per unit effort (CPUE) (kg/hr); and factors that influence bycatch catch rate of Namibian hake bottom trawl bycatch species. The specific objectives were: to assess the spatial distribution of the bycatch species in the hake-directed bottom trawl fishery; to assess monthly variations of hake bycatch species/ species groups based on catch rate data that spans over 18 years; to assess inter-annual variations of hake bycatch species’/ species groups, catch rate over a period of 18 years and to identify and assess the relative importance of factors (latitude, month and year ) that best explain the variation in catch rates of hake bottom trawl bycatch species. Data were extracted from the database: Fisheries Information Management System (FIMS) at Namibia’s Ministry of Fisheries and Marine Resources, where all commercial information is captured and stored by scientists. The data extracted and used in the study was from 1997-2014. The study covered the whole fishing grounds within the Namibian Economic Exclusive Zone (EEZ), from the Kunene River to the Orange River. Data analysis methods used in this study included: mapping and density plots; Hierarchical Cluster Analysis (HCA); Multidimensional Scaling (MDS); Similarity Percentage (SIMPER) and Analysis of similarity (ANOSIM) to tackle the following objectives: spatial distribution of bycatch and monthly variations of the Namibian hake bottom trawl bycatch species. These analyses are used to find natural groupings and to give statistical significance in groups. In addition, Generalized Additive Models (GAMs) analyses are used for: monthly and inter-annual variations in the bycatch catch per unit effort (CPUE) (kg/hr); and for the significance of factors that influence bycatch catch rates of Namibian hake bottom trawl bycatch species. These analyses are done by incorporating various factors (latitude, month and year) that may have had an influence on bycatch. In this study twenty-four species are recorded as bycatch in the hake-fishery, with a combined catch amounting to about 9120.90 metric tons for the period 1997-2014. Among all bycatch species, the species that had the highest total catch rate was ribbonfish (Trachipterus trachypterus), while blacktail (Diplodus capensis) was the species with the lowest catch rates in the study period. Most bycatch species were caught along the entire Namibian coast, with some species having higher catch rates in the Northern (17°S - 20°59’59” S), Central (21°S - 24°59’59”) and the majority in Southern regions southern (25°S - 29°59’59”). Blacktail (Diplodus capensis) and silver kob (Argyrosomus inodorous) were only encountered in the Northern parts of Namibia while, yellowfin tuna (Thunnus albacores) was only caught in Northern and Central Namibia. Three major bycatch assemblages/groups were identified along latitudes. The three bycatch groups were mostly distributed at around 29°S (A), 22°S (B) and 17°S (C). Kingklip (Genypterus capensis), west coast sole (Austroglossus microlepsis), species belonging to the family Raja (Skates) and large-eye dentex (Dentex macrophthalmus) were the species that contributed most to the dissimilarity in groups for spatial distribution for A, B and C respectively. Variations in monthly CPUE were observed in all bycatch species. Some species had noticeable variations in the trends increasing /decreasing. CPUE was explained by various factors: latitude; month and year that had an influence on it. This was done for all bycatch species. The common factor that influenced the CPUE of most species was latitude while month and year had the least influence on CPUEs of species. Overall, this study shows that bycatch varies considerably between different species. It also shows that hake fishing has the potential to negatively influence the functioning of the Benguela ecosystem on the basis of the number of species that it has an influence on. Bycatch management measures will, therefore, need to be species-specific in order to tackle specific factors that may have an influence on the different bycatch species. Each species has an influence on the structure and function of food web and the ecosystem at large.
Spatial distribution and diversity of soft-bottom benthic invertebrates from demersal trawl surveys off the coast of Namibia
(University of Namibia, 2022) Mateus, Ndamononghenda Lipuleni
This study assessed the spatial distribution and diversity of soft-bottom benthic invertebrates collected during the Ministry of Fisheries and Marine Resources' annual monkfish, hake, and deep-sea red crab demersal surveys for the years 2018-2020, along the northern, central, and southern coast of Namibia. The study aimed to determine the spatial distribution and diverse composition of soft-bottom benthic invertebrates as affected by environmental factors such as depth, latitude, and temperature. 324 trawls were sampled, resulting in the identification of 130 taxa belonging to 17 classes, namely Arthropoda (44 species), Mollusca (33 species), Echinodermata (28 species), Cnidaria (23 species), Porifera (5 species), Annelida (4 species), Chordata (2 species), Sipuncula (1 species) and Bryozoa (1 species). The diversity of benthic invertebrate taxa along Namibia's coast was found to increase in a south-to-north direction. High abundances of arthropods were observed in the northern region, whilst ascidians dominated the central region, and Brissopsis lyrifera capensis and Suberites dandelenae in the south. In terms of depth, relatively high taxa richness was observed between the inner and middle shelf (101 - 400 m). Benthic invertebrate diversity distribution across regions and depths varied for each survey. For the monkfish survey, diversity was significantly different across regions (Kruskal test = 5.79; df = 2; SD = 0.44; p < 0.01) but was not significant across depths (Kruskal test = 1.23; df = 7; SD = 0.44; p = 0.36). From the hake survey, diversity was significant across both, region (Kruskal test = 35.24; df = 2; SD = 0.55; p < 0.001) and depth (Kruskal test = 17.78; df = 6; SD = 0.55; p < 0.007). The crab survey, diversity overlap across the regions (Kruskal test = 4.46; df = 3; SD = 0.33; p = 0.32) and depths (Kruskal test = 6.36; df = 3; SD = 0.33; p = 0.95). x The PERMANOVA test revealed a significant difference in the community structure across regions (Pseudo-F = 9.09, p < 0.001) and depths (Pseudo-F = 10.34, p < 0.001). DistLM of the environmental factors showed a weak correlation with the benthic invertebrate assemblage overall AICs reveal depth and latitude were strong predictor variables in the taxa assemblage. The study provides important information on taxonomic composition, spatial distribution, and abundance of benthic invertebrates along the Namibian coast. This thesis contributes a comprehensive assessment of offshore benthic invertebrate community structure and can be used to further inform marine conservation and spatial planning assessments along the Namibian coast.
Spatial, seasonal, interannual variability and long-term trends in the diet of Cape fur seals along the Namibian coast (1994 – 2018)
(University of Namibia, 2022) Mwaala, Diina N.
To fully understand how the Cape fur seal population responds to changing ecological and environmental conditions, and be able to comprehend their functional roles in the ecosystem, a thorough understanding of their diet composition, seasonality, interannual variation and long-term trends, is necessary. The diet of the Namibian Cape fur seal population was studied from prey remnants recovered across a 25-year time-series (1994 – 2018) of monthly scat samples (n = 1270) collected at various colonies situated along the entire Namibian coastline. Colonies in close proximity had similar prey matrices, and all colony samples were grouped into four geographical regions, namely the Far North (FN), Central North (CN), Central South (CS) and Far South (FS) regions. The study found that bearded goby Sufflogobius bibarbatus was the most abundant prey species at all four regions. This was followed by Cape horse mackerel Trachurus capensis in northern Namibia (CN and FN) and the lanternfish Lampanyctodes hectoris in southern Namibia (CS and FS). Cape hake Merluccius capensis was the third most abundant prey species in three regions, but negligible in the Far North. All the other prey species combined contributed less than 7 % of the total numerical diet composition. The monthly contribution of the three main prey species varied in all regions, but, bearded goby was dominant during all months in the Far North and Far South regions. The study indicated that the bearded goby abundances were less variable in relation to other species in all the regions. However, the bearded goby seemed to be the least preferred prey. This is because when Cape horse mackerel or lanternfish were available, the Cape fur seals readily switched and fed more on those than the more abundant bearded goby. There were significant interannual variations in the contribution of the first two most important prey species in each region, being negatively correlated to each other (r = -0.84, n = 10, P < 0.002; r = -0.62, n = 25, P < 0.001) for bearded goby and Cape horse mackerel in the FN and CN respectively, (r = -0.52, n = 25, P < 0.007; r = -0.63, n = 22, P < 0.002) for bearded goby and lanternfish in the CS and FS regions, respectively. The spatial, seasonal andlong-term variations in the contributions of the main prey species were related to the geographical distribution, abundances as well as the calorific values of each prey species (the bearded goby has the least calorific content of the considered species). There was a long-term increase in the proportion of the bearded goby in the diet of the Cape fur seals in the Central North and Far South regions (t = 3.21, n = 10, p < 0.001; t = 4.06, n = 22, P = 0.001), an increase in Cape horse mackerel in the Far North region (t = -5.01, n = 10, P < 0.001) and a decline in lanternfish (t = -2.19, n = 22, P = 0.04) in the Far South region. Lanternfish and bearded goby are currently not exploited in Namibia. With minimal pressure from fishing, their variability in the diet is thus likely related to their natural variability in abundance, with possible alterations due to changing environmental conditions affecting their abundance directly or their availability to the fur seals through depth distribution. The study indicated that the abundance of the lanternfish declinedwhereas that of the bearded goby increased in southern Namibia (CS and FS). Therefore, to understand the decline in lanternfish abundance in the Cape fur seal diet in southern Namibia, the study further investigated the link between its discrete monthly and long term diet contribution and to the coastal upwelling index and sea surface temperature recorded across the same timeframe in the same area. There was a significant positive correlation between the discrete monthly lanternfish abundance and coastal upwelling in the Central South region (r = 0.41, n = 90, P < 0.001) and a significant negative correlation between the discrete monthly lanternfish abundance and in situ SSTs in both the Central South (r = -0.27, n = 300, P < 0.001) and Far South region (r = -0.27, n = 300, P < 0.001). The long-term monthly lanternfish abundances showed significant correlations with the SSTs in the Far South region (r = -0.60, n = 12, P = 0.04). This was the first attempt to relate the abundance of the lanternfish in the diet of the Cape fur seal to fluctuations in the coastal upwelling and sea surface temperature in Namibia. These correlations provided a view of how climate variability affected the diet of higher trophic levels. Furthermore, they revealed that the diet composition of Cape fur seals inferred from scat sampling do not only help us understand their dietary composition and variations over time, but also contribute to better understanding the underlying causes of abundance fluctuations - especially for species that are currently commercially unexploited (and thus not actively assessed and/or monitored in Namibia). The knowledge assimilated by this study therefore helps in better understanding the relative importance of prey species, especially the non-commercially important species to the diet of a top predator in a functional ecosystem, as well as insights into the relationships that these species have with environmental variables such as coastal upwelling and sea surface temperature.
Climate-growth relationships of Argyrosomus spp. found along the Namibian coast
(University of Namibia, 2024) Jagger, Charmaine Eleonora
Understanding how fish growth responds to changing environments is critical for monitoring and predicting the impacts of climate change on fish populations. The ocean warming hotspot in the northern part of the northern Benguela provides an excellent opportunity to examine the responses of fishes to rapid environmental change. This research focused on the congeneric Argyrosomus inodorus and Argyrosomus coronus as these are commercially and recreationally important fishery species in Namibia. Standard otolith age determination techniques were used to determine if the age and growth of A. inodorus had changed from the 1990s to 2019. The results showed that there was a significant difference (Likelihood-ratio chi square test, χ 2 = 33.36, df = 4, p < 0.001) for the historical (1994-1996) and contemporary (2018-2019) period, thus indicating that A. inodorus stock used to grow older and faster in the historical period when compared to the contemporary. Kruskal Wallis test showed that there was a significant difference between A. inodorus and A. coronus lengths at age 4 (H (1) = 8.264, p = 0.004 < 0.05). The linear mixed-effects models fitted to the A. inodorus otolith biochronology indicated that a best linear unbiased predictor (BLUP) of growth was significantly positively correlated (r = 0.580, n = 25, p < 0.05) with mean SSTs (July, year of formation) and significantly negatively correlated (r = -0.473, n = 25, p < 0.05) with mean spring-summer SSTs (October y-1 to December y-1) in the area 20-24◦S, 12-14◦E. Thus, faster annual growth was observed during cooler periods (above 15℃) and slower growth was noted when exposed to warmer temperatures >17℃. These findings suggest that changes in A. inodorus growth was associated with a change in temperature and that its growth and productivity will be negatively affected with the increasing frequency of extreme climate events and ocean warming

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