Dr Fridtjof Nansen Research Articles

Present Status and Future Developmental Perspectives of Pole and Line Fishery in Sri Lanka

Present Status and Future Developmental Perspectives of Pole and Line Fishery in Sri Lanka

Diet and condition of juvenile deep-water hake Merluccius paradoxus on the west coast of southern Africa

The composition of the diet of juvenile deep-water hake Merluccius paradoxus is described and compared for three body-condition groups (below-average, average, and above-average). Diet was analysed using three metrics: occurrence, numeric and gravimetric. Juvenile hakes were caught in January and February of 2012 by means of 28 trawl deployments from the RV Dr Fridtjof Nansen between Orange Banks and Hondeklip Bay on the west coast of southern Africa at depths of 30-232 m. The 300 individuals analysed were a representative sample of all juveniles caught (3 114 fish), ranging in length from 52 to 205 mm (average length 107 mm). Their diet consisted of the euphausiid Euphausia lucens, the hyperiid amphipod Themisto gaudichaudii, the stomatopod Pterygosquilla armata capensis, and mesopelagic fishes Maurolicus muelleri and Lampanyctodes hectoris. The only significant difference in diet between the body-condition groups was for euphausiids and hyperiid amphipods in the numeric metric. A new method of assessing fish condition using otolith weight was tested. Because of the large variability in the data and small sample size, this method could not be applied effectively in this study. However, otolith weight has potential as a new fish condition metric when the variability can be lowered.

Ocean circulation over the Saya de Malha Bank in the South West Indian Ocean

The Saya de Malha Bank is one of the major banks of the Mascarene Plateau in the South West Indian Ocean. It is known for its unique ecosystem, remoteness (nearest island is about 300 km away) and complex oceanographic conditions. This study presents the results of a survey conducted in May 2018 on-board the R/V Dr Fridtjof Nansen, and aims to provide a descriptive overview of the interaction of the large-scale South Equatorial Current (SEC) over the shallow Saya de Malha Bank. The analysis of the current pattern revealed a two-layered structure of the current over the shallow topography of the bank compared to the vertically rigid-structure of the SEC throughflow in its deeper region. This two-layered flow consists of a surface layer and a sub-thermocline layer. The top layer flow, carrying the lower salinity mass (Tropical Surface Water) is driven by the Ekman dynamics observed in the southern hemisphere whereas the sub-thermocline current layer is most likely governed by the tidal and internal wave dynamics generated by the topographic relief of the bank.

Grain size analysis and total organic matter and carbonate contents of sediments on Saya de Malha and Nazareth Banks

The Mascarene Plateau is one of the least studied regions of the Western Indian Ocean. The aim of this paper is to describe the qualitative grain size characteristics and the total organic matter and carbonate contents of sediments on the two underwater banks known as the Saya de Malha and Nazareth Banks. Sediment samples were collected during the R/V Dr Fridtjof Nansen expedition in 2018, using a Video-Assisted Multi-Sampler (VAMS). Sieving techniques and GRADISTAT software were used for granulometric analysis and grain size statistics, respectively. Total organic matter and carbonate contents were determined with the sequential weight loss on ignition method. Grain size parameters from 15 stations (depths ranging from 37 to 381 m) are reported in this study. The results showed that sand was the dominant grain size on both banks. Eleven of the 15 stations had a mean grain size of coarse sand, three stations had a mean size of medium sand and one station had fine sand. The average total organic matter and carbonate contents on the Banks ranged from 2.1 to 7.7 % and from 53.9 to 58.3 %, respectively. This study provides baseline information that will help to classify benthic habitats and better understand the ecosystem functioning and dynamics on of the Saya de Malha and Nazareth Banks.

Grammatonotus bianchi, a new species of splendid perch (Percoidei: Callanthiidae) from Myanmar, northeastern Indian Ocean.

A new splendid perch, Grammatonotus bianchi sp. nov. is described on the basis of two specimens (45.968.7 mm SL) collected at 184 m depth in the Andaman Sea off the coast of Myanmar during bottom surveys conducted by the R/V Dr Fridtjof Nansen in 2018. The new species can be distinguished from all congeners by its large head (37.738.6% SL), large orbit (14.415.3% SL), caudal-fin shape, and fresh coloration. A key to Indian Ocean species of Grammatonotus is provided.

Description of two new species of sandlances, genus Bleekeria (Perciformes, Ammodytidae) from the Andaman Sea (northeastern Indian Ocean)

Two new species of Bleekeria Günther, 1862 are described from specimens collected in the Andaman Sea off the coast of Myanmar during bottom surveys conducted by the R/V Dr Fridtjof Nansen in 2015 and 2018. They are distinguished from each other and from congeners by a combination of morphological and meristic characters as well as fin coloration and genetic variance. Bleekeria albicauda sp. nov. has pelvic fins, 40–41 dorsal-fin rays, 54–55 total vertebrae, no teeth in jaws, 4 scale rows between dorsal-fin origin and lateral line, a single row of about 10 scales on mid-upper part of opercle, scales on central part of body clearly shorter than their height, caudal fin with white upper and lower lobes when fresh (unique within the genus). Bleekeria nigrilinea sp. nov. has no pelvic fins, 37–39 dorsal-fin rays, 49–50 total vertebrae, 2½ scale rows between dorsal-fin origin and lateral line (the smallest count within the genus with B. estuaria of Mozambique brackish water), 5–6 scales on mid-upper part of opercle arranged in a single row, scales on central part of body clearly longer than their height, upper and lower margins of caudal fin black when fresh (unique within the genus). The COI gene sequences of the two new species showed clear genetic divergence (pairwise K2P, >10 %) from Bleekeria estuaria Randall & Ida, 2014 and Bleekeria mitsukurii (Jordan & Evermann, 1902). A key to the species of Bleekeria is provided.

Seasonal variations in the diversity and structure of decapod communities in the changing hydrological scenario of the northwest African upwelling

Multidisciplinary surveys conducted on-board RV Dr Fridtjof Nansen in the northwest Africa region during November 2011 and June 2012 allowed exploration for the first time of seasonal variations in diversity and structure of decapod communities inhabiting the changing hydrological scenario of the shelf and upper slope off Mauritania (20–750 m depth). Decapods were gathered with a commercial bottom trawl at 95 stations, 39 of which were visited in both surveys. In addition, 249 oceanographic profiles and 95 sediment samples were collected. Shelf and slope assemblages, respectively characterized by the dominance of benthic (63 species) and swimming decapods (37 species), were segregated at 100–150 m depth, coinciding with the shelf-break and the hydrological frontier separating the surface mixed layer from the deeper water masses. The assemblages showed a certain stability in their structure but significant seasonal differences in their composition. Abundance and biomass, and in particular specific richness, were strongly correlated with depth in the case of swimming decapods, which were clearly dominant during the warm season, especially in the southern zone. The most significant seasonal variations were mainly due to the Penaeidae family and its associated species, Parapenaeus longirostris (Lucas, 1846). The latitudinal decreasing trend in upwelling permanence and productivity toward the south, responding to the strength and displacement of trade winds along the Mauritanian coast, which are observable at the surface and sea-bottom, were the main reason explaining the seasonal variations observed in the distribution patterns and assemblages structure of Mauritania decapods.

A new deep-bodied species of Antigonia (Teleostei, Antigoniidae) from the Andaman Sea (northeastern Indian Ocean).

A new species of boarfish, Antigonia emanuela, is described from two specimens collected in the Andaman Sea off the coast of Myanmar during bottom trawl surveys conducted by the R/V Dr Fridtjof Nansen in 2015 and 2018. It is unique within the genus in the combination of the deeply rhomboid body (1.1-1.2 times deeper than long), nine dorsal-fin spines, premaxilla strongly bent in middle, scales with deep apical crest armed with numerous spinules, soft dorsal, anal and caudal fins bright-yellow in life. Two other Antigonia species captured during these surveys are A. indica and A. ovalis, the latter species being a first report for the Andaman Sea.

Oxygen and temperature influence the distribution of deepwater Cape hake Merluccius paradoxus in the southern Benguela: a GAM analysis of a 10-year time-series

Generalised additive models (GAMs) were applied to survey data to assess the influence of dissolved oxygen, water temperature and year of sampling upon the presence/absence of small (≤15 cm TL), medium (16–34 cm TL) and large (≥35 cm TL) size classes of deepwater Cape hake Merluccius paradoxus captured off the west coast of South Africa. Data were obtained from surveys using the RV Dr Fridtjof Nansen conducted in 2003 and from 2005 to 2013 during summer (January–February). Among the variables investigated, oxygen was the most important for the small size class (juveniles), with both low and high constraints (two-sided, ‘just right’ option), whereas for the medium and large size classes the oxygen effects were one-sided (avoiding lows). This finding, in combination with other published information, suggests that the Orange Banks is a nursery ground for juvenile M. paradoxus and that the area covered by this nursery ground can vary with the optimal oxygen concentration. The temperature constraint was generally wider and weaker than that for oxygen, being two-sided for the small and medium hake and one-sided (avoiding highs) for the large hake. The medium hake displayed the greatest tolerance to the investigated variables, which resulted in the widest distribution for this size class. Temperature, oxygen and sampling year play an important role in determining the distribution of M. paradoxus, but details of the biology (life cycle) of the species, such as its pelagic–demersal transition and associated movements, are no less important.

Smooth fan lobster Ibacus novemdentatus in the southwestern Indian Ocean: an overlooked fisheries resource?

Bottom-trawl data collected by the RV Dr Fridtjof Nansen off Mozambique (in 1990, 2007 and 2018) and off eastern South Africa (in 2018) were analysed to investigate the distribution, abundance and size composition of the smooth fan lobster Ibacus novemdentatus as a potential fisheries resource. The lobsters occurred shallower (depth 53–297 m) than traditional deep-water trawl grounds, were concentrated in ‘hotspots’ at the seaward edges of sand/mud banks at the Thukela Bank, Delagoa Bight and Sofala Bank, and exhibited segregated juvenile and adult populations. Three other high-value lobster species and several marketable teleost species occurred in trawl nets where I. novemdentatus was present. Further exploration by trawling should be considered to better define hotspots of adults of this species and to investigate the contribution that I. novemdentatus could make to mixed-species commercial catches.

Intercalibration of survey methods using paired fishing operations and log-Gaussian Cox processes

Abstract We present a statistical method for intercalibration of fishery surveys methods, i.e. determining the difference in catchability and size selectivity of two methods, such as trawl gears or vessels, based on data from paired fishing operations. The model estimates the selectivity ratios in each length class by modelling the size distribution of the underlying population at each station and the size-structured clustering of fish at small temporal and spatial scales. The model allows for overdispersion and correlation between catch counts in neighbouring size classes. This is obtained by assuming Poisson-distributed catch numbers conditional on unobserved log-Gaussian variables, i.e. the catch is modelled using log-Gaussian Cox processes. We apply the method to catches of hake (Merluccius paradoxus and M. capensis) in 341 paired trawl hauls performed by two different vessels, viz. the RV Dr Fridtjof Nansen and the FV Blue Sea, operating off the coast of Namibia. The results demonstrate that it is feasible to estimate the selectivity ratio in each size class, and to test statistically the hypothesis that the selectivity is independent of size or species. For the specific case, we find that differences between size classes and species are statistically significant.

Pseudanthias emma new species, with notes on a collection of anthiadine serranid fishes from off Myanmar (eastern Indian Ocean).

Recent exploratory trawling off the coast of Myanmar by the R/V Dr Fridtjof Nansen has resulted in the collection of six species of anthiadine serranid fishes. Four of the species are represented by specimen vouchers: Pseudanthias emma sp. nov., P. gibbosus (Klunzinger), Odontanthias rhodopeplus (Günther) and Plectranthias sp. 1. The remaining two species are represented only by photographs: Plectranthias sp. 2 and Sacura sanguinea Motomura, Yoshida Vilasri. Pseudanthias emma is described from the 107 mm SL holotype. It is distinguished from congeners in having the following combination of characters: dorsal rays X,16; pectoral rays 18; lateral-line scales 42; third dorsal-fin spine longest, prolonged; no papillae on posterior margin of orbit; soft part of dorsal with low scaly sheath; subopercle and interopercle indistinctly serrated; caudal fin weakly concave centrally, with elongate filaments extending from second to uppermost and second to lowermost branched rays. Plectranthias sp. 1 is a probable new species closely allied to P. sagamiensis (Katayama), from which it appears to differ in having fewer segmented dorsal rays and fewer rows of cheek scales. Plectranthias sp. 2 is a probable new species closely allied to P. alcocki Bineesh, Akhilesh, Gopalakrishnan Jena, from which it appears to differ in lacking a large black spot on the operculum and dusky ventral spot on the abdomen. New character, habitat and distribution information is provided for all six species. Pseudanthias vizagensis Krishna, Rao Venu is suggested as a probable junior synonym of P. pillai Heemstra Akhilesh.

Composition, distribution and abundance of zooplankton and ichthyoplankton along the Senegal-Guinea maritime zone (West Africa)

A study of zooplankton and ichthyoplankton was carried out from samples collected along the Senegal-Guinea maritime area on board the Norwegian research vessel 'Dr Fridtjof Nansen' in November 2011. Nineteen groups of zooplankton were identified with a dominance of copepods representing 68.5%. The Copepods included 35 species, where Paracalanus parvus was dominant (21.5%). As to ichthyoplankton, 29 fish families were encountered, and Clupeidae were dominant (35.8%). They were represented by Sardinella spp (98.0%) and by Sardina pilchardus (2.0%). An area of high density of copepods (5250 individuals/m3) was located at the mouth of the Gambia River. Two areas of high densities of fish eggs were identified in the waters off Senegal and Guinea; one was characterized by high surface temperature and salinity (29 °C and 35) and moderately oxygenated (3.8 ml/l), while the other was slightly warm (25.5–27.5 °C), less salty (31.5) and highly oxygenated (4.8 ml/l). Areas of high zooplankton biomass correspond to the zones of high dissolved oxygen concentration (4–4.5 ml/l) of low salinity (31.5–34.5). This pattern is supported by the results of the PCA, through the station PL4.

Diversity and systematics of philinid snails (Gastropoda: Cephalaspidea) in West Africa with remarks on the biogeography of the region

West Africa is often considered one of the least studied regions of the world concerning marine biodiversity. Knowledge about the philinid snails of the region has largely been based on shells, but shells can be insufficient to discriminate between species. In this paper, we review the diversity and revise the systematics of the West African species of Philinidae sensu lato based on a comprehensive literature review and the study of shells, anatomy and DNA sequence data of novel specimens obtained by the R/V Dr Fridtjof Nansen (2005–2012) between Morocco and Angola. We use scanning electron microscopy together with a cytochrome c oxidase subunit I molecular phylogeny and molecular species delimitation methods (ABGD and SDP Geneious Plug In) to define species. Twenty species (19 formally named) were found to occur in the region, including four new species we describe here: Laona nanseni sp. nov., Philine cerebralis sp. nov., Philine schrammi sp. nov. and Spiniphiline caboverdensis sp. nov. We identified a complex of four cryptic species under the name P. quadripartita (P. guineensis, P. quadripartita, Philine schrammi sp. nov., Philine sp.). A biogeographical break is observed around Cape Verde and the Sahelian upwelling system coinciding with the West African Transition Province; eight species have their northern geographical limit or are restricted to this Province, while ten species have their southern limit here or just further north. Only one species seems to span across this biogeographical breaking point (P. scabra), but we speculate that this might result from taxonomic misidentifications.

Life cycle of hake and likely management implications

Despite its economic and social importance for Namibia and South Africa, limited documented information exists regarding key aspects of the biology of deep-water hake, including its life cycle. This study utilizes data collected through the demersal surveys of the R/V Dr Fridtjof Nansen in South Africa and F/V Blue Sea 1 in Namibia to describe the migratory patterns of deep-water hake in space and time. Furthermore the study investigates aspects of the life cycle of this important species in the Benguela region. Results show that deep-water hake spawns between the western Agulhas Bank and Elands Bay in South Africa with the main nursery ground between Hondeklip Bay and the northern tip of Orange Banks. Deep-water hake in Namibia (up to the Kunene River) and along the south coast of South Africa (eastwards to Port Alfred) originate from these grounds, and undertake long-range migrations across latitudes and longitudes, respectively. This hypothesis is supported by the finding that spawning has not been observed in Namibia and there are no small juveniles along the South African south coast from the eastern border of the Agulhas Bank. The proposed pattern implies an interconnection between the Namibian and the South African components of the stock and the consequent need for a revision of the present management regime based on the assumption of stocks confined within the respective national jurisdictions. This study has used length frequency distributions in space and time in order to investigate the life cycle, in terms of origin, movement and population structure in particular, an approach that may also be useful for other widely distributed species.

First record of Tanaidacea (Crustacea) from a deep-sea coral reef in the Gulf of Guinea.

Four undescribed species of Tanaidacea were discovered during a baseline monitoring program conducted off the coast of Ghana. The specimens came from a deep-water reef largely composed of the ahermatypic coral, Lophelia pertusa. The tanaidacean material was collected during November 2012 onboard the RV Dr Fridtjof Nansen using a van Veen grab in depths of between 375 and 386 m. Three of the new species described herein are tanaidomorphans belonging to the genera Bathyleptochelia (Leptocheliidae), Pseudotanais (Pseudotanaidae) and Cryptocopoides (Cryptocopidae). The fourth species, an apseudomorphan, belongs to Calozodion (Metapseudidae), a genus hitherto known only from shallow waters (<200 m). This report constitutes the first records of tanaidaceans from a deep-sea Lophelia reef.

Multiregional Periodic Matrix for Modeling the Population Dynamics of Sardine (Sardina pilchardus) Along the Moroccan Atlantic Coast: Management Elements for Fisheries

In this paper, we present a deterministic time discrete mathematical model based on multiregional periodic matrices to describe the dynamics of Sardina pilchardus in the Central Atlantic area of the Moroccan coast. This model deals with two stages (immature and mature) and three spatial zones where sardines are supposed to migrate from one zone to another. The population dynamics is described by an autonomous recurrence equation N(t + 1) = A.N(t), where A is a positive matrix whose entries are estimated using data collected during biannual acoustic surveys carried out from 2001 to 2003 onboard the Norwegian research vessel "Dr Fridtjof Nansen". The dominant eigenvalue lambda of A that gives the long-term growth rate of fish population is smaller than one. This agrees with the stock decrease observed in the data collected. We show that lambda is highly sensitive to the recruitment rate and much less sensitive to the reproduction rate. These results can clearly be used to define an efficient scenario in order to fight for instance against a stock decrease.

Observations on the near-surface behaviour of sardinella schools in Angolan waters

The schooling dynamics and swimming behaviour of sardinella Sardinella maderensis and S. aurita schooling near the surface in Angolan waters were recorded visually, using a calibrated echo integration system and a 95kHz high resolution sonar on board RV Dr Fridtjof Nansen during three cruises; July 1996, May 1997 and May 1998. The mean packing density of the sardinella schools was about 3 fish m−3, but this figure varied by two orders of magnitude among the schools. There was a linear relationship between the area and the biomass of the schools, which can be used in future surveys to convert recordings of school area by camera, lidar or sonar to school biomass. The sardinella schools swam at an average horizontal speed of 1.24m s−1, mostly towards or away from the coast. The schools seemed to be feeding near the surface and behaved rather dynamically. Intra-school events such as change of shape, reorganisation, splitting and leaving occurred every 2.4 minutes on average. Inter-school events such as approach and join was every 14.2 minutes on average. The schools were visible on the surface in the morning and in the afternoon, but seemed remarkably undisturbed by predating birds, fish and seals during the day. It is suggested that the low predation activity near the surface enables the sardinella to exploit plankton prey that is concentrated at surface fronts of internal waves and along the convergence lines of coastal and oceanic currents.

Precision in systematic trawl surveys as assessed from replicate sampling by parallel trawling off Namibia

Following independence of Namibia in 1990, abundance of that country's hake stocks was monitored by trawl surveys conducted by the Norwegian F.R.V. Dr Fridtjof Nansen using a systematic survey design. Precision from such designs is considered to be better than with a random design, but it cannot be quantified through standard analysis of variance. In 1998 and 1999, the trawl surveys were duplicated in full using a commercial trawler that operated in parallel with the Dr Fridtjof Nansen . Both vessels had the same fishing gear and rigging. In both years, biomass and distribution patterns estimated by the vessels were similar. The paired datasets collected from the surveys were used to analyse variability in the point samples. Absolute differences between pairs of catches were, in general, proportional to the catch, but they varied randomly following a normal distribution around the catch level. Local variability was analysed as random noise, modelled and later reapplied on single vessel data series to evaluate the effect of sample size and replicates on the survey mean. In a survey with 200 stations, local sample noise accounted for about 4% of the variability in the survey mean. Alternatively, running series of simulated surveys by bootstrapping on pairs of catch data gave similar results, when a small systematic vessel effect was adjusted for. The main statistical techniques applied were less susceptible to outlier catches than straightforward correlations or regressions and could therefore, perhaps with some advantage, also be used to estimate the vessel factor when intercalibrating trawl survey vessels. Keywords: intercalibration, point precision, replicate sampling, trawl surveys African Journal of Marine Science 2001, 23: 385–396

Dynamics of the pelagic component of Namibian hake stocks

A study was undertaken to investigate the spatial, diurnal and interannual dynamics of the pelagic component of Namibian hake stocks. Data were derived from trawl surveys conducted on board the R.V. Dr Fridtjof Nansen during the period 1991–1999. In addition to trawl surveys, acoustic methods were applied concurrently to quantify fish biomass off the bottom (above the headline). The pelagic component of hake was variable in both time and space – it was denser in the north and at depths of 200–500 m. Pelagic densities also correlated well with the density of adult fish (&#8805 30 cm) at the bottom, but not with that of young hake (>18 and Keywords: acoustic correction, bottom trawl surveys, Cape hake, vertical migration African Journal of Marine Science 2001, 23: 337–346