Deep scattering layers (DSL) in oligotrophic systems are typically comprised of a variety of coexisting organisms, including fish, zooplankton, jellyfish, and squid, and hence there is uncertainty about the proportion of the acoustic backscatter in these layers that can be attributed to mesopelagic fish. Here, acoustic targets were classified using a multi-frequency acoustic classification algorithm based on data collected in the Indian Ocean at 18, 38, 70, and 120 kHz during three acoustic surveys in 2018. Frequency-dependent backscattering strength information ( increment Sv) was extracted from acoustic data that coincided with trawl hauls dominated by mesopelagic fish. Five increment Sv frequency pairs were used to separate the acoustic backscattering into three broad scattering categories, i.e. mesopelagic fish, crustacean-like/tunicates, and squids/others. Results indicated that the DSL is highly diverse and dominated by mesopelagic fish, with average densities at a regional scale ranging from 12.0 (+/- 10.9) to 26.0 (+/- 21.7) g m-2, and proportions of the acoustic backscatter attributed to mesopelagic fish ranging from 0.5 to 0.7. These estimates are generally lower than previously estimated for other regions of the Indian Ocean. The situation may well be similar elsewhere, particularly in oligotrophic systems, with potential ramifications for global mesopelagic fish biomass estimates.

On the western shore of the semi-enclosed coral-reef rich Red Sea, the 850 km coastline of the Red Sea State of the Republic of Sudan provides livelihoods to artisanal fishers, but the present state of the living natural resources and the impact of fisheries are poorly known. To provide a baseline on the biodiversity and fish abundance three fisheries research surveys spanning the entire Sudanese coast were carried out in 2012-13 designed around the seven Sudanese fisheries management areas. Baited traps and gillnets were employed to sample the various reef habitats and fish assemblages from inshore to deeper outer reef archipelagos. The highest species richness, functional diversity, as well and the highest catch rates with both traps and gillnets were observed in the protected Dungonab Bay area in the north, while the management area closest to the main population center along the coast - Port Sudan - showed the lowest levels of biodiversity and catch rates. The Dungonab bay area and adjacent northern areas therefore seem more pristine than areas closer to the main human population center. Thus the present study has provides a necessary knowledge baseline and highlights the opportunity for establishing effective ecosystem-based management before the resources and habitats are irreversibly impacted.

The semi-enclosed Red Sea harbours one of the longest coral-reef ecosystems on the planet. The ≈ 850 km section of the western shore, comprising the coastline of the Red Sea State of the Republic of Sudan, has however been sparsely studied. Sudan’s coral reef fishery provides livelihoods to fishers and business opportunities by means of local and regional trade, however, the knowledge level of the state of the natural resources and the impacts of fisheries are poorly known. Here we report the results from the first three comprehensive fisheries research surveys spanning the entire Sudanese coast in 2012-13, representing a new baseline for the western coast fisheries resources. The surveys covered the entire coast from inshore down to about 150 m bottom depth using a combination of baited traps, gillnets and handlines to sample the various reef habitats and fish assemblages. The results demonstrate a uniform latitudinal species distribution with peak catch per unit effort rates in and around the Dungonab Bay area in the north and the outer Suakin archipelago in the south. Functional diversity (Rao’s Q index) was found to be highest in and around the Dungonab Bay area, thus coming through as a regional hot-spot of biodiversity. The results form a baseline for future research and monitoring, thus representing key input for an ecosystem approach to management of Sudan’s coastal artisanal fisheries.

13 One of the most effective ways to alter catch and length compositions in trawls is to change the 14 selectivity of the gear, either by adjusting the mesh size or the addition of selective devices such as 15 sorting grids and selective netting panels. These changes are often introduced into the fishery in a top 16 down manner whereby fishermen are forced to comply with specific legislation. However, fishermen 17 have also introduced gear modifications that have contributed to improving species selectivity in trawls. 18 One of the simplest and most effective modifications that came from industry was the development and 19 introduction of twin and multi-rig trawls. Here we analyse catch rates of four target species, Norway 20 lobster (Nephrops norvegicus), cod (Gadus morhua), plaice (Pleuronectes platessa) and haddock 21 (Melanogrammus aeglefinus), to try and understand how the above mentioned change in gear design has 22 altered catch rates within the Danish demersal trawl fishery over the last 16 years (1997-2012). Results 23 showed that catch rates of Nephrops in twin trawls were significantly higher (1.89-2.03) than those in 24 single trawls. For cod, haddock and plaice there was no significant effect of gear type. The results are 25 discussed in relation to the CFP reform and the increasing importance of industry introduced gear 26 modifications. 27

Demersal fish, shrimp and cephalopod assemblages on the continental shelves and slopes off Angola, Namibia and the southern and western coasts of South Africa have been monitored in terms of fisheries-independent trawl surveys since the 1980s. The time series have provided vital input to stock assessments and are widely used in studies of ecology and biodiversity. The objectives of this study were to evaluate the technical specifications of the vessels and trawls used, to examine effects of modifications on catching efficiency, and to assess implications of these modifications over time. We find that the demersal trawl data collected in South Africa are not comparable with those of Namibia and Angola, and that the time series of Angola and Namibia contain inherent differences in terms of catchability of bottom dwellers. The introduction of smaller bobbins gear on the RV Dr. Fridtjof Nansen in 1994 increased the catchability of bottom-dwelling species, and catch rates of monkfish and sole were higher in surveys with commercial vessels than the RV Dr. Fridtjof Nansen. We recommend that temporal trends are interpreted with caution and that time series for the three countries are viewed in isolation.

Using long-term survey data, changes in demersal faunal communities in the Benguela Current Large Marine Ecosystem were analysed at community and population levels to provide a comparative overview of the occurrence and timing of regime shifts. For South Africa, the timing of a community-level shift observed in the early 1990s, and of a lesser shift observed in the mid-2000s, corresponded well with the results of other studies that showed environmental, community-level or population-level changes at similar times, suggesting that environmental forcing had played a role. Several population-level shifts were detected for Namibia; these and a regime shift in the overall community identified for this country corresponded well to the timing of severe environmental perturbations and an extensive regime shift in the pelagic ecosystem of this area. However, the interpretation of these shifts was confounded by changes in sampling gear; closer scrutiny of the types of species affected and the direction of shifts (increase/decrease) in relation to the timing and nature of sampling gear modifications, revealed that the observed shifts were potentially an artefact of gear changes. This highlighted the importance of accounting for changes in sampling protocols during the analysis and interpretation of long-term data. For Angola, a community level shift in the mid-2000s and population-level changes for a few species (mainly positive), could not have been influenced by gear changes which took place mainly before the onset of the time series under consideration. However, no clear environmental or anthropogenic changes that could have influenced these shifts were obvious.

The Norwegian Sea is a migration and feeding ground for fin whales (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae) in summer. During the last decade, significant structural changes in the prey community, including northerly expansion and movement in the distribution of pelagic fish species, have been reported from this ecosystem. However, little information on whale feeding ecology exists in the Norwegian Sea and surrounding waters. A total of 59 fin whales and 48 humpback whales were sighted during 864hof observation over an observation distance of about 8200 nmi (15,200km) in the Norwegian Sea from 15 July to 6 August 2006 and 2007. The fin whale group size, as mean (+/- SD), varied between one and five individuals (2.1 +/- 1.2 ind.) and humpback whale group size varied between one and six individuals (2.5 +/- 1.7 ind.). Fin- and humpback whales were observed mainly in the northern part of the study area, and were only found correlated with the presence of macro-zooplankton in cold Arctic water. Humpback whales were not correlated with the occurrence of adult Norwegian spring-spawning herring (Clupea harengus) except for the northernmost areas. Despite changes in the whale prey communities in the Norwegian Sea, no apparent changes in fin- or humpback whale distribution pattern could be found in our study compared to their observed summer distribution 10-15years ago.

Distribution and aggregation patterns of pelagic fish in marine ecosystems are diffi- cult to predict and understand, particularly since focussed studies are typically limited to single field efforts. Our study targeted coastal pre-spawning herring Clupea harengus in a small semi- enclosed coastal marine ecosystem, investigating their spatial and intra-school dynamic charac- teristics during the pre-spawning period over 6 consecutive years by means of hydro-acoustic sur- veying. Prior to spawning, the herring were consistently located at a specific site characterised by a deep trench that led to the deepest basin in the system and provided a potential escape route from predators. As the herring approached spawning, they moved in the direction of the spawning grounds, mimicking the long-distance spawning migration of Norwegian spring-spawning her- ring. There were consistent trends across years in the school characteristics demonstrating state- dependent schooling dynamics. Schools were found closer to the bottom and more densely packed during late than early pre-spawning. Furthermore, a higher diversity in maturity stage be - tween individual herring, assumed to be associated with stronger motivational conflicts, resulted in increased heterogeneity in school packing density. Our repeated investigations from a small site pinpoint some key behavioural traits in herring, trading off survival, energy saving and repro- ductive success.

Distributional change, expressed as range expansion or contraction, has been observed in many marine populations and related to changes in the environment. The extent of such distributional changes is also expected to increase in response to future climate change. The Benguela Current Large Marine Ecosystem (BCLME) which adjoins the south-western coast of Africa is a global marine hotspot with long-term warming occurring over a large area. The area is also an important centre of marine food production for three countries—South Africa, Namibia and Angola and is considered to be vulnerable to future climate change or increased climate variability. In this study we analysed change in distribution and range size of several demersal fish species in the BCLME over the period 1985–2010, including both commercial and non-commercial fish populations. Some of the observed changes in distribution and range size correspond to what is expected with increased warming whereas others appear to the contrary. Overall the results of the study highlight the complex nature of the response of fish population to climate change.

This study made use of distribution and abundance data of demersal fish and cephalopod species targeted during trawl surveys off Angola, Namibia and the west coast of South Africa, to determine species richness patterns including the location of diversity hotspots in the Benguela Current Large Marine Ecosystem. The reliability of alternative techniques for determining species richness patterns over the study domain, including geostatistical and non-geostatistical interpolation methods and regression type modelling, was tested using a cross-validation method. Generalized additive models were found to be the most effective method and were used to generate horizontal maps of species richness for different periods in each country. Despite changes in community structure that have been documented during the study period and which may be associated with climatic changes, this study showed the presence of consistently predictable hotspot areas over a 20-30-year study period (depending on country). The relationship between species richness and physical/environmental variables was inconsistent between countries, but generally hotspots of species richness were associated with greater depths and cooler bottom temperatures. Range shifts of species associated, for example, with warming of temperatures could conceivably affect the spatio-temporal persistence of hotspots in the long term.

There has been debate in the literature about whether jellyfish abundance has increased in the northern Benguela upwelling system, or not, over the past five decades and what impact they are having on pelagic fish. Here we review old expedition literature as well as more recent spatial and temporal patterns in distribution of jellyfish off Namibia at a number of different scales, using both published and previously unpublished data. Specifically, we have used data from fishery-dependent sources of both the demersal (359 638 trawls) and pelagic fisheries (11 324 purse-seine sets) that cover the period 1992–2006, supported by data from fishery-independent demersal (6 109 trawls) and pelagic trawls (1 817 trawls) from 1996 to 2006. Using frequency of capture as an index of abundance, it is clear that jellyfish are not randomly distributed within the northern Benguela ecosystem, but show specific areas of concentration that broadly reflect regional oceanography and the distribution of other zooplankton. Although jellyfish are present throughout the year, peaks in abundance are shown that often coincide with peaks in the spawning activity of fish of commercial importance. Interannual changes in jellyfish abundance observed from all sources do not agree, with some showing increases, others declines, and still others showing no change, which suggests caution should be exercised in their interpretation. Based on the multiple lines of evidence synthesised here, we conclude that jellyfish abundance has increased concomitant with a decline of pelagic fish stocks. We conclude that future recovery of the pelagic fishery off Namibia is likely to be considerably challenged because of significant overlaps in space and time between fish and jellyfish, and through the effects of competition and predation effects of jellyfish on fish.

Predator-prey interactions of mammals and their fish prey in marine ecosystems are rarely identified and recorded. We document for the first time the underwater behaviour of hunting dusky dolphins Lagenorhynchus obscurus and the responses of their prey, Cape horse mackerel Trachurus capensis, observed in open ocean waters off northern Namibia. Predator-prey interactions were monitored acoustically during a continuous period of 2 h with a split-beam scientific echo sounder, and the surface behaviour of the dolphins was observed from the ship's deck. In total 54 predator-prey events were observed (mean, 0.45 events min(-1)). The maximum burst speed during attack was 9.9 m s(-1), while the average attack speed was 3.4 m s(-1) (mean swimming speed: 1.4 m s(-1)). Dolphin traces were predominantly located underneath the fish aggregation (63% of the time), as the dolphins attacked the schools from underneath (mean depth: 120 m; maximum depth: 156 m). The dolphin target strength at 38 kHz was, on average, -31.5 dB (95% CI: -33.4 to -30.2 dB). The attacks caused immediate reactions in the Cape horse mackerel aggregations: subschools were forced towards the surface where they were herded into dense aggregations by the dolphins. Observed predator response patterns included 'Vacuole' (n = 21), 'Split' (n = 19), 'Bend' (n = 10) and 'Hourglass' (n = 4). Packing densities changed significantly before (0.4 fish m(-3)), during (2.0 fish m(-3)) and after (0.3 fish m(-3)) the dolphin attacks, and when predators burst into the fish aggregations, mean (+/- SD) intraschool packing densities of horse mackerel were significantly lower in front of the predators (0.33 +/- 0.17 fish m(-3)) than behind (6.65 +/- 1.76 fish m(-3)).

The fatty acid composition of the blubber of five dusky dolphins Lagenorhynchus obscurus and five Cape fur seals Arctocephalus pusillus pusillus from the northern Benguela ecosystem (South-East Atlantic) and their main prey was determined. Differences in fatty acid composition of the inner and outer blubber layer of the dolphins were substantial, with higher relative amounts of monoenic fatty acids with 14, 16 and 18 carbons in the outer layer and higher relative amounts of saturated, long-chain monounsaturated and polyunsaturated fatty acids in the inner layer. This stratification is similar to the general pattern that has been observed in a variety of marine mammals, but the degree of stratification is the highest yet observed, with the long-chained (n3) fatty acids being more than 20 times more abundant in the inner than in the outer layer. On the other hand, the seals' blubber consisted of only one uniform layer, densely supported by connective tissue. The whole-body fatty acid composition of the prey was species specific. The dolphins and seals had significantly different fatty acid composition of their blubber. In both species, the blubber fatty acid composition was different from the composition of the prey, indicating that a phylogenetic component is involved in the determination of the fatty acid composition of the blubber.

The assessment and the management of the Cunene horse mackerel in Angola rely heavily on abundance estimates from hydroacoustic surveys. Acoustic data collected from 1994 to 1999 were analysed to quantify diurnal variation in relative acoustic densities at 38 kHz. The nautical-area scattering coefficient (s(A), m(2) nautical mile(-2)) was characterized by clear day night differences: s(A) values recorded during the day were significantly higher (mean s(A): 135 m(2) nautical mile(-2)) than the corresponding night-time values (mean s(A) 83 m(2) nautical mile(-2)). This pattern is associated with differences in behaviour and horizontal and vertical distributions between day and night: by day, the fish school near the seabed, and by night, they move into the pelagic zone and disperse into widespread scattering layers. More than 40% of the total backscatter by day originated from the bottom 10 m, but at night this proportion decreased to <10%. The findings demonstrate considerable influences of behaviour and aggregation dynamics on acoustic measurements. Possible implications for the estimates of acoustic abundance are discussed in the light of the differences.

(Current Biology 16, R492–R493; July 11, 2006) The units in the y-axis label of Figure 1A, which shows historic fish landings in the Benguela (South East Atlantic Major Area 47, Western Coastal Subarea, Divisions 1.3 Cunene, 1.4 Cape Cross, and 1.5 Orange River) from data compiled by the United Nations Food and Agriculture Organization (FAO), are mislabeled as 106 tonnes instead of 105 tonnes. The text that refers to the figure (paragraph 2, lines 11–12) is similarly in error by a factor of 10 too high; fish landings actually fell from about 1.7 million metric tonnes to 0.1 million metric tonnes. This error has no bearing on the main thrust or conclusion of our paper—that jellyfish biomass in the Benguela now exceeds that of once prolific finfish—because the recent estimates of fish and jellyfish biomass we report are from our own observations and not our interpretation of FAO data. Jellyfish overtake fish in a heavily fished ecosystemLynam et al.Current BiologyJuly 11, 2006In BriefOver the past half century fishing has led globally to a reduction in the mean trophic level of commercially landed species, with a significant decline from large predatory fish toward plankton-eating pelagic species and low trophic-level invertebrates [1]. An implied endpoint of this ‘fishing down marine food webs’ is a proliferation of previously suppressed gelatinous plankton (jellyfish) [2] thriving on the food no longer consumed by fish. We report here that, in the heavily exploited northern Benguela off Namibia, a transition towards this endpoint has occurred, and jellyfish biomass (12.2 million tonnes (MT)) now exceeds the biomass of once-abundant fish (3.6 MT). Full-Text PDF Open Archive

We studied the influence of a surveying vessel on the behaviour of shallowly distributed local pre-spawning herring (Clupea harengus L.) in a protected area in the Trondheimsfjord (West Norway). The vessel passed an acoustic buoy at standard survey speed at a distance of 10–40m four times at night and seven times by day. Bottom depths ranged from 40 to 60m and a herring layer consisting mostly of pre-spawners was recorded from transducer depth (2.5m) down to 15–30m. The herring mass centre was deeper during vessel passage than during undisturbed reference periods, indicating a diving reaction. Backscattered echo energy also decreased during most passages, but the trend was less consistent than increased centre depth, especially during night-time passages. In summary, indications of vessel avoidance were weaker than those previously reported for herring in the same period. We interpret vessel avoidance as the reaction to a perceived threat, and the weak response was unexpected. Pre-spawning herring should act in order to maximise their chances of survival until reproduction and a strong reaction to perceived threats as documented from earlier studies was therefore expected. The shallowness of the location in combination with many gadoids close to the bottom, as observed acoustically, may have been important factors that attenuated the reaction of the fish to the vessel in our study.