Distribution patterns,stock size and life-history strategies of Cape horse mackerel Trachurus trachurus capensis,based on bottom trawl and acoustic surveys

Research surveys of Cape horse mackerel Trachurus trachurus capensis abundance on the south coast of South Africa are complicated because changes in the species' vertical and horizontal distribution limit the value of stock assessments based a single survey method. Annual bottom trawl surveys conducted in spring provide estimates of the abundance of fish close to the bottom over trawlable grounds. Between 1991 and 1994, hydroacoustic surveys conducted in spring have been used to estimate the pelagic portion of the stock, as well as the portion over untrawlable grounds. These two research datasets, as well as data from purse-seine, mid water and bottom trawl commercial landings, are reviewed to elucidate distribution patterns of horse mackerel and their migratory and spawning strategies. The problems and advantages of bottom trawl and acoustic surveys are discussed in the context of fluctuations in estimates of the size of the stock between 1991 and 1994 and the prevailing environmental conditions. It is concluded that combined acoustic and bottom trawl surveys are the only effective means of surveying horse mackerel, and that effort should be concentrated east of 22°E to assess the spawner stock. It is suggested that research effort directed at improving understanding of exchanges between West Coast (including Namibia) and South Coast population of horse mackerel, as well as of the role of vertical migrations in modulating these exchanges, would be beneficial.     

Vertical migration,catchability and acoustic assessment of semi-pelagic Cape horse mackerel Trachurus trachurus capensis in the southern Benguela

The vertical distribution and migration of the Cape horse mackerel Trachurus trachurus capensis were investigated on the basis of midwater and bottom trawl collections taken over diel cycles at two sites on South Africa's south coast. Attempts were also made to investigate the relationship between bottom trawl catch rates and acoustic abundance estimates. During the first cycle, horse mackerel were dispersed in midwater during the night, and towards sunrise they coalesced into schools and migrated to the seabed where they remained throughout the day. During that cycle, horse mackerel did not appear to feed, despite the prevalence of potential food, mainly Calanus copepods and euphausiid larvae, in the water column. In contrast, during the second cycle, horse mackerel remained near the seabed during the 24-h experiment, but they exhibited a marked feeding periodicity, with most feeding taking place in the late afternoon. They appeared to select for larger copepods in the water column. Several reasons are advanced to explain the difference in the vertical migratory behaviour of horse mackerel between the two sites, on the basis of the vertical distribution of their potential prey. Diel differences in acoustic estimates of horse mackerel abundance were significantly reduced after applying an acoustic deadzone correction factor. Trawl catches, however, could not be reconciled with acoustic estimates during the day, when horse mackerel were too close to the bottom for effective echo-integration.     

Variable spatial structure of schooling pelagic fish off Namibia: implications for acoustic surveys

Schools of horse mackerel Trachurus trachurus capensis, sardine Sardinops sagax and round herring Etrumeus whiteheadi were surveyed by conventional echo-integration along systematic transects. The transects were surveyed twice, in opposite directions. Clupeoid biomass was concentrated in a few dense aggregations, whereas horse mackerel were less dense and distributed over larger areas. The influence of aggregation patterns of the different species on the precision of the acoustic estimates was analysed with respect to spatial variability and diurnal effects. Isotropic variograms computed from values of acoustic back-scattering strength showed little or no structure for all three species. Indicator variograms showed relatively large structures (>15 miles) for all three species at the lowest cut-off level, confirming the influence of high values on the variograms. The autocorrelation range decreased with an increase in cut-off value for horse mackerel, whereas variograms of sardine and round herring showed no structure at larger cut-off values. There were large variances in estimated biomass for sardine (77%) and round herring (90%) between surveys, whereas biomass estimates of horse mackerel were relatively similar between surveys.     

Analysis of bycatch in the South African midwater trawl fishery for horse mackerel Trachurus capensis based on observer data

The South African midwater trawl fishery targets adult horse mackerel Trachurus capensis. The bulk of the catch is taken by a single freezer-trawler, the biggest fishing vessel operating in South African waters. As fishing takes place off the south coast in ecologically sensitive areas, there are concerns about the potential impacts of this fishing operation on non-target species. Fishing behaviour and bycatch of this fishery from 2004 to 2014 were investigated by analysing observer records with regard to catch composition, volume and temporal and spatial patterns. The midwater trawl fishery was estimated to have caught 25 415 tonnes annually, with a bycatch of 6.9% of the total catch, by weight. There are species overlaps with various fisheries, namely the demersal trawl, small-pelagic, line, shark longline and squid fisheries, yet the total bycatch estimates from this fishery are generally small relative to catches taken in the target fisheries. Bycatch species with the highest average annual catches were chub mackerel Scomber japonicus, redeye roundherring Etrumeus whiteheadi, ribbonfish Lepidopus caudatus and hake Merluccius spp. Large-fauna bycatch species included sunfish Mola mola as well as a number of CITES II- and IUCN-listed species, such as Cape fur seal Arctocephalus pusillus, dusky shark Carcharhinus obscurus, smooth hammerhead shark Sphyrna zygaena and thresher sharks Alopias spp. The 97.9% observer coverage is high and the 6.9% bycatch rate low compared to other South African fisheries; however, due to the large size of the individual hauls (average of 46.3 t), the average sampling rate of 1.56% is low. Our analyses suggest that bycatch in the South African midwater trawl fishery has been lower than in other South African fisheries and similar fisheries elsewhere, but due to the combination of high catch volumes and low sampling rates, estimation errors for rare species are high and there is a substantial risk of incidental unmonitored bycatch of rare large fauna and aggregations of small fauna. This could be mitigated by spatio-temporal management of this fishery, to avoid fishing in high-risk areas, and the introduction of an electronic monitoring programme.     

An overview of the living marine resources of Namibia

This paper gives an overview of the main living marine resources of Namibia. It focuses on the scientific research conducted during the past decade as input to the management of these resources. The distribution and habitats of the most important harvested species and the main seabird populations are briefly described and discussed. The life histories of the major exploited species are summarized, with emphasis on spatial and temporal spawning patterns, dispersal of early life stages, migration patterns of recruits and adults, and diet, the latter particularly as it relates to potential competition between species. A number of commercially important species, such as the hake Merluccius capensis and M. paradoxus, deep-sea red crab Chaceon maritae, West Coast rock lobster Jasus lalandii, skipjack tuna Katsuwonus pelamis, southern albacore Thunnus alalunga and to a lesser extent Cape horse mackerel Trachurus capensis, southern African sardine Sardinops sagax and Cape anchovy Engraulis capensis, are distributed across national boundaries, requiring regional cooperation in research and management. The history and current status of the major fisheries is discussed. Over the past 30–40 years total annual catches have declined from a peak of around 2 million tons in the late 1960s to less than a million tons in the 1990s. This decline has been due, mainly, to a collapse in the sardine stock in the late 1960s and 1970s, and a reduction in the catches of hake and horse mackerel under a conservative management strategy in the past decade. Changes in the abundance and distribution of commercially important species, as determined by acoustic and trawl surveys and catch-based analytical methods, are presented. The effect of major environmental anomalies on the distribution and abundance of the resources in recent years is discussed. The most dramatic anomaly in recent years was the wide-scale advection of low-oxygen water into the northern Benguela from the Angola Dome in 1994, and the subsequent Benguela Niño of 1995, which appear to have severely impacted the Namibian sardine population and many other resources. The present socio-economic value of the Namibian fishing industry is given together with the broad policy, legislation and formal structures for managing the living marine resources.     

The ecology, distribution, and abundance of midwater fishes of the Subarctic Pacific gyres

We describe the distribution and abundance of the midwater fish community, between 200 m and 500 m, in the North Pacific. The main area of interest was the Subarctic Pacific gyres, but we include species from the Bering Sea and the Sea of Okhotsk. There were 196 species identified in each gyre, 38 of which were common to both gyres. The most abundant species belong to the family Myctophidae and the most ecologically important myctophid probably is Stenobrachius leucopsarus. This species could have a biomass of approximately 21 million tonnes (t) in the Subarctic Pacific (including the Bering Sea and Sea of Okhotsk). S. leucopsarus is a small (about 8 cm maximum length) fish that lives up to 7 years. It is prey for a variety of other fishes, birds and mammals and may migrate into the mixing layer each evening where it feeds mainly on euphausiids and copepods.The total abundance of midwater fishes appears to be large relative to total catches of other fish in the same areas. The vertical migratory behaviour of some of the residents provides a mechanism to transfer production out of the mixing layer. The movement into the surface layer by some fishes at night indicates that dynamic changes occur in the midwater community between the day and night, and the ecosystem dynamics in the surface layer are different in the day and in the night. This behaviour and the huge biomass relative to commercial species means that the dynamics of fish communities in the Subarctic Pacific are complex and need to be studied over a 24 hour period. The large biomasses may eventually attract commercial interest, thus it is important to establish international, cooperative programs now to learn more about the dynamics of these populations and the relationships with other species.     

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.     

Bycatch in the gillnet and beach-seine fisheries in the Western Cape,South Africa,with implications for management

Interview questionnaires and access point surveys were conducted in order to describe and quantify the catch composition of the inshore net-fisheries in the Western Cape, South Africa. A total of 138 562 fish, representing 29 species from 20 families, was recorded in 141 monitored commercial gillnet fishing operations between February 1998 and October 1999. Numerically, the legal target species, harders Liza richardsonii, dominated the catches, contributing 94.87% of the total gillnet catch. Elf, Pomatomus saltatrix, horse mackerel Trachurustrachurus capensis, gurnard Chelidonichthys capensis and barbel Galeichthys feliceps were the most common bycatch species, and contributed 4.2% to the total catch numerically and occurred in 12–47% of the marine 44–64-mm gillnet catches that were monitored. Five species most frequently targeted by shore-anglers on the West Coast: galjoen Dichistius capensis, white stumpnose Rhabdosargus globiceps, hottentot Pachymetopon blochii, silver kob Argyrosomus inodorus and white steenbras Lithognathus lithognathus, also occurred in gillnet catches. Most of the bycatch consisting of immature, undersized fish that were often injured during entanglement and were not released alive. L. richardsonii also numerically dominated the beach-seine hauls that were monitored (>99%) with only four bycatch species being recorded in low numbers. Beach-seine questionnaire respondents, however, reported sporadic catches of at least 17 bycatch species, including occasional appreciable catches of the important linefish species L. lithognathus and A. inodorus.     

Stock assessment and management implications of horse mackerel (Trachurus japonicus) in Korean waters, based on the relationships between recruitment and the ocean environment

This study presents an example of horse mackerel (Trachurus japonicus) stock to demonstrate that marine environmental factors are important in stock assessment for the new Korean Total Allowable Catch (TAC)-based fisheries management system. The estimated survival rate (S) of horse mackerel ranged from 0.25 to 0.36. The instantaneous coefficient of natural mortality (M) was 0.48/year, and the age at first capture was 0.83 year. Annual biomass of horse mackerel in Korean waters was estimated by a biomass-based cohort analysis using annual catch in weight at age during 1965–1995. Yield-per-recruit and spawning biomass-per-recruit were estimated under various harvest strategies at F_max, F_0.1, F_30% and F_40%. A method for estimating acceptable biological catch (ABC) is proposed for dealing with the large differences in the quality and quantity of information and data available. Using recruitment of horse mackerel estimated from various spawner–recruitment relationship models combined with salinity, volume transport, and zooplankton biomass as environmental factors, the ABC under the best information available was estimated to range from 3100 to 3800 mt.     

Direct hydroacoustic observations of chokka squid Loligo vulgaris reynaudii spawning activity in deep water

Unusual and distinct hydroacoustic targets were observed in November 1996, May 1998 and November 1998 during routine pelagic biomass surveys off the south coast of South Africa. During the November 1996 survey, seven such targets were observed near the bottom at depths of 115–125 m, directly south of the traditional inshore spawning grounds of chokka squid Loligo vulgaris reynaudii at Cape St Francis. The targets were close to prominent seabed ridges and extended 30–40 m off the bottom. In May 1998, three similar targets were observed at depths 55–80 m off Plettenberg Bay, another well-known squid spawning site. The shallowest target was identified, by means of a midwater trawl, as a mixture of mature male and female chokka. During the November 1998 survey, nine similar targets were again observed on the squid spawning grounds at Cape St Francis, also adjacent to seabed ridges. Drawing on fisheries hydroacoustic experience and knowledge of chokka squid spawning behaviour, the targets are believed to be aggregations of spawning squid.     

Use of particle-size data to predict potential pelagic-fish yields of some southern African areas

A particle-size spectrum model is used to estimate standing stocks of some pelagic fish from measurements of phytoplankton chlorophyll in two exploited regions in southern Africa (southern Benguela and off South West Africa/Namibia) and in two unexploited regions (the Agulhas Bank and off the eastern Cape Province). The model is based on the assumption that equal biomasses occur in logarithmically equal size classes in the pelagic marine environment. Phytoplankton, with an equivalent spherical diameter ranging from 1 to 128 μm, occupy 21 size classes on the logarithmic scale. Two different size ranges are assumed for some commercial, pelagic fish species, equivalent to exploitation with two different purse-seine mesh sizes. A mesh of 12,7 mm would catch 8 size classes of pilchard, horse mackerel and anchovy whereas a mesh of 32 mm would catch only 3,3 size classes of pilchard and horse mackerel. From the model, the potential biomass of these commercial pelagic fish is estimated, after allowing for the presence of other commercial and non-commercial fish and other taxa in the exploited size range. Total pelagic fish production is estimated by assuming constant turnover rates of 1·y^?1 and 1,5·y^?1 when exploited with 32 and 12,7 mm mesh nets respectively. Consideration of the maximum and mean reported catches in the exploited areas indicates that only some 25 per cent of pelagic fish production is exploitable by man. On this basis, the unexploited Agulhas Bank region may yield some 400 000 metric tons (wet) of pelagic fish of the species considered, and the East Coast region some 90 000 tons. Exploitation in these regions cannot be recommended, because the Agulhas Bank is an important spawning ground for many pelagic species, and the fish in both regions probably act as a reserve buffer for the heavily exploited pelagic resource of the Western Cape.     

Diel effects on bottom-trawl survey catch rates of shallow- and deep-water Cape hakes Merluccius capensis and M. paradoxus off Namibia,using solar zenith angle

Diel patterns in survey trawl catches for the Cape hakes Merluccius capensis and M. paradoxus off Namibia were studied in order to examine the effect of diel bias on catchability, and its implication for survey abundance estimation and the consistency of the survey time-series. Catch rates (numbers per haul) by species and length from summer demersal biomass surveys conducted during the period 2002–2013 were used, together with a computation of the corresponding light-level data from which the solar zenith angles were obtained. Generalised additive models were fitted to assess the relationship between the catches and a number of explanatory variables. Significant covariates were zenith angle, depth and geographical position. The final models explained 78% and 59% of the variability in catch rates of M. capensis and M. paradoxus, respectively. For M. capensis, the response to zenith angle increased sharply for values above 100°, which represents the time between sunset and sunrise. For M. paradoxus there was a moderate increase in the response to zenith angle during the night. In cases where some fishing took place at night in shallow water, the survey results for M. capensis were more greatly affected than was the case for M. paradoxus, which is related to the different depth preference of the two species. Fishing in depths shallower than 400?m outside daylight hours should therefore be avoided in order to reduce bias and ensure consistency in abundance estimates from surveys.     

The distribution and abundance of seven commercial trawlfish from the Cape south coast of South Africa, 1986–1990

Spatial and size/age distributions of seven teleost species are described from a series of five annual trawling surveys made over the continental shelf and upper slope (20–500 m) of the South African south coast between Cape Agulhas and Port Alfred. The catch was quantified and scaled up to an estimate of an annual biomass index for each of the principal species. The fish studied were the two species of Cape hake Merluccius capensis and M. paradoxus, kingklip Genypterus capensis, Agulhas sole Austroglossus pectoralis, Cape horse mackerel Trachurus trachurus capensis, monk Lophius vomerinus and panga Pterogymnus laniarius. Each showed varying patterns of biomass fluctuation annually, although the surveys constituted too short a data series to reveal significant trends. Nevertheless, declines in abundance of panga and kingklip supported data collected from other sources over a longer period. Interpretation of the data is tempered by a realization that species which are patchily distributed or less vulnerable to the sampling gear may have been inadequately sampled. The information can be used for more-effective management of the multi-species demersal trawl fishery of the region.     

Food and population variability in five regions supporting large stocks of anchovy,sardine and horse mackerel

The Benguela, California, Humboldt and Canary currents and the coastal waters of Japan support large stocks of sardine or pilchard Sardinops spp. and Sardina sp., anchovy Engraulis spp., horse mackerel Trachurus spp. and chub mackerel Scomber japonicus, with hake Merluccius spp. also abundant in each of the systems except off Japan. In these systems many of the more numerous fish, birds and mammals are opportunistic feeders having catholic diets. As a result species often have overlapping diets, and when a particular food item is plentiful it may be consumed by a wide variety of organisms at higher trophic levels. Frequently the performances of predator populations have been related to the availability of forage species, but predators have not always been influenced similarly by gross changes in the systems. Predators with the most generalized feeding strategies appear best buffered against collapses of particular prey resources. Although seabirds are generally opportunistic feeders, limitations in their foraging range may make them more susceptible than most other predators to system perturbations. Opportunistic feeding by species at lower trophic levels suggests that they could be advantaged by an increased food supply, and it may not be unreasonable to interpret species replacements in catches as reflecting shifts in the dominance of species. Overall catches from the systems have usually been more stable than the catches of individual species, which have shown wide fluctuations. Shifts in the dominance of species in the catches have often been sustained over a number of years. Generalized feeding creates the possibility that more than one species may benefit from energy made available by the collapse of an abundant resource, and there is evidence that species more abundant in former years have in some instances been partially replaced by a number of organisms. The likelihood of a particular organism becoming the main replacing species will probably be influenced by the degree to which it is utilized during and after the collapse of the originally dominant resource.     

Comparison of the abundance of pilchard in Cape gannet diet and commercial catches off the Western Cape,South Africa

A hypothesis is presented that, in the absence of quota restrictions, commercial catches of pilchard Sardinops ocellatus in the western Cape and the abundance of pilchard in the diet of Cape gannet Morus capensis obtained at three South African colonies are directly related to pilchard biomass when the latter is low. There were significant correlations (p < 0,05) between the abundance of pilchard in gannet diet at Lambert's Bay and Malgas Island combined, and the total catch and CPUE at annual, though not monthly, time-scales. Trends in levels of correlation are discussed in relation to short-term variability, sampling intensity, statistical chance and the distribution of pilchard. Gannet diet and commercial catches reflect regional, but not local, changes in biomass. The 1983 pilchard year-class was relatively strong. Gannet diet is suggested to be a reliable monitor of the trend in pilchard biomass at low pilchard biomasses, a time at which direct surveys are least reliable.     

Distribution,abundance and population structure of Hexaplex trunculus and Bolinus brandaris (Gastropoda: Muricidae) in offshore areas of the Gulf of Gabès,southern Tunisia

The banded murex Hexaplex trunculus and purple dye murex Bolinus brandaris are frequently caught as bycatch by bottom trawlers operating in the Gulf of Gabès, southern Tunisia. Although these muricid gastropods might importantly constitute an additional source of income for fishermen as well as help to diversify local shellfish production, information on their catches is relatively scarce. Hence, the present study aimed to assess the status of local populations of H. trunculus and B. brandaris, including data on their spatial and depth distributions, abundance, biomass, population structure and mortality. Fishing surveys using bottom trawls were performed between May 2006 and June 2007 in offshore areas of the Gulf of Gabès. In total, 198 fishing hauls were analysed, corresponding to a total fishing duration of 333.3 hours and a total prospecting area of 1 534 ha at depths of 20–86 m. Altogether, 3 026 H. trunculus and 3 599 B. brandaris of a broad size range (22.9?91.1 and 22.4?97.0 mm shell length, respectively) were sampled during the study period. GIS-produced maps were employed to determine the species’ spatial and bathymetric distributions by extrapolating their estimated fishing yield (bycatch per unit effort), abundance, biomass and mortality in relation to the total area surveyed in the gulf. The results provide valuable baseline information that may be applied to management proposals for regulating this untargeted resource, ultimately assisting sustainable exploitation and long-term preservation of this alternative fishing resource in the offshore areas of the Gulf of Gabès.     

黄海夏季不同取样网具渔获物组成比较分析

根据 2 0 0 0年夏季双拖渔船和“北斗”号专业调查船对应站位拖网调查资料 ,进行了两种不同取样网具情况下渔获物组成的比较分析 ,结果表明 :渔船的单位扫海面积渔获量、几种重要种类的长度范围、平均个体长度、5 0 %选择长度、重量计优势度均高于“北斗”号 ,而渔获种类数则少于“北斗”号 ,且两者种类组成相似性较低 ,仅为 0 .4 5 8;而且“北斗”号的底层、近底层鱼类和无脊椎类占总渔获量的比例高于渔船 ,这主要是由于渔具的选择作用造成的。结果说明应用渔船针对渔业生产进行的渔业资源监测 ,对带鱼、小黄鱼、鱼等重要生产种类是适合的 ,能够反映其资源量状况 ,但对群落结构、生物多样性等生态系统水平上的研究却存在着明显的不足     

Relationship between measurements of hake biomass and sea surface temperature off southern Namibia

A good relationship between the variability in measurements of Cape hake Merluccius capensis biomass during winter and summer swept-area research surveys and sea surface temperature off southern Namibia for the years 1983–1990 is presented. The results call into question the hypothesis that hake density is related to biomass and imply that abnormally warm summers could support higher catches, regardless of the state of the stock. A hypothesis that anomalous warm conditions could induce hake to concentrate closer to the sea bed, making them more susceptible to bottom trawling, is presented.     

Population genetic studies of horse mackerel Trachurus trecae and Trachurus trachurus capensis off Angola

Genetic variability of the Kunene horse mackerel Trachurus trecae and the Cape horse mackerel Trachurus trachurus capensis was examined using starch gel electrophoresis of enzymatic proteins on individuals collected along the Angolan coast. Six polymorphic enzyme loci were found in both species. The idh-2* locus displayed differences between the two species, and several alleles at different loci exhibited different allele frequencies between the two species, indicating that they are genetically different. Significant differences found in the distribution of allele frequencies between Kunene horse mackerel from the Benguela region of Angola indicated that this species consist of more than one randomly mating population in Angolan waters.     

Pilchard distribution in South African waters, 1983–1985

Seasonal movements of South African pilchard Sardinops ocellatus in the relatively abundant 1982/83 year-class are inferred by means of length frequency information from predator diet analyses and from commercial and research catches in various localities. Pilchard of approximately one year of age were most abundant in the vicinity of Algoa Bay on the south-eastern Cape coast in the summer of 1983/84. The pilchard length frequency distribution in gannet diet and research midwater trawl samples in this area provided a clear linkage, through the known juvenile growth rate, with fish of an average 6 months of age, which were abundant off the West Coast during May 1983. Pilchard in this year-class were subsequently found to be widely distributed over the Agulhas Bank, with a tendency for them to inhabit deeper water towards the shelf edge as they reached the age of first spawning (about 18 months). During 1985, a noticeable increase in the abundance of large, two-year-old and older pilchard in the diet of West Coast gannets and in West Coast commercial catches indicated a movement of 1982/83 year-class fish into that area. The changes in distribution of pilchard in this year-class are discussed in relation to seasonal differences in the marine environment off South Africa.