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1.
Namibia’s most important commercial fisheries resource, the shallow-water Cape hake Merluccius capensis, is currently assessed using statistical catch-at-age analysis. Age data obtained from otoliths constitute an important data component of this model. Recent age-validation studies of M. capensis showed that growth was previously underestimated. We investigated this new fast-growth hypothesis (FGH) by using measurements and counts of translucent zones (T1 to T14, from otolith core to edge) on two survey otolith samples covering the entire range of fish lengths. We compared three hypotheses of periodicity of otolith zone formation and show that, if all zones are counted, T2 (at 9.0 mm otolith length), T5, T8, T11 and T14 are most likely to be annuli. A conversion from the slow-growth-hypothesis (SGH, currently used) age data was calculated as: FGH age group = round (0.41[SGH age group] + 0.25), and this formula should be applied to compute and test updated catch-at-age data in a future hake stock assessment. Additional adjustment for the hake stock assessment following the FGH, such as the timing of recruitment in winter and catches in summer, should be considered in future assessments.  相似文献   

2.
Within international markets for fish, the past decade has witnessed a significant growth and proliferation of products labelled to be sustainable or responsibly sourced. These terms encapsulate a range of criteria concerning the state of the stocks and, inter alia, how the fish have been captured. Of the different modes of capture ‘line-caught’ is one of the longer standing and with associations to lesser impacts upon the environment. Yet despite this position, there appears to have been little assessment of any price premiums realised for fish marketed with environmental, responsibly-sourced, line-caught or other such credentials. This paper is the first published study to examine whether such attributes of chilled fish products command any price premium at the supermarket level of the value chain. The study is based on 68 weekly observations of chilled pre-packed cod and haddock in seven different supermarkets in the UK. The study also examines possible price premiums for other observable attributes such as product form, processing and country of origin, in addition to any differences in pricing between the supermarkets. The results show that the ‘line-caught’ attribute gives cod and haddock a price premium of 18% and 10%, respectively. The MSC ecolabel gives a 10% price premium on haddock products.  相似文献   

3.
During intensive biannual demersal fish-sampling cruises since January 1983 over the South African west coast shelf region (Orange River to Cape Agulhas, coast to 500-m isobath), feeding studies on both species of Cape hake (Merluccius capensis and M. paradoxus) were undertaken. These studies were of two forms, random sampling of the dietary preference and constituents of the hake caught during daylight only and intensive sampling of stomachs with food throughout a 28-hour cycle. From the index of relative importance, crustaceans were shown to be most important in the majority of length classes of hake studied, with mesopelagics (fish and cephalopods) and other hake also important. Differences by hake species, by predator size, by geographical location and by season were evident. The Cape hakes, which constitute some three-quarters of the demersal fish biomass in the area, are deduced to play a major role in the marine faunal environment, both as predators and prey, and some base-line figures of the quantities of different food items consumed by the resource are presented. Hake are largely opportunistic feeders and it is concluded that they can adapt to perturbations in the availability of prey, whether they be triggered by environmental cause or by, for example, purse-seining of particularly mesopelagic fish.  相似文献   

4.
Literature on trophic relationships in the Benguela ecosystem has stressed the importance of cephalopods as prey of groundfish. The groundfish community of the shelf and upper slope of southern Africa is dominated by the Cape hakes, and the results presented (1984–1991) confirm that both species of hake are important predators of cephalopods, especially taking into consideration the abundance of hake in the ecosystem. However, geographic, seasonal and species variability are evident in the patterns observed. The main prey species are Sepia spp. (predominantly Sepia australis), Loligo vulgaris reynaudii, Todaropsis eblanae and Lycoteuthis ?diadema. The last-named is an important food organism for fish. Its systematic status needs revision, however. Qualitative results of studies of cephalopod predation are also provided for kingklip and monkfish.  相似文献   

5.
Population genetic data of marine genetic resources add up to establish their pattern of connectivity. Such pattern determines the choice of a suitable management strategy to those resources in space and time. This study addresses the pattern of connectivity among European hake grounds upon the largest sampling effort so far developed in this species. Bayesian inference made on multilocus genotypic data provides evidence that a large genetic connectivity exists among Atlantic grounds and is mediated by significant migration rates stepping up from the Celtic Sea towards its adjacent Atlantic grounds. Therefore, the spawning biomass of the northern hake population could play a crucial role at ensuring the sustainability of southern hake fish grounds. The deepest restriction to gene flow was observed at the easternmost side of the Alboran Sea what suggests that the Almeria–Oran Oceanographic Front is an effective barrier keeping apart the two major gene pools so far shown in this species. A molecular clock calibrated for cytochrome b sequences of fishes and applied to the divergence between hake pools (0.3%) suggests that the Atlantic–Mediterranean population split might date back to the Middle Pleistocene (150,000 years BP). Complementarily, Bayesian treatments of multilocus genotypes indicate that such barrier to gene flow remains active nowadays. This study shows the upmost interest of future integrative efforts to incorporate population genetic data into current assessment practices, in order to better manage European hake populations and make its exploitation sustainable.  相似文献   

6.
The hake resource is the most important commercial fish species in the demersal sector of Namibia's fisheries, both in terms of annual catch and contribution to Gross Domestic Product (GDP). The fishery now spans four decades. In the 1960s and 1970s, hake were exploited heavily by mainly foreign fleets, total catches peaking at more than 800 000 tons in 1972. The first control measures, the use of a minimum mesh size of 110 mm and the allocation of quotas to each member country participating in the hake fishery, were implemented by the International Commission for the Southeast Atlantic Fisheries in 1975. In 1990, the Namibian Government took action to control fishing activities in Namibian waters, and the enactment of its Fisheries Policy (1991) and Sea Fisheries Act of 1992 provided for the control measures to be taken. The conservative management strategy adopted between 1990 and 1993 resulted in gradual increase in hake biomass, but thereafter the stock declined. The hake fishery is currently managed on the basis of a total allowable catch that takes into consideration the rate of increase or decrease in the size of the resource. Since 1990, the demersal trawl fishery has accounted for approximately 90% of the total hake catch. The resource is subjected to both directed fishing and bycatch, the latter taken in directed fisheries for species such as horse mackerel, monkfish and sole.  相似文献   

7.
A trawl catches only a portion of the fish in its path. The term catchability therefore refers to the fraction of the available fish caught. A method was developed and tested to establish catchability constants for trawl surveys of Namibian hake Merluccius spp. A catchability constant can be expressed as a simple relationship between hake area densities calculated from trawl catches and acoustic biomass estimates. Initial values were on an order of magnitude of 0.8, meaning that the catch takes 80% of the hake available to the trawl. The methodology depends on careful area selection (flat bottom, homogenous fish distribution), following the same trawl lane during subsequent hauls in an area, and thorough acoustic post-processing. A pronounced and repetitive pattern in catchability within surface daylight hours was found. Early morning and early afternoon catches were low, and the best catches of the day were made around noon, a result that may influence stock assessments based on trawl data, because morning and afternoon data would under-represent actual abundance.  相似文献   

8.
The diet of the Patagonian hake Merluccius australis polylepis in 1986 was determined by analysis of stomach samples collected from commercial trawl catches. The major prey items were Patagonian grenadier Macrouronus magellanicus and, to a lesser extent, southern blue whiting Micromesistius australis. Large (81–100 cm) hake are mainly piscivorous and consume more grenadier than smaller (51–80 cm) hake. The smaller hake feed both on fish and invertebrates. There was no difference by sex in terms of stomach contents. The daily feeding cycle was also investigated from samples collected during 1991. Hake feed principally on 1-, 2-, and 3-year-old grenadier, which they seem to catch mainly at night. The daily ration of grenadier was between 0,043 and 0,26 per cent of body mass (1,692–9,849 g). The results are similar to those for other hake species in terms of composition of the diet, daily feeding cycle and daily ration.  相似文献   

9.
Almost 1 000 South African fur seals Arctocephalus pusillus pusillus were collected at sea (inshore and offshore) for examination of stomach contents and other biological parameters. The main prey of seals above the age of one year was teleost fish, of which at least 28 species were identified. Overall, 74,6 per cent of the diet was teleost fish, 16,7 per cent cephalopods, 4,8 per cent crustaceans such as shrimps, prawns, amphipods and isopods and 3,2 percent rock lobster. Within this overall dietary pattern there were marked regional differences. The importance of individual fish species in the diet was investigated from the numbers and the sizes of otoliths occurring in the stomachs, from which the identity and size classes of the prey species could be calculated. Again there were regional differences in the importance of different prey species. On the west coast of South Africa the most important species were anchovy and Cape hakes (each 23% of total diet), whereas on the south coast the important species were anchovy (17%), horse mackerel (14%), pilchard (12%) and Cape hake (14%). Off South West Africa the two dominant species were pelagic goby (52%) and horse mackerel (23%), anchovy and Cape hake constituting only 4 and 5 per cent respectively of total diet. The total annual consumption of anchovy and Cape hake by seals in South African waters was calculated at 125 000 and 120 000 tons respectively.  相似文献   

10.
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11.
Marine Protected Areas (MPAs) are generally considered as one of the most important tools, among the many regulations, designed to preserve marine resources as well as enhance fisheries. In the southern Bay of Biscay, local French fishermen requested creation of a restricted area to help settle disputes between the various métiers operating there. This restricted area, which lies mainly in French waters, covers part of a deep submarine canyon off the French and Spanish coasts, known to have a large population of mature hake. This study aims to better understand the effects of a restricted area upon French fleets operating there, particularly upon three main métiers—longliners, gillnetters and trawlers. The study area includes three ICES statistical rectangles. The data, based upon reported landings and auctions sales for the period 1985–2008, were analyzed using multivariate analysis. The fishing activity is more important in one rectangle which includes the restricted area. Bottom longliners and gillnetters, operate mainly in this one while trawlers are less dependent. The first métier concentrates particularly on hake and the second has targeted other species and has become less dependent on hake. Trawlers target a wider range of species. Over the past ten years, the restricted area has contributed to maintain the fleets operating here. The border with Spain adds other constraints over the issue of access to regional fisheries and makes management a little more complicated.  相似文献   

12.
Measuring and forecasting recruitment are central to the understanding and management of fish stocks. Kainge et al. (2013) studied the effect of spawning stock size and environmental fluctuations on the recruitment levels of the Cape hake Merluccius capensis in Namibia. However, their study contains some flaws that undermine the conclusion that Cape hake recruitment is under the influence of upwelling in summer. Until those flaws are properly addressed, this conclusion, in our view, should be treated with caution.  相似文献   

13.
Seasonal pattern of fish recruitment can vary over years, and the coupling of such pattern with favourable trophic-environmental conditions may trigger non-stationary and non-linear effects on the yearly recruitment variability. The relative contribution of intra-annual discrete recruitment events (REs) to recruitment is often the basis of the inter-annual fluctuations in abundance and biomass of exploited fish populations which are based on a small number of age classes. In this study, REs of European hake (Merluccius merluccius L.) were identified in the north and south of the Balearic Islands (NW Mediterranean) during the 2004 recruitment season and investigated for growth and physiological and morphological condition. Five REs were identified with different contributions in abundance to the annual recruitment in each area coming from different intra-annual spawning batches. Growth rates were similar between the REs with the exception of the last event of the season. Conversely, large differences in morphological and physiological condition were observed between REs and were due to: different environmental-trophic scenarios encountered, and ontogenetic changes related to the recruitment to the shelf-break grounds (ca. 11 cm, 5–6 months of life) which triggered a trade-off in the energy allocation strategy. The spatial-temporal variability in the physiological condition among REs at the beginning of the recruitment season might be explained in the basis on a ‘condition expanded match–mismatch hypothesis’. We advance that individuals recruiting in the north, which were in better condition and temporally and spatially differenced from the south, have most probably hatched in separated locations with different food availability for larvae and early life stages. We propose that hake recruiting to the north and south of the island are from different origins, mainland and the islands respectively, and have benefited from different environmental conditions. This results in a different condition but not in different growth rates.  相似文献   

14.
Ecosystem-based management of marine fisheries requires the use of simulation modelling to investigate the system-level impact of candidate fisheries management strategies. However, testing of fundamental assumptions such as system structure or process formulations is rarely done. In this study, we compare the output of three different ecosystem models (Atlantis, Ecopath with Ecosim, and OSMOSE) applied to the same ecosystem (the southern Benguela), to explore which ecosystem effects of fishing are most sensitive to model uncertainty. We subjected the models to two contrasting fishing pressure scenarios, applying high fishing pressure to either small pelagic fish or to adult hake. We compared the resulting model behaviour at a system level, and also at the level of model groups. We analysed the outputs in terms of various commonly used ecosystem indicators, and found some similarities in the overall behaviour of the models, despite major differences in model formulation and assumptions. Direction of change in system-level indicators was consistent for all models under the hake pressure scenario, although discrepancies emerged under the small-pelagic-fish scenario. Studying biomass response of individual model groups was key to understanding more integrated system-level metrics. All three models are based on existing knowledge of the system, and the convergence of model results increases confidence in the robustness of the model outputs. Points of divergence in the model results suggest important areas of future study. The use of feeding guilds to provide indicators for fish species at an aggregated level was explored, and proved to be an interesting alternative to aggregation by trophic level.  相似文献   

15.
In this study we examine physiological responses of Peruvian hake (Merluccius gayi peruanus) to changes in their population structure and physical environment during the period 1971–2004. In particular, we assess the relative investment of energy in growth and/or reproduction of small (20–35 cm) and large (35–50 cm) hake. We calculated the (i) condition (Fulton’s K), (ii) gonad and (iii) gut fullness indices for 42,761 female hakes sampled from commercial landings; these indices indicate fish somatic, reproductive and feeding condition, respectively. Using Generalized Additive Models we then examined potential relationships between these indices and sea surface temperature anomalies and date. Drastic energy exhaustion and a decrease in female hake fecundity were observed during El Niño events. The long-term trend showed a general increase in condition factor and a decrease in gonad index for large hake between 1971 and 2004. Small hake exhibited a different trend with an increase in reproductive activity, which was accompanied by an earlier maturation. We hypothesise that the observed low investment of energy in reproduction by large female hake might be related to the lack of large males, due to a sex-selective fishery and the impact of El Niño. We suggest that fishing diminished hake reproductive capacity, modified the sex ratio in favour of females and increased population vulnerability to environmental stress, in particular to the El Niño. The impact of multidecadal variability and predators like the squid, Dosidicus gigas, remain unresolved until longer time series become available.  相似文献   

16.
Short spatio–temporal variations in the feeding intensity and the diet of the European hake, Merluccius merluccius, together with the abundance of their potential prey were studied between August 2003 and June 2004 at two locations, northwest (Sóller) and south (Cabrera), off the island of Mallorca (Balearic Islands, Western Mediterranean) at depths between 150 and 750 m. The two areas present different oceanographic conditions. Hake was mainly distributed along the shelf-slope break and the upper slope (between 166 and 350 m) where recruits (TL<18 cm) were dominant. The hake's diet varied as a function of size. Recruits fed mainly on micronektonic prey, and the diet was influenced primarily by seasonality, with two dietary patterns (identified by MDS analyses) corresponding to August–September 2003 (summer) and to November 2003/February–April 2004 (autumn–winter). The summer pattern was consistent with a thermally stratified water column, while November and April were consistent with homogenized temperature and salinity throughout all the water column. The main prey of recruits were the euphausiid Meganyctiphanes norvegica and the midwater fish Maurolicus muelleri in autumn–winter and myctophids (mainly Ceratoscopelus maderensis) in summer. In contrast to recruits, the geographic factor (NW vs. S) was the main factor influencing the diets of post-recruits (TL between 18 and 21.9 cm) and adults (TL?22 cm). Hake recruits (and to a lesser extent post-recruits) and their preferred prey occupied different depth ranges during daylight periods. Meganyctiphanes norvegica and Ceratoscopelus maderensis were, for instance, distributed as much as 500 m deeper than hake that had eaten them. All these trends were especially obvious at NW, an area with a more abrupt slope and with a greater influence by northern winter intermediate water (WIW) inflow in early spring than the S area. These factors probably enhanced micronekton aggregation in April, when feeding intensity (stomach fullness) increased among recruits and post-recruits only at NW. All these factors may have a crucial role in the diet, distribution and probably recruitment success of small hake. Biological factors were also important in trophic shifts in the diet and feeding of hake. Multi-linear regression models pointed to a trend of higher fullness with higher hepato-somatic index (HSI). Therefore greater food consumption by hake may enhance its metabolic condition. Within the framework of shelf-break and slope ecology, we show how the ‘boundary’ mesopelagic community inhabiting the middle slope sustains the trophic requirements of hake, a species distributed at shallower depths along the shelf-slope break. Mesopelagic euphausiids and myctophids are often found in the diets of shelf-break fish. Because the boundary mesopelagic community is distributed worldwide, the high levels of fish biomass often found at shelf-slope breaks could be sustained trophically by deeper, offshore mesopelagic communities, an inverse energy transfer from deep to shallow-water marine ecosystems.  相似文献   

17.
Fish communities over continental shelves are of three types: pelagic, rocky reef and soft substrata. The pelagic community of the South African west coast is dominated by clupeiform fish, which are the principal prey of snoek, the dominant piscivorous teleost in the area, and other fish, including tuna and pelagic sharks. Keystone prey species are broadly similar on the South African south and west coasts, but predator composition differs. Studies of rocky reef fish suggest that the fauna of the South Coast is more diverse than that of the West, the West Coast having fewer species and only two reef fish species being taken in linefisheries. Feeding interactions of some of the dominant species have been established, but the status of commercially less important fish such as elasmobranchs and small species is poorly known. The community and feeding interactions of fish living over soft substrata are better known and part of this fauna is taken by demersal trawlers. The two species of Cape hake dominate the communities on both coasts, but the fauna is more diverse on the Cape south coast. Generally, much of the present ichthyological knowledge is derived from the ecologically narrow perspective of commercial fisheries. Nevertheless, the patterns of diversity appear to conform to previous work, which has indicated a lower species diversity in the Atlantic than in the Indian Ocean. Knowledge of commercially unimportant fish is fragmentary, and understanding of the ecological interactions on the South Coast generally lags that of the West.  相似文献   

18.
We analyzed recent food web and fish stock changes in the central Chile marine ecosystem, comparing the roles of jumbo squid (Dosidicus gigas) as predator, the environment, and fishing. To accomplish this we used food web modeling and the Ecopath with Ecosim software (EwE). The principal fish stocks have experienced wide decadal fluctuations in the past 30 years, including stock collapses of horse mackerel (Trachurus murphyi) and hake (Merluccius gayi), and there was a large influx of jumbo squid during the mid-2000s. We used two EwE models representing the food web off central Chile to test the hypothesis that predation by jumbo squid has been significant in explaining the dynamics of the main fishing resources and other species in the study area. Results indicate that predation by jumbo squid on fish stocks is lower than that of other predators (e.g. hake) and the fishery. Long-term fluctuations (1978–2004) in the biomass of the main fish stocks (as well as other components of the food web) seem to be related to fishing and to variation in primary production, rather than to predation by jumbo squid alone. Jumbo squid seems to play a role as predator rather than prey in the system, but its impacts are low when compared with the impacts of other predators and fishing. Therefore, we conclude that jumbo squid predation on its prey was not the primary force behind the collapse of important fish stocks off central Chile. Future efforts should be directed to better understanding factors that trigger sudden increases in jumbo squid abundance off central Chile, as well as modeling its trophic impacts.  相似文献   

19.
Three fish species, anchovy, hake and lanternfish, occupy different habitats in the neritic zone of the Benguela Current ecosystem. Compatibility between the predicted environmental variability in each habitat and the collection of life history traits found in each species is examined with simulation models. Variability in early-stage survival is introduced into the models by means of both random and non-random ("red") spectra. Traits exhibited by lanternfish are shown to be inefficient in filtering out random variability and the species therefore cannot prevent collapse in a non-randomly variable environment. By comparison, the traits found in anchovy and hake filter out part of both random and non-random variability in survival and allow a fairly stable population size to be maintained. It is argued that, as life-history traits are adapted to the spectrum of variability associated with the habitat, only anomalies with respect to this spectrum are likely to have a large influence on stock size.  相似文献   

20.
A balanced trophic flow model of the southern Benguela ecosystem is presented, averaging the period 1980–1989 and emphasizing upper trophic levels. The model is based largely on studies conducted within the framework of the Benguela Ecology Programme and updates the results of an expert workshop held in Cape Town in September 1989. Small pelagic fish other than anchovy Engraulis capensis and sardine Sardinops sagax, mainly round herring Etrumeus whiteheadi and mesopelagic fish, were important components of the food web in the southern Benguela. Severe balancing difficulties were encountered with respect to the semi-pelagic resources (hake Merluccius spp.) and demersal top predators (sharks), indicating the need for further research on the interaction of these groups with their ecosystem. The model is compared to other existing trophic flow models of ecosystems in major upwelling areas, i.e. the northern Humboldt Current (4–14°S), the California Current (28–42°N) and the southern Canary Current (l2–25°N), and to two independently constructed models of the northern Benguela ecosystem. These models are compared using network analysis routines of the ECOPATH software, focusing on the interactions between the five dominant fish species (anchovy, sardine, horse mackerel Trachurus trachurus capensis, chub mackerel Scomber japonicus and hake) that support important fisheries in all systems. The upwelling systems rank by size rather than species dominance. The ratio of catches and primary production differs between systems, partly because of differences in fishing regimes. Predation on the five dominant fish groups by other fish in the system was the most important cause of fish mortality in all models. Fishery catches are generally a larger cause of mortality for these groups than predation by mammals. The ecological cost of fishing appears to be comparatively low in the southern Benguela, because catches are low compared with the primary production, but also because the fishery is relatively low in the foodweb. However, in view of the very tight foodweb demonstrated in the model. it is likely that an increase in fishing pressure would cause severe trade-offs with respect to other components of the southern Benguela ecosystem.  相似文献   

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