Hake Merluccius Capensis Research Articles

Diel feeding periodicity, daily ration and vertical migration of juvenile Cape hake off the west coast of South Africa

Feeding periodicity, daily ration and vertical migration of juvenile Cape hake Merluccius capensis are investigated from midwater and bottom trawl collections taken during a 42‐h period between 29 February and 2 March 1992 at a fixed position off the west coast of South Africa. Feeding of 10‐20 cm hake intensified during the night when they ascended into subsurface layers to prey on recruits of anchovy Engraulis capensis. Larger hake remained close to the bottom, were partially cannibalistic and exhibited no diel feeding periodicity. M. capensis appear to migrate vertically and feed asynchronously in midwater, as individuals, and not as a population, returning to the bottom when satiated. Based upon the exponential rate of decline in stomach fullness throughout the day, the evacuation rate by hake <20 cm was estimated as 0.054 h−1; 90% evacuation of anchovy prey required an estimated 43 h. Using the Elliott & Persson and Eggers methods, the daily ration was estimated as 5.51 and 4.15% of wet body weight respectively. The effect of the foraging behaviour of M. capensis on the appropriateness of acoustic sampling for estimates of their abundance is discussed.

The diet of Cape hakeMerluccius capensison the south coast of South Africa

In stomach contents of Cape hake Merluccius capensis collected during demersal research surveys (1989–1994) on the south coast of South Africa, fish remains accounted for 92% by mass, crustaceans for 5% and cephalopods for 3%. Fish prey consisted of 51% pelagic, mainly anchovy (33%), round herring (11%) and pilchard, 24% horse mackerel and 17% demersal fish, mainly M. capensis. This dietary composition contrasts with that documented for the same species on the West Coast, where crustaceans and mesopelagic fish are more important in the diet of small and medium-sized (<50 cm) M. capensis, and horse mackerel are of minor importance in the diet of larger hake. Cannibalism was a more prominent feature in M. capensis on the South Coast (accounting for up to 25% of the diet) than it is on the West Coast, where interspecific (hake-on-hake) predation on Merluccius paradoxus is common. On the South Coast, M. capensis adapt their diet not only to exploit larger prey species but also larger prey items within a species, the length and size range of prey increasing significantly with predator length. The maximum ingestible prey size was around 50% of body length. Hake as small as 15 cm were piscivorous and cannibalistic, but the minimum length at which hake preyed on horse mackerel was 30 cm, and generally fish >15 cm were consumed at that length. Horse mackerel was an important food source for large M. capensis (>50 cm) on the South Coast.

Effect of prey densities on cannibalism in Cape hake (Merluccius capensis) off Namibia

The preference of large (>60 cm total length) Cape hake (Merluccius capensis Castelnau, 1861) for small conspecifics under natural conditions was analyzed off southern Namibia (S.E. Atlantic) between 1983 and 1989. The results indicate that cannibalism is not related to the density of the small conspecifics nor to the density of alternative prey, but rather that large hake have a dietary preference for small conspecifics. The main consequence of such cannibalism is a lack of density-dependent regulation of the M. capensis population.

Determination of Composition and Vertical Structure of Fish Communities Using in situ Measurements of Acoustic Target Strength

A split-beam echo-sounder was used to estimate in situ acoustic target strengths (TS) of fish from a number of different fish communities on the South African continental shelf. The TS and size distributions (obtained by aimed trawling) were used to describe the size structure and vertical distribution of these communities. TS distributions obtained from several monospecific populations of juvenile Cape hake (Merluccius capensis), the dominant species present, were self-consistent, and there was good correspondence between modes in the TS and length distributions of juvenile hake, adult round herring (Etrumeus whiteheadi), anchovy (Engraulis capensis) recruits, pelagic goby (Sufflogobius bibarbatus), horse mackerel (Trachurus trachurus capensis), and ribbon fish (Lepidopus caudatus). Average TS values for all these species, both per individual and normalized by weight, are presented and compared with published values. The use of TS information in studies of the small-scale community structure and dynamics of fish populations is discussed. It is concluded that the method can be effective on low-density, multispecific assemblages such as those in our study, avoiding many of the pitfalls of conventional net sampling.

Feeding selectivity of juvenile Cape hakeMerluccius capensisin the southern Benguela

Prey selectivity and feeding behaviour of juvenile shallow-water Cape hake Merluccius capensis are investigated on the basis of material collected at two sites of contrasting feeding regimes in coastal waters off the South African west coast. Hake collected from a site abundant in zooplankton fed preferentially on relatively large crustaceans (amphipods, euphausiids and Pasiphaea semispinosa), but apparently ignored the smaller copepods and translucent prey (chaetognaths), despite these taxa being both numerically and by mass the dominant potential prey items. At a site dominated by recruits of anchovy Engraulis capensis, the hake were almost exclusively piscivorous. Small hake (10–25 cm) migrated vertically into subsurface layers to feed on anchovy, whereas larger fish appeared to remain near the bottom where cannibalism was prevalent. The present observations demonstrate the importance of vertical migratory behaviour in the foraging strategy of juvenile M. capensis.

Estimation of the annual consumption of food by Cape hake Merluccius capensis and M. paradoxus off the South African west coast

Information on hake stomach contents collected during research cruises off the west coast of South Africa between 1988 and 1990 is analysed. Estimates of the annual consumption and daily ration of the Cape hakes Merluccius capensis and M. paradoxus are obtained. Coefficients of variation (CVs) for these estimates are estimated by a bootstrap variance-estimation procedure. Cape hakes are opportunistic feeders. Juveniles feed mainly on crustaceans and the diet becomes increasingly piscivorous with age. Anchovy were the dominant dietary item of juvenile M. capensis, although this may be a reflection of the fact that anchovy were readily available at the time of the surveys. The large number of anchovy found in the diet during this study highlights the dangers of extrapolating consumption estimates for opportunistic feeders to other time periods. Pooling of data across geographic and seasonal strata introduces substantial bias in estimates of consumption and daily ration for only the pelagic prey species. Est...

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.

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.

Estimates of some biological parameters for the Cape hakes off the South African west coast

Maturation and growth data for Cape hakes Merluccius capensis and M. paradoxus collected during direct research surveys over the period 1986 – 1990 are analysed. A logistic model is fitted to the fraction of mature fish by length. Males mature earlier than females, but there is little difference in length at different levels of sexual maturity for the two species. The Schnute, Von Bertalanffy and linear growth equations are fitted to the age-length data by maximizing the likelihood function. The Schnute growth equation occasionally provides a statistically superior fit to the data than the Von Bertalanffy equation. However, both these equations provide a reasonable fit over the age ranges considered. The growth curve is near linear for females, but less so for males. It appears that females grow slightly faster than males. However, the difference in growth rate is less than that suggested by some previous age-and-growth analyses. It is not possible to determine whether or not the values of the growth and ...

Growth of Cape hakeMerluccius capensisoff Namibia determined by means of length frequency analysis and age/length data

Growth of Cape hake Merluccius capensis off Namibia is determined by means of length frequency analysis and age/length information. A time-series of length frequencies collected during 1987 was used for the analysis together with age data collected concomitantly. Estimated growth fluctuated seasonally, the minimum growth rate being in autumn. Application of age data in the analyses notably improved the fit of the growth curve. The results are similar to published estimates of Cape hake growth.

The influence of environmental factors on microstructure of otoliths of three demersal fish species caught off Namibia

The microstructure of otoliths of the hakes Merluccius capensis and M. paradoxus and kingklip Genypterus capensis was studied by means of a scanning electron microscope. The otoliths of all three species displayed distinct daily growth rhythms. Starting from the core, regular microstructural patterns of deposition produced distinct increments which could be enumerated from the rugose surface of prepared otoliths. During the early stages of growth, deposition of the increments was daily, but this regularity was lost with age. Checks or discontinuities, which may record periods of perturbation or stress of the fish, are frequent in both species of hake and in kingklip. In the two Merluccius species the growth increments contain sub-daily units and patterns of increment deposition, probably dependent upon the combined effect of temperature variation and migratory behaviour. Sub-daily units were not found in the increments of kingklip otoliths. As kingklip always remain close to the bottom, the absence of sub...

Genetic stock structure of the southern African hakes Merluccius capensis and M. paradoxus

Genetic stock structure of the southern African hakes Merluccius capensis and M. paradoxus

Photographic guide for the determination from otoliths of the age of young hakeMerluccius capensisfrom South West Africa (Namibia)

In an attempt to reduce subjectivity and arbitrary interpretation in the reading of hake Merluccius capensis otoliths from South West Africa (Namibia), interpreted photographs of a large random sample of young otoliths of age groups 0 — III are presented as a guide for readers. The various rings and zones and their positioning are discussed.

Demersal trawling in the nearshore region between Cape Seal and Klippen Point, Republic of South Africa, 1977-1979.

Annual landings of fishes trawled demersally in the vicinity of the Tsitsikamma Coastal National Park increased by almost 75 between 1977 and 1979. Catches of sole Austroglossus pectoralis showed a large decrease over the same period, but catch rates for other species remained relatively constant suggesting that they have not yet been overexploited. Hake Merluccius capensis and horse mackerel Trachurus trachurus dominated the landings in all three years.

Assessment of the South African hake resource taking its two-species nature into account

The commercially valuable hake fishery off South Africa consists of two morphologically similar species, the shallow-water Cape hake Merluccius capensis and the deep-water Cape hake M. paradoxus. Because catch-and-effort statistics collected from the fishery are not species-disaggregated, previous published quantitative assessment methods have treated the two hake species as one. Furthermore, recent evidence suggests that (although treated as two separate populations in past assessments) the South and West coasts components of each species form a single stock. This paper describes the development of the first fully species-disaggregated coast-wide baseline assessment of the South African hake resource. M. paradoxus is estimated to be currently at <10% of its pre-exploitation level whereas M. capensis is estimated to be well above its maximum sustainable yield level. By taking into consideration the primary sources of uncertainty in this assessment, a Reference Set of 24 operating models is developed to be used in Operational Management Procedure testing.

Ecosystem approach to fisheries management in the northern Benguela: the Namibian experience

The northern Benguela marine resources have a long history of questionable management primarily because of the limitations of management measures under the political framework of the time (either free access or under an international authority with no national representation, authority or enforcement power). Only after its Independence in 1990 could Namibia exercise its own national management policies, but by that time the most important commercial stocks were severely depleted. Since Independence, and despite strong management measures being implemented, the recovery of the stocks has not been as successful as expected. Some of the possible reasons are the effect of environmental variability on some stocks, ecosystem effects of fishing and unforeseen trophic interaction effects. To date, fisheries management has been based largely on a single-stock approach, but Namibia is committed to implement, in addition, an ecosystem approach to fisheries (EAF) management. The work leading to this implementation is described, in particular an ecosystem modelling study undertaken to summarize the current understanding of the northern Benguela ecosystem, to provide a basis for future work towards an EAF in Namibia. Model simulation results suggest that, given the present assumed trophic structure of the northern Benguela, altering the major fisheries would not result in recovery of the small pelagics to levels seen 40 years ago, suggesting that the original foodweb in the region may have been altered dramatically. Cape hake Merluccius capensis and M. paradoxus were negatively impacted when a large fishery on small horse mackerel Trachurus trachurus capensis was simulated. Model simulations illustrate the important finding that ecosystem effects of altered fishing scenarios are often not of a magnitude or direction that would be expected by considering predator-prey relationships in the absence of indirect trophic effects. Trophic effects may have large indirect consequences for some components of the system, for example seabirds.

Simulations of fishing effects on the southern Benguela fish community using an individual-based model: learning from a comparison with ECOSIM

By applying an individual-based model (OSMOSE) to the southern Benguela ecosystem, a multispecies analysis is proposed, complementary to that provided by the application of ECOPATH/ECOSIM models. To reconstruct marine foodwebs, OSMOSE is based on the hypothesis that predation is a size-structured process. In all, 12 fish species, chosen for their importance in terms of biomass and catches, are explicitly modelled. Growth, reproduction and mortality parameters are required to model their dynamics and trophic interactions. Maps of mean spatial distribution of the species are compiled from published literature. Taking into account the spatial component is necessary because spatial co-occurrence determines potential interactions between predatory fish and prey fish of suitable size. To explore ecosystem effects of fishing, different fishing scenarios, previously examined using ECOSIM, are simulated using the OSMOSE model. They explore the effects of targeting fish species in the southern Benguela considered to be predators (Cape hake Merluccius capensis and M. paradoxus) or prey (anchovy Engraulis encrasicolus, sardine Sardinops sagax, round herring Etrumeus whiteheadi). Simulation results are compared and are generally consistent with those obtained using an ECOSIM model. This cross-validation appears to be a promising means of evaluating the robustness of model outputs, when separate validation of marine ecosystem models are still difficult to perform.