Cape Hake Merluccius Research Articles

Low hypoxia tolerance in larvae of the sardine Sardinops sagax and anchovy Engraulis encrasicolus may limit their stock recovery in the northern Benguela upwelling system

Physiological traits of five key fish species (Cape hake Merluccius spp., pelagic goby Sufflogobius bibarbatus, Cape horse mackerel Trachurus capensis, European anchovy Engraulis encrasicolus and sardine Sardinops sagax) from the northern Benguela upwelling system (NBUS) were compared during their larval stage by measurements of enzymatic activities of key metabolic enzymes (citrate synthase [CS] and pyruvate kinase [PK]). Two distinct age classes (early larvae: 8‒14 days old; late larvae: 22‒29 days old) for each species and from two areas were compared: Terrace Bay (20° S), the main spawning ground of Cape horse mackerel and anchovy, and Kunene (17° S), near the Angola–Benguela frontal zone, where warm and hypoxic water masses intrude into the NBUS. The results revealed significantly higher CS activity in both larval age classes in Cape horse mackerel, anchovy and sardine compared with Cape hake and pelagic goby. Pelagic goby and Cape horse mackerel had significantly higher PK activity compared with Cape hake, anchovy and sardine, apparent in both larval age classes and both areas. Results for anaerobic metabolism indicate higher capacity for pelagic goby and Cape horse mackerel to recover from oxygen debt built up in prey capture and predator escape behaviour and a higher potential for hypoxia tolerance when compared with Cape hake, anchovy and sardine. These results suggest higher survival probability for pelagic goby and Cape horse mackerel compared with the other species under conditions of a shoaling oxygen minimum zone and may explain their current dominance in the NBUS.

Effects of Modified Atmosphere Packaging, Storage Temperature, and Absorbent Pads on the Quality of Fresh Cape Hake Fish Fillets

This study investigated the effects of modified atmosphere packaging (MAP), storage temperature, and the use of absorbent pads (PAD) on the quality attributes of Cape hake (Merluccius capensis) fish fillets. Fresh Cape hake fillets were packaged under active-MA (40% CO2 + 30% O2 + 30% N2) or passive-MA (0.039% CO2 + 20.95% O2 + 78% N2), with and without PAD, and stored at 0 °C, 4 °C, and 8 °C (to mimic abuse temperature). The control fresh fillets were stored under passive-MAP without PAD at 0 °C, 4 °C, and 8 °C. Headspace O2 gas composition continuously decreased below critical limits under passive-MAP, with an increase in storage temperature. Similarly, O2 levels decreased under active-MAP but did not reach critical levels, with the lowest being 9.5% at 0 °C. The interaction of storage temperature and modified atmosphere had a significant effect on quality attributes of Cape hake fillets. Drip loss was higher in active-MAP packaged fillets without PAD (0.64%) than passive-MAP packaged fillets without PAD (0.27%). Drip loss was significantly reduced when using absorbent pads (p < 0.05). Firmness, color, and pH were better maintained under active-MAP at the lowest temperature of 0 °C. Firmness (work of shear) of active-MA packaged fillets on day 12 at 0 °C and 4 °C was 527 N/s and 506 N/s, respectively. Fillets packaged under active-MAP at 0 °C had longer shelf-life than control passive-MAP fillets.

New age-determination method based on fast growth rate of Cape hake Merluccius capensis in Namibia

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.

Biannual otolith zonation of Cape hake (Merluccius capensis) in response to fish physiology and environment in the northern Benguela

Abstract Understanding and validating annual structures in fish otoliths are important for stock assessments and fisheries ecology. Biannual translucent zone formation has been demonstrated for 3–21 months old Namibian shallow-water hake Merluccius capensis. This paper addresses the hypothesis that the pattern continues in older fish. Otolith zone periodicity was studied for four cohorts hatched in July of 1996, 1998, 2002 and 2005, based on modal progression analysis (n = 1059). Edge analysis and marginal increment analysis were performed on otoliths representing eleven months of the year from port samples (n = 1153) from the years 2007–2015. Two to three translucent (T)-zones are formed on the hake otoliths each year, one or two between austral summer and autumn (January to April) and one in winter–spring (between July and October). Annual zones are usually the T2, T5, T8 zones (likely every third T-zone). Annual zones are not distinguishable from pseudo-annuli under the light microscope, and therefore otolith length measurements (OL) at each T-zone should be used as a guide for assigning age (9, 15 and 19 mm OL for ages 1, 2 and 3 respectively). Otolith T-zones are associated with both warm and cold periods, when they are presumably limited by temperature or dissolved oxygen concentration as well as food availability and feeding efficiency at both ends of their tolerance range. The most noticeable factor influencing the formation of translucent zones in otoliths is fish condition, not always linked with spawning (not always linked with high GSI). M. capensis otolith zonation is thus linked with their endogenous adaptation to the ecosystem, their fish physiology and also regulated by environmental variability.

The effects of stock size and environmental variability on Cape hake recruitment in Namibia: an unsolved puzzle (a comment on Kainge et al. 2013)

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.

Stock-environment recruitment analysis for Namibian Cape hake Merluccius capensis

The factors modulating recruitment success of Cape hake Merluccius capensis in Namibian waters are still unresolved. In this study, we used generalised additive models, regression tree analysis and the conventional Ricker model to examine the effect of environmental indices and spawning stock biomass (SSB) on hake recruitment success for the period 1984–2012. Results indicated that upwelling strength explained 51% of the recruitment variability, whereas SSB had no significant influence. The effect of SSB on recruitment only became significant when combined with upwelling strength, explaining 89% of the recruitment variability. SSB influenced recruitment during periods of strong upwelling. Optimal conditions for hake recruitment were associated with moderate upwelling strength. Low and high upwelling intensities reduced recruitment success. Our results are consistent with those of other studies suggesting a significant influence of environmental conditions on recruitment at a low spawning stock level. Our study highlights the importance of assessing the combined non-linear effects of both biotic and abiotic factors on hake recruitment.

On the feasibility of a directed trap-fishery for panga Pterogymnus laniarius (Sparidae) in South Africa

The feasibility of a directed trap-fishery for panga Pterogymnus laniarius was investigated with special emphasis on minimising the bycatch. A total of 1 302 fish, representing 19 species, was captured in 59 trap deployments at three locations along the Eastern Cape coast of South Africa between September 2002 and July 2004. Panga comprised over 55% of the total catch, with an average of 12 panga per trap haul. Depth and substrate type had a significant effect on the species composition of the catch. Panga comprised 77% of the catch in depths >50m, whereas other commercially important species contributed 11% to the total catch in water >50m. No kingklip Genypterus capensis were captured in the traps and shallow-water Cape hake Merluccius capensis accounted for only 0.2% of the total catch composition. The findings suggest that trap-fishing provides a new fishing opportunity in South Africa with a high catch per unit effort for the target species and a low level of bycatch.

Diet of Cape fur seals Arctocephalus pusillus pusillus at three mainland breeding colonies in Namibia. 1. Spatial variation

The diet composition of Cape fur seals Arctocephalus pusillus pusillus was investigated from three breeding colonies in Namibia between January 1994 and April 2002 using scat analysis. Otolith numbers were corrected for those lost during digestion before determining the percentage numerical abundance of prey in each sample. Otolith diameters were corrected for erosion during digestion and then converted to fish length and mass to determine the percentage mass of prey in samples. In all, six main groups of teleost prey were identified, consisting of at least 48 species and 19 families. Cape Cross seals mainly consumed Cape horse mackerel Trachurus trachurus capensis, whereas lanternfish Lampanyctodes hectoris were negligible in their diet. At Van Reenen Bay and Atlas-Wolf Bay, the two southerly colonies, lanternfish were numerically prevalent in scats but not important in terms of mass. Cape hake Merluccius spp. and goby Sufflogobius bibarbatus featured in scats from all three colonies. The species in the pelagics prey group were of lesser dietary importance, with round herring Etrumeus whiteheadi being most prevalent in the south at Van Reenen Bay and anchovy Engraulis encrasicolus and sardine Sardinops sagax most prevalent in scats from Cape Cross. The difference in the prevalence of prey in seal diets seems to be related to fish distributions.

Diet of Cape fur seals Arctocephalus pusillus pusillus at three mainland breeding colonies in Namibia. 2. Temporal variation

Scat analysis was used to assess temporal variability in the diet composition of Cape fur seals Arctocephalus pusillus pusillus from three breeding colonies in Namibia (January 1994 to April 2002). The diet displayed significant inter- and intra-annual variation in composition at each of the study colonies. The diet was not predictable on an annual pattern, and little consistent seasonality was apparent at each of the study sites. However, some prey species tended to display some seasonality, possibly as a result of their life-history patterns. At the Cape Cross colony, small Cape hake Merluccius spp. in the diet coincided with their spawning season. At the Atlas-Wolf Bay and Van Reenen Bay colonies, lanternfish Lampanyctodes hectoris were most prevalent in the diet during the upwelling season (August–January). Outside that period, pelagic goby Sufflogobius bibarbatus were most abundant in the diet at those colonies. This pattern may be a result of the gobies' southwards migration from northern Namibian waters where smaller goby are dominant in the diet at the Cape Cross colony in the upwelling season. The prevalence of Cape horse mackerel Trachurus trachurus capensis in the diet at Cape Cross was coincident with the pelagic phase of juvenile horse mackerel.

Distribution patterns of key fish species of the southern Benguela ecosystem: an approach combining fishery-dependent and fishery-independent data

Within the context of an ecosystem approach for fisheries, there is a need for quantitative information on distributions of key marine species. This information is valuable input for modelling species interactions in the southern Benguela ecosystem. In the present study, a method is described for mapping the density distribution of 15 key species: anchovy Engraulis encrasicolus, sardine Sardinops sagax, round herring Etrumeus whiteheadi, chub mackerel Scomber japonicus, horse mackerel Trachurus trachurus capensis, lanternfish Lampanyctodes hectoris, lightfish Maurolicus muelleri, albacore Thunnus alalunga, bigeye tuna Thunnus obesus, yellowfin tuna Thunnus albacares, silver kob Argyrosomus inodorus, snoek Thyrsites atun, Cape hake Merluccius spp., kingklip Genypterus capensis and chokka squid Loligo vulgaris reynaudi. The purpose was to make use of all available sources of data to extend the spatial and temporal coverage of the southern Benguela. Six sources of data were combined on a 10′ × 10′ cell grid in a Geographical Information System: acoustic and demersal surveys conducted by Marine and Coastal Management (MCM), and pelagic, demersal (including midwater trawl), hake-directed and tuna-directed longline commercial landings data collected by MCM. Comparisons of distributions between two periods (1980s and 1990s) and between two semesters (April – September and October – March) were conducted, but biases as a result of major differences in sampling strategy prevented detailed analysis for certain species. Maps of density distributions are nevertheless presented here and the method to determine them is discussed.

Comparison between micro‐ and macro‐structure readings in the age estimations of Cape hake

By counting otolith daily increments juvenile Cape hake Merluccius capensis were found to grow faster than European hake Merluccius merluccius. The mean frequency of translucent zones in juveniles 130–240 mm LT were higher (×3·2) than the annual frequency, although their mean duration was shorter (1 month) than a season. Comparison between micro‐ and macro‐structure readings showed that the latter tended to overestimate the age of juvenile hake, with 1 year old hake being overestimated by 18% in individuals of 160 and 210 mm LT.

Depth-stratified density estimates and distribution of the Cape hake Merluccius capensis and M. paradoxus off Namibia deduced from survey data, 1990–1999

In this study demersal survey data for the period 1990–1999 are used to investigate the average distribution of the Cape hake Merluccius capensis and M. paradoxus off Namibia in terms of density and mean length. Further, biomass estimates are compared on the basis of depth and density stratification. The main distribution of M. capensis was north of 27°S and that of M. paradoxus south of 24°S. M. paradoxus was deeper than M. capensis. For both species, average length increased with depth. M. paradoxus expanded its range to the north through the 1990s as its population size (off South Africa and Namibia) increased. In Namibian waters, small M. paradoxus were found only south of 25°S. Mean length of M. capensis increased north of 21°S, largely as a result of decreased numbers of small fish in shallower water. Abundance estimates stratified by depth were no different from those post-stratified on similar densities.

Investigation into the declining trend in chokka squid Loligo vulgaris reynaudii Catches made by South African trawlers

The trawl fishery, which targets Cape hake Merluccius spp. and Agulhas sole Austroglossus pectoralis, takes chokka squid Loligo vulgaris reynaudii as by-catch. Catch and effort data from the trawl fishery for the period 1978–1996 are used to obtain annual estimates of catch rate (catch per unit effort cpue) for that period. Examination of the cpue trend shows a sharp decline in the early 1980s and, in order to identify factors that could have influenced that decline, the distribution of fishing effort is investigated both temporally and spatially. There is a possible change in the incidence of squid-directed catches over time, but their overall scarcity could have had only a small impact on the annual cpue trend. Further, using distribution of fishing effort to evaluate the effects of possible changes in fishing patterns, rather than changes in resource abundance, on the trend in trawl cpue, it became clear that there had been a contraction of the trawling grounds and changes in fishing patterns in relati...

Considerations on the feasibility of a directed fishery for panga Pterogymnus lantartus (Pisces: Sparidae)

Recent assessments of the status of the panga Pterogymnus laniarius stock on the Agulhas Bank, South Africa, showed that catches could be sustained at considerably higher levels than those harvested at present. Although the stock could be successfully harvested using available fishing methods, the sympatry of this species with other commercial species, such as shallow-water Cape hake Merluccius capensis, Cape horse mackerel Trachurus trachurus capensis and other deep-reef species was of concern, because these species would form a significant bycatch in a panga-directed fishery. These findings highlight the problems of bycatch management and emphasize the need for creativity by both scientists and fishers in designing new and improved methods for selectively exploiting bycatch fish resources.

Serial spawning and batch fecundity of Merluccius capensis and M. Paradoxus

Histological analysis of the ovaries and the presence of multiple modes in oocyte size frequency distribution of Cape hake Merluccius capensis and M. paradoxus indicates that they are serial spawners. Batch fecundity, calculated by means of the "hydrated oocyte method", was positively correlated to ovary-free fish mass and total fish length. The mean relative fecundity (ovary-free mass) of M. capensis and M. paradoxus was 160 ± 12 and 306 ± 25 eggs·g−1 respectively.

Observations of severe hypoxia and offshore displacement of Cape hake over the Namibian shelf in 1994

In 1994, persistent and pronounced hypoxic conditions developed off the coast of central and northern Namibia, with oxygen levels <0.5 mℓ·ℓ−1 in bottom water over much of the continental shelf. These conditions apparently displaced juvenile Cape hake Merluccius capensis offshore from their typical inshore habitat, subjecting them to heavy mortalities from predation by larger hake and from commercial trawling.

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 &lt;20 cm was estimated as 0.054 h−1; 90% evacuation of anchovy prey required an estimated 43 h. Using the Elliott &amp; 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.