Dynamics of recruitment of larval and juvenile Cape stumpnose Rhabdosargus holubi (Teleostei: Sparidae) into the Swartkops and Sundays estuaries,South Africa

Recruitment of early life stages into estuaries is an integral part of the life cycle of many marine fish species. Although estuaries are naturally environmentally dynamic, they also are subject to anthropogenic disturbances, including land use and climate change, which may affect recruitment. Rhabdosargus holubi is an endemic marine-spawning species predominantly associated with freshwater-rich estuaries which serve as nursery areas for postflexion larvae and juveniles. This study assessed the effect of environmental variables on the dynamics of recruitment of R. holubi larvae and juveniles into the Swartkops and Sundays estuaries, South Africa. Over a period of two years, fyke nets were set at each estuary mouth to monitor movement into the estuaries at each tidal phase over a 24 h cycle during two sampling sessions per season. Rhabdosargus holubi larvae recruited into estuaries primarily in summer and autumn and during the ebb tide at night, while juvenile movements showed no pattern. Salinity, turbidity and temperature were significantly important factors affecting R. holubi recruitment, with pH having no significant effect. Turbidity and salinity are affected by rainfall and freshwater abstraction which may thus influence recruitment. Rhabdosargus holubi is dependent on estuaries, so a combination of future changes in turbidity, salinity and temperature due to predicted climate change may place populations at risk.     

Evaluating the effects of catch-and-release angling on Cape stumpnose Rhabdosargus holubi in a South African estuary

Fisheries managers are increasingly promoting catch-and-release (C&R) to manage recreationally angled fish stocks. Despite this, there is a scarcity of information on the effects of C&R on estuarine-dependent species. Cape stumpnose Rhabdosargus holubi dominates the recreational fisheries catch and provides an important source of food for subsistence fishers in some temperate South African estuaries. The health and survival of R. holubi exposed to a C&R event was investigated by examining their physiological stress response (blood glucose and lactate), reflex impairment (reflex action mortality predictors [RAMP]) and short-term (12-hour) survival. Fish were captured and exposed to one of three air-exposure treatments: 0 s, 30 s or 90 s. Stress and health were measured either immediately (immediate) or one hour after (delayed) the C&R event. There was no significant difference in blood glucose between air-exposure treatments, but there was a significant difference between the mean immediate and delayed glucose levels within each treatment (F_(2,143) = 81.8, p < 0.01). In contrast, blood lactate level was significantly higher in the 90-s treatment (p < 0.05). Immediate blood lactate levels were significantly lower than the delayed samples for each treatment (F = 4.29, p = 0.02; n = 169). Although all fish exhibited at least one reflex impairment, the RAMP score was significantly higher in the 90-s air-exposure treatment (H_(2,86) = 9.73, p = 0.007). Also, RAMP scores were significantly lower in the delayed samples (p < 0.01). Although short-term mortality was relatively low (2.3%) for this species, it was highest in the 90-s treatment (7%). These results suggest that physiological stress is higher when R. holubi are exposed to longer periods of air exposure and that the physiological stress of fish subject to a C&R event is best measured after a delay.     

Age,growth and natural mortality of blue jack mackerel Trachurus picturatus (Carangidae) from the Canary Islands,Spain (NW Africa)

Status assessment of exploited fish populations is a general requirement to ensure sustainability of fisheries in both global and local contexts. The blue jack mackerel Trachurus picturatus (Bowdich, 1825) remains unassessed in a data-limited fishery in the eastern central Atlantic, within the waters of the Canary Islands (Spain) off the coast of northwest Africa. This is the most southwestern targeted fishery in the species’ distributional range. To obtain basic life-history information, a total of 2 472 specimens were analysed, ranging from 10.2 to 31.9 cm total length, from samples collected monthly between March 2005 and March 2006. Length–weight relationships (a = 0.0023; b = 3.418; R ² = 0.97) showed no significant difference between sexes. Deposition periodicity and coherence of the ageing criteria were verified by analysis of the monthly evolution of the otolith edge. Ages were estimated by counting annual growth rings on whole otoliths, with these fish ranging from age 0 to 6 years. The von Bertalanffy growth model, which was used to estimate growth parameters from both direct readings and back-calculated length-at-age data, gave good statistical fits. The parameters from the direct readings were: L∞ = 34.9 cm; k = 0.214 y^–1; t ₀ = ?2.545 y (R ² = 0.84), and the natural mortality rates estimated from empirical methods varied between 0.20 and 0.55 y^–1.     

The age and growth of hottentot seabream Pachymetopon blochii before and after the South African linefish state of emergency in 2000

The hottentot seabream Pachymetopon blochii is a small-sized (maximum 2.67 kg) sparid endemic to southern Africa. It is an important target in South Africa's Western Cape traditional linefishery, particularly in the absence of more valuable pelagic species (such as Thyrsites atun and Seriola lalandi). In 2000, South Africa's linefishery was declared to be in a state of emergency, and commercial fishing effort was consequently reduced by 70%. A subsequent increase in stock biomass and intraspecific competition, coupled with environmental changes, were hypothesised to have thereafter altered the growth rate of hottentot, from 2000 to 2010. This study aimed to revise outdated age–growth models for the hottentot by using modern techniques (sectioned otoliths), and to compare age–growth relationships before and after the declared linefish state of emergency. The maximum age observed was 19 years, with no difference in the growth rate between sexes (p = 0.39–0.43) or time-periods (p = 0.96). Although the growth rate did not change, there is evidence that the age structure of the stock changed between time-periods as a result of changes in fishing pressure between 2000 and 2010. The enhanced recent growth model for hottentot, described as L_t = 418.063 (1 – e^{?0.104(t – [?4.709])}) (pooled sexes; n = 206), indicates a considerably slower growth rate for this species than was proposed previously using whole otoliths and has major implications for effective stock management.     

Age and Growth of <Emphasis Type="Italic">Sillago aeolus</Emphasis> in Okinawa Island,Japan

In a study of the age and growth of Sillago aeolus, a total of 414 fish were collected from December 2000 to March 2002, around Okinawa Island. Ages of 403 specimens were determined by otoliths. Monthly changes in the percentage of otoliths with an opaque zone in the outer margin indicated that an opaque ring formed once a year, occurring between February and May. The rings can therefore be defined as annual rings. Ages of females were estimated to be 0+ to 4+, while those of males were 0+ to 2+. The von Bertalanffy growth curves were well fitted to age-standard length data of males and females, as follows: Male: L_t = 209.6 [1 − exp{−0.70 (t + 0.58)}]. Female: L_t = 297.7 [1 − exp{−0.42 (t + 0.61)}].     

Stomach content analysis of juvenile Cape stumpnose Rhabdosargus holubi in the Swartkops Estuary,South Africa

Stomach content analyses were performed on the Cape stumpnose Rhabdosargus holubi. The contents were identified to species level and the dietary importance of each component assessed by employing unrelated quantifying methods, which were subsequently combined to determine an index of relative importance (IRI). The index was determined on a seasonal basis as well as for individual sampling stations. The aquatic macrophyte Zostera capensis was the single most abundant item and, together with filamentous algae, constituted the major proportion (IRI = 7 314) of the diet. Crustacea were the next most abundant (IRI = 6 534), followed by Mollusca (IRI = 160). Stomach contents from the sampling stations in the lower reaches were dominated by aquatic macrophytes and algae, whereas those from the middle and upper reaches had a higher incidence of Crustacea. Mollusca in the diet were uniformly distributed throughout the estuary. There was considerable flexibility in the feeding pattern of R. holubi.     

Spatial variation in elemental composition of otoliths of three species of fish (family Sparidae)

Determining the nursery habitat of fishes that have moved from estuarine nursery habitats is difficult. The elemental fingerprints of otoliths of three species of sparids were determined to investigate their utility as a natural tag of the nursery habitat. Juvenile Pagrus auratus (snapper), Rhabdosargus sarba (tarwhine) and Acanthopagrus australis (bream) were collected from two sites in each of 15, six and three estuaries, respectively, and their otoliths analysed by solution-based inductively coupled plasma-mass spectrometry. Significant differences in otolith chemistry were found for all three species of juveniles collected from different estuaries. The same patterns among estuaries were not seen for all species, although it was not possible to sample the same sites within an estuary for all species. For bream, significant differences in otolith chemistry were found among all three estuaries, whereas for tarwhine the six estuaries were separated into three groups. For snapper, a number of estuaries could be separated, but there was some overlap for other estuaries. All three species were collected from the same site within one estuary and their otoliths analysed. Significant differences were found among species, but the implication of this finding remains unclear as the three species show differences in microhabitat use and may also differ in age. Because the elemental fingerprints of juveniles vary among estuaries or groups of estuaries, the nursery or recruitment estuary of adult fish could now be determined by analysing the juvenile region of adult otoliths. Thus, connectivity between estuaries and open coastal populations could be determined. Such information will have major implications for fisheries management because it will provide information on the distance that fish have moved from their recruitment estuary and the number of estuaries that contribute to each adult population.     

Assessment of the recreational linefishery in selected Eastern Cape estuaries: trends in catches and effort

Boat-based creel surveys were used to collect catch and effort data on the shore- and boat-based linefisheries of the Kromme, Gamtoos, Swartkops, Sundays, Bushmans, Kariega, Kowie and Great Fish estuaries in the Eastern Cape, South Africa, from January 1996 to April 1997. The surveys, which were performed primarily during routine fisheries law enforcement patrols by a regional conservation agency, recorded the catch and effort of 2 468 individual angler outings (12 840 angling hours) during 337 angler-count patrols. Although 26 species were recorded, a large proportion of the catch (85%) comprised only four species. Spotted grunter Pomadasys commersonnii (43%) was most commonly caught, followed by Cape stumpnose Rhabdosargus holubi (16%), dusky kob Argyrosomus japonicus (14%) and white seacatfish Galeichthys feliceps (12%). In terms of mass, the overall catch was dominated by A. japonicus (50%) and P. commersonnii (33%). Overall catch per unit effort (cpue) by number was highest during summer and lowest during winter, and vice versa for cpue according to mass. Angler counts showed the Sundays Estuary had the highest mean angling effort for both weekdays and weekends (10.1 and 22.9 anglers count^?1 respectively). The Gamtoos Estuary had the lowest count for weekdays (5.5 anglers count^?1) and the Kariega Estuary the lowest for weekends (9.8 anglers count^?1). Shore-based was more popular than boat-based angling (59% compared to 41%), and all estuaries had substantially higher angling effort on weekends than on weekdays. Angling with bait (93.3%) was more popular than lure- or fly-angling (6.7% combined). Considerations for management of A. japonicus in the Eastern Cape estuarine linefishery are presented.     

Relationship between somatic growth and otolith growth: a case study of the ornate jobfish Pristipomoides argyrogrammicus from the coast of Réunion (SW Indian Ocean)

The ornate jobfish Pristipomoides argyrogrammicus Valenciennes 1832 occurs in the Indo-West Pacific Ocean, where it is harvested by small-scale coastal fisheries. Management of this species is hindered by lack of adequate biological data. We sampled a total of 113 individuals from the landings of local artisanal fishers on the island of Réunion (southwestern Indian Ocean), from March 2014 to March 2015. The relationships between two types of body length (total and standard length, cm) and total weight (g) were shown to be significant (p < 0.05). The length–weight relationship was described by a power function, with the scaling factor estimated to be 0.008 and the exponent 3.146. Age was determined using whole otoliths. The von Bertalanffy growth equation was estimated to be TL_t = 30.68(1 – e^?0.52(t)). Otolith morphometry variables (length, width and area) were significantly correlated with age estimates (p < 0.05). No significant difference in age estimates was observed between left and right otoliths used as predictors. Readings from observed age and the estimates from modelled age indicated relatively good agreement, suggesting the potential to use whole otoliths for age estimation.     

Age and life histories of black and smooth oreos inferred using bomb radiocarbon dating and stable isotope analysis

Black oreo (Allocyttus niger) and smooth oreo (Pseudocyttus maculatus) are commercially exploited deepwater fishes in New Zealand. They are considered to be long-lived with unvalidated maximum otolith growth zone counts of 153 years and 86 years, respectively. Bomb radiocarbon dating results supported the otolith age estimates for A. niger, but those for P. maculatus provided only partial support. Oxygen isotope (δ¹⁸O) results for A. niger otoliths suggested that juveniles were in near-surface waters and adults in depths of 700–1300 m. In contrast, P. maculatus otoliths suggested a variable depth history (500 to at least 1500 m) throughout life. Carbon isotope (δ¹³C) results showed a steady enrichment from core values to c. 20 years for both species, owing to changes in diet and metabolic rate from the juvenile to the adult fish, associated with a depth increase. Von Bertalanffy growth parameters are provided for both species.     

A preliminary study of age and growth of the monkfish Lophius upsicephalus (Pisces: Lophiidae) on the Agulhas Bank,South Africa

The sagittal otoliths of Lophius upsicephalus, although extremely variable in shape, were found to be the only structures suitable for age estimation. The annuli are multi-ringed zones separated by "wider than normal" opaque zones. They are difficult to interpret, especially those of fish older than nine years. Periodicity of annulus formation could not be determined by standard methods. By inference, annulus formation could be related to feeding habits and reproductive seasonality. The growth in length (sexes combined) is best described by

Length-at-age = 733,7 (1 – e^{?0,1054 (t + 1,879)})mm.

There is a significant difference in growth by mass between the sexes. The respective equations describing growth by mass (in g) for males and females are:

Weight-at-age = 8616 (1 – e^{?0,1054 (t + 1,879)})^2,805 and

Weight-at-age = 9499 (1 – e^{?0,1054 (t + 1,879)})^2,890.     

Growth of red gurnard (Teleostei: Triglidae) from Pegasus bay,Canterbury, New Zealand

Growth of the red gurnard, Chelidonichthys kumu (Lesson and Garnot), from Pegasus Bay, Canterbury, was measured during 1966–67. Otoliths were used as an indicator of fish growth; mean length‐at‐age data were obtained from back‐calculated fish lengths at the time of formation of successive annual rings in the otoliths. Growth in length was found to be adequately expressed by the von Bertalanffy growth equation :

l_t = 52.0 [1 ‐ e‐^{0.406 (t‐o.291)}]

(where l_t is the fork length in cm at age t). The length: weight relationship was:

w = 78.56 × 10^‐4 l ^3.072

(where w is the weight in grams). From this relationship, growth in weight was described by the equation:

w_t = 1469 [1 ‐ e‐0.406 (t‐0.291)]³.     

Biology of the redspotted tonguesole Cynoglossus zanzibarensis (Pleuronectiformes: Cynoglossidae) on the Agulhas Bank,South Africa

The biology of the redspotted tonguesole Cynoglossus zanzibarensis, a common African cynoglossid inhabiting the Agulhas Bank, South Africa, is described. Growth studies based on sectioned sagittal otoliths revealed that C. zanzibarensis is relatively fast-growing and long-lived, attaining ages >8 years. Growth in length was rapid in immature fish, fish attaining 56% of their maximum size within their first year. By sexual maturity, fish had attained 28% of their maximum age and 68% of their maximum length. Total length-at-age was best described by the Von Bertalanffy growth model with combined-sex growth described as L_t = 354.78(1?e^{?0.43 (t+1.17)}) mm TL. Sexually dimorphic growth patterns were evident, females attaining larger lengths, but at a slower growth rate than males. Despite the similar mean size of adult fish, the trawl-sampled adult population was dominated by females, with a sex ratio of 1 male:2.4 females. Female C. zanzibarensis mature in their second year of life (275 mm TL), after which they spawn small, pelagic eggs throughout the year. Approximations of the rates of total, natural and fishing mortality were estimated to be 0.62, 0.48 and 0.14 year^?1 respectively.     

Value and management of the subsistence fishery at Knysna Estuary,South Africa

Knysna Estuary supports an estimated 30 full-time and 200 part-time subsistence fishers involved in bait collection, mud crab harvesting, and fishing. The mud prawn Upogebia africana dominates the bait-fishery, with estimated catches amounting to about 3% of the standing stock, suggesting sustainable use. Harvest of Marphysa spp. and Gorgonorhynchus dayi is conducted in a destructive manner. Most of the value of fishing lies in the setline catches of spotted grunter Pomadasys commersonnii and white steenbras Lithognathus lithognathus, whereas small species and individuals, particularly Cape stumpnose Rhabdosargus holubi, are also caught with handlines. Indications are that the linefishery is also sustainable at present. The subsistence fishery is worth an estimated R0.7-R1.1 million per annum, with full-time fishers earning at least R11 000-R17 000 per annum from the estuary. Currently operating under recreational regulations, the fishery is poorly controlled and fails to reach its full potential. The main threat to the estuary from damaging harvests of bait species, such as Marphysa spp. and G. dayi, should be discouraged through heavy penalisation of buyers and traders. Sales of mud prawn could probably be legalised. If off-take rates are found to be sustainable, sales of certain fish species could be legalised if access to the fishery could be effectively controlled under a co-management arrangement.     

Growth of Cape horse mackerel Trachurus trachurus capensis off South Africa

Recent studies have provided evidence that the Cape horse mackerel Trachurus trachwus capensis in South African waters is a unit stock. Therefore, an age and growth study of the entire stock throughout its distributional range was undertaken in 1988/89. Growth is described by the equation

L_t = 487,989 (1 – e^{?0.556(t – 0,229)}) mm.     

Aspects of the biology and fisheries of an economically important sparid Dentex macrophthalmus (Bloch 1791) in the Namibe province,Angola

The sparid Dentex macrophthalmus is a widespread, important fishery species along most of the West African coast from southern Namibia to the Mediterranean. In southern Angola it is an important artisanal species targeted predominantly by handline fishers. A biological and fisheries study was conducted on this species in southern Angola between June 2008 and July 2009. It was the dominant species in the artisanal fishery, accounting for 99% of the sparids captured and 67% (by mass) of the total catch. The life history of D. macrophthalmus was characterised by slow growth (females: L_t = 309[1 ? e^{?0.06(t + 5.43)}], males: L_t = 248[1 ? e^{?0.16(t ? 1.77)}]), advanced age at maturity (females: 7.4 years, males: 6.0 years) and high longevity (females: 36 years, males: 38 years). The sex ratio was 1:1 male:female. The length- and age-frequency distributions and macroscopic observations suggested that the species is a late gonochorist. Males and females reached 50% maturity at 151 and 166 mm fork length respectively. Although individuals with ripe gonads were found during most of the year, the peak spawning period appeared to be in December and January. Despite a life history that renders D. macrophthal-mus vulnerable to overexploitation, only 38% of artisanal fishers noticed a decline in the catches of this species. Potential reasons for this include: technology creep; limited pressure on the juvenile portion of the stock as a result of late recruitment (above the length of 50% maturity); the ‘basin affect’, whereby depleted areas are reseeded by areas (e.g. deeper water) inaccessible to the linefishery; and a deep-water reserve of large individuals. It is recommended that precautionary management strategies be implemented until the age estimates are revised by the countries in which this species forms significant fisheries.     

Age and growth of redeye round herring Etrumeus whiteheadi off the South African coast

Ages of redeye round herring Etrumeus whiteheadi were estimated with associated errors and used to infer life-history information, such as age composition, age-at-maturity and instantaneous mortality rate. Samples were collected in November 2005 during a research survey aimed at estimating the biomass of spawning pelagic fish off South Africa’s west and south coasts. Replicate age estimates obtained from sagittal otoliths were collected with slight bias and relatively high precision. A von Bertalanffy model describing growth of the combined sexes, including juveniles, was L_t = 20.41(1 ? e^{?0.41(t ? 1.92)}). Kimura’s likelihood ratio test revealed no statistically significant differences between growth parameters of males and females. Results suggested that otolith length is a better predictor of age than otolith weight. Maturity estimates for E. whiteheadi were similar to those previously documented.     

Catch and effort of the shore and skiboat linefisheries along the South African Eastern Cape Coast

An assessment of catch and effort in the Eastern Cape shore and skiboat linefisheries was undertaken between 1994 and 1996 by means of roving creel and access point surveys. Catch-and-effort data were obtained from direct observation of 3 273 shore-fishers, 172 recreational and 223 commercial skiboat outings. Total effort in the region was high at 903 186 fisher-days year^?1 in the shore fishery and 64 266 and 24 357 fisher-days year^?1 in the commercial and recreational skiboat sectors respectively. The fisheries are multispecies in nature. The shore fishery consisted of 66 species, the recreational skiboat fishery 44 species and the commercial skiboat fishery 48 species. Just 10 species accounted for 75, 83 and 90% of the catch of the shore, recreational and commercial skiboat fishery respectively. The average catch per unit effort (cpue) was low in all sectors, 1.15 kg fisher^?1 day^?1 in the shorefishery, 9.4 kg fisher^?1 day^?1 in the recreational skiboat fishery and 21.5 kg fisher^?1 day^?1 in the commercial skiboat fishery. Catch data showed that professional and club anglers are more successful fishers. Comparisons with historic records for Port Elizabeth revealed that the cpue in the shore fishery had declined markedly, whereas the total effort increase was negligible (0.1%). In addition, the species composition of the fishery has changed.     

Population dynamics of the venerid bivalve Callista chione (L.) in a coastal area of the eastern Mediterranean

The venerid bivalve Callista chione is the most prominent suspension-feeding bivalve inhabiting the soft bottom of coastal areas in the northern Euboikos Gulf (Aegean Sea), where it is exploited by small-scale fishery. The parameters of the von Bertalanffy growth function were estimated and the somatic production-to-biomass ratio was found to be 0.45 y⁻¹. The maximum age of adult individuals was 17 years. The species is gonochoristic, its oocyte diameter ranged from 54.5 to 120 μm with a conspicuous jelly-coat. Microscopical examination of the gonads showed that gonadal production did not start until the second year of life. The observations on the gonadal cycle indicate that this bivalve is capable of year-round reproduction.