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Age and growth of the Cape knifejaw Oplegnathus conwayi , an endemic South African teleost


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The South African spearfishery targets a variety of data-deficient species, which are consequently poorly managed. This study aimed to describe the age and growth of one of these species, the Cape knifejaw, Oplegnathus conwayi, which is endemic to the southern and eastern coasts of South Africa. Monthly biological samples were collected through research spearfishing (n = 170) and augmented by recreational spearfishers’ catches (n = 135). The results indicated that the O. conwayi population sex ratio was skewed towards males (1M:0.6F). The length- and age-frequency distributions were similar between sexes. Oplegnathus conwayi is a relatively slow-growing species, with a maximum-recorded age of 27 years. No significant differences were observed between male and female growth, with the overall population growth curve being best described as L(t) = 697.15(1 − e−0.06(t−6.30)). The slow growth observed in this species is characteristic of a species that is vulnerable to overexploitation, and accordingly a precautionary approach to future management is recommended.
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African Zoology
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Age and growth of the Cape knifejaw Oplegnathus
conwayi, an endemic South African teleost
RM Foster, A-R Childs, BQ Mann & WM Potts
To cite this article: RM Foster, A-R Childs, BQ Mann & WM Potts (2022): Age and growth of the
Cape knifejaw Oplegnathus�conwayi, an endemic South African teleost, African Zoology, DOI:
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Published online: 03 May 2022.
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African Zoology 2022, 57(1): xxx–xxx
Printed in South Africa — All rights reserved
Copyright © Zoological Society
of Southern Africa
ISSN 1562-7020 EISSN 2224-073X
African Zoology is co-published by NISC (Pty) Ltd and Informa UK Limited (trading as Taylor & Francis Group)
This is the nal version of the article that is
published ahead of the print and online issue
Age and growth of the Cape knifejaw Oplegnathus conwayi, an endemic
South African teleost
RM Foster1 , A-R Childs1 , BQ Mann2 and WM Potts1*
1 Department of Ichthyology and Fisheries Science, Rhodes University, Makhanda (formerly Grahamstown), South Africa
2 Oceanographic Research Institute, South African Association for Marine Biological Research, Durban, South Africa
The South African spearfishery targets a variety of data-deficient species, which are consequently poorly managed.
This study aimed to describe the age and growth of one of these species, the Cape knifejaw, Oplegnathus conwayi,
which is endemic to the southern and eastern coasts of South Africa. Monthly biological samples were collected
through research spearfishing (n = 170) and augmented by recreational spearfishers’ catches (n = 135). The
results indicated that the O. conwayi population sex ratio was skewed towards males (1M:0.6F). The length- and
age-frequency distributions were similar between sexes. Oplegnathus conwayi is a relatively slow-growing species,
with a maximum-recorded age of 27 years. No significant differences were observed between male and female
growth, with the overall population growth curve being best described as L(t) = 697.15(1 − e−0.06(t−6.30)). The slow
growth observed in this species is characteristic of a species that is vulnerable to overexploitation, and accordingly
a precautionary approach to future management is recommended.
Keywords: citizen science, growth curves, management, otoliths, spearfishery
Life history information, such as growth rates, age and size
of maturity allows fisheries scientists to understand the
potential resilience of a species to exploitation (Winemiller
2005). Furthermore, information on the size structure and
age-and-growth of a species can be incorporated into the
assessment of fish stocks to enable fisheries managers
to make informed management decisions (Griffiths et al.
1999; King and McFarlane 2003; Winemiller 2005; Young
et al. 2006). Management decisions made without a good
understanding of the life history of target species have
been detrimental in the past and have sometimes led to
population collapses (Hilborn et al. 2020). Therefore, the
collection of species-specific life history information is
imperative for the promotion of sustainable fisheries.
South Africa has a rich history of biological research on
marine fishes (Palmer et al. 2008) and there is detailed
information available for the majority of important fishes
captured in the linefishery (Mann 2013). However,
there is limited information for several species that
are exclusively captured in the spearfishery. These
include species like Oplegnathus conwayi, O. robinsoni,
Chirodactylus jessicalenorum and C. grandis (RSA 1998;
Mann 2013). This lack of knowledge has undoubtedly
contributed to the absence of stock assessments and the
development of appropriate species-specific regulations
for these species.
One of the primary reasons for the lack of life history
information for fish that are predominantly targeted in the
spearfishery is the difficulty associated with collecting
sufficient specimens (Carruthers et al. 2014). This is
because these species are not readily captured using
traditional sampling methods. However, citizen science
projects, which have encouraged anglers to provide
specimens (both whole and without fillets) have recently
been effectively implemented in Australia (Fairclough et al.
2015) and South Africa (Hewett 2019) and have enabled
the collection of sufficient samples for life history and
stock assessment research. The ‘Send us your Skeletons’
(SUYS) programme in Australia has proven to be an
extremely successful example of this and has dramatically
increased data collection, as well as substantially
decreasing the costs (Fairclough et al. 2015). Although
large-scale programmes, such as this have not been
developed in South Africa, the collection of fish skeletons
by the recreational sector holds much promise both here
and elsewhere in the developing world, where research
funding is not always readily available for extensive field
data collection (Potts et al. 2021).
There are three species of Oplegnathus endemic to
southern Africa, two of which (O. conwayi and O. robinsoni)
are commonly targeted by South African recreational
spearfishers (Chater et al. 1995; Mann et al. 1997; Lloyd
et al. 2012). Oplegnathus robinsoni is an important target
species in KwaZulu-Natal, whereas O. conwayi is important
along the eastern seaboard of South Africa south of the
Thukela River, with the greatest catches and abundance
occurring in the Eastern Cape and Western Cape (Mann
and Maggs 2013). These species are seldom captured by
anglers, because of their mouth morphology and feeding
habits (Chater et al. 1995). Previous research conducted
Published online 03 May 2022
Foster, Childs, Mann and Potts
on the South African spearfishery between 1984 and 1995
(Mann et al. 1997) showed that Oplegnathus spp. made up
28% of spearfishing competition catches in KwaZulu-Natal,
whereas O. conwayi made up 30% of competition catches
by spearfishers in the Eastern Cape and 16% in the
Western Cape. Mann et al. (1997) also showed that O.
conwayi bag limits were often reached, and it was the sixth
most important species targeted by spearfishers in both the
Eastern Cape and Western Cape. Current species-specific
regulation for O. conwayi includes a daily bag limit of five
fish person−1 d−1 and a ban on the sale of the species.
There has been very little biological data collected on
O. conwayi (Chater et al. 1995; Mann and Maggs 2013).
Chater et al. (1995) conducted a preliminary assessment
on the biology of both knifejaw species (O. conwayi and
O. robinsoni) and examined growth, feeding behaviour
and reproduction. The study suggested that O. conwayi
predominantly feed on sponges, barnacles, sea cucumbers
and algae, using their beak-like mouths to bite off, and
scrape the reef of sessile invertebrates. Unfortunately, as is
the case with many spearfishery species, they were unable
to collect sufficient specimens of O. conwayi to draw robust
conclusions on their growth. However, they estimated a
maximum age of 13 years for O. conwayi, based on their
readings of whole otoliths.
The aim of this study was to provide a description of the
age and growth of O. conwayi. To achieve this, O. conwayi
samples were collected during monthly research spearfishing
excursions and voluntary donations from recreational
spearfishers between February 2019 and January 2020.
Materials and methods
Fish sampling
Oplegnathus conwayi specimens were collected monthly
between Kenton-on-Sea (30 km west of Port Alfred) and
East London (Figure 1) by research spearfishing between
February 2019 and January 2020 (Rhodes University
Animal Ethics Committee clearance: No. 2020-2814-
4858). In addition, local affiliated spearfishing club
members (Border Undersea Club and Gully Jumpers)
and unaffiliated spearfishers were asked to donate their
catches for biological sampling (Appendix 1). Fresh fish
frames were collected from these volunteers whenever
possible, otherwise, the frames were frozen and collected
at a later stage. Additionally, sampling was conducted at
two spearfishing competitions between Durban and East
London, one at Hole in The Wall (HITW) (Eastern Cape)
(161 km east of East London) and one in Durban (KwaZulu-
Natal) (Figure 1), where specimens of O. conwayi brought
to the weigh-in were sampled.
Biological data collection
The fork length (FL) and total length (TL) of each fish was
measured to the nearest mm. Fish were weighed whole,
with and without viscera, respectively, to the nearest gram
and sexed macroscopically (juvenile, male, female). Where
only the fish frame was available, no weight or sex could
be determined. Sagittal otoliths were removed and stored in
paper envelopes for later preparation and analysis.
Age and Growth
Otolith preparation and reading
A pilot study was conducted to determine the optimal
sectioning plane and preparation of the otoliths using the
methods proposed by Winkler (2013). Fifteen random
otolith pairs were selected and set in clear polyester resin
in latex setting trays. Once set, the resin was removed and
one of each pair of otoliths was sectioned transversely
through the nucleus, whereas the other longitudinally, at a
thickness of between 0.4 and 0.5 mm, to assess the best
sectioning plane using a twin-bladed, diamond-edged,
geological saw. The sectioned otoliths were mounted on
glass slides using a DPX mounting medium. The otoliths
were read under transmitted light, using a low-power
dissecting microscope (10–35 × magnification). Two
independent readers counted the visible opaque zones.
A readability index (from 0 = unreadable to 5 = easily
readable) was assigned to each otolith to determine which
section was most accurate, and average readability indices
(ARI) were calculated for the transversely and longitudinally
sectioned otoliths, respectively. The reliability of each
growth zone count was assessed through an index of
average percentage error (IAPE) calculation (Beamish and
Fournier 1981), using Equation 1:
ij j
ji j
= =
Equation 1
where n = aged fish, R = number of times each j fish is
aged, Xij = ith age determined for the jth fish, and ×
j = the
mean age calculated for the jth fish.
Transversely sectioned otoliths had the highest ARI
(3.2) and the lowest IAPE (33.8%), compared with
longitudinally sectioned otoliths and consequently the
remaining (283) otoliths were sectioned transversely.
All the otoliths were read by three independent readers.
Otolith counts were accepted if two of the reader’s counts
coincided, and where counts followed a succession
(e.g. 2, 3 and 4), the middle count was accepted. When
assessing older fish (>12 years), if all three counts
differed by two or less (e.g. 14, 15 and 17), the counts
were accepted, and the middle count was chosen. When
counts exceeded 20 and differed by more than two, the
closest two counts were averaged (e.g. 26, 28 and 39
was accepted as 27). Sectioned otoliths with counts out of
these ranges were discarded.
IAPE values were also calculated for fish sampled
in different biogeographic zones (i.e. subtropical,
warm-temperate and cool-temperate zones) and in different
size classes (i.e. small 10–250 mm FL, medium 251–450
mm FL and large 451–600 mm FL) to account for potential
differences in the difficulty of age estimation.
Length-weight relationships
The relationship between fork length (FL) and total length
(TL) was expressed using a linear relationship:
FL mTL c= +
Equation 2
African Zoology 2022, 57(1): xxx–xxx 3
where m is the slope and c is the intercept coefficient.
The relationship between FL and whole weight (Wt) was
described using the following power relationship:
Equation 3
are the model parameters to be estimated.
Age validation and increment analysis
Marginal zone analysis was used to determine the
seasonality of growth zone deposition. The proportion of
otoliths with hyaline or opaque edges was plotted by month
over the year.
Growth Model
A three-parameter von Bertalanffy Growth Function (VBGF)
(Ricker 1975) was used to model the growth of O. conwayi
from the observed length-at-age data. This model was
selected above other similar growth models, because it
had the lowest Akaike’s Information Criterion (AIC) value
(Bozdogan 1987), making it more statistically robust. The data
were modelled using the VBGF equation represented below:
( )
( )
( )
kt t
Lt L e
= −
Equation 4
where L(t) is the length of an individual at a given time, L()
is the asymptotic maximum length of the population, k is
the growth coefficient and t0 is the theoretical length at
age zero.
A downhill simplex search routine was used to
estimate the model’s three parameters (Nelder and
Mead 1965). The model’s variability was estimated using
a parametric bootstrapping procedure (Efron 1982) with
1 000 iterations, from which 95% confidence intervals
were constructed. All juveniles were included in both the
male and female models to maintain biological realism.
Differences in the model parameters of the sexes were
tested with a likelihood ratio test (LRT).
30° S
35° S
30° E25° E20° E Credits: Tayla Dominy
Cape Town
Port Elizabeth
East London
Port Edward
0 210 420 km
Hole in the Wall
Cape St. Francis
Port Alfred
Number of fish
Figure 1: Map of South Africa showing the sites where Oplegnathus conwayi were collected during the sampling period (February 2019–
January 2020). The proportions of samples collected per site are represented by the size of circle at each site. The location of the three
biogeographic zones is indicated in different colours
Foster, Childs, Mann and Potts
All analyses were run, and graphs created, in R studio
and R 3.6.1 (R Development Core Team).
Population structure and morphometrics
In total, 305 fish were sampled, 254 fish from the
warm-temperate coastal zone (East London–Struisbaai),
28 from the subtropical zone (Durban–East London)
and 23 from the cool-temperate zone (Cape Agulhas–
Cape Point) (Figure 1). The largest proportion of samples
(50.1%, n = 154) came from Kenton-on-Sea in the
Eastern Cape (see Appendix 1, Figure 1). Of the 305
individuals, 105 were males, 58 females, 87 juveniles and
55 were unsexed (because of fish frames being provided
that had already been eviscerated). A total of 170 fish
were collected by research spearfishing, whereas 108
specimens were collected from volunteer spearfishers
(citizen scientists) and 27 specimens were sampled at
spearfishing competitions (see Appendix 1). The average
minimum size of specimens collected by citizen scientists
was larger than that of specimens collected by research
spearfishing. The overall population sex ratio was male
dominated at 1 M:0.6 F. Juveniles were observed up to
a length of approximately 400 mm FL. Generally, males
dominated the adult population, particularly in the larger
(>450 mm FL) size classes (Figure 2).
The mean FL and whole weight of sampled specimens
was 392 mm (range: 104–600 mm) and 1 321 g (range:
75–3 356 g), respectively. The relationship between FL and
TL was best described by the equation: TL = 1.071(FL)
2.599 with ( = 0.99) (Figure 3a), whereas the relationship
between FL and mass was best described by the exponential
equation: Wt = 0.000029(FL)2.945 (= 0.97) (Figure 3b).
Age and Growth
Of the 298 otoliths read, 76.9% (229) were accepted,
with 89.7%, 73.6% and 95.7% being accepted in the
subtropical, warm-temperate and cool-temperate
biogeographic zones, respectively. The IAPE of otoliths
from larger fish (451–600 mm FL) was much lower
(9.9%) than small- (26.3%) and medium-sized (24.1%)
fish (100–450 mm TL) (Figure 4). Within the medium-
sized fish, the IAPE was considerably higher (26.9%)
in the warm-temperate biogeographic zone compared
with the cool-temperate (9.9%) and subtropical (9.3%)
biogeographic zones.
Age validation and increment deposition
There was a high proportion of otoliths with hyaline
edges during late summer (February to March), whereas
opaque margins dominated during the remainder of the
year (Figure 5). The hyaline peak in October is likely to
be an anomaly, because of the small sample size (n = 4).
Although not definitive, the inverse relationship between
hyaline and opaque edges suggests that only one hyaline
and one opaque band are deposited annually.
The oldest unsexed, male and female fish were 27, 24
and 18 years old, respectively. The von Bertalanffy growth
equation was L(t) = 673.76(1 − e−0.07(t + 5.48)) for males
(Figure 6a, Table 1) and L(t) = 603.33(1 e−0.07(t + 5.64)) for
females (Figure 6b, Table 1). There was no significant
difference in growth between males and females (LRT, p >
0.05). The growth curve for the pooled dataset was best
described as L(t) = 697.15(1 − e−0.06(t + 6.30)) (Figure 6c, Table 1).
The collection of biological data on O. conwayi by research
spearfishing and with the added assistance of volunteer
spearfishers was effective, with the results providing
valuable information that can be incorporated into the
assessment and subsequent management of this species.
Slow growth and high longevity are features of O. conwayi’s
life history, as they are for many other targeted inshore reef
fish in South Africa (inter alia Buxton and Clarke 1992;
Bennett 1993; Mann and Buxton 1997; Potts and Cowley
2005) and this should be considered when reviewing
species-specific regulations.
Previously the maximum age recorded for O. conwayi,
based on whole otolith readings from a fish of 510 mm FL
was 13 years (Chater et al. 1995). This is considerably
100 200 300 400 500 600
5 10 15 20 25
Figure 2: Length (n = 305) and age (n = 229) frequency
distributions of Oplegnathus conwayi, collected between Durban
and Struisbaai between February 2019 and January 2020
African Zoology 2022, 57(1): xxx–xxx 5
younger than the 27-year-old fish of 600 mm FL observed
in this study. The growth curve obtained in this study
predicts that a 510 mm FL fish would be 16 years of age.
This difference in estimates could reflect sampling error or
natural variation, but we suspect that Chater et al (1995)
underestimated the age, as a result of reading whole
otoliths. Numerous studies have shown that reading whole
otoliths generally underestimates the age of older fish
(reviewed in Winkler et al. 2019). This underestimation can
be detrimental, if used in subsequent stock assessment
models, because it would result in an overestimation
of stock productivity, which could ultimately lead to the
overexploitation of the population/species. Therefore, it is
desirable to use sectioned otoliths to accurately determine
the age of the fish in the population and every attempt
should be made to obtain samples of the largest fish in the
population to enable determination of the maximum age.
In this study, otoliths from fish sampled in the
cool-temperate and subtropical biogeographic zones
were generally easier to read than those from the
warm-temperate zone based on IAPE calculations. The
reason might be the high variability in water temperatures
TL = 1.071(FL) − 2.599
R2 = 0.99
Wt = 0.000029(FL)2.945
R2 = 0.97
100 200 300 400 500 600
Figure 3: The relationship between (a) fork length (FL) and total length (TL) (n = 305) and (b) fork length and weight (Wt) (n = 215) for
Oplegnathus conwayi collected between Durban and Struisbaai between February 2019 and January 2020
Foster, Childs, Mann and Potts
in the warm-temperate zone, especially during the
summer months (Lutjeharms 2006), which may result in
the deposition of check rings (e.g. following cold upwelling
events) making interpretation of annual growth zones
more difficult (Mann-Lang and Buxton 1996). In addition
to the region where fish were sampled, the readability of
growth zones proved to be better in larger fish, compared
with smaller- and medium-sized fish. This observation is
not uncommon in fish and has been attributed to the rapid
growth of fish at an early age and the associated difficulty
in determining the position of the first annulus (Campana
and Thorrold 2001; Richardson 2010; Farthing et al 2018).
There were several limitations in this study that should
be considered when assessing the validity of the findings
presented. The validation of the periodicity of growth zone
deposition is critical for accurate aging and although a
variety of methods can be used for validation, many, such
as chemical marking using oxytetracycline (OTC) (Lang
and Buxton 1993), were not possible, because of the short
duration of this study and the collection methods used.
Marginal zone analysis (MZA) is generally not considered
to be a robust method of validating the periodicity of growth
zone deposition (Campana 2001), particularly when sample
sizes are low in some months. Although inconclusive in this
study, there was a predominance of hyaline edges in late
summer (February and March) indicating faster somatic
growth and a greater proportion of opaque growth during
Figure 4: The index of average percentage error (IAPE) for small
(100–250 mm FL), medium (251–450 mm FL) and large (451–600
mm FL) fish in each of the three biogeographic regions (ST:
subtropical; WT: warm-temperate; CT: cool-temperate) and overall.
Numbers at the top of each bar indicate sample sizes
49 21 23 18 35 24 24 40 4 10 20 21
JanFeb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 5: The monthly proportion of hyaline and opaque edges
on the otoliths of Oplegnathus conwayi, collected between Durban
and Struisbaai between February 2019 and January 2020. Monthly
sample sizes are shown along the top
510 15 20 25
AGE (years)
n = 58
n = 227
n = 105
L(t) = 603.33(1e0.07(t+5.84))
L(t) = 673.76(1e0.07(t+5.48))
L(t) = 697.15(1−e0.06(t+6.30))
Figure 6: von Bertalanffy growth curves for (a) male, (b) female and
(c) full population model (combined sexes) for Oplegnathus conwayi
collected between February 2019 and January 2020 along the east
and south coast of South Africa. The dotted lines represent the 95%
confidence intervals, estimated from the parametric bootstrap
African Zoology 2022, 57(1): xxx–xxx 7
winter, spring and early summer indicating slower somatic
growth (Figure 5). Nevertheless, the deposition of a single
opaque and hyaline band each year is common among
many coastal fish species that share similar habitats to O.
conwayi (Table 2). A peak of opaque growth deposited
during spring and early summer has also been documented
for several southern African linefishes (Buxton and Clarke
1992; Bennett 1993; Mann-Lang and Buxton 1996, Mann
and Buxton 1997; van Zyl 2013). This has been attributed to
the onset of spawning in these species at this time of year
and the associated energetic requirements of spawning
resulting in slower somatic growth (Mann-Lang and Buxton
1996). If this assumption is valid, then the protracted
spawning season observed in O. conwayi by Chater et al
(1995) may well be the reason for the predominance of
opaque growth seen at the margins of many of the otoliths
observed. Therefore, although age validation was not
conclusive in this study, the assumption that O. conwayi
deposits one hyaline and one opaque zone annually
is probably correct. It is advised that, because of the low
number of samples obtained during the spring and early
summer months in this study, future research should
attempt to reassess the validation of otolith increment
deposition, particularly with large fish and in the subtropical
and cool-temperate regions, where the lowest IAPE values
were observed. Alternatively, the collection of specimens
by hand netting, injecting them with OTC and allowing
them to grow out in captivity with natural seasonal variation
in daylight and water temperature for a year may be an
alternative approach for validating the age of O. conwayi
(see James et al. 2003).
The very low t0 value estimates for this species is
problematic and likely results in an overestimate of early
growth and a ‘shallow’ growth curve. This bias can most
likely be ascribed to the selectivity associated with the
gear (spear) used in this study. This is because larger
individuals are targeted and therefore only the fastest
growing individuals in the early age classes are represented
(Farthing et al. 2018). This is not unusual and has been
documented in several other reef associated species that
are targeted with hook and line in South Africa (see Pulfrich
and Griffiths 1988; Brouwer and Griffiths 2004; Farthing et al.
2018). Stock assessment scientists and fisheries managers
should be aware of this ‘early growth bias’, while every effort
should be made to improve these estimates by employing
different capture methods (e.g. traps, mist nets, eugenol) to
capture a representative sample of the earliest age classes.
The Gallucci–Quinn (GQ) index, which is estimated as
L × k, is recognised to be a reliable index for comparing
the growth of fishes (Charnov 2010) and was used to
Summary Statistics
Parameter Point estimate Standard error Lower 95% CI Upper 95% CI
Overall population
(n = 227)
L(mm FL) 697.15 66.01 599.98 902.88
K (years) 0.06 0.01 0.03 0.08
t̥ (years) −6.30 0.97 −8.50 −4.74
(n = 105)
L(mm FL) 673.76 80.44 566.34 959.42
K (years) 0.07 0.01 0.03 0.11
t̥ (years) −5.48 1.05 −8.00 −3.82
Females (n = 58)
L(mm FL) 603.33 107.83 478.07 1 154.09
K (years) 0.07 0.03 0.02 0.14
t̥ (years) −5.64 1.47 −9.41 −3.36
Table 1: Summary statistics and point estimates for the parameters from the von Bertalanffy growth function for the whole population
(combined sexes), male and female Oplegnathus conwayi captured during the sampling period (February 2019–January 2020) between
Durban and Cape Point
Species Common
Name Distribution Validation
Method Reference
Dichistius capensis Galjoen Southern Angola–Durban OTC Potts and Cowley (2005)
Umbrina robinsoni Baardman Western Indian Ocean
(Cape Point–Kosi Bay)
MZA, MIA, OTC Hutchings and Griffiths (2010)
Chrysoblephus cristiceps Dageraad Cape Point–Durban MZA Buxton (1993)
Chrysoblephus gibbiceps Red stumpnose Cape Point–East London MZA Attwood et al (2019)
Chrysoblephus lacticeps Red roman Cape Point–Port St Johns OTC Potts and Cowley (2005)
Cymatoceps nasutus Black musselcracker Cape Agulhas–Southern Mozambique OTC Potts and Cowley (2005)
Diplodus capensis Blacktail Cape Point–Southern Mozambique MZA, OTC Mann and Buxton (1997)
Pachymetopon blochii Hottentot Angola–Port Alfred OTC Farthing et al (2018)
Pachymetopon grande Bronze bream Cape Agulhas–Southern Mozambique MZA Buxton and Clarke (1992)
Table 2: Various southern African reef fish species that occupy similar reef habitats to Oplegnathus conwayi, with confirmed single
opaque and hyaline depositions occurring annually in their otoliths. Each species’ distribution and validation method is shown by OTC
(Oxytetracycline), MZA (marginal zone analysis) and MIA (marginal increment analysis)
Foster, Childs, Mann and Potts
Family Species Common name IUCN
listing Stock status
Size at
Carangidae Lichia amia Leervis (Garrick) LC Collapsed (<25%) 80 10 180 Fast R,S,SB,B All
Seriola lalandi Cape yellowtail LC Optimally exploited 47–84 21 143 Fast S,SB,B,C All
Dichistiidae Dichistius capensis Galjoen N/A Collapsed (<25%) 31–34 12 47 Slow R,S,SB WT,CT
Umbrina robinsoni Baardman DD Collapsed (<25%) 37–48 16 60 Slow R,S,SB All
Scombridae Scomberomorus commerson King mackerel (couta) NT Optimally exploited 68 14 220 Fast S,B ST
Scomberomorus plurilineatus Queen mackerel (Natal snoek) DD Optimally exploited 75 6 117 Fast S,B ST
Serranidae Epinephelus andersoni Catface rockcod NT Optimally exploited 43–50 11 87 Medium R,S,SB,B,C ST,WT
Sparidae Chrysoblephus gibbiceps Red stumpnose EN Not assessed
(assumed <5%)
21–25 48 45 Slow S,B,C WT,CT
Chrysoblephus lacticeps Red roman NT Optimally exploited 18 17 51 Slow S,B,C WT,CT
Cymatoceps nasutus Black musselcracker VU Not assessed
(assumed <20%)
53 45 109 Slow R,S,SB,B All
Diplodus capensis Blacktail LC Not assessed 15–16 20 35 Slow R,SB All
Lithognathus lithognathus White steenbras EN Collapsed (<25%) 65 25–30 138 Medium R,S,SB All
Pachymetopon blochii Hottentot LC Optimally exploited 20–22 21 54 Slow S,SB,B,C WT,CT
Pachymetopon grande Bronze bream NT Not assessed 30 38+ 57 Slow R,S,SB ST,WT
Sparodon durbanensis White musselcracker NT Not assessed
(assumed <20%)
35 31 103 Medium R,S,SB All
Table 3: Some of the main target species in the South African recreational linefishery with IUCN (International Union for Conservation of Nature) Red Listing, stock status and growth
rates. IUCN Red Listing: EN (endangered), VU (vulnerable), NT (near threatened), LC (least concern), DD (data deficient), NA (not assessed). Harvesting methods: R (shore-based rod
and line), S (spearfishing), SB (small-scale/subsistence), B (boat-based rod and line), C (commercial linefishing). Biogeographic regions: ST (subtropical), WT (warm-temperate) and CT
(cool-temperate). Source: Mann (2013)
African Zoology 2022, 57(1): xxx–xxx 9
illustrate the comparative growth of O. conwayi (Table 3).
The GQ index value for O. conwayi was far lower than fast
growing pelagic gamefish species like Seriola lalandi and
Scomberomorus commerson, but similar (although still
lower) than the slowest growing reef associated species,
such as Chrysoblephus gibbiceps, Cymatoceps nasutus
and Pachymetopon blochii. Longevity and slow growth are
common in many of South Africa’s linefish species (Table 3)
and Attwood et al. (2019) proposed that these traits have
been retained, because they have allowed these species
the best chance of survival in a highly dynamic and variable
coastal environment.
Unfortunately, life-history traits, such as slow growth
and extended longevity, have also made such species
susceptible to overexploitation (Buxton and Clarke 1986;
Griffiths 2000; Murray 2012). These life history traits have
often been associated with the collapse of fish stocks
targeted by the multisectoral commercial, recreational and
small-scale South African linefisheries (Potts et al. 2020).
However, O. conwayi is only targeted by the spearfishing
sector and their population may not be subject to the
same rates of fishing mortality as other species targeted
by larger fishing sectors. Furthermore, the depth limitation
of spearfishers (Mann et al. 1997) may provide a natural
refuge for O. conwayi on deeper reefs. However, as
demonstrated by a large proportion of South African sparids,
slow growth has the potential to exacerbate the effects of
exploitation and can lead to significant population declines
if management is ineffective and suitable control measures
are not put in place (Smale and Punt 1991; Bennett 1993;
Griffiths 2000; Murray 2012; Hewett 2019). Additionally, as
participation in the spearfishery increases and other target
species decline (van der Elst 1989), it is possible that the
O. conwayi population may also become more depleted.
This suggests that, if not managed correctly, O. conwayi
could suffer the same fate as many South African sparids.
This study provided additional important information on
the age and growth of O. conwayi for its future management.
The use of citizen scientists (recreational spearfishers who
voluntarily provided their catches for research) greatly
assisted the collection of O. conwayi samples for this study.
Such collaboration can strengthen the relationship between
researchers and fishers and ultimately improve both
understanding and compliance with subsequent management
regulations that stem from such research (Sbragaglia et al.
2021). In the absence of a robust stock assessment, which
is generally required before determining species-specific
regulations, some of the information collected in this study
can be used to implement precautionary management
regulations. For example, the introduction of a reduced daily
bag limit (DBL) is advised. Mann et al. (1997) showed that
the current DBL for O. conwayi was frequently reached and
consequently the reduction of the DBL could be considered
a precautionary measure, which will help reduce fishing
mortality on this potentially vulnerable reef fish until a
stock assessment has been conducted. Although valuable
age-and-growth data were collected in this study, information
gaps, such as determining the population status (using a
per-recruit stock assessment approach), spawning season,
spawning locality and size-at-maturity, should be research
priorities for this species in future.
Acknowledgements — We thank the spearfishers who donated
specimens for this study. Terence Bellingan, Angus Patterson
and Hannes Le Roux are thanked for the use of their vessels and
skippering expertise. Ryan Foster was funded by the National
Research Foundations student bursary.
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Amber-Robyn Childs:
Bruce Mann:
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Manuscript received: 21 September 2021, revised: 21 January 2022, accepted: 24 January 2022
Associate Editor: N James
African Zoology 2022, 57(1): xxx–xxx 11
Date Location Biogeographic
of specimens
Range of lengths
2019/02/02 Port Alfred WT CS 1 600
2019/02/05 Boknes WT RS 9 362–455
2019/02/05 East London WT CS 5 380–433
2019/02/06 Kenton-on-Sea WT RS 5 199–489
CS 3 400–415
2019/02/14 East London WT CS 3 411–429
2019/03/16 Kenton-on-Sea WT RS 7 145–495
CS 4 322–480
2019/03/19 Kenton-on-Sea WT RS 6 329–454
CS 5 344–472
2019/03/19 Port Alfred WT CS 2 400–580
2019/04/26 Kenton-on-Sea WT RS 4 293–455
2019/04/27 Kenton-on-Sea WT RS 15 200–475
CS 4 369–455
2019/05/13 Kenton-on-Sea WT RS 3 272–445
CS 2 346–361
2019/05/19 Boknes WT RS 4 298–460
CS 3 401–445
2019/05/20 Kenton-on-Sea WT RS 6 298–485
2019/06/01 Kenton-on-Sea WT RS 5 310–471
2019/06/04 Boknes WT RS 2 319–487
2019/06/15 Hole in the wall ST C 21 440–587
2019/06/16 Port Alfred WT RS 2 420–485
CS 2 460–491
2019/06/17 Kenton-on-Sea WT RS 3 291–438
2019/07/15 2019/07/19 Durban (Nationals) ST C 6 500–600
2019/07/24 Boknes WT RS 7 231–526
CS 3 394–428
2019/07/29 Kenton-on-Sea WT RS 10 290–526
2019/08/05 Kenton-on-Sea WT RS 8 306–480
2019/08/06 Kenton-on-Sea WT RS 4 164–240
2019/08/08 Port Alfred WT CS 4 349–441
2019/08/10 Boknes WT RS 10 232–472
2019/09/09 Boknes WT CS 4 389–491
2019/09/10 Kenton-on-Sea WT RS 12 277–509
2019/09/10 Kenton-on-Sea WT CS 6 241–530
2019/09/10 Port Alfred WT CS 4 350–499
2019/09/12 Kenton-on-Sea WT RS 7 239–445
2019/09/17 Port Alfred WT RS 7 104–467
2019/10/04 Kenton-on-Sea WT RS 2 223–525
2019/10/21 Kenton-on-Sea WT RS 2 185–402
2019/11/24 Kenton-on-Sea WT RS 2 249–471
2019/11/30 Kenton-on-Sea WT RS 3 165–284
2019/11/30 Cape St. Francis WT CS 3 392–460
2019/11/30 Port Alfred WT CS 2 360–374
2019/12/08 Kenton-on-Sea WT CS 2 355–384
2019/12/26 Kenton-on-Sea WT CS 6 370–475
2019/12/26 Port Alfred WT CS 4 371–503
2019/12/28 Port Alfred WT CS 7 359–470
2019/12/31 Kenton-on-Sea WT CS 3 176–454
2020/01/04 Cape St. Francis WT RS 9 251–443
2020/01/15 Kenton-on-Sea WT RS 8 179–472
CS 5 271–468
2020/01/26 Kenton-on-Sea WT RS 2 425–479
2020/02/14 Struisbaai CT RS 6 391–458
CS 21 296–459
Appendix 1: Table of all sampling events between February 2019 and January 2020. CS (Citizen scientists), RS (Research
Spearfishing), C (Competitions), FL (Fork Length). Biogeographic regions: ST: subtropical; WT: warm-temperate; CT: cool-temperate)
... The samples were then embedded in paraffin wax, sectioned at 5-6 µm, stained using haematoxylin and eosin (HE) and mounted on glass slides. Note that otoliths were also kept for an age and growth study but this forms part of a separate publication (Foster et al., 2022). ...
Fishers’ ecological knowledge (FEK) has become a popular method for collecting fisheries data. Yet, standard biological sampling by researchers is still the most commonly used method. South Africa’s recreational fishing sector currently has a wealth of biological and demographic information on species captured with hook and line, while several species targeted in the spearfishery are data limited. The aim of this study was to incorporate specialised spearfisher FEK to improve the understanding of the reproductive biology of Oplegnathus conwayi in South Africa. This was achieved using standard biological sampling and FEK data collected from specialist spearfishers using a questionnaire. A total of 305 O. conwayi were collected along the South African coastline. Fish were dissected, sexed and staged macroscopically and gonad staging was verified histologically. One hundred and three survey responses were received, of which 90 were regarded as being from specialised spearfishers. The combination of both traditional sampling and FEK data complemented each other well, with key reproductive data such as spawning season, length-at-maturity and spawning locality being collected. The combination of these methods is strongly advised for future life history studies, particularly, in countries with data-deficient species and limited research funding.
Full-text available
Marine fish stocks are an important part of the world food system and are particularly important for many of the poorest people of the world. Most existing analyses suggest overfishing is increasing, and there is widespread concern that fish stocks are decreasing throughout most of the world. We assembled trends in abundance and harvest rate of stocks that are scientifically assessed, constituting half of the reported global marine fish catch. For these stocks, on average, abundance is increasing and is at proposed target levels. Compared with regions that are intensively managed, regions with less-developed fisheries management have, on average, 3-fold greater harvest rates and half the abundance as assessed stocks. Available evidence suggests that the regions without assessments of abundance have little fisheries management, and stocks are in poor shape. Increased application of area-appropriate fisheries science recommendations and management tools are still needed for sustaining fisheries in places where they are lacking.
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While otoliths are considered the most reliable structure to accurately age fish, a variety of otolith preparation techniques are available, which have consequences on the otolith's optical properties and therefore interpretation of growth bands. Recently, numerous studies from a variety of authors have criticised the use of whole otoliths in ageing sparids with sectioned otoliths subsequently acknowledged as the most reliable preparation technique. Despite this criticism; ageing data is still being generated from whole otoliths and other unreliable structures such as scales. In an attempt to understand the severity of this issue we conducted a global literature review of otolith preparation protocols used for sparids. We identified global spatial inconsistencies in otolith preparation techniques with some regions predominately using methods other than sectioned otoliths to age sparids. The review highlights the need for a standardisation of otolith preparation methods and a move towards the use of sectioned otoliths, or at least valid support where alternative structures or preparation techniques are used. Given that large numbers of studies have been conducted on whole otoliths in certain regions, it may be necessary to revaluate the existing growth parameters to ensure that accurate information is incorporated into management structures.
Full-text available
Ages were estimated for the tropical sparid, Acanthopagrus berda, caught in northern KwaZulu-Natal estuaries. Whole otoliths were used in the age determination. Age estimates were validated by marginal increment analysis and oxytetracycline labelling, which indicated that opaque deposition occurs primarily from September to November each year. The reproducibility of age estimates was described by a coefficient of variation of 10%. The von Bertalanffy growth curve was found to best describe the growth of A. berda. The parameters of the von Bertalanffy growth curve indicated that A. berda in northern KwaZulu-Natal is a slow-growing species, capable of reaching at least 16 years of age. Longevity of the species, coupled with sex change, late maturation and estuarine dependency, give cause for concern for the continued sustainable utilization of this species.
Full-text available
Citizen science offers a potentially cost-effective way for researchers to obtain large data sets over large spatial scales. However, it is not used widely to support biological data collection for fisheries stock assessments. Overfishing of demersal fishes along 1,000 km of the west Australian coast led to restrictive management to recover stocks. This diminished opportunities for scientists to cost-effectively monitor stock recovery via fishery-dependent sampling, particularly of the recreational fishing sector. As fishery-independent methods would be too expensive and logistically-challenging to implement, a citizen science program, Send us your skeletons (SUYS), was developed. SUYS asks recreational fishers to voluntarily donate fish skeletons of important species from their catch to allow biological data extraction by scientists to produce age structures and conduct stock assessment analyses. During SUYS, recreational fisher involvement, sample sizes and spatial and temporal coverage of samples have dramatically increased, while the collection cost per skeleton has declined substantially. SUYS is ensuring sampling objectives for stock assessments are achieved via fishery-dependent collection and reliable and timely scientific advice can be provided to managers. The program is also encouraging public ownership through involvement in the monitoring process, which can lead to greater acceptance of management decisions.
Spearfishing is practiced by a small fraction of younger recreational fishers and has received considerably less scientific attention than angling. This knowledge gap may negatively affect the ability for developing sustainable marine recreational fisheries. We address this through a global systematic review of the literature pertaining to marine spearfishing (both recreational and otherwise) and providing an integrative overview of key research topics of ecological, social, and economic dimensions. The systematic review indicated an increasing number of papers related to marine recreational spearfishing, with the majority exclusively focused on ecological impacts of spearfishing. The integrative review identifies the most relevant ecological impacts and possible strategies to minimize them to develop sustainable marine recreational spearfishing. Marine recreational spearfishing fosters connection with the underwater environment, but more research on the social aspects is needed. Results also show a growing research interest in assessing the economic contribution of marine recreational spearfishing. Finally, we argue that recreational spearfishers represent a widespread network of underwater observers whose extensive knowledge may help to identify and track changes in marine ecosystems. Overall, we highlight key points to consider when conducting multi- and interdisciplinary research regarding marine recreational spearfishing.
South Africa has a long history of engagement in citizen science (CS), particularly marine CS. This review examines the contributions made by marine CS, from the 1930s through to the current era, where websites, social media and mobile apps provide a wide range of opportunities. Largescale marine CS projects, such as the Oceanographic Research Institute’s Cooperative Fish Tagging Project, have made enormous contributions to marine scientific research. Individual citizen scientists have also made considerable contributions, particularly in taxonomy and the publication of field guides. Marine CS has also contributed towards the popularisation of science and improved scientific literacy through the active engagement of many citizens. These benefits align well with the visions of policies that currently guide the South African marine research agenda. However, marine CS in the developing world is not without challenges, and practitioners should be cognisant of the time and effort required to initiate and maintain viable CS initiatives. Especially, long-term successful CS projects depend on secure, ongoing funding, institutional support and enthusiastic champions. Participation by almost exclusively the urban and middle-class sectors of society is also of concern. These challenges can be addressed through stakeholder-inclusive planning, development of novel methods that engage with broader sectors of society, and regular critical evaluations of CS projects. Where global projects on the intended taxa/subject of study already exist, it may also be preferable to enter into collaborative data-sharing agreements with these to reduce operational costs and avoid duplication.
Linefish' is a uniquely South African term used to describe marine fishes that are captured using hook and line. The South African linefishery is a complex socio-ecological system that has a considerable impact on the coastal marine environment while generating social and economic benefits for commercial, small-scale and recreational fishers. Like many fisheries, this complex system is under threat from the combined impacts of increasing levels of exploitation and climate change associated with the Anthropocene. The Southern African Marine Linefish Symposium (SAMLS) provides a platform for linefish scientists, managers, conservation officers, individuals from nongovernmental organisations, and other stakeholders to meet and exchange knowledge about the state and development of linefisheries. This overview discusses some of the long-term trends in linefish research during the last five symposia and highlights salient outcomes of the 5th SAMLS, which was held in July 2019. While the recovery and management success for some of South Africa's commercial linefish species are recognised, the lack of policy and management in the recreational sector will not only undermine the implementation of the country's new small-scale fisheries policy, but also the resilience of the socio-ecological system. To promote a resilient linefishery in the Anthropocene it is recommended that the fishery policies be strengthened to cover all sectors in the linefishery and that the general principles of the ecosystem approach to fisheries, including the incorporation of the human dimension and the implementation of co-management, are promoted. Improved communication between fishers, scientists, and managers is necessary, and recreational permit revenue should be used for research and monitoring to improve the management and stock assessment of species important to the recreational and small-scale sectors.
The red stumpnose Chrysoblephus gibbiceps (Sparidae) is a South African endemic seabream that has been severely depleted by fishing. A total of 678 C. gibbiceps were sampled by line and trawl fishing for a study of their morphology, age, growth, reproduction and diet. In addition, catch records from three time-periods since 1897 were interrogated for distribution patterns and movement behaviour. The sex ratio was 1.5:1 in favour of males. The length–weight relationship, W = 9.32 × 10⁻⁵LF2.811, indicates hypoallometric growth. The bulbous head of large males is a secondary sexual characteristic. A von Bertalanffy growth model was fitted to age data obtained using otolith analyses: LF = 429.9 × (1 – e−0.113(t – (−3.799)). The maximum recorded age was 48 years, which is among the highest in seabreams, though the species matures after only 3 years. The gonadosomatic index (GSI) of ripe females (4.5%) was comparatively low, and the two-month-long early-summer spawning season was short compared with that of other sympatric seabreams. The very low GSI of ripe males (1.6%) suggests courtship battles and polygamy. The species feeds over low-profile reefs and consolidated sediments. The principal prey are ophiuroids. Although its trophic level is 3.7, C. gibbiceps has a low-nutrition diet. Historical and current catch data confirm a distribution from Cape Point (southeastern Atlantic) to southern KwaZulu-Natal Province (western Indian Ocean). There is evidence for localised migratory patterns, now partly lost due to severe population depletion. Whereas protogyny and resident behaviour have been suggested as traits that render seabreams vulnerable to fishing pressure, C. gibbiceps is a shoaling gonochorist that has collapsed due to fishing.
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 Lt = 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.
Data on the seasonal abundance, size composition, reproduction, growth and diet of Lithognathus lithognaihus are presented to outline the life history of the species. Spawning appears to be localized on the Transkei and Eastern Cape coasts during a short period in late winter. Small juveniles (150 mm) are semi-resident in the surf zone of sandy and mixed shores for about five years until maturing at a length of 650 mm, when they commence annual migrations. During autumn and early winter these mature fish migrate eastwards to near the north-eastern limit of their distributional range, to spawn during late winter. The return migration takes place during spring and large numbers of mature fish arrive in the South-Western Cape during summer. The high degree of estuarine dependence, the confinement of juveniles and subadults to the surf-zone, the large size at maturation and the predictable aggregations of mature individuals in particular areas are considered to render L. lithognathus particularly vulnerable to estuarine degradation and exploitation by fishermen.