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African Journal of Marine Science

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The biology, life history and management needs of a large sciaenid fish, Argyrosomus coronus, in Angola

W. M. Pottsab; W HH Sauera; R. Henriquesc; S. Sequessequed; C. V. Santosd; P. W. Shawc a Department of Ichthyology and Fisheries Science, PO Box 94, Rhodes University, Grahamstown, South Africa b South African Institute of Aquatic Biodiversity, Grahamstown, South Africa c School of Biological Sciences, Royal Holloway University of London, Egham, UK d Faculdade Ciências da Universidade Agostinho Neto (FCUAN), Luanda, Angola Online publication date: 08 November 2010 To cite this Article Potts, W. M. , Sauer, W HH , Henriques, R. , Sequesseque, S. , Santos, C. V. and Shaw, P. W.(2010) 'The

biology, life history and management needs of a large sciaenid fish, Argyrosomus coronus, in Angola', African Journal of Marine Science, 32: 2, 247 — 258 To link to this Article: DOI: 10.2989/1814232X.2010.501567 URL: http://dx.doi.org/10.2989/1814232X.2010.501567

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African Journal of Marine Science 2010, 32(2): 247–258 Printed in South Africa — All rights reserved

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AFRICAN JOURNAL OF MARINE SCIENCE ISSN 1814–232X EISSN 1814–2338 doi: 10.2989/1814232X.2010.501567

The biology, life history and management needs of a large sciaenid fish, Argyrosomus coronus, in Angola WM Potts1,2*, WHH Sauer1, R Henriques3, S Sequesseque4, CV Santos4 and PW Shaw3 Department of Ichthyology and Fisheries Science, PO Box 94, Rhodes University, Grahamstown 6140, South Africa South African Institute of Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa 3 School of Biological Sciences, Royal Holloway University of London, Egham, TW20 OEX, UK 4 Faculdade Ciências da Universidade Agostinho Neto (FCUAN), Avenida 4 de Fevereiro 71, Luanda, Angola * Corresponding author, e-mail: [email protected]

1

2

Downloaded By: [Rhodes University] At: 13:34 12 November 2010

Manuscript received December 2009; accepted April 2010 The West Coast dusky kob Argyrosomus coronus is an understudied yet important fishery species in Angola. During a five-year study (2005–2009), the species was recorded in all fishery sectors, but was most important in the inshore recreational fishery in southern Angola (Cunene Estuary to Namibe). Early juveniles (870 mm TL) were captured in all fisheries as far north as Namibe, and shoals of large adult fish (>1 000 mm TL) were occasionally captured in the offshore purse-seine commercial fishery between the Cunene Estuary and Lucira. Because some Argyrosomus species are morphologically cryptic, a DNA barcoding method was used to confirm the taxonomic status of the biological samples used in this study. The male:female sex ratio of examined samples was 1:1.4 (n = 225). The length-at-50% maturity was 823 mm and 904 mm TL for males and females respectively. Age-at-50% maturity was 4.4 and 4.3 years for males and females respectively. The periodicity of otolith ring formation was confirmed to be one year using a marginal zone and a chemical marking analysis. Growth (in mm TL) was best described by: Lt = 1 826(1 – e−0.12(t + 1.60)). Argyrosomus coronus fed predominantly on fish, mainly Sardinella aurita (62% frequency of occurrence). Early juveniles appeared to frequent the offshore zone (50–100 m depth), moving into the inshore region at approximately 300 mm TL. Juveniles and subadults were resident (57% recaptured at the same site) and were particularly abundant around the mouth of the Cunene Estuary as well as in central and northern Namibia. Adults undertake migrations that correspond with the movement of the Angola–Benguela frontal zone, moving north as far as Gabon in winter and returning to southern Angola in spring, when spawning appeared to take place offshore. There are currently no catch restrictions on A. coronus in Angolan waters. However, declining catches and increasing fishing effort suggest that some management intervention is required, commencing with a proposed closure of the Cunene Estuary mouth region to fishing. Keywords: fisheries, growth, management, reproduction, Sciaenidae

Introduction Little is known of the biology of the West Coast dusky kob Argyrosomus coronus, a sciaenid recently described by Griffiths and Heemstra (1995). This inshore species has a limited distribution and is mainly found between northern Namibia (north of Cape Frio) and southern Angola (south of Lucira), although individuals have been observed as far south as St Helena Bay on the South African west coast (Lamberth et al. 2008) and as far north as the coast of Gabon by Poll (1954) (Figure 1). Off Namibia, it is found concomitantly with the more dominant Argyrosomus inodorus at a ratio of less than 1:10 (Holtzhauzen et al. 2001), but it occurs almost exclusively in Angolan waters. Argyrosomus coronus is a large species (reaching 77 kg, Griffiths and Heemstra 1995) and is captured in fisheries

throughout its primary distribution. Currently, there are no fishery regulations controlling the capture of A. coronus in Angola and, despite good catch rates in the recreational fishery (Potts et al. 2009), there is a concern that the characteristic catch decline and fishery collapse observed for many large sciaenid species worldwide (McHugh and Conover 1986, Cisneros-Mata et al. 1995, Griffiths 1996a, 1997a, Holtzhausen and Kirchner 2001, Sadovy and Cheung 2003, Silberschneider et al. 2009) may also occur in Angolan waters. Generally, the effect of fishing is manifest in many ways, such as constriction in geographical range (Clark et al. 2000), reduction in abundance (Hutchings and Reynolds 2004), a truncation in the size and age structure of the population (Haedrich and Barnes 1997, Miethe et al. 2010), loss

African Journal of Marine Science is co-published by NISC (Pty) Ltd and Taylor & Francis

Material and methods Fisheries information Fisheries information for A. coronus was collected from the recreational (tourists and locals), artisanal (hook and line, beach-seine and gillnet), subsistence (hook and line) and commercial (purse-seine) fisheries from May 2005 to June 2009 between the Cunene Estuary mouth and Baia Farte (Figure 1). The tourist-based recreational fishery was monitored at two sites between May 2005 and December 2006. The first site, based at Flamingo Lodge, comprised a 23 km section of coast between the towns of Namibe and Tombua (Figure 1),

GABON Poll, 1948

Luanda Kw an za

AFRICA

Kwanza

Ang ola Cur ren t Angola– Benguela Front

DEM. REP. of CONGO

ANGOLA 0

80

160 km

Benguela Cunene Skeleton Coast National Park

ATLANTIC OCEAN

Baia Farte ANGOLA

Lucira Bentiaba

14° S

NAMIBIA

Lubango

nt rre Cu

of large highly fecund individuals (Palumbi 2004), alteration in life-history characteristics (Jennings et al. 1998, Miethe et al. 2010) and a loss of genetic diversity (Kenchington et al. 2003). Although there is a limited understanding of the impact of exploitation on large sciaenid species (Sadovy and Cheung 2003), it is likely that a combination of these effects has led to large-scale depletions. In some cases, these depletions occurred before any fishery crisis was perceived and even before the biology of the species was described (Sadovy and Cheung 2003). With a paucity of information on large sciaenid species, many researchers have relied on transdisciplinary information to make inferences on their biological and life-history characteristics (e.g. Cisneros-Mata et al. 1995, Sadovy and Cheung 2003). Whereas this type of data collection and analysis is useful for the conservation of virtually extinct species, its value in terms of fisheries management is questionable as it only provides retrospective information. With limited biological and life-history information, it is not surprising that previous management interventions directed at large sciaenid fisheries have been met with limited success. For example, the timing of the unsuccessful closed season for Totoaba macdonaldi in Mexico, which was implemented in 1940 (Cisneros-Mata et al. 1995), was not based on their reproductive biology. Even when the fisheries regulations have been based on rigorously collected biological data, such as the size and bag limits for A. japonicus in South Africa, there has been no noticeable improvement in the stock status of the species (Palmer and Snowball 2009). This suggests that the regulations were either implemented too late for the species to recover or that there was limited compliance to the regulations (CisnerosMata et al. 1995). With a poor fisheries management history for large sciaenid species, it is clear that the timely collection of life-history and biological information and the early implementation of a precautionary management approach is a prerequisite. In the case of A. coronus, the sparsely populated coastline in southern Angola and the protracted civil war has provided an almost unprecedented opportunity to collect biological and life-history information and implement a precautionary management approach prior to major population depletion. This study provides information on population size structure, relative abundance, age, growth, sexual maturity, reproductive seasonality and diet of A. coronus. Observations on its fisheries, distribution and movement patterns are discussed and potential threats and precautionary management strategies are identified.

Potts, Sauer, Henriques, Sequesseque, Santos and Shaw

la ue ng Be


248

Namibe Flamingo Lodge Praia Pinda a Tombua oc ATLANTIC or C OCEAN Iona National SOUTH Park AFRICA Cunene N e River nen E W Cu mouth St Helena Bay NAMIBIA S Cape Frio

0

300

600 km

12° E

Figure 1: Map of the south-western coast of Africa showing the extent of the known distribution of A. coronus, selected oceanographic and physical features, and the towns and collection sites mentioned in the text

and was sampled by rod and line whenever fishing took place. The second, a 5 km section of coast situated north of the Cunene Estuary in the terrestrial Iona National Park (Figure 1), was sampled opportunistically for two consecutive days on 10 occasions: June, July, August, September, November 2005 and February, April, June, July, August and October 2006. Fish were captured using conventional shoreangling tackle, a range of baits and/or artificial lures with hook sizes ranging from 1/0 to 8/0. Catch-and-effort and spatial data were obtained by recording the location of fishing effort (GPS coordinates), number of anglers, and the start and end time of each fishing outing. Sea surface temperature was measured on each fishing day between 07:00 and 08:00. The catch composition of the beach-seine net fishery just south of Flamingo Lodge was assessed at least once per week between May 2005 and December 2006. The beach-seine net (500 m × 3 m) had a stretched mesh size of 60 mm and a large codend (8 m long) with a stretched mesh of 45 mm. It was deployed by boat in waters not deeper than 3 m and brought to shore by between 15 and 18 fishers. Argyrosomus coronus present in the catches were counted, measured and inspected for tags.

249


African Journal of Marine Science 2010, 32(2): 247–258

Catches in the artisanal boat-based hook and line fishery were inspected during a two-week period in June 2008, October 2008, January 2009 and May 2009. Inspection sites were situated at artisanal landing sites in the towns of Namibe, Praia Pinda and Tombua. Boat-based hook and line fishers use 2–4 handlines (0.9–1.2 mm diameter), with between 2 and 20 hooks ranging in size from 1/0–6/0. Artisanal fishers in Namibe and Tombua use motorised vessels and mainly target reef-associated species such as Dentex macropthalmus at depths between 100 and 200 m. In contrast, fishers at Praia Pinda use non-motorised vessels and target a variety of species (e.g. Atractoscion spp., sand steenbras Lithognathus mormyrus and elf Pomatomus saltatrix) on a muddy substrate at depths between 50 and 80 m. Argyrosomus coronus present in the artisanal catches were counted, measured and inspected for tags. When tags were found, information including the date of capture, site of capture, capture technique and length of fish was recorded. The catches of 46 subsistence and 30 groups of local recreational fishers were examined during 24 shore-based roving creel surveys conducted quarterly between Flamingo Lodge and Namibe from June 2008 to May 2009. In addition, the catch of six groups of local recreational fishers was examined on an ad hoc basis at the Cunene Estuary mouth between June 2005 and December 2009. Subsistence fishers mainly use handlines (0.65–0.90 mm diameter) with 1–2 hooks ranging in size from number 2 to 2/0 and predominantly target reef-associated species (e.g. blacktail Diplodus capensis) in shallow water (1–3 m). Recreational fishers used conventional tackle and hook sizes ranging from 1 to 8/0. All fish captured were identified, measured and inspected for tags. In addition, catches in the subsistence and artisanal fishery were inspected at landing sites in Bentiaba, Lucira, Baia Farte and Benguela (Figure 1) during June 2009. Fishers from each of the sectors were interviewed at the landing sites to determine the abundance of a range of inshore fishery species (including A. coronus) in their catches. Visual aids were used to avoid misnaming or misidentification. Argyrosomus coronus observed in the catch were noted and genetic samples were taken. Four commercial purse-seine net fisheries were visited in Tombua (3) and Baia Farte (1) in January and June 2009 respectively. Owners were interviewed and A. coronus specimens were measured and genetic samples were taken.

Biological data collection Ideally, biological data should be collected throughout the distribution of the species. However, the paucity of fish in the catches of all fisheries north of Namibe precluded cost-effective data collection. Because the mtDNA control region sequence data indicated that A. coronus in northern Angola are not differentiated from the population sampled in southern Angola (RH unpublished data), it was assumed that biological samples of fish collected in southern Angola were representative of the entire Angolan population. Biological samples of A. coronus were therefore primarily collected at or near the two tourist-based recreational fishery sites in southern Angola between May 2005 and December 2006. To minimise the impact of the research activity, only fish that were injured or caught for consumption at the lodge were retained for biological sampling. However, some fish in certain length classes were retained specifically for the biological analysis to ensure a representative sample of the population. To augment the sample size, a further 46 samples were collected for the aging component from the local recreational fishery between January 2007 and June 2009, and from the artisanal fishery at Praia Pinda in January 2009. All fish retained in the tourist-based recreational fishery were killed immediately and later, in the laboratory, measured (total length) and weighed. Fish were dissected, sexed and the gonads categorised according to six developmental stages (Table 1). The stomach contents of each fish were removed and preserved in 10% formalin. Saggital otoliths were removed from each fish and stored dry in manila envelopes. A genetic sample (fin clip) was taken and stored in 95% ethanol. Between August 2007 and May 2009, 237 fish were injected intramuscularly with high-tet 120 oxytetracycline (OTC; 0.1 ml kg–1), and tagged and released at the Flamingo Lodge site. Fish movement information Fish tagging was restricted to the Flamingo Lodge site. Fish 700 mm were tagged with PDA-type tags. A unique identification number and the lodge name and contact telephone number were printed on each tag. Recaptured fish were either caught by lodge guests in the 23 km fishing zone or elsewhere by artisanal, subsistence and local recreational fishers.

Table 1: Macroscopic criteria used to stage gonad development for A. coronus in southern Angola Stage I II

Development Juvenile Immature

III

Resting

IV

Developing

V

Ripe

VI

Spent

Macroscopic appearance Not possible to visibly distinguish sex; gonad appears as a translucent, gelatinous strip Ovaries translucent orange tubes; oocytes not macroscopically distinguishable Testes are discernible as thin, flat, white bands Ovaries slightly larger; oocytes visible as tiny yellow granules in gelatinous orange matrix Testes noticeably broader and appears as a flat, broad, white band Ovaries larger in diameter and opaque yellow in colour; oocytes readily visible and occupying the entire ovary Testes broadened, distended and mottled and creamy-beige in colour Ovaries large in diameter; oocytes of maximum size, dark orange and hydrated Testes swollen to maximum size; creamier in colour due to considerable quantities of sperm which extruded when pressure was applied to the abdomen Ovaries pinky-red, flaccid and sac-like with few vitellogenic oocytes visible Testes shrivelled, reduced in size and grey in colour

250



Fishing effort outside the research area is concentrated mainly around the coastal towns and recaptures in these areas were either reported telephonically (using the number printed on the tags) or personally to lodge staff. Public awareness posters were placed at the fish markets in the coastal towns. Laboratory analysis Due to misidentification problems associated with sympatric, congeneric Argyrosomus species (Griffiths and Hecht 1993), a molecular barcoding examination of 178 randomly chosen specimens from this study was conducted to confirm that the life-history description was based on only A. coronus specimens. The preserved stomach contents of each fish were sorted, identified to the lowest possible taxon and counted. Otoliths were embedded in clear polyester casting resin and sectioned (0.4 mm) longitudinally through the nucleus with a double-bladed, diamond-edged saw. The sections were mounted onto glass slides with DPX mountant, and examined for opaque growth zones on three occasions under transmitted light at a magnification of between 20× and 50×. The median of the three readings was accepted as the age estimate, but the otolith was rejected if all three readings differed. The periodicity of opaque zone formation was validated using a marginal zone analysis and a chemical marking technique. For the marginal zone analysis, the outer margin of the otoliths was examined and the optical appearance of the outer margin (either opaque or hyaline) was noted and expressed as a fraction of the monthly sample. Four fish that were injected with OTC were recaptured after 10, 77, 146 and 371 days at liberty respectively. The otoliths of these fish were kept in the dark and then sectioned (as described above), examined and photographed using the methods described by Potts and Cowley (2005). Data analysis The information from the tourist-based recreational fishery was used to gain an understanding of seasonal trends in the catch rate. Catch per unit effort (CPUE) was calculated as the total number (or mass) of fish captured each month (at Flamingo Lodge) or each trip (at the Cunene Estuary mouth) and divided by the total effort for the month or trip respectively. The coefficient of variation was calculated by dividing the standard deviation of the CPUE (fish/mass angler–1 h–1) by the arithmetic mean. Differences in the mean length of A. coronus in the different fisheries and habitats were tested using an ANOVA where a length frequency for more than 50 fish was obtained. A Tukey range test was used to determine which means were different from one another. Length- and age-at-50% sexual maturity for A. coronus followed the method of Potts et al. (2008) and was determined using macroscopic staging information (Table 1). All fish in Stages I and II were considered immature, whereas fish in Stages III, IV, V and VI were regarded as mature. Fish were separated into 50 mm length or one-year age classes. The proportion of sexually mature fish (PMi) by length (Li) or age was fitted with a logistic ogive of the form: PMi = 1/(1 + e−(l − l i

50)/δ

)

where PMi is the proportion of mature fish in the ith length (or age) class, li is the ith length (or age) class, l50 is the mean length or age-at-50% maturity and δ is the width of the logistic ogive (this describes the rate at which the population changes from 0% to 100% mature). Maximum likelihood estimates of the parameters were obtained by minimising a binomial likelihood. Reproductive seasonality was determined using the macroscopic staging information. Due to the limited number of fish captured in December and January, monthly macroscopic staging information was pooled into seasons for comparative purposes (March–May = autumn, June–August = winter, September–November = spring, December–February = summer). The precision of the aging process was measured using the index of average percentage error (APE; Beamish and Fournier 1981) and the mean coefficient of variation (CV; Chang 1982). The monthly proportion of otoliths with a hyaline edge were fitted to a periodic logistic regression using the methods described in Beamish et al. (2005). A loglikelihood ratio test was used to test the hypothesis that the annual hyaline ring deposition was unimodal (rejection was p < 0.05). Because there was a lack of early juvenile fish for this study, small Age-0 individuals were not represented in the sample. To reduce this bias, the age of all fish was first calculated in days (not years). This was done by subtracting the date of capture from a theoretical birth date of 15 November (middle of the month was when most ripe and running fish were observed) and assuming that opaque zones are deposited on the 1st of July. The age in days was then divided by 365.25 to obtain the estimated age in years for each fish. Modelling of growth followed the methods described by Potts et al. (2008). The von Bertalanffy (Ricker 1975) growth model was fitted to the observed length-at-age data using a downhill simplex search (Nelder and Mead 1965), which comprises a non-linear minimisation routine to obtain model parameter estimates. Model fits were obtained by minimising the negative normal log-likelihood of the observed and predicted lengths-at-age. To compare the model fits, a non-parametric, one-sample runs test for residual randomness and the Bartlett’s test for their homoscedascity were applied. In addition, variance estimates were calculated using the (conditioned) parametric bootstrap resampling method (Efron 1982) with 1 000 bootstrap iterations. Standard errors and 95% confidence intervals were constructed from the bootstrap data using the percentile method described by Buckland (1984). A likelihood ratio test (Cerrato 1990) was used to compare growth model parameters between males and females. The catch records from the tourist-based recreational fishery and tag return information (up until May 2009) were used to describe movement patterns of A. coronus. Recaptured fish were grouped into adults or juveniles based on their total length at recapture. Ontogenetic changes in their movement patterns were investigated by comparing the minimum, mean and maximum number of days at liberty and distance travelled by each group. Seasonal movements were elucidated using information from fish recaptured after less than three months at liberty and pooled for all years.

251


(>870 mm) A. coronus were only present in the catch of the subsistence and artisanal fishery during winter between Flamingo and Namibe. Adult fish were captured near Lucira by the commercial purse-seine fishery in June. Recreational fishers captured A. coronus throughout the year at the Cunene Estuary mouth and artisanal fishers captured early juveniles (0.1* >0.1*

Flamingo–Namibe

Jun 2008–May 2009

S LR

H&L R&L

Inshore Inshore

46 30

11 23

525 (417–1 010) 652 (405–890)

1.3 3.2

Flamingo

May 2005–Dec 2006

Namibe

Jun 2008–May 2009

FR A A

R&L BS H&L

Inshore Inshore Offshore

Monitored 32 122

778 5 102

836 (344–1 650)D 450 (420–505) 764 (550–1 280)C

26.5 >0.1* >0.1

Bentiaba

June 2009

A S

GN H&L

Offshore Inshore

2 2

0 0

0 0

NE

Lucira

June 2009

A C

H&L PS

Offshore Offshore

12 1

0 87

0 1 152 (1 080–1 230)E

NE

Baia Farte

June 2009

S A

H&L H&L

Inshore Offshore

4 26

0 0

0 0

NE

Dissimilar superscripts indicate significant differences (p < 0.05) in mean length where n > 50 NE = Not estimated * = Visual estimation only (all fish not counted)

252


(a) Cunene River

(b) Flamingo Lodge CV CPUE

0.25

3.5

0.2

3 2.5

0.15

2

0.1

1.5 1

0.05

TEMPERATURE (°C)

0.5

25 23 21 19 17

1200 TOTAL EFFORT (h)

1000 800 600 400

D ec

ct O

Au g

Ap r Ju n

b Fe

D ec

ct O

Au g

D e Ju c n

ct O

Au g

b

Ap r Ju n

Fe

D ec

ct O

Au g

200

Ju n


CV (%)

CPUE (fish angler−1 h−1)

4

MONTH Figure 2: Monthly trends in CPUE, coefficient of variation (CV), mean monthly sea surface temperature (bars denote minimum and maximum values) and total monthly fishing effort of the recreational linefishery at (a) the Cunene River mouth and (b) Flamingo Lodge between June 2005 and December 2006

The smallest mature female and male were 668 mm TL (3.1 kg) and 546 mm TL (1.1 kg) respectively. Lengthat-50% maturity for all fish was 870 mm TL with males maturing at a smaller size (823 mm TL) than females (904 mm TL). The pattern of maturation for females and males was best described by logistic curves (Figure 4). The male:female sex ratio was skewed towards females (1:1.4) (χ2 = 6.1, df = 1, p > 0.05). This was mainly because there were significantly more female fish (χ2 = 5.8, df = 1, p > 0.05) in the adult population (1:1.6) than in the juvenile population (1:1.2) (χ2 = 1.1, df = 1, p > 0.05). Only two (20%) of the 10 fish dissected during autumn had gonads in a developing state (Figure 5). In winter, however,

almost 50% of the 91 fish dissected had developing gonads. During spring, the gonads of one female and eight males (all captured in November) were in a ripe and running condition. Only two adult fish (both with gonads in a resting phase) were captured during the summer months (Figure 5). The otoliths of 239 fish were used for age determination. Of these, only two were rejected because the number of opaque rings observed during all three readings differed. The APE was 3% and the CV was 1%. The margin of the otoliths was dominated by opaque zones in winter and hyaline zones in summer. The periodic regression estimated a period of 11.6 months for the formation of one opaque and one hyaline zone (Figure 6)

253


40 30

Mass = (4E−09) × TL3.14 r 2 = 0.97 n = 225

20 15 10 5 400

600

Spent

Ripe n = 44

n=2

60 40 20

800 1000 1200 1400 1600 TOTAL LENGTH (mm)

Autumn

Figure 3: Relationship between total length and mass of A. coronus captured in southern Angola between June 2005 and December 2006

Winter Spring SEASON

Summer

Figure 5: Seasonal macroscopic categorisation of the gonads of A. coronus in southern Angola between June 2005 and December 2006

Observed Predicted

FRACTION HYALINE

0.9

(b) Male

0.8 0.7

4

10

0.5 0.4

5 3

0.6 6

10

0.3 0.2

5

2

12

6

38

4 5

16

17 0.1 3 J J A S O N D J F M A M J J A S O N D MONTH

TOTAL LENGTH (mm)

16 00

14 00

12 00

10 00

Figure 6: Periodic regression fitted to the marginal zone analysis of A. coronus collected between June 2005 and December 2006. Numbers indicate the number of individuals per month

80 0

90 80 70 60 50 40 30 20 10

(a) Female

60 0

90 80 70 60 50 40 30 20 10

40 0

MATURITY (%)


Developing n = 91

80

25

MATURITY (%)

MASS (kg)

35

Resting n = 10

Figure 4: Maturation pattern (by length) of (a) female and (b) male A. coronus in southern Angola between June 2005 and December 2006

and the hypothesis that one opaque zone was formed per annum was accepted (p = 0.96). Of the four recaptured fish that had previously been injected with OTC, only one individual was at liberty for over one year (371 days). The OTC band on the otolith of this fish was deposited in an opaque (dark) zone (Figure 7). After this opaque zone, one hyaline zone could be identified between the OTC band and the otolith edge (Figure 7), further confirming the annual periodicity of opaque zone formation. The oldest female was 13 years, the oldest male 9 years, and the mean individual age 4.2 years. A comparison of the von Bertalanffy growth parameters, using likelihood ratio

Figure 7: Cross-section of a sagittal otolith from a 795 mm TL A. coronus that was tagged and injected with OTC and recaptured after 371 days at liberty. Insert taken under reflected ultraviolet light only and main image under transmitted white, reflected light. ● = opaque zone; arrow indicates the OTC band


1 600 n = 232

1 400 1 200 1 000 800 600 400 200 2

4

Discussion There are 10 species of the genus Argyrosomus worldwide, many of which are morphologically similar. Due to these similarities, two sympatric South African species, A. japonicus and A. inodorus, were originally described as one species, A. hololepidotus, despite having very different life-history characteristics. This misidentification had a significant impact on the A. japonicus population in particular, because the South African fishery regulations were designed for A. inodorus, a species that matures at a considerably smaller size (Griffiths and Hecht 1993). In Namibian waters, A. coronus is sympatric to A. inodorus (van der Bank and Kirchner 1997) and can be easily confused, leading to a similar management problem. Therefore this study made use of a molecular method (barcoding) to confirm the identification of A. coronus. It is recommended that future life-history studies on sympatric species should incorporate this type of analysis to avoid the problems associated with misidentification. Compared with other sciaenid species, the life history of A. coronus most closely resembles that of A. japonicus (in South Africa), A. regius and Totoaba macdonaldi (Table 4) with a fast growth rate, large maximum size and a similar size- and age-at-maturity. These and other large sciaenid species such as the Chinese bahaba Bahaba taipingensis have a similar life cycle, including a marine adult spawning phase (Moal 1957, cited in Griffiths and Heemstra 1995, Cisneros-Mata et al. 1995, Griffiths 1996a, Sadovy and Cheung 2003), a juvenile estuarine phase and a migratory subadult and adult phase (Quero and Vayne 1987, CisnerosMata et al. 1995, Griffiths 1996a, Sadovy and Cheung 2003). A combination of the fisheries, movement and historical data can be used to describe the life history of A. coronus.

MATURITY (%)

methods, showed that there was no significant difference in the Brody coefficient (k) (p = 0.89) or in asymptotic lengths (L∞) of female and male A. coronus (p = 0.87). Samples were therefore pooled and the growth of the population (in mm) was best described by the von Bertalanffy growth equation: Lt = 1 826(1 − e −0.12(t +1.60)), where Lt = length (TL) at time t (Figure 8). The youngest mature male and female were 2 and 3 years old respectively. The age-at-50% maturity was 4.4 and 4.3 years for males and females respectively, and the pattern of maturation was best described by the logistic curves in Figure 9. In all, 61 (27%) of the 225 A. coronus stomachs examined contained food items. The round sardinella Sardinella aurita was the dominant prey item (62% frequency of occurrence and 63% by number), whereas other teleost items included the Cunene horse mackerel Trachurus trecae (10% frequency of occurrence and 8% by number), snake eel Opichthidae (8% frequency of occurrence and 4% by number), blacktail Diplodus capensis (5% frequency of occurrence and 3% by number) and unidentified fish (5% frequency of occurrence and 16% by number). Crustaceans in the diet included the estuarine prawn Nematopalaemon spp. (3% frequency of occurrence and 3% by number) and swimming crab Callinetes spp. (3% frequency of occurrence and 1% by number). In total, 66 fish were recaptured after being at liberty for between 2 and 694 days between September 2005 and March 2009. Of these, 33% were recaptured at the release site, 18% were recaptured within 1 km, 42% between 1 km and 10 km, and 7% over 10 km from the release site. If fish were not recaptured within three months of first capture, they were mostly recaptured during the same month, but one or two years later (Figure 10). Juvenile fish were more resident, with 57% recaptured at the release site compared with only 13% of the adults (Table 3, Figure 10). Adult fish moved more than twice the distance as juveniles (Table 3). There was a seasonal trend in movement with fish generally moving in a northerly direction in early winter (June, July) and in a southerly direction from late winter into spring (August–November) (Figure 11).

TOTAL LENGTH (mm)


254

6 8 AGE (y)

10

12

Figure 8: Growth of A. coronus collected between June 2005 and December 2006 in southern Angola. Dashed lines = 95% confidence intervals from the bootstrapped predicted lengths-at-age

90 80 70 60 50 40 30 20 10 90 80 70 60 50 40 30 20 10

(a) Female

(b) Male

0

2

4

6 8 AGE (y)

10

12

Figure 9: Maturation pattern (by age) of (a) female and (b) male A. coronus in southern Angola

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Catch information suggests that it is predominantly an inshore species although large adults also may move offshore. Juvenile and subadult A. coronus (300–600 mm TL) are found in great abundance inshore and occur at the Cunene Estuary mouth throughout the year. Although this appears to be a very important area for juveniles, these fish are also abundant in the inshore zone of northern and central Namibia (Griffiths and Heemstra 1995). The mark-recapture information confirms that juvenile and subadult A. coronus are more sedentary than the adults, often remaining within 200 m of their catch site for up to two years. On reaching maturity, adults disperse and appear to undertake an annual return migration, beginning with a northerly movement from the southern region of their distribution in early winter and a return migration in late winter and early spring. The extent

Juveniles Adults

Summer

600

Autumn

Winter

Spring

50

500

DISTANCE (km)

DAYS AT LIBERTY

700 Downloaded By: [Rhodes University] At: 13:34 12 November 2010

of the movement appears to be dependent on the extent of the northward movement of the Angola–Benguela Front, which has a suitable temperature range (16–19 °C) for this species (Figure 2). In winter, adults were observed (although not in great numbers) near the Kwanza Estuary mouth, and have been observed in the past as far north as Gabon (Poll 1954). The recapture information suggests that their return migration appears to coincide with the southerly movement of the Angola–Benguela Front (Boyd et al. 1987, Veitch et al. 2006) and increasing water temperatures. Argyrosomus coronus appear to spawn in late spring (November), during or after their return migration. Although there is no information on the spawning habitat of this species, the closely related A. japonicus are thought to spawn in the nearshore environment, on the east coast of South Africa (Griffiths 1996a) and possibly in estuaries in Australia (Parsons et al.

400 300 200 100

40 30 20 10

−15 −10 −5 South

0

5 10 15 20 25 30 35 40 North

J

DISTANCE MOVED (km)

Figure 10: Relationship between distance moved and days at liberty of recaptured juvenile and adult A. coronus in southern Angola

F

M

A

M

J J MONTH

A

S

O

N

D

Figure 11: Seasonal movement patterns of recaptured A. coronus in southern Angola

Table 3: The minimum, mean and maximum distance travelled and days at liberty for recaptured juvenile and adult A. coronus between June 2005 and March 2009

Maturity stage Juvenile Adult Combined

Minimum 0.0 0.0 0.0

Distance travelled (km) Mean (±CV) Maximum 1.71 (±1.77) 6.3 4.65 (±1.69) 40.0 3.40 (±1.96) 40.0

Days at liberty Mean (±CV) 147 (±1.24) 122 (±1.39) 132 (±1.32)

Minimum 3 3 3

% with no displacement 57.1 12.9 33.3

Maximum 694 644 694

Table 4: Life-history characteristics of selected large sciaenid fish

Species Argyrosomus japonicus Argyrosomus inodorus Argurosomus inodorus Argyrosomus regius Totoaba macdonaldi Argyrosomus japonicus Argyrosomus coronus

Country South Africa South Africa Namibia Mauritania Mexico South-East Australia Angola

Max. TL (mm) 1 8101 1 4501 1 82011 1 98013 1 69014 1 9001

L∞ (mm) 1 4272 1 0875 1 1608 2 10012 1 69913 1 31714 1 88315

Max. age (y) 422 255 288 2513 2414 1315

k 0.242 0.415 0.128 0.0912 0.1513 0.2014 0.1115

L50 (mm) ±1 0004 300–3507 350–36010 >70012 1 20013 510–68014 87015

Sex ratio (M:F) 1.0:1.04 1.0:1.6–2.17

A50 6–84 1.3–2.45 1.5–1.610 >3 6–713 2–514 4–515

1.0:1.415

Griffiths and Heemstra 1995; Griffiths and Hecht 1995; Griffiths 1997a; Griffiths 1996a; Griffiths 1996b; Griffiths 1997b; Griffiths 1997c; Kirchner and Voges 1999; 9Kirchner 2001; 10Kirchner et al. 2001; 11Quero and Vayne 1987; 12Tixerant 1974; 13Cisneros-Mata et al. 1995; 14 Silberschneider et al. 2009; 15this study

1

8

2

3

4

5

6

7


256

2009, Lagardère and Mariani 2006). It is unlikely that the Cunene Estuary is a suitable spawning area for A. coronus due to the high volumes of freshwater flow (350–450 m3 s–1) during the summer period (Hughes and Hughes 1992). Early juveniles (>300 mm TL) of this species were absent in the inshore zone and captured on muddy substrata offshore (50–80 m depth) in the artisanal fishery. In a research trawl conducted in 1949, Poll (1954) identified 11 early juvenile (185–285 mm TL) specimens of Sciaena aquila (later described as A. coronus by Griffiths and Heemstra 1995). These fish were captured in depths between 55 and 100 m on a muddy or sandy-mud substrate (Poll 1954). Because the nursery area for this species appears to be offshore it is most likely that spawning occurs in this region. As very few A. coronus with ripe gonads were captured at Flamingo Lodge, their spawning grounds are most probably in the offshore zone south of this area. After an early juvenile phase, at a length of approximately 300 mm TL (one year of age), juveniles recruit into the inshore zone between central Namibia and southern Angola, where they are targeted by the inshore fisheries. The intrinsic vulnerability of large sciaenid fish to fishing (Sadovy and Cheung 2003) suggests that A. coronus may be highly susceptible to fishing pressure. The large average size and high CPUE in the tourist-based recreational fishery suggests that the population of A. coronus off Angola is in a relatively healthy state. However, with increasing fishing pressure in the region (Potts et al. 2009), reports of decreasing catches by local recreational fishers (WMP unpublished data) and the current lack of larger fish (>50 kg) captured by any of the fishing gears (despite the observations by Griffiths and Heemstra [1995] that this size class was captured regularly), the population appears to be under threat. According to the guidelines proposed by the FAO (2001), which use life-history traits to classify fishery species by productivity, A. coronus falls into the medium productivity category and would be expected to sustain medium levels of fishing effort. However, a number of life-history traits make A. coronus and other large sciaenid fish particularly vulnerable. Large piscivorous species such as A. coronus are considered more likely to decline faster than species lower down the food chain (Pauly et al. 2001). A large proportion of juvenile A. coronus appears to occupy a restricted and variable environment (the Cunene Estuary mouth area). This is an area of vulnerability and may have serious consequences for the species. In addition to Angolans who operate a shore-based commercial fishery for this species, there are an increasing number of foreign recreational fishers (from Namibia and South Africa) who travel to the Cunene Estuary mouth from Namibia and target A. coronus. Given that 88% of the fish captured at the Cunene Estuary site were below the size at sexual maturity during this study, it is likely that this increased fishing pressure may result in recruitment overfishing, a situation that is mirrored in South Africa with the estuarine dependent A. japonicus (Griffiths 1997a). Furthermore, the Namibian fisheries regulations for kob (both A. coronus and A. inodorus), which allow fish of over 400 mm TL to be retained, will place additional pressure on the juvenile and subadult population of this species.


Fisheries are generally intrinsically diverse and complex, and few more so than those targeting A. coronus. These include the capture of early juveniles in the artisanal fishery at Praia Pinda, juveniles and subadults inshore in proximity to the Cunene Estuary mouth by recreational shore-fishers, the capture of a wide size range of fish by subsistence, artisanal and recreational fishers between Tombua and Namibe, and the occasional capture of large adult fish offshore by the purse-seine fishery between the Cunene Estuary mouth and Lucira. This presents a complex management challenge and highlights the susceptibility of this fish, because the collapse of many other large sciaenid fisheries was preceded by the capture of the target species at more than one life-history stage (Cisneros-Mata et al. 1995, Sadovy and Cheung 2003). During the study period, small pelagic fish, particularly Sardinella aurita, dominated the diet of A. coronus. These fish characteristically have short life cycles, and their population size is heavily dependent on environmental conditions (Zeeberg et al. 2008). The resulting variability in their dominant food source may periodically subject A. coronus to periods of non-optimal feeding, resulting in decreased growth and production. Despite these threats, the distribution of A. coronus provides an inherent degree of protection. The Iona National Park coastal belt (Figure 1), although not policed or regulated, is largely uninhabited and there is limited access to the shore (with the exception of the Cunene Estuary mouth). Although boat access is possible, artisanal boat-fishing activity is primarily concentrated around a 50 km radius of the towns of Namibe and Tombua. This provides large refuge areas that are mostly unfished by local fishers. Similarly, the greatest abundance of this species in Namibia appears to be in the Skeleton Coast National Park, where fishing is prohibited, except for two isolated fishing camps (Torra Bay and Terrace Bay), which are only open for restricted periods. Climate-induced changes are expected to have a major influence on the distribution of marine fish species in the future (Cheung et al. 2008). The northward and southward seasonal migration of A. coronus observed during our study appears to be correlated to the movement of the Angola– Benguela Front, which moves in a northerly direction in early winter and a southerly direction in spring (Boyd et al. 1987). With climate change predicted to be more pronounced in biogeographical transitional zones (Allen and Breshears 1998), environmental changes may have a major impact on the migration and reproductive patterns of this species. The correlation between the CPUE of A. coronus and sea surface temperature suggests that this fish avoids or does not feed in temperatures above 19 °C. This suggests that any change in temperature may result in a change in distribution of the species, which may disrupt the life cycle and decrease the level of protection. Despite the potential threats to A. coronus and the economic value of its fisheries to local communities (Potts et al. 2009), there is limited regional management directed at this species. Currently, as this is a multiple-gear, open-access fishery, there is a strong likelihood of overfishing. Indeed, long-term data collected on the Namibian side of the Cunene Estuary mouth shows a reduction in CPUE for this species from over 15 fish angler–1 h–1 in 1991 to