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African Journal of Marine Science
ISSN: 1814-232X (Print) 1814-2338 (Online) Journal homepage: https://www.tandfonline.com/loi/tams20
Aspects of the reproductive biology of monkfish Lophius vomerinus off Namibia
L Maartens & AJ Booth
To cite this article: L Maartens & AJ Booth (2005) Aspects of the reproductive biology of
monkfish Lophius�vomerinus off Namibia, African Journal of Marine Science, 27:1, 325-329, DOI:
10.2989/18142320509504090
To link to this article: https://doi.org/10.2989/18142320509504090
Published online: 08 Jan 2010.
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ISSN 1814–232X
Short Communication Aspects of the reproductive biology of monkfish Lophius vomerinus off Namibia
L Maartens1*and AJ Booth2
1Formerly Department of Natural Resources and Conservation, University of Namibia, Private Bag 13301, Windhoek, Namibia; now De Beers Marine Namibia, PO Box 23016, Windhoek, Namibia.
2Department of Ichthyology and Fisheries Science, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
* Corresponding author, e-mail: [email protected]
Aspects of the reproductive biology of monkfish Lophius vomerinus are described from material collected during hake Merlucciusspp. biomass surveys and from commercial monkfish and sole Austroglossus microlepis vessels between January 1996 and June 2000 off Namibia at depths between 97m and 686m.
Length-at-50% sexual maturity for males and females were estimated at 39.9cm and 58.2cm respectively, males maturing faster than females. The adult sex ratio of fish >50cm total length was strongly biased towards females. There was reproductive activity throughout the year, peaking slightly between autumn and spring.
Keywords: Lophius vomerinus, reproductive biology, sexual maturity
Introduction
Two species of monkfish are found in Namibian waters Lophius vomerinus and L. vaillanti. The distribution of L.
vomerinusextends from northern Namibia (21°S) to Durban on the east coast of South Africa (30°S, 31°E; Leslie and Grant 1990) and that of L. vaillanti from north of Walvis Bay (23°S) to the Gulf of Guinea. L. vomerinusis a demersal species that lives in areas from the tidal zone to depths of more than 600m, and is the more important lophiid species in terms of abundance, landed mass and value to the commercial Namibian trawl fisheries (Maartens and Booth 2001a, 2001b).
Little is known of the life history of L. vomerinus in southern African waters. Available data on the reproductive biology of L. vomerinus in Namibia are restricted to the geographical positions of recruitment areas, i.e. areas with high abundance of 0-aged fish. The International Commi- ssion of the Southeast Atlantic Fisheries (ICSEAF) and specifically Spanish researchers identified two separate recruitment areas, one off Walvis Bay (23–25°S) at depths between 150m and 300m and the second near the Orange River (28°35’S) at depths between 100m and 300m (Anon.
1984, 1985). Recruitment took place during winter. These observations confirm independent data collected by the Norwegian research vessel Dr Fridtjof Nansen during bottom trawl surveys during the 1990s (NAN-SIS database, unpublished data).
This lack of information and the need for a biologically
based management strategy have prompted an investigation into aspects of its life history, including its growth patterns (Maartens et al. 1999) and reproductive biology. This study investigates the reproductive biology of L. vomerinusin Namibian waters and includes reproductive seasonality, length frequency distribution of the different sexes, juvenile and adult sex ratios, and size- and age-at- sexual maturity.
Material and Methods
Biological data were collected between March 1997 and June 2000 from commercial vessels targeting monkfish and sole Austroglossus microlepis, as well as during five hake Merlucciusspp. biomass surveys on board the RV Dr Fridtjof Nansen(January/February 1996, September/October 1996, January/February 1997, January/February 1998 and January/February 1999) off Namibia. In total, 1 541 (15–95cm total length TL) and 2 784 monkfish (6–99cm TL) were sampled from the commercial (measured to the nearest mm) and research vessels (measured to the nearest cm) respec- ively. All fish were weighed to the nearest g. Because monkfish are not sexually dimorphic, all the fish were sexed macroscopically. Fish too small to sex were classified as juveniles. Gonads were dissected from the fish, weighed (g), staged visually and assigned to one of five maturity stages (Table 1).
Published online 08 Jan 2010
Maartens and Booth 326
The length at which 50% of fish were sexually mature (l50) was estimated for reproductively active fish (Stages 3–5) per cm size-class by fitting a logistic ogive of the form
where p(li) is the probability that fish in length-class i are mature, lithe midpoint of length-class i and δthe steepness of the ogive, essentially a length-based rate of sexual maturity. Parameter estimates of l50 and δ were obtained using a downhill search routine (Press et al. 1997) that minimises the binomial log-likelihood of the form
where miis the number of fish mature in size-class i, nithe total number of fish sampled in size-class i and x the number of size-classes investigated. The null hypothesis that sex-
specific estimates of l50 and δ were equal was assessed using a likelihood ratio test.
Population sex ratio was determined from the commercial and survey length frequency data separated by sex into 5- cm size-classes. The null hypothesis that numbers of fish per size-class were equal was tested using a Chi-squared test.
Reproductive seasonality was determined by monthly inspection of the macroscopic development stages, and by 0.2
0.4 0.6 0.8 1.0 1.2
GSI (%)
0.5 1.0 1.5 2.0 2.5 3.0 3.5
J F M A M J J A S O N D
Male (n = 735)
Female (n = 793)
15 30 45 60
FREQUENCY (%) Developing Ripe Spent
15 30 45 60
Jan.
Testes (n = 735)
Ovaries (n = 793)
Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
Figure 1:Mean monthly gonosomatic indices (GSI) for male and female L. vomerinussampled off Namibia from commercial vessels between March 1997 and June 2000. Vertical bars indicate 95%
confidence intervals
Figure 2:Monthly frequency of occurrence of testes and ovaries in developing, ripe and spent stages of L. vomerinussampled off Namibia from commercial vessels between March 1997 and June 2000
Maturity Stage Description
1. Immature 2. Resting
3. Developing 4. Ripe 5. Spent
Ovaries are greyish-pink in colour, relatively small, ribbon-like and appear almost empty with no vascularisation. Testes are white to tan in colour and very small
Ovaries are orange-pink, larger than the immature stage and with little vascularisation. No ova are visible. Testes are white to tan in colour, much larger than the immature stage, and a small amount of milt is sometimes present when dissected
Ovaries are orange, highly vascular and ova are discernible by eye. Testes are blotchy cream- to tan-coloured and very firm in texture. Moderate to large amounts of milt are present when dissected
Ovaries are straw-coloured to almost clear and highly vascular. Distinct ova are present. Testes are blotchy cream- to tan-coloured with areas of pink, extremely firm in texture and copious amounts of milt are present when dissected Ovaries are grey in colour, extremely flaccid, moderately vascular and appear almost empty. Atretic ova appear as black or white dots. Testes are greyish-tan, extremely flaccid, edges appear translucent and a small amount of milt is sometimes present when dissected
Table 1:Description of the various macroscopic maturity stages for male and female L. vomerinus based on visual observations
a gonadosomatic index (GSI), calculated monthly for each sex, by expressing gonad mass (only available for commer- cial samples) as a percentage of eviscerated body mass.
Temporal trends in GSI were investigated using periodic regression (Batschelet 1981). To test the null hypothesis that there was no cyclic trend in GSI throughout the year, GSI was linearly regressed against the transformed month of the year data, such that
where the independent variables represent angular trans- formations in radians of the month (MOY)i when the GSI data were collected, such that
SINMOYi=
Results and Discussion
The two sources of data on gonadal development indicate that L. vomerinus spawn throughout the year, with female fish exhibiting a slight increase in spawning intensity between autumn and spring (Figures 1, 2). Whereas no significant cyclic trend was evident for male GSI (F = 0.64, df = 2, 237, p > 0.05, r2 = 0.01), a weak, but statistically significant, trend was noted for females (F = 7.138, df = 2, 95, p < 0.05, r2 = 0.13). Female GSI remained high between July and September (Figure 1), coincident with the months of greatest incidence of ‘ripe’ fish (Figure 2). Distinct peaks in the frequency of ‘spent’ ovaries were found during January, August–September and December, indicating the presence of possible spawning seasons from late-winter to early-spring and summer. The frequency of ‘developing’
ovaries peaked in May and June (Figure 2).
For lophiids, spawning seasonality appears to vary between species as well as geographical areas. Griffiths and Hecht (1986) suggested that L. vomerinus from the Agulhas Bank, South Africa, had a well-defined summer breeding season and Armstrong et al. (1992) indicated that L. americanus spawn during May–June between Cape Hatteras and southern New England. Duarte et al. (2001) and Azevedo (1996) identified the main spawning period for L. budegassa off the Portuguese and Spanish Atlantic coasts to be from November to February and October to March respectively. In the Bay of Biscay, the peak spawning period for L. budegassa was May–July (Quincoces et al.
1998a). The peak season of spawning for L. piscatoriusoff the Portuguese and Spanish Atlantic coasts is from January to June (Duarte et al. 2001), from November to May off the north-west coast of Scotland (Afonso-Dias and Hislop 1996) and from May to August (peaking in May and June) in the Bay of Biscay (Quincoces et al.1998b).
Male and female lengths-at-50% sexual maturity and maturation rates were significantly different from each other (λ(l50) = 402.34, p < 0.001; λ(δ) = 40.67, p < 0.001; Figure 3). Male and female lengths-at-50%-maturity were estimated at 39.9cm and 58.2cm respectively, whereas maturation rate was higher for males (6.0cm–1) than for females (9.3cm–1).
The estimated lengths-at-sexual maturity compares well with that found for L. americanus and L. budegassa (Table 2), with female fish maturing at larger sizes than males.
The sex ratio for males, females and juveniles varied between 1.00:1.08:0.02 (commercial samples), 1.00:1.
29:0.12 (survey samples) and 1.00:1.21:0.08 (commercial and survey samples combined, Figure 4). Females domi- nated larger size-classes (>50cm TL) for both commercial and survey samples (χ2= 355.12, df = 7, p <– 0.001) and attained a greater size than males, i.e. 95cm vs 67cm TL (commercial samples) and 91cm vs 63cm TL (survey samples).
Leslie and Grant (1990) found thatL. vomerinus spawn flat gelatinous egg masses (called veils) in South African waters, which float near the surface. According to Pietsch
0.8 1
0.6 0.4 PROPORTION MATURE0.2
20 40 60 80 100
TOTAL LENGTH (mm) V
+
Figure 3: Proportion of sexually mature L. vomerinusmales and females sampled off Namibia by research and commercial vessels between March 1997 and June 2000. Length-at-50% maturity was estimated using a logistic model
In
2 4 6 8 10
10 20 30 40 50 60 70 80 90 LENGTH (cm)
Male (n = 1 890) Female (n = 2 279) Juvenile (n = 156)
FREQUENCY (%)
Figure 4: Length frequency distribution of L. vomerinussampled off Namibia by research and commercial vessels between March 1997 and June 2000
Maartens and Booth 328
and Grobecker (1978), all females of the Lophiiformes, with the possible exception of one species of antenariid angler- fish, are thought to expel these non-adhesive, mucoid egg veils or rafts. It has been proposed that the advantages of releasing eggs in these veils may be, inter alia, to facilitate the broadcasting of a large number of eggs over great distances, so providing for development in relatively produc- tive surface waters, as well as the protection of eggs against predators (Pietsch and Grobecker 1978, Armstrong et al.
1992). To the authors’ knowledge, veils have never been recorded in Namibian waters.
Female monkfish mature at a larger size and at a greater age (8 years vs 5 years) than males (Maartens et al. 1999), a trend typical among teleosts (Moyle and Cech 2000).
According to Stearns and Crandall (1989), organisms mature along a trajectory of age and size, depending on demographic conditions, and not at a fixed size or age. The decreased growth rate in female fish after the onset of maturity could be explained by the relationship between maturation and size (Maartens et al.1999). Although the growth rate in males is generally less than that of immature females, male fish continue to grow after maturity. In the case of monkfish, a large female body size would clearly be at an advantage to accommodate a large egg veil. The considerable energetic contribution by female lophiids to reproduction is also noticeable in the maximum GSI values obtained for female L. vomerinus, i.e. 30% (this study), 51% for L. americanus (Armstrong et al. 1992), 37% for L. budegassa(Tsimenidis 1980) and 30% for L. piscatorius (Staalesen 1995).
Acknowledgements — We thank the Ministry of Fisheries and Marine Resources, Namibia, for funding the study. Special appreciation is extended to the officers and crew of the RV Dr Fridtjof Nansen, as well as those of the commercial vessels for their assistance with the collection of data.
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