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

Hofmeyr, MD

Japp, DW

Melo, YC

Osborne, RF

English

AJOL - African Journals Online

Two species of Cape hake, the shallow-water speciesMerluccius capensis and the deep-water species M. paradoxus are the main target species of the SouthAfrican demersal fishery, contributing around 70% bymass of the catches (Stuttaford 1998). M. capensis isgenerally caught on the South Coast, and off the WestCoast in waters shallower than 300 m. M. paradoxusform the bulk of the catches on the West Coast atdepths of 200–600 m, but there is considerable overlapin the distribution of the two species at depths betweenapproximately 150 and 450 m (Botha 1980).Studies on the spawning behaviour of the Capehake include the work of Hecht (1976) on the SouthAfrican east coast, who used gonadosomatic index,frequency occurrence of ripe gonads, and conditionfactor to infer reproductive seasonality and spawningfrequency in M. capensis. Hecht (1976) derived aseven-point maturity scale for M. capensis. Frommacroscopic examination of the gonads of hake fromthe West and South-East coasts, Botha (1980, 1986)and Payne (1986) determined the length and age at50% and complete maturity, as well as the durationand time of the spawning season of M. paradoxusand M. capensis. However, there is, no understandingof the fecundity or the mode of spawning in Cape hake.Histological techniques and oocyte size frequencyanalyses are used in the present study to investigatespawning type in M. capensis and M. paradoxus.Batch fecundity is estimated by means of the “hydratedoocyte method”. MATERIAL AND METHODSSpawning typeSpawning type was investigated by means of histo-logical analysis of ovaries from hake collected on thesouth coast of South Africa in June 1996 (for M. capen-sis) and on the West Coast in July 1996 (for M. para-doxus). Cape hake are believed to spawn throughoutthe year (Botha 1980). For histological analysis, theovaries were dehydrated through a series of ethanolimmersions, cleared with toluene and embedded inparaplast. Sections (5–7 µm thick) were stained withHarris’s haematoxylin, followed by eosin counterstain.Each ovary was histologically classified according tothe most advanced oocyte stage present in the ovary.Oocyte stages were analogous to the six ovarian maturitystages described by Herrera et al. (1988) for M. gayigayi and Balbontín and Bravo (1993) for M. australis:1 – virginal, 2 – immature, 3 – maturing, 4 – mature/ripe, 3a – partially spent and 5 – regressing/resting.In the present study, the partially spent stage (3a) isreferred to as Stage 5 and the regressing/resting stage(5) as Stage 6.Oocyte diameter was measured, by means of an ocularmicrometer, from five females per maturity stage. Thediameter of 50 oocytes per female was measured.The measurements of oocytes found in hydrated(Stage 4) ovaries were taken from formalin-preservedS. Afr. J. mar. Sci. 21: 211–2161999 211SERIAL SPAWNING AND BATCH FECUNDITY OF MERLUCCIUS CAPENSISAND M. PARADOXUSR. F. OSBORNE*, Y. C. MELO†, M. D. HOFMEYR‡ and D.W. JAPP§Histological analysis of the ovaries and the presence of multiple modes in oocyte size frequency distributionof Cape hake Merluccius capensis and M. paradoxus indicates that they are serial spawners. Batch fecundity,calculated by means of the “hydrated oocyte method”, was positively correlated to ovary-free fish mass andtotal fish length. The mean relative fecundity (ovary-free mass) of M. capensis and M. paradoxus was 160 ± 12and 306 ± 25 eggs.g-1 respectively.* Marine & Coastal Management (formerly Sea Fisheries), Private Bag X2, Rogge Bay 8012, South Africa. Department of Zoology,University of the Western Cape, Private Bag X17, Bellville 7535, South Africa. E-mail: rosborne@sfri.wcape.gov.za† Marine and Coastal Management. E-mail: ymelo@sfri.wcape.gov.za‡ Department of Zoology, University of the Western Cape. E-mail: retha@botany.uwc.ac.za§ Formerly Marine & Coastal Management; now Fisheries and Oceanographic Support Services, 16 Orchard Avenue, Lakeside 7945,South Africa. E-mail: jappy@iafrica.comManuscript received: May 1998material, because the hydrated oocytes collapsed duringhistological processing.Batch fecundityCape hake were collected on the South Coast (M. capensis – April 1996, 1997; M. paradoxus –May 1997). The hydrated ovaries of nine M. capensisand 14 M. paradoxus were used to determine batchfecundity, according to the “hydrated oocyte method”(Hunter et al. 1985). Fish and gonad mass were mea-sured to the nearest 1 g and total fish length to thenearest 1 cm. Fresh ovaries were preserved in 10%buffered formalin. For determination of batch fecundity,two samples of 0.5 g each, positioned about one-third of the distance from each end of the ovary, wereremoved from the right ovary and weighed to the212 South African Journal of Marine Science 21 19991020304010203040506010203040OOCYTE DIAMETER (mm)246810121416a) Stage 2 n = 100b) Stage 3 n = 250c) Stage 4 n = 249d) Stage 5 n = 250102030405060701020304050246810121410203040OOCYTE DIAMETER (mm)a) Stage 2 n = 100b) Stage 3 n = 249c) Stage 4 n = 250d) Stage 5 n = 100Fig. 1: Frequency distribution of oocyte size classes in thevarious ovarian stages of M. capensisFig. 2: Frequency distribution of oocyte size classes in thevarious ovarian stages of M. paradoxusnearest 0.01 g. The hydrated oocytes, identified bytheir translucent straw-coloured yolk and oil droplet,were counted under a dissecting microscope (Table I).Histological examination of the ovaries showed theabsence of new post-ovulatory follicles. The presenceof such follicles in the ovaries indicates that spawninghas commenced, resulting in decreased numbers ofhydrated oocytes. Such ovaries are not suitable forbatch fecundity determination (Murua et al. 1996).RESULTSDescription of maturity stages Four of the above-mentioned six classes of ovarieswere found in the hake under study, the virginal(Stage 1) and regressing (Stage 6) being absent. Theoocyte stages were identical for each species, but theoocyte diameters differed slightly between speciesfor each stage. The following lists the characteristicsof each ovarian stage present. The measurementsgiven in parenthesis refer to those of M. capensis andM. paradoxus respectively. Stage 2 (immature ovaries, Figs 1a, 2a) – In bothspecies, immature ovaries were characterized by apredominance of perinuclear and previtellogenicoocytes. The perinuclear oocytes were round to irregularly shaped, with a large nucleus and a thinsurrounding layer of cytoplasm (diameter 0.08 and0.02 – 0.09 mm). In the previtellogenic oocyte(0.10–0.24 and 0.10–0.20 mm), the volume of cyto-plasm had increased and yolk vesicles had developedas a single row of vacuoles around the periphery ofthe cytoplasm.Stage 3 (maturing ovaries, Figs 1b, 2b) – Vitellogenicoocytes were present in both species (0.24–0.37 and0.24–0.40 mm), characterized by the appearance ofyolk granules in the cytoplasm. Early mature oocytes(0.40 and 0.44 mm) were present, in which thenucleus had started to migrate to the animal pole. Alarge oil droplet was evident in the mature oocyte.Stage 4 (mature/ripe ovaries, Figs 1c, 2c) – Matureand hydrated oocytes predominated, with fewer vitel-logenic oocytes present. In the mature oocyte, nuclearmigration was almost complete and the yolk granuleshad started to fuse (0.60 and 0.84 mm). In the hydratedoocyte (diameter 0.70–1.10 and 0.88–1.08 mm), thenuclear membrane had disintegrated, and the yolkgranules first fused to form yolk plates and then ahomogenous mass that covered the entire oocyte.Stage 5 (partially spent ovaries, Figs 1d, 2d) –Ovaries were similar to those of Stage 3. They containvitellogenic oocytes (0.24–0.40 and 0.34–0.48 mm)in the process of yolk accumulation (indicative of an-other spawning) and developing perinuclear (0.08 and0.09 mm) and previtellogenic oocytes (0.12–0.24 mmfor M. capensis) for a subsequent spawning batch. Asa result of poor histological preparation, no measure-ments were obtained for previtellogenic oocytes inthe partially spent gonads of M. paradoxus. However,new post-ovulatory follicles were also present in bothspecies, indicating that the fish had already spawned.  Osborne et al.: Spawning and Fecundity of Cape Hake1999 213Table I: Estimated number of hydrated oocytes in two subsamples (a and b) taken from the right ovary of M. capensis andM. paradoxusM. capensis M. paradoxusFish Mean of three countsOverall mean SEMean of three countsOverall mean SEnumberSubsample a Subsample b Subsample a Subsample b01 646 00* 646 629 590 610 19.202 665 700 682 17.5 535 498 517 18.703 739 678 709 30.8 633 642 637 04.304 610 606 608 01.7 642 617 630 12.205 804 619 712 92.5 590 594 592 2.006 696 665 681 15.7 595 688 642 46.307 774 755 765 09.7 632 709 671 38.708 0000*00 918 918 685 703 694 08.809 715 633 674 41.0 489 511 500 10.810 653 632 643 10.511 629 693 661 32.212 860 726 793 66.813 658 617 638 20.814 473 550 512 38.2* Eggs collapsed in one sampleBatch fecundityBatch fecundity for both M. capensis and M. para-doxus was positively correlated with ovary-free fishmass and with total fish length (Figs 3, 4). The meanbatch fecundity was 417 205 ± 64 568 (SE) eggs.fe-male-1 (range = 147 600–723 658) for M. capensisand 374 375 ± 45 562 eggs.female-1 (range = 121 731–710 901) for M. paradoxus. The sample size for the two species was too smallto extrapolate the batch fecundity-mass relationshipto the entire hake population. Therefore, batch fecunditywas expressed at the individual level, relative to fe-male mass, i.e. relative batch fecundity. Mean relativebatch fecundity was estimated at 160 ± 12 eggs.g-1(ovary-free mass) and 143 ± 10 eggs.g-1 (whole fishmass) for M. capensis and 306 ± 25 eggs.g-1 (ovary-free mass) and 243 ± 17 eggs.g-1 (whole fish mass)214 South African Journal of Marine Science 21 1999JJJJJJJJJOVARY-FREE FISH MASS (g)JJJJJJJJJ45 50 55 60 65 70 75FISH LENGTH (cm)600a) y = 190.51 x – 71 915 r.2 = 0.75 n = 9400200600400200b) y = 0.15x3.49 r.2 = 0.80 n = 9JJJJJJ JJJJJJJJOVARY-FREE FISH MASS (g)JJJJJJ JJJJJJJJ40 50 60 70FISH LENGTH (cm)600a) y = 230.26 x – 81 379 r.2 = 0.66 n = 14400200600400200b) y = 8.02x.2.67 r.2 = 0.79 n = 14Fig. 3: Relationship between batch fecundity and (a) ovary-free fish mass (SE of slope = 41.7, mean mass offish = 2 567 g) and (b) fish length for M. capensis(April 1996 and 1997)Fig. 4: Relationship between batch fecundity and (a) ovary-free fish mass (SE of slope = 48.1, mean mass offish = 1 272g) and (b) fish length for M. paradoxus(May 1997)for M. paradoxus. Batch fecundity did not differ sig-nificantly (Mann-Whitney U-test, p > 0.05) betweenthe species, but there were significant differences inthe relative fecundity (p < 0.05). The batch fecundity,ovary-free fish mass and fish length regression rela-tionships did not differ significantly between the twospecies (p > 0.05). There was also no significant relationship between relative fecundity and fish size(p > 0.05).DISCUSSIONThe presence of oocytes in different maturitystages in the ovaries of fish (asynchronous ovaries) isusually considered to be evidence for serial spawning(Hunter and Goldberg 1980, Hunter and Macewicz1985, Melo and Armstrong 1991). Serial spawninghas been shown for several species of Merluccius, e.g.M. hubbsi (Ciechomsky 1967, Christiansen andCousseau (1971), M. gayi gayi (Balbontín and Fischer1981) and M. merluccius (Murua et al. 1996).Serial spawners usually possess indeterminate fecundity (Hunter et al. 1985), i.e. the annual fecundity(total number of eggs spawned by a female per year)is not fixed prior to the onset of spawning and un-yolked oocytes continue to mature and be spawnedduring the spawning season (Hunter et al. 1992).Therefore, estimation of the standing stock of advanced oocytes in the ovary is meaningless if, duringthe spawning season, oocytes are recruited to thatstock (Murua et al. 1996). Given that identificationof a predetermined annual spawning batch is probablyimpossible for hake, the only useful measure of theirfecundity is the number of eggs produced in a singlespawning batch, i.e. batch fecundity (Hunter et al.1985).Histological examination showed that M. capensisand M. paradoxus contained oocytes in different stagesof development in all the ovarian classes examined.The presence of post-ovulatory follicles, togetherwith vitellogenic oocytes, was also evident in the hakeunder study, a feature indicative of serial spawning(Goldberg 1985, Melo and Armstrong 1991). No hiatusin development was observed between the immature,previtellogenic and vitellogenic oocytes in the matureovaries examined, which is suggestive of indetermi-nate fecundity (West 1990, Murua et al. 1996).The positive correlations between batch fecundityand ovary-free fish mass and fish length for M. capensisand M. paradoxus have been shown for similar rela-tionships in other species of hake, e.g. M. merluccius(Murua et al.1996), M. productus (MacGregor 1966),M. hubbsi (Grunwaldt 1986), M. australis (Balbontínand Bravo 1993) and M. gayi gayi (Balbontín andFischer 1981). Also, similar relative batch fecundities(ovary-free mass) to those estimated for M. capensisand M. paradoxus (i.e. 160 and 306 eggs.g-1 re-spectively) have been reported for M. merluccius(165 eggs.g-1, Murua et al.1996), M. productus(192 eggs.g-1, MacGregor 1966) and M. australis(334 eggs.g-1, Balbontín and Bravo 1993). The finding that M. paradoxus had a significantlyhigher mean relative fecundity than M. capensiscould be explained by the fact that the M. capensisexamined had already spawned one or more batchesof eggs, reducing the number of eggs available forthe next spawning, whereas the M. paradoxus understudy had not shed any eggs. Alternatively, M. para-doxus may be a more prolific spawner than M. capensis.In conclusion, both M. capensis and M. paradoxusare serial spawners. The estimates of batch fecundityfor Cape hake are preliminary and more detailed in-formation, using larger sample sizes, is required toquantify their reproductive capacity. Appropriate research on the spawning fraction, the frequency ofspawning and duration of the spawning season hasbeen initiated in an attempt to apply the egg productionmethod in addition to trawl data for future spawnerbiomass estimates of the Cape hakes.ACKNOWLEDGEMENTSWe thank the technical staff of the OffshoreResources Biology Section of Marine and CoastalManagement (MCM) and Mr J. Wissema (MCM) forproviding the samples. We are grateful to Ms J. van derPoel (MCM), who assisted with the histology prepa-ration and counting of oocytes. The paper benefitedfrom the critical appraisals of Prof. W. Veith (Universityof the Western Cape), Dr R. L. Tilney (MCM) and ananonymous referee. The work was partially fundedby the Foundation for Research Developement,South Africa, and the Royal Society, London.LITERATURE CITEDBALBONTÍN, F. and R. BRAVO 1993 —  Fecundidad, talla de laprimera madurez sexual y datos biométricos en la Merluzadel sur Merluccius australis. Revta. 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Serial spawning and batch fecundity of merluccius capensis and m. Paradoxus

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Serial spawning and batch fecundity of merluccius capensis and m. Paradoxus

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