Biology and ®shery of Octopus vulgaris

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    Relini, 1984; Tursi and D'Onghia, 1992; Belcari and

    Sartor, 1993). However, the trawlers can

    only exploit the part of the population living on a

    soft, sandy or muddy bottom. Octopuses inhabit-ing rocky substrates are caught with pots on a com-

    mercial scale, where several km2 areas are covered by

    a web of strings holding thousands of pots (Mangold,

    1983b).

    The biology and shery of O. vulgaris in the

    Mediterranean Sea have been previously studied,

    but the majority of studies refer to individuals kept

    in laboratory conditions (Nixon, 1966, 1971, 1979;

    Boucaud-Camou et al., 1976; Nixon and Maconna-

    chie, 1988) or caught in littoral waters (Kayes, 1974;

    Ambrose and Nelson, 1983). Some studies deal withthe biology of this species taken by trawlers. Thus,

    Mangold-Wirz (1963) made a detailed study of the

    general biology, Mangold and Boletzky (1973) dealt

    with growth and reproduction biology, Guerra (1975)

    determined the sexual development and Guerra (1978)

    analysed the diet.

    Sanchez and Obarti (1993) studied the biology and

    shery of O. vulgaris populations living in littoral

    waters from 535 m in the central Spanish Mediter-

    ranean coast, where the species is collected with pots

    by local shermen.The present work has been developed in a nearby

    area of the Western Mediterranean and is a general

    study on the biology and shery ofO. vulgaris caught

    by trawlers at depths of 50100 m.

    2. Material and methods

    The sampling programme was carried out from

    August 1995 to August 1996, on-board commercial

    bottom trawling vessels operating off the port ofPalma de Mallorca (Fig. 1). The haul data (date,

    position, duration, depth and course) and the weight

    by species of the total commercial catch were

    recorded. All the hauls were performed between a

    depth of 50100 m.

    Monthly sizefrequency distributions ofO. vulgaris

    were measured on-board. Apart from this species,

    another octopus, Eledone moschata Lamarck, 1799,

    is present in this shery but, although shermen

    recognise them, they are not classied into species.

    Thus, both species appear pooled together under the

    `octopuses' category in the statistics of the central

    auction wharf of Mallorca. In order to determine theproportion of these two species in the catches, the

    weight of each one was estimated from representative

    samples.

    Monthly samples of O. vulgaris were taken to the

    laboratory for processing. For each specimen the

    following measurements were noted: dorsal mantle

    length (ML, in mm), total body weight (BW, in g), sex

    and maturity stage.

    To calculate the relationship between dorsal mantle

    length and total body weight, the formula TWaMLb

    was used. Calculations were made for each sex sepa-rately and also for both sexes pooled. The slopes and

    the intercepts for males and females were compared

    using the methods described in Zar (1984). The allo-

    metry of the growth in weight was tested by a Stu-

    dent's t-test for males, females and both sexes pooled.

    The sex ratio was estimated for each season of the year

    and it was tested by a Chi-square test. In all the

    statistical tests applied in this study, a signicance

    level () of 0.05 was considered.

    The following three-stage maturity scale (adapted

    from Sanchez and Obarti, 1993) was used:

    Fig. 1. The study area in the Balearic Sea (Western Mediterra-

    nean).

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    Immature (I): ovary whitish, very small and with

    no signs of granulation in females; spermatophoric

    organ transparent in males.

    Maturing (II): ovary yellowish with a granularstructure; spermatophoric organ with white streaks.

    Mature (III): ovary very large with plenty of

    eggs; spermatophoric sac with spermatophores.

    The weight of the gonads was recorded by taking

    the weight of the testis and the spermatophoric com-

    plex for males and the weight of ovary for females. For

    males, the relationship between total body weight and

    gonad weight was obtained. For females, a logarith-

    mic transformation of the ovary weight (OvW) was

    made: log (1OvW), and used to eliminate negativeOvW values (Bartlett, 1947).Stomach contents were analysed and prey identied

    from their remains (eyes, mandibles or appendages in

    Crustacea; cephalopod beaks; otoliths) after making a

    comparison with a reference collection and published

    descriptions (Zariquiey-Alvarez, 1968; Perez-Gan-

    daras, 1983; D'Angello and Gargiullo, 1991). The

    following indices were used (Hyslop, 1980; Cortez

    et al., 1995):

    Occurrence index (OCI): the ratio between the

    number of stomachs with one type of prey presentand the total number of stomachs with food, eachstomach being counted as many times as the

    number of different types of prey it contained.

    Emptiness index (EMI): the percentage of

    specimens with no food in their stomachs.

    Finally, monthly statistics of octopus catches (in kg)from January 1981 to August 1996 for the total

    Mallorca eet were collected. In order to determine

    if landings showed any periodicity, two time-series

    analysis techniques (Bloomeld, 1976) were used.

    The rst one was the least-squares tting, which

    consists of nding the sinusoidal function that best

    ts the data. The function has the following form:

    y "x A cos3t B cos3t

    where 3 is the frequency of the series searched for, "x

    the mean of the catch data, and A and B are as follows:

    A 2

    n

    t

    n1

    xt "xcos 3tY B 2

    n

    t

    n1

    xt "xsin 3t

    n and xt being the number of points and the value ofx

    in time t (in months), respectively.

    The second technique was spectrum analysis, which

    consists in computing the Fourier transform of the

    series. The spectrum obtained indicates the relative

    importance of each frequency in a time series. The

    peaks in the spectrum indicate the existence of moreenergetic frequencies, being the importance of a given

    frequency determined by the high of its corresponding

    peak.

    Table 1

    Parameters of the relationship between mantle length (ML) and body weight (BW) from previous studies and the present work

    Sex a b n r Area Size range

    (cm)

    Source

    M 0.350 2.988 584 0.979

    F 0.542 2.804 434 0.969 Catalonia (Western Mediterranean) Guerra and Manrquez, 1980MF 0.420 2.917 1018 0.969 322

    M 0.757 2.74 37 0.95 4.921.5

    F 0.587 2.83 55 0.97 South Africa (Atlantic Ocean) 4.621.5 Smale and Buchan, 1981

    MF 0.718 2.80 92 0.97 4.621.5

    M 3.306 2.323 155 0.90 822

    F 1.654 2.576 165 0.92 Valencia (Western Mediterranean) 926 Sanchez and Obarti, 1993

    MF

    M 0.442 2.882 168 0.95 516

    F 0.413 2.916 175 0.94 Mallorca (Western Mediterranean) 516 Present work

    MF 0.437 2.889 343 0.94 516

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    3. Results

    3.1. Lengthweight relationships and size-frequency

    distributions

    The results of the relationship between dorsal man-

    tle length and total body weight are shown in Table 1.

    No signicant differences were observed when the

    slopes and the intercepts were compared between

    sexes (0.20

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    maximum number in MayJune and a minimum in

    NovemberDecember. Maturing females were caught

    from May to August.

    Formales, the relationship between total bodyweight

    and gonad weight (GoWtestisspermatophoriccomplex weight) gave these parameters (Fig. 4(a)):

    GoW 0X541 0X014TWY with n 167

    and r 0X92

    For females, the relationships between log (1OvW)and log (ML) are presented in Fig. 4(b).

    3.3. Diet

    The occurrence-index (OCI) values for the prey

    items found in the stomachs are shown in Table 2.

    Major taxonomic groups are summarised in Fig. 5(a).

    O. vulgaris fed basically on crustaceans (mainly dec-

    apods) and shes, although it occasionally included

    gastropods and cephalopods in its diet. Percentages of

    the number of prey types found in the stomachs are

    shown in Fig. 5(b). Stomachs with one or two types of

    prey were most common, but those with three or four

    types were also rather common. The emptiness index

    (EMI) was analysed seasonally (Fig. 6). There was agradual decrease of this index from winter to summer,

    increasing again in autumn.

    3.4. Fishery

    Two species of octopuses, O. vulgaris and E.

    moschata, were caught in this shery which targeted

    sh. Both species were pooled for sales, although they

    were separated into sizes. It was noticed that another

    octopus, E. cirrhosa Lamarck, 1798, would sporadi-

    cally appear, but it generally inhabits deeper waters,

    Fig. 2. (Continued).

    A. Quetglas et al. / Fisheries Research 36 (1998) 237249 241

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    and is caught in small quantities, mostly by trawlers

    targeting the Norway lobster (Nephrops norvegicus

    Linnaeus, 1758).

    Bimonthly percentages ofO. vulgaris, E. moschataand the rest of the commercial catch are shown in

    Fig. 7. The relative importance of octopuses increased

    gradually from $20%, in the rst months of the year,up to $40% in JuneJuly. Subsequently, theydecreased abruptly to $20% in AugustSeptember,before increasing again thereafter.

    Although E. moschata appeared regularly through-

    out the year (Fig. 7), its catches were lower than those

    ofO. vulgaris. In addition, the importance ofOctopus

    in relation to Eledone increased gradually from

    December to July.The mean and standard deviation of monthly land-

    ings from January 1981 to August 1996 are shown in

    Fig. 8(a). Catches increase progressively from Janu-

    ary to March, going down thereafter. Finally, after a

    clear minimum in AugustSeptember they increased

    again. The catch rates (kg/h) throughout the year are

    shown in Fig. 8(b). The highest catch rates were

    obtained from April to July, while during the rest of

    the year they remained at low levels.

    As suggested by the visual analysis of the monthly

    octopus landings, two oscillations seemed to exist: an

    approximately annual periodicity superimposed on a

    higher oscillation. To characterise this higher oscilla-

    tion, the series was analysed by the least-squares

    tting method. As a result, a period of 92 monthswas obtained, with "x 1521X04 kg, A4710.64 kgand B1626.95 kg. Monthly landings from January1981 to August 1996 and the sinusoidal function

    obtained (extrapolated until January 2000) are shown

    in Fig. 9(a). If this periodicity were to be maintained,

    an increase of landings until the year 2000 would be

    expected.

    In order to determine the lower oscillation, a

    spectrum analysis was applied to the series, and the

    trend and the larger periodicities in the data were

    eliminated by ltering the periods longer than 24months. The spectrum analysis (Fig. 9(b)) revealed

    a clear peak at a frequency of 310.083 months1

    (corresponding to a period of 12 months) with

    two other peaks at 320.167 and 330.250 months1

    (periods of 6 and 4 months, respectively). These

    two last peaks were located at frequencies that are

    multiples of the rst one (32231 and 33331),thus indicating that they were probably related

    to the harmonics of the 12-month periodicity.

    The presence of harmonics indicates that the

    periodicity of 12 months is not strictly sinusoidal.

    Fig. 3. Bimonthly percentage of maturity stages of (a) males and (b) females.

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    Other higher frequency peaks may not be signi-

    cant.

    4. Discussion

    It is known that O. vulgaris migrates to the coast

    during the rst months of the year, and remains close

    to it (mainly at a depth between 30 and 60 m) during

    the reproductive period (Mangold-Wirz, 1963). The

    results obtained in the present work show that this

    migration is reected in some aspects of the biology

    and shery population exploited by trawlers, which

    are forbidden to sh over 50 m depth.

    The rst effect of this displacement can be observed

    in the sizefrequency distribution. Octopuses

    Fig. 4. (a) Scatter diagram of gonad weight vs. total body weight for males. (b) Relationship between the logarithmic transformation of ovary

    weight (OvW) and the logarithm of mantle length (ML).

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    Table 2

    The occurrence index (OCI) for the prey items found in the

    stomachs

    Occurrence Index (OCI)

    CRUSTACEA 65.75

    AMPHIPODA 4.50

    Gammaridea 4.50

    ISOPODA 1.00

    DECAPODA 60.25

    Decapoda indeterminate 8.75

    DECAPODA NATANTIA 3.25

    Natantia indeterminate 2.25

    Caridea indeterminate 0.25

    Alpheidae indeterminate 0.25

    Thoralus cranchii 0.25

    Philocheras sculptus 0.25

    DECAPODA REPTANTIA 48.25

    ANOMURA 19.25

    Anomura indeterminate 2.00

    Galathea sp. 1.75

    Galathea intermedia 11.50

    Galathea strigosa 0.25

    Galathea bolivari 0.75

    Paguridea indeterminate 2.00

    Paguristes eremita 0.75

    Pagurus prideaux 0.25

    BRACHYURA 29.00

    Brachyura indeterminate 15.00

    Liocarcinus sp. 0.25

    Liocarcinus corrugatus 8.75

    Liocarcinus pusillus 0.25

    Pilumnus spinifer 0.25

    Xantho pilipes 0.25

    Ebalia sp. 0.75

    Ebalia granulosa 0.25

    Ebalia tuberosa 1.25

    Oxyrhyncha indeterminate 0.25

    Parthenopidae indeterminate 0.25

    Inachus dorsettensis 0.25

    Eurynome spinosa 0.25

    Atelecyclus rotundatus 1.00

    MOLLUSCA 6.50

    POLIPLACOPHORA 0.50

    GASTROPODA 3.25

    Trachidae indeterminate 1.50

    Turritella communis 0.25

    Raphitoma reticulata 0.25

    Naticarius intricatoides 0.25

    Naticarius hebraeus 0.50

    Calliostoma granulatum 0.50

    CEPHALOPODA 2.75

    Cephalopoda indeterminate 1.75

    Sepiolidae indeterminate 0.50

    Alloteuthis media 0.25

    Table 2

    (Continued)

    Occurrence Index (OCI)

    Loligo vulgaris 0.25

    TELEOSTEI 27.00

    Teleostei indeterminate 12.00

    Gobidae indeterminate 13.00

    Carapus acus 0.25

    Ophichthus rufus 0.25

    Gaidropsarus vulgaris 0.25

    Blennius ocellaris 0.75

    Capros aper 0.25

    Centracanthus cirrus 0.25

    Not identified 0.75

    Fig. 5. Percentages of (a) the major taxonomic groups and (b) the

    number of prey types found in the stomach contents.

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    disappear progressively from trawling grounds

    when they reach a 1112 cm ML size. This size

    coincides with the minimum mean length obtained

    by Sanchez and Obarti (1993), whose work was

    carried out in a depth range of 5 to 35 m in a nearby

    area of the Spanish Mediterranean coast. The largest

    individuals analysed in our study (16 cm ML) were

    clearly smaller than those obtained by these authors

    (26 cm ML).

    The movement to the coast is probably related to theneed of rocky substrates where females could lay eggs

    (Mangold-Wirz, 1963), so mature females would

    always be found in littoral waters. The fact that no

    mature females were caught throughout the year of

    sampling is in accordance with this. Mangold and

    Boletzky (1973) and Guerra (1975) found mature

    females but their specimens were caught by trawlers

    working between depths of 2090 m and 25100 m,

    respectively. The absence of mature females in ourwork could be explained if they were in waters

    shallower than the minimum depth sampled (50 m).

    The results of Sanchez and Obarti (1993) conrm this

    because important percentages of mature females

    appeared regularly in their samples.

    Sanchez and Obarti (1993) found a protracted and

    somewhat irregular reproductive period, lasting from

    January to July. Guerra (1975) suggested that this

    period could occur from March to September, with

    a maximum in May to July. Mangold and Boletzky

    (1973) extended this period to October. Taking intoaccount the results obtained by all these authors it can

    be noticed that, although reproduction could last from

    January to October, it reaches a maximum from April

    to July. The only maturing females caught during the

    present work were from May to August, in good

    agreement with these results.

    Bearing in mind the results about the reproductive

    period, it is now possible to interpret the sizefre-

    quency distribution obtained. Recruits, in the sense of

    small animals, are mainly present from September to

    April. Following Mangold-Wirz (1963), the speci-mens of 6.5 to 7 cm ML are 8 months old. Thus,

    the octopuses of 67 cm ML caught from September

    to April, would have been spawned from January to

    August, which coincides with the reproductive period.

    The interruption of spawning from September to

    Fig. 6. Seasonal changes of the emptiness index (EMI) for the

    digestive tracts analysed.

    Fig. 7. Bimonthly composition of octopus catches (O. vulgaris and E. moschata) and the rest of the commercial catch.

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    December is reected in the low percentage of speci-

    mens of that size from May to August.

    It is known that data on the feeding habits of

    octopuses are biased by the sampling method used

    (Sanchez and Obarti, 1993; Cortez et al., 1995). Thus,

    stomach contents studies would underestimate the

    proportion of molluscs, while studies based on debris

    found near the middens would overestimate it. The

    results obtained in this work conrm the importance of

    crustaceans in the diet of octopuses, as have studiesfrom other authors (Nigmatullin and Ostapenko, 1976;

    Guerra, 1978; Sanchez and Obarti, 1993). The impor-

    tance of shes was higher when compared to the

    values obtained by Guerra (1978), Smale and Buchan

    (1981) and Sanchez and Obarti (1993), but similar to

    those found by Nigmatullin and Ostapenko (1976).

    Seasonal changes in feeding intensity were in

    accordance with Cortez et al. (1995). The EMI was

    higher during the colder seasons (especially in winter).

    These results agree with the fact that, in general,

    cephalopods respond to temperature increases by

    increasing their food intake (Mangold and Boletzky,

    1973; Mangold-Wirz and Boucher-Rodoni, 1973;

    Mangold, 1983a).

    From the point of view of shery, apart from O.

    vulgaris, another species, E. moschata, occurs regu-

    larly throughout the year. The results of the present

    study show that O. vulgaris is always more abundant

    and its importance, in relation to Eledone, increases

    gradually from December to July, although it

    decreases thereafter.Octopuses represented 2040% of the total catch

    for the trawlers. Sanchez and Obarti (1993) recorded

    that 36.26% annual catch of O. vulgaris from the

    Spanish Mediterranean coast was made by pots, the

    rest being caught by trawl. The high percentage of clay

    pots catches in the total octopus catch is due to the fact

    that this kind of shing takes only large specimens,

    whereas trawls catch all sizes, specially small ones. In

    our work, the highest catch rates were obtained in

    spring and at the beginning of the summer, while

    during the rest of the year they remained at low levels.

    Fig. 8. (a) Mean and standard deviation for monthly octopus landings of the total Mallorcan fleet from January 1981 to August 1996. (b)

    Catch yields (kg/h) obtained during the sampling period.

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    Sanchez and Obarti (1993) observed that the most

    productive times were at the end of the spring and

    beginning of summer when the octopuses caught were

    larger, and in autumn when the number of specimens

    was higher.

    Landings of octopuses show a cyclic behaviour

    throughout the year. After a minimum in August

    September, they increase gradually until March,

    before decreasing. This minimum could be explained

    by the fact that until August, as was cited above,

    octopuses disappear progressively from trawling

    grounds and, moreover, they are not replaced by

    recruits from May to August. The lack of recruits

    during these months would also explain the decrease

    Fig. 9. (a) Monthly octopus landings from January 1981 to August 1996 and the sinusoidal function that best fits the data. (b) The spectrum

    obtained from the time series; DF16.

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    of catches from April to August. Octopus population

    would recover only by means of the recruitment since,

    like the majority of cephalopod species, they die after

    reproduction (Mangold and Boletzky, 1973; O'Dorand Wells, 1987). The September to April catches

    would increase by recruitment and also by the growth

    in weight of individuals.

    Although many of the exogenous changes that

    affect a shery occur at time scales much shorter than

    a year (Mendelssohn and Cury, 1987), applications of

    time-series analysis to monthly catches are scarce

    (Saila et al., 1980; Mendelssohn, 1981; Mendelssohn

    and Cury, 1987; Jeffries et al., 1989).

    To our knowledge, this is the rst study where time-

    series analysis is applied to a cephalopod shery.Since octopuses landings showed a cyclic behaviour

    throughout the year, as cited above, the 12-month

    cycle revealed by the spectrum analysis would simply

    reect the annual biological cycle of the species. This

    marked seasonality in landings has also been observed

    in other cephalopod species (Sanchez and Martn,

    1993; Cunha and Moreno, 1994; Guerra et al.,

    1994; Pierce et al., 1994), being related to their short

    life span, rapid population turnover and the reproduc-

    tion behaviour of the species (Sanchez and Martn,

    1993).It is only in recent years that the catch data from

    ofcial statistics is well-documented. This allows us to

    observe uctuations in landings along the year, but

    little can be done to analyse trends for longer periods

    of time. The periodicity of 92 months found in the time

    series could be signicant, but a longer series would be

    needed to conrm the signicance of this periodicity.

    Acknowledgements

    This study was carried out within the framework of

    the project `Discards of the Western Mediterranean

    trawl eets' (Contract ref. DG-XIV, MED/94/027).

    We wish to express our gratitude to the crew of the

    trawlers Bellver and Mar Jupe II for their kindness

    during the on-board boat sampling. Special thanks

    also to Dr. Sebastia Monserrat (Department of Phy-

    sics, Universitat Illes Balears) for his help in the

    analysis of the time-series data and to Dr. Chris

    Rodgers and Catalina Ballester for the English

    version.

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