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Biology and fishery of Octopus vulgaris Cuvier, 1797, caught by trawlers in Mallorca (Balearic Sea, Western Mediterranean)
Fisheries Research 36 (1998) 237±249
Biology and ®shery of Octopus vulgaris Cuvier, 1797, caught by trawlers in Mallorca (Balearic Sea, Western Mediterranean) Antoni Quetglasa, Francesc Alemanya, Aina Carbonella,*, Paolo Merellaa, Pilar SaÂnchezb b
a IEO, Centre OceanograÁ®c de Balears, Apdo. 291, 07080 Palma de Mallorca, Spain CSIC, Institut de CieÁncies del Mar, Passeig Joan de Borbo s/n, 08039 Barcelona, Spain
Received 28 May 1997; accepted 29 January 1998
Abstract Some aspects of the biology and ®shery of Octopus vulgaris caught by trawlers in the Balearic Sea (Western Mediterranean) are studied. The analysis of the size±frequency distribution followed the growth of specimens from January (6±7 cm ML) to August (11±12 cm ML). The sex ratio was estimated for each season and it was not signi®cantly different from 1 : 1 in any of them. The stomach contents revealed that the octopus fed predominantly on crustaceans and ®shes. Another octopus species, Eledone moschata, is present in this ®shery but its catches were clearly lower than those of O. vulgaris. The analysis of the importance of these two species in relation to the rest of the commercial catch showed that octopuses represent between 20± 40% of the total catch for trawlers. The highest catch rates (kg/h) were obtained in spring and at the beginning of summer. Time-series analysis of monthly catches from January 1981 to August 1996 showed two main oscillations. The lower one, with a periodicity of 12 months, re¯ects the annual biological cycle of the species; on the other hand, the higher one has a periodicity of 92 months, the time series available being too short to con®rm the signi®cance of this period. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Octopus vulgaris; Eledone moschata; Biology; Fishery; Time-series analysis; Balearic Islands; Western Mediterranean
1. Introduction The common octopus, Octopus vulgaris Cuvier, 1797 has a world-wide distribution in tropical, subtropical and temperate waters of the Atlantic, Indian and Paci®c oceans; it is also present in the Mediterranean Sea (Mangold, 1983b). O. vulgaris is a typical inhabitant of littoral waters, existing up to the limit of the continental shelf *Corresponding author. 0165-7836/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0165-7836(98)00093-9
(Mangold-Wirz, 1963). In very shallow waters, the species occurs mostly in coral reefs or rocks, but in many areas it is equally, or even more abundant, over sandy and muddy bottom or in seagrass (Mangold, 1983b). The octopus is an important species for the ®sheries of many countries, being captured by various methods, but mainly by otter trawl. In the Mediterranean Sea, octopus catches from trawlers that operate on the continental shelf constitute an important fraction of the total landings of these ships (Relini and Orsi-
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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 inhabiting rocky substrates are caught with pots on a commercial 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 Maconnachie, 1988) or caught in littoral waters (Kayes, 1974; Ambrose and Nelson, 1983). Some studies deal with the 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. SaÂnchez and Obarti (1993) studied the biology and ®shery of O. vulgaris populations living in littoral waters from 5±35 m in the central Spanish Mediterranean 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 of O. vulgaris caught by trawlers at depths of 50±100 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 of Palma 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 50±100 m. Monthly size±frequency distributions of O. 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 classi®ed into species. Thus, both species appear pooled together under the
Fig. 1. The study area in the Balearic Sea (Western Mediterranean).
`octopuses' category in the statistics of the central auction wharf of Mallorca. In order to determine the proportion 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 separately 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 allometry of the growth in weight was tested by a Student'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 signi®cance level ( ) of 0.05 was considered. The following three-stage maturity scale (adapted from SaÂnchez and Obarti, 1993) was used:
A. Quetglas et al. / Fisheries Research 36 (1998) 237±249
± 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 granular structure; 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 complex 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 logarithmic transformation of the ovary weight (OvW) was made: log (1OvW), and used to eliminate negative OvW values (Bartlett, 1947). Stomach contents were analysed and prey identi®ed 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; PeÂrez-GaÂndaras, 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 present and the total number of stomachs with food, each stomach being counted as many times as the number of different types of prey it contained.
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± 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 (Bloom®eld, 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 cos !t B cos !t where ! is the frequency of the series searched for, x the mean of the catch data, and A and B are as follows: A
t 2X xt ÿ xcos !t; n n1
B
t 2X xt ÿ xsin !t n n1
n and xt being the number of points and the value of x 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 more energetic 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
M F MF
0.350 0.542 0.420
2.988 2.804 2.917
M F MF
0.757 0.587 0.718
2.74 2.83 2.80
M F MF
3.306 1.654 Ð
M F MF
0.442 0.413 0.437
n
r
Area
0.979 0.969 0.969
Catalonia (Western Mediterranean)
Ð Ð 3±22
Guerra and ManrõÂquez, 1980
37 55 92
0.95 0.97 0.97
South Africa (Atlantic Ocean)
4.9±21.5 4.6±21.5 4.6±21.5
Smale and Buchan, 1981
2.323 2.576 Ð
155 165 Ð
0.90 0.92 Ð
Valencia (Western Mediterranean)
8±22 9±26 Ð
SaÂnchez and Obarti, 1993
2.882 2.916 2.889
168 175 343
0.95 0.94 0.94
Mallorca (Western Mediterranean)
5±16 5±16 5±16
Present work
584 434 1018
Size range (cm)
Source
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3. Results 3.1. Length±weight relationships and size-frequency distributions The results of the relationship between dorsal mantle length and total body weight are shown in Table 1. No signi®cant differences were observed when the slopes and the intercepts were compared between sexes (0.200.50 for males and females; 0.20
The growth of octopuses can be followed from January, with 6±7 cm ML, to August, when they reach 11±12 cm ML. From September, the octopuses of these latter sizes become rarer and the individuals of 6±7 cm ML predominate again. 3.2. Sex ratio, maturation and reproduction The sex ratio was not signi®cantly different from 1 : 1 in any of the seasons (0.90
Fig. 2. Bimonthly size±frequency distributions of O. vulgaris throughout the year of sampling.
A. Quetglas et al. / Fisheries Research 36 (1998) 237±249
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Fig. 2. (Continued).
maximum number in May±June and a minimum in November±December. Maturing females were caught from May to August. For males, the relationship between total body weight and gonad weight (GoWtestisspermatophoric complex weight) gave these parameters (Fig. 4(a)): GoW ÿ0:541 0:014TW; with n 167 and r 0:92
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 a gradual decrease of this index from winter to summer, increasing again in autumn.
For females, the relationships between log (1OvW) and log (ML) are presented in Fig. 4(b).
3.4. Fishery
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-
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 sporadically appear, but it generally inhabits deeper waters,
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Fig. 3. Bimonthly percentage of maturity stages of (a) males and (b) females.
and is caught in small quantities, mostly by trawlers targeting the Norway lobster (Nephrops norvegicus Linnaeus, 1758). Bimonthly percentages of O. vulgaris, E. moschata and 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 June±July. Subsequently, they decreased abruptly to 20% in August±September, before increasing again thereafter. Although E. moschata appeared regularly throughout the year (Fig. 7), its catches were lower than those of O. vulgaris. In addition, the importance of Octopus in relation to Eledone increased gradually from December to July. The mean and standard deviation of monthly landings from January 1981 to August 1996 are shown in Fig. 8(a). Catches increase progressively from January to March, going down thereafter. Finally, after a clear minimum in August±September 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 oscillation, the series was analysed by the least-squares ®tting method. As a result, a period of 92 months was obtained, with x 1521:04 kg, Aÿ4710.64 kg and B1626.95 kg. Monthly landings from January 1981 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 24 months. The spectrum analysis (Fig. 9(b)) revealed a clear peak at a frequency of !10.083 monthsÿ1 (corresponding to a period of 12 months) with two other peaks at !20.167 and !30.250 monthsÿ1 (periods of 6 and 4 months, respectively). These two last peaks were located at frequencies that are multiples of the ®rst one (!22!1 and !33!1), 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.
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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).
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 re¯ected 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 size±frequency distribution. Octopuses
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Table 2 The occurrence index (OCI) for the prey items found in the stomachs Occurrence Index (OCI) CRUSTACEA AMPHIPODA Gammaridea
65.75
ISOPODA DECAPODA Decapoda indeterminate DECAPODA NATANTIA Natantia indeterminate Caridea indeterminate Alpheidae indeterminate Thoralus cranchii Philocheras sculptus DECAPODA REPTANTIA ANOMURA Anomura indeterminate Galathea sp. Galathea intermedia Galathea strigosa Galathea bolivari Paguridea indeterminate Paguristes eremita Pagurus prideaux BRACHYURA Brachyura indeterminate Liocarcinus sp. Liocarcinus corrugatus Liocarcinus pusillus Pilumnus spinifer Xantho pilipes Ebalia sp. Ebalia granulosa Ebalia tuberosa Oxyrhyncha indeterminate Parthenopidae indeterminate Inachus dorsettensis Eurynome spinosa Atelecyclus rotundatus MOLLUSCA POLIPLACOPHORA GASTROPODA Trachidae indeterminate Turritella communis Raphitoma reticulata Naticarius intricatoides Naticarius hebraeus Calliostoma granulatum CEPHALOPODA Cephalopoda indeterminate Sepiolidae indeterminate Alloteuthis media
4.50 4.50 1.00 60.25 8.75 3.25 2.25 0.25 0.25 0.25 0.25 48.25 19.25 2.00 1.75 11.50 0.25 0.75 2.00 0.75 0.25 29.00 15.00 0.25 8.75 0.25 0.25 0.25 0.75 0.25 1.25 0.25 0.25 0.25 0.25 1.00
6.50
0.50 3.25 1.50 0.25 0.25 0.25 0.50 0.50 2.75 1.75 0.50 0.25
Table 2 (Continued) Occurrence Index (OCI) Loligo vulgaris TELEOSTEI Teleostei indeterminate Gobidae indeterminate Carapus acus Ophichthus rufus Gaidropsarus vulgaris Blennius ocellaris Capros aper Centracanthus cirrus Not identified
0.25 27.00
0.75
12.00 13.00 0.25 0.25 0.25 0.75 0.25 0.25
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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Fig. 6. Seasonal changes of the emptiness index (EMI) for the digestive tracts analysed.
disappear progressively from trawling grounds when they reach a 11±12 cm ML size. This size coincides with the minimum mean length obtained by SaÂnchez 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 the need 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
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Boletzky (1973) and Guerra (1975) found mature females but their specimens were caught by trawlers working between depths of 20±90 m and 25±100 m, respectively. The absence of mature females in our work could be explained if they were in waters shallower than the minimum depth sampled (50 m). The results of SaÂnchez and Obarti (1993) con®rm this because important percentages of mature females appeared regularly in their samples. SaÂnchez 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 into account 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 size±frequency distribution obtained. Recruits, in the sense of small animals, are mainly present from September to April. Following Mangold-Wirz (1963), the specimens of 6.5 to 7 cm ML are 8 months old. Thus, the octopuses of 6±7 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. 7. Bimonthly composition of octopus catches (O. vulgaris and E. moschata) and the rest of the commercial catch.
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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.
December is re¯ected in the low percentage of specimens 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 (SaÂnchez 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 con®rm the importance of crustaceans in the diet of octopuses, as have studies from other authors (Nigmatullin and Ostapenko, 1976; Guerra, 1978; SaÂnchez and Obarti, 1993). The importance of ®shes was higher when compared to the values obtained by Guerra (1978), Smale and Buchan (1981) and SaÂnchez 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 regularly 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 20±40% of the total catch for the trawlers. SaÂnchez 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.
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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.
SaÂnchez 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
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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'Dor and 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 timeseries 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 re¯ect the annual biological cycle of the species. This marked seasonality in landings has also been observed in other cephalopod species (SaÂnchez and MartõÂn, 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 reproduction behaviour of the species (SaÂnchez and MartõÂn, 1993). It is only in recent years that the catch data from of®cial 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 signi®cant, but a longer series would be needed to con®rm the signi®cance 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. SebastiaÁ Monserrat (Department of Physics, 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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A. Quetglas et al. / Fisheries Research 36 (1998) 237±249 Nigmatullin, Ch.M., Ostapenko, A.A., 1976. Feeding of Octopus vulgaris Lam. from the Northwest African coast. ICES, C.M. 1976/K 6, 1±15. Nixon, M., 1966. Changes in body weight and intake of food by Octopus vulgaris. J. Zool., Lond. 150, 1±9. Nixon, M., 1971. Some parameters of growth in Octopus vulgaris. J. Zool., Lond. 163, 277±284. Nixon, M., 1979. Hole-boring in shells by Octopus vulgaris Cuvier in the Mediterranean. Malacologia 18, 431±443. Nixon, M., Maconnachie, E., 1988. Drilling by Octopus vulgaris (Mollusca: Cephalopoda) in the Mediterranean. J. Zool., Lond. 216, 687±716. O' Dor, R.K., Wells M.J., 1987. Energy and nutrient flow. In: P.R. Boyle (Ed.), Cephalopods Life Cycles. Vol. II. Comparative Reviews, Academic Press, pp. 109±133. PeÂrez-GaÂndaras, G., 1983. Estudio de los cefaloÂpodos ibeÂricos. SistemaÂtica y bionomõÂa mediante el estudio morfomeÂtrico comparado de sus mandõÂbulas. Tesis Doctoral, Universidad Complutense de Madrid. Pierce, G.J., Boyle, P.R., Hastie, L.C., Shanks, A., 1994. Distribution and abundance of the fished population of Loligo forbesi in UK waters: Analysis of fishery data. Fish. Res. 21, 193±216.
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Relini, G., Orsi-Relini, L., 1984. The role of cephalopods in the inshore trawl fishing of the Ligurian Sea. Oebalia, 10, N.S. 37± 58. Saila, S.B., Wigbout, M., Lermit, R.J., 1980. Comparison of some time series models for the analysis of fisheries data. J. Cons. Int. Explor. Mer. 39, 44±52. SaÂnchez, P., MartõÂn, P., 1993. Population dynamics of the exploited cephalopod species of the Catalan Sea (NW Mediterranean), Sci. Mar. 57(2-3), 153±159. SaÂnchez, P., Obarti, R., 1993. The Biology and Fishery of Octopus vulgaris Caught with Clay Pots on the Spanish Mediterranean Coast. In: Okutani, T., R.K. O'Dor, T. Kubodera (Eds.), Recent Advances in Fisheries Biology. Tokai University Press, Tokyo., pp. 477±487. Smale, M.J., Buchan, P.R., 1981. Biology of Octopus vulgaris off the east coast of South Africa. Mar. Biol. 65, 1±12. Tursi, S., D'Onghia, G., 1992. Cephalopods of the Ionian Sea (Mediterranean Sea). Oebalia 18, 25±43. Zar, J.H., 1984. Biostatistical analysis. Prentice-Hall Inc., Englewood Cliffs, pp. 718. Zariquiey-Alvarez, R., 1968. CrustaÂceos decaÂpodos ibeÂricos. Inv. Pesq., 32, pp. 510.
Biology and ®shery of Octopus vulgaris Cuvier, 1797, caught by trawlers in Mallorca (Balearic Sea, Western Mediterranean) Antoni Quetglasa, Francesc Alemanya, Aina Carbonella,*, Paolo Merellaa, Pilar SaÂnchezb b
a IEO, Centre OceanograÁ®c de Balears, Apdo. 291, 07080 Palma de Mallorca, Spain CSIC, Institut de CieÁncies del Mar, Passeig Joan de Borbo s/n, 08039 Barcelona, Spain
Received 28 May 1997; accepted 29 January 1998
Abstract Some aspects of the biology and ®shery of Octopus vulgaris caught by trawlers in the Balearic Sea (Western Mediterranean) are studied. The analysis of the size±frequency distribution followed the growth of specimens from January (6±7 cm ML) to August (11±12 cm ML). The sex ratio was estimated for each season and it was not signi®cantly different from 1 : 1 in any of them. The stomach contents revealed that the octopus fed predominantly on crustaceans and ®shes. Another octopus species, Eledone moschata, is present in this ®shery but its catches were clearly lower than those of O. vulgaris. The analysis of the importance of these two species in relation to the rest of the commercial catch showed that octopuses represent between 20± 40% of the total catch for trawlers. The highest catch rates (kg/h) were obtained in spring and at the beginning of summer. Time-series analysis of monthly catches from January 1981 to August 1996 showed two main oscillations. The lower one, with a periodicity of 12 months, re¯ects the annual biological cycle of the species; on the other hand, the higher one has a periodicity of 92 months, the time series available being too short to con®rm the signi®cance of this period. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Octopus vulgaris; Eledone moschata; Biology; Fishery; Time-series analysis; Balearic Islands; Western Mediterranean
1. Introduction The common octopus, Octopus vulgaris Cuvier, 1797 has a world-wide distribution in tropical, subtropical and temperate waters of the Atlantic, Indian and Paci®c oceans; it is also present in the Mediterranean Sea (Mangold, 1983b). O. vulgaris is a typical inhabitant of littoral waters, existing up to the limit of the continental shelf *Corresponding author. 0165-7836/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0165-7836(98)00093-9
(Mangold-Wirz, 1963). In very shallow waters, the species occurs mostly in coral reefs or rocks, but in many areas it is equally, or even more abundant, over sandy and muddy bottom or in seagrass (Mangold, 1983b). The octopus is an important species for the ®sheries of many countries, being captured by various methods, but mainly by otter trawl. In the Mediterranean Sea, octopus catches from trawlers that operate on the continental shelf constitute an important fraction of the total landings of these ships (Relini and Orsi-
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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 inhabiting rocky substrates are caught with pots on a commercial 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 Maconnachie, 1988) or caught in littoral waters (Kayes, 1974; Ambrose and Nelson, 1983). Some studies deal with the 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. SaÂnchez and Obarti (1993) studied the biology and ®shery of O. vulgaris populations living in littoral waters from 5±35 m in the central Spanish Mediterranean 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 of O. vulgaris caught by trawlers at depths of 50±100 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 of Palma 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 50±100 m. Monthly size±frequency distributions of O. 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 classi®ed into species. Thus, both species appear pooled together under the
Fig. 1. The study area in the Balearic Sea (Western Mediterranean).
`octopuses' category in the statistics of the central auction wharf of Mallorca. In order to determine the proportion 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 separately 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 allometry of the growth in weight was tested by a Student'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 signi®cance level ( ) of 0.05 was considered. The following three-stage maturity scale (adapted from SaÂnchez and Obarti, 1993) was used:
A. Quetglas et al. / Fisheries Research 36 (1998) 237±249
± 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 granular structure; 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 complex 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 logarithmic transformation of the ovary weight (OvW) was made: log (1OvW), and used to eliminate negative OvW values (Bartlett, 1947). Stomach contents were analysed and prey identi®ed 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; PeÂrez-GaÂndaras, 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 present and the total number of stomachs with food, each stomach being counted as many times as the number of different types of prey it contained.
239
± 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 (Bloom®eld, 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 cos !t B cos !t where ! is the frequency of the series searched for, x the mean of the catch data, and A and B are as follows: A
t 2X xt ÿ xcos !t; n n1
B
t 2X xt ÿ xsin !t n n1
n and xt being the number of points and the value of x 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 more energetic 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
M F MF
0.350 0.542 0.420
2.988 2.804 2.917
M F MF
0.757 0.587 0.718
2.74 2.83 2.80
M F MF
3.306 1.654 Ð
M F MF
0.442 0.413 0.437
n
r
Area
0.979 0.969 0.969
Catalonia (Western Mediterranean)
Ð Ð 3±22
Guerra and ManrõÂquez, 1980
37 55 92
0.95 0.97 0.97
South Africa (Atlantic Ocean)
4.9±21.5 4.6±21.5 4.6±21.5
Smale and Buchan, 1981
2.323 2.576 Ð
155 165 Ð
0.90 0.92 Ð
Valencia (Western Mediterranean)
8±22 9±26 Ð
SaÂnchez and Obarti, 1993
2.882 2.916 2.889
168 175 343
0.95 0.94 0.94
Mallorca (Western Mediterranean)
5±16 5±16 5±16
Present work
584 434 1018
Size range (cm)
Source
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3. Results 3.1. Length±weight relationships and size-frequency distributions The results of the relationship between dorsal mantle length and total body weight are shown in Table 1. No signi®cant differences were observed when the slopes and the intercepts were compared between sexes (0.20
The growth of octopuses can be followed from January, with 6±7 cm ML, to August, when they reach 11±12 cm ML. From September, the octopuses of these latter sizes become rarer and the individuals of 6±7 cm ML predominate again. 3.2. Sex ratio, maturation and reproduction The sex ratio was not signi®cantly different from 1 : 1 in any of the seasons (0.90
Fig. 2. Bimonthly size±frequency distributions of O. vulgaris throughout the year of sampling.
A. Quetglas et al. / Fisheries Research 36 (1998) 237±249
241
Fig. 2. (Continued).
maximum number in May±June and a minimum in November±December. Maturing females were caught from May to August. For males, the relationship between total body weight and gonad weight (GoWtestisspermatophoric complex weight) gave these parameters (Fig. 4(a)): GoW ÿ0:541 0:014TW; with n 167 and r 0:92
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 a gradual decrease of this index from winter to summer, increasing again in autumn.
For females, the relationships between log (1OvW) and log (ML) are presented in Fig. 4(b).
3.4. Fishery
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-
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 sporadically appear, but it generally inhabits deeper waters,
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Fig. 3. Bimonthly percentage of maturity stages of (a) males and (b) females.
and is caught in small quantities, mostly by trawlers targeting the Norway lobster (Nephrops norvegicus Linnaeus, 1758). Bimonthly percentages of O. vulgaris, E. moschata and 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 June±July. Subsequently, they decreased abruptly to 20% in August±September, before increasing again thereafter. Although E. moschata appeared regularly throughout the year (Fig. 7), its catches were lower than those of O. vulgaris. In addition, the importance of Octopus in relation to Eledone increased gradually from December to July. The mean and standard deviation of monthly landings from January 1981 to August 1996 are shown in Fig. 8(a). Catches increase progressively from January to March, going down thereafter. Finally, after a clear minimum in August±September 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 oscillation, the series was analysed by the least-squares ®tting method. As a result, a period of 92 months was obtained, with x 1521:04 kg, Aÿ4710.64 kg and B1626.95 kg. Monthly landings from January 1981 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 24 months. The spectrum analysis (Fig. 9(b)) revealed a clear peak at a frequency of !10.083 monthsÿ1 (corresponding to a period of 12 months) with two other peaks at !20.167 and !30.250 monthsÿ1 (periods of 6 and 4 months, respectively). These two last peaks were located at frequencies that are multiples of the ®rst one (!22!1 and !33!1), 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.
A. Quetglas et al. / Fisheries Research 36 (1998) 237±249
243
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).
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 re¯ected 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 size±frequency distribution. Octopuses
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A. Quetglas et al. / Fisheries Research 36 (1998) 237±249
Table 2 The occurrence index (OCI) for the prey items found in the stomachs Occurrence Index (OCI) CRUSTACEA AMPHIPODA Gammaridea
65.75
ISOPODA DECAPODA Decapoda indeterminate DECAPODA NATANTIA Natantia indeterminate Caridea indeterminate Alpheidae indeterminate Thoralus cranchii Philocheras sculptus DECAPODA REPTANTIA ANOMURA Anomura indeterminate Galathea sp. Galathea intermedia Galathea strigosa Galathea bolivari Paguridea indeterminate Paguristes eremita Pagurus prideaux BRACHYURA Brachyura indeterminate Liocarcinus sp. Liocarcinus corrugatus Liocarcinus pusillus Pilumnus spinifer Xantho pilipes Ebalia sp. Ebalia granulosa Ebalia tuberosa Oxyrhyncha indeterminate Parthenopidae indeterminate Inachus dorsettensis Eurynome spinosa Atelecyclus rotundatus MOLLUSCA POLIPLACOPHORA GASTROPODA Trachidae indeterminate Turritella communis Raphitoma reticulata Naticarius intricatoides Naticarius hebraeus Calliostoma granulatum CEPHALOPODA Cephalopoda indeterminate Sepiolidae indeterminate Alloteuthis media
4.50 4.50 1.00 60.25 8.75 3.25 2.25 0.25 0.25 0.25 0.25 48.25 19.25 2.00 1.75 11.50 0.25 0.75 2.00 0.75 0.25 29.00 15.00 0.25 8.75 0.25 0.25 0.25 0.75 0.25 1.25 0.25 0.25 0.25 0.25 1.00
6.50
0.50 3.25 1.50 0.25 0.25 0.25 0.50 0.50 2.75 1.75 0.50 0.25
Table 2 (Continued) Occurrence Index (OCI) Loligo vulgaris TELEOSTEI Teleostei indeterminate Gobidae indeterminate Carapus acus Ophichthus rufus Gaidropsarus vulgaris Blennius ocellaris Capros aper Centracanthus cirrus Not identified
0.25 27.00
0.75
12.00 13.00 0.25 0.25 0.25 0.75 0.25 0.25
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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Fig. 6. Seasonal changes of the emptiness index (EMI) for the digestive tracts analysed.
disappear progressively from trawling grounds when they reach a 11±12 cm ML size. This size coincides with the minimum mean length obtained by SaÂnchez 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 the need 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
245
Boletzky (1973) and Guerra (1975) found mature females but their specimens were caught by trawlers working between depths of 20±90 m and 25±100 m, respectively. The absence of mature females in our work could be explained if they were in waters shallower than the minimum depth sampled (50 m). The results of SaÂnchez and Obarti (1993) con®rm this because important percentages of mature females appeared regularly in their samples. SaÂnchez 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 into account 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 size±frequency distribution obtained. Recruits, in the sense of small animals, are mainly present from September to April. Following Mangold-Wirz (1963), the specimens of 6.5 to 7 cm ML are 8 months old. Thus, the octopuses of 6±7 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. 7. Bimonthly composition of octopus catches (O. vulgaris and E. moschata) and the rest of the commercial catch.
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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.
December is re¯ected in the low percentage of specimens 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 (SaÂnchez 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 con®rm the importance of crustaceans in the diet of octopuses, as have studies from other authors (Nigmatullin and Ostapenko, 1976; Guerra, 1978; SaÂnchez and Obarti, 1993). The importance of ®shes was higher when compared to the values obtained by Guerra (1978), Smale and Buchan (1981) and SaÂnchez 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 regularly 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 20±40% of the total catch for the trawlers. SaÂnchez 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.
A. Quetglas et al. / Fisheries Research 36 (1998) 237±249
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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.
SaÂnchez 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
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A. Quetglas et al. / Fisheries Research 36 (1998) 237±249
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'Dor and 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 timeseries 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 re¯ect the annual biological cycle of the species. This marked seasonality in landings has also been observed in other cephalopod species (SaÂnchez and MartõÂn, 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 reproduction behaviour of the species (SaÂnchez and MartõÂn, 1993). It is only in recent years that the catch data from of®cial 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 signi®cant, but a longer series would be needed to con®rm the signi®cance 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. SebastiaÁ Monserrat (Department of Physics, 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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