a comparative study of the diet of loligo vulgaris (lamarck, 1799) (mollusca:cephalopoda) from the...

ELSEVIER Fisheries Research 29 (I 997) 245-255

A comparative study of the diet of Loligo vulgaris ( Lamarck, 1799) ( Mollusca:Cephalopoda) from the south coast of Portugal

and the Saharan Bank ( Central-East Atlantic)

M. Coelho a, P. Domingues a, E. Balguerias b, M. Femandez b, J. P. Andrade a, * a UCTRA, Universidade do Algurve, Cumpus de Gambelas, 8000 Furo, Portugal

b I.E.O.. Centro Oceanogrujco de Cat&us, Ctu de San An&&, s/n, 38120 Santa Cruz de Tenerije, Isius Cunrrrins. Espaiia

Accepted 2 August 1996

Abstract

This paper presents data on the diet of the squid, Loligo vulgaris (Lamarck, 1799) from the south coast of Portugal (Algarve) and from the Saharan Bank (Central-East Atlantic). A total of 964 squid was collected from the Algarve coast,

between March 1993 and October 1994, from bottom trawling. An additional sample of 70 stomachs was obtained in the Algarve coast from the hand jigging fishery during 1991 and 1992. In the Saharan Bank, 848 squid were obtained, between June 1993 and January 1994, with bottom trawling. Stomachs with contents for trawled squid from the Algarve coast, and the Saharan Bank represented 28.1% and 40.8%, respectively. Fish was always the main component of the diet in both regions, representing 88.6% of the total weight of the prey found in the stomachs, for the Algarve coast, and 70.9% for the Saharan Bank. The occurrence percentage and the percentage in number indicated that Trachurus truchurus and fish belonging to the family Gobiidae were the most frequent fish found in the Algarve samples, while flatfish were the most

common fish in the Saharan Bank samples. Loligo vulgaris was the dominant cephalopod found in the stomachs for both regions. No differences in the diet were found between males and females. The comparison between immature and mature squid showed that the importance of fish was higher in mature squid, for both regions. The squid captured from the Algarve coast by bottom trawling (at greater depths and offshore) fed primarily on fish, while those captured with hand jigging (inshore, at lower depths) contained an important percentage of crustaceans. 0 1997 Elsevier Science B.V.

Keywor& Algarve; Saharan bank; Feeding; Loligo vulgaris

1. Introduction

Cephalopods are marine predators and voracious carnivores (Okutani, 1990) with very high metabolic

Corresponding author. Tel.: + 351 89 800967; fax: + 351 89 8 18353; e-mail: [email protected].

and conversion rates, and feeding on live prey throughout their life cycle (Amaratunga, 1983). Fur- ther, a great number of sea birds, marine mammals and fish feed on cephalopods (Furness, 1994; Gonza- lez et al., 1994; Pierce et al., 1994). Therefore, these organisms play an important role in the trophic relations within marine ecosystems (Boletzky and Hanlon, 1983; Caddy, 1983) and the determination

0165-7836/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO1 65.7836(96)00540- 1

246 M. Coelho et ul./ Fisheries Research 29 (1997) 245-255

of their food spectrum is a prerequisite to understanding these relations (Lipinski, 1992).

During their life cycle, cephalopods prey on a variety of species and on a large range of sizes (Boucher-Rodoni et al., 1987), widening their range of prey species as they grow (Nixon, 1987; Lipinski, 1987, 1992). Adults feed mainly on fish and cephalopods, while crustaceans are the most important prey of juveniles (Nixon, 1987). Cannibalism by large squids on smaller ones is also known for some species (Worms, 1983; Baddyr, 1988; Dawe, 1988; Lipinski and Linkowski, 1988; Sauer and Lipinski, 1991).

Few published data exist on the diet of L. uul- gutis. Guerra and Rocha (1994) and Rocha et al. (1994) have examined Lofigo spp. in Galicean waters (NW Spain) indicating the occurrence of a wide range of fishes and crustaceans in the diet of L.

vulgaris. Pierce et al. (1994) reported that L. vul-

garis from the northeast Atlantic fed largely on fishes, cephalopods and crustaceans. Studies on L.

vulgaris reynaudii from the South African coast indicate that fishes and crustaceans are also the main prey taxa (Lipinski, 1987; Augustyn, 1990, 1991).

This paper provides information on the diet of Loligo uutgmis (Lamarck, 1799) from the south coast of Portugal and the Saharan Bank. The basic components are described in relation to depth, geo- graphic area, sex and stage of maturity.

2. Materials and methods

A total of 964 individuals were obtained from the south coast of Portugal (Algarve) from the bottom trawl fishery, between March 1993 and October 1994 (Fig. 1). The fishing grounds are near Vila Real de Santo Antbnio, between 6 miles and 12 miles offshore, at depths between 120 m and 180 m. Seventy additional stomachs were obtained from squid collected during 1991 (May and August) and 1992 (March, May and June) from the hand jigging fishery, using small boats operating in the same area at depths less than 25 m. From the Saharan Bank (Fig. 1), 848 I d d 1 ‘n ivi ua s were collected from commercial fishing trawlers in June, September, Novem- ber and December 1993, and January 1994, usually deeper than 100 m.

ATLANTIC OCEAN

400

Fig. 1. The two sampling areas: A, Algme, B, Saharan Bank, located in the eastern Atlantic Ocean.

For each individual, the dorsal mantle length (DML) was measured to the nearest millimetre and the total weight to the nearest gram. The individuals were sexed and the sexual maturity was assessed according to the maturity scale of Lipinski (1979).

Stomach contents were weighed to the nearest 0.01 g. Stomach fullness was scaled l-4, as follows: 1. The contents in the stomach were up to 0.25 of

the total volume of the stomach; 2. The contents were 0.5 of the volume of the

stomach; 3. The contents were 0.75 of the volume of the

stomach; 4. The contents filled the stomach.

After this scaling, the material was preserved in 70% ethyl alcohol and analyzed a few days later. The stomachs of the individuals obtained from the hand jigging were frozen for l-2 years. For all the stomachs, the basic contents (fish, crustaceans, cephalopods, and unidentified items) were sorted. These were further identified, when possible, to the lowest taxon (family, genus or species). Otoliths, statoliths, beaks, scales, vertebrae and other hard parts were used to identify the stomach contents, using keys provided in Echeverrya (1950), Clark (1978, 1986) and Hark&en (1986). Algae and sedi- ments were not considered as a prey group, in spite of their occasional presence in the stomachs.

The following indices were used to quantify the diet: * Percentage occurrence: the number of stomachs

M. Coelho et ui./Fisheries Research 29 (1997) 245-255 247

where a determined prey item occurred, expressed as percentage (Hyslop, 1980);

- Weight percentage: the wet weight of a particular prey item divided by the total wet weight of all prey, in the whole sample, expressed as a percentage (Herran, 1988);

- Number, in percentage: the percentage of a determined prey item in relation to the total number of prey (Hyslop, 1980; Herran, 1988);

* The Fullness Index: expressed by the weight of the stomach contents divided by the whole weight (stomach included) of the animal (Berg, 1979; Herran, 1988). For the stomachs obtained from the squid caught

with hand jigging (n = 701, the results refer only to the number and the weight percentages as well as frequency of occurrence, as these stomachs had been preserved from samples collected much earlier. As this information is restricted to the stomach contents, the percentage of stomachs with contents, the fullness index, the differences between sexes and the maturity stages could not be calculated.

Comparisons of the diet of squids of different sex, depth (only in the samples from the Algarve), geo- graphic area and stage of maturity (immature and mature individuals) were made applying the chi- square test (Sokal and Rohlf, 1969) to the frequency of occurrence index of the prey grouped by major taxa (fish, crustaceans, cephalopods, polychaetes and others). Maturity curves usually follow a logistic

1 function of the type Pi = 1 + expc _ ca + Liblj t

where a and b are constants, and Pi the percentage of mature individuals at length Li. The resolution of this equation was made by correlation analysis of variables P, all Li, after linearization. The length at maturity was calculated from the ratio between the

constants a and b CL,, = - “1 (see details in Coelho

et al., 1994). b

3. Results

3.1. Stomach fullness

3.1. I _ Algarve: bottom trawling

From the 964 stomachs examined, 70.9% were empty. The stomach contents with more than one

type of prey (fish, crustaceans, cephalopods or unidentified prey items) were 12.9% of all the stomachs with food remains. Of the 271 stomachs with contents, 173 had Stomach Fullness 1, 61 Stomach Fullness 2, 29 Stomach Fullness 3 and eight Stomach Fullness 4 (Table I>.

3.1.2. Saharan Bank: bottom trawling

Of the 848 individuals sampled in the Saharan Bank, 59.2% of the stomachs were empty. The stomachs with more than one prey item represented 5.1% of the total. Of the 346 stomachs with contents, 199 had Stomach Fullness Index 1, 84 had Stomach Fullness Index 2, 47 had Stomach Fullness Index 3 and 16 had Stomach Fullness Index 4 (Table 1).

3.2. Prey spectrum

Table 2 shows the overall qualitative content of squid stomachs caught along the Algarve coast and in the Saharan Bank, using the occurrence and number percentages.

3.2.1. Algarve: bottom trawling

Trachurus trachurus was the most frequent among the fish prey items, occurring in 7.7% of all the stomachs with contents. The gobies were relatively common, occurring in 4.4% of the stomachs. The occurrence percentage of L. vulgaris, 2.6%, was the highest among the cephalopod prey items. Poly- chaetes were found in only 1.8% of the stomachs with contents. From all the individuals of all groups of prey in the stomach contents, only 25.4% were identified, since they consisted of hard parts.

Considering the fish, about 38% of the prey items in number were identified. The most abundant species

Table 1 Values calculated for the Fullness Index of L. vulgaris from the

Algarve coast and the Saharan Bank

Stomach fullness Algarve a Saharan Bank a

Number Per cent Number Per cent

I 173 63.8 199 57.5 2 61 22.5 84 24.3

3 29 10.7 47 13.6

4 8 3.0 16 4.6

a Bottom trawling.

248

Table 2

M. Coelho et al./ Fisheries Research 29 (1997) 24.5-255

The nercentage occurrence and nercentage in number of the different srouns of nrev in the diet of L. uulearis

Taxon Percentage occurrence

Abt Ahg SB

Percentage in number

Abt Ahj SB

Pisces Clupeidae Sardina pilchardus Engraulis encrusicolus Myctophidae Diuphus dumerilii Hemiramphidae Carangidae Trachurus trachurus Sparidae Scomber scombrus Gyrnnocephulus cernuus Gobiidae Callionymidae Atherinu sp. Paleomonas sp. Pleuronectidae Microchirus sp. Microchirus boscanion Chelidonichthys sp. Unidentified fish

Crustacea lsopoda Gammaridae Euphausiacea Decapoda Unidentified crustaceans

Cephalopoda Sepio sp. Sepiu elegans Loliginidae Loligo vulgaris Cranchiidae octopus sp. Unidentified cephalopods

Polychaetae Nereis sp. Perinereis sp. Nephthys sp. Unidentified polychaetes

1.1

0.4

0.4

7.7 1.5

0.4 4.4 0.4 1.1

0.4

48.6

0.4

0.4 0.7

21.7

0.4

1.1 2.6 0.4

21.7

0.7 1.1 0.4 0.4

1.4 1.4

8.6

1.4 4.3 7.1

7.1 1.4

35.7

1.4

10.0

4.3

10.0

1.4 1.4

1.7 0.3 0.8 1.4

1.4

1.0 0.2

0.3 0.6 0.3

0.3 0.3 0.6 0.3

5.5 I .o

4.2 0.2 0.3 0.2

2.9 0.3 3.1 0.3 0.8

0.7 2.8 4.9

3.5 0.7

1.7

6.1 3.5 0.3 1.4

39.9

0.3

41.3

3.3 1.9 0.2 0.8

38.0

1.4 5.5 1.7 5.5

16.5

0.3

30.1

0.7

0.3 0.5

19.6 4.9

0.8 7.0 1.0 3.5

19.4

0.3 0.3

0.3

9.2 0.3 0.3

16.5

0.8 1.8 0.3

2.8

19.6 4.9

0.2 0.2

5.2 0.2 0.2

19.4

3.1 6.5 0.3 0.3

2.8 4.9

Abt is Algarve, bottom trawling; Ahj is Algarve, hand jigging; SB is Saharan Bank, bottom trawling.

by number were T. rrachurus (5.5%), Gobiidae (3.1%) and Sardina pilchardus (0.8%). Other fish belonged to the families Sparidae, Myctophydae, Callionymidae and Atherinidae. Of the cephalopods identified, 1.8% were L. uulgaris and 0.8% were unidentified loliginids. For the crustaceans, 29.5% were identified, the decapods being the most abun-

dant group, although scarcely represented. The gen- era Perinereis (6.5%) and Nereis (3.1%) dominated among the polychaetes.

3.2.2. Algarve: hand jigging The occurrence percentage indicated that the most

common fish was T. trachurus, presenting 8.6% of

M. Coelho er al. / Fisheries Research 29 C 1997) 245-255 249

I

Cephalopods, 19.0 %

Others, 1.1 %

Fish, 72.7 56

Cephalopods, 30.5 %

Fish, 5 1.0 %

halopods, 25.6 %

II

Crustaceans, 7.4 “3

Cephalopods, 7.7

others, 0.2 %

Abt

\ .___/- / Fish 84.6 %

Ahj

Fish_ 88.9 %

crustaceans. 9.1 % Cephalopods, 18.9 %

Others, 1.1 %

SB

Fish, 70.9 %

Fig. 2. Occurrence percentage (1) and percentage weight (II) of the different prey groups in the diet of L. uulpu-is caught in Algawe coast

(A) and in the Saharan Bank (SB); (bt, bottom trawling; hj, hand jigging).


the stomachs with contents. The families Gobiidae and Atherinidae (7.1%) were also frequent and S. pilchardus was present in 1.4% of the stomachs. L. vulgaris and the isopods occurred in 4.3% and 1.4%, respectively, of the stomachs with contents.

Only 30.8% of the fish, by number, were identified, with gobies the most abundant (4.9%), followed by T. truchurus (4.2%) and the family Atherinidae (3.5%). L. vulgar-is corresponded to 2.8% of the cephalopod prey items. About 86.1% of the crustaceans remained unidentified and the isopods corresponded to 0.7% of the prey identified, by number.

3.2.3. Saharan Bank: bottom trawling The values calculated for the occurrence percent-

age show that Pleuronectidae flatfishes were the most common (6.1%) with Gobiids, Myctophids and Spar-ids also present. The occurrence percentage of L. vulgaris was highest among the cephalopods

Fig. 3. Variation with growth of the main groups of prey in the diet of L.Vulguris caught with bottom trawling in the South Coast of Portugal (A) and in the Saharan Bank (B).

Table 3 Comparison between the diet of males, females, immature and mature L. udgaris caught in the Algarve and in the Saharan Bank with bottom trawl

Fish Cepha- Crusta- Poly- Others lopods ceans chaetes

Algarve Immature males Immature females Mature males Mature females Males (total) Females (total)

Saharan Bank Immature males Immature females Mature males Mature females Males (total) Females (total)

36.5 29.4 31.2 2.0 0.9 72.1 15.5 12.0 0.2 0.2 96.3 2.8 0.3 0.6 95.8 3.9 0.3 83.6 8.9 6.4 0.9 0.2 86.6 9.4 3.5 0.1 0.1

35.1 24.9 30.5 9.5 54.1 18.7 20.3 6.9 76.8 22.1 1.0 0.1 93.1 5.5 1.7 72.1 21.1 6.1 0.7 70.3 16.0 9.1 4.6

The results are expressed in weight percentages for each group of

prey.

(9.2%) while Gammarids were the most abundant among the crustaceans, occurring in 5.5% of the stomachs with contents.

Only 28% (number percentage) of the fishes represented in the sample were identified and flatfishes accounted for 5.4% of the total. Only 40.4% of all cephalopods were identified, with L. vulgaris representing 5.1% of the total. The primary crustacean component of the diet were Gammarids (7.0%) of which 9.1% were unidentified.

3.3. Prey composition

The values calculated for the occurrence percentage and weight percentage of the different prey groups in the diet of L. vulgaris caught along the Algarve coast and in the Saharan Bank are shown in Fig. 2.

3.3.1. Algarve: bottom trawling Fish were the most important item as shown by

occurrence percentage, occurring in 72.7% of the stomachs, followed by the cephalopods (19.0%) and the crustaceans (7.2%). Fish corresponded to 84.6%

M. Coelho et al./ Fisheries Research 29 (19971245-255 251

of the total prey weight, followed by the cephalopods

(7.7%), the crustaceans (7.4%) and others, including

the polychaetes (0.2%).

3.3.2. Algarve: hand jigging

The occurrence percentage of the main prey items indicated that fish occurred in 54.9% of the stomachs

with contents. The cephalopods were also frequent (30.5%) and the crustaceans were present in 12.2%

of the stomachs. The weight percentage of the diet

indicates that fish were the dominant group (88.9%),

followed by the crustaceans (5.7%) and the

cephalopods (5.2%).

3.3.3. Saharan Bank: bottom trawling

The currence percentage of fish was 5 l.O%, fol-

lowed by the cephalopods (25.6%) and the crustaceans (19.8%). In terms of weight percentage, fish

was the main group (70.9%), followed by the

cephalopods (18.9%) and the crustaceans (9.1%).

3.4. Effect of size and maturation

3.4.1. Algarve: bottom trawling

The variation of the diet with the increasing mantle length shows that, as squid grew, the fish portion

of the diet increased. For individuals larger than 25.0

cm DML, fish were the dominant component of the diet, while the cephalopods and the crustaceans rep-

resented a smaller part, when compared with smaller

squid (Fig. 3).

The DML calculated for the mean size at matura-

tion was 20.6 cm (r2 = 0.69) for the males, and 21.8

cm (r* = 0.68) for the females. The composition of the diet of immature and mature individuals, by sex,

classified according to the length at maturity is shown

in Table 3. For the immature males the three most common groups of prey were equally represented:

fish (36.5%), crustaceans (3 1.2%) and cephalopods (29.4%). The polychaetes and unidentified remains

were scarcely represented. For the immature females,

Table 4

Comparison a between the diet of males, females, immature and mature L. vulgaris from the Algarve and the Saharan Bank

Sex Maturity stage Region

Males Females Males Females Algarve Saharan Bank

Immature Mature Immature Mature

Algawe

Fish 56 69 30 26 52 17 178 154

Cephalopods 16 22 II 5 15 7 44 96

Crustaceans 6 II 3 3 9 2 38 82

Polychaetes 4 2 3 1 2 0 9 0

Others 2 I 2 0 I 0 9 0

Number 84 105 49 35 79 26 271 346

Chi-square 2.47 3.29 I .83 46.76

d.f. 4 4 4 4

P 0.25 <P < 0.50 0.5 < P < 0.75 0.75 < P < 0.90 > *** P<O.OOl

Saharan Bank

Fish 45 45 19 24 35 10

Cephalopods 33 33 21 12 31 2

Crustaceans I3 30 10 4 29 1

Others 4 6 3 2 6 0

Number 95 114 5 42 101 13

Chi-square 5.44 4.60 8.85

d.f 3 3 3

P O.lO<P<O.25 O.lO<P<O.25 * P < 0.005

a The chi-square test is used for occurrence percentages of the main groups of prey.

252 M. Coelho et al. / Fisheries Research 29 (1997) 24.5-255

the fish proportion was considerably larger (72.1%) than for immature males, with the cephalopods (15.5%) and the crustaceans (12.0%) also well represented. For mature squid, of both sexes, fish were the most abundant group, representing more than 95.0% of the weight of the whole prey. The cephalopods were second with 2.8% and 3.9%, for males and females, respectively. Crustaceans were rare in the diet of the mature squid, accounting for 0.3% of the total weight in both sexes. No unidentified remains were found in the stomachs of the squids caught at Algarve with bottom trawling, and the polychaetes were present only in the stomachs of mature males, representing 0.6% of the prey weight.

The chi-square test showed no significant difference in the diet of immature and mature squid, both for males (0.50 < 0.75) and females (0.75 < 0.90). No significant differences (0.25 < 0.50) were found when comparing male and female squid (Table 4).

3.4.2. Saharan Bank: bottom trawling Similar to the Algarve samples, the importance of

fish increased with growth (Fig. 3). When squid reached 25.0 cm DML, both crustaceans and cephalopods decrease their importance in the diet. The highest value of the fullness index (7.9%) was calculated for a male with 24.6 cm DML.

The mean size at maturation was 16.5 cm (r2 = 0.96) for males, and 22.2 cm (r2 = 0.52) for females. For the immature males, the three most common groups of prey were equally represented: fish (3 1.5%), crustaceans (30.5%), cephalopods (24.9%) and unidentified remains (9.5%). For the immature females, fish was the most important diet constituent, accounting for 54.1% of the weight of the food remains in the stomachs. The crustaceans, the cephalopods and the unidentified items accounted for 20.3%, 18.7% and 6.9% of the total prey weight, respectively. Fish was the most important group of prey for mature squid of both sexes, weighing 76.8% of the prey remains for males, and 93.1% for females. The cephalopods represented 22.1% of the total prey weight for males and 5.5% for females. The crustaceans were occasional prey for mature squid of both sexes: 1.0% for males and 1.7% for females. The unidentified remains were found only in the stomach of the males, weighing 0.1% of the total prey weight (Table 3).

The chi-square test showed significant difference in the diet of immature and mature females (*P < 0.05). No differences were calculated when comparing the diet of the male and female squid (Table 4).

4. Discussion

4.1. Stomach jidlness

While relatively low in both study areas, the percentage of stomachs with contents was greater in the Saharan Bank samples than in the Algarve. This difference could be explained by the higher produc- tivity in the former, which is described as an up- welling zone (Bravo de Laguna, 1985), compared with the Algarve, where the availability of prey is probably lower. Low percentage of stomach with contents were also obtained for L. uulgaris reynaudii by Lipinski (1987) and Augustyn (1990, 1991) in South Africa, and for L. uulgaris by Guerra and Rocha (1994), in Spain. According to Lipinski (1990>, the low percentage of stomachs with contents, as well as the high incidence of stomachs with low fullness indices, could indicate a very high digestion rate.

4.2. Prey spectrum

Prey identification of L. vulgaris was very diffi- cult because they were highly macerated and often the hard parts that are normally used in the identification process were not present. However, it is clear that, in general, stomach contents were not very diverse. The low percentage of stomachs with more than one prey item could indicate that this species probably follows large schools of similar prey, possi- bly as a way of maximizing their capture effort. This feeding behaviour would allow squid to spend less energy in chasing the prey, as well as to increase the probability of a successful hunt. Low percentages of different prey items in the stomachs were also found by Lipinski (1987), for L. uulgaris reynaudii, in South Africa, for L. vulgaris by Guerra and Rocha (1994) and Rocha et al. (1994) in northwestern Spain, and Pierce et al. (1994) in the northeast Atlantic.

The type of prey identified, both along the Al-

M. Coelho et ok/ Fisheries Reseurch 29 (1997) 245-255 253

garve and in the Saharan Bank, indicates that L. vulgaris could feed either in the pelagic zone, or forage along the bottom. This behaviour was sup- ported by the presence of semipelagic species (T. trachurus), pelagic species (S. pilchardus and Athe- rina sp), and benthic species, respectively.

4.3. Prey composition

Stomach contents of L. vulgaris caught with bottom trawling along the Algarve coast and in the Saharan Bank were significantly different, both qualitatively and quantitatively (chi-square test). Along the Algarve coast, T. trachurus and S. pilchardus are very important prey items, both being very abundant in this area. In the Saharan Bank, flatfish are also an important component of the diet. As for the Portuguese coast, S. pilchardus is also an abundant species and an important contributor to the Saharan Bank fishery.

In spite of the qualitative and quantitative variation in the diet found between the two areas considered in this study, the comparison must also take into consideration that the bottom trawls in the Saharan Bank were conducted at depths of less than 100 m (E. Balguerias, personal communication, 199.51, whereas samples from theAlgarve coast were collected at 150 m. This fact, associated with the geo- graphic variation, could explain the differences in the diet found between these two regions.

Cannibalism in L. vulgaris has been reported by Burukovski et al. (1979) in the Saharan Bank, Guerra and Rocha (1994) and Rocha et al. (1994) in Spain, and for L. vulgaris reynaudii by Lipinski (19871, Augustyn (1990) and Sauer and Lipinski (1991) in South Africa. In the Saharan Bank, the occurrence of both cannibalism and other cephalopod items in the diet appears to be higher than along the Algarve coast, probably because cephalopods are more abundant in the former (Cart and Perez-Gandaras, 1973). Cannibalism could be strongly related to the availability of juveniles all year round (Coelho et al., 19941, rather than the lack of alternative prey that would force L. vulgar-is to feed on their own species. However, most of the beaks of L. vulgaris identified were from a single sample (September 1993), which was probably collected in a place where squid had been feeding on a large school of juveniles of their

own species, resulting in the overestimate of cannibalism.

Comparing the diet of squid caught in deep water by bottom trawls with inshore hand jigging indicates that crustaceans are an important prey item near the coast. It was not possible to quantify statistically the differences between these two sets of data, due to the differences in sampling gear, squid sizes and the low number of jigging samples. A more representative sample from the hand jigging fishery should be studied in order to examine this difference. More- over, year of sampling differed between these data sources and there are unknown effects of month of sampling and size of squid. Nevertheless, the weight percentage of fish in the diet for the Algarve sample is similar, both inshore and offshore, and clearly dominant with respect to the other prey items.

4.4. Effect of size and maturation

The comparison of the diet between immature and mature squid showed that the percentage of fish consumed increased with growth. Similar results were observed in Loligo forbesi by Collins et al. (1994). According to Pierce et al. (19941, the dominance of fish in the diet, as squid grow, reflects the greater energetic profitability of feeding on fish, rather than on crustaceans. The fact that a statistically significant variation was only found for females and, even so, just for the Saharan Bank, indicates that diet require- ments change with growth. Energy savings are criti- cal to cephalopods and squid paralarvae are much better suited to optimal energy utilization in locomo- tion than adults (Lipinski, 1987). Nevertheless, they have a relatively low food spectrum. On the other hand, adults experience more problems than paralarvae in saving energy, but their food spectrum broad- ens proportionally and it changes as squid mature (Lipinski, 1987; Sauer and Lipinski, 1991). The change of prey preference referred to above is also described by Burukovski et al. (1979) and Augustyn (19911, which supports the higher energetic require- ments associated with the maturation of the gonads.

The mean size at maturity calculated in this study was smaller for males than for females, in both study areas. It would be interesting to investigate whether this result is related to the changes in diet during growth and if the selection of prey is a function of size or maturity.


No statistically significant differences were found between the diet of males and females. It was not possible to study the food intake, but data from Worms (1983) showed no decrease in food intake during maturation of females. Rocha et al. (1994) described higher food intake in mature females than in mature males or immature specimens and based this result to the increasing demand of energy and nutrients for egg production.

L. vulgaris feeds on several exploited fish species, such as T. trachurus, S. pilchardus and flatfish. Therefore, a better understanding of the diet of L. vulgaris is required to assess the impact of this species on the fish stocks. A study of the daily feeding rhythms, and the density dependent factors, such as food availability and predation must be taken into consideration in future studies.

Acknowledgements

This work was funded by the Commission of the European Communities within the frame of the EEC research programme in the fisheries sector (AIR, Contract No AIRl-CT 92-0573). We would also like to thank Drs. A. Bolten, M. Collins, A. Guerra and M. Lipinski who reviewed the manuscript and of- fered suggestions, encouragement, and criticism as well. The manuscript benefited from comments of two anonymous referees.

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a comparative study of the diet of loligo vulgaris (lamarck, 1799) (mollusca:cephalopoda) from the...

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