European hake Merluccius merluccius (L., 1758) feeding in the Cantabrian Sea: seasonal, bathymetric and length variations

Fisheries Research 38 (1998) 33±44

European hake Merluccius merluccius (L., 1758) feeding in the Cantabrian Sea: seasonal, bathymetric and length variations F. Velasco*, I. Olaso Instituto EspanÄol de OceanografõÂa, Laboratorio Oceanogra®co de Santander, Apdo. 240, 39080, Santander, Spain Received 4 November 1997; accepted 7 April 1998

Abstract In 1994, 5828 stomachs of European hake, Merluccius merluccius (L., 1758) were analysed on commercial trawl and gill-net vessels in the Cantabrian Sea (northern Spain). Data were analysed quantitatively using fullness indices, and differences in feeding intensity and diet composition were compared statistically by quarter, by depth strata, and throughout the predator length-range. The results show that feeding is more intense during the second quarter among specimens longer than 30 cm, which may be related to recovery from the spawning season. There was very little seasonal variation in diet composition, this being centred on blue whiting from 30 cm, as well as on horse mackerel Trachurus trachurus (L., 1758) and silvery pout Gadiculus argenteus (Guichenot, 1850) at shorter lengths. By depth, there was no noteworthy variation in feeding intensity, but an increase in the predominance of blue whiting was observed as depth increased, with a greater intake of horse mackerel and clupeids at less than 200 m. Regarding predation on blue whiting Micromesistius poutassou (Risso, 1826) in the Cantabrian Sea, there is apparently no selection of prey lengths, since length distributions in the stomachs closely coincided with those of the total catches of this species in the study area. Compared with another study conducted in the northern Bay of Biscay, the present ®ndings show marked differences in the composition of this species's diet. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Hake; Fish food; Cantabrian Sea; Merluccius merluccius (L.)

1. Introduction The ecological position of European hake as a major predator in the demersal ecosystem of the Cantabrian Sea (Olaso, 1993), together with its importance to the ®shery, make the study of its diet particularly interesting. In addition, the high consumption of certain ®sh species by hake, particularly blue whiting in the Cantabrian Sea, requires detailed study *Corresponding author. Tel.: +34 942 291060; fax: +34 942 275072; e-mail: [email protected]

due to its possible implications in the inter-relationships between the two species. Studies currently being conducted on consumption rates, of great importance to the multi-species models that are beginning to be used in ®sh management, require a broad knowledge of seasonal diet and its variations. Throughout 1994, a sampling of 5828 hake stomachs was carried out, following several other studies of hake feeding since 1980 (Pereda et al., 1981; AlcaÂzar et al., 1981; GonzaÂlez et al., 1985; Olaso, 1990); however, none of these studies had covered an entire year in the Cantabrian Sea, like the study on the

0165-7836/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0165-7836(98)00111-8

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F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

French continental shelf of the Bay of Biscay by Guichet (1995). 2. Material and methods 2.1. Sampling In 1994, 5828 hake stomachs were analysed. The samplings, mainly aimed at discard estimation, were carried out monthly on commercial trawl and gill-net vessels, as part of the project `Discards of the Spanish ¯eet in ICES divisions', ®nanced by the European Union. The quarterly distribution by length ranges and by depth strata of stomach contents analysed is summarised in Table 1. The unequal coverage of length ranges and depth strata is due to this species' abundance, and to individual vessels' varying decisions regarding where to ®sh and target species. Therefore, in certain combinations of length ranges, depth strata and quarters, suf®cient data for statistical analysis were not collected. These combinations are included in the graphs, but excluded from tables and statistical analyses. The samplings correspond to ICES area VIIIc, and hauls were made at depths of between 43 and 823 m, day and night without interruption, although the unequal duration of hauls prevents the analysis of possible differences in feeding timetables. 2.2. Stomach content analysis Stomach contents were analysed quantitatively on board, using a trophometer (Olaso, 1990) to measure volume. Wherever possible, the whole of the catch of predators measuring longer than 20 cm was analysed,

as well as a minimum of 10 predators from each length-group of juveniles (<20 cm), according to the catch. However, such individuals were very scarce, because sampling was carried out on commercial vessels: these lengths are not very accessible to commercial trawl gear, and are inaccessible to gill nets. For each predator, details were noted regarding total length to the lowest centimetre, sex, maturity and possible regurgitation according to the gall bladder criteria of Robb (1992), given the hake's tendency to regurgitate (Hickling, 1927 and Bowman, 1986 with other species of hake). Stomachs were thus classi®ed as full, regurgitated or empty. In the case of full stomachs, total volume was measured, and prey were separated in an attempt to classify them to species level in the case of ®sh and decapod crustaceans, and to higher taxa in remaining cases. From each prey the mean percentage of total volume of stomach content was measured, and the state of digestion was noted (1, undigested or fresh; 2, in process of digestion; 3, highly digested). Total length of the prey or the otolith length (chelae in decapod crustaceans) was measured when the state of digestion did not permit the measurement of total length. When identi®cation was impossible, prey were assigned to the highest taxa level. Prey with the appearance of having been ingested on the cod-end were not included in the study. 2.3. Indices used To compare diet composition quarterly and by depth strata, the partial mean volume of prey i (PMVi) was used. To compare different length ranges, the partial fullness index of prey i (PFIi), proposed by Bowering and Lilly (1992), was used. It was assumed that regurgitated stomachs have the same mean full-

Table 1 Quarterly distribution by length ranges and depth strata of stomach contents analysed in the present study Length ranges (cm)

1st quarter 2nd quarter 3rd quarter 4th quarter Total a

Depth strata (m)

Total

10±20

20±30

30±40

40±50

50±90

<100

101±200

201±300

301±500

>500

76 27a 1a 8a 112

180 72 10a 162 424

467 311 191 769 1738

490 692 375 895 2452

210 612 155 125 1102

68 2a 0a 0a 70

243 507 12a 300 1062

487 363 83 277 1210

557 579 404 977 2517

68 263 233 405 969

Data insufficient to be included in statistical analysis.

1423 1714 732 1959 5828

F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

ness as non-regurgitated and non-empty ones. Although some bias may be introduced with this approach, because stomachs with small items or with food well broken up have less probability of being regurgitated (Bowen, 1983), this is probably not the case, given the particular feeding habits of hake, which feed mostly on one or two big items. A correction factor was introduced in both indices to avoid possible deviation of the different percentages of regurgitated stomachs, taking each haul as a sample and obtaining both indices as the arithmetic mean of all hauls: PH PFIi ˆ

Haulˆ1

35

variance analysis were not ful®lled and the index is too complex to apply other transformations), and we could establish the possible statistical signi®cance of differences in feeding intensity. TFIH ˆ TFI ˆ

k X …F ‡ R† Vj 4  10 F…F ‡ R ‡ E† jˆ1 L3j H X 1  TFIH H Haulˆ1

(3)

where Vj is the total volume of stomach j, k being the number of stomachs in the haul H; thus, the partial

h i P …F ‡ R†=F…F ‡ R ‡ E†  kjˆ1 …Vij =L3j †  104

H h i Pk Haulˆ1 …F ‡ R†=F…F ‡ R ‡ E†  jˆ1 Vij

PH PMVi ˆ

where F is the number of full stomachs; R the number of regurgitated stomachs; E the number of empty stomachs; Vij the volume of prey i in the stomach j, k being the number of stomachs in the haul, Lj the length of the predator j and H the number of hauls in which the predator appears. The corresponding total fullness indices for the predator, which permit the comparison of feeding intensity, are usually obtained from the formulae (1) in the following way: TFI ˆ

(1)

H

x X iˆ1

PFIi

TMV ˆ

x X

PMVi

(2)

iˆ1

where x is the total number of prey consumed by the predator throughout the study. The dif®culty with these formulae (2) is estimating whether the differences found between samples or sub-groups are signi®cant, because the fullness means by prey (PFIi) do not permit the use of statistical tests, since they generally deal with different prey in different proportions. To solve this problem, when the number of hauls sampled for each sub-group was high enough, we calculated TFI as a mean of the partial TFIs of each haul (TFIH), as shown in the formulae (3); thus a series of TFIH were obtained for each total TFI, so that statistical tests of mean comparison could be carried out (non-parametric, since the conditions for multi-

totals of prey i are eliminated, given that it is the fullness of the predator, its feeding intensity, and not its diet composition which is of interest in this case. The same procedure can be followed with TMV or to compare PFI of a speci®c prey among different subgroups. 3. Results 3.1. Emptiness and regurgitation percentages The emptiness and regurgitation percentages by length ranges, depth strata and quarters are shown in Table 2. Emptiness increases with length, with signi®cant differences throughout the distribution (24 ˆ16.13, p<0.01), but not between the two lower ranges (10±30 cm, 21 ˆ0.69, pˆ0.40) nor among the three higher ranges (30 to 90 cm, 22 ˆ3.43, pˆ0.18). Emptiness also seems to increase with depth, although, as observed in the means of length by stratum included in Table 2, the increase is because hake length distribution varies with depth (Pereiro and FernaÂndez, 1983). There are no signi®cant differences among the three deepest strata (22 ˆ1.05, pˆ0.59), in which mean length is practically invariable; however, signi®cant differences appear upon including strata of lesser depth (24 ˆ38.01, p<0.01). Emptiness varies

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F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

Table 2 Emptiness and regurgitated percentages per length ranks, depth strata and quarters Emptiness and regurgitated percentages per length ranges 10±20 % Empty % Regurgitated % Full Number

33.93 5.36 60.71 112

20±30 38.21 9.2 52.59 424

30±40

40±50

50±90

43.33 11.68 44.99 1738

45.88 15.95 38.17 2452

46.28 20.78 32.94 1102

% Empty % Regurgitated % Full Number Mean length (cm)

Emptiness and regurgitated percentages per depth strata <100 100±200 200±300 25.71 37.48 46.86 12.86 15.07 14.88 61.43 47.46 38.26 70 1062 1210 26.54 37.43 43.16

300±500 45.57 13.98 40.44 2517 44.02

% Empty % Regurgitated % Full Number

Emptiness and regurgitated percentages per quarter 1st quarter 2nd quarter 3rd quarter 44.62 39.96 52.32 19.75 16.63 17.76 35.63 43.41 29.92 1423 1714 732

4th quarter 45.18 8.88 45.94 1959

quarterly (23 ˆ31.14, p<0,01), being greater in the third quarter and lower in the second, while in the ®rst and the fourth it is very similar (21 ˆ0.0008, pˆ0.978). Regurgitation increases with length ± there were statistically signi®cant differences for the total of the distribution (24 ˆ65.32, p<0.01) ± but not with depth (24 ˆ1.05, pˆ0.19). This may be because abrupt changes in pressure that provoke regurgitation only occur at the greatest depths. By quarters, regurgitation is clearly lower in the fourth quarter (23 ˆ93.31, p<0.01); among the other three, the differences

>500 47.27 17.23 35.50 969 45.50

are lower, while remaining signi®cant (22 ˆ4.89, pˆ0.09). 3.2. Quarterly variation in feeding: intensity and diet composition Fig. 1 shows variation in TMV and TFI by quarters and length ranges, whereas Table 3 summarises TFI data by quarters and lengths when calculated according to the formula (3) and the number of hauls used in each sub-group (excluding data from the 10±20 cm length range).

Fig. 1. Quarterly variation in the fullness indices per length ranges.

F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

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Table 3 TFI data per length ranges and quarters Length ranges (cm) Quarter

Data

20±30

30±40

40±50

50±90

Total

1st

TFI (average of TFIH) StdDev of TFIH No. of hauls TFI (average of TFIH) StdDev of TFIH No. of hauls TFI (average of TFIH) StdDev of TFIH No. of hauls TFI (average of TFIH) StdDev of TFIH No. of hauls

2.3916 2.7877 27 1.5131 2.2139 20 2.8348a 3.2320 6 2.5044 3.7081 40 2.2798 3.1271 93

1.9288 1.6362 46 3.6043 3.1776 57 1.6654 2.0297 38 1.7576 1.8615 84 2.2448 2.3798 225

1.2982 1.3967 42 2.0738 1.5517 84 1.6396 2.1710 44 1.3122 1.4479 80 1.6234 1.6494 250

0.6701 1.0230 40 1.2150 1.4275 69 0.8478 1.6353 33 0.8103 0.9778 46 0.9356 1.3023 188

1.5137 1.8063 155 2.1467 2.2762 230 1.4910 2.0807 121 1.5602 2.1040 250 1.7180 2.1133 756

2nd 3rd 4th Total TFI (total average of TFIH) Total StdDev of TFIH Total No. of hauls a

No. of hauls insufficient to make any statistical tests.

The main quarterly difference in the quantity of food ingested is an increase in the second quarter for specimens longer than 30 cm, both in TMVand in TFI. The rest of the quarters are similar, with the exception of a slight fall in TFI during the second quarter for specimens of 20±30 cm. On applying the Kruskal± Wallis test to the TFI totals by quarters, signi®cant differences are obtained for the four quarters together (23 ˆ19.63, p<0.01), but not when the second quarter is excluded (22 ˆ3.420, pˆ0.18). When the Kruskal± Wallis test is applied to quarters within each length range, the differences remain signi®cant for ranges over 30 cm; however, and these differences cease to be signi®cant if the second quarter is excluded. In the 20± 30 cm length range there are no signi®cant differences (when the third quarter is excluded, 22 ˆ2.565, pˆ0.28). With respect to the length ranges, together, the four ranges are clearly different (23 ˆ59.72, p<0.01), and in this case, applying the Mann±Whitney U two-bytwo, signi®cant differences are not found between the 20±30 cm range and the 30±40 cm range (Uˆ9649.0, pˆ0.2748) and 40±50 cm range (Uˆ11160.0, pˆ 0.568). All other combinations are signi®cantly different. The comparison between the two graphs in Fig. 1 clearly shows that, as expected, hake increases its food quantity with length, as can be seen in the

TMV graph. Nevertheless, when the in¯uence of the length factor is eliminated by using TFI, it can be observed that feeding intensity in relation to length reaches its maximum in the 20±30 cm range, and later diminishes as the length increases. This is logical, because younger specimens have a higher growth rate, requiring a greater feeding intensity in relation to their length. The exception to this is the second quarter, during which, in addition to the length factor, other factors (discussed below) have to be taken into consideration. Fig. 2 shows how hake diet composition presents little quarterly variation, particularly from 30 cm. In hake smaller than 30 cm a higher consumption of horse mackerel was found in the ®rst and fourth quarters. The importance of this species diminishes greatly in the diet of larger hake, practically disappearing in specimens longer than 40 cm, at which point blue whiting becomes the fundamental basis of diet throughout the year, with very little presence of other species. It can also be seen that the importance of crustaceans, mainly decapod prawns (6.86% of the total volume of the 10±20 cm range) and Euphausiacea (4.84% of the total volume in the same range), is limited to predators measuring less than 20 cm. Other invertebrates such as molluscs and polychaetes are uncommon in the diet of this species.

38

F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

Fig. 2. PFI of the most important prey groups per length ranges and quarters.

3.3. Variation by depth strata Fig. 3 shows the importance of the main prey groups at the different depth strata. Once again, the importance of blue whiting as the main prey from

100 m stands out. Both horse mackerel and clupeids play a dominant role in the shallowest depth strata (<100 m), because at this depth, blue whiting's abundance is very low in the Cantabrian Sea (SaÂnchez, 1993). The difference between the two graphs

F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

39

Fig. 3. PMV and PFI of the most important prey groups per depth strata.

indicates that, although a positive relationship seems to exist between feeding intensity and depth, this is due to the previously explained difference in length distribution by depths. The TFI (Table 4) shows how feeding intensity is slightly higher at less than 200 m, although this difference is not statistically signi®cant between the strata of 100±200 m and the rest (Kruskal±Wallis: 23 ˆ1.625, pˆ0.654); the strata of less than 100 m is excluded because only four hauls are available (70 predators in total). Given that the length and depth are the main factors affecting hake's diet composition, Table 5 shows the PMVi of main hake preys per depth strata and length range. The three deeper strata, from 200 m to more than 500 m, have been combined into a single one because of the small diet variability found between them; the two longer length ranges have also been combined for the same reasons. 3.4. Cannibalism Cannibalism is a known phenomenon in hake (Hickling, 1927; GonzaÂlez et al., 1985; Guichet, 1995). However, in the present study it was of little importance, reaching a maximum at the 100±200 m strata, where it was 3.23% of total volume and its PFI was 0.86% of TFI, and occurring neither at less than

100 m nor at more than 500 m. By length ranges, it appeared from 20 cm and increased with size, reaching its greatest intensity at 30±40 cm with 1.25% in volume, and at 40±50 cm with respect to PFI, 2.3% of TFI. By quarters, the fourth quarter stands out with 1.15% of volume, and with respect to PFI, 0.58% of TFI. 3.5. Relationship between predator and prey length In the present study, suf®cient data were obtained to analyse only the relationships between hake predator length and horse mackerel and blue whiting preys. The results of the regression analysis and their corresponding curves can be seen in Table 6 and Fig. 4. In both cases, we con®rmed the existence of a signi®cant relationship, already indicated by GonzaÂlez et al. (1985) and Guichet (1995), among others. In the case of horse mackerel, the best ®t was obtained with a linear model. Most of the observed cases were found in predators measuring less than 40 cm (as indicated in comments on PFI in Fig. 2), which consume horse mackerel of less than 15 cm. For blue whiting, the best ®t was obtained with an asymptotic model, although this ®t was considerably worse than that of horse mackerel. This is because, as GonzaÂlez et al. (1985) pointed out, hake predators

Table 4 TFI data per length ranks and quarters Depth Strata (m)

TFI (average of TFIH) StdDev of TFIH No. of hauls

Total

<100

100±200

200±300

300±500

>500

2.0776 1.4631 4

2.1514 1.8616 72

1.7465 1.2818 58

1.8223 1.4559 108

1.8231 1.4586 43

1.8937 1.5351 285

40

F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

Table 5 Partial mean volume (PMVi) of main hake preys per depth strata and length range Depth strata: <100 m

Length range (cm)

Prey taxa

10±19

Crustaceans Decapods (Natantia) Euphausiids Fishes Isospondyli Trachurus trachurus Other fishes TMV Predator No. Predators mean length

0.1000 0.0333 0.0667 1.6667 Ð 1.3334 0.3333 1.7667 9 17.89

Ð Ð Ð 2.4042 0.2917 1.5225 0.5901 2.4043 39 24.54

0.1333 0.1333 Ð 12.2017 5.8096 5.0365 1.3556 12.3350 19 31.58

Ð Ð Ð Ð Ð Ð Ð 0.0000 3 46.33

Depth strata: 100±200 m Prey taxa Crustaceans Natantia Other decapods Euphausiids Mysids Other crustaceans Molluscs (Cephalopods) Fishes Gadiculus argenteus Micromesistius poutassou Merluccius merluccius Gobiids Sardina pilchardus Trachurus trachurus Other fishes TMV Predator No. Predators mean length

10±19 0.2437 0.1330 0.0000 0.1071 Ð 0.0036 0.0650 0.4673 Ð 0.1250 Ð 0.0347 Ð Ð 0.3076 0.7762 82 17.05

20±29 0.0687 0.0498 0.0003 0.0130 0.0056 0.0000 Ð 4.1759 0.0451 2.5685 0.0500 Ð Ð 0.9899 0.5224 4.2448 202 25.01

30±39 0.0381 0.0334 0.0002 0.0038 0.0007 0.0000 0.0008 11.8143 0.0049 6.9486 1.1724 0.0005 0.0984 1.2776 2.3119 11.8538 360 34.69

40±90 0.0698 0.0301 0.0397 Ð Ð 0.0000 0.0340 12.8653 0.0123 4.5690 1.4619 0.0463 1.2443 0.7442 4.7873 12.9693 418 49.8

Depth strata:>200 m Prey taxa Crustaceans Molluscs (Cefalopod‡gastropod) Others (plastic, mud, unidentified) Fishes Gadiculus argenteus Micromesistius poutassou Merluccius merluccius Sardina pilchardus Trachurus trachurus Other fishes Polychaetes TMV Predator No. Predators mean length

10±19 Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð 0.0000 0 Ð

20±29 0.0172 Ð Ð 4.6213 0.0227 3.2606 Ð Ð 0.5147 0.8460 Ð 4.6386 128 27.33

30±39 0.0470 Ð 0.0093 10.1737 0.1211 9.1704 0.0692 0.0582 0.1759 0.7000 0.0016 10.2312 1142 35.77

40±90 0.1094 0.0362 0.0055 15.7764 0.1152 13.7638 0.0316 0.3004 0.4230 1.2576 Ð 15.9271 3426 47.51

20±29

30±39

40±90

F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

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Table 6 Regression analysis between predator and prey length for horse mackerel and blue whiting n Horse mackerel r2ˆ0.56939 Blue whiting r2ˆ0.26050

b

a

115

0.331

815

ÿ426.816

1.7898 30.886

Model

Analysis of variance

DF

Sum of square

Mean square

F-ratio

p-level

a‡bx

Model Residual Model Residual

1 113 1 813

1425.73 1078.22 3288.26 9334.65

1425.73 9.54 3288.26 11.48

149.42

<0.01

286.39

<0.01

a‡b/x

reach a size at which, although they continue to grow, they cannot not ®nd larger blue whiting, since such individuals do not exist in the population. As shown in the graph of blue whiting in Fig. 4, this limit is reached approximately in predators measuring 40 cm, which consume blue whiting of up to 33 cm, with a notable drop in consumption from 29 cm. The limit of the asymptotic model, when hake's length tends toward the in®nite, is 30.88 cm. Due to the low abundance in the area of blue whiting larger than this size (SaÂnchez, 1994 on 1991 data and pers. commun. on 1994 data), hake consume more prey per predator. In the present study, of 10 stomachs in which more than two blue whiting were found, only one predator was shorter than 40 cm, and of 26 stomachs with more than 2 prey, only three corresponded to hake shorter than 40 cm. 3.6. Length range of fish prey Fig. 5 shows the length distribution by species of the main ®sh prey in hake stomachs. These graphs show that, while preys shorter than 13 cm consumed by hake are mainly horse mackerel and silvery pout,

blue whiting predominates among preys longer than 13 cm, increasing from 17% of ®sh prey to 83%, a percentage that even reaches 100% over much of the distribution. From 4 cm up to 8 cm, silvery pout predominate, and from this length up to 13 cm, horse mackerel predominate. Sardine Sardina pilchardus (Walbaum, 1792) and hake itself also appear as prey at somewhat greater lengths, above all between 17±23 cm, but their importance in diet compared with blue whiting is very small. In blue whiting distribution two modes are clearly observed, which would correspond to two age cohorts, suggesting that hake prey on the whole length-distribution of blue whiting, at least on individuals accessible to trawl gear (blue whiting catches in gill nets are rare). This can be seen clearly in Fig. 6, which shows the coincidence between the length distribution of blue whiting found in stomachs and those found in the total catch (catch and discard) throughout the discard project that comprises this study. Such a coincidence of distributions, also indicated for the area during autumn by Olaso et al. (1994), suggests

Fig. 4. Regression curves and observed values for hake length versus horse mackerel and blue whiting length.

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F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

Fig. 5. Length distributions in the stomach contents of the most important fish preys of hake. (Blue whiting is shown apart due to its high abundance).

that hake do not select the blue whiting length to prey on, but are limited to catching the blue whiting they can encounter. 4. Discussion 4.1. Regurgitation The fact that signi®cant variations were not found in the percentage of regurgitation over depth contradicts the observations of Hickling (1927) and Bowman (1986) in other hake species, whose data supported the hypothesis that regurgitation increased with depth. These differences may be due, particularly in the case of Hickling, to the use of Robb's criteria (1992), which permits a more precise determination of this factor, con®rming that only the last metres are decisive in regurgitation. In the case of Bowman, the differences

may be explained by the study having been carried out with species of less depth, and these differences may be more related to this factor. In any case, the gallbladder analysis permits a much clearer determination of fullness indices, both on correcting the real percentage of emptiness and permitting the use of correction factors that consider regurgitated stomachs within the full stomachs, such as those used in TFI and in TMV, problems which appear in Hickling. The increase in the regurgitated percentage with length may be due, as Bowen (1983) points out, to the fact that large preys are regurgitated more often than small ones, and because large and extendable oesophaguses make regurgitation fairly easy. Both characteristics are applicable to hake, and probably they are more important as length increases, since large hake usually have larger preys than smaller ones and often it is a large blue whiting.

Fig. 6. Length distributions of blue whiting found in hake stomachs and blue whiting caught and discarded during the 1994 discard sampling project (N. PeÂrez, pers. comm.).

F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

4.2. Seasonal feeding As noted, signi®cantly higher feeding intensity was observed both in the percentage of full stomachs and in TFI during the second quarter. This may be related to recovery from the spawning period which, in the Cantabrian Sea, stretches from December to April, with a maximum of post-spawning females in the second quarter (AlcaÂzar et al., 1983; PeÂrez and Pereiro, 1985). This hypothesis, already noted by Hickling (1927) in hake, is con®rmed by the present study, because this increase was observed from the 30±40 cm range, at which males reach ®rst maturity, according to most studies (among others MartõÂn, 1991; PeÂrez and Pereiro, 1981). It would be expected that the increase in feeding intensity carries over to females, which reach ®rst maturity at a greater length (between 49±70 cm, according to the studies cited), although the increase in the second quarter occurs in exactly the same way in males and females. This feeding pattern differs, nevertheless, from those found by another author (Bowman, 1984) in other species from the same genus, Merluccius bilinearis (Mitchill, 1814). This species feeds more in preparation for spawning and less during spawning, and then recovers feeding intensity at the end of this period, although without rising again to pre-spawning levels. 4.3. Comparison with the northern Bay of Biscay Comparing the results from the present study with those obtained in the northern Bay of Biscay during a study of similar characteristics (Guichet, 1995), a series of outstanding differences can be observed in hake diet in the two areas. In the northern area, the changes in diet with predator length are much more varied than in the Cantabrian Sea, as shown in Table 7.

43

Thus, blue whiting, a major prey in the Cantabrian Sea among predators longer than 30 cm, predominates in the north of the Bay of Biscay only among hake of 35±50 cm, with anchovy Engraulis encrasicholus (L., 1758) dominating among smaller hake and horse mackerel dominating among larger individuals. Cannibalism also appears to have greater importance in the northern area, since in winter it reaches 13.99% of weight, and even higher percentages (20.8% in the whole study) have been reported off the western coast of Ireland and Britain (Hickling, 1927). However, in the Cantabrian Sea in the fourth quarter, the highest percentage reached was only 1.15% of volume. These two differences in hake diet in the two neighbouring areas may be explained by the abrupt bathymetry of the Cantabrian continental shelf, on which hake would be distributed at greater depths than in the north. Thus, the coincidence of habitat with species favouring shallower water (horse mackerel, sardine, anchovy and small hake) would be lower, and predation on these species would be lower as well, favouring predation on blue whiting. In the case of Hickling's work, the major decrease in the abundance of this species since 1927 may also be an in¯uence. The other outstanding difference found is the length distribution of the ®sh prey, which seem to be much smaller in the northern area, where most ®sh prey measure between 9±15 cm, while blue whiting are found mainly between 15±24 cm, with a maximum between 19±21 cm. These data contrast with those obtained in the Cantabrian Sea, where the maximum is between 20±25 cm. This explains the difference between the regressions obtained in both areas, being linear in the northern area with r2ˆ0.538, while in the Cantabrian Sea it is potential with r2ˆ0.251, given that in the Cantabrian area the consumption of blue whiting continues throughout the whole length distribution

Table 7 Main preys in the diet of hake along the length distribution in the Cantabrian Sea and the northern Bay of Biscay (Guichet, 1995)

10±20 cm 20±30 cm 30±40 cm >40 cm

Cantabrian Sea

Northern Bay of Biscay (from Guichet, 1995)

Crustaceans (Natantia and Euphausiidae) Silvery pout Horse mackerel Horse mackerel and blue whiting Blue whiting and horse mackerel Blue whiting

<10 cm 10±11 cm 12±13 cm 15±34 cm 35±44 cm >45 cm

Euphausiidae Euphausiidae and small fish Small Gadoids Anchovy Blue whiting and Clupeidae Horse mackerel

44

F. Velasco, I. Olaso / Fisheries Research 38 (1998) 33±44

of hake, while in the northern area it is limited to a 10 cm range. In the Cantabrian Sea, hake dependence on blue whiting seems much greater than in the northern Bay of Biscay, where, however, a positive correlation has been found between hake abundance and the blue whiting population (Guichet and Meriel-Bussy, 1970). Both this, and the fact that in other species, such as whiting, (Hislop et al., 1983), signi®cant relationships have been found between predator and prey, mean that Cantabrian hake may be affected more easily by variations in the blue whiting population. Therefore, this factor must be taken into account when managing both species in the ®shery. References AlcaÂzar, J., Carrasco, J., de la Hoz, M., Ortega, J., VizcaõÂno, A., 1981. ReÂgimen alimenticio de la merluza (Merluccius merluccius L.) (Pisces Gadidae) del mar CantaÂbrico. Int. J. IctiologõÂa IbeÂrica (LeoÂn, Spain). AlcaÂzar, J.L., Carrasco, J.F., Liera, E.M., MeneÅndez, M., Ortea, J.A., VizcaõÂno, A., 1983. BiologõÂa dinaÂmica y pesca de la merluza en Asturias. Recursos Pesqueros de Asturias 3, 135 pp. Bowen, S.H., 1983. Quantitative description of the diet. In: Nielsen, L.A., Johnson, D.L. (Eds.), Fisheries Techniques. American Fisheries Soc., Maryland, USA. pp. 325±336. Bowering, W.R., Lilly, G.R., 1992. Greenland halibut (Reinhardtius hippoglossoides) off southern Labrador and northeastern Newfoundland (northwest Atlantic) feed primarily on capelin (Mallotus villosus). Neth. J Sea Res. 29(1±3), 211±222. Bowman, R.E., 1984. Food of silver hake Merluccius bilinearis. Fish. Bull. 82(1), 21±35. Bowman, R.E., 1986. Effect of regurgitation on stomach content data of marine fishes. Environ. Biol. Fish 16(1±3), 171±181. GonzaÂlez, R., Olaso, I., Pereda, P., 1985. ContribucioÂn al conocimiento de la alimentacioÂn de la merluza (Merluccius merluccius L.) en la plataforma continental de Galicia y el CantaÂbrico. Bol. Inst. Esp. Oceanogr. 29(1±3), 211±222. Guichet, R., 1995. The diet of European Hake (Merluccius merluccius) in the northern part of the Bay of Biscay. ICES J. Mar. Sci. 52, 21±31.

Guichet, R., Meriel-Bussy, M., 1970. Association du merlu Merluccius merluccius (L.) et du merlan bleu Micromesistius poutassou (Risso) dans le Golfe de Gascogne. Rev. Trav. Inst. PeÃches Mar. 34(1), 69±72. Hickling, C.F., 1927. The Natural History of the Hake ± Parts I and II. Fishery Investigations, Series II., Vol. X., No. 2, 79 pp. Hislop, J.R.G., Robb, A.P., Brown, M.A., Armstrong, D., 1983. A preliminary report on the analysis of the whiting stomachs collected during the 1981 North Sea stomach-sampling project. ICES C.M. 1983/G:59. MartõÂn, I., 1991. A preliminary analysis of some biological aspects of hake (Merluccius merluccius L. 1758) in the Bay of Biscay. ICES C.M. 1991/G:54. Olaso, I., 1990. DistribucioÂn y abundancia del megabentos invertebrado en fondos de la plataforma cantaÂbrica. Publ. Espec. Inst. Esp. Oceanogr. No. 5, 128 pp. Olaso, I., 1993. PosicioÂn troÂfica de la merluza en la Plataforma CantaÂbrica. In: GonzaÂlez, GarceÂs, Pereiro (Eds.) Estado actual de los conocimientos de las poblaciones de merluza que habitan la plataforma continental atlaÂntica y mediterraÂnea de la UnioÂn Europea con especial atencioÂn a la penõÂnsula ibeÂrica. Private publication, 1994, IEO y AIR, Vigo, pp. 193± 206. Olaso, I., SaÂnchez, F., PinÄeiro, C.G., 1994. Influence of anchovy and blue whiting in the feeding of Northern Spain hake. ICES C.M. 1994/P:9. Pereda, P., GonzaÂlez, R., Olaso, I., 1981. Studies on the feeding of the southern stock of hake (Merluccius merluccius L.): First results. ICES C.M. 1981/G:26. PeÂrez, N., Pereiro, F.J., 1981. First data on sexual maturation and sex-ratio of hake (Merluccius merluccius L.) from ICES Divisions VIIIc and IXa. ICES C.M. 1981/G:37. PeÂrez, N., Pereiro, F.J., 1985. Aspectos de la reproduccioÂn de la merluza (Merluccius merluccius L.) de la plataforma gallega y cantaÂbrica. Bol. Inst. Esp. Oceanogr. 2(3), 39±47. Pereiro, F.J., FernaÂndez, A., 1983. RelacioÂn entre las edades y la profundidad, e Âõndices y aÂreas de reclutamiento de la merluza, en GalõÂcia y aguas adyacentes. Bol. Inst. Esp. Oceanogr. 1, 131±143. Robb, A.P., 1992. Changes in the gall bladder of whiting (Merlangius merlangus) in relation to recent feeding history. ICES J. Mar. Sci. 49, 431±436. SaÂnchez, F., 1993. Las comunidades de peces de la plataforma del CantaÂbrico. Publ. Espec. Inst. Esp. Oceanogr. No. 13, 137 pp. SaÂnchez, F., 1994. CampanÄa de evaluacioÂn de recursos pesqueros demersales 0991. Inf. TeÂc. Inst. Esp. Oceanogr. 155, 51.