Fisheries Research 73 (2005) 29–36
Vertical distribution and feeding patterns of the juvenile European
hake, Merluccius merluccius in the NW Mediterranean
Anna Bozzano ∗ , Francesc Sardà, José Rı́os
Institut de Ciències del Mar (CSIC), Passeig Marı́tim de la Barceloneta 37-49, 08003 Barcelona, Spain
Received 4 February 2004; received in revised form 31 December 2004; accepted 11 January 2005
Abstract
Diel vertical migration of the European hake Merluccius merluccius on the Catalan coast (north-western Mediterranean) was
studied by pelagic trawls at a single location during two cycles of 24 h each. Diurnal bottom trawls were also employed to
determine which part of the population performed vertical migration. The size range of the population caught by both nets did
not match exactly, since very small individuals (<5 cm TL) were obtained only with the pelagic net, indicating that this part of
the population had not present on the bottom. In addition, the 95.5% of the pelagic captures were obtained during the night, while
the maximum catches of the bottom net were obtained at midday, indicating that juvenile hake may display nocturnal vertical
migration. Pronounced quantitative and qualitative variations in the diet of M. merluccius were observed between pelagic and
bottom trawl samples belonging to the same size range. Differences in the stomach fullness and in the state of digestion of
the food between benthic and pelagic individuals were also noticed. These results suggest that feeding represented one of the
possible constraint factors controlling the vertical migration of juvenile hake.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Merluccius merluccius; Pelagic and benthic trawls; Feeding patterns; Mediterranean
1. Introduction
Classical population dynamics assumes that the
stocks in a certain area have a homogeneous distribution (Sparre et al., 1989). Indeed, the resource is frequently aggregated by size or sex or by the fact that
only one part of the fish population moves towards
∗ Corresponding author. Tel.: +34 93 2309500;
fax: +34 93 2309555.
E-mail address: [email protected] (A. Bozzano).
0165-7836/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.fishres.2005.01.006
deeper or shallower waters, or towards the surface or
the bottom for predation avoidance or in response to
feeding or spawning (Neilson and Perry, 1990). Thus,
depth distribution and the onshore–offshore or vertical
migration are important for understanding the biology
and ecology of marine fishes and for optimising their
management.
In this respect, the species of the genus Merluccius
represent an interesting example because many of them
make migrations during one or more phases of their
lifecycle (Cohen et al., 1990). However, the vertical
30
A. Bozzano et al. / Fisheries Research 73 (2005) 29–36
movement of the European hake Merluccius merluccius (Linnaeus, 1758) is poorly documented, although
this species occupies a main role in the north-eastern
Atlantic fishery economy and especially in the Mediterranean. Its biology is quite well known (Bozzano et al.,
1997; Recasens et al., 1998; Orsi Relini et al., 2002) and
many studies indicate that M. merluccius is a dominant
predator, having an important role in the Mediterranean
trophic web. Notwithstanding, only indirect information is available concerning its migratory capability
along the shelf and the slope and especially on its vertical movement (Olaso, 1990; Abad and Franco, 1996;
Orsi Relini et al., 1997). Although it seems clear that the
European hake is able to make diel vertical migration,
the factors controlling this movement remain unclear.
The purpose of the present study was to investigate
the patterns of European hake diel vertical movement
using pelagic and bottom trawls successively in a single
location in order to determine if all or only a part of the
population moves off the bottom. In addition, the diet of
the individuals caught with both nets was compared to
determine if feeding represented a constraint factor that
might control the vertical movement of this species.
2. Materials and methods
Two consecutive 24 h pelagic trawl cycles were
made at a single location (41◦ 18 N, 2◦ 20 E) in September 1999 on the Catalan Sea shelf (north-western
Mediterranean), at a depth of 100–200 m. Samples
were taken every 2 h. Successively, diurnal bottom
trawls were made for 3 days in the same area at the same
depth. No night bottom trawls were carried out because during a previous 3 day–night bottom trawl cruise
(November 1998, unpublished report) in the same area
and at the same depth, 5.5% of the total hake were
caught nighttime and only 2.1% of them were juveniles.
The opening of the pelagic net was 9 m × 6 m and a
lifter of 12 mm stretch mesh covered the codend. The
trawling velocity was 6–7 knots. The characteristics
of the benthic net were 12 × 1.8 opening and 2.5–2.7
trawling velocity. The same 12 m stretch mesh covered the lifter. The footrope of the pelagic net worked
10–15 m above the bottom to assure that the sampled
layer was above the layer sampled by the trawl net without overlapping it. The trawl time varied between 30
and 60 min. The water volume filtered by the two nets
was quantified by remote control system (SCANMAR)
readings, and these data were used to compute the abundance (number and weight) of the catches × 10,000 m3 .
The Kruskal–Wallis test was employed to compare the
catches of the nets and the Bray–Curtis similarity index
(Bray and Curtis, 1957) was used to compare the percentage in number and weight of the captured species.
The size frequencies of hake were compared with the
Kolmogorov–Smirnov test.
To compare the diet of the pelagic and benthic hake,
the individuals in the size range 9–15 cm were selected
since this was the best-represented range in the samples
obtained by the two gears. A fullness order between 0%
(empty), and 100% (full) was assigned to each stomach, and the digestive state of the food was classified
between 1 (very little digested) and 5 (unidentifiable digested prey). Both characteristics were compared with
a t-test. The contribution of each food category (species
or higher taxonomic group) to the diet of the hake was
examined through the frequency of occurrence (F) and
the percentage number (N) and weight (W) of each prey.
To integrate these three parameters, the index of relative importance (IRI) was used, where IRI = F(N + W).
The percentage contribution of IRI for each prey item
was chosen as a descriptor of diet similarity. The
Bray–Curtis similarity index was used to compare the
trophic spectra of pelagic and benthic samples.
3. Results
3.1. Hake in the pelagic and benthic trawling
During the pelagic trawling 178 hake were captured,
while 316 were obtained from the bottom trawls. The
trawling details are shown in Table 1. The diel variability of hake catches with pelagic and benthic nets
is indicated in Fig. 1. The pelagic trawl caught 170
individuals at night, while minimum or void catches
were obtained during the day. The maximum catches
of the bottom net were obtained at midday while minimum catches around sunrise. Significant differences
were found between the catches of hake with the
two nets, both in number (P = 0.007) and in weight
(P = 0.0006). The adjusted catches indicated that the
mean abundance of hake obtained with the bottom nets
was 8.2 ± 8.3 individuals × 104 m−3 h−1 , correspond-
A. Bozzano et al. / Fisheries Research 73 (2005) 29–36
31
Table 1
Details of the pelagic and bottom trawls that took place at a depth of 100–200 m
Hauls code
Pelagic trawl
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Bottom trawl
7
4
3
6
5
2
1
20
19
15
16
17
13
Date
Latitude
(N)
Longitude
(W)
Depth
Initial
time
Final
time
Footrope distance
from the bottom (m)
September
15, 1999
September
15, 1999
September
15, 1999
September
15, 1999
September
15, 1999
September
15, 1999
September
15, 1999
September
16, 1999
September
16, 1999
September
16, 1999
September
16, 1999
September
16, 1999
September
16, 1999
September
16, 1999
September
16, 1999
September
17, 1999
September
17, 1999
September
17, 1999
41◦ 12 4
2◦ 6 10
110
10:43
12:00
11.8
3
183
0.1
8.5
41◦ 10 17
2◦ 2 10
100
12:37
14:01
11.8
0
0
0.0
0.0
41◦ 11 40
2◦ 5 58
130
14:56
15:58
11.9
1
10
0.04
0.4
41◦ 16 41
2◦ 12 49
135
17:05
18:00
12.2
0
0
0.0
0.0
41◦ 14 6
2◦ 9 55
140
20:01
21:00
11.9
6
88
0.2
5.6
41◦ 11 14
2◦ 5 52
152
22:18
23:19
11.9
1
68
0.04
2.5
41◦ 12 52
2◦ 8 60
187
00:35
01:39
11.9
3
27
0.1
1.1
41◦ 10 20
2◦ 4 26
167
02:55
03:45
12.3
6
51
0.3
2.7
41◦ 11 43
2◦ 6 55
September
17, 1999
September
17, 1999
September
18, 1999
September
18, 1999
September
18, 1999
September
18, 1999
September
18, 1999
September
18, 1999
September
18, 1999
September
18, 1999
September
22, 1999
September
22, 1999
September
22, 1999
◦
◦
41 10 18
41 12 33
◦ 41 8 55
◦
◦
◦
◦
◦
41 13 45
41 17 60
41 18 35
41 18 34
41 18 28
41 18 51
41◦ 18 15
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
41 19 14
41 20 13
41 22 13
41 23 18
41 25 17
41 24 16
41 19 03
41 18 29
41 17 23
41 15 04
41 14 53
41 16 03
Weight Adjusted
(g)
catches (N)
Adjusted
weight (g)
172
05:00
05:40
11.5
11
62
0.9
4.8
◦ 155
11:26
12:26
11.1
0
0
0.0
0.0
◦ 185
13:24
14:24
10.5
3
17
0.1
0.8
◦ 137
15:25
16:20
7.6
1
160
0.04
0.5
◦ 2 4 16
2 8 22
2 3 14
◦
Hake
catches (N)
2 9 55
163
17:32
18:39
11.8
0
0
0.0
0.0
◦
132
19:48
21:00
12.2
38
347
1.2
10.8
◦
170
22:23
23:28
15.2
28
375
1.2
15.7
◦
195
00:29
01:30
11.2
28
492
1.4
24.6
◦
170
02:27
03:27
15.6
27
904
1.2
39.4
◦
146
04:41
05:18
12.8
22
133
1.3
70.8
2◦ 21 54
2 16 11
2 20 48
2 25 16
2 20 16
2 21 17
195
16:54
17:24
0.0
21
207
8.0
79.3
◦
118
18:17
18:49
0.0
15
247
5.5
90.7
◦
77
8:36
9:06
0.0
1
385
0.4
153.7
◦
69
9:59
10:30
0.0
3
176
1.1
65.1
◦
66
11:07
11:39
0.0
0
0
0.0
0.0
◦
60
12:16
12:47
0.0
27
1237
8.6
396.1
◦
62
13:21
13:52
0.0
3
14
0.5
6.6
◦
123
15:00
15:31
0.0
17
518
6.6
201.2
◦
150
16:30
17:07
0.0
69
1696
25.0
606.3
◦
150
17:47
18:09
0.0
91
1001
19.9
604.5
◦
95
13:37
14:07
0.0
24
1342
11.1
620.2
◦
117
14:53
15:23
0.0
42
2969
18.5
1309.1
◦
140
16:03
16:23
0.0
3
78
1.9
48.5
2 21 19
2 18 40
2 20 52
2 20 17
2 20 54
2 18 22
2 16 56
2 18 17
2 14 24
2 09 03
2 10 35
2 12 23
Sunrise and sunset were at 07:31 and 20:01, respectively (September 15, 1999; Spanish Forecast Institute). Adjusted catches and weight referred to 1 h trawl for 10,000 m3
sea-water filtered. Bad weather conditions obliged a stop between the second and the third bottom trawling sampling day.
32
A. Bozzano et al. / Fisheries Research 73 (2005) 29–36
ing to a weight of 321.6 ± 379.5 g × 104 m−3 h−1 ,
while hake caught by the pelagic net were 0.5 ± 0.5
individuals × 104 m−3 h−1 , corresponding to a weight
of 6.9 ± 10.4 g × 104 m−3 h−1 . The minimum size of
hake caught with the pelagic net was 2.5 cm TL, while
5 cm was the smaller hake caught with the benthic net.
The maximum size was 40 cm for both nets (Fig. 2).
The individuals between 9 and 15 cm TL constituted
78 and 60% of the pelagic and benthic catches, respectively. No significant difference was found between the
two size frequencies (P = 0.1).
3.2. Feeding
Fig. 1. Diel variability of hake catches with pelagic and benthic
nets. Each point represents the mean catches and standard deviation
(where it existed) obtained at the same hour on different consecutive
days. Dark dots: nighttime catches; white dots: daytime catches.
For the stomach contents analysis, 121 and 194 individuals were chosen from pelagic and benthic catches,
respectively. The high number of everted stomachs reduced the stomach contents available for analysis to 26
and 79. The diet of the juvenile hake was composed
in both cases of crustaceans and teleosts, but their proportion in the trophic spectrum varied depending on
whether the hake were captured close to or away from
the bottom. In fact, crustaceans characterised 82% of
the benthic hake diet, whereas fish comprised almost
18%. On the contrary, in the diet of the pelagic hake, the
Fig. 2. Size frequency distributions of hake captured with pelagic and benthic nets.
A. Bozzano et al. / Fisheries Research 73 (2005) 29–36
importance of crustaceans and fishes was very similar
(approximately 50%).
The taxonomic characterisation of the prey showed
a diversified diet for benthic hake (Table 2), while
a more specialised diet was observed in the pelagic
33
hake, whose feeding preferences were mainly focused
on two species of benthopelagic and bathypelagic fish
(G. argenteus (Guichenot, 1850) and the pearlsides
M. muelleri (Gmelin, 1789)) and one crustacean decapod (Chlorotocus crassicornis (Costa, 1871)). The
Table 2
Taxonomic list of the prey characterising the diet of juvenile hake captured with the pelagic and benthic nets
Pelagic trawl
%N
%W
Bottom trawl
%F
IRI
%IRI
%N
%W
%F
IRI
%IRI
Cephalopoda
Sepia orbignyana
Alloteuthis media
Sepiolidae
2.17
–
–
9.10
–
–
3.85
–
–
43.34
–
–
1.90
–
–
–
0.97
0.97
–
5.94
4.82
–
1.92
1.92
Copepoda
Copepoda unid.
2.17
0.01
3.85
8.39
0.37
–
–
–
–
–
Isopoda
Gnathia sp.
Isopoda unid.
–
2.17
–
0.14
–
3.85
–
8.90
–
0.39
0.97
–
0.01
–
1.92
–
1.88
–
0.10
–
Amphipoda
Amphipoda unid.
–
–
–
–
–
2.91
0.18
3.85
11.89
0.65
Euphausiacea
Euphasia krohnii
Nyctiphanes couchii
Euphausiacea unid.
6.52
4.35
6.52
1.03
0.08
0.03
7.69
3.85
7.69
58.05
17.04
50.42
2.55
0.75
2.21
–
–
6.80
–
–
0.13
–
–
7.69
–
–
53.26
–
–
2.92
Mysidacea
Lophogater typicus
Anchialina agilis
Gastrosaccus sp.
Mysidacea unid.
4.35
8.70
4.35
–
1.22
4.13
0.21
–
7.69
3.85
3.85
–
42.86
49.35
17.52
–
1.88
2.16
0.77
–
1.94
3.88
–
6.80
0.11
0.18
–
0.26
3.85
5.77
–
9.62
7.88
23.47
–
67.88
0.42
1.29
–
3.72
Crustacea decapoda
Chlorotocus crassicornis
Solenocera membranacea
Processa nouveli
Pasiphaea sivado
Philocheras sculptus
Brachyura
Scyllarus arctus
Scyllarus posteli
Crustacea Decapoda unid.
Crustacea unid.
10.87
4.35
–
–
–
–
–
–
–
–
13.56
5.51
–
–
–
–
–
–
–
–
19.2
7.69
–
–
–
–
–
–
–
–
469.8
75.81
–
–
–
–
–
–
–
–
20.61
3.32
–
–
–
–
–
–
–
–
1.94
0.97
2.91
0.97
1.94
10.68
0.97
0.97
3.88
22.33
3.41
6.33
1.86
6.77
0.36
0.23
1.52
1.14
0.55
1.79
3.85
1.92
5.77
1.92
3.85
9.62
1.92
1.92
7.69
25.00
20.59
14.03
27.55
14.89
8.86
104.9
4.79
4.05
34.14
603.0
1.13
0.77
1.51
0.82
0.49
18.93
0.26
0.22
1.87
33.064
Osteychthyes
Gadiculus argenteus
Maurolicus muelleri
Myctophidae
Deltentosteus quadrimaculatus
Lesueurigobius friesii
Callionymus maculatus
Antonogadus megalokynodon
Paralepididae
Osteichthyes unid
15.22
10.87
4.35
2.17
–
–
2.17
–
2.17
22.1
19.03
9.19
10.81
–
–
3.44
–
0.17
26.9
15.4
7.69
3.85
–
–
3.85
–
3.85
1006.
459.9
104.1
49.93
–
–
21.59
–
9.00
44.13
20.17
4.57
2.19
–
–
0.95
–
0.39
4.85
0.97
1.94
1.94
0.97
0.97
–
0.97
5.05
12.67
1.15
9.47
15.67
0.28
3.27
–
5.90
11.48
9.62
1.92
3.85
3.85
1.92
1.92
–
1.92
10.64
168.5
4.08
43.91
67.72
2.41
8.15
–
13.21
175.9
9.42
0.22
2.41
3.71
0.13
0.45
–
0.72
13.6
–
13.29
11.13
–
0.73
0.61
34
A. Bozzano et al. / Fisheries Research 73 (2005) 29–36
Bray–Curtis similarity index of 23.7 (with a range between 0 (no overlap) and 100 (total overlap)) indicated
a low level of similarity between the trophic spectrum of hake captured close to and away from the bottom. Moreover, the stomach fullness and the digestive
state of the prey also showed significant differences
(P < 0.001) between the two groups. In fact, 79% of the
stomachs analysed from the pelagic samples were totally full with less digested prey, while only 21% of the
stomachs from the benthic samples were full and 47%
of them had digested prey that occupied 25% or less of
the stomach volume. Finally, the by catch species captured with the pelagic and the benthic nets were also
analysed to detect some of the potential resources available to the juvenile hake. Several differences in catch
composition indicated that the community changed according to the depth (Table 3). In the pelagic catches,
more than 55% in number of the fish was constituted
Table 3
Taxonomic list of species captured with the pelagic and benthic nets
Pelagic trawl
Bottom trawl
%N
%W
%N
%W
Cephalopoda
0.7
1.5
10.9
9.3
Crustacea Decapoda
14.6
3.4
1.9
0.9
Chlorotocus crassicornis
Plesionika heterocarpus
Solenocera membranacea
Pasiphaea sivado
Parapenaeus longirostris
Sergestes arcticus
Brachyura
Pagurus prideaux
Othera
Osteichthyes
Micromesistius poutassou
Myctophidae
Maurolicus muelleri
Gadiculus argenteus
Engraulis encrasicolus
Argentina sphyraena
Merluccius merluccius
Trisopterus minutus
Mullus surmuletus
Trachurus mediterraneus
Trachurus trachurus
Lepidopus caudatus
Pagellus bogaraveo
Sardina pilchardus
Glossanodon leioglossus
Deltentosteus quadrimaculatus
Scyliorhinus canicula
Other benthic speciesa
%N (Crust. Dec.)
%W (Crust. Dec.)
%N (Crust. Dec.)
%W (Crust. Dec.)
47.82
13.35
11.33
9.91
8.84
6.72
–
–
1.86
50.88
17.31
7.31
7.41
13.95
1.49
–
–
1.4
–
19.93
–
–
2.99
–
40.28
34.72
2.07
–
6.46
–
–
2.14
–
47.55
43.43
0.41
83.5
95.0
87.1
89.7
%N (Osteichthyes)
%W (Osteichthyes)
%N (Osteichthyes)
%W (Osteichthyes)
32.48
25.46
17.69
11.88
6.06
2.60
1.67
0.23
–
0.11
0.08
0.07
–
63.32
2.74
0.74
2.73
6.02
1.04
3.22
0.29
–
0.10
0.24
0.68
–
37.73
–
0.02
17.36
1.57
0.02
3.69
5.57
0.76
8.85
3.51
3.54
1.15
1.51
3.61
2.28
0.77
7.5
31.99
–
0.01
2.44
0.54
0.01
8.13
4.86
4.55
3.51
1.58
19.90
3.13
0.79
0.54
0.50
6.10
10.35
a
a
–
–
–
1.13
–
–
–
4.57
In bold letter the percentage in number (%N) and weight (%W) of each class upon the total catch are presented. The importance of each species
was calculated in relation to the percentage of its corresponding class and not on the total catch. The species are arranged in relation to their
importance (%N) obtained in the pelagic trawl.
a Species whose N and W were both lower than 0.1%.
A. Bozzano et al. / Fisheries Research 73 (2005) 29–36
by myctophids, G. argenteus and M. muelleri and almost 50% of the crustaceans by C. crassicornis. On the
other hand, the benthic catches were characterised by
a wide range of benthic species. The Bray–Curtis similarity index of 43 (calculated on n) indicated a quite
low level of similarity between the captures of both
nets.
4. Discussion
The comparison between pelagic and benthic
catches shows that the juveniles of the European hake
M. merluccius may display nocturnal migration, moving away from the bottom, at least 10–20 m and staying closer to the sea bed during the day. In fact, 78%
of the individuals caught in the water column ranged
between 9 and 15 cm TL, thus confirming the hypothesis postulated by Orsi Relini et al. (1997) for young
hake in the Ligurian Sea (N Mediterranean). This behaviour has also been noticed in juveniles of several
species of gadiforms (Pillar and Barange, 1993, 1995;
Lough et al., 1989; Sogard and Olla, 1996). In general,
as Neilson and Perry (1990) pointed out, such movements are often more evident during the first year of
the fish life.
The differences in the total catches and in the size
of hake captured with pelagic and benthic nets indicate that one part of the population (2.5–4.5 cm TL)
has not yet moved towards the bottom at this length,
while another part (5–40 cm TL) is able to move into
midwater during the night. In any case, it seems that
only a part of the population moves into the water column, since daytime catches were generally greater than
nightly ones. This result could be an artefact of the difference in catchability between the pelagic and benthic
nets, since these gears are technically different. However, Lough et al. (1989) noticed that only one fraction
of juvenile cod and haddock rose off the bottom, probably in relation to the movement of their main prey,
while Huse et al. (1998) observed that juvenile cape
hake remained higher in the water column to avoid being eaten by larger individuals. However, cannibalism
of the European hake is very low (Bozzano et al., 1997;
Velasco and Olaso, 1998) and does not seem to justify
this behaviour, although juvenile hake could be escaping from other potential predators or be following their
prey.
35
In relation to the part of the population that moved
from the bottom into the water column, Orsi Relini
et al. (1997) postulated that very young fish might be
more influenced by the light condition than the older
ones. As Blaxter (1975) and Neilson and Perry (1990)
pointed out, diel vertical migration can be related to the
circadian rhythmicity that is synchronised by a natural
cyclical phenomenon such as light. In any case, the response of fish to light could be modified by a secondary
environmental factor, such as prey availability.
Lombarte and Popper (1994) and Bozzano and
Catalán (2002) analysed the ontogenetic changes in
hake hearing and vision, respectively and observed that
the two sensorial systems undergo striking changes during development, probably related to the shift in feeding habits. The increase in sensory capabilities should
allow juvenile hake to explore the water column, thus
widening the range of their available prey. In effect, at
this size range, the trophic spectrum of juvenile hake
begins to include fishes and crustacean decapods. The
analysis of the diet of the individuals caught with the
pelagic net demonstrates that vertical migration of the
juvenile European hake also takes place in response
to this new feeding requirement. The differences in
the trophic spectra and in the status of food digestion
between pelagic and benthic hake indicate that these
fish could feed twice a day at least and they probably feed close to sunrise and sunset, as many fishes
do (Hobson, 1972; Lough et al., 1989), probably in response to the endogenous rhythms in behaviour that
tend to ‘anticipate’ the start of the daylight or night
period (Woodhead, 1966). Pillar and Barange (1995)
found that juvenile of M. capensis feed at night in the
water column, need 43 h to evacuate 90% of their stomach content. These results evidence that conclusions
regarding feeding periodicity of fish are often contradictory.
Finally, although the European hake has always
been considered an opportunistic predator (Bozzano
et al., 1997; Velasco and Olaso, 1998), the results of
the present study indicate that juvenile hake move into
midwater at night in response to similar movement of
their prey. In fact, a close relationship seems to exist
between the main prey that hake captured close to or
away from the bottom and the most abundant by catch
species captured by the benthic and pelagic net, respectively, confirming that temporal variation in hake diet
reflect differences in prey availability. A similar feed-
36
A. Bozzano et al. / Fisheries Research 73 (2005) 29–36
ing behaviour was also observed in other hakes (Pillar
and Barange, 1993; Huse et al., 1998).
In conclusion, feeding is one of the main factors that
forces the juvenile hake of the north-western Mediterranean to be displaced in the water column during the
nighttime. Nevertheless, multiple physical and biological factors such as light intensity and the development
of the sensory organs may also affect the behaviour of
the fish.
Acknowledgements
This study was conducted as part of the project CEFAIR CT97-3522 financed by the E.U. The authors
would like to thank all the members of the cruises
Lluçet IV and Pelagic II for their help during the sampling. Dr. Rodgers kindly reviewed the final English
version.
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