ICES Journal of Marine Science, 55: 331–341. 1998
Importance of discarded blue whiting (Micromesistius poutassou)
in the diet of lesser spotted dogfish (Scyliorhinus canicula) in the
Cantabrian Sea
I. Olaso, F. Velasco, and N. Pérez
Olaso, I., Velasco, F., and Pérez, N. 1998. Importance of discarded blue whiting
(Micromesistius poutassou) in the diet of lesser spotted dogfish (Scyliorhinus canicula)
in the Cantabrian Sea. – ICES Journal of Marine Science, 55: 331–341.
The blue whiting is the most important demersal species caught by the Spanish trawler
fleet in the southern Bay of Biscay. Lesser spotted dogfish are not a commercial
by-catch, and are discarded immediately after sorting. During 1994 the contents of
1094 dogfish stomachs were analysed. The results show that lesser spotted dogfish with
a total length of >17 cm eat blue whiting throughout the year. The length range of blue
whiting found in lesser spotted dogfish stomachs was similar to the length range
discarded by the fishery. Blue whiting account for 20% of the stomach volume in lesser
spotted dogfish 30–50 cm long. From the state of digestion of blue whiting prey, and
the comparative analysis of the size distributions of blue whiting in lesser spotted
dogfish stomachs and in the fishery, it appears that many of these blue whiting are
discards, consumed when already dead or damaged. The impact of this supplementary
food on lesser spotted dogfish abundance is discussed.
1998 International Council for the Exploration of the Sea
Key words: blue whiting, diet, discard, dogfish, scavenger.
Received 8 January 1997; accepted 8 May 1997.
Ignacio Olaso and F. Velasco: Instituto Español de Oceanografı́a, Apdo 240, 39080
Santander, Spain. N. Pérez: Instituto Español de Oceanografı́a, Apartado 1552, 36 Vigo,
Spain. Correspondence to I. Olaso: tel: +34 42 275033, fax: +34 42 275 072.
Introduction
The Cantabrian Sea, to the north of Spain, corresponds
to ICES Division VIIIc. In this Division there is a
varied commercial fleet involving different gillnet, hook
and trawl fisheries. The bottom-trawl fishery, which
lands around 15 000 t of fresh fish annually, comprises
vessels which use pair and single bottom trawls; it is a
mixed fishery in which landings include hake, horse
mackerel, and monkfish, but the main catch is blue
whiting.
Several abundant species are discarded after capture,
either because of their size or because of their low
commercial value. Some of these individuals survive,
while others die (Kaiser and Spencer, 1995), and they
may provide an important food resource for scavengers
(Kaiser and Spencer, 1994).
The records of Division VIIIc trawlers (single bottomtrawling vessels) from 1994 (Pérez et al., 1996) show that
blue whiting have both the highest catch and discard
rates, representing 33% of total weight and 22% of total
discards. Lesser spotted dogfish are also commonly
1054–3139/98/030331+11 $30.00/0/jm970263
caught by the trawlers; the species has little commercial
interest and, since it is a large fish, it is usually discarded
at the beginning of sorting the catch. It is the second
most important discarded species.
Bottom-trawl surveys series show that blue whiting is
the most abundant fish in the area (Sánchez, 1993) and
plays an important role in the diets of the Cantabrian
fish community, according to Olaso and Velasco (personal communication). These authors studied the diet of
27 fish species which made up the trophic demersal
structure in the area of study, and found that 19 of these
species prey on blue whiting and eight of them are
predators which actively feed on blue whiting, while the
other 11 are opportunist feeders, probably preying on
dying, dead or decomposing individuals.
Because the lesser spotted dogfish is the most commonly caught benthic shark in the area, we considered it
to be of interest to study its particular ecological role.
The aims of this study were to determine the proportion
of the dogfish diet which comes from discards and to
interpret the observed changes in the abundance of this
species.
1998 International Council for the Exploration of the Sea
332
I. Olaso et al.
Table 1. Summary of Spanish trawler fleet estimated discards and catches in weight in 1994 (Pérez et al., 1996).
Discard weight (tonnes)
Quarters
Species
Blue whiting
Dogfish
Other species
Total
1
2
3
4
Discard
Catch
Discards
% of
total catch
1680
708
3423
5811
181
204
1560
1945
493
348
1735
2576
1056
199
2384
3639
3410
1459
9102
13 791
13 078
1572
13 342
27 982
26.1
92.8
68.2
49.9
Material and methods
Discard sampling
The discard sampling programme (Pérez et al., 1996) was
based on monthly random stratified sampling of single
bottom sampling in Division VIIIc, from February to
December 1994. Fourteen commercial trawlers made
301 hauls during 33 trips over the 109-day sampling
period. The average duration of each commercial trip
was 3 days, during which an average of 9.1 hauls was
made, i.e. a daily average of 2.8 hauls. Catches and
discard sampling of all taxa, including target species and
low-interest or non-commercial species, were quantified.
Data were standardised to 100 fishing hours (f.h.), with
a mean of 5.3 f.h. per haul.
One observer went on each trip, collecting, by species,
data on the quantities discarded and retained. Fish and
decapod crustaceans were identifed to the species level
wherever possible, and non-commercial invertebrates
usually to family or order. The total lengths of discarded
blue whiting and lesser spotted dogfish, and blue whiting
retained in the catch, were also measured to the
nearest cm.
Stomach sampling and analysis
Lesser spotted dogfish stomachs were collected from
each commercial haul, and during a bottom-trawl
research survey conducted in October 1994. On the
commercial vessels, hauls were made by day and by
night, while on the bottom-trawl survey, all hauls were
carried out by day. For each haul, stomachs were
selected from 10 dogfish covering the length range. If
there was any evidence of predation by dogfish during
the haul, the predator was rejected and, whenever possible, another specimen of the same length was selected.
Empty stomachs of dogfish with remains of food in the
mouth were considered to have been regurgitated, but
this was unusual (13 regurgitated stomachs in 1094
sampled predators).
Stomach contents were analysed on board both the
commercial and research vessels. The volume (ml) of
total prey groups in the stomach was measured using
Total (tonnes)
a trophometer, a calibrated instrument consisting of
several different-sized half-cylinders built into a tray, so
that they form horizontal cylindrical moulds (Olaso,
1990). The approximate percentage of the total volume
made up by the individual identified prey categories was
then determined, and the relationship between estimated
volumes and actual weights of the stomach contents was
derived from linear regression (a=0.932735, b=0.99324,
r2 =0.99); Bowman (1982) was the first to use this
methodology. Prey were identifed to the lowest possible
taxa for decapod crustaceans, cephalopods and fish;
other invertebrates were classified to higher taxonomic
levels. For each prey type, the following data were
quantified: the percentage of stomach volume, the state
of digestion (1=intact prey; 2=partially-digested prey;
3=well-digested), and the number of individuals. Fish,
decapod crustaceans and cephalopods were measured
to the nearest mm. In the case of fish, when the state of
the prey made determination of total length impossible,
the otolith size was measured to the nearest mm, and the
length was estimated from regressions of fish length on
otolith length (Pereda and Villamor, 1991).
Data analysis
Fishing intensity and the analysis of catch data, discards
and landings by species were reported by Pérez et al.
(1996). The most representative data are shown in
Table 1. In order to establish the minimum number of
stomachs for each length group (minimum size of the
sample) to obtain a reliable result, the accumulated
number of prey types was plotted against the number of
stomachs, in such a way as to consider an adequate
number when the curve reaches its asymptote (Modde
and Ross, 1983).
The relative importance of individual prey taxa was
assessed with indices of volume (Hyslop, 1980). Total
stomach fullness (%BW) and mean partial fullness index
for prey i (PFIi) were also calculated to permit comparison of quantities of prey in stomachs of dogfish
of various sizes during different quarters of the year
(Bowering and Lilly, 1992). The method used to calculate the mean partial fullness index of prey category i is:
Blue whiting in diet of lesser spotted dogfish
333
Table 2. Number of stomachs sampled by dogfish size class and
quarter in 1994.
Size class
where H is the number of hauls, F is the number of
stomachs containing food, R is the number of regurgitated stomachs, E is the number of empty stomachs, Vij is
the volume of prey i in fish j, and Lj is the length of fish j.
The relationship between prey size and predator size
was analysed by means of regressions analysis, and
statistical analysis (chi-square) was used to analyse
differences in empty stomach frequency.
Additional information from the fishery (Pérez et al.,
1996) was used to compute catch rates (numbers per
fishing hour per area) and length compositions, and
these data were used to compare the blue whiting length
distributions in the stomachs with those from landings
and discards.
Results
Sampling intensity
Most of the fish sampled throughout the year were of
similar size, except in the third quarter (Table 2). Plots
of cumulative taxa vs. the number of stomachs examined (Fig. 1) indicate that sample sizes sufficient for
description of prey type were obtained for all length
groups, with the exception of the stomachs in the third
quarter.
Dogfish diet
Lesser spotted dogfish consumed a wide variety of prey:
65 different taxa were found in the diet. Of these, 22 taxa
were fish (Fig. 2). Crustaceans, almost all of which were
decapods, were represented by 29 taxa. Small lesser
spotted dogfish generally presented a small number of
taxa, although, as shown in Figures 2 and 3, the taxa in
all prey groups (crustaceans, fishes and other invertebrates) increased up to 50 cm. In the diet of larger-sized
lesser spotted dogfish, crustaceans, other invertebrates
and fishes diminished in species richness.
The diet of lesser spotted dogfish, in terms of percentage volume, comprised mostly decapod crustaceans and
fish, the remainder being invertebrate groups, such as
polychaetes and cephalopods (Table 3). Fish, of which
blue whiting was the most important species, represented more than 50% of the percentage in volume. The
diet of smaller-sized lesser spotted dogfish (14–30 cm)
was 63% euphausiids and shrimps, and about 20% fish.
As size increased, the consumption of crustaceans
diminished slightly, while fish volume increased, reaching over 60% in dogfish of >50 cm in length.
Most of the potential prey groups found in the lesser
spotted dogfish diet are also discarded by the trawl
Quarter
Quarter
Quarter
Quarter
Total
1
2
3
4
14–30 cm
30–50 cm
50–70 cm
57
40
8*
73
178
104
127
8*
197
436
121
130
39*
180
470
*No minimum sampling size.
fleet, specifically, 21 of the 22 species of prey fish
identified. The relative importance of the individual
species varied seasonally. Figure 4 shows the most
representative prey groups, separating the fish species
which are important in discards and differentiating
between other fish and undetermined fish, since the state
of digestion did not usually permit their identification.
The importance of the euphausiids was evident during
the first months of the year for dogfish measuring less
than 30 cm, and as the year progressed, other, mainly
decapod, crustaceans tended to dominate. During the
summer, the percentage of brachyuran crabs was more
significant in medium and large-sized lesser spotted
dogfish, although they fed on decapod crustaceans
throughout the year.
Cephalopods appeared in the diet during the winter,
and less so in autumn; other invertebrate groups were
preyed on all year, but small dogfish did not prey on
them in summer. Blue whiting were consumed by all
length groups during autumn, but only lesser spotted
dogfish of >30 cm preyed on blue whiting throughout
the year. Lesser spotted dogfish of <30 cm had a high
percentage of undetermined fish in their diet; since they
have smaller mouths, much of this material may have
been blue whiting pieces bitten from whole carcasses or
left by other scavengers.
The incidence of empty stomachs and state of
food digestion
As shown in Table 4, 86% of lesser spotted dogfish
stomachs contained food. Taking into account the fact
that dogfish from commercial vessels are caught in hauls
of long duration, those coming from the fourth quarter
were compared with those from the October research
cruise. The emptiness percentage is clearly similar,
because there are no significant differences between
quarters and surveys (÷24 =7.7306, p=0.102), while
there are significant differences between sizes classes
(÷23 =11.7389, p<0.01).
The stomach contents were found in a highly digested
state (Fig. 5). Comparing the distribution of the states of
digestion between the autumn samples from commercial
334
I. Olaso et al.
Number of prey taxa
50
Length group 14–30 cm
40
First quarter
Second quarter
Third quarter
Fourth quarter
30
20
10
0
20
40
60
Number of stomachs
80
100
50
Number of prey taxa
Length group 30–50 cm
40
20
10
0
50
Number of prey taxa
First quarter
Second quarter
Third quarter
Fourth quarter
30
20
40
60
Number of stomachs
80
100
Length group 50–70 cm
40
30
First quarter
Second quarter
Third quarter
Fourth quarter
20
10
0
20
40
60
Number of stomachs
80
100
Figure 1. Number of prey taxa accumulated in stomachs with food, by quarter and length group.
vessels and the survey samples, no significant differences
were found (÷22 =0.56927, p=0.75).
The stomach fullness index (BW) was 2.92. On transforming the length of blue whiting prey to live weight, it
was found that it only made up 24% of stomach content
weight. This low value was due to the highly digested
state of the prey, and to the fact that, in many cases,
only pieces had been eaten.
The importance of blue whiting in the diet of
dogfish
From a length of 17 cm, lesser spotted dogfish preyed on
blue whiting of 7–32 cm, although the regression line
obtained for the relationship between the dogfish length
and the length of blue whiting prey found in the
stomachs is not representative, given the large variations
Blue whiting in diet of lesser spotted dogfish
No. of taxa
25
20
15
10
5
0
14–30 cm
30–50 cm
50–70 cm
Length of lesser spotted dogfish
Figure 2. Relation between the size of lesser spotted dogfish and
prey taxa. Crustacea; other invertebrata; pisces.
in the data (Fig. 6). However, there is a statistical
tendency for growth in prey size as predator size
increases (Table 5).
The length distributions of blue whiting in discards,
landings and as prey, for the whole of 1994, are shown in
Figure 7. The length ranges of discarded blue whiting
and blue whiting found as prey were similar (6–31 cm
for blue whiting found in stomachs, and 7–39 cm for the
discarded blue whiting), but we observed that the
highest mode of the length composition of the blue
whiting prey was 13 cm, while the modes of the discarded blue whiting were between 19 and 23 cm.
To determine the importance of blue whiting in the
diet of lesser spotted dogfish throughout the year and at
different sizes, we used the fullness index PFI. Figure 8
shows only four groups of taxa, since a large proportion
of the fish were unidentified, although many of these
were probably blue whiting, since the mean weight
of a blue whiting is 10.472.63 g, compared with
5.80.026 g for other fish species, while the mean of the
undetermined fish was 8.980.067 g. Applying the
Kruskal–Wallis test to the three groups gave significant
differences (÷22 =22.7469, p>0.01), while applying the
335
Mann–Whitney test to compare the mean volume of
blue whiting prey and the mean volume of undetermined
fish gave no significant differences (U=3850, W=8431.0,
z= 1.6376, 2-tailed p=0.1015). For this reason, we
separated prey fish into three groups and allocated
another to invertebrates. The total fullness index
increased throughout the year, varying from 0.47 during
the winter to 1.55 during autumn in the smallest specimens, with fish having very little importance during the
first six months of the year, and only invertebrates
forming the main food. From summer on, there was an
increase in the fullness index due to the availability of
blue whiting and undetermined fish. Lesser spotted
dogfish of >30 cm consumed undetermined fish and blue
whiting year round (although during the third quarter
the only fish analysed were undetermined) and their
total fullness index was similar throughout the year,
between 0.41 and 0.43, except during the summer, when
it was 0.21.
Figure 8 shows the seasonal length distributions of
blue whiting discarded by the trawl fleet in 1994. The
length range landed does not vary over the year, remaining above 16 cm. These data indicate that throughout
the year the mean length of discarded blue whiting
drops, for specimens greater than 20 cm in winter, which
have a mode of around 22 cm, to specimens with a wider
length range in the second half of the year, with a mode
of around 15 cm. These changes in the length of discarded blue whiting are related to the results shown in
Figure 9. In winter, the importance of blue whiting prey
was not evident in smaller-sized lesser spotted dogfish,
while in the spring they began to prey on undetermined
fish. In summer and autumn the increase in small-sized
discarded blue whiting coincided with the consumption
of undetermined fish and blue whiting in small dogfish,
while lesser spotted dogfish of greater size continued to
prey on blue whiting.
50
Number of prey taxa
Length group 50–70 cm
40
30
12–30 cm
30–50 cm
50–70 cm
20
10
0
20
40
60
80
100
Number of stomachs
Figure 3. Comparison of the number of taxa accumulated in 100 stomachs of lesser spotted dogfish, for each length group.
336
I. Olaso et al.
Table 3. Diet (% volume) of Scyliorhinus canicula by size class (cm) and fauna discarded and caught (k/100 f.h.) by the trawler fleet
in 1994 (*<0.5 k/100 f.h.). Only the prey with more than 1% at least in one length range are shown.
Diet of Scyliorhinus canicula
Taxa
Crustacea
Decapoda
Munida spp.
Pagurus prideaux
Goneplax rhomboides
Liocarcinus depurator
Polybius henslowi
Alpheus glaber
Chlorotochus crassicomis
Pasiphaea multidentata
Pasiphaea sivado
Processa spp.
Solenocera membranacea
Other decapoda
Euphausiacea
Other crustacea
Mollusca
Cephalopoda
Octopus defilippi
Other cephalopoda
Other mollusca
Polychaeta
Sipunculida
Other invertebrata
Fish
Gadiculus argenteus
Micromesistius poutassou
Merluccius merluccius
Engraulis encrasicholus
Scyliorhinus canicula
Trachurus trachurus
Cepola rubescens
Capros aper
Other fish
Average length (cm)
Number of regurgitated stomachs
Number of empty stomachs
Number of total stomachs
Fishery discards and catches
14–30 cm
30–50 cm
50–70 cm
63.1
43.6
2.4
10.5
2.4
1.8
0.4
4.5
41.8
37.7
3.4
9.7
0.5
4.0
1.1
3.0
0.2
1.1
0.0
1.2
1.3
12.2
2.3
1.8
3.3
2.9
24.0
20.5
1.1
5.1
0.1
0.8
1.0
1.3
1.1
0.9
0.3
0.4
2.0
6.4
2.7
0.8
8.7
8.5
1.2
7.3
0.2
1.9
0.7
31.5
27.1
1.9
6.8
0.3
1.9
1.0
2.0
0.8
0.9
0.2
0.7
1.7
8.9
3.1
1.3
6.7
6.5
0.7
5.8
0.2
4.2
0.9
62.3
0.7
19.8
0.1
3.8
1.2
6.7
1.4
1.1
22.7
56.2
11
83
470
54.6
1.1
17.0
0.5
2.9
0.8
4.7
1.1
0.9
21.5
44.0
13
151
1094
0.4
0.6
0.6
20.0
15.7
3.8
4.0
4.0
4.0
8.5
22.4
1.6
11.0
0.8
8.9
24.0
1
17
188
2.9
0.4
8.3
1.4
44.1
1.7
12.4
1.3
1.4
1.4
0.7
0.6
20.6
39.4
1
51
436
Discussion
Relationships between dogfish diet and material
discarded in trawling
Lesser spotted dogfish prey on a wide range of megabenthic fauna (echinoderms, polychaetes, molluscs,
sipunculids, tunicates, crustaceans), although decapod
crustaceans and fish are their main food. This kind of
general, opportunistic and scavenger feeding has been
described by other authors (Lyle, 1979; Olaso and
Rodrı́guez-Marı́n, 1995), but Kaiser and Spencer (1994)
were the first to suggest that dogfish feed on damaged
or dead animals from fishing operations or on other
scavengers attracted to the trawled area.
Total
Catch
Discard
Discard/
catch (%)
138
33
*
41
83
138
33
*
41
83
100
100
100
100
100
1
1
4
25
1
*
*
1
4
24
100
100
100
100
100
96
*
*
100
18
2
14
2
78
100
71
71
100
135
2775
473
135
530
16
100
23
3
931
578
16
63
569
225
10
16
63
93
24
2
100
100
16
*
*
On the basis of this information, the 1994 discard data
of the Division VIIIc trawl fleet were analysed (Pérez
et al., 1996), and it was observed that of 216 taxa caught
(104 fish, 41 decapod crustaceans, 35 molluscs, and 36 of
other groups of invertebrates), 150 taxa were discarded,
each one representing, more than 90% of their total
catch (52 fish, 35 decapod crustaceans, 27 mollsucs, and
36 other groups of invertebrates). If we compare these
results with the 65 taxa found in the dogfish diet (Table
3), we find that all the preyed fish and a large majority of
the other phylum prey were discarded, with the exception of the groups of small size, such as non-decapod
crustaceans. But many megabenthic species (Munida
intermedia, Pagurus prideaux, Liocarcinus depurator,
Solenocera membranacea, Rossia macrossoma and Sepia
Blue whiting in diet of lesser spotted dogfish
337
Length group of lesser spotted dogfish 14–30 cm
80
60
40
20
0
30–50 cm
80
% Volume
60
40
20
0
50–70 cm
80
60
40
Figure 4. Percentage volume of major prey taxa in each predator size class and quarter. Winter;
orbignyana) were abundant, although their abundance in
the environment (Olaso, 1990; Garcı́a-Castrillo and
Olaso, 1995) should be taken into account, since they
can be consumed alive. However, the presence in the
lesser spotted dogfish diet of infaunal species (Alpheus
Pisces
undetermined
Other
pisces
Anchovy
Blue
whiting
Silvery
pout
Other
invertebrata
Cephalopoda
Euphausiacea
0
Other
crustacea
20
spring; summer;
autumn.
glaber, Goneplax rhomboides, polychaetes), benthopelagic fish species with a high discard rate (M. poutassou, G. argenteus) or pelagic species that suffer from
slipping (discard catches that are not taken on board,
but released into the water, i.e. a part of the total catch
338
I. Olaso et al.
Table 4. Percentage of empty stomachs of dogfish, by length
group and quarter in 1994.
Size class
Quarter 1
Quarter 2
Quarter 3
Quarter 4
Cruise surv.
Total
14–30 cm
30–50 cm
50–70 cm
10
0
0
10
10
8
13
7
0
11
11
11
19
19
15
14
13
14
Digestion states
3
2
1
0
14–30
30–50
Length groups (cm)
50–70
Figure 5. Digestion states of lesser spotted dogfish stomach
contents, by length range and quarters. Winter;
spring;
summer; autumn; cruise.
Table 5. Analysis of variance of the regression.
Regression
Residuals
DF
Sum of
squares
Mean
square
1
83
1.5688584
7.9368303
1.5688584
0.0956245
F=16.40645; Signif F=0.0001.
which returns to the sea and may survive, Anon., 1991),
such as E. encrasicholus, shows that they are likely to be
discarded. The highest percentage in volume of anchovy
occurs in the spring, which is when fishing for this
species is at its most active, and so at this time discards
and slipping take place.
As dogfish size increased, the number of prey taxa
increased, the greatest variety being found in fish
>30 cm. This may be the length at which discarded
material becomes particularly beneficial, since small
dogfish feed on a limited number of taxa (Fig. 3, Table
3), because their smaller mouth size restricts the consumption of large prey whole. The mean length of fish
prey was 140.0611.04 mm (n=126), and that of crustaceans was 32.712.85 mm (n=202). Nevertheless, we
must consider that these small-sized prey (e.g. Goneplax
rhomboides, Alpheus glaber and other shrimp) may
350
Blue whiting prey length (mm)
300
250
200
150
100
50
0
10
20
40
50
30
Lesser spotted dogfish length (cm)
60
70
Figure 6. Predator–prey relationships between lesser spotted dogfish and blue whiting. y=84.890exp(0.0125X); r2 =0.1650;
n=85 indiv.
Blue whiting in diet of lesser spotted dogfish
339
40
35
30
%
25
20
15
10
5
0
9
7
11
13
15
17
19
21
23
25
Length blue whiting (cm)
27
29
31
33
Figure 7. Length distribution of blue whiting in lesser spotted dogfish stomachs, discarded and landed in 1994. In the sample
there were: 85 blue whiting prey, 6305 discarded blue whiting and 5178 landed blue whiting. —— Discarded; – –– – landed;
—— prey.
60
%
45
30
15
0
6
12
18
24
30
Length of discarded blue whiting (cm)
36
Figure 8. Seasonal variation in the length distribution of blue whiting discarded by the ICES Division VIIIc trawler fleet in 1994.
—— Winter; · · ·· · · spring; – –– – summer; —— autumn.
remain available as food, since their availability may be
favoured by the effect of trawls on the sea bottom, as is
the case of the amphipod Ampelisca spinipes (Kaiser and
Spencer, 1994). The importance of fish prey increased
with dogfish size, but even though many of the fish prey
could not be identified, the majority would probably be
blue whiting.
Influence of the supply of blue whiting prey on
the feeding habits of lesser spotted dogfish
The high percentage of fish prey, in particular blue
whiting, consumed by the lesser spotted dogfish on the
northern coast of Spain contrasts with the small percentage of fish in this species’ diet in other Atlantic areas: 4%
(Lyle, 1979), 16% (Ellis et al., 1996). Bearing in mind
that the type and size of prey cause digestion to be faster
or slower, it is considered that invertebrates, which have
a lower energy value than fish, can be digested more
rapidly (Macpherson et al., 1989 point out that a fish
like the silvery pout takes twice as long to digest as a
natantian crustacean).
Feeding experiments carried out in the laboratory
using lesser spotted dogfish show that there is a close
relationship between the gastric evacuation rate and the
return of appetite, so that a single meal is evacuated 2–5
times more quickly than a double meal (Sims et al.,
1996). Studying the feeding habits of the lesser spotted
dogfish in areas in which its diet mainly comprises
invertebrates, Lyle (1979) considered the near absence
of empty stomachs and different states of digestion of
the prey to be due to the fact that these fish eat
340
I. Olaso et al.
Partial fullness index
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Winter
Blue whiting
Pisces
undetermined
Other pisces
Invertebrata
Pisces
undetermined
Other pisces
Invertebrata
Pisces
undetermined
Other pisces
Invertebrata
Pisces
undetermined
Other pisces
Invertebrata
Spring
Blue whiting
Summer
Blue whiting
Autumn
Blue whiting
Figure 9. Seasonal variation in partial fullness index for major prey groups, and total fullness index (as the sum of partial fullness
indices), with increasing dogfish length during 1994. Length groups: 14–30 cm; 30–50 cm; 50–70 cm.
intermittently throughout the day. However, in the
lesser spotted dogfish studied for the present paper,
although the percentage of empty stomachs was also
low, there was a major component of fish prey in their
diet, found in an advanced state of digestion. This would
seem to indicate that the supply of discarded fish prey in
the species’ diet, above all of blue whiting, modifies the
energetic content of the food ingested, and lesser spotted
dogfish may reach an overfed state.
These results indicate that, although fishing activity
tends to have a negative impact on the majority of
species, in this case, we find a certain benefit. Although
the lesser spotted dogfish is indeed affected by fishing
mortality, this effect is not as severe as it is for other
species, since nearly all specimens caught are discarded,
and their survival rate can be as high as 90% in half-hour
hauls (Kaiser and Spencer, 1995); meanwhile, they take
advantage of the food provided by discards and prey
killed or damaged by fishing.
Acknowledgements
We would like to thank Antonio Punzón for his participation in lesser spotted dogfish stomach sampling,
Blue whiting in diet of lesser spotted dogfish
and the co-ordinator of the ICES Mini-Symposium
‘‘Ecosystem Effects of Fisheries’’, Henrik Gislason, for
his positive contribution and suggestions for revising the
manuscript.
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