1732
Incidental bycatch of short-beaked common dolphins (Delphinus
delphis) by pairtrawlers off northwestern Spain
M. M. Fernández-Contreras, L. Cardona, C. H. Lockyer, and A. Aguilar
M. M. Fernández-Contreras, L. Cardona, C. H. Lockyer, and A. Aguilar. 2010. Incidental bycatch of short-beaked common dolphins (Delphinus
delphis) by pairtrawlers off northwestern Spain. – ICES Journal of Marine Science, 67: 1732– 1738.
The numbers of short-beaked common dolphins captured annually by pairtrawlers operating off Galicia (northwestern Spain) and the
operational factors influencing the bycatch were evaluated using on-board observations. Hauling time, fishing depth, and season of the
year were identified as the key factors involved in the incidental capture. The dolphins were most vulnerable to trawls at night from
May to September, around the continental shelf break. Most of the dolphins in the bycatch were males, and the average age was
13.4 + 4.4 years for males and 11.5 + 4.8 years for females. The sex ratio was male-biased owing to a few capture events involving
several males each, supporting the notion that bachelor groups exist in the area. The annual bycatch in 2001 and 2002 was an estimated 394 dolphins [95% confidence interval (CI) 230 – 632], most taken from May to September (mean 348 dolphins, 95% CI 200 –
590) and just a few from October to April (mean 46 dolphins, 95% CI 0– 132). This level of bycatch could be reduced significantly if
trawlers were restricted to operating in water deeper than 250 m and likely avoided entirely if they were restricted to water deeper
than 300 m.
Keywords: bycatch, northwestern Atlantic, pairtrawlers, short-beaked common dolphin.
Received 14 August 2009; accepted 11 May 2010; advance access publication 16 June 2010.
M. M. Fernández-Contreras, L. Cardona, and A. Aguilar: IRBIO and Department of Animal Biology, Faculty of Biology, University of Barcelona,
Avinguda Diagonal 645, 08028 Barcelona, Spain. C. H. Lockyer: Age Dynamics, Huldbergs Allé 42, 2800 Kongens Lyngby, Denmark.
Correspondence to L. Cardona: tel: +34 93 4031368; fax: +34 93 4034426; e-mail: [email protected].
Introduction
The exploitation of marine ecosystems is causing rapid depletion
of top predators worldwide (Pauly et al., 1998; Jackson and Sala,
2001; Myers and Worm, 2003), and small cetaceans are no exception. However, this is attributed to unsustainable, incidental
bycatch rather than direct exploitation (Waring et al., 1990;
Perrin et al., 1994).
Common dolphins (Delphinus delphis) are found in all the
major ocean basins except the high-latitude Arctic and Southern
Oceans (Heyning and Perrin, 1994; Perrin, 2002; Jefferson et al.,
2008, 2009). For a long time, all common dolphins worldwide
had been assumed to represent a single pan-global species,
Delphinus delphis, but Heyning and Perrin (1994) showed that,
at least in the northeastern Pacific, two species were represented
by long- and short-beaked forms. Although the taxonomic
status of the long-beaked populations elsewhere is unclear
(Natoli et al., 2006), studies indicate that only the short-beaked
common dolphin (Delphinus delphis) is found in the Atlantic
north of 308N, although there is some differentiation between
the eastern and western populations (Natoli et al., 2006;
Jefferson et al., 2009; Mirimin et al., 2009).
Short-beaked common dolphins are numerically the most
abundant cetaceans in the eastern Atlantic (Hammond et al.,
2002; López et al., 2003; SCANS-II, 2008) and account for most
of the incidental small cetacean catch made by fishers in the area
(Goujon, 1996; Tregenza et al., 1997; Morizur et al., 1999; López
et al., 2003) as well as in the adjoining regions of the western
Mediterranean (Silvani et al., 1999; Tudela et al., 2005). High
levels of bycatch have been reported in some areas (Goujon,
1996; Tregenza et al., 1997; Silvani et al., 1999; López et al.,
2003; Tudela et al., 2005), but the actual impact on the population
remains unquantified (Murphy et al., 2009), although published
population estimates exist for several areas (Goujon, 1996;
Hammond et al., 2002; SCANS-II, 2008).
Galicia (northwest Spain) is a well-known fishing area with a
large fishing fleet. A questionnaire-based survey estimated that
the fleet incidentally captures 1629 small dolphins annually,
most likely short-beaked common dolphins (López et al., 2003).
Although the incidental capture of marine mammals by a trawl
fishery is considered to be rare (Morizur et al., 1999; Gonzalvo
et al., 2008), the questionnaire-based survey (López et al., 2003)
identified pairtrawling as the fishing technique involved most
often in the capture of small cetaceans off Galicia. The survey
was uncertain about the identity of the small dolphins taken by
trawlers, so the operational factors leading to the incidental
capture of short-beaked common dolphins could not be determined (López et al., 2003). However, on-board observers monitoring a few fishing operations confirmed the incidental capture of
short-beaked common dolphins by pairtrawlers (López et al.,
2003).
Here, our objectives were to quantify the number of shortbeaked common dolphins captured annually by pairtrawlers operating off the coast of Galicia and to identify the operational factors
influencing the bycatch.
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1733
Incidental bycatch of Delphinus delphis by pairtrawlers off NW Spain
Material and methods
The coastal waters off Galicia are characterized by the presence of
filaments of cool upwelled water, which collectively form a coastal
transition zone between shelf and open ocean waters (Haynes
et al., 1993). Filaments are considered to be narrow (10 –
30 km), long (100 –500 km), and relatively shallow (50–100 m)
surface features that locally transport relatively cool water from
coastal upwelling regions towards the open sea. Following the
onset of seasonal upwelling off the coast of Iberia in either May
or June, filaments begin to develop in July or August. By
September, they have grown to lengths of 200 km (Haynes et al.,
1993), but quickly disappear in October, when the wind pattern
changes and upwelling weakens. Further, the water column is stratified from May to September (Herrera et al., 2008). As a consequence, two clear periods exist annually off Galicia: stratified
(May–September) and wind-mixed (October–April). Examining
these two periods separately is important because the behaviour
of the fishing fleet changes seasonally (see below), although
seasonality has a minor effect on the structure of fish communities
(Fariña et al., 1997; Serrano et al., 2008).
Upwelling promotes high productivity, so Galicia is a commercially important fishing region in the Atlantic, hosting about a
million fishing trips annually (López et al., 2003). Most of the
fishing effort consists of gillnets and traps, and the highest rates
of cetacean bycatch have been reported for offshore gillnets
and offshore trawling (López et al., 2003). Pairtrawlers have
operated off Galicia since the mid-1980s and target mainly blue
whiting (Micromesistius poutassou), along with mackerel
(Scomber scombrus), hake (Merluccius merluccius), and horse
mackerel (Trachurus spp.) as secondary targets. Each pair of trawlers tows a net 200 m long with a mouth opening of 90 m × 30 m.
The net is towed over the seabed in areas ranging from 125 to
700 m deep, corresponding to the deeper shelf and the upper
and mid slope. In the early 2000s, the fleet consisted of 14 pairs
of trawlers, and Riveira was their base port throughout most of
the year, although some pairs tended to move 150 km north to
La Coruña in summer.
Study design
On-board observers monitored 891 fishing operations conducted
by 12 pairtrawlers from March 2001 to December 2003. Sampling
was discontinuous from March 2001 to July 2002, during which
time three surveys were conducted: March–April 2001, July
2001, and July 2002. Each survey lasted 1 month and was conducted by five observers, who shifted randomly every day among
the 12 pairs of trawlers. Sampling was then continuous from
August 2002 to December 2003, when three observers based at
Riveira shifted randomly every day among the 12 pairs of trawlers,
and observers covered some 50 fishing trips monthly.
The observer assigned to a particular pair was aboard the
trawler where the catch was loaded and sorted, to have first-hand
access to any dolphin caught incidentally. Observers reported the
date and the time of setting and hauling, the latitude and longitude
of the setting and hauling points, the average depth of net deployment, and the number of dolphins captured in each tow. Tow duration was computed from the setting and hauling times. The
species and sex of each dolphin taken were determined on board
by the observer, and several teeth were collected from the central
part of the lower left jaw for age determination. Skin, blubber,
and muscle were also collected for research purposes, but genetic
analysis was not used to confirm species identity. On-board observers watched systematically for marine mammals during the
fishing operations conducted in daylight and reported date,
time, latitude and longitude, species, and pod size of each sighting.
During the survey periods, the skippers of the pairtrawlers that
operated without an on-board observer were interviewed once
daily in the port to identify any incidental bycatch. Dolphins incidentally caught by vessels without an on-board observer were
brought to the harbour, where a skilled observer identified the
species, determined the sex, and collected teeth for age
determination.
Age determination
Soft tissues around the teeth were allowed to decompose in water
and, once cleaned, the teeth were stored dry. In the laboratory, the
teeth were decalcified in Rapid Bone Decalcifier for the
Preparation of Histological Material (RDO), a commercially prepared mixture of acids, for 2– 8 h, depending on tooth volume.
Teeth were then sectioned longitudinally in a freezing microtome,
stained with haematoxylin, blued in a weak ammonia solution,
and mounted onto gelatine-coated slides. Age was determined
by counting growth layer groups (GLGs) in dentine, assuming
that each GLG corresponded to 1 year, as described by Lockyer
(1995a, 1995b).
Data treatment and statistics
The survey was designed to sample the wind-mixed and the stratified periods throughout several years to include all fishing scenarios. However, on 13 November 2002, the sinking of the tanker
“Prestige” caused an oil spill. Oil-spill fishing-exclusion areas
were implemented for trawlers on 30 November 2002 and lasted
until July 2003. As a consequence, the pairtrawler fleet operated
in an anomalous situation in terms of both fishing grounds and
fishing effort, and four contrasting sampling periods were considered when the data were analysed: wind-mixed conditions
before and after the oil spill, and stratified conditions before and
after the oil spill.
A Student’s t-test was used to investigate differences in the
depth and the duration of tows capturing and not capturing dolphins. Discriminant analysis (Legendre and Legendre, 1998) was
used to test the hypothesis that month, hauling time, and depth
were good predictors of short-beaked dolphin bycatch. Month
and time are circular variables and had to be cosine-transformed
to linearize them. Data are shown as mean + s.d. (standard deviation) unless stated otherwise.
Although the catch per unit effort (cpue) was calculated for
each period, the numbers of dolphins captured by the whole
fleet were estimated only for the wind-mixed and stratified
periods before the oil spill (see below). The total number of shortbeaked common dolphins taken incidentally and the total number
of capture events were used to calculate the average number of dolphins per capture event, because differences in the average number
of the dolphins in a capture event did not change after the
“Prestige” oil spill (see below); bootstrapping was used to calculate
the 95% confidence interval (CI) of the average number of dolphins per capture event because the data were not normally distributed. The probability of capturing at least one short-beaked
dolphin in a fishing day in a particular period was calculated
from the number of fishing days surveyed and the number of
capture events. The 95% CI of that probability was calculated
according to Fowler et al. (1998).
1734
M. M. Fernández-Contreras et al.
The number of short-beaked common dolphins captured seasonally by the whole fleet was calculated by multiplying the probability of a capture event per fishing day, the average number of
dolphins per capture event, and the number of fishing days for
the whole fleet. However, this procedure would have ignored
variability in the average number of dolphins per capture event
and the uncertainty of the probability of a capture event. This
was solved by randomizing the data as follows: first, 1000 simulated surveys were generated for each period by sampling fishing
days randomly from our database. These simulated surveys each
consisted of 136 fishing days for the wind-mixed period and 300
fishing days for the stratified period, the same as the surveys,
and the only information retained from the sampled fishing days
was whether a capture event had been recorded. Therefore, each
simulation generated a probability of a capture event in a fishing
day, by dividing the simulated number of capture events by the
number of simulated fishing days. This probability of capture
was then multiplied by the actual fishing effort (fishing days ×
number of pairs in the fleet) for each period (1862 vessel-days in
the wind-mixed period, 1400 vessel-days in the stratified period)
to generate a dataset consisting of 1000 estimates of the number
of capture events experienced by the whole fleet in each season.
These calculations assume that each pair of trawlers operated on
average 20 d per month in the stratified period and 19 d per
month in the wind-mixed period, as shown by the records in
the skippers’ logbooks. In the second step, the numbers of shortbeaked common dolphins in each capture event in the simulated
datasets were randomly sampled from the numbers of that
species of dolphin taken in the capture events observed in this
study (see below). All observed capture events were considered,
because the average number of short-beaked common dolphins
captured in an event did not change after the “Prestige” oil spill.
The 1000 estimates of the total number of short-beaked
common dolphins captured in the wind-mixed period were combined randomly with the 1000 estimates for the stratified period to
calculate 1000 estimates of the total number of short-beaked
common dolphins captured annually by the whole fleet and the
corresponding 95% CI.
2001 and December 2003. Fishing effort decreased dramatically
from December 2002 to June 2003, when only four of the 12
pairs of trawlers involved in the monitoring programme operated.
This reduction in fishing effort resulted in better coverage of the
fishing operations, because the three observers shifted daily
among a smaller fleet. The other pairs of trawlers resumed activity
in July 2003.
Most fishing trips surveyed by the on-board observers involved
just a single tow, but 53 (6%) involved a second tow. A second tow
was less frequent in the wind-mixed period (4% of the surveyed
fishing trips) than in the stratified period (7% of the surveyed
fishing trips). In both periods, the first tow usually started
between 04:00 and 06:00 (Figure 1) and lasted an average of
8.9 h (range 0.5 –19.1 h). Therefore, boats operated most of the
time in daylight, except at the very beginning of each tow or
during extremely long tows. The second tow usually started
between 17:00 and 18:00 and lasted an average of 5.8 h (range
0.6 –8.4 h), so taking place at least partially at night. The average
tow depth was 299 + 121 m (Figure 2), corresponding to the
upper slope, although the first tow was deeper than the second
(average depth of the first tow 303 + 120 m, average depth
of the second tow 225 + 108 m; Student’s t-test, t ¼ 3.05,
d.f. ¼ 455, p ¼ 0.002). The average depth of the fishing grounds
also changed seasonally and in relation to the “Prestige” oil spill.
Before the oil spill, the fleet exploited slightly deeper fishing
grounds in the stratified period than in the wind-mixed period
(average tow depth in the stratified period 393+ 148 m, average
tow depth in the wind-mixed period 339 + 92 m; Student’s
t-test, t ¼ 2.05, d.f. ¼ 42, p ¼ 0.047). After the oil spill, the fleet
operated roughly at the same depth in the wind-mixed period,
but moved shallower in the stratified period (average tow depth
Results
Table 1 summarizes the fishing trips surveyed by on-board observers and the fishing trips reported by skippers to port observers,
along with the capture events observed by on-board observers
and the capture events reported by skippers to port observers.
On-board observers surveyed 891 fishing trips between March
Table 1. Summary of the fishing trips and the capture events
surveyed by on-board observers (surveyed) and reported by
skippers to port observers (reported) during this study.
Before “Prestige” oil spill
Parameter
Wind-mixed
Fishing trips
Surveyed
73
Reported
63
Total
136
Capture events
Surveyed
0
Reported
1
Total
1
After “Prestige” oil spill
Stratified
Wind-mixed
Stratified
137
163
300
275
472
747
406
188
594
9
4
13
5
1
6
15
0
15
Figure 1. Setting (top panel) and hauling times (bottom panel) of
the tows in the pairtrawler fishery off Galicia.
1735
Incidental bycatch of Delphinus delphis by pairtrawlers off NW Spain
Table 2. Summary of the canonical discriminant equations
extracted by discriminant analysis for predicting dolphin bycatch
based on operational factors (month, hauling time, and depth).
Test and value
Canonical
correlation
Wilk’s lambda
Chi-square
Degrees of freedom
p-value
Before “Prestige” oil
spill
0.557
After “Prestige” oil
spill
0.181
0.690
44.010
3
,0.001
0.967
11.096
3
0.011
The p-values refer to significant differences between the centroids of the
two groups considered (no bycatch/at least one dolphin taken as bycatch).
Month and hauling data are cosine-transformed.
Figure 2. Depth of the tows that captured or did not capture
short-beaked common dolphins.
in the wind-mixed period 294 + 85 m, average tow depth in
the stratified period 266 + 110 m; Student’s t-test, t ¼ 2.71,
d.f. ¼ 305, p ¼ 0.007).
On-board observers witnessed 29 capture events, all involving
short-beaked common dolphins. All the dolphins were free
within the codend, and not entangled. Most of the capture
events happened during daylight (daylight events 22, or 76%;
night-time events 7, or 24%), an expected result given that 93%
of the tows surveyed were made during daylight (Figure 1).
However, nocturnal capture events were more often than expected
(x 2 ¼ 13.4, d.f. ¼ 1, p , 0.001). Before the “Prestige” oil spill, the
tows capturing at least one dolphin were much shallower than
those not capturing dolphins (average depth of tows with no
dolphins 402 + 130 m, average depth of tows with dolphins
180 + 62 m; Student’s t-test, t ¼ 9.06, d.f. ¼ 16, p , 0.001), but
they did not differ in duration (average duration of tows that
captured dolphins 10.1 + 1.8 h, average duration of tows that
did not capture dolphins 8.8 + 2.2 h; Student’s t-test, t ¼ –1.68,
d.f. ¼ 205, p ¼ 0.094). After the “Prestige” oil spill, the tows capturing at least one dolphin and those that did not capture dolphins
did not differ in average depth (average depth of tows without
dolphins 277 + 104 m, average depth of tows with dolphins
224 + 58 m; Student’s t-test, t ¼ 1.61, d.f. ¼ 354, p ¼ 0.108) or
duration (average duration of tows with dolphins 8.8 + 3.0 h,
average duration of tows without dolphins 8.7 + 2.3 h; Student’s
t-test, t ¼ –0.44, d.f. ¼ 646, p ¼ 0.661).
The occurrence of dolphins in the fishing area was a poor predictor of a capture event both before and after the “Prestige” oil
spill, because dolphins were not observed in any of the 29
fishing trips that resulted in a capture event, but were observed
in 38 of the 845 fishing trips that did not result in a capture
event. The average depth of the areas where observers sighted
short-beaked common dolphins was 398 + 138 m. Other marine
mammals sighted by the on-board observers were bottlenose dolphins (Tursiops truncatus, seven sightings) and long-finned pilot
whales (Globicephala melas, five sightings).
Discriminant analysis confirmed the relevance of month,
hauling time, and depth as predictors of dolphin bycatch before
the “Prestige” oil spill (Table 2), because the analysis classified correctly 97% of the tows not capturing dolphins and 78% of the tows
capturing at least one dolphin. The centroid of the tows capturing
at least one dolphin was negative (22.36) and that of the tows not
capturing dolphins was positive (0.19). This was because the
Figure 3. Group size of the capture events of short-beaked common
dolphins by pairtrawlers.
matrix of canonical structure of the discriminant equation
opposed depth and cos(month) to cos(hauling time), so revealed
that shallow and nocturnal tows [cos(hauling time) ≥0] conducted during the stratified season [cos(month) ≤0] were most
likely to result in the capture of at least one dolphin. A new discriminant analysis including the same variables correctly classified
77% of the tows not capturing dolphins and 70% of those capturing at least one dolphin after the “Prestige” oil spill, but the canonical correlation was much lower (Table 2).
Fishers reported the bycatch in an additional 886 fishing trips
throughout the same period, another six capture events
(Table 1). The cetaceans captured in five of those events were
brought to harbour and identified by observers as short-beaked
common dolphins. The small cetacean captured in the sixth
capture event was not brought to the harbour so remained unidentified, although it was thought not to have been a common
dolphin according to the description given by the fishers. That
dolphin was not considered further in analyses. Fishers did not
report any capture event during the stratified period after the
“Prestige” oil spill, when on-board observers witnessed 15
capture events (Table 1). This discrepancy suggests that the attitude of fishers towards the monitoring programme might have
changed after the “Prestige” oil spill, and that data reports on
capture events may have become unreliable then.
Most of the observed and reported capture events of shortbeaked common dolphins involved one or two animals, but
some events involved up to 15 (Figure 3). The average number
of short-beaked common dolphins captured in an event did not
1736
Figure 4. Age distribution of the short-beaked common dolphins
incidentally taken by pairtrawlers.
change after the “Prestige” oil spill (Mann –Whitney test; U ¼
124.5, n1 ¼ 14, n2 ¼ 21, p ¼ 0.410), so all the capture events
observed were used to calculate the average number of shortbeaked common dolphins captured in an event. The sex ratio of
the dolphins taken incidentally was highly skewed in favour of
males (51 males:28 females) and was statistically different from
unity (x 2 ¼ 6.1, d.f. ¼ 1, p ¼ 0.013). This unbalanced sex ratio
was caused by two all-male mass-capture events in July 2001
that included 7 and 15 short-beaked common dolphins. Overall,
the average age of males was 13.4 + 4.4 years (n ¼ 49) and that
of females was 11.5 + 4.8 years (n ¼ 26; Figure 4). Interestingly,
the average age of the males captured in the two all-male masscapture events (7.4 + 3.2 years) was lower than the average age
of males taken in all capture events.
Before the “Prestige” oil spill, the percentage of fishing trips
capturing at least one short-beaked common dolphin was much
higher during the stratified period (4.3%, 95% CI 2.1– 6.5) than
during the wind-mixed period (0.7, 95% CI 0 – 2.2). The probability of capturing at least one short-beaked common dolphin
in a fishing trip did not change after the “Prestige” oil spill (stratified period: 3.7%, 95% CI 1.9 –5.5; wind-mixed period: 1.8%,
95% CI 0.2 –3.4), although the latter calculations are based only
on the fishing trips surveyed, given the above reported suspicion
that fishers became less likely to report dolphin bycatch after the
“Prestige” oil spill. When the percentage of fishing trips capturing
at least one short-beaked common dolphin before the “Prestige”
oil spill was combined with fleet size, the number of fishing trips
in each period, and the number of short-beaked dolphins in a
capture event, the total number of short-beaked common dolphins captured annually by the fleet in 2001 and 2002 was estimated to have been 394 dolphins (95% CI 230 –632), most
taken incidentally during the stratified period (mean 348 dolphins,
95% CI 200 –590) and only a few during the wind-mixed period
(mean 46 dolphins, 95% CI 0 –132). Dolphin bycatch after the
“Prestige” oil spill has not been calculated, because the fleet was
not operating as usual.
Discussion
Most of the research on incidental bycatch of common dolphins
has focused on gillnets (Perrin et al., 1994; Ferrero and Walker,
1995; Tregenza et al., 1997; Silvani et al., 1999; Tudela et al.,
2005; Rogan and Mackey, 2007). Morizur et al. (1999) conducted
M. M. Fernández-Contreras et al.
the first study specifically aimed at quantifying marine mammal
bycatch in the trawl fisheries of European fishing fleets. That
study revealed that some of the pelagic trawl fisheries operating
off western Europe captured short-beaked common dolphins
and other species of small cetaceans in 4.8% of the tows. The
current study indicates a similar rate of cetacean bycatch for pairtrawlers targeting blue whiting off the coast of Galicia and confirmed that almost all the cetaceans incidentally captured by that
fleet are short-beaked common dolphins.
Morizur et al. (1999) suggested that the incidental capture of
dolphins by pelagic trawlers happened close to or during hauling
because (i) the dolphins were not entangled in the lines or the
net, but usually free within the lighting bag, (ii) the dolphins
had a high body temperature, revealing relatively recent death,
and (iii) there was no correlation between tow duration and
dolphin bycatch. Similarly, the dolphins taken incidentally by pairtrawlers surveyed off the coast of Galicia in the current study were
not entangled, and there was no relationship between tow duration
and the probability of bycatch, thus indicating that they were incidentally caught because they failed to abandon the gear when the
net was hauled. However, most of the dolphins interacting with the
trawlers are probably not caught, because the average pod size of
short-beaked common dolphins in the eastern Atlantic is 10.8
animals (Hammond et al., 2002), and the number of short-beaked
dolphins involved in a capture event while trawling is usually one
or two (Figure 3 and Morizur et al., 1999).
Quite commonly, the vulnerability to fishing gear seems to be
very dependent on sex and age of the dolphins, and the bycatch
of common dolphins is usually dominated by males, not only in
trawl fisheries (Morizur et al., 1999; this study), but also in
gillnet fisheries (Ferrero and Walker, 1995; Rogan and Mackey,
2007; Westgate and Read, 2007; but see Silvani et al., 1999). This
suggests sexual segregation either within the schools or in the
use of habitat, or suggests that there is sex-related variation in
vulnerability to bycatch attributable to sex-related differences in
behaviour. However, the actual reasons for such a systematic
bias are poorly understood.
Calves also appear more vulnerable to gillnets than adults
(Ferrero and Walker, 1995; Silvani et al., 1999; Rogan and
Mackey, 2007), but this is not true for the pairtrawlers operating
near Galicia. Remarkably, the males involved in the two all-male
mass-capture events reported here were younger than the
average, and also younger than the age-at-first-maturity of male
short-beaked common dolphins in the eastern Atlantic (Murphy
et al., 2005, 2009), although the actual maturity stage of most of
the short-beaked common dolphins taken incidentally was not
checked. Moreover, both of these mass-capture events were in
July, during the apparent peak breeding season of short-beaked
common dolphins in the eastern Atlantic (Murphy et al., 2005,
2009), suggesting that bachelor groups of immature males are
formed during the breeding season in the region where pairtrawlers operate. It is worth noting too that Murphy and Rogan (2006)
and Rogan and Mackey (2007) recorded an absence of juveniles
(4–8 years old) in the tuna driftnet fishery that operates west
and north of the fishery covered in this manuscript, reinforcing
the hypothesis that short-beaked common dolphins segregate by
age and sex in the Northeast Atlantic.
Whatever the reason for the bycatch of short-beaked common
dolphins by trawlers, we determined three operational factors that
significantly influenced the rate of pairtrawler capture in this
study. The most obvious factor was depth, because all the dolphins
1737
Incidental bycatch of Delphinus delphis by pairtrawlers off NW Spain
were captured during tows made in water shallower than 300 m.
Most of the sightings of the short-beaked common dolphins
during the surveys were over the continental shelf break and on
the upper slope, in water ranging from 183 to 585 m deep.
However, dolphins were captured over a much narrower depth
range (128 –294 m), indicating greater vulnerability in shallow
water. The net used by pairtrawlers is 200 m long and intercepts
the entire water column when hauled over the shelf break or lower
shelf, which may reduce the opportunity for dolphins to escape.
The probability of a capture event did not change significantly
after the “Prestige” oil spill, despite the profound impact of
that incident on shelf communities (Serrano et al., 2006;
Martı́nez-Gómez et al., 2009) and the reduction of food
availability for other top predators (Velando et al., 2005). This
appears to confirm the notion that short-beaked common
dolphins are not particularly vulnerable to the effects of oil spills
(Ridoux et al., 2004), but it is worth noting that the fleet of
pairtrawlers shifted in the stratified period after the “Prestige”
oil spill to fishing grounds close to the shelf break, where the probability of a capture event was the highest both before and after the
“Prestige” oil spill. Under this scenario, the stability of the bycatch
rate might be interpreted as evidence of lesser abundance of shortbeaked common dolphins in the area. Therefore, caution is needed
in trying to conclude that the population was not affected by the
“Prestige” oil spill.
Time of day was the second operational factor that influenced
dolphin capture significantly. Morizur et al. (1999) and López
et al. (2003) reported that all capture events observed on trawlers
operating in the Northeast Atlantic were at night, but most of the
capture events observed during this study were by day. This was
simply because most of the tows by pairtrawlers were made in daylight. However, the percentage of night-time tows that captured
dolphins was significantly higher than expected, in accord with
the distribution of fishing during day and night, indicating a
greater vulnerability of short-beaked common dolphins to pairtrawlers at night.
The third factor that influenced bycatch was seasonality,
because the rate of bycatch of short-beaked common dolphins
was significantly higher during the stratified period both before
and after the “Prestige” oil spill. As pairtrawlers operated in
deeper water during the stratified period, at least before the
“Prestige” oil spill, and the hours of operation did not change seasonally, the higher rate of bycatch observed in the stratified period
is unlikely to be caused by operational factors, but may reflect
changes in the distribution and/or behaviour of the short-beaked
common dolphins.
The data reported here indicate that the rate of bycatch of
short-beaked common dolphins by pairtrawlers off Galicia could
be reduced if the vessels modified their operational strategy
slightly. From such a perspective, regulation of fishing hours and
seasonal closures would be difficult to implement, because the
catch of target species would reduce dramatically and fishers
would not be favourably disposed towards such regulation.
Conversely, a ban on trawling in water shallower than 250 m is
more likely to be accepted, because the average depth of pairtrawler tows is 299 m and the primary target species, blue whiting, is
more abundant on the slope than on the shelf (Fariña et al.,
1997; Serrano et al., 2008). According to our dataset, if such regulation had been in effect, 68% of the capture events would not have
taken place and 78% of the short-beaked common dolphins would
not have been caught. A ban on trawling in water shallower than
300 m would have prevented all bycatch of these dolphins, but
such a limit would probably be less acceptable for fishers, considering the average depth of the tows.
Reducing the incidental bycatch of short-beaked common dolphins off Galicia is not only fairly easy, but also necessary.
Recently, the SCANS-II programme estimated the population of
short-beaked common dolphins over the continental shelf off
Portugal, Atlantic Spain, and Southwest France to be 17 916
(SCANS-II, 2008). Removals exceeding 2% of the total are likely
to be unsustainable (López et al., 2003), so the number of shortbeaked common dolphins taken incidentally in the whole area
should be no more than 358 per year. However, the pairtrawlers
operating off Galicia captured up to 394 animals annually, so
the total rate of bycatch by all fleets operating in the area is
almost certainly unsustainable. Moreover, the more frequent use
of pairtrawlers as an alternative fishing method to driftnets,
which were banned by the European Union to reduce the
bycatch of small cetaceans, may result in even higher levels of
bycatch in some regions.
Acknowledgements
The authors are indebted to Juan Pérez, President of the Fishermen
Association of Riveira, for facilitating contact with the skippers.
The authors also thank Maria Victoria Tornero, Miguel Ángel
Sánchez, Nuria Garcı́a, Joan Gonzalvo, Marta Coll, Marı́a Valls,
and Griselda Julián for their assistance.
References
Fariña, A. C., Freire, J., and González-Gurriarán, E. 1997. Demersal
fish assemblages in the Galician continental shelf and upper
slope (NW Spain): spatial structure and long-term changes.
Estuarine, Coastal and Shelf Science, 44: 435– 454.
Ferrero, R. C., and Walker, W. A. 1995. Growth and reproduction of
the common dolphin, Delphinus delphis Linnaeus, in the offshore
waters of the North Pacific Ocean. Fishery Bulletin US, 93:
483– 494.
Fowler, J., Cohen, L., and Jarvis, P. 1998. Practical Statistics for Field
Biology. John Wiley, Chichester, UK. 272 pp.
Gonzalvo, J., Valls, M., Cardona, L., and Aguilar, L. 2008. Factors
determining the interaction between common bottlenose dolphins
and bottom trawlers off the Balearic Archipelago (western
Mediterranean Sea). Journal of Experimental Marine Biology and
Ecology, 367: 47 – 52.
Goujon, M. 1996. Captures accidentelles du filet maillant derivant et
dynamique des populations de dauphins au large du Golfe de
Gascogne. Thèse Ecole Nationale Superieure Agronomique de
Rennes, Laboratoire Halieutique de Rennes. 239 pp.
Hammond, P. S., Berggren, P., Benke, H., Borchers, D. L., Collet, A.,
Heide-Jorgensen, M. P., Heimlich, S., et al. 2002. Abundance of
harbour porpoise and other cetaceans in the North Sea and adjacent waters. Journal of Applied Ecology, 39: 361 –376.
Haynes, R., Barton, E. D., and Pilling, I. 1993. Development, persistence and variability of upwelling filaments off the Atlantic Coast
of the Iberian Peninsula. Journal of Geophysical Research, 98:
22681– 22692.
Herrera, L., Rosón, G., Varela, R. A., and Piedracoba, S. 2008.
Variability of the western Galician upwelling system (NW Spain)
during an intensively sampled annual cycle. An EOF analysis
approach. Journal of Marine Systems, 72: 200 – 217.
Heyning, J. E., and Perrin, W. F. 1994. Evidence for two species of
common dolphins (genus Delphinus) from the eastern North
Pacific. Contributions in Science (Los Angeles), Natural History
Museum of Los Angeles County City Contributions in Science,
442: 1 – 35.
1738
Jackson, J. B. C., and Sala, E. 2001. Unnatural oceans. Scientia Marina,
65: 273– 281.
Jefferson, T. A., Fertl, D., Bolaós-Jiménez, J., and Zerbini, A. N. 2009.
Distribution of common dolphins (Delphinus spp.) in the western
Atlantic Ocean: a critical re-examination. Marine Biology, 156:
1109– 1124.
Jefferson, T. A., Webber, M. A., and Pitman, R. L. 2008. Marine
Mammals of the World: a Comprehensive Guide to their
Identification. Academic Press, San Diego. 573 pp.
Legendre, P., and Legendre, L. 1998. Numerical Ecology. Elsevier,
Amsterdam. 853 pp.
Lockyer, C. 1995a. Investigation of aspects of the life history of the
harbour porpoise Phocoena phocoena in British waters. Reports
of the International Whaling Commission, Special issue 15:
189– 197.
Lockyer, C. 1995b. A review of factors involved in zonation in odontocete teeth, and investigation of the likely impact of environmental factors and major life events on harbour porpoise tooth
structure. Reports of the International Whaling Commission,
Special issue 16: 511– 529.
López, A., Pierce, G. J., Santos, M. B., Gracia, J., and Guerra, A. 2003.
Fishery by-catches of marine mammals in Galician waters: results
from on-board observations and an interview survey of fishermen.
Biological Conservation, 111: 25 – 40.
Martı́nez-Gómez, C., Fernández, B., Valdés, J., Campillo, J. A.,
Benedicto, J., Sánchez, F., and Vethaak, A. D. 2009. Evaluation of
three-year monitoring with biomarkers in fish following the
Prestige oil spill (N Spain). Chemosphere, 74: 613– 620.
Mirimin, L., Westgate, A., Rogan, E., Rosel, P., Read, A., Coughlan, J.,
and Cross, T. 2009. Population structure of short-beaked common
dolphins (Delphinus delphis) in the North Atlantic Ocean as
revealed by mitochondrial and nuclear genetic markers. Marine
Biology, 156: 821– 834.
Morizur, Y., Berrow, S. D., Tregenza, N. J. C., Couperus, A. S., and
Pouvreau, S. 1999. Incidental catches of marine-mammals in
pelagic trawl fisheries of the Northeast Atlantic. Fisheries
Research, 41: 297– 307.
Murphy, S., Collet, A., and Rogan, E. 2005. Mating strategy in the male
common dolphin (Delphinus delphis): what gonadal analysis tells
us. Journal of Mammalogy, 86: 1247– 1258.
Murphy, S., and Rogan, E. 2006. External morphology of the shortbeaked common dolphin Delphinus delphis: growth, allometric
relationships and sexual dimorphism. Acta Zoologica
(Stockholm), 87: 315– 229.
Murphy, S., Winship, A., Dabin, W., Jepson, P. D., Deaville, R., Reid,
R. J., Spurrier, C., et al. 2009. Importance of biological parameters
in assessing the status of Delphinus delphis. Marine Ecology
Progress Series, 388: 273 – 291.
Myers, R. A., and Worm, B. 2003. Rapid worldwide depletion of predatory fish communities. Nature, 423: 280– 283.
Natoli, A., Cañadas, A., Peddemors, V. M., Aguilar, A., Vaquero, C.,
Fernández-Piqueras, P., and Hoelzel, A. R. 2006. Phylogeography
and alpha taxonomy of the common dolphin (Delphinus sp.).
Journal of Evolutionary Biology, 19: 943– 954.
M. M. Fernández-Contreras et al.
Pauly, D., Christensen, V., Dalsgaard, J., Froese, R., and Torres, F. 1998.
Fishing down marine food webs. Science, 279: 860– 863.
Perrin, W. F. 2002. Common dolphins Delphinus delphis, D. capensis,
and D. tropicalis. In Encyclopedia of Marine Mammals, pp.
282– 289. Ed. by W. F. Perrin, B. Würsig, and J. G. M. Thewissen.
Academic Press, San Diego.
Perrin, W. F., Donovan, G. P., and Barlow, J. 1994. Gillnets and cetaceans. Reports of the International Whaling Commission, Special
issue 15: 1 – 53.
Ridoux, V., Lafontaine, L., Bustamante, P., Caurant, F., Dabin, W.,
Delcroix, C., Hassani, S., et al. 2004. The impact of the “Erika”
oil spill on pelagic and coastal marine mammals: combining demographic, ecological, trace metals and biomarker evidences. Aquatic
Living Resources, 17: 379 – 387.
Rogan, E., and Mackey, M. 2007. Megafauna bycatch in drift nets for
albacore tuna (Thunnus alalunga) in the NE Atlantic. Fisheries
Research, 86: 6– 14.
SCANS-II 2008. Small cetaceans in the European Atlantic and North
Sea. Final report to the European Commission under project
LIFE04NAT/GB/000245. Available from SMRU, Scottish Oceans
Institute, University of St Andrews, St Andrews, Fife KY16 8LB,
UK. http://biology.st-andrews.ac.uk/scans2/.
Serrano, A., Preciado, I., Abad, E., Sánchez, F., Parra, S., and Frutos, I.
2008. Spatial distribution patterns of demersal and epibenthic
communities on the Galician continental shelf (NW Spain).
Journal of Marine Systems, 72: 87– 100.
Serrano, A., Sánchez, F., Preciado, I., Parra, S., and Frutos, I. 2006.
Spatial and temporal changes in benthic communities of the
Galician continental shelf after the Prestige oil spill. Marine
Pollution Bulletin, 53: 315 – 331.
Silvani, L., Gazo, M., and Aguilar, A. 1999. Spanish driftnet fishing and
incidental catches in the western Mediterranean. Biological
Conservation, 90: 79 – 85.
Tregenza, N. J. C., Berrow, S. D., Leaper, R., and Hammond, P. S. 1997.
Common dolphin, Delphinus delphis L., bycatch in bottom set gillnets in the Celtic Sea. Reports of the International Whaling
Commission, 47: 835– 839.
Tudela, S., Kai Kai, A., Maynou, F., El Andalossi, M., and Guglielmi, P.
2005. Driftnet fishing and biodiversity conservation: the case study
of the large-scale Moroccan driftnet fleet operating in the Alboran
Sea (SW Mediterranean). Biological Conservation, 121: 65– 78.
Velando, A., Muilla, I., and Leyenda, P. M. 2005. Short-term indirect
effects of the Prestige oil spill on a marine top predator: changes
in prey availability for European shags. Marine Ecology Progress
Series, 302: 263 – 274.
Waring, G. T., Gerrior, P., Payne, P. M., Parry, B. L., and Nicolas, J. R.
1990. Incidental take of marine mammals in foreign fishery activities of the northeast United States, 1977 – 88. Fishery Bulletin US,
88: 347– 360.
Westgate, A. J., and Read, A. J. 2007. Reproduction in short-beaked
common dolphins (Delphinus delphis) from the western North
Atlantic. Marine Biology, 150: 1011– 1024.
doi:10.1093/icesjms/fsq077