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SCIENTIFIC ADVISORY COMMITTEE
Sub-Committee on Marine Environment and Ecosystems (SCMEE)
Sub-Committee on Stock Assessment (SCSA)
Transversal Working Group on by catch/incidental catches
FAO Headquarters, Rome (Italy), 15-16 September 2008
Evaluation of by-catch and fishing mortality for threatened species in the Mediterranean
Seals-fisheries interactions in the Mediterranean monk seal (Monachus monachus):
related mortality, mitigating measures and comparison to dolphin-fisheries interactions
By Dr. Daniel CEBRIÁN
GENERAL FISHERIES COMMISSION FOR THE
GENERAL FISHERIES COMMISSION FOR THE GENERAL FISHERIES COMMISSION FOR THE
GENERAL FISHERIES COMMISSION FOR THE
MEDITERRANEAN
MEDITERRANEANMEDITERRANEAN
MEDITERRANEAN
COMMISSION GÉNÉRALE DES PÊCHES POUR
COMMISSION GÉNÉRALE DES PÊCHES POUR COMMISSION GÉNÉRALE DES PÊCHES POUR
COMMISSION GÉNÉRALE DES PÊCHES POUR
LA MÉDITERRANÉE
LA MÉDITERRANÉELA MÉDITERRANÉE
LA MÉDITERRANÉE
1
Seals-fisheries interactions in the Mediterranean monk seal
(Monachus monachus): related mortality, mitigating
measures and comparison to dolphin-fisheries interactions
By Dr. Daniel CEBRIÁN*
* Marine Biology Expert. PhD
SAP BIO Programme Officer
UNEP-Mediterranean Action Plan
Regional Activity Centre for
Specially Protected Areas (RAC/SPA)
B.P. 337 - 1080 Tunis Cedex. TUNISIA
Email: daniel.cebrian@rac-spa.org
2
Abstract
The frequency of interactions with trammel nets by Mediterranean monk seals
(Monachus monachus) and dolphins was recorded at the island of Zakynthos, located
in the south Ionian Sea, Greece.
Monk seals interact in the region mainly with static fishing gear. Zakynthos fishers
endured an overall damage rate of 4.96% caused by monk seals out of 1632 net
settings. Dolphins caused an overall damage rate of 6.19%. This rate is similar to the
one attributed to seals, but the level of damage to each net was more severe.
Interaction of monk seals with trammel nets and related by-catch risk is related to the
distance of the net placements to the caves where the seals rest. Damage becomes
very low at distances along the coast higher than 5 nautical miles from the caves, and
insignificant for distances higher than 10 nm. It might be possible to strongly reduce
the level of this interaction, the main drive to extinction through by-catch and killing
by fishers, by management of coastal fisheries based on this result.
Conservation actions for the seals could consider this knowledge as a tool to properly
design MPAs or to create static net restricted Important Seal Areas, with marine
boundaries according to the tolerable level of interaction with nets accepted by
managers.
Fish obtained by the seals from the predated nets during the study would reach as a
maximum 20.81 Kg/month. Such catch would hardly provide 1 Kg fish/seal to the
seal population monitored. Hence, we disagree with the hypothesis that monk seals in
the Mediterranean search for nets as a reaction to a depletion in the fishing shoals.
3
Introduction
The Mediterranean monk seal (Monachus monachus) is the most endangered
pinnipedian worldwide, considered critically endangered by the I.U.C.N. Its
remaining world population is located mainly in Greece, where 234-300 individuals
out of less than 550 survive. They breed in marine caves, and because peak pupping
in Greece occurs in September and October, they are very linked to the nearshore
habitat in autumn and winter (Cebrián, 1998a).
The interactions of the species with fishing gears have negative effects both for
humans and for the seals. Monk seals damage nets to eat fish trapped in them, and
they frequently die either entangled ( by-caught) or killed by fishers (Ronald and
Duguy, 1979; Berkes et al., 1979; Harwood, 1987; Avellá, 1986; Avellá and
González, 1989; Panou et al., 1993; Cebrián and Vlachoutsikou, 1992; Ozturk and
Dede, 1995; Cebrián, 1998a; Ozturk, 1998). Entanglement rates in active fishing gear
depends on the local characteristics of fisheries and does not seem to be a very
significant cause of death in Hawaiian monk seal (Monachus schauinslandi) owing to
protective measures. However some fisheries interactions occur, including
entanglement in active and derelict nets (Gilmartin et al., 1983; Twiss and Reeves,
1999). Other species, like the harbour seal (Phoca vitulina) in Norwegian waters
(Bjorge et al., 2001), endure the highest by-catch mortality rates in active gear.
The level of interactions between seals and trammel nets is traditionally exaggerated
by the artisanal fishers. In contrast, damages to purse seines or trawls are reported to
be rare by these fishers.
The fact that monk seals approach trammel nets with a certain frequency is a major
problem for the future of the species.
This study addresses the magnitude of that problem, contrasts it with the damages
attributed to dolphins, and suggests solutions to mitigate negative interactions, aimed
to enhance the survival chances for the species.
Methods
The population studied
Zakynthos island, in the South Ionian Sea, was chosen as sample area for the study. It
hosts a breeding population of monk seals, which we estimated by capture-recapture-
method to be between 18 and 23 individuals during the nineties, excluding yearlings.
The population is declining due mainly to illegal shooting related to their interaction
with trammel nets. The species rests and breed in the south and west of the island
with a yearly production of two-three pups along the nineties.
The National Marine Park of Zakynthos was declared in December 2000. Its main
purpose is the protection of the populations and habitats of both the loggerhead turtle
(Caretta caretta), which breeds on Laganas Bay, and the monk seal. However the
4
declared protected area embraces only the marine sector and the coast around the bay.
The seal caves located at the Laganas bay and at the northeast of the island were
abandoned in the middle 90's because of shooting, habitat degradation by new public
roads building and mass tourism activities. At present, the monk seal habitat is
completely excluded, except for a few caves used only sporadically.
There is not a resident population of dolphins near the island. Damages can be
sustained both by bottlenose dolphin (Tursiops truncatus), a coastal species
uncommon in Zakynthos, and by striped dolphin (Stenella coeruleoalba). Although
the latter is an oceanic species, Politi et al. (1992) found them to be very frequent in
the Greek Ionian and they felt surprised by its systematic presence in shallow waters
and close to the shore. We recorded several strandings of this species on the island
due to the epizootic that affected them during the study period (Cebrián, 1995a).
Common dolphins (Delphinus delphis) are frequent in the Central Ionian but we have
not recorded them around Zakynthos, in the South Ionian.
Working area
The frequency of damages to nets produced by monk seals and dolphins was recorded
at the island of Zakynthos, located in the south Ionian Sea, Greece (Fig. 1). For that
purpose, the activity of artisanal fishing boats was monitored in the main fishing
harbours on the island, Limni Keriou and Aghios Sostis, both of them located in the
southern island bay of Laganas. These boats are wooden vessels usually less than 10
m long, with license to fish with trammel nets and bottom long lines. Usually only
one fisherman and seldom two of them are onboard. These vessels fish on the south
and west coasts of the island, as well as within the big bay of Laganas, open to the
southeast. The nets are placed at dusk and pulled up shortly after dawn.
Data on monitored interactions presented in this study were collected from autumn
1990 to summer 1993. Some additional data on interactions and by-catch of monk
seals with other fishing gear were collected from the Aegean Sea between 1990 and
2003.
For the monitoring, the location of the net settings on each trip, as well as the damage
events to nets attributed to seals or dolphins were recorded from the fishers when they
returned to port in the morning. Although the island underwater topography is steep,
most of the trammel nets were set at depths shallower than -50m, since these fishers
do not venture very far from the shore to fish. Interactions with long lines were not
monitored since it is not possible to assign them with certainty to marine mammals.
In order to calculate the frequency of damages per trip and to relate the geographical
location of the damages to the fishing effort, data were used only if more than 50% of
the fishing trips of a particular vessel had been properly recorded that month.
The damage events recorded were used to create a contingency table and analyzed
with a G test, to relate their location with the presence of the closest caves inhabited
by seals.
5
Results
Monk seal interaction with different fishing gears
Monk seals interact in the seas surrounding Greece mainly with static fishing gear.
From 1990 to 2003 years we only have the following few records supporting the
contrary: a fisherman from northeast Zakynthos killed at least two seals trapped in his
small haul seine; an adult male in 1992 and another animal of uncertain age in winter
1994. The former seal was trapped together with a juvenile who managed to escape
through a hole open in the net sac. Another juvenile was trapped with a similar gear,
and released unharmed, in winter 1991 offshore of Spetches island in the east
Peloponnesus, Aegean Sea. Seals also eat fishes trapped in bottom long lines. Fishers
relate that the fish head is usually left together with the hook in the line, although we
have two reports of seals eating the whole fish together with the hook. In one of these
cases, near Naxos island, Aegean Sea on September 1991, a hooked juvenile seal was
lifted onboard and released after cutting the long line. Although the animal had
swallowed the hook, we did not record any seal death in the area during that period.
On the contrary heavy seal mortality related to interactions with fishing gear is
endured by the species owed to direct killing (Cebrián, 1998b). The worst seal
mortality record in Zakynthos for the last two decades was between summer 2000 and
winter 2000-2001 with two adults of sexes, a subadult male and a youngster killed by
humans. In spite of the severe decline, on October 2001 we counted at least 13
surviving seals in just two caves close to each other: three adult males, one subadult
male, three adult females, four juveniles and two pups, without implementing a
complete census of the island.
Difference between damages inflicted to nets by seals and dolphins
Damages by seals
When a monk seal captures a fish trapped in a trammel net, it usually produces three
or more holes. One hole is in the place where the fish is pulled out and two or more
other smaller ones are located to both sides of the former because the animal stands
vertically holding the net with its forelimbs. The fish is usually swallowed together
with a small piece of net as confirmed by the rests from nets found sometimes by us
in the excrements and inside the stomachs of seal carcasses. For that reason the net
can lack some filament fragments in the big holes and is just ripped in the small ones.
This pattern repeats with each fish taken from the net, so the damage can be mended
when there are not many fishes trapped, which is usual in these fisheries. Simple
small holes, unassociated with other holes also occur, so the net presents a pattern of
scattered small holes.
We have several reports from fishers around Greece that describe seals pulling fish
out of nets and dropping them to the bottom to eat latter, whenever there are many
fishes trapped. The only fish reported to be always left in the net is the scorpion fish
(Scorpaena spp.) These animals have poisonous bones on their dorsal fins and
opercula, and are represented in Greece by three species.
6
Damages by dolphins
Damages inflicted by dolphins are bigger in quality than the ones inflicted by seals.
Dolphins usually catch the fish while passing through these weak nets, consequently
leaving a very large hole on it for every fish eaten. They also rip big extensions of the
net, possibly pulling it to become untangled. These gillnets are very thin and easily
breakable if the dolphin's body is not too twisted in the net mantle or entangled with
the float or weight-holding ropes delimiting the net. Dolphin attacks are inflicted in
herds and the nets usually become completely destroyed, while attacks by groups of
seals are less common.
Quantification of interactions with trammel nets
Relation between damages to nets and number of fishing trips
Zakynthos fishers endured 81 seal damage events out of 1632 settings. This means an
overall damage rate of 4.96% (Table 1). It is worthy to mention that 48 of these
events happened off the south and west coasts, which have rocky cliffs. Despite a
lower level of fishing activity, about half level of fishing activity inside the bay with
its mixed cliffs and sandy beaches, the former area accumulates a much higher
proportion of damages than the latter, 8.53% and 3.09% respectively (Chi-square =
23.14, p<0.001)
In 65.6% of those cases in which a net had been predated by seals, fishers saw one or
more seals in the setting area (other sightings during the fishing trip were not
considered). This strongly suggests that seals usually predate on nets after sunrise.
When damages were due to dolphins, the value ranged from 80% (bay) to 100%
(west coast). Only in eight cases were dolphins seen around the net placement area
without damaging them. Dolphins are much more conspicuous than seals and can be
seen further away because they jump much more frequently than monk seals and are
usually in pods, which are sometimes numerous. Seals that are reported attacking nets
are usually alone, which makes them more difficult to see.
Dolphins caused an overall damage rate of 6.19% and this loss rate is similar between
cliff and smooth coasts (Chi-square = 0.03, p= NS). The value is not significantly
greater than the 4.96% damage rate obtained for seals (Chi-square = 2.33, p= NS) but
the level of damage to each net was much severe, as stated above.
To further examine differences between seal and dolphin interactions with nets we
calculated the correlation coefficients r for the damages relative to nets deployments.
The association between quantity of damages and number of nets available every
sampled season is stronger for the dolphins (r = 0.8, p<0.001, n = 12 seasons) than for
the seals (r = 0.6, p<0.05, n = 12 seasons). This could indicate active searching and
better efficiency at locating nets by the dolphins, maybe thanks to the help of
echolocation and group foraging in contrast with seals, which lack echolocation and
are usually solitary foragers.
Damages by seals were associated with proximity to the seal caves existing on the
island. As Table 1 shows, from the 563 nets deployed in the west coast damages were
inflicted to 48 (8.53%), while from 1069 nets set in the bay damages affected 33
7
(3.1%). All monk seal caves permanently occupied in the study area are located on
the west coast. The greatest difference was in summer 1991, when 25% of the nets
were affected by seals in the west coast, while the only damage within the bay
happened in its western limit. Even excluding this exceptional season from the data,
the total level of damages on the west coast was double the level from the bay.
A contingency table was created to estimate the effect of the coastal distance to used
caves on the frequency of damages by seals (Table 2). Four categories of coastal belts
have been displayed, each one having a length of 5 nautical miles. The first one
utilizes data on damages incurred within areas with occupied caves at the time when
the damage occurred. The other three ones utilize data on damages incurred at
sequentially higher distances.
The first category, with inhabited caves, includes data from the west coast within the
cave area and between the southernmost occupied cave to 5 nm north from it. This
embraces all the caves monitored by us in the area. Data collected more than 5 nm
north are rejected, since some caves not monitored there could be inhabited by seals
and distort results. A total of 117 data points were rejected because their location did
not allow a certain assignment to a specific category in relation to cave use by seals.
Data collected in the west half of the bay when seals inhabited its caves are also
included. A broken net within this last area would be 2 nm at most from an inhabited
cave and probably less.
The second category includes all data from the coastal zone between the southern
occupied cave in the west coast and the south cape of the island, which is 5 nm away
from that cave. It also includes the data from the east half of the bay during periods in
which its west half had seal presence on its caves. A damaged net would be from a
few meters to 5 nm away from the closest used cave. In fact, the distance would be
usually at least 2 nm, which is the distance from the more frequented caves to the
inhabited belt limit.
The third category includes data from the west half of the bay, collected during
periods when there were not seals inhabiting caves in this area (every winter, autumn
1992 and spring and summer 1993). A net predated here would be 5-10 nm away
from the closest used cave.
The last category includes data from the east of the bay, collected during periods
when there were not seals inhabiting the bay caves, all of them located on the western
side of the bay. The caves used by seals were at those times at a minimum distance of
10-15 nm from a net damaged in this area.
A G test on the resulting table (Table 2) tested the null hypothesis that damages are
independent from areas. The result (G=25.54>X2(05) 3) is significant, demonstrating
that the damages are not independent from the belts. Consequently, damages to nets
become less frequent as nets are placed further from an occupied cave. The frequency
of damages on areas separated 5 nm would be respectively: 10.56%; 7.03%; 3.22%;
1.05%. The damage ratio would be given by the following equation:
Y = 10.312-0.65 * X +
ε
8
Where X is the distance in nautical miles and Y is the damage expressed in %, and
ε
is the random error associated with the measure. In theory, we would not expect to
have predation on nets at coastal distances higher than 16 nm from caves.
Phenological differences between seal and dolphin damages
The percentage of net predation by dolphins and seals by season is shown in Fig. 2.
The main difference between dolphins and seals is the minimum number of damages
produced by seals in spring, while there is a maximum for the dolphins during that
season.
Discussion
Phenology and damages
Since there is not a resident population of dolphins near the island, their damage peak
in spring may be related to higher dolphin presence in that season and not necessarily
to higher predatory activity in relation to other seasons.
The minimum damage in spring for seals may be related to the peak of moulting, that
we found to be in that season (Cebrián, 1998a). All the seal species where moulting
have been studied fast or hardly eat during moult (Bonner, 1989).
The maximum for the seals in winter suggest that predation is higher in this season,
when the animals forage in groups with pups. The biggest group sizes are recorded in
the caves when the pups are still very dependant on land and can not swim very long
distances, although they can swim inside the cave less than one week after birth. The
biggest group size recorded resting together in Zakynthos was 10 individuals,
including one pup. Maybe the shortening of foraging trips offshore because of the
presence of pups or the group foraging itself, or both, increase the chances to find
nets along the coast. However, this maximum could be just an artefact, since absolute
values of damages in this season are low (
X
= 4.67, S.D.= 3.79, n= 3) but available
nets in winter are five times less than in spring and summer. Considering that nets are
a food resource that is in limited supply, each net would have a greater chance of
predation.
Seal-nets interactions and learning
Damages to nets may increase due to cultural factors related to learning from other
individuals. During a survey of the Adriatic Sea fishers did not report damages to
their trammel nets, before the seals became extinct in Dalmatia as a breeding
population. Instead seals used to break reed fish traps by crushing them with their
bodies against the seabed. The only recorded damages to trammel nets had been
produced by a vagrant juvenile in 1993, which we concluded was a dispersing
individual from the Ionian Sea, the closest breeding area being located roughly 300
nm to the south south-east (Cebrián, 1995b). This does not mean that seals from the
Adriatic never ate fish from nets, but just that the last remaining populations did not
seem to do it. In the oldest records known to us on interactions of monk seals with
nets, Brusina (1889) reports a poem by Mavro Vetranic Cavcic (1482-1576), which
9
refers to seal predation on nets off St. Andrija Island, close to Dubrovnik. Also Orbini
(1601) records the report of "big damages to the fishermen" whenever monk seals
entered in Meleda lagoon, on Mljet Island, as well as the trapping of the seals in nets
deployed for that purpose across the lagoon entrance.
Learning from adults would explain why monk seals eat dead fish from nets, since
they do not eat fishes thrown to them by fishers, at least in the Mediterranean. We
recorded several reports of pups eating from the nets together with adults. The biggest
group reported mentioned more than five seals, including pups, in SW Zakynthos.
The group composition was verified by us during cave monitoring; six hours later
there was a group of three females and two moulted pups resting in a cave just in
front of the spot where the net had been broken that morning. Another pup was seen
several times with an adult, which we assume was its mother, eating from the nets in
the Bay of Milos Island, in the Aegean Sea. We also have several records of pups
drowned in nets (Cebrián et al.,1995): one pup in a net off Keros Island, and two pups
in different nets off Iraklia Island, both in the Aegean. These sightings show that pups
forage together with their mothers and probably learn to eat fish from nets that way.
Interactions and seal foraging behaviour
The results of the contingency table suggest that seals predate on nets whenever they
find them by chance on their foraging trips. Should they actively look for nets, a
higher frequency of interactions than the one recorded would be expected at long
distances, and the animals would have learnt to look for them in the bay, where nets
are more abundant. A juvenile monk seal can travel more than 15 nm in less than one
day, as we can infer from radiotracking data recorded by Reijnders and Ries (1989);
also Mursaloglu (1984) reports 20 nm displacements in less than 24 hours. Therefore,
at least some of the adults within our population would be physically able to reach the
farther limit of our study area in less than one day.
The spatial distribution of damages suggests that the seals usually travel offshore
from their caves to forage in unknown areas, possibly far away from the coast as grey
seals (Halichoerus grypus) do (McConnell et al., 1999). The expected cumulative
pattern of the population foraging trips would show a radial plotting centred in the
caves proximity, towards unknown offshore feeding areas and not a linear one along
the coast starting from them. The damages in the coastal trenches would be
consequent with encounters when seals leave and especially when they return from
these offshore trips. The departure is always from a haul out site, but the return to it
depends on navigation skills, which surely are very good but cannot be 100% perfect,
so the returning animal would travel along the coastline trench until it relocates the
cave. That would explain the steep decrease in interactions found at distances
relatively close for seal foraging capabilities. Active foraging along the coast would
result in a homogeneous level of damage, since the distance is short enough to be
covered every day.
The proportion of seal predation on nets might reflect the probability of nets being
found as seals forage along the coast to the mentioned distances from the occupied
caves. Given a frequency of 100% in the caves sector, where they leave from, we
might assume a seal presence frequency of only 9.47% along the coast at a distance
10-15 nm from the caves. We do not know how far away the seals leave offshore
10
from the coast in their forages, but we have not managed to see them with binoculars
from cliffs even 300m high. It is possible that the seals travel more frequently to good
feeding grounds far offshore than along the coast near to the caves. Proper studies of
home range and foraging behavior for the species would need the use of telemetry
techniques.
Nevertheless, we have found temporary preferred hunting grounds for individual
seals besides the shore on the northeast and the southwest of the island. These
foraging areas were visited regularly by up to two individuals at a time, always males,
but exact foraging locations did not overlap. These locations were respectively 3 nm
and 3.5 nm from the caves where those seals habitually rested. Marchessaux and
Muller (1987) also found in the West Sahara coastal foraging areas where the
individuals kept contiguous territories which did not overlap, although at least some
of those individuals rested on the adjacent secluded beaches and not in caves, as the
individuals studied by us did. The pelage pattern of those individuals recorded by the
authors indicates that all of them could be also males.
In our sample island, whatever net deployment on the way between the caves and the
coastal foraging territory should have been actively located with very little foraging
effort. However, we recorded a total of 21 and 20 net placements in the northeast and
the southwest respectively that did not register damages during the period in which
the seals were using the territories. Those records support our theory that seals do not
actively search for nets, even in areas with high probability to find them.
The existence of offshore foraging areas would also explain the scarcity of coastal
foraging areas close to the coast in relation to the size of the population studied.
Four of the five net predations inside the bay were recorded in spring and summer
when its caves were not occupied (Table 2). Considering the seasons, it is possible
that vagrant juveniles dispersing from their birth areas after weaning incurred all the
damages to distances higher than 10 nm from used caves. In that case, damages to
nets placed in the coasts away from the resident seals cave area would be even rarer
than suggested by the contingency table.
Efficiency locating nets seems to be lower for seals than for dolphins. In these clear
seas, light reaches usually more than 40 m depth, as we could verify by the presence
of Posidonia oceanica prairies at –42 m in the bay of Zakynthos, a Mediterranean
phanerogam. In spite of that transparency, a net is usually invisible underwater from a
distance higher than 30 m. Monk seals might see under lower light conditions than
humans underwater, since their eyes possess tapetum. On the other hand seals and
dolphins lack blue sensitive cones, so the detection of contrast and brightness (i. e.
non-chromatic cues) is very poor in the blue part of the spectrum (Peichl et al., 2001).
Therefore, the distance reached cannot be much farther than mentioned above. Blind
seals can survive and forage in the wild (King, 1983; Riedman, 1990; Mcconnell et
al., 1999), but it is possible that their special senses (vibrissae use?, low frequency
sounds?) allow them to locate benthic organisms like walruses do, but not locate
pelagic prey. Under the low visibility range produced by the water environment,
(little contrast and brightness dominated by blue wavelength) active foraging for nets
does not seem to be more productive for a monk seal than foraging for fish,
crustaceans and cephalopods. It seems easy for seals to pass close to a net without
11
noticing it, whereas cetaceans can echolocate nets.
Other studies on monk seal -nets interactions
Other researchers (Panou et al., 1993) obtained a level of predation on nets in the
Central Ionian of 7.3% in 1864 fishing trips in an area near to occupied seals caves.
This value falls close to the one obtained by us in our area, which seems to host a
larger seal population than the Central Ionian.
It has been suggested that monk seals increasingly attack nets because of the
depletion of fish stocks in the Mediterranean (Boudouresque, 1991; Karavellas, 1995;
Ozturk and Dede, 1995; Karavellas, 1996), but none of the authors above present
proof for their argument. The latter author recorded damage rates by seals in
Zakynthos of 18.1% in 1994 and 21.7% in 1995 and concluded that damages are
increasing as a reaction to impoverishment of the island shoals, so fishers complains
are fully justified given the high proportion of damages. However, those conclusions
are based on a record of less than 11 fishing trips per month (291 trips) (Karavellas,
1995 and 1996). Since fishing activity on those years did not decrease, we believe
that the sampling effort (a fifth of ours) was at least too low to determine the real rate
of net damages.
Boudouresque (1991) suggests that the monk seal switches its foraging strategy
towards an active net search when the fish density decreases to a critical level. The
weakness that we find in this argument is that nets are density-dependent traps and
not active-attraction devices (as fish traps with bait), so the level of fish capture with
nets would be very low, as seen already in Greece. Only bait attraction or active
fishing as with well-trained speargunners or experienced long line fishers in Greece
can provide satisfactory capture rates with low fish density. It does not seem that
monk seals are more frequently hooked in long lines now than before, although the
extent and severity of this interaction has not been properly evaluated yet in the
Mediterranean seal. Hooked Hawaiian monk seals have been found both in the
Leeward and the main Hawaiian Islands, and have caused deaths in some cases
(Twiss and Reeves, 1999).
The Mediterranean monk seal is a stalk demersal hunter, which search behind reef
corners, marine tunnels and holes, or waits for hours floating over selected shallow
reefs, in order to surprise its prey from a short distance. They are astonishing fast
sprinters, but probably slower than most fishes for long runs. In any case their natural
foraging strategy seems much more efficient for hunting demersal preys than looking
for almost empty nets.
Sociological factors must also be considered in these studies. Fishers tend to inform
authorities (and complain) every time they endure damages in their nets, but not when
they implement a good amount of fishing trips without damages. Many fishers cannot
be trusted at all for collaborating with researchers. As an example, 80 fishing trips
rejected for our study because of their doubtful net locations and dates comprised
together a damage rate of 28.75% (filtering criteria for fishing trips data were always
independent from the damage rate recorded on them). Only intensive monitoring of
fishing activity can result in reliable damage rate data.
12
Exaggeration of damage rates produced by marine mammals to fishing gear incurred
by scientists can become an additional threat for these species, since culling is then
socially justified for the fishers, at the same time that authorities feel the impossibility
of finding management solutions to mitigate the competition.
The 81 nets predated in our three-year sampling give an average of 2.25 nets/month.
We calculated the catch for trammel nets in an eight years study of coastal fisheries in
the Marine Park of North Sporades, Aegean Sea (Cebrián and Anagnostopoulou,
1995). The value was 3367 Kg/vessel-year, what means 9.25 Kg/vessel-day, caught
with several Km. of trammel net. Zakynthos shoals are much poorer, but supposing a
similar level of catch the fish obtained by the seals from the predated nets would
reach as a maximum 20.81 Kg/month (eating all the fish from the nets, which rarely
happens). Such catch would hardly provide 1 Kg fish/seal monthly to the seal
population recorded at the time. Foraging for a single fish or an octopus would more
easily render that amount of capture. The species might easily eat 10 Kg fish/day
since their stomachs can hold much more than that amount (Cebrián, 1998a). Hence,
we disagree with the hypothesis that monk seals in the Mediterranean search for nets
as a reaction to a depletion in the fishing shoals.
Mitigating measures
By-catch by gill nets and trammel nets alone does not seem to constitute the most
serious threat to the species but it plays an additive role, considering the presently
spread presence of these gears in the sea. Essays on Population Viability Analysis
considering a theoretical human related mortality provoked only by fishing nets
bycatch (without intentional killing) still render high values of 83,5% extinction risk
in Greece after 122 years (UNEP, 2005).
Reduction of by-catch might be achieved through management actions addressed to
keep net settings away from main seal caves, where interactions concentrate.
If comparative Population Viability Analysis (PVA) essays are done considering all
human related mortality and a situation without by-catch mortality in fishing gear
(UNEP, 2005), it is verified that the difference in time passed until reaching
extinction is less than an additional decade to the forty years predicted under the
action of all causes.
Extinction risk would not be much reduced only by eliminating all by-catch by static
nets, so why to attempt it? The answer is that seals are mainly killed by fishermen
who consider them a threat for their nets and the motivation to kill the seals would not
exist in those areas where such nets would not be used, while other methods with
unsound interaction (e.g. long lines) might be allowed.. In such situation all human
induced causes would disappear and the risk of extinction might be negligible.
The problem of implementing a full banning of static nets in certain regions inhabited
by seals may be socially conspicuous.
A feasible mitigating measure to consider is eliminating the setting of static nets only
in the proximity of areas where seal caves exist. Population Viability Analysis results
allow to predict a reduction of by-catch through this method of at least 25%, without
13
closing areas too wide around the important caves. That might allow a strong
reduction of the motivation to kill seals since damages to nets would be also strongly
reduced.
An alternative mitigating measure would be banning static nets in coastal regions
with sound seal populations, except in certain marine reserves, where fishermen
would have the right to use them as far as intentional killing is not incurred (this is a
real situation in the Greek Marine Park of North Sporades). That situation would
make useless the fishermen practise to kill seals. Should such reserves embrace 25%
of the seals population, that would imply a 75% reduction in seal by-catch. Having as
an example Greece, three Marine areas cover such percentage of that country seal
population North Esporades (North Aegean), Milos-Kimolos-Polyegos (South
Aegean) and Zakynthos (Cebrian 1998b).
Theoretical low values of 6% risk of extinction might be reached, and that only after
166 years (Cebrian 1998a). That is a much acceptable risk. The advantage of such
measure would be that static nets banning in other areas would be soundly justifiable
by the tolerance in areas respectful to the species.
Conclusion
Interaction of monk seals with trammel nets in the sampled population is related to
the distance of the net placements to the caves where the seals rest. The present study
suggests that damage becomes very low at distances along the coast higher than 5 nm
from the caves, and insignificant for distances higher than 10 nm. It might be possible
to strongly reduce the level of this interaction, the main drive to the species
extinction, by management of coastal fisheries based on this result. Conservation
actions for the species could consider this tool to properly design MPAs or to create
static net restricted Important Seal Areas, with marine boundaries according to the
tolerable level of interaction with nets accepted by managers.
14
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17
Table 1. Trammel nets placed, damages and sightings recorded from autumn 1990 to
summer 1993.
Interactions Total % West coast % Bay %
Fishing trips 1632 563 1069
Seal damage 81 4.96 48 8.53 33 3.10
Seal sighted 53 3.25 31 5.51 22 2.06
Dolphin damage 101 6.19 34 6.04 67 6.27
Dolphin sighted 101 6.19 27 4.80 74 6.92
Table 2. Contingency table, showing events of seal damage to nets in contiguous coastal
trenches inhabited by monk seals in relation to distance from their resting caves. nm:
nautical miles.
Coastal Trench
Seal caves (A) 0 to 5 nm from A 5 to 10 nm from A
10 to 15 nm from A
Damage 36 23 12 5
No damage 305 304 361 469
18
Fig. 1. Location of the study area
Ionian Sea
Aegean Sea
GREECE
ZAKYNTHOS
19
Fig. 2. Seasonal frequency of predation on trammel nets by dolphins and monk seals in
Zakynthos Island.
0 5 10 15 20
Autumn
Winter
Spring
Summer
% Seal damages
% Dolphin damages
