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IMPACT OF CLIMATE CHANGE ON NW MEDITERRANEAN FISHERIES
Isabel Palomera & Josep Lloret
Institut de Ciències del Mar. CSIC. Barcelona. Spain
SUMMARY
The Mediterranean Sea is almost a closed sea connected with the Atlantic by the Strait of
Gibraltar. Climatically, the Mediterranean is characterized by warm summer temperatures,
winter or autumn-dominated rainfall, dry summers and a profusion of microclimates due to
local terrain. Mediterranean waters are generally warm and oligothropic (i.e. have relatively
low concentrations of nutrients and low productivity) except in the vicinity of the rivers
mouths, where river discharges bring nutrients to sea, and in areas where wind mixing and
upwelling allows for vertical transport of nutrients. One of the most productive areas is the
north-western Mediterranean owing to important river discharges from the Rhône and Ebre
rivers and strong wind mixing. It is very likely that the effects of the climate change (e.g. sea
warming, sea-level rise, reduced river runoff, etc.) will cause changes within the fisheries of
this region, where pelagic, demersal and benthic species are targeted by artisanal, semiindustrial and industrial fleets. Furthermore, the consequences of climate change will
reinforce the other human-related impacts such as overfishing, tourism activities or river
modifications that are already threatening the sustainability of Mediterranean fisheries.
THE FISHERIES
Mediterranean fisheries have existed since ancient times. The north-western Mediterranean is
one of the areas where anthropogenic and environmental impacts from fishing, tourism,
industry, agriculture and farming activities have been more noticeable and diverse. Generally
speaking, the change in structure of the catch in the past years has been related to the
overexploitation of some species and, to some extent, to the impact of some environmental
factors. Exploitation takes place at sea (continental shelf and upper slope), estuaries and
coastal lagoons.
Which are the fundamental features of the NW Mediterranean fisheries?
Landings are multispecific: a large number of species (more than 100) are caught, landed and
commercialized. The absence of large monospecific stocks comparable to those inhabiting
other oceans (e.g. cod stocks in the North Atlantic) is noticeable. There is also a high diversity
of fishing gears (e.g. pelagic and bottom trawls, longlines, gillnets, purse seines).
The artisanal fleet (i.e. small-capital business, usually the fishermen’s property, exploiting
areas close to the ports where the fishermen are based, with low tonnage boats) coexists with
the industrial fleet (i.e. big-capital business, usually owned by companies or financial groups,
exploiting areas far away from their ports, with large boats). While the former is represented
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by small boats that use a multitude of fishing gears (e.g. trammel nets, gillnets, small
longlines, etc), the later is made of large seiners that target tuna, many of which come from
non-Mediterranean countries such as Japan. There is also an intermediate group between the
artisanal and the industrial fleet (the so-called semi-industrial fleet), which is composed of
trawlers, purse seiners and big longliners.
There is a large density of landing sites (e.g. 30 fishing ports within the 600 km Catalan
coast). The fisheries yield is comparatively low (compared to other oceans), probably as a
result of the relatively low primary productivity and generally narrow continental shelves.
The estimated Mediterranean fisheries production potential in relation to shelf area is 2.4
tonnes / km2 compared to 3.0, 4.9 and 5.5 tones / km2 for the Atlantic, Indian and Pacific
oceans, respectively. The majority of the catch includes fish in the first years of life, with high
exploitation rates on recruits. Thus, compared to other areas of the world oceans, juveniles
support most of the fishery. This has lead to “growth overfishing” of many stocks, i.e. the
maximum yield is not attained.
Which are the main species caught?
Small pelagics such as sardine and anchovy, and medium size-pelagics such as mackerel and
bonite are the main contributors to total landings (about 50%). Sardine and anchovy, which
have a schooling behaviour, are the main pelagic species landed in the Western
Mediterranean. These species are mainly caught with purse seines (light attraction) during
nighttimes) and trawls (during daytimes). Large size pelagics such as bluefin tuna and
swordfish, which are exploited with driftnets, longlines and seines, are economically very
important, even though landings of those species only represent around 4% of the total
landings. Atlantic Bluefin tuna is highly profitable because of extremely high prices. This
species migrates within the Mediterranean and between the Mediterranean and the North
Atlantic. Spawning takes place in the Mediterranean during spring and early summer, when
most catches take place.
Many demersal species have commercial value and are abundant, although none of them
counts more than 3% of total Mediterranean landings. The main gear to exploit demersal
species is the bottom trawl, even though artisanal gears such as trammel nets, gillnets and
bottom longlines are used too. Exploitation takes place on the continental shelf and the upper
slope. Among demersal species, red mullet, hake, blue whiting, Norway lobster and red
shrimp are the most important ones. Hake is the most important demersal species and the
most studied fish in the NW Mediterranean Sea. This gadoid species inhabits the soft bottoms
of the continental shelf and the upper slope, between 30 and 700 m depth. Juveniles of hake
are mainly caught with trawls on the shelf, while adults are mostly caught in the upper slope
with longlines and gillnets. Recent investigations have showed that wind mixing during the
spawning season of this species had a positive impact on the recruitment of this species. Red
shrimp constitutes the most valuable fishery species caught in the deep-sea. It is fished by
trawlers in the upper slope, especially near the submarine canyons.
Landings of benthic species are low but very profitable due to high market prices. Benthic
species like shellfish are exploited in some sandy coastal areas by artisanal boats that use
small drags. Recent investigations have shown that the productivity of some clams inhabiting
close to river mouths is affected positively by river discharges. Other benthic species like sea
urchins are exploited on rocky shores by artisanal fishermen. Finally, some holothurids are
caught by trawlers and commercialized.
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THE ENVIRONMENT
Which are the main environmental characteristics?
Climatically, the Mediterranean is characterized by warm summer temperatures, winterdominated rainfall, dry summers and a profusion of microclimates due to local terrain.
Oceanographically, the Mediterranean is a semi-enclosed sea globally considered as
oligotrophic. Its continental shelf is most frequently reduced to a narrow coastal fringe.
Because of the oligotrophic character of the Mediterranean, local environmental events like
wind-driven mixing and river discharge can have an important role on the local fertilization.
Because of the seasonal and scarce rains, high rates of evaporation and the correspondingly
low runoff of the relatively few short rivers, the Mediterranean has a deficient hydrological
balance, with loss through evaporation exceeding the input of water through runoff and
precipitation. This deficiency is mainly compensated by the flow of Atlantic surface waters
through the Strait of Gibraltar. Except in very few areas, the Mediterranean is characterized
by very weak tides.
The north-western Mediterranean is one of the most productive zones of the Mediterranean
Sea due to a number of environmental features which includes a wide shelf, strong vertical
mixing in winter, occasional coastal upwelling and important river runoff. This area can be
considered as a very complex region, because several intense and highly variable phenomena
compete simultaneously:

The strong continental winds. An intense water mixing induced by the strong, dry and
cold north-westerly winds blowing at the NW Mediterranean during winter. Coastal
upwelling may also occur under specific winds circumstances. Therefore, these winds
contribute to the vertical transport of nutrients. They have shown important seasonal
and interannual fluctuations but have not displayed any important trend over the last
decades.

The strong currents. An important component of the general circulation in the NW
Mediterranean consists of a current that follows the continental slope south-westward
most of time describing a cyclonic circulation. This stream displays a clear seasonal
variability, with about double the flux in winter. Associated to this current there is a
density front where nutrients are enhanced.

The formation of dense water. The cyclonic circulation pattern, the vertical structure,
the characteristics of the surface waters and the influence of the dry and cold winds are
the driving mechanisms of deep water formation during winter.

The fresh waters from the rivers. The large fresh water input in the area comes mainly
from the rivers Rhône and Ebre, which contribute to the largest fresh water and
nutrient input in the area as they collect water from large snow-covered mountains
(Alps and Pyrenees respectively) and flow through areas where agriculture and
farming are important. Rhône and Ebre river discharges display important seasonal
and interannual fluctuations, with prolonged low discharges recorded during some
years. In addition, the Ebre runoff has shown a progressive decline from the beginning
of the last century owing to the dam’s construction. Annual discharge of these rives
appears to vary inversely with the North Atlantic Oscillation (NAO) index: discharges
tend to be lower during high NAO episodes. High NAO episodes occur when the
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high-pressure system residing near the Azores strengthens and covers the southwestern parts of Europe, thus inducing lower than average rainfall in much of the area.

Nearly all of the earth has experienced during the last century an increase of global
mean air temperature of about 0.5 ºC in the 20th century. The world ocean has
warmed significantly during the past 40 years too. The water in the upper 3000 meters
of the world ocean warmed on average by 0.06 ºC. Closer to the surface (in the upper
300 meters), oceans waters increased in temperature by 0.31 ºC on average. This rise
in water temperatures could be responsible for a significant portion of the observed
global sea level rise of about 10 to 25 cm ove the last 100 years (0.1 to 0.25 cm per
year) due to thermal expansion of the oceans and melting of small glaciers. An
increasing concentration of greenhouse gases in the atmosphere is expected to raise
global temperatures in the 21st century and cause accelerated sea level rise: recent
studies estimate that global sea level has a large chances of rising about half a meter
by the year 2100. In the NW Mediterranean, the most noticeable feature is that air and
sea temperatures have shown an upward trend in all levels, which appears to be
stronger than in other oceans. Air temperatures at sea level have increased 2.1ºC in 30
years (0.07º per year). The sea temperatures have increased 1.1ºC in 27 years (0.04ºC
per year) at surface; at 80 m depth the tendency shows an increase of 0.7ºC in 27 years
(0.025ºC per year), and below 400 m depth the tendency shows an increase of 0.12ºC
in 30 years (0.004ºC per year). Warming has been accompanied by an increase of 3.3
cm of sea level in 11 years (0.3 cm per year). It also appears that the NW
Mediterranean is getting saltier: the salinity of western Mediterranean deep water in
the Gulf of Lions is increasing at a rate of 0.007 p.s.u. per decade.
Which are the main effects of environmental factors on fisheries?
Scientific studies have demonstrated that environmental factors such as river runoff, wind
mixing or water temperature can play a role on the productivity of stocks and the distribution
of fish species. Results have showed that enhanced hydroclimatic conditions (i.e. increased
river runoff, wind mixing and upwelling) in the NW Mediterranean are favourable for the
productivity of the fish and invertebrate stocks. Landings are generally low when river runoff
or wind mixing and upwelling is reduced.
The loss of essential fish habitats due to anthropogenic effects (e.g. tourism, trawling,
pollution, etc) has, however, the most adverse impact on fisheries. Thus for example, seagrass
meadows of Posidonia oceanica, wetlands, estuaries and rocky bottoms are very important
fish and invertebrate production areas that are fragile and receive multiple negative impacts
from human activities.
The two most important rivers discharging into the western Mediterranean (Ebre and Rhône)
have relatively high runoff and high suspended and dissolved sediment and nutrient yields.
Large inputs of nutrients from river flows in the 1970s and 1980s owing to the development
of intensive agriculture and farming and tourist activities might have contributed to the
continuous rise of the overall Mediterranean landings. Recent studies have demonstrated that
recruitment of several demersal and pelagic species in the NW Mediterranean is positively
influenced by runoffs of rivers Rhône and Ebre (e.g. anchovy, red mullet, octopus). Thus for
example, an additional 1 m3 / s of Ebre runoff during the reproduction season of anchovy
leads to about 200 kg of added landings of this species in Tarragona harbour after one year.
These rivers are important sources of phosphorus, nitrogen and other nutrients that are
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introduced at the surface, thus becoming directly available for phytoplankton. Phytoplankton
will be then consumed by zooplankton, which is the main food items for many fish larvae of
many species, and for small pelagic fishes such as anchovy and sardine. Similar positive
effects of river runoff on fish local production have been reported in other Mediterranean
areas, e.g. the northern Adriatic Sea (Po River outflow), the Black Sea (several rivers) and the
south-eastern Mediterranean (Nile River). In addition, condition (or “fitness”) of many fish
species has been found to be better in those areas affected by riverine inputs (such as the Gulf
of Lions). The effect of riverine inputs on marine fisheries does not only concern the area
close to the river mouths. It is also detectable in adjacent areas as the riverine discharges are
spread across these areas in the form of freshwater plumes that are nutrient enriched. Thus, for
example, major spawning grounds and large quantities of eggs and larvae of some fish species
are associated with river plumes.
Recent investigations have shown that recruitment of many demersal and pelagic species in
the NW Mediterranean is positively influenced by wind mixing induced by cold and dry
winds blowing in the area. Thus for example, an additional 1 m3 / s3 of the wind mixing index
during the reproduction season of sardine leads to about 6000 kg of added landings of this
species in Tarragona harbour after one year and a half. The vertical mixing induced by these
winds will carry nutrients deposed in deep waters to the surface (euphotic zone), thus
becoming available to phytoplankton and contributing to the enhancement of the productivity.
Although globally wind mixing favours primary productivity through the enhancement of
nutrient recycling, calm conditions over some seasons are favourable for the primary
production: a phytoplankton bloom, for example, develops in late winter in the NW
Mediterranean during a period of high atmospheric pressure.
The influence of water temperature on productivity of Mediterranean plants and animals is
quite complex. In general, colder years tend to be more productive, partly because mixing in
the water column may reach a greater depth and incorporate more nutrients and partly because
the formation of deep water may occur over a larger area. Regarding fish, significant effects
of temperature on productivity of some species have been established in some areas of the
Mediterranean. In some cases the increase of water temperature affected positively the
productivity of some species (e.g. anchovy in the Adriatic and red mullet in the Strait of
Sicily), whilst in other species the effect was negative (e.g. sprat and horse mackerel in the
Black Sea). Water temperature may affect directly the growth and condition of fishes and the
survival rates of their pelagic eggs and larvae, or may be a proxy for other processes affecting
fish productivity (such as vertical and/or horizontal water motion or freshwater input).
HOW ANTHROPOGENIC CLIMATE CHANGE CAN IMPACT FISHERIES.
Evidence for on-going global climate change owing to anthropogenic effects in our planet is
overwhelming. Rapid rising emissions of carbon dioxide and other greenhouse gases are
having a dramatic impact on climate, both by raising air and water temperatures and by
increasing extreme meteorological events. One of the most striking consequences of a
warming climate will be the rise in global-mean sea-level coming from the expansion of
warmer waters and melting land glaciers. Changes in evaporation, precipitation, salinity, river
runoffs, current and wind patterns and strengths would also probably occur.
It is very likely that the expected climatic changes will cause changes within the ecosystem.
Due to global change, existing ecological balances and chains can be broken, and new ones
would be formed. The biomass of some species will decline, benefiting other species whose
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biomass will increase. However, nobody can guarantee that the favoured species will be
exploitable or marketable (it may be, for example, a jellyfish that consumers will refuse to
eat).
Because of rapid population and economic growth, Mediterranean fisheries face major
problems in the coming years that will add to the effects of climate change. The most
important problems are: overfishing, which will continue to threaten economically important
species, the loss of high quality habitats for fish production (e.g. seagrass beds), which will
continue due to boat anchoring, pollution, trawling and other anthropogenic effects; and the
change of European rivers and their drainage basins in the coming years in order to supply
freshwater to the growing tourist and agriculture activities. Compared to these immediate
impacts, the effects of global climate change on fish populations will probably be only
remarkable in the medium and long-term. In short, any measure for the management of the
Mesiterranean fisheries has to consider the effects of climatic change on marine ecosystems.
Which are the possible specific effects of climate change on NW Mediterranean
fisheries?
Global warming may force some cold water species such as hake and poor cod to disperse and
shift their distribution ranges, or may alter their ability to survive, and thereby disrupting
existing ecosystems. In contrast, termophilic species such as dusky grouper (Epinephelus
marginatus), round sardinella (Sardinella aurita) and the ornate wrasse (Thalassoma pavo),
benefit from warmer water temperatures and are expanding their distribution to areas where
they could not live otherwise. The observed warming of western Mediterranean waters may
have already contributed to the changes in the recruitment pattern of the dusky grouper, which
now recruits regularly on the northwesternmost coasts (while it seems it never did before due
to relative historic cold conditions in the area).
The increase in water temperatures may reduce water mixing and upwelling of nutrients
because water stratification would be enhanced, thus affecting negatively phytoplankton and
zooplankton productivity. This would finally impact adversely on fish populations whose
adults (e.g. sardines and anchovies) or larvae (most of fishes and invertebrates) feed on
plankton.
The temperature change will probably mean an increase in the frequency of extreme events
such as droughts, floods and storms that would have a negative impact on coastal habitats
such as estuaries and seagrass meadows, thus affecting fish productivity in these areas. These
impacts could be attenuated, however, by the fact that Mediterranean biota is somewhat
adapted to common high seasonal and interannual variability in hydrographic variables (e.g.
torrential rainfall).
The rise in global-mean sea-level due to warming will erode essential fish habitats such as
deltaic areas and coastal lagoons (e.g. the Rhône and Ebre deltas and the coastal lagoons and
saltmarshes of the southeast of France) that are important for breeding, feeding or resting of
many fish species (e.g. anchovy, eel and many sparids). Erosion of these key habitats for fish
production will be exacerbated by the decrease of riverine sediments flowing into the NW
Mediterranean due to local reversion of agricultural land to forest, as well as planned river
diversion and dam construction (e.g. Ebre River). In addition, most lowland coastal areas are
subject to a degree of slow tectonic subsidence that will also accentuate the predicted sealevel rise. Salt-water intrusion due to sea-level rise and decreasing riverine sediments will
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cause increased salinity in these areas, thus limiting the life of many species in these
environments.
Changes in all climate and oceanographic parameters such as evaporation, precipitation,
salinity, river runoffs, current and wind patterns and strengths, etc., will necessarily
accompany changes in global-mean temperature. At present, however, our capability for
predicting these changes is limited and we must resort to the use of scenarios that must be
interpreted cautiously One of the parameters that could show large fluctuations that will affect
fish populations is river runoff, which depends, to some extent, on precipitations. Some
projections show that there will not be major precipitation changes whilst other projections
predict a general decrease of precipitation and river runoff. Although local rainfall has not
shown any important trend, large variability in runoff of some rivers has been observed in the
last decades owing to water use for human activities (e.g. decreasing trend of Ebre River
runoff). Any further decrease of freshwater from the rivers due to climatic changes will
reduce the amount of nutrients available to phytoplankton, which will finally adversely affect
zooplankton productivity and hence fish populations, as mentioned before. In many coastal
Mediterranean areas, river discharges are the only source of nutrients in surface waters during
spring and early summer, i.e. the periods in which many marine species spawn.
Assuming that the entire Mediterranean region will experience a decrease in precipitation and
an increase in evaporation, then salinity would continue to rise, which could upset the ecology
of the coastal areas. A rise in both the salinity and temperature of the sea will result in a
decrease of oxygen solubility and increases organic matter decomposition. This may enhance
the oxygen depletion in some coastal, shallow areas (e.g. bays), which may negatively affect
benthic species. Global warming may also induce changes in currents that may cause
subsequent changes in the migratory habits of some exploited fish species such as tuna.
Everything point to the hypothesis that the climate change would first affect coastal fisheries,
especially in deltaic areas (e.g. Ebre’s delta) and coastal lagoons (e.g. southeast of France).
These areas are very sensible to the expected changes in temperature and salinity, the decrease
of river runoff, erosion, rise of the sea level, etc. and thus the effects on natural resources
inhabiting there would occur very fast.
Further reading:
Bethoux, J. P., Gentili, B., Raunet, J. and Tailliez, D. (1990). Nature 347:660-662.
Bryden, H. L., and Boscolo, R. (2002). Understanding climate changes in Mediterranean water
masses. In: Tracking long-term hydrological change in the Mediterranean Sea (ed: F. Briand).
CIESM Workshop series 16, 134p.
Caddy, J. F., Refk, R. and Do-Chi. T. (1995). Productivity estimates for the Mediterranean: evidence
of accelerating ecological change. Ocean Coastal Management 26:1-18.
Duarte, C.M., Agustí, S., Kennedy, H. and Vaqué, D. (1999). The Mediterranean climate as a template
for Mediterranean marine ecosystems: the example of the northeast Spanish littoral. Progress
in Oceanography 44, 245-270.
Grainger, R.J.R. and Garcia, S.M. (1996). FAO Fisheries Department Chronicles of Marine Fishery
Landings (1950-1994): Trend Analysis and Fisheries Potential. FAO Fisheries Technical
Paper 359.
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Jeftic, L., Milliman, J.D. and Sestini, G. (1992). Climate change and the Mediterranean. Edward
Arnord (publ.), London, 664 pp.
Lleonart, J. and Maynou, F. (2003). Fish stock assessments in the Mediterranean: state of the art.
Scientia Marina 67(suppl. 1):37-49.
Lloret, J., Palomera, I., Salat, J. and Solé, I. (2004). Impact of freshwater input and wind on landings
of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) in shelf waters
surrounding the Ebre River delta (northwestern Mediterranean). Fisheries Oceanography
13(2):102-110.
Lloret, J. Lleonart, J., Solé, I. and Fromentin, J.M. (2001). Fluctuations of landings and environmental
conditions in the north-western Mediterranean Sea. Fisheries Oceanography 10(1):33-50.
Milliman, J. (2001). Delivery and fate of fluvial water and sediment to the sea: a marine geologist’s
view of European rivers. Scientia marina 65(suppl. 2):121-132.
Nicholls, R. J. and Hoozemans, F.M.J. (1996). The Mediterranean: vulnerability to coastal
implications of climate change. Ocean & coastal Management 31(2-3):105-132.
Salat, J. and Pascual, J. (2002). The oceanographic and meteorological station at l’Estartit (NW
Mediterranean). In: Tracking long-term hydrological change in the Mediterranean Sea (ed: F.
Briand). CIESM Workshop series 16, 134p.
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