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[Tuna Ecology and Biology] ->
[Environmental
Relationships and Modelling]
Climate and tuna fisheries
From a general point of view, to answer the question why the apparent abundance of tuna species (as inferred from catch rates) has changed in a given time and space, we have to consider three categories of factors: factors related to changes in fishing techniques that cause changes in species catchability (e.g., changing the depth exploited by the
longline), environmental (climate-linked) factors inducing spatial changes in the distribution and movements of fish, both in the vertical and horizontal dimensions (e.g., in relation to the depth of the
thermocline, or the seasonal or ENSO-related extension of warm waters), and finally real changes in abundance of the stock, with low or high levels of recruitment, in relation either to an environmental change or to the size of the spawning stock biomass (stock-recruitment relationship). In the latter case however, it is generally admitted that, given their very high fecundity and extensive spawning seasons and areas, tropical tunas do not exhibit a marked stock-recruitment relationship.
Spatial variability
Vertical
distribution
Recruitment and (absolute) population abundance
Long-term Climate Change
(greenhouse warming)
References
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Spatial variability
Nearly 70% of the world’s annual tuna harvest, currently
3.2 million tonnes, comes from the Pacific Ocean. Skipjack tuna (Katsuwonus
pelamis) dominate the catch. Although skipjack are distributed in the
surface mixed layer throughout the equatorial and subtropical Pacific, catches
are highest in the western equatorial Pacific warm pool, a region characterized
by low primary productivity rates that has the warmest surface waters of the
world’s oceans. Assessments of tuna stocks indicate that recent western
Pacific skipjack catches approaching one million tonnes annually are
sustainable. The warm pool, which is fundamental to the El Niño Southern
Oscillation (ENSO) and the Earth’s climate in general, must therefore also
provide a habitat capable of supporting this highly productive tuna population.
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Most
of this surface tuna catch is made by fleets of large purse-seiners in the
equatorial band (5oN-5oS). Aggregating fishing data
between these latitudes allows to show a high correlation between the skipjack
catch rate (CPUE) and the oscillation of ENSO illustrated by the climate index
SOI (negative during El Niño) and the longitudinal location of high catch rates
(green to red on the figure).
These displacements appear to be correlated to the zonal
(east-west) movement of the eastern edge of the warm pool approximated by the 29oC
Sea Surface Temperature (SST) isotherm. There is an obvious impact for the
Pacific Island Nations through the level of catch in their Exclusive Economic
Zones and consequently on their economical revenue, according to the ENSO
situation.
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 Data from the large tagging experiments conducted by the
Oceanic Fisheries Programme of the SPC were used to determine if the spatial
variability in abundance observed from fishing statistics was effectively
related to physical displacement of tuna. By selecting displacements of fish
tagged and released during El Niño and La Niña phases, it is shown that tuna
can cover the large distances (in thousands of km) corresponding to the
displacement of the eastern edge of the warm pool. In addition, tuna movements
observed during these phases are in agreement with the direction of the warm
pool displacement.
The mechanisms that could explain these
displacements have been investigated through oceanographic analyses and
simulations with a spatial environmental population dynamics model [more on SEPODYM …]. Even with a simplified
parameterization, this first simulations have been helpful in the interpretation
of observed ENSO-related spatio-temporal changes in the distribution of skipjack
population and allowed to propose a conceptual model of the biological
consequences of ENSO in the warm pool – cold tongue pelagic ecosystem (Lehodey
2001).
During a normal situation, the cold tongue extends to 180o,
and the well–developed equatorial divergence is the source of high primary
productivity. In the western Pacific, the relatively low primary productivity of
the mixed layer is associated with the presence of an atmospheric convective
zone and a barrier layer. The equatorial divergence superimposed upon the mean
westward zonal flux creates a spatial shift in the planktonic communities both
on the meridional (poleward) and zonal (westward) axes. With the development of
El Niño, the system shifts eastward; the water masses of the warm pool extend
towards the central Pacific while the intensity of the equatorial divergence
decreases and the cold tongue retreats eastward. Secondary production, which
previously had been developing in the cold tongue, moves into the warm pool, and
eventually merges with secondary production originating in the western Pacific.
Under the influence of currents, the organisms remain aggregated in a large
zonal band associated with the convergent front along the eastern edge of the
expanding warm pool. A longer residence time in the warm pool, clear waters and
a physical subsurface barrier are all highly favorable to the feeding of surface
tuna and this may explain the observed displacement of tuna populations linked
to the movement of the convergence zone. As the atmospheric convective zone is
displaced towards the central equatorial Pacific, a positive wind stress anomaly
leads to the development of coastal upwelling along the north coast of Papua New
Guinea, and more generally to an increase of primary production in the far
western Pacific between Indonesia and 160oE-180o,
according to the intensity of the event.
When eastward displacement of the
system stops and starts to reverse, the eastern edge of the warm pool becomes
less attractive. The westward
displacement of the warm pool - cold tongue system limits the potential
enrichment of surface waters of the eastern warm pool with the secondary
production of the cold tongue. In addition, the decreased intensity of the
equatorial divergence during El Niño is likely lead to a decrease in secondary
productivity with a shift in time from a few weeks (zooplankton) to a few months
(micronekton). On the other hand, the Papua New Guinea - Indonesian region that
was enriched during the El Niño phase will have an increase in zooplankton and
micronekton biomass at the time of the return to normal oceanographic
conditions. Consequently, the attractiveness (in terms of tuna habitat) of the
Papua New Guinea - Indonesian region should increase while as it decreases at
the eastern boundary of the warm pool. This situation could explain the westward
movement of tagged tuna observed during this phase (Lehodey et al.
1997). The probable enrichment in zooplankton of the western Pacific during the
El Niño phase is a condition highly favorable for survival and larval
development. Such conditions might therefore be expected to produce strong tuna
recruitment to the fisheries six to twelve months later. Preliminary
investigations seem to confirm that El Niño events have a favorable impact on
skipjack recruitment in the western Pacific (see below).
In summary, the western Pacific warm pool displays
remarkable dynamic in biological productivity, balancing the "Papua New
Guinea – Indonesian" region and the warm pool-cold tongue convergence
zone in an out of phase pattern. By contrast, in the eastern Pacific, primary
production and "down-stream" tuna forage are high and stable in space
and time, except during the most powerful El Niño events. From a skipjack
habitat point of view, the lower but sufficient secondary production of the warm
pool compared to the eastern Pacific is likely compensated by better
environmental conditions in terms of temperature, oxygen, and water clarity. In
the eastern Pacific, the high forage level is probably tempered by sub-optimal
values of other environmental parameters. While El Niño events have
catastrophic consequences in the eastern Pacific, it appears that they have
beneficial impacts on the biological productivity in the western equatorial
Pacific.
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Vertical
distribution
Oceanographic
conditions in four different geographical boxes identified as: the Western
Equatorial Pacific (W: 120oE-165oE; 10oN-10oS),
the Central Western Equatorial Pacific (CW: 165oE-150oW;
10oN-10oS), the Central Eastern Equatorial Pacific
(CE: 150oW-125oW; 0oN-20oS)
and the Eastern Equatorial Pacific (E: 125oW-80oW; 0oN-20oS)
have been investigated (Lehodey 2000). In summary, the major changes
observed in the vertical thermal structure are a strong seasonal signal in
the eastern and central eastern region in the surface layer both for SST
and mixed-layer, and a relatively low ENSO-related change in the central
eastern region for the same variables. During El Niño events, in the
western and central western areas there is a general "shallower"
thermal structure, particularly well marked and correlated with SOI in the
western region, and with a potential delay of 2 months between the two
regions. In the eastern region, there is an important deepening of the
thermal structure below the mixed-layer. The effects are opposite during
La Niña events.
 To help
in analysing how these temperature changes can affect the vertical and
horizontal distribution of tuna species, a temperature habitat was defined for
the four tuna species, skipjack, yellowfin, bigeye, and albacore. Temperature
functions have been defined for these species according to the knowledge on the
relationships between tuna physiology, distribution and ambient temperature.
The environmental changes in the vertical thermal
structure directly linked to SOI likely have a low influence on the skipjack
catchability (at least at the scale of the fishery), or the impact is masked by
much more large fluctuations due to other factors (i.e., horizontal movement or
stock abundance). The major change is a horizontal extension or contraction of
the skipjack habitat during El Niño and La Niña phases respectively.
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Yellowfin CPUE of the pole and line fishery is much lower
than for skipjack but shows interesting large fluctuations. Considered together
with the purse seine CPUE time series, there is a general coherent pattern with
higher CPUE during El Niño in the both western areas.
For the
three fleets, cross-correlation with SOI is negative, direct in the western area
and with a lag of 3 months in the central western area, consistently with the
lag observed in the rising of the vertical thermal structure between these two
regions. Therefore, the raising (deepening) of the mixed-layer depth related to
El Niño
(La Niña) is associated with an increasing (decreasing) pole and line
and purse seine CPUE of yellowfin in both western and central western regions
but with a concomitant delay of ~2-3 months. The raising is associated with a
vertical extension of the vertical temperature habitat.
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Before analysing the ENSO variability of the longline
fisheries, it is necessary to consider
the general trends due to the transition from the regular shallow
longline, e.g. in the first decade of the series, to the deep longline,
e.g. in the last decade. This change in the fishing technique resulted in
a large increase of bigeye CPUE in the western region, while there is no
apparent strong effect in the other regions. In the same time, the
yellowfin CPUE was reduced in the western area. These mirrored changes can
likely be attributed to the difference in the vertical extension of the
bigeye and yellowfin habitat; while the two habitats are overlapping,
bigeye are distributed deeper than yellowfin. |
Therefore, it seems possible to draw some coherent
pattern in the west. In the two western regions, El Niño (La Niña) events
would have a positive (negative) effect on bigeye CPUE, and conversely a
negative (positive) effect on albacore CPUE. The temperature habitats show
during El Niño events a vertical extension for bigeye and a raising of the very
narrow habitat for albacore in the west (between 10oN-10oS).
In the eastern areas, results are ambiguous, since there
is an apparent positive El Niño direct effect on yellowfin longline CPUE in the
central eastern box, while in the eastern box the CPUE increases when the depth
of isotherm 15oC decreases, i.e. during La Niña events. The
temperature habitat in the east shows a slight contraction and deepening of the
yellowfin vertical temperature habitat during El Niño events.
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Recruitment and (absolute) population abundance
Concerning skipjack, large fluctuation in stock size, i.e.
recruitment, is a major potential explanation for the CPUE fluctuations. It is
interesting to note that a recurrent pattern in CPUE time-series is a high peak
6 to 12 months following an El Niño event (Lehodey, in press). This is
reflected by the significant cross-correlation between SOI and the Japanese
purse seine skipjack CPUE time series for the western area (R= -0.314, lag 8
months). Skipjack of ~35 cm, (~9 months) are usually the first size class of the
purse seine catch in the WCPO. Therefore, this lag is coherent with a positive
effect of El Niño on the recruitment.  Since
the warm pool is the major spawning area of skipjack, a plausible hypothesis to
explain the ENSO effect on skipjack recruitment could be an increase of survival
rates of juvenile skipjack correlated to the increase of primary and zooplankton
production during El Niño events in this region. The effect will be delayed by
6 to 12 months, at which time skipjack are recruited to the fishery. The
mechanisms explaining this correlation are investigated with a spatial
environmental population dynamics model (SEPODYM). Recent simulations confirm
the impact of the ENSO variability with a positive (negative) effect of El Niño
(La Niña) events on the recruitment that is propagated into the stock in the
following two years.
[more on SEPODYM
recent developments...]
For the south Pacific albacore, a positive effect of La
Niña on the recruitment has been proposed on the basis of the estimated
recruitment by the length-based, age-structured population dynamic model
MULTIFAN-CL (Fournier et al. 1998). To test this hypothesis, a very simple
(quarterly-based) population model has been used with a recruitment directly
proportional to the SOI values (positive values during La Niña). Therefore,
artificial fluctuations of stock biomass due to the variation of the SOI values
are produced,  but without
consideration of the impact of fisheries. When compared to the albacore stock
biomass series provided by the Multifan-CL estimate (Fournier et al. 1998,
Hampton and Fournier, 2000), there is a spectacular agreement between the series
with a biomass slightly increasing in the 1960s, peaking in the mid-1970s, and
decreasing thereafter at a level lower than in the 60s. Given the extreme
simplicity of the model used to test the ENSO - recruitment relationship, this
result is really encouraging. Moreover, it is interesting to consider how the
changes in the frequency of ENSO events has thus been propagated in the
population structure suggesting a decadal change.
Indeed, there is a direct and opposite
correlation between the interannual ENSO and decadal PDOsignals and the
recruitment of these species that suggests a possible new regime for the
next coming years. Until 1976, cold La Niña events were dominating in the
tropical Pacific while the situation reversed from 1977 to day with less and
weaker La Niña events and strong more frequent El Niño events.
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Long-term Climate Change
(greenhouse warming)
Tuna distributions and abundance have been shown to
be sensitive to environmental variability (see above). Based on these findings,
we can also envision large impacts related to changes associated with the global
warming. Scenarios of climate change due to greenhouse warming used in several
coupled atmosphere-ocean simulations have suggested that the changes in the mean
state of the tropical Pacific Ocean would result in climate conditions similar
to present-day El Niño conditions with an increased interannual variability (Timmermann et al.
1999). Resulting scenarios could include increasing temperature, changes in the
illumination of the surface layer where photosynthesis takes place, increasing
stratification of the upper ocean, and changes in the oceanic circulation,
reducing the nutrient input in the euphotic layer. Some potential impacts of
these changes on the tropical Pacific fisheries for tunas include (anonym 2000) the extension
of present fisheries to higher latitudes, a decrease in productivity, mainly in
the eastern Pacific, increasing variability in the catches, changes in the
catchability of the different species, and increasing fishing pressure,
particularly on bigeye and yellowfin.
But a large uncertainty
remains, particularly with the change in the productivity of the western
equatorial Pacific, the impact on recruitment and spawning migrations, or the
connection with the extra-tropical areas. Nevertheless, in terms of scientific
investigations and predictions of the impacts of the global warming, encouraging
perspectives appear in the various scenarios proposed for the tropical Pacific
Ocean, mainly because the observations and the study of the present ENSO events
give us the ability to understand the mechanisms and interactions that link the
ocean and atmosphere, and also because considerable progress has been achieved
in the understanding of ENSO during the last decades. A challenge exists now to
integrate the numerous parameters related to the biology and fisheries in a
comprehensive framework. This "grand challenge" requires a
multi-disciplinary approach and a considerable scientific effort that can be
produced only through a large international scientific collaboration. Such an
initiative has been recently developed with the Oceanic Fisheries and Climate
Change Project (OFCCP) of the international GLOBEC programme, a component of the
International Geosphere-Biosphere Programme (IGBP), sponsored by the IOC and
SCOR.
OFCCP GLOBEC will investigate the effect of climate
change on the productivity and distribution of oceanic tuna stocks and fisheries
in the Pacific Ocean with the goal of predicting short- to long-term changes and
impacts related to climate variability and global warming. The ultimate goal of
the project is to conduct simulations with ecosystem models that include the
main tuna species, using an input data set predicted under a scenario of climate
change induced by greenhouse warming (Bopp et al., 2001) as defined by
the IPCC. This should lead to the first tentative understanding how greenhouse
warming will affect, at the ocean and global scales, the abundance and
productivity of marine populations in the pelagic ecosystem, focusing on the
major exploited species and fisheries, by a real coupling between atmospheric,
oceanic, chemical and biological processes. Potential feedbacks from the changes
in the pelagic ecosystem, and socio-economical consequences will be investigated
to propose adaptation measures for the future.
However, analyses of simulations based on
retrospective series of oceanic and fishing data sets are necessary intermediate
steps to increase the reliability in the predictive capacity of the models. In
particular, realistic prediction by the models of changes and fluctuations
observed at short (e.g., ENSO) and decadal (e.g., Pacific Decadal Oscillation,
PDO) time scales in the ocean ecosystem and the tuna populations are necessary
before prediction based on the global warming projection are incorporated. In
addition, diverse studies are needed to improve the parameterization (e.g.,
energy transfer from primary to secondary production), the modelling of key
processes (e.g., recruitment, movements, and feeding), and to validate the
results of the simulations. Four major components have been identified to
achieve these objectives (cf. details in Appendix 1): (i) monitoring the upper
trophic levels of the pelagic ecosystem, (ii) food web structure in pelagic
ecosystems, (iii) modelling from ocean basin to individual scale, and (iv)
socio-economical impacts. The modelling component has a pivotal role and
requires original approaches in modelling movement and spatial dynamics.
[more on OFCCP GLOBEC...]
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