Download Oceanography of the WCPO and its influence on tuna dynamic

Oceanography of the WCPO and
its influence on tuna dynamics
How do oceanographic and climatic processes impact upon tuna
fisheries
(and stock assessment)?
Overview
1.
Introduction
2.
Basic Principles of Physical Oceanography
•
3.
4.
Oceanography of the Pacific
•
Currents, Warm pool/cool tongue system
•
ENSO
Relationship between oceanographic processes and fish populations
•
5.
Ocean properties, movement, productivity
Survival, Growth, Movement, Recruitment
Oceanographic impacts on fisheries
•
Catchability and catch rates
•
Movement and distribution
6.
Implications for stock assessments
7.
Climate change
Introduction
•
Oceanographic and climatic factors influence the distribution
and abundance of pelagic fish (through impacts on
recruitment, growth and mortality), and subsequently, the
distribution and activity of the fisheries that target them
•
Understanding the relationship between fish
abundance/distribution and oceanographic and climatic factors
can provide fishers, managers and scientists with an better
understanding of fishery variability.
•
This may allow management and development plans to
consider fluctuations in fish biomass and availability.
•
With climate change, understanding these relationships may
become even more important.
Basic principles
•
There are four key features of the ocean about which we need to be
aware if we are to understand how fish populations and fisheries are
influenced by the ocean:
•
The oceans are not uniform water masses but have a physical structure,
both vertically and horizontally
•
This structure results from spatial differences in the properties (e.g.
temperature, salinity, water pressure and other factors) of the oceans
water
•
Oceanic waters are constantly moving and this movement can be
horizontal (e.g. wind driven surface currents) or vertical (e.g. upwellings
or downwellings).
•
The properties, structure and movement of oceanic waters are strongly
influenced by climatic and atmospheric processes, and conversely have
a strong influence upon these…ocean and atmosphere are a coupled
dynamic system
What are the key properties and
structure of oceanic waters?
Key properties of sea water - Temperature
•
The temperature of the oceans varies by latitude
•
Ocean surface temperature strongly correlates with latitude because
insolation, the amount of sunlight striking Earth’s surface, is
directly related to latitude, and is highest in the tropics, hence
tropical waters are warmer.
Key properties of sea water - Temperature
Warmer waters are generally less dense than cooler water and
therefore “sit on top” of the cooler waters (i.e. so temperature
decreases with depth). In tropical and subtropical waters, there is a
thermocline (depth at which rapid temperature change)
Surface Mixed Layer
(warm)
Deep Layer
(cold)
Key properties of sea water - Density
•
•
•
•
The density of oceanic waters varies by depth
Low density waters (due to heating and precipitation) lie at the surface, denser waters below.
Pycnocline is a rapid change in density with depth (similar principle to thermocline)
The pycnocline is transitional between the surface and deep layers
• In the low latitudes (tropics), the pycnocline coincides with the thermocline.
• Surface water in high latitudes cools, becomes dense, sinks (convects) to the sea floor
and flows outward (advects) across the ocean basin.
(warmer, less salty and less dense)
Source: http://www.tulane.edu/~bianchi/Courses/Oceanography/
Ocean water movement
•
The waters of the ocean are constantly in motion….why is this?
•
There are different types of ocean water movement, including:
• horizontal currents, gyres and eddies, and
• vertical upwellings and downwellings.
•
These movements are caused by two main factors:
• Gravity
• Wind
•
The following section will discuss how these factors drive the
movement of water in the ocean
Ocean movement – Horizontal currents
High pressure
(Cooler air)
Solar radiation and wind
creation
Low pressure
(Warmer air)
High pressure
(Cooler air)
Ocean movement – Horizontal currents
• Wind is created to stabilize the atmospheric pressure and moves from
the high to low pressure area
• Wind-driven currents - As wind moves across the water, it drags on the
water. Water moves at about 3-4% of the wind speed.
Ocean movement – Currents
Eddies
Surface
Mixed Layer
Surface currents
Gyres
Coastal
Upwelling/
Downwelling
Seamount/
Ridge
Upwelling
Subsurface
currents
Divergence based
upwelling
Convergence/
Gravity based
downwelling
Oceanographic and Primary production
Primary production refers to the amount of inorganic C (mainly carbon
dioxide) converted to organic C (e.g. simple sugars) by microscopic
algae in a process known as photosynthesis (photo – (sun) light;
synthesis = to make something)
Primary producers represent the base of the oceanic food chain and their
abundance is critical to the abundance of animals higher in the chain
Oceanography and Primary
production
Low productivity in the gyres, due to
downwelling (opposite of process
needed to bring nutrients to surface)
Source:
http://www.tulane.edu/~bianchi/Cours
es/Oceanography/
Oceanography and fish populations in
the Pacific
……the following sections are going to discuss:
1. Some of the key features of oceanographic and climate
processes (and ocean habitats) of the Pacific Ocean in more
detail.
2. What we currently know about the relationship between
oceanographic processes and the key species targeted by
pelagic fisheries in the Pacific
3. The implications of these relationships for the fisheries targeting
them, and,
4. For assessments of their status and subsequent management of
the fisheries.
Pacific Ocean - Surface currents
Three major current features are the North Pacific Subtropical Gyre, the South
Pacific Subtropical Gyre, and the equatorial currents
The strength and direction of the equatorial and subequatorial currents is
dependant on the prevailing winds and climatic conditions
60o
Major shifts occur in currents
due to changes between
South East Trade Wind
and North West
Monsoon seasons
warm pool
cold tongue
Subarctic Gyre
convergence
40o
KUR
divergence
Subtropical Gyre
20o
NEC
NECC
0o
SEC
SECC
20o
EAC
Subtropical Gyre
HBT
40o
60o
120o
140 o
160 o
180o
160 o
140o
120 o
100 o
80o
The strength and direction of
the wind driven currents
play a major role in the
location and size of the
warm-pool/cold tongue
convergence zone.
Pacific Ocean – Warm Pool / Cold Tongue
Western equatorial Pacific - low primary production, extreme uniformity of high sea surface
temperatures (SST) (up to 28° C year-round). This water mass is referred to as the
“warm pool”.
In the eastern and central Pacific, wind driven movement of currents along the equator
creates an upwelling that extends westward from South America…this feature is called
the “cold tongue”. These two water masses meet at the “convergence zone”.
Convergence zone
Warm pool
Cold tongue
Pacific Ocean - ENSO
El Nino conditions – expansion of the warm pool eastwards, resulting in warmer than
average waters in the central and eastern Pacific, higher rainfall in that region, a deepening
of the thermocline in the east and rising of thermocline in the western region, and cooler
than average waters in the western Pacific.
Neutral conditions, and
La Nina conditions –characterised by stronger Pacific Trade winds, the contraction of the
warm pool into the equatorial western Pacific, higher rainfall in the western Pacific and lower
rainfall in the eastern Pacific, a deepening of the thermocline in the west and rising of
thermocline in the eastern region.
Pacific Ocean – Inter-annual Primary Production
During El Nino conditions, the Eastern upwelling is suppressed, but coastal upwelling and
productivity in the far western area (PNG, Phillipines, Palau) can be enhanced.
During La Nina, upwelling induced primary production is enhanced in the equatorial Eastern
pacific and brought by wind driven surface waters across to central and Western Pacific, which
at the same time diverge north and south of the equator.
Sea Surface
Temperature
El Nino
(Jan 98)
La Nina
(Jan 99)
Chlorophyll a
Summary – Oceanography of the Pacific
There are three key and interacting features of the Pacific
ocean-climate system that have a large influence on the
distribution and abundance of the target tuna species.
These are:
1. The direction and strength of the major surface
currents,
2. The size and location of the warm-pool-cold tongue
interaction, and
3. The overall influence/interaction of prevailing climatic
conditions (in particular ENSO on these).
These are play a large role in fishing success, as will be
see in following sections
How are oceanographic and climatic
processes relevant to fish population
dynamics and stock assessment in
the Pacific?
WCPO tuna stock assessments take account of:
1. Growth
2. Recruitment
3. Survival
4. Movement
5. Catch rates (Abundance indices)
Oceanographic/environmental impacts on
fish populations – Growth
What environmental factors impact GROWTH?
1.
2.
3.
Primary production (Food availability)
Water temperature (Metabolic rate)
Current/turbulence (Energy expenditure ex: holding position in water column)
Oceanographic/environmental impacts on
fish populations – Recruitment
What environmental factors impact RECRUITMENT?
1.
2.
3.
4.
5.
6.
Primary production/Food availability (Parental condition, larvae survival)
Water temperature
Current speed (turbulence) and advection
Salinity
Oxygen
Predation/cannibalism (of eggs, larvae)
Oceanographic/environmental impacts on
fish populations – Survival
What environmental factors impact SURVIVAL?
1.
2.
3.
4.
5.
6.
Primary production/Food availability
Water temperature
Current speed and advection
Salinity
Oxygen
Predation
Impacts vary depending on stage of development
•
Oceanography impacts on fish populations
Numerous oceanographic factors have a significant influence
on the recruitment, growth and survival of pelagic fish
species; the degree of influence often varies between
different life history stages
Spawning and
fertilisation
Adults
Eggs
Maturation
Hatching
Larvae
Survival/growth
Juvenile stages
Survival/growth
Oceanographic/environmental impacts on
fish populations – Movement
1- Horizontal Movement
2- Vertical Movement
Oceanography impacts on fish populations - SKIPJACK
Horizontal movement
Displacements of tagged skipjack
tuna during representative El Nino
(top) and La Nina (bottom) periods.
Thick arrows indicate the direction
and magnitude of displacement of the
skipjack CPUE gravity centre during
the tag recapture periods.
El Nino period
From (Lehodey et al, 1997)
La Nina period
Skipjack tuna and climate/oceanographic processes– Basin wide and EEZ impacts
2000 (+)
2002 (-)
Changes in the depth of the thermocline also impacts
on catchability of fish by longline fisheries
Bigeye and yellowfin vertical movements and longline
susceptibility
BET CPUE higher on
deep sets (in WCPO)
5S
50
YFT CPUE higher on
shallow sets
5N
150
Catch rates depend
on depth of longline
gear relative to the
thermocline depth.
15N
15S
250
Yellowfin
predominantly above
thermocline, bigeye
predominantly below
(during the day)
140E
5
10
160E
15
20
25
180
30
160W
140W
120W
Climate related changes in habitat volume in the Pacific
There are implications of habitat volume and variation in species
horizontal and vertical movements for interpretation of CPUE data and its
use in stock assessments. The effect of increasing/decreasing
thermocline depth on catch rates, for example, is very much species
specific, and also dependant on the gear type being used.
Impacts on Stock Assessment
The stock assessments conducted by SPC have 3 aspects that might be impacted
by oceanographic and climatic processes:
1.
Calculation of CPUE index – as an index of abundance to which the
model is fitted, is a critical component of stock assessments. Variation in
oceanographic processes effect catch rates of pelagic tuna due to:
a. Changing the volume of the species habitat (effects their density)
b. Effecting the ability of fishers to target their fish appropriately
2.
Incorporation of movement –tuna might move into/out of stock
assessment regions in response to oceanographic/climatic shifts. Stock
assessment models need to incorporate variation in fish movement and
migration that occurs due large scale oceanographic and climatic changes
3.
Recruitment Estimation – the least easily estimated but most important
biological factor in SA. Variations in recruitment can be very substantial and
are predominantly due to environmental processes. Current efforts are
attempting to use oceanographic analyses to predict recruitment to feed into
stock assessment models.
Climate Change
Climate Change
Fact: Increases in global average sea temperature have been observed. Sea
surface temperatures affect the patterns in atmospheric pressure, which in
turn are responsible for wind generation.
Hypothesis: Changes in wind generated surface currents would not only
modify the weather conditions but also alter the timing, location and
extent of the upwelling processes upon which much oceanic primary
productivity is reliant. Some studies suggest that primary productivity in
tropical oceans would decline due to increased stratification between
warmer surface waters and colder deeper water (and consequent
reduction in upwelling) (Bopp et al., 2001), but further research is
required.
Implication: Decline in the upwelling system of the central and eastern
equatorial Pacific may lead to reduced productivity that is normally
advected westwards and upon which pelagic fish stocks depend. This
decreasing production could lead to a decline in tuna abundance
Climate Change
Fact: El Nino events appear to have become more frequent in recent decades,
(possibly linked to climate change) and are associated with an eastward
shift of major tuna resource (e.g. skipjack) in the WCPO.
Hypothesis: Climate change may imply more permanent El Niños, which are
likely to increase the annual fluctuations of the spatial distribution and
abundance of tuna. First simulations of global warming on skipjack
suggest a global improvement of its habitat conditions east of the date
line and a spatial redistribution of this species to higher latitudes (Loukos
et al., 2003).
Distant water fishing fleets should be able to adapt to changes in the spatial
distribution and abundance in tuna stocks. But domestic fleets would be
vulnerable to fluctuations of tuna fisheries in their Exclusive Economic
Zones.
Climate Change
Uncertainty remains on the change in the productivity of the western equatorial
Pacific. The impact of climate change on tuna recruitment and spawning
migration is also poorly known.
Current thinking is that climate change may lead to a shift in the spatial tuna
distribution, as well as possible changes in total abundance and total catch in
different regions.
Session Summary and Conclusions
The ocean is not a homogenous uniform water mass. We have seen that
it moves in a complex manner, and that it has different properties
(of temperature, salinity, density) in different areas and at different
times.
These sets of conditions constitute mobile, sometimes transient, but
differing habitats within the ocean
Target tuna species have different habitat preferences (which vary over
life history), and different behaviours evolved to exploit these
habitats.
Because each species differs in its relationship to the ocean
environment, each species will be impacted differently by large
scale changes in climate and oceanography.
Session Summary and Conclusions
The impacts of oceanographic phenomena on species recruitment, survival
and growth have very large implications for population dynamics and
status, irrespective of the fisheries impacts.
Furthermore, the movement of tuna, as a result of changes in
oceanography, have significant implications for catchability of these
species.
It is imperative that we understand the relationship between oceanographic
and climatic phenomena and fisheries catches for ensuring more
reliable assessments, and for ensuring management decision making
processes that take account of this.
THANKS !
ENSO Variability
Time series of skipjack biomass is lagged by 8 months  potential for
forecasting
Impacts of Climate Change
1950
1950
2000
2000
2050
2050
2099
2099
Skipjack
Bigeye
Change in adult distribution
Using IPCC Scenarios to force SEAPODYM