ICES Journal of
Marine Science
ICES Journal of Marine Science (2013), 70(6), 1097– 1107. doi:10.1093/icesjms/fst065
The influence of oceanographic processes on jack mackerel
(Trachurus murphyi) larval distribution and population structure
in the southeastern Pacific Ocean
Sebastián Vásquez 1,2*, Marco Correa-Ramı́rez 3, Carolina Parada1,4, and Aquiles Sepúlveda 1
1
Instituto de Investigación Pesquera (INPESCA), PO Box 350, Talcahuano, Chile
Programa de Magı́ster en ciencias mención Pesquerı́as, Departamento de Oceanografı́a, Universidad de Concepción, Casilla 160-C, Concepción, Chile
3
Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaı́so, Valparaı́so, Chile
4
Departamento de Geofı́sica, Universidad de Concepción, Concepción, Chile
2
*Corresponding Author: Instituto de Investigación Pesquera (INPESCA), PO Box 350, Talcahuano, Chile. Tel: +56 41 292 0410; fax: +56 41 292 0411;
e-mail: [email protected]
Vásquez, S., Correa-Ramı́rez, M., Parada, C., and Sepúlveda, A. 2013. The influence of oceanographic processes on jack mackerel (Trachurus murphyi)
larval distribution and population structure in the southeastern Pacific Ocean. – ICES Journal of Marine Science, 70: 1097– 1107.
Received 17 April 2012; accepted 6 April 2013; advance access publication 27 May 2013.
The distribution of jack mackerel larvae in the main oceanic spawning area of the southeastern Pacific Ocean was investigated through three
consecutive spring bio-oceanographic surveys (2003–2005). In this study, otolith microstructure analysis revealed a spatial age gradient with
the smallest/youngest larvae specimens found primarily in the offshore area and the largest/oldest found in the coastal area, implying offshoreinshore larval drift. This suggests a connection between the oceanic spawning area and the historical coastal nursery ground (north of 308S).
In order to understand the oceanographic processes that drive this larval transport, we inferred circulation patterns from two data sources:
mesoscale eddy trajectories identified by applying the Okubo-Weiss parameter to satellite geostrophic currents, and 20 years of satellite tracking data of drifters. Our results showed that eddy trajectories lead to net northwestward offshore transport (the opposite direction of larval
connectivity). In addition, mean circulation associated with the subtropical anticyclonic gyre and recurrent energetic meandering structures
seem to be the major mechanisms driving the spatial dynamics of the early jack mackerel life history, determining a net transport to nursery
grounds. These mechanisms could play a key role in recruitment, which supports the continuity of the jack mackerel population.
Keywords: larvae, meandering currents, mesoscale eddies, Okubo-Weiss parameter, spatial recruitment dynamics, transport.
Introduction
The jack mackerel (Trachurus murphyi) is a medium-sized pelagic
fish of ecological and commercial importance to the region,
broadly distributed in the South Pacific Ocean. The overall distribution of the species in the southeastern Pacific Ocean (SEP) ranges
from the Chilean and Peruvian coasts to more than 2000 km off
the central coast of Chile (Cárdenas et al., 2009). The population
structure of jack mackerel, inferred from the distribution of eggs
and larvae, juveniles and mature females, consists of three main
habitats (Arcos et al., 2001; see Figure 1): (i) a nursery ground
located in the coastal zone off southern Peru and northern Chile
(north of 308S); (ii) a feeding ground located in the central-south
zone off Chile, where the recruitment of 2– 3 year-old individuals
occurs (308–408S); and (iii) an oceanic spawning area off central
Chile, extending up to 1800 km offshore during spring.
# 2013
The abundance of marine fish populations is highly dependent
on recruitment strength, which is largely determined by survival
rates during early life stages. The transport of fish eggs and larvae
from spawning areas to nurseries has often been cited as key among
the mechanistic processes affecting recruitment (Pineda et al., 2007;
Kasai et al., 2008). The connectivity between the spawning and
nursery grounds requires the transport of eggs and larvae to specific
locations where conditions for growth are favourable (Hare and
Cowan, 1993). Larvae transported to less productive areas experience
higher mortality rates than those transported to suitable nursery
grounds. This transport process is particularly relevant when
nursery grounds are distant from the spawning areas, as is the case
for the jack mackerel (Figure 1). The large distance (up to 1800 km)
separating jack mackerel spawning and nursery areas suggests the existence of an efficient transport mechanism for eggs and larvae.
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S. Vásquez et al.
well described (Cubillos et al., 2008). However, larval transport
and the oceanographic processes affecting early jack mackerel life
history are not fully understood. Due to the large distance and the
spatial uncoupling between jack mackerel spawning and nursery
grounds, deeper understanding is required of the physical mechanisms that force the transport of eggs and larvae that promote connectivity between the spawning and recruitment fractions of the
population.
The aim of this study is to attain an understanding of the spatial
structure of spawning and nursery grounds and to assess the likely
physical factors that drive successful transport from spawning to
nursery grounds. Therefore, this study will (i) describe jack mackerel
larval distribution based on data from bio-oceanographic cruises,
(ii) explore larval age-specific spatial patterns through microstructure analysis of daily deposition of sagittal otoliths, and (iii) characterize the oceanographic structures involved in larval transport.
Finally, we will discuss the effects of transport processes on the connectivity between jack mackerel spawning and nursery grounds and
the population structure.
Figure 1. Conceptual spatial model of the jack mackerel population in
the southeastern Pacific Ocean (modified from Arcos et al., 2001).
The oceanographic structures involved in larval transport can
affect larval survival and subsequent recruitment of pelagic fish.
Therefore, to understand interannual variability in stock abundance
it is essential to have a deeper understanding of the ways in which
oceanographic processes affect eggs and larvae trajectories from
spawning to nursery grounds (Kim and Sugimoto, 2002). The
spatial locations of jack mackerel spawning and nursery grounds
overlap with the anticyclonic subtropical gyre that characterizes
the regional circulation in the SEP. This ocean current system
encompasses the eastward South Pacific Current (SPC) from 30 –
408S, the Chile Peru Current (CPC) flowing equatorward along
the coast and the westward South Equatorial Current (SEC) north
of 258S (Chaigneau and Pizarro, 2005a). The SPC is an extension
of the East Australian Current and is related to the Subtropical
Convergence or Subtropical Front (STF) that separates relatively
warm and salty subtropical water from colder and fresher
Subantarctic water. The STF surface velocities are characterized by
a predominant flow towards the coast and high mesoscale activity
(Chaigneau and Pizarro, 2005a), which could promote ocean –
coast exchange. The jack mackerel spawning ground has been
strongly linked to the southward shift of the STF in spring –
summer (Evseenko, 1987; Cubillos et al., 2008). Significant mesoscale activity has been observed along the SEP in the Coastal
Transition Zone (CTZ) off south-central Chile (30– 388S) extending 600 km offshore (Hormazábal et al., 2004). This highly
dynamic area is characterized by distinctive mesoscale oceanographic features, such as mesoscale eddies and strong energetic
meanders that remain as coherent spatial structures for several
months, influencing exchanges (Leth and Schaffer, 2001;
Correa-Ramı́rez et al., 2007; Morales et al., 2010). Oceanic gyres,
large-scale meanders, jet currents and mesoscale eddies have been
observed as processes that influence the distribution of larval fish
(Watanabe, 1982; Nakata et al., 2000; Bruce et al., 2001; Logerwell
and Smith, 2001; Bakun, 2006; Atwood et al., 2010) and fish recruitment, and consequently they can modulate fluctuations in fishstock abundance (Watanabe, 1982, Nakata et al., 2000). The
spatial structure of the oceanic jack mackerel spawning has been
Methods
In this section we present the data and analyses used to describe the
spatial distribution of jack mackerel larvae, the larval age-specific
spatial patterns in the west–east direction and the oceanographic
processes that potentially connect spawning and nursery areas.
Field work: study area, sampling methods and larval
distribution
Information from intensive bio-oceanographic cruises in the
oceanic zone off central Chile was used to describe the distribution
of jack mackerel larvae. Jack mackerel larvae were sorted from planktonic samples, collected in the main spawning area during three research cruises using industrial fishing vessels in November–
December (the main reproductive season for jack mackerel) of
2003, 2004 and 2005. All the cruises aimed to evaluate jack mackerel
spawning biomass based on the daily egg production method.
Latitudinal survey transects were systematically allocated 37.0 km
apart, while plankton stations were sampled each 33.3 km along
each transect, covering over 850 000 km2 in less than two weeks
(Figure 2; Table 1). At each plankton station, larvae were collected
by vertical hauls with a WP-2 net (60-cm diameter, 303-mm mesh
size) from a depth of 100 m. All samples were fixed in a 10% buffered
formaldehyde/seawater solution neutralized with borated sodium.
Laboratory procedures: larval classification and ageing
Otolith microstructure analysis was used to describe larval age and
then to back-calculate spawning dates. Once in the laboratory, jack
mackerel larvae were identified following Santander and Castillo
(1971) and placed in 90% alcohol. The abundance was standardized
to a unit of sea surface (10 m2) based on the volume filtered and the
maximum depth sampled. All larvae were measured (to the nearest
0.1 mm) using an ocular micrometer. Length measurements were
not corrected for shrinkage. Jack mackerel larvae were aged by examining sagittal otolith microstructures, following Campana (1992).
Incremental deposition was assumed to be daily, based on previous
studies of congeneric species in which an age validation procedure
has been documented (Theilacker, 1986; Jordan, 1994; Xie et al.,
2005). Otoliths were removed from 100, 112 and 118 larvae in
2003, 2004 and 2005 samples, respectively. Otholiths were found
undamaged and growth rings were perfectly legible. The sagittal
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Influence of oceanographic processes on jack mackerel larvae
Figure 2. Study area located in the oceanic zone off central Chile, indicating the planktonic sampling stations carried out for 2003– 2005.
Table 1. Description of the surveys performed in the jack mackerel oceanic spawning area.
Year
Date
2003
2004
2005
11 Nov–21 Nov
20 Nov–29 Nov
22 Nov–01 Dec
Latitudinal
range
33800’ –38800’S
31840’ –38800’S
33800’ –38840’S
Inter-transect
distance (km)
37.0
37.0
37.0
otoliths were identified using a polarized light filter and removed
with fine dissecting needles, placing them onto glass slides and covering with DePex (a transparent mounting medium). Otolith increments were observed under a Nikon eclipse E200 binocular
microscope at a magnification of × 1000. The best-defined otoliths
(shape, sharpness and definition of the increments) were selected
and the number of increments counted in triplicate. When the
number of increments among the replicates differed by at least
three, the otolith information was discarded. Increment age (i.e.
number of increments) differs from true age by the number of
days between hatching and the formation of the first increment.
To adjust otolith ageing to the true larval age, three days were
added to the number of increments based on the first increment deposition after hatching data obtained for Trachurus symmetricus
(Hewitt et al., 1985), Trachurus declivis (Jordan, 1994) and
Trachurus japonicus (Xie et al., 2005) larvae. True larval age was
used in the calculations of growth rate, larval age assignments and
back-calculations of spawning dates. A Gompertz model was
Number
of stations
694
910
784
Positive
stations (%)
38.9
53.1
47.0
Study area
(km2)
871 179
1 385 613
1 222 143
fitted to the data, as a function of length to describe growth and estimate the age of jack mackerel larvae. Larval abundance was classified into age groups in order to assess age-specific spatial trends
(west–east direction) and later linked to oceanographic processes.
Environmental conditions and oceanographic features
Two sources of information were used to describe the circulation
patterns and oceanographic conditions in the study region:
namely satellite data and satellite-tracked surface drifters. Satellite
data of sea surface temperature (SST) and chlorophyll-a (Chl-a)
were obtained for each sampling period from the MODIS-Aqua
Level 3 Global Program (4 × 4 km of spatial resolution). Gaps
were filled with 3-D kriging (longitude, latitude, time) interpolation
(Marcotte, 1991, Correa-Ramı́rez et al., 2007). Wind regimes were
obtained from QuickScat Ocean Surface Winds derived from the
SeaWinds Scatterometer (IFREMER) with a spatial resolution of
27.8 × 27.8 km. In addition, geostrophic currents were obtained
from the AVISO program (http://www.aviso.oceanobs.com).
1100
Eddies with diameters up to 70 km were identified by applying
the Okubo-Weiss parameter (W) over the geostrophic current
field, which allows separating vorticity-dominated regions
(Isern-Fontanet et al., 2004). A threshold value of 1 × 10212 was
used to distinguish and isolate coherent eddy structures from the
background field, which is mainly associated with meander currents
flowing around eddies. Eddies older than one month were tracked
from 2003– 2005 to construct their trajectories. Finally, mean regional surface circulation was inferred using satellite-tracked drifters from 1979–2010. The surface satellite-tracked drifter dataset
is part of the Global Drifter Program/Surface Velocity Program
(see http://www.aoml.noaa.gov). A total of 225 drogue drifters
were used in the analyses covering the region from 70 –958W and
25 –458S. Drifter positions were reported at uniform 6-h intervals
by the Atlantic Oceanographic and Meteorological Laboratory in
Miami (Hansen and Poulain, 1996). Velocity components were calculated by a centred difference scheme. High-frequency variability
was removed by a daily averaging of the data, resulting in around
65 000 positions and velocities. Mean surface circulation was calculated by averaging the velocities on a 18 × 18 grid.
Results
Ageing and back-calculated spawning dates
Sagittal otoliths samples of 87, 102 and 106 larvae (out of 100, 112
and 118) were selected and processed for ageing in 2003, 2004 and
2005, respectively. The otoliths were round-shaped for all the analysed larvae. The daily increments were inferred, identifying alternating zones between translucent and opaque rings that form
clear and regular patterns located beyond the first ring associated
with the core. No obvious signal of subdaily increments was
observed. Larval age increments ranged from 2 –24 in 2003, from
2 –29 in 2004, and from 2 – 37 in 2005. The Gompertz model was
fitted to the data for all years, the results of which were highly significant (p , 0.01; Table 2). However, the larvae found during
2003 were younger and showed a better fit to a linear model than
those reported for 2004 and 2005. Consequently, to compare the
length –age relationship among all years studied, a linear function
was used for larvae , 25 days old (Table 2). Intercepts (a) and
slopes (b) were calculated by regression analyses. Slopes were
not significantly different among the years (ANCOVA, F ¼ 2.466,
p ¼ 0.12; Table 2), implying no significant differences in growth
rates. Spawning dates were calculated for aged larvae during the
2003–2005 spawning seasons. The timing of spawning was consistent among the years. In 2003, spawning was recorded over a 28-day
period (17 October to 13 November), with a peak in early
November. In 2004, spawning was recorded over a 41-day period
(17 October to 26 November, peaking in mid-November). In
2005, the spawning date distribution was similar to that observed
in 2004, ranging from 15 October to 26 November and also
peaking in mid-November (Figure 3).
Larval distribution
Surveys conducted in November of 2003, 2004 and 2005 are
described in Table 1. The spatial coverage of the surveys was lower
in 2003 than 2004 and 2005. The absolute number of larvae was
much higher in 2004 (2163) and 2005 (1798) than in 2003 (913).
Jack mackerel larvae were widely distributed in the entire survey
region (Figure 4a –c) with maximum abundances of 1379 (2003),
1698 (2004) and 2122 (2005) larvae 10 m22. Larvae were mainly
recorded in the surface strata with .80% of larvae captured in the
S. Vásquez et al.
first 50 m of the water column (not shown here, see Arcos et al.,
2004 for details). Larvae were predominantly distributed in
frontal waters associated with STF, with high densities up to
1000 km offshore (Figure 4a–c), decreasing in abundance toward
the coast, with the exception of 2003, where two peaks were detected
(Figure 4d– f). Jack mackerel larvae length ranged from 2.2 –8.2,
2.0 –10.0 and 1.9 –14.9 mm TL in 2003, 2004 and 2005 respectively
(Figure 4g– i). Larvae showed unimodal size distribution for all
three years, centred at 3.0 mm in 2003 and 2004, while in 2005 it
was centred at 3.5 mm. We explored an age-specific spatial
pattern based on the length–age relationship. There was a significant increment in larval age with inshore distance (R2 ¼ 0.7, R2 ¼
0.43 and R2 ¼ 0.3, for 2003, 2004 and 2005, respectively). The
oldest individuals were captured predominantly at the inshore
limit (600 km) of the study area, suggesting that the distribution
of that fraction of larvae extends to the coastal zone beyond the sampling region (Figure 5a–c).
Environmental conditions and oceanographic features
Wind-driven processes determine the Peru –Chile current system
(PCCS) where the jack mackerel grows and reproduces. Wind patterns during the jack mackerel spawning season were analysed.
The wind field showed marked anticyclonic circulation with a dominant component parallel to the coast (meridional). In the oceanic
zone around 388S, the zonal component showed high velocities
reaching values of 5 m s21, while in the coastal zone, the meridional
component reached velocities around 10 m s21 (Figure 6a–c). We
observed interannual variability in the wind field. The influence of
anticyclonic circulation was stronger in 2005 (Figure 6c) than in
2003 (Figure 6a) and 2004 (Figure 6b). SST was widely characterized
by the STF that separates cold Subantarctic waters in the south from
warm subtropical waters in the north. The STF extends southward
with a broad leading edge, with positions that are highly correlated
with the anticyclone southern boundary (Figure 6d–f). Alongshore
equatorward wind stress triggers intensive upwellings along the
coast, which in turn generate offshore Ekman transport.
Upwelling in the coastal region promotes a highly productive
zone, bringing cold and nutrient-rich water from depths below
150 m to the surface. A conspicuous coast–ocean gradient was
observed with high Chl-a concentration at the surface in the
coastal region (Figure 6g–i). Strong mesoscale variability was
observed in the region of oceanic and coastal water interaction,
which forms a CTZ. Mesoscale eddies, meandering currents and
filaments were observed as recurrent structures during the study
period, which seem to contribute in expanding the area of high
Chl-a concentrations beyond the upwelling area off central Chile
(especially in 2003 and 2004, Figure 6g and i, respectively).
The geostrophic currents, along with the vorticity field and the
eddy structures defined by the Okubo-Weiss parameter, were used
to identify the mesoscale eddies in the study area from 2003–
2005. Cyclonic (CEs) and anticyclonic eddies (AEs) were distinguished by the vorticity sign (2 or + respectively) in their centres.
To identify possible effects on the larval distribution and transport
process, the trajectories of all eddies with time spans longer than
two months were tracked following eddy rotational centres. On
average, 106 CEs (7.7 × 103 km2) and 105 AEs (7.9 × 103 km2)
were observed daily, travelling mainly westward at mean speeds of
1.41 km d21 (Figure 7a and b, respectively). These eddies showed residence times from 2 months to .1 year in the CTZ, travelling distances
of up to 1000 km offshore. Assuming a 200-m vertical extension and a
speed of 1.41 km d21, each eddy produces a mean offshore transport
1101
Influence of oceanographic processes on jack mackerel larvae
Table 2. Growth models used to describe growth schedules of larval jack mackerel during the 2003, 2004 and 2005 main spawning seasons.
Model
Gompertz
L(t) ¼ L0 exp (G (1 – exp (– gt)))
Year
2003
2004
2005
Linear
L(t) ¼ a + bt
2003
2004
2005
Parameter
L0 ¼ 2.195
G ¼ 2.168
g ¼ 0.032
L0 ¼ 2.218
G ¼ 3.233
g ¼ 0.019
L0 ¼ 2.281
G ¼ 2.816
g ¼ 0.027
a ¼ 1.831
b ¼ 0.218
a ¼ 1.671
b ¼ 0.2301
a ¼ 1.775
b ¼ 0.278
RSS
3.39
R2
0.950
p
,0.01
Error
0.202
12.03
0.958
,0.01
0.331
30.31
0.944
,0.01
0.511
3.42
0.958
,0.01
0.199
10.82
0.936
,0.01
0.321
27.79
0.897
,0.01
0.503
RSS ¼ residual sum of squares, R2 ¼ coefficient of determination.
Figure 3. Back-calculated spawning dates for Trachurus murphyi
larvae collected in the oceanic zone off central Chile during the main
spawning season of 2003, 2004 and 2005.
of 0.31 Sv (Table 3), promoting potential coastal–ocean exchanges
and oceanic retention.
The mean regional current circulation pattern was inferred
from the trajectories of surface drifters. Figure 8 shows a detailed
description of the currents, terms of velocity, direction and
strength over a broad area encompassing jack mackerel spawning
and nursery grounds. Surface currents were mainly oriented eastwardly with a progressive velocity increment (from south of 358S
and west of 808W) toward higher latitudes, reaching maximum velocities around 458S (23 cm s21). A secondary maximum is
observed at 388S and seems to be associated with the STF, with
mean velocities of the order of 18 cm s21. In the CTZ, east of 808W,
the circulation pattern showed a branch of northeastwardly current,
with increasing velocities from 378S (15 cm s21) to lower latitudes,
reaching a primary maximum between 288 and 328S with velocities
. 25 cm s21. The circulation derived from these surface drifter measurements showed a marked anticyclonic pattern that could promote
the connectivity between the oceanic and the coastal areas of the SEP
apparently related to jack mackerel larval distribution.
Discussion
The transport process of early life stages from spawning grounds to
suitable nursery areas has been recognized as a key factor in the
success of year classes of marine fish stocks. Therefore, oceanographic processes/features involved in larval dispersal and population connectivity are considered primary factors in larval fish
survival, and hence a main contributor to the recruitment
dynamic of marine fish populations (Watanabe, 1982; Hutchings
et al., 1999; Nakata et al., 2000; Bruce et al., 2001). Physical oceanographic processes influence the distribution of larval fish and invertebrates on a variety of scales, ranging from a few metres (Kingsford,
1990) to thousands of kilometres (Johnson, 1960; Cowen, 1985;
Scheltema, 1986; Victor, 1986; Newman and McConnaughey,
1987), and their importance for population connectivity increases
as the spawning zone is spatially uncoupled from the nursery
grounds (Cowen et al., 2007).
Although jack mackerel spawning has been recorded in oceanic
waters all along the South American coast between 78S and 418S
(Evseenko, 1987; Serra, 1991), egg densities in oceanic waters off
central Chile are higher than those reported off Peru (Santander
and Flores, 1983; Gorbunova et al., 1985; Muck et al., 1987) and
northern Chile (Gretchina et al., 1998). Thus, the surveyed area in
oceanic waters off central Chile can be considered the most important spawning area of jack mackerel. On the other hand, according to
Elizarov et al. (1993) and Gretchina (1998), high abundance of
young jack mackerel of ,25 cm fork length have been observed
almost exclusively inhabiting warmer waters in the coastal region
to the north of 308S, which constitute the only known nursery
ground. Consequently, offshore early life history strategies of jack
mackerel, and the major distance separating the main jack mackerel
spawning zone from the nursery ground, imply that efficient physical mechanisms are required to transport eggs and larvae from the
oceanic to coastal zones to reduce losses or expatriation of larvae out
of the nursery grounds. In addition, the consistent interannual
pattern of offshore larval distribution for T. murphyi off central
Chile, the significant inverse relationship between age and distance
from shore, and the lack of larvae younger than 20 days old in the
inshore sampling boundary, suggest a regular strategy of early life
history transport. Therefore, the identification of the physical processes becomes a major challenge in understanding the mechanisms
controlling jack mackerel larval dispersal and ultimately, its effect on
recruitment strength variability.
The jack mackerel spawns over a consistent period from
October–December, peaking in November, during the austral
1102
S. Vásquez et al.
Figure 4. Larval distribution of Trachurus murphyi during the main spawning season of (a) 2003, (b) 2004, and (c) 2005. Integrated jack mackerel
larvae density (ind m22) in (d) 2003, (e) 2004, and (f) 2005. Length frequency of jack mackerel larvae sampled in November –December of (g) 2003,
(h) 2004, and (i) 2005.
spring (Gretchina et al., 1998; Oyarzún et al., 1998). Back-calculated
spawning dates from larvae captured during November 2003, 2004,
and 2005 revealed consistent spawning between mid-October and
November. The narrow range in estimated spawning dates may be
explained by an underestimation of the abundance of large larvae,
considering that larvae older than 25 days have enhanced swimming
and net avoidance capabilities. Furthermore, in this study larvae
were collected by vertical hauls that presumably are less effective
than other gear (oblique tows) in capturing large larvae. Despite
the bias created by net avoidance by larger larvae, the data suggests
that the spatial age-distribution differences are related to an
offshore-inshore transport process.
The high degree of heterogeneity observed in the larval spatial
distribution of jack mackerel is probably the result of several processes, including ecological interactions, the population dynamic
of the spawning fraction, behaviour, and physical processes
(Huntley et al., 2000). We focused on the potential effect of oceanographic processes on jack mackerel larval distribution under two
scales: (i) the regional scale of mean surface circulation estimated
from tracked drifters deployed in the SEP, and (ii) the mesoscale,
characterized by the trajectories of mesoscale eddies and their duration based on satellite identification of eddy-like structures.
The regional surface circulation pattern, based on the trajectories
of satellite-tracked drifters, provides an inshore mechanism for jack
mackerel larvae. Once inshore, the behaviour of these drifters suggests that surface circulation patterns also facilitate the retention
of jack mackerel larvae within the coastal zone and provide an efficient mechanism to bring them to the historical nursery ground
situated in northern Chile and southern Peru. Circulation derived
from surface drifter measurements is strongly influenced by winddriven currents (Chaigneau and Pizarro, 2005a). In the SEP the
wind stress has a pronounced seasonal signal. On the large scale,
wind variability over the SEP originates from the meridional movement of the Eastern Pacific Subtropical Anticyclone (EPSA) from
approximately 328S during the austral summer to 288S during the
austral winter (Karstensen and Ulloa, 2008). Wind fields confirm
the presence of the EPSA, with moderate interannual variability in
its location, beginning with a marked eastward wind pattern
through the oceanic zone and turning northward in the coastal
region. The EPSA is the dominant physical feature in the SEP
and encompasses the westward SEC north of 258S, the SPC from
30 –408S, and the CPC flowing equatorward along the coast
(Chaigneau and Pizarro, 2005a). The SPC is related to the
Subtropical Convergence or STF that separates relatively warm
and salty subtropical water from colder and fresher Subantarctic
water. The jack mackerel spawning in the STF has been confined,
extending from the Chilean coast to between 150 and 1608W
(Evseenko, 1987; Bailey, 1989; Elizarov et al., 1993; Cubillos et al.,
2008). This evidence is consistent with our results since the main
bulk of jack mackerel larvae was strongly associated with the presence of the STF (observed in SST imagery) for all three years
analysed. In addition, back-calculated spawning dates were consistently associated with the seasonal variability of the location of the
STF, suggesting that jack mackerel reproductive activity is highly
correlated temporally to this oceanographic feature. On the other
hand, the surface current velocity in the study region increased significantly in relation to the STF (around 378S) oriented eastwardly
with mean values of 18 cm s21. Considering this mean current
Influence of oceanographic processes on jack mackerel larvae
Figure 5. Relationship between jack mackerel larval age and distance
offshore in (a) 2003, (b) 2004, and (c) 2005. Bars indicate the standard
deviation for each age-class.
velocity, the time frame for the movement of drifters between the
oceanic zone and the CTZ (about a month and a half) is strongly
related to age differences observed for jack mackerel larvae from
the two areas. In addition, the results are consistent with what was
reported by Parada et al., (2010), who evaluated connectivity
between jack mackerel spawning and nursery grounds through a
biophysical modelling approach.
Mesoscale eddies have been recognized as a potentially important pelagic habitat for fish eggs and larvae in the ocean (Bakun,
2006), enhancing primary and secondary production (Huntley
et al., 2000; Teira et al., 2005) and consequently providing improved
feeding habitats. Eddies can retain fish larvae near their spawning
areas (Lobel and Robinson, 1986) in favourable feeding environments (Logerwell and Smith 2001), or provide mechanisms to transport them to suitable nursery grounds (Sponaugle et al., 2005). In
our study, most of the eddies generated in the CTZ off Chile
ranged from two to several months in age, coinciding with the features previously discussed (Correa-Ramı́rez et al., 2007). Eddy trajectories (cyclonic and anticyclonic) revealed a marked pattern in
their displacement, travelling mainly westwardly and promoting
significant regional offshore transport. Seaward eddy propagation
may drive the major offshore transport of coastal waters (Leth and
Shaffer, 2001). The trajectories, recurrence and origin of eddies
associated with the CTZ off central Chile described in this study
1103
are consistent with the findings of Chelton et al. (2011). Recently,
Correa-Ramı́rez et al. (2007) studied the association between mesoscale eddies and Chl-a concentrations and concluded that these features seem to account for 50% of winter Chl-a in the CTZ off central
Chile as a result of advection or in situ generation (eddy pumping).
This links the upwelling system with oceanic waters and could
extend the high productive coastal zone further offshore. In parallel,
this physical–biological mechanism was recently verified by
Morales et al. (2010), who noted that some copepod species
common to shelf waters display relatively high densities in adjacent
oceanic waters. The extension of their distribution is the result of an
initial entrainment of waters containing copepod species, and the
offshore propagation of mesoscale eddies contributes significantly
to expanding the area of high biological production. Coastal water
intrusion and eddy propagation bring nutrient-rich waters from
the shelf to adjacent oceanic waters. This enrichment seems to
result in enhanced primary productivity. Cyclonic eddies entrain
coastal zooplankton (i.e. copepods) from the shelf to oceanic
waters. As these eddies propagate northwestwardly, a succession
of zooplankton communities can be driven by enhanced primary
productivity (Govoni et al., 2010). This extension of zooplankton
distribution may promote jack mackerel larval survival by providing
an exceptional and more continuous food supply for larvae that are
spawned in the oceanic zone off central Chile and transported to the
coast. Nevertheless, jack mackerel larvae that are entrained into
eddies could be transported offshore and consequently lost from
nursery grounds in the coastal zone. This scenario could promote
oceanic retention and under suitable conditions could generate an
oceanic habitat for juvenile jack mackerel (Parada et al., unpublished data).
Recent studies have demonstrated that larval orientation with
respect to the current direction (Staaterman et al., 2012) and vertical
behaviour of larvae (Grioche et al., 2000; Vikebø et al., 2007; Parada
et al., 2008) has an impact on larval drift trajectory and environmental conditions, which in turn affects final larval destination, growth
performance and survival. The role of jack mackerel larval behaviour in the SEP (i.e. diel or ontogenetic vertical migration, horizontal orientation) and its association with horizontal and vertical
current flows remain unclear. No information on horizontal movement patterns is evident for this species (as has been reported for
other species of perciform fish and some reef fish. See Paris and
Cowen, 2004, and Leis, 2006 for example). Trachurus larvae have
been observed in the upper 50 m above the seasonal thermocline,
with abundance peaks in the depth range of 10 –30 m (Coombs
et al., 2001). Vertical distributional ranges of Trachurus murphyi
larvae in the SEP are mostly in the first 50 m (Arcos et al., 2004),
but no records of vertical movement have been found. Although
little information is available, based on vertical oceanographic structures of the jack mackerel larvae habitat, their swimming speeds, and
the distributional larval ranges, we can postulate how potential
vertical behaviour may impact final larval destination. Firstly,
since coastal eddies propagate northwestwardly with significant
mass transport, extending vertically down to around 2000 m
(Chaigneau and Pizarro, 2005b), it can be expected that larvae
trapped in eddies never reach the coastal nursery area. Despite this
apparent mechanism of larval loss, surface circulation patterns are
persistently northeastward, which promotes potential ocean –
coastal exchanges and facilitates the transport of jack mackerel
larvae. This passive mechanism, which promotes onshore coastal
transport in the oceanic zone, can be enhanced or reduced if
larvae have a directed behaviour, especially considering that
1104
S. Vásquez et al.
Figure 6. Distribution of (a–c) satellite surface wind (in m s21), (d–e) sea surface temperature (in 8C), and (g–h) surface chlorophyll-a (in mg m23)
during the period of each cruise (2003, 2004 and 2005, respectively). The geostrophic velocity is superimposed on chlorophyll images.
Figure 7. (a) Cyclonic, and (b) anticyclonic eddy trajectories inferred from the Okubo-Weiss parameter with a time span longer than two months,
from January 2003 to December 2005.
Trachurus larvae seem to increase their swimming speed during ontogeny (Masuda, 2006). On the other hand, the vertical structure of
the current around the spawning area presents a consistent coastal
orientation that decreases vertically at around 200 m (S. Vasquez,
unpublished data). Since jack mackerel larvae are found in
shallow waters, we can expect overall larval transport to be in that
direction. However, once larvae are successfully transported to
coastal upwelling regions characterized by near-surface offshore
transport, vertical larval movement, which has been identified for
other species in upwelling regions as a mechanism that enhances
coastal-area retention (Grioche et al., 2000; Emsley et al., 2005),
can substantially change the final destination of individuals.
However, for SEP jack mackerel larvae, this mechanism is only
speculative. Field and laboratory studies need to be conducted to
1105
Influence of oceanographic processes on jack mackerel larvae
Table 3. Basic statistics for eddies in the oceanic zone off central
Chile (25800’ – 42800’S and 71800’ – 95800’W) in 2003, 2004 and 2005.
Parameter
Number (d – 1)
Area (310 – 3 km 2)
Vorticity (310 – 4 s – 1)
u speed (km d – 1)
v speed (km d – 1)
Speed (km d – 1)
Azimuth (º)
Transport (Sv)
Cyclonic eddies
Anticyclonic
eddies
Mean
106.10
7.67
3.73
1.09
0.66
1.40
253.16
0.306
Mean
105.99
7.90
3.86
1.11
0.67
1.42
252.79
0.314
Std
6.34
4.98
1.67
0.87
0.63
0.90
79.09
0.095
Std
5.91
5.09
1.73
0.82
0.63
0.87
78.90
0.097
Transport is in Sv units (1 Sv ¼ 106 m23 s21) and was obtained at 200 m
(conservative depth).
Figure 9. Summary of larval advection processes for Trachurus
murphyi in the southeastern Pacific Ocean.
Figure 8. Surface velocities (in cm s21) and velocity variance ellipses
from drifter measurements.
improve our knowledge about directed swimming abilities, orientation patterns and vertical behaviours. This is an interesting unresolved question that could be addressed using biophysical
modelling tools and data collection to disentangle the role of physical features and active behaviour and swimming abilities in overall
larval distribution.
The jack mackerel is a highly migratory transregional pelagic
species with broad distribution in the SEP, from the Chilean and
Peruvian coasts to beyond 2000 km from the central coast of Chile
(Gretchina, 1998). There are conflicting interpretations of the
population structure and number of populations of T. murphyi in
the SEP. Serra (1991) proposed that the jack mackerel is structured
into two self-sustaining populations within the SEP, one located in
Peruvian waters and the other off Chile, the oceanic fraction being
included in the latter. On the other hand, according to Arcos et al.
(2001) the jack mackerel constitutes a single population widely distributed in the South Pacific Ocean. They described it as a straddling
stock with several subpopulations that constitute different fishery
units. Recently, genetic analyses using mitochondrial DNA and
microsatellite techniques of adult specimens of jack mackerel collected from across the Pacific Ocean (Chile, New Zealand and
open ocean) indicated no genetic differentiation in the Chilean
jack mackerel T. murphyi across its entire distribution range
(Cardenas et al., 2009), supporting the hypothesis that a single
population of T. murphyi inhabits the entire South Pacific.
Furthermore, the results suggest the absence of the genetic structure
that can result from the long-distance dispersal of the early development stages. The latter involves the seasonal offshore migration of
the adult fraction to spawn in oceanic waters (austral spring), and
a recruitment process in the coastal region dependent on the advection of jack mackerel from the oceanic area to coastal nursery
grounds. Oceanic jack mackerel spawning behaviour, combined
with the regional surface circulation, provides possible physicalbiological mechanisms to explain offshore-inshore larval transport
indicated by otolith microstructures. Our results support the hypothesis that the juvenile fraction observed in northern Chile and
southern Peru originates in the oceanic zone off central Chile and
is transported to the coastal nursery ground, thus modulating
annual recruitment and establishing a single population of
T. murphyi in the SEP (Figure 9).
In summary, the transport process of T. murphyi in waters off
Chile appears to be linked to mean surface circulation (as a
product of geostrophic flow and wind-driven currents) rather
than eddy self-propagation transport from the CTZ. Mean surface
velocities consistently explain the age heterogeneity in the spatial
distribution of jack mackerel larvae. On the other hand, mesoscale
processes (i.e. mesoscale eddies and meander currents) may influence early jack mackerel life stages by modifying the dispersal
pattern, generating oceanic retention zones and/or providing
larval food supply through the extension of the productive coastal
upwelling area. Larval distribution and advection processes
support the single population hypothesis suggested by genetic analyses. Since recruitment success seems to be in essence a problem of
biophysical coupling, it is necessary to conduct research that incorporates these aspects dynamically, such as implementing biophysical models that allow testing connectivity hypotheses for this
population.
1106
Acknowledgements
The authors thank the collaborative effort from the fishing industry
for the development and execution of this research program. Thanks
also to the captains and the crew of each of the fishing vessels who
participated in the research program and to the technical staff of
the Instituto de Investigación Pesquera for their work during the different stages of the program, particularly P. Ruiz, C. González and
S. Núñez. Special thanks to Pablo Sangrá for providing guidance
in the buoys analyses. Thanks also go to Billy Ernst for reviewing
the results of this study.
Funding
Research cruises carried out for this study were supported by the
“Fondo de Investigaciœn Pesquera (FIP)”, through grants FIP
2005-11, FIP 2002-12 and FIP 2004-33 (www.fip.cl). SV was partially supported by a CONICYT Masters fellowship. MC-R was funded
by postdoctoral project FONDECYT 3110173. CP was funded by
CONICYT project 78090007 entitled “Modelaciœn biofı́sica del
jurel en el Pacı́fico suroriental y su impacto en la actividad pesquera
chilena”.
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Handling editor: Claire Paris