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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.68 n.2 supl.TIIProc Concepción  2004

http://dx.doi.org/10.4067/S0717-65382004000300023 

  Gayana 68(2): 444-449, 2004

RELATING SPATIAL DISTRIBUTION OF CHILEAN JACK MACKEREL EGGS AND ENVIRONMENTAL FACTORS IN THE OCEANIC WATERS OFF CHILE

 

Sergio Núñez E.1, Jaime Letelier P.2,3, David Donoso Q.4, Aquiles Sepúlveda O.1 & Dagoberto Arcos R.1

1. Instituto de Investigación Pesquera. P.O. Box 350, Talcahuano, Chile. snunez@inpesca.cl
2. Departamento de Oceanografía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
3. Departamento de Física de la Atmósfera y del Océano, Universidad de Concepción, Casilla 160-C, Concepción, Chile
4. Pontificia Universidad Católica de Valparaíso. Casilla 1020, Valparaíso, Chile.


ABSTRACT

Chilean jack mackerel (Trachurus murphyi) is a highly migratory pelagic species that inhabits the Southern Pacific Ocean, constituting the most important fishery for Chile. This species exhibits an onshore migration during the summer related to coastal food availability, and an offshore migration towards reproductive oceanic areas (beyond Chilean EEZ) in early spring. During the spawning peaks (November) of 1998, 1999, 2000 and 2001, jack mackerel eggs were collected from four systematic surveys using 8-10 fishing vessels, deployed in the spawning area (76-92 W) off central Chile. Spatial distribution of eggs was modeled by geostatistical techniques. Environmental information including SST, wind, sea level anomalies and chlorophyll-a, as well as SST gradients, turbulence and currents were calculated.

Exploratory results suggest a higher egg density related to warmer waters (16-19 C), moderate winds (4-8 m s-1) and low currents (< 15 cm s-1) as well as SST gradients (< 0.3 C 10 km-1). Statistical analysis developed with the purpose of providing evidence of habitat-species association, reveal a significant relationship (p<0.05) between egg density and both SST and wind magnitude. The spatial coupling between eggs and environmental data was accomplished by using the spatial Empirical Orthogonal Functions method (EOF), showing that high amplitudes of the first mode were spatially coherent with high density of eggs, warm waters and low magnitudes of wind, suggesting a bio-physical coupling among these variables in the jack mackerel spawning area.


INTRODUCTION

Chilean jack mackerel (Trachurus murphyi Nichols) is a migrating pelagic species which inhabits the Southern Pacific Ocean, and constitutes the most important fishery for Chile. This fishery attained the maximum historical catch in 1995 (4.4 million t.), but since 1998 total catches decreased to 1.3 million t. because it was heavily regulated. This species presents a wide distribution, revealing a fairy broad band from Chile to New Zealand and Tasmania (Bailey 1989, Elizarov et al. 1993, Arcos & Grechina 1994). Along the Chilean waters, Serra (1991) pointed out a single self-sustained population, which includes the oceanic fraction off central-south Chile. According to fishery and biological data, Chilean jack mackerel exhibits a strong seasonal migration pattern (Serra 1991, Arcos et al. 2001), showing an offshore migration towards the reproductive oceanic habitat in early spring, extending along the southeastern Pacific, but mainly in oceanic waters off central Chile from 32° to 40° S and beyond 90°W (Cubillos 2002); and an onshore migration during the summer related to coastal food availability. During fall and winter, jack mackerel aggregates in compact schools in coastal and oceanic waters off central Chile, being more available for the Chilean purse-seiner fleet (Arancibia et al. 1995). Spawning occurs mainly between October and December, although it can extend from September to February (Grechina et al. 1998).

In this contribution, we describe the spatial distribution of jack mackerel eggs for the 1998-2001 period, and we attempt to examine the relationships between egg densities and environmental conditions during the spawning season in oceanic waters off central Chile.

DATA AND METHODS

From 1998 to 2001, four surveys were deployed in oceanic waters off central Chile by using 9-10 fishing vessels simultaneously during a 6-12 days period, to determine the abundance and distribution of jack mackerel eggs (Table 1).

Surveys were conducted during the main spawning period for jack mackerel, as determined from historical data (Grechina et al. 1998). All cruises were strategically deployed by using a systematic linear-tracking design, except for a 1998 pilot study, characterized by a systematic zig-zag tracking design (see Cubillos 2002 for details).

 

Table 1: Sampling design for 1998-2001 cruises and mean density of jack mackerel eggs. The percentage of positive stations and the coefficient of variation (%) are shown in parentheses.

Latitude (S) Area (nm2) Sampling design Total Positive Geostatistic mean egg

Cruises

Longitude (W) (N° vessels) (Dist. between transects) stations stations
(CV,%)
density (eggs/10 m2)

Dec. 1998

33°00'-39°00' 231,340 Zig-Zag 173 125 (72,3%) 372.2
75°00'-86°00' (5) (75 nm) (14.4)
Nov. 1999 33°06'-38°12' 284,526 Lineal 751 546 (72,7%) 662.4
75°00'-92°00' (9) (18 nm) (2.5)
Nov. 2000 32°06'-37°48' 261,815 Lineal 880 660 (75,0%) 515.7
75°00'-92°00' (10) (18 nm) (2.6)
Nov. 2001 31°48'-36°54          
  75°00'-92°00' 257,280 Lineal 661 478 (72,3%) 734.9
    (9) (18 nm)     (3.4)


Each station was separated by 18 nm inside the surveyed area, and vertical tows (WP-2 nets, 0.6 m diameter, 0.33 mm mesh size) were used for collecting planktonic samples. Geostatistical techniques were used to describe the spatial structure of egg distribution and to estimate egg density and precision. A spherical model was fitted according to the weighted least-square minimization criterion. Satellite information of SST, wind, sea level and chlorophyll was used to relate jack mackerel eggs to environmental conditions in the spawning area (30 - 40°S, 75 - 92°W). In addition, thermal gradients, turbulence and currents were calculated.

To explore the association between eggs and the environment, data were analyzed by using the cumulative frequency distribution method and Monte Carlo randomization (Perry & Smith 1994). Likewise, the spatial coupling between both normalized egg density and environmental data was accomplished through spatial Empirical Orthogonal Functions to find spatial patterns (modes) of variability.

RESULTS AND DISCUSSION

Spatial distribution of eggs

Evseenko (1987) suggested that the spawning of jack mackerel is related to the Subtropical Convergence Zone, which extends from Chile to southwestern Pacific (150°W). The spatial distribution of jack mackerel eggs for the 1998-2001 spawning period, showed a high incidence of positive stations in the surveyed area (>72%; Table 1), revealing that the bulk of jack mackerel spawning occurs in oceanic waters off central Chile, centered between 80 - 90°W and 33 - 38°S. During the 1998 pilot study, five industrial vessels covered 231,340 nm2 in nine days, showing important aggregations of eggs, which distributed from 78°W to 84°W along the 36°30'S. In November 1999, three nucles of high density were found farther offshore than 84W, limiting with the western boundary of the surveyed area. In contrast, a wide extension of eggs and several small nuclei of high densities were observed in November 2000, with the bulk of jack mackerel eggs distributed offshore 80W. Similar results were observed for November 2001, but in this survey the most important aggregations of jack mackerel eggs were distributed northward 36S (Figure 1). For the 1998-2001 spawning period, Cubillos et al. (2004) showed a nonlinear dome-shaped relationship between eggs and both latitude and SST, concluding that spawning is confined within 35 to 37°S and associated with SST between 16 and 19 °C, while the offshore boundary of spawning was clearly unsolved due to continuous increases in the egg density with longitude.


 
Figure 1: Spatial distribution of jack mackerel egg density (egg 10 m-2) for 1998-2001 spawning period as a result of the spatially stochastic process by kriging.

Spatial analysis of eggs showed an omni-directional spherical variogram. The spatial continuity at short distance was solved with the range fluctuating between 143 and 250 nm. The unsolved spatial continuity at short distances was normally low (nugget < 36 %). The geostatistical mean egg density fluctuated between 372.2 and 734.9 eggs 10 m-2 (14.4 - 2.5 % CV) (Table 1). These estimates were compared to the simple arithmetic mean and the bootstrap mean because of the high incidence of positive stations (>70 %), although geostatistical estimates provided lower coefficients of variation.

Eggs density - environment associations

The relationships between the huge oceanic spawning area and environmental conditions, as well as the mechanisms controlling their spatial-temporal variability have not been clearly established. It has been suggested that spawning is confined to the Subtropical Convergence Zone (northern 40S), characterized by small food, warm waters and low variability in plankton biomass (Evseenko 1987).

In this study, exploratory analysis showed that higher densities of eggs were found in oceanic waters (farther offshore than 84 W) associated with warmer waters (16-19 C), moderate values of wind speed (4-8 m s-1), as well as low turbulence (< 150 m3 s-3), current speed (< 15 cm s-1) and thermal gradients (< 0.3 C 10km-1). However, cumulative frequency distributions (Perry & Smith 1984) only showed a significant association (p<0.05) between eggs and both SST and wind speed (except for 1998), revealing a preferential range of temperature (14.9 - 18.5 C) and wind magnitude (4.4 - 8.6 m s-1) (see Table 2). Results showed a non-significant association for most part of the other cases (turbulence, currents speed, SST gradient and chlorophyll-a). The range of thermal preference for jack mackerel eggs, was coherent with results of habitat selection by SST (van der Lingen 1999), which included all cruises combined (not showed here), verifying the association between higher egg density and warmer waters in the oceanic spawning area off Chile, and was also coincident with previous results suggesting that the position of the 15 - 16 °C isotherm could be used as a southern limit of high egg concentration of jack mackerel (Evseenko 1987; Elizarov et al. 1993). These SST spatial changes could also be related to oceanic upwelling due to mesoscale eddies and shear vortices (Hormazábal et al. 2004; Cubillos et al., in press), constituting an habitat where jack mackerel larvae could be favored from alternated patterns of divergence and convergence occurring in the oceanic spawning habitat (Cubillos et al. 2002).


2II: Results of univariate randomization test of association between egg density and environmental conditions for spawning period of 1998-2001. Environmental preference range. Maximum absolute difference between g(t) and f(t). p-value is shown in parentheses.

Cruises SST
(°C)
Wind speed
(m s-1)
Turb. Index
Current speed
(cm s-1)
SST gradient
(°C 10 km-1)
Chlorophyll-a
(mg m-3)

1998

14.9-16.4 5.0-6.2 240-355 3.9-8.0 0.06-0.12 0.25-0.37
 

(0.004)

(0.469) (0.954) (0.607) (0.615) (0.883)
1999 16.4-17.1 4.4-5.6 83-173 7.9-13.0 0.06-0.15 0.11-0.15
  (0.007) (0.052) (0.041) (0.097) (0.020) (0.655)
2000 15.9-18.5 5.6-7.0 60.8-145.9 6.0-11.5 0.06-0.2 0.15-0.24
  (0.000) (0.053) (1.000) (0.013) (0.504) (0.999)
2001 15.9-17.4 6.4-8.6 190.9-245 5.2-6.4 0.06-0.14 0.16-0.54
  (0.006) (0.005) (1.000) (0.996) (0.628) (0.652)

Figure 2 shows the spatial structure of the EOF first mode amplitude between SST, wind magnitude and egg density for November 2000, which represented a 62.2 % of the total spatial variability. In this case, the first EOF mode contains the variability which is common among all variables, determining a measurement of their spatial covariance. It was observed that high amplitudes in the first mode were spatially coincident with higher egg density, warmer waters and low magnitudes of wind in the spawning area, suggesting a possible bio-physical coupling between these variables. The spatial distribution of the first EOF mode shows the relationship between eggs and oceanic waters associated with slow changes and not with strong variability of the coastal waters. The contributions of each variable to this mode are indicated for the first eigenvector structure (Table 3), showing positive and negative contributions between these variables and the EOF first mode. The example presented for 2000 was also true for the other years, except for November 1999, that evidenced a high density nucleus clearly restricted to the western limit of the surveyed area.


 
Figure 2: Spatial distribution of a) SST, b) wind speed, c) egg density and d) spatial EOF first mode between egg density and environment (SST, wind magnitude) for 2000.

 


Table 3: Eigenvalues and eigenvectors of the EOF analysis between egg density, sea surface temperature (SST) and wind speed, for the 1998-2001 spawning period in the oceanic waters off Chile.

Cruices EOF modes Explained
variability (%)

Egg density
(eggs/10 m2)
EOF eigenvectors
SST (°C)

Wind speed
(m s-1)

1998 1 61.7 0.2920 0.6804 0.6721
2 30.8 - 0.9554 0.1748 - 0.2381
3 7.7 0.0455 - 0.7117 - 0.7011
1999 1 52.7 - 0.0510 - 07103 0.7020
2 34.3 -0.21 51 - 0.1159 -0.1882
3 13.1 0.9753 - 0.6942 - 0.6869
2000 1 62.2 0.5078 0.6088 - 0.6095
  2 23.3 - 0.0039 0.3622 - 0.3560
  3 14.5 - 0.8614 - 0. 7058 - 0.7084
2001 1 50.4 0.5458 0.5072 - 0.6669
  2 30.0 - 0.6626 0.7485 - 0.0270
  3 19.7 - 0.5129 - 0.4271 - 0.7446

 

REFERENCES

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Bailey, K., 1989. Description and surface distribution of juvenile Peruvian jack mackerel, Trachurus murphyi, Nichols from the Subtropical Convergence Zone of Central South Pacific. Fishery Bulletin, U.S. 87,273-278.         [ Links ] [4]

Cubillos, L. (ed). 2002. Condición biológica de jurel en alta mar, año 2001. Informe Final FIP 2001-12, Informes Técnicos FIP - IT/2001-12, 168 p.         [ Links ] [5]

Cubillos, L., S. Núñez & D. Arcos. 2002. Relating Chilean jack mackerel recruitment to sea surface temperature (SST): looking for spatial spawning pattern and SST indexes. WG Product 4. Report of the Second meeting of the SPACC/IOC study group on "Use of environmental indices in the management of pelagic fish populations". 9-11 November 2002, Paris, France. GLOBEC Special Contribution N6. GLOBEC Report Series. 156 p.         [ Links ] [6]

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Elizarov, A.A., A.S. Grechina, B.N. Kotenev & A.N. Kuzetsov. 1993. Peruvian jack mackerel, Trachurus symmetricus murphyi, in the open waters of the South Pacific. Journal of Ichtyology, 33(3), 86-104.         [ Links ] [9]

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Grechina, A.S., S. Núñez & D. Arcos. 1998. Biología reproductiva del jurel (Trachurus symmetricus murphyi) en el Pacífico sur. In: Arcos, D. (ed.) Biología y ecología del jurel en aguas chilenas, Instituto de Investigación Pesquera, Talcahuano, Chile, p. 77-99.         [ Links ] [11]

Hormazábal, S., G. Shaffer & O. Leth. 2004. The coastal transition zone of Chile. Journal of Geophysical Research, Volumen 109, C01021, doi:10.1029/2003JCC001956.         [ Links ] [12]

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Serra, J.R., 1991. Important life history aspects of the Chilean jack mackerel, Trachurus symmetricus murphyi. Investigación Pesquera (Chile) 36, 67-83.         [ Links ] [14]

Van der Lingen, C.D., L. Hutchings, D. Merkle, J.J. Van der Westhuizen & J. Nelson. 1999. Comparative spawning habitats of anchovy (Engraulis capensis) and sardine (Sardinops sagax) in the Southern Benguela upwelling. In: Kruse, G.H., N. Bez, A. Booth, M.W. Dorn, S. Hills, R.N. Lipcius, D. Pelletier, C. Roy, S.J. Smith & D. Witherlls (eds). Ecosystem spatial processes and management of marine populations. Lowel Wakefield Dish. Symp. Ser. 17: 185-209.         [ Links ] [15]
 
 

* Research was partially supported by the Fondo de Investigación Pesquera, through grant FIP 99-14, FIP 2000-10 and FIP 2001-12, and by the Instituto de Investigación Pesquera.

 

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