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VARIATION OF YELLOWFIN TUNA (Thunnus albacares) ABUNDANCE RELATED TO EL NIÑO AT THE ENTRANCE TO THE GULF OF CALIFORNIA.

Ernesto Torres-Orozco Centro de Investigaciones Biológicas del Noroeste, S.C. Mar Bermejo No. 195, Col. Playa Palo de Santa Rita Apdo. Postal 128; La Paz, BCS 23090, México Phone: (52) (612) 123-8484 Fax: (52) (612) 125-3625 Email: etorres@cibnor.mx and Facultad de Ciencias Marinas. Universidad de Colima. Manzanillo, Colima. México. Armando Trasviña Oceanografía Física, CICESE en BCS Miraflores 334 e/Mulegé y La Paz Fracc. Bella Vista. La Paz, BCS 23050, México. Arturo Muhlia-Melo Centro de Investigaciones Biológicas del Noroeste, S.C. Mar Bermejo No. 195, Col. Playa Palo de Santa Rita Apdo. Postal 128; La Paz, BCS 23090, México. Sofía Ortega-García Centro Interdisciplinario de Ciencias Marinas. La Paz, BCS. México, 23000 Key words: Yellowfin tuna, Gulf of California, El Niño, interannual variability

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Abstract. We examine capture data of yellowfin tuna for the period of 1990 to 1999, and for an area

off the mouth of the Gulf of California, in the northeastern tropical Pacific (18-24o N, 112-

104o W). The data were extracted from a database gathered from logbook records of the

Mexican tuna purse-seine fleet. Standardized catch-per-unit-effort in a zonal band of high

catches is strongly correlated with El Niño-Southern Oscillation (ENSO) events. The

latitudinal distribution of tuna catches increase from south to north for the 10-year period.

Highest effort, and catches, concentrate between 22 and 23o N. This area accumulates 50%

of the total relative abundance for the 10-year period expressed as capture-per-unit-

standard-effort (81380t). At least two periods of exceptionally high relative abundance are

found. They occur for two consecutive springs, following El Niño events starting in 1991

and 1997. Catch peaks are triggered by the arrival of positive anomalies of sea surface

temperature (SST) to the area. A delay of 2 to 4 months is observed between the occurrence

of maximum SST anomalies at the equator and peaks of relative abundance. Prior to these

two events, negative SST anomalies were the dominant feature in the study area and

catches were extremely low. We attribute this behavior to northward migration patterns

associated to El Niño events.

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Introduction

The entrance to the Gulf of California (from 18oN to 24oN and 104oW to 112oW), is located

in the convergence zone of the North Pacific Gyre, where the California Current separates

from the coast to feed the North Equatorial Current. It has a complex hydrographic

structure due to the confluence of different water masses (Roden and Groves, 1959; Roden,

1964, 1972; Wyrtki, 1966; Álvarez-Borrego and Schwartzlose, 1979; Bray and Robles,

1991; Torres-Orozco, 1993). This region is highly responsive to El Niño phenomenon. Its

main response is characterized by positive sea level anomalies, warming of the upper layer

and a general alteration of marine current patterns (Baumgartner and Christensen, 1985;

Robles and Marinone, 1987; Torres-Orozco, 1993; Lavín et al., 1997; Ortega-García et al.,

1999; Castro et al., 2000; Bernal et al., 2001, Trasviña et al., 1999).

High abundance of yellowfin tuna Thunnus albacares (YFT) is reported in the area by a

number of authors (see, for instance Allen and Pusly, 1984; Castro-Ortiz and Quiñones-

Velásquez, 1987; Muhlia-Melo, 1993; Ortega-Garcia, 1998). However, studies about the

interaction of the YFT with the physical environment of the Mexican Pacific are scarce.

Blackburn (1965, 1969) considers that the abundance of the YFT is given among 20°C and

30°C, but it can also be present in regions with temperature between 18°C and 31°C.

Ortega-García (1998) reported that the YFT is distributed in regions where the sea surface

temperature (SST) ranges from 17°C to 31°C, with more frequent occurrence (mode) at

about 28°C. Other studies like that Laevatus and Rosa (1963) and Castro-Ortiz y Quiñones-

Velasquez (1987) mentioned that YFT is found in commercial concentrations in regions

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where the SST ranges from 20ºC to 28ºC. Blackburn (1969) considers 30ºC as an optimal

estimate of the maximum temperature of occurrence of YFT. In summary, YFT catches are

reported to occur at SST values ranging from 17 to 30°C.

Bautista-Cortes (1997) studied the relationships between varying vertical temperature

structure and YFT catches in the Mexican Pacific to 140° longitude W. He found the depth

of the 23°C isotherm to be related with these catches and concludes that the shallower the

isotherm the larger the tuna catches.

Studies relating the interannual variability to the abundance and distributions of YFT are

also few. A study of the Northeastern Pacific (Castro-Ortiz and Quiñones-Velasquez, 1987)

report smaller catches during the El Niño 1982-1983, as compared to the following (1984-

1985) seasons. In the annual report of 1989 of the Inter American Tropical Tuna

Commission (IATTC) it is concluded that the 1982-83 El Niño caused the largest decrease

availability-vulnerability index on record. This is consistent with the resource being less

susceptible to fishing in the Northeastern Pacific during an intense El Niño. Lehodey

(2000) analyzes data of purse seine catch during 2 El Niño and 2 La Niña events, from

1982 to 1997 and for the whole Tropical Pacific (-20 to +20 latitude). In the western

equatorial Pacific rising and vertical extension of temperature during the El Niño increases

the catchability of YFT by surface fishing gears. In the eastern Pacific, however, El Niño

events have a negative impact on the purse seine fishery. This also appears to be the case

with the longline fishery. Such negative effect on both surface and deep fishery would

suggest a horizontal displacement of the resource. In a more recent work, Hsueh-Jung Lu et

al. (2001) analyze catches of YFT and bigeye tuna of the longline fleet of the tropical

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Pacific Ocean. They conclude that high hook rates of both species (YFT and bigeye tuna)

were mostly associated with regions where SST increased during El Niño or La Niña years.

During La Niña episodes YFT undergo a meridional displacement and expand their

preferred range northwards.

For the region of interest, Ortega-García et al. (1999) studied the impact of the El Niño

event of 1997-1998 on the spatial variability of the Mexican tuna fishery. They report an

increase in YFT oceanic sets during the first months (July-December 1997) of the El Niño

event. At the same time a smaller number of sets were made in the coastal zone, near the

entrance to the Gulf of California. During the second part of this El Niño (January-June

1998) oceanic sets were few while the number of sets along the west coast of the Baja

California peninsula was larger than elsewhere. Joseph and Miller (1988) analyze capture

data of the Eastern Pacific and find positive anomalies in tuna recruitment after El Niño

events. These authors find large recruitment in 1971, 1974, 1978 and 1985 all of which

were preceded by the El Niño events of 1969, 1972, 1976 and 1983.

In summary, at the basin scale the YFT fishery of the Eastern Pacific is reported to

diminish during El Niño events. The response of both longline and purseine cathches

suggest a horizontal displacement of the resource. Regional studies such as that by Ortega-

García et al. (1999) are consistent this hypothesis. Large recruitment after El Niño events is

also reported to occur in the Eastern Pacific. Joseph and Miller (1988) report positive

anomalies in tuna recruitment happening consistently during a 22-year period. The

objective of this work it is to study the effect of El Niño events on YFT catches in an area

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of great relevance for the industry and for the ecology of the resource in the Eastern Pacific:

the entrance to the Gulf of California.

Materials and methods

Environmental data in the form of monthly means of sea surface temperature anomalies

(SSTA) from 1990 to 1999 were extracted from NOAA NCEP EMC CMB GLOBAL

Reyn_SmithOIv1 monthly Sea Surface Temperature Anomaly database

(http://ingrid.ldeo.columbia.edu). The monthly optimum interpolation (OI) fields are

derived by a linear interpolation of the weekly OI fields to daily fields then averaging the

daily values over a month. A description of the OI analysis can be found in Reynolds and

Smith (1994). SSTA data for El Niño region 3 (NIÑO3, from 150°W to 90°W and from

5°N to 5°S) were obtained from Climate Prediction Center databases (CPC, NOAA)

(http://www.cpc.ncep.noaa.gov/).

The classification of El Niño intensities was taken from the web page of the Climate

Preditiction Center:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.html

Yellowfin tuna catches were obtained from the database of the Big Pelagic Research

Project of CICIMAR (the Interdisciplinary Research Center of Marine Science of the

National Polytechnic Institute in La Paz, Mexico). This database has information on daily

fishing activities of about 80% of the Mexican purse-seine fleet operating at the Eastern

Pacific. The data used in this study includes carrying capacity, catch by species, location

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and school type (free swimming - breeze -, dolphin associated, or associated to floating

objects), from 1990 to 1999, includes 11,690 records (Fig. 1).

Assumptions:

(a) The target species fishing effort applied by the tuna purse-seine fleet is yellowfin

tuna.

(b) CPUE (catch-per-unit-effort) is used as index of relative abundance of YFT.

Standardization of fishing effort

The standardization of fishing effort follows the methodology of Ehrhardt (1981) and

Ortega-García and Muhlia-Melo (1992). Catch per unit effort (CPUE) was standardized

using the carrying capacity of different vessels and each set is taken as unitary effort.

To test for significant differences in average CPUE between vessels of different sizes an

analysis of variance was applied to the whole data base. Results show two significantly

different classes of vessels (F(1, 11690); =395.84 P<0.05): a) vessels with carrying capacity

equal or smaller than 680 metric tons (t) and b) vessels with carrying capacity greater then

than 680t. They are denominated in this work as class 1 and 2 vessels, respectively.

The relative fishing power (P) was estimated considering as the standard unit class 2 of

fishing vessel (more than 680t). The CPUE for this class of vessels was more constant, this

corresponds to the highest percentage of the fleet. They have a greater autonomy, greater

carrying capacity, and they use in most of their sets aerial help.

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Therefore we estimate catch-per unit-effort as:

NC

CPUE jj = (1)

Where: CPUEj is the catch per unit of nominal effort for the jth set (j=1 to n). Cj is the

reported catch of yellowfin tuna for the jth set (j=1 to n) and N, taken here as a set, is

nominal effort.

Then, the fishing power for each vessel category was determined as:

==

2;1

1

2

2

2

1

class to vessel responds if j cor CPUECPUE

lass vessel cponds to j corres ; if CPUECPUE

Pj (2)

where 1

CPUE and 2

CPUE are the averages of the catch per unit effort of vessels class

1 and class 2 respectively.

Standardized sets:

The standardized effort (E) was estimated by the product of the fishing power (P) and

nominal effort (N).

*j jE P N= (3)

Catch per standard unit effort:

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jEj

j

CPUECPSUE = (4)

Therefore, the CPSUE for vessel class 1 and 2 is simplified in the following way:

=

2

1

classvesseltoscorrespondjif;C

classvesseltoscorrespondjif;PC

CPSUEj

j

j

J (5)

Results

Interannual variation of abundance.

The catch per standard unit effort (CPSUE) shows clear interannual periodicity (figure 2).

Yellowfin tuna abundance in the area increases 185% from 1991 to 1992, 202% from 1995

to 1996 and 232% from 1997 to 1998. In particular, the sum of the two El Niño years in the

record (1991-93, 1997-98) represent 56.2% (91,133 t) of the total catches for the 10 years.

Latitudinal variation of abundance.

Figure 3 describes zonal variation of catches from 1990 to 1999. Vertical bars include

catches from different zonal bands between 18° and 23.9°N and for meridional ranges

between 104° to 112°W (see Fig 1). With the exception of the 18°-19°N band there is a

linear increase of CPUE with latitude. Greatest fishing effort is concentrated in the 22°-

23°N and 23-24°N zonal bands. Both account for 50% (81,380 t) of the total capture for the

10-year period in this region. From 18° to 21°N CPSUE values average 12.4% (20,269 t).

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Yellowfin tuna abundance and El Niño event.

Sea surface temperature anomalies (SSTA) were used to investigate the effect of

interannual warming or cooling events on the variability of the YFT abundance. Anomalies

for the entrance to the Gulf of California were calculated from anomalies of the zonal band

between 22° and 23°N and from 103° to 112oW. These anomalies were extracted from the

Reynolds-Smith 1994 Monthly 1-degree SSTA climatology. A complete description on the

SST climatology is in Reynolds and Smith (1994). This variable is then compared to catch

per standard unit effort (CPSUE). We do not examine SSTAs south of 21°N because they

fall within the Mexican Warm Pool (Trasviña et al., 1999). In this warm pool the

persistently high SSTs (above 27°C all year long) mask the propagation of warm and cold

signals from the Equator. The 10-year time series (1990 to 1999) is presented here

detrended and filtered to eliminate periods shorter than 3 months. We use the filter

described in Godin (1972).

SSTAs in the NINO3 region (Fig 4a) show cold and warm conditions associated with 5

events. These include warm El Niño events in 1991-1992 (strong), 1994-1995 (moderate)

and 1997-1998 (strong). Moderately cold La Niña events took place in 1995-1996 and

1998-1999. Strong events are marked with a black line located at each respective maximum

SSTA. The 1997-98 event was the strongest warm episode in the 1990-1999 decade.

Now we compare NINO3 anomalies with SSTAs in our region (Fig 4b). These are found

to be positive and larger than 1°C from 1990-1992 and from 1997-1998. The maximum

intensity of those warm episodes is marked with dashed lines on the figure 4b. Near-normal

conditions are found to occur from 1993 to 1996. And the coldest and more persistent event

of this decade took place in the study area from July 1998 to December 1999.

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The temporal behavior of the YFT relative abundance shows significant interannual

variability (Figure 4c). This figure shows the time series of monthly values of relative

abundance (CPSUE) filtered and detrended by eliminating the mean for the 10-year period.

Actual CPSUE values are obtained by adding the 10-year mean (1363 t). Within this period

we observe 5 years of exceptionally high relative abundance. These are the years of 1992,

1993, 1996, 1998 and 1999. These years account for 69.2% (112,366 t) of the total capture

in the 10-year period. Peaks of abundance (greater than 1000 t) are indicated by thin lines

on the figure. Thick lines indicate maximum SST anomalies in NINO3. Dashed lines

indicate peaks of SSTA in the area of study. Several features are apparent from these

diagrams:

a) Abundance Peaks in 1992 and 1998 occur 4 and 4.5 months, respectively, after the

onset of an El Niño event in the equator (time span between thin and thick lines)

and 2 and 3 months, respectively, after the SST anomaly signal reaches the entrance

to the Gulf (c thin and dashed lines).

b) Peaks occur during the springs following an El Niño winter.

c) Peaks of YFT relative abundance are observed in 1993 and 1999. These occur one

year after the El Niño event, however, the 1993 peak took place with nearly normal

SSTs conditions while the 1999 peak occurred with extremely negative SST

anomalies (La Niña event, compare figures 4b and 4c). These peaks of relative

abundance are higher than the one recorded in the previous year.

d) The 1996 peak cannot be explained in terms of the variability of SST anomalies.

e) There is a delay from one to two months among the mature phase of the El Niño

event in the Equator and the presence of high SSTAs in our region (time span

between thick and dashed lines in figures 4a and 4b).

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Conclusions

These results contrast with the known behavior of the fishery in the eastern Pacific. Instead

of showing small catches (and relative abundance) after an El Niño event, two springs

following successive warm episodes present high catches of YFT. This, and the dramatic

decrease in catches south of 22°N in our region of study (see figure 3), is evidence

consistent with the poleward horizontal motion of the fishery. The existence of high catches

of YFT one year after El Niño episodes occurs in contrasting environmental conditions.

During the spring of 1993 SSTAs at the entrance to the Gulf of California were normal (see

Fig. 4b), whereas the spring of 1999 SSTAs corresponded to those of a moderate cold

episode (La Niña). We believe this can be attributed to enhanced recruitment after the

warm episode, consistent with reports for the Eastern Pacific such as that by Joseph and

Miller (1988).

El Niño events appear to be related to an increase in abundance of yellowfin tuna at the

entrance to the Gulf of California. Maximum relative abundance occurs four months after

the mature phase of El Niño in the equator, as indicated by maximum SSTAs in the NINO3

region (see figure 4). Clearly, more detailed studies of yellowfin tuna recruitment and

migration patterns are necessary in this area. These findings describe a little known but

important behavior of the fishery.

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Acknowledgments. The first author was a CONACYT grant holder for the duration of his doctoral studies at CIBNor (Reg. 60997). The Universidad de Colima and PROMEP also supported this effort. We are grateful to the Big Pelagics Research Project (CICIMAR) for providing the capture data. The second author is a SNI grant holder and wishes to acknowledge the support of the Oceanology Division of CICESE and of CICESE’s campus at Baja California Sur.

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Torres-Orozco, Fig. 1

Distribution of the sets made by the Mexican purse seine fleet from 1990 to 1999.

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Torres-Orozco, Fig. 2

Total catch per standard unit effort (CPSUE) during 1990-1999.

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Torres-Orozco, Fig. 3

Latitudinal distribution of CPSUE for the period 1990-1999.

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Torres-Orozco, Fig. 4

SSTA in the El Niño region 3 (A), SSTA in the study area (B) and relative abundance of yellowfin tuna (C). The vertical lines are referred in the text. Bars at the top show the equatorial central Pacific episodes as compiled by the Climate Prediction Center (see methods section): Strong warm (Sw), Moderate warm (Mw) and cold episode.