Baja California’s Biological Transition Zones:

8/10/2019 Baja Californias Biological Transition Zones:

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503

Journal of Oceanography , Vol. 59, pp. 503 to 513, 2003

Keywords:

California Current,

Gulf of California,

California sardine,

productivity,

interchange,

transition zones.

* Corresponding author. E-mail: [email protected]

Copyright The Oceanographic Society of Japan.

Baja Californias Biological Transition Zones: Refugesfor the California Sardine

DANIELLLUCH-BELDA1*, DANIELB. LLUCH-COTA2and SALVADORE. LLUCH-COTA2

1Centro Interdisciplinario de Ciencias Marinas (CICIMAR-IPN),

P.O. Box 592, 23000 La Paz, Baja California Sur, Mexico2Centro de Investigaciones Biolgicas del Noroeste, S.C. (CIBNOR),

P.O. Box 128, 23000 La Paz, Baja California Sur, Mexico

(Received 31 May 2002; in revised form 7 January 2003; accepted 17 January 2003)

The information on the transitional areas between the temperate and tropical do-

mains at the southern extent of the California Current System is reviewed and de-

scribed, particularly searching for the relative isolation or interchange between the

western coast of the Baja California peninsula and the Gulf of California, as well as

mechanisms that permit the existence of sizeable stocks of California sardine. Bio-

logical Action Centers that have high productivity throughout the year, as opposed tothe rest of the coastal area, are found in both the western coast of the peninsula at the

Sebastin VizcanoPunta Eugenia region and in the Ballenas Channel inside the

gulf; these features support large biomasses of sardine throughout the full year and

serve as long term refuges during adverse periods. The role of the Sebastin Vizcaino

sardine stock as the primary group for expansion is examined.

The CalCOFI (California Cooperative Fisheries In-

vestigations) program has been a major investigative ef-

fort since the late 1940s and its contribution has been in-

valuable. Other institutions have participated in the study

of the northern limits of the CCS and beyond into thenorth Pacific, resulting in a comprehensive knowledge

of the area. Thus, what may be called the temperate-

subarctic transition is well known.

However, while during the 1950s to the early 1970s

the CalCOFI grid covered most of the latitudinal extent

of the CCS (mostly south to Magdalena Bay, plus some

cruises within the Gulf of California; Fig. 2), since the

late 1970s it has been restricted to the north of the inter-

national boundary, and the southern part has lacked that

continued research effort (Figs. 3 and 4).

Partly due to this, the southern part of the CCS (the

temperate-tropical transition) has been poorly defined;

further, the Gulf of California has been perceived as an

isolated body of water, mostly because it is surrounded

by high topography, connected to the open ocean only at

its southern end. The perception of its uniqueness has

resulted in its designation as a Large Marine Ecosystem

(Anon., 1991).

Oceanographic descriptions of the CCS normally

reach south to about 25N (Hickey, 1979; Lynn and

Simpson, 1987), the southern extent of most CalCOFI

cruises. Equatorward from this latitude (southward from

the north of Magdalena Bay), there are no equivalent data

1. Introduction

The California Current System (CCS) is one of the

better known marine areas of the world. It is a huge tran-

sitional area, the eastern boundary of the North Pacific

Gyre (Lynn and Simpson, 1987). Basically, it consists ofa surface current (down to 300 m) transporting water from

the subarctic divergence equatorward, together with quan-

tities of eastern North Pacific Central water entering from

the west along its path; Equatorial Pacific water penetrates

through the southern limit of the system in the form of a

deeper countercurrent. Seasonally variable wind-driven

upwelling incorporates cool, nutrient-rich waters

alongshore (Huyer, 1983). Inshore, a narrow countercur-

rent often flows poleward during fall and winter (Lynn

and Simpson, 1987, Fig. 1).

Essentially, four major faunal assemblages result

from the oceanographic conditions in the region: transi-

tional, or the California Current System itself, limited to

the north by the subarctic, to the west by the centraland

by the equatorialdomains at the south. The California

Current System fauna is a mixture of species from each

of these domains plus some endemic, reflecting its tran-

sitional nature (Moser et al., 1987). It is a huge subarctic-

tropical ecotone, in the sense defined by Odum (1959).


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504 D. L luch-Belda et al.

to be analyzed in similar manner.

The principal aims of the present paper are: a) to re-

view the main spatial and temporal features of the tem-

perate-tropical transition, and b) to asses its role as a ref-

uge area for the California sardine, as previously sug-

gested (Lluch-Belda et al., 1991).

1.1 The temperate-tropical transition

At the west coast of the peninsula, the southernmost

flow of the California Current (CC) has been described

as turning west and merging into the North Equatorial

Current (Wyrtki, 1965; Moser et al., 1987), mostly dif-

fusing by turbulence and mixing. This description hasoften obscured the fact that the CC does reach the tip of

the peninsula and the mouth of the Gulf of California;

modified water of the CC is recognizable in the vicinity

of the Revillagigedo Islands (Lluch-Cota et al., 1994)

about 19N, particularly from February to June when SSTs

are lower, upwelling is maximum and the current intensi-

fies. Biological evidence of this southward extent of the

CC is suggested by the fact that juvenile California sar-

dine and northern anchovy schools, both temperate affin-

ity species, have been reported from the Revillagigedo

Fig. 1. Sketch of the main currents in the northeastern Pacific

Ocean. The Subarctic Current, Alaska Current, northern part

of the California Current and North Equatorial Current re-

drawn from various sources. The southern part of the Cali-

fornia Current is from Lynn and Simpson (1987). Biogeo-graphical domains from Moser et al. (1987), originally de-

fined by Brinton (1962).

Fig. 2. Southern part of the California Current System and the

Gulf of California.

Fig. 3. Latitudinal extension of the CalCOFI cruises. Each bar

represents one cruise; numbers at the bottom are years. Map

at right shown for reference.

Fig. 4. CalCOFI stations in the egg survey database; shadowed

area, 1areas used for photosynthetic pigment concentra-

tion averages.


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Baja Californias Biological Transition Zones: Refuges for the California Sardine 505

Islands, about 19N (Whitehead and Rodrguez-Snchez,

1995). Thus, at least a part of the CC during part of the

year follows a southeasterly path, reaching the mouth of

the Gulf and beyond.The CC also penetrates the Gulf of California in a

number of complex structures, including lenses of water

10 km wide and 50 m thick (Collins et al., 1997), a flow

entrained in the cyclonic flow (Griffiths, 1968) and

mesoscale eddies breaking through the Cabo San Lucas

front and reaching the central part of the Gulf (Angel

Jimnez-Illescas, pers. comm., CICIMAR, La Paz,

Mxico). Water from the Costa Rica Current enters the

Gulf along its eastern coast.

Along the Pacific coast the CC relaxes during sum-

mer while the inshore countercurrent intensifies and

warms up to a maximum during September. During the

cold part of the year water from the California Current

extends along the peninsulas southwestern coast, past its

tip and beyond, and enters the Gulf. During the warm

part of the year, tropical water enters the Gulf and ex-

tends along the southern part of the peninsulas west coast.

The temperate-tropical transition occurs throughout the

year with the alternating dominance of warm and tem-

perate water (Fig. 5).

It is not only temperature that changes considerably

at this transition; productivity is closely associated with

it. On the Pacific side, the main source of enrichment in

the northern part of the California Currentas reflected

by macrozooplankton displacement volumeshas beenshown to be southward advection, with a north-south de-

clining trend (Bernal, 1979), tropical water being far less

productive. In the southern part of the CCthe peninsu-

lar coastphytoplankton and consequently zooplankton

productivity is more nutrient-limited, the bulk of enrich-

ment is mostly a result of local upwelling (Roesler and

Chelton, 1987; Lluch-Belda, 2000). Wind induced

upwelling and consequent enrichment occurs essentially

during the colder part of the year (March to June), simul-

taneously with the most southward advection of the CC

and the period of lower sea level of the year (Fig. 6).

During summer upwelling relaxes to a minimum and

warm, impoverished water covers most of the area.

On the eastern side of the peninsula, the Gulf of Cali-

fornia is about 1,000 km long and 100 to 200 km wide.

Its northern part is separated from the rest by two large

midriff islands (Angel de la Guarda and Tiburn, Fig. 2)and an irregular sill. The northern part is mostly shallow

(


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number of taxa which characterize the Californian prov-

ince, including coastal fishes (Hubbs, 1960), brachiura

(Garth, 1960), briozoa (Soule, 1960) and molluscs (Hall,

1964; Valentine, 1966) as discussed by Hernndez-Rivas

et al. (2000). Ahlstrom (1965) recognized Punta Eugenia

as the northern limit of distribution of warm water (both

tropical and subtropical) species from the south and thesouthern limit of temperate affinity species. Other tem-

perate area taxa reach south to about 27N, around Punta

Eugenia, where giant kelp (Macrocystis pirifera) and most

abalones (Haliotisspp.) have their extreme equatorward

reach (Casas et al., 1996; Len and Mucio, 1996).

Roesler and Chelton (1987), in their analysis of

macrozooplankton volume seasonal variability in

CalCOFI samples, found a region of maximum variance

at about 29N, coinciding with the biogeographical

boundary between high-biomass northern and low-

biomass southern species of zooplankton, as previously

described by Bernal (1979) and McGowan and Miller(1980).

Tropical fauna extend along both coasts of the Gulf

of California and northward mostly to Magdalena Bay in

the Pacific side. Penaeid blue (Litopenaeus stylirostris)

and brown (Farfantepenaeus californiensis) shrimps are

fished extensively along the continental shelf of the east

coast of the gulf, together with white shrimp (Litopenaeus

vannamei) south of Topolobampo Bay. Blue and brown

shrimps are also abundant in the wide soft bottom of the

northern part of the gulf. Although most of the coast is

rocky along the west coast of the gulf, shrimp trawling is

also practiced where soft bottoms exist, particularly at

the mouth of bays like Concepcin and La Paz. Along thewest coast of the peninsula, shrimp are also taken in

Magdalena Bay and in lesser amounts at other smaller

coastal lagoons north to Punta Abreojos (about 27.5N),

as reported by Garca et al. (1996). The threadfin herring

(Opisthonema spp.) and other tropical pelagic species

spawn northward to Punta Eugenia (Whitehead, 1985).

Between the extremes of these two faunal commu-

nities, the temperate California Current on one hand and

the tropical on the other, Brusca and Wallerstein (1979)

regarded the area between Vizcano and Magdalena bays

as the transition zone between the temperate and tropical

fauna in the eastern Pacific. Moser et al. (1987) groupedalmost 200 taxa of fish larvae in the CalCOFI collections

into three distinct groups; a northern complex including

subarctic-transitional fauna, a coastal pelagic fauna and

its associates, a southern group that incorporates transi-

tional, warm-water cosmopolitans and eastern tropical

Pacific taxa and a separate group, associated with the

extensive continental shelf area of Sebastin Vizcano Bay

and Punta Abreojos-Cabo San Lzaro Bight (essentially

the Gulf of Ulloa).

Inside the Gulf of California, Brusca (1973) found

that its invertebrate fauna has a mixture of endemics

(21%), eastern tropical Pacific (41%) and northern tem-

perate species (18%), plus other minor constituents. The

temperate component is mostly found at the northern gulf,

with a considerable number of disjunct distribution spe-

cies that are also found along the west coast south to about

Magdalena Bay (Walker, 1960). The existence of thisgroup of species (including some fish) has been explained

by two principal mechanisms: one, migrating across past

water connections between the west coast and the gulf

(Garth, 1961) and the second, southward displacement

of isotherms during cold eras, permitting entrance around

the tip of the peninsula and to the northern gulf where

they became trapped when temperatures warmed again

(Brusca, 1973). Nevertheless, many other species show

continuous distribution around the Cabo San Lucas from

the west coast and into the gulf.

1.2 Biological interchange between the California Cur-rent and the Gulf of California

The Gulf of California has often been regarded as a

mostly isolated, distinct body of water. Even though Cali-

fornia sardine populations are harvested in the Gulf, they

have been considered separately, implicitly assuming iso-

lation from the west coast populations (Schwartzlose et

al., 1999). The same has been true for mackerel (Scomber

japonicus), northern anchovy (Engraulis mordax) and

pelagic red crab (Pleuroncodes planipes) populations,

among others. Two species of hake (Merluccius productus

andM. angustimanus) also exist along the west coast and

the Gulf (Cohen et al., 1990).

Sokolov and Wong (1973) proposed a model for thereproductive cycle of California sardine population in the

Gulf, including spawning along the mainland coast dur-

ing the upwelling season, which extends from mid-Octo-

ber to early May. Advection was assumed to transport eggs

and larvae to the central and western Gulf. The linkage

between this model and the surface oceanographic envi-

ronment was shown by Hammann et al. (1988). This

scheme led to the belief that Gulf sardines mostly spawned

along the Sonora-Sinaloa coast and were thus far from

the peninsulas tip. However, Nevrez-Martnez (1990)

reported sardine eggs along the peninsular coast south of

La Paz Bay and the CICIMAR ichthyoplankton group hasfound abundant eggs and larvae of California sardine and

round herring (Etrumeus teres) larvae south of Magdalena

Bay along the west coast and south of the La Paz Bay in

the eastern coast, reaching the tip of the peninsula at each

side (R. Saldierna, pers. comm., CICIMAR, La Paz,

Mxico). Fishing boat owners have reported sardine

schools around the Cape San Lucas region, at depths out

of the reach of purse seiners (M. Hernndez R., pers.

comm., CICIMAR, La Paz, Mxico) and during March,

1999 more than 300 t of large adults (160 to 170 mm)


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where caught north of the Espritu Santo Island, just out-

side La Paz Bay (M. O. Nevrez-Martnez, pers. comm.,

Centro Regional de Investigaciones Pesqueras, Guaymas).

Further evidence of interchange is presented by

Gluyas-Milln and Quinez-Velzquez (1996), showing

that mackerel (Scomber japonicus) collected at Magdalena

Bay came from two different stocks, one distributed fromsouthern California to Vizcano Bay and the other in the

Gulf of California. Flix-Uraga et al . (1996) and

Alvarado-Castillo et al. (1995) analyzed sardines fished

at Magdalena Bay and found two distinct groups, one of

northern origin and the other presumably moving between

Magdalena Bay and south of it. This might be similar to

the mackerel case and this second group might be from

the Gulf.

The analysis of data on catches of juvenile small

pelagic fishes caught by the tuna clipper fleet for bait

along the American west coast has been suggested to show

sardine stocks massively invading the Gulf of Californiaduring the cold period of the 1970s (Rodrguez-Snchez

et al., 2001). Scale deposition rates at the Santa Barbara

Basin (just south off Point Conception), the Soledad Ba-

sin (just north of Magdalena Bay) and in the Gulf of Cali-

fornia just off Guaymas, show long records of sardine,

anchovy, mackerel and hake (Holmgren-Urba and

Baumgartner, 1993); further, there are indications of

migrational shifts from one region to another over peri-

ods of several decades.

1.3 The role of the Baja California Biological Transi-

tion Zones as refuge areas for California sardine

As early as 1936 Tibby (1937) found sizable Cali-fornia sardine spawning in the Sebastin Vizcano region;

Ahlstrom (1954) stated that at that time there were two

major areas of sardine spawning, a compact area of in-

tense year-around spawning off central Baja California

(the Sebastin Vizcano area), and a larger area of diffuse

spawning off southern California and adjacent Baja Cali-

fornia (the southern California Bight area). Lluch-Belda

et al. (1991) proposed that the Sebastin VizcanoPunta

Eugenia area is the major region for sardine survival, the

southern California bight being the secondary one during

favorable periods. Further, they suggested that the

Vizcano area is the refuge for the sardine populationduring adverse intervals.

If productivity declines from north to south as de-

scribed above, particularly equatorward from 29N

(Bernal, 1979; McGowan and Miller, 1980) and this area

impoverishes during summer, in part due to the penetra-

tion of subtropical water, the existence of sizeable sar-

dine spawning throughout the year must be supported by

other mechanisms than southward advection or wind-in-

duced coastal upwelling.

There is an area within the transition zone that main-

tains high productivity throughout the year, the Sebastin

Vizcano Bay and Punta Eugenia (Lluch-Belda, 2000).

Morales-Zrate et al. (2000) found that this spot has

higher productivity than the rest of the coastal area

throughout the year, while other high productivity areas

have sizeable enrichment values only during part of the

annual cycle. These small, permanently highly produc-tive areas characteristic of certain upwelling regions have

been described as Biological Action Centers (BAC) by

Lluch-Belda (2000).

Other coastal areas, such as Punta Baja (~30N) and

the Gulf of Ulloa also show considerable enrichment dur-

ing the upwelling season, but it diminishes noticeably

when the winds relax.

On the other hand, inside the Gulf of California

Lluch-Cota and Arias-Archiga (2000) examined the

monthly distribution of photosynthetic pigment concen-

tration and reviewed the regionalization schemes of pre-

vious authors. They proposed to divide the Gulf into fourmain regions: a northern one, under the predominant in-

fluence of tidal forcing; a central one, mostly under the

effect of winds; and a southern one influenced by the

Pacific Ocean. A fourth one, around the Ballenas Chan-

nel, shows high productivity during the whole year; thus

is also considered as a Biological Action Center. Enrich-

ment of the full gulf begins about October in the central

area and expands, reaching a maximum during January,

decaying afterwards to minimum values during the sum-

mer.

During the coastal upwelling enrichment season, the

actively swimming populations of sardine expand their

area of distribution both north and south of their refugearea, schools found from Puerto Libertad in the north and

fished during cold years (for instance, 197172) as far

south as Mazatln (about 23.5N); further, juveniles have

been collected in Puerto Vallarta (about 21N; Rubn

Rodrguez S., pers. comm., CICIMAR, La Paz, Mxico);

when productivity decays they retreat to the Ballenas

Channel area, which is permanently productive (Lluch-

Belda et al., 1986).

The sustained productivity of these two areas

(Sebastin Vizcano at the west coast and the Ballenas

Channel inside the Gulf) permits the existence of com-

paratively large populations of species with temperateaffinity, surrounded by tropical-like impoverished areas

where warm water species show high diversity and lower

biomass during summer.

In order to review the hypothesis that the central Baja

California area is a refuge for sardine, we looked into the

monthly distribution of photosynthetic pigment and lati-

tudinal macrozooplankton displacement volumes as in-

dices related to primary and secondary productivity, and

compared them to the relative abundance of sardine eggs.


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south decay, the rich season has high concentrations, with

major peaks at about 24+N (north of Magdalena Bay)

and about 28N (at Punta EugeniaSebastin Vizcano

Bay). However, during the poor season there is only one

major peak: that of the Sebastin Vizcano Bay.

Gridded photosynthetic pigment concentration is

shown in Fig. 8 for the richest and poorest months in thewest coast and inside the gulf.

Latitudinal averages of macrozooplankton displace-

ment volumes are shown in Fig. 9. The high values north

of line 100 (the southern limit of the Southern California

Bight) to San Francisco (line 60) contrast with those be-

tween lines 100 and 120 (Sebastin Vizcano Bay) and

between 130 to about 150 (Magdalena Bay).

Figure 10 shows the relative abundance of sardine

eggs (as an index of spawning) per CalCOFI station in

the best sampled period (see Fig. 3). The radii of the filled

circles are proportional to the log (number of eggs) and

the heavy crosses show the yearly averaged line of spawn-ing. The warming event associated with the 195759

ENSO resulted in increased spawning at the northern ar-

eas and a northward shift of the average spawning lati-

tude; no less dramatic is the cooling period from 1958 to

1965, with a noticeable decrease in northern spawning

and increase in the south. It should be noted that the

Sebastin Vizcano Bay area was an active spawning

center throughout the full period. Another point to be high-

lighted refers to the seasonality of spawning; while eggs

are abundant in the middle months of the year north of

line 110 during 1954 to 1957, it also occurred during the

early months of the year during the 1958 to 1960 period

at the northern area. In the Sebastin Vizcano region,though, spawning occurs throughout the full year.

Fig. 9. Station-averaged macrozooplankton displacement vol-

umes per CalCOFI line number. Bottom names for approxi-mated geographical references.

Fig. 10. Relative abundance of California sardine eggs per sampled station for each line/month of the CalCOFI grid, 19511967.

Small dots show negative stations for sardine eggs; the radii of filled circles are proportional to log (number of eggs). Heavy

crosses show the averaged line of spawning per year. Left axis: CalCOFI grid lines. Bottom axis: years. Map at right shown

for reference.

SST yearly averaged anomalies were estimated from

data at coastal stations and were obtained from the

PACLIM database, including Point Hueneme, Crescent

City, Pacific Grove, Los Angeles, San Francisco and San

Diego. Yearly anomalies were estimated as the averaged

departures from the monthly means of the full series, then

they were standardized (std. score = (raw score mean)/std. deviation) and averaged.

3. Results

Figure 7 shows the latitudinal averaged concentra-

tion of photosynthetic pigments during the rich and poor

seasons. The rich season is that of intense southward

advection of the CC and intense upwelling (April to June,

Fig. 5); the poor one is the warm, subtropical period of

August to December. While there is a noticeable north-


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A longer, but less resolved perspective is shown in

Fig. 11 where annual average egg abundance is shown

per CalCOFI grid line, both in circles (proportional to

the log of the average) and gridded values. Unfortunately,

there is a critical gap in the data after the 1970s and south

of line 90, but the northward expansion of sardine spawn-

ing could be expected to be similar to that of the 1950s to1960s. A commonly utilized environmental index, SST

annually averaged anomalies at coastal stations, is shown

in the upper panel of the figure. There is a good corre-

spondence between the warming periods from 19561959

and 19761980 and sardine spawning expansion north-

ward; the major difference being that while there was a

sustained cooling trend after 1959, coinciding with sar-

dine spawning being progressively restricted to the south,

after the late 1970s warming there has been a continuous

lapse of warm conditions during which sardine spawning

has been present in the northern area.

4. Discussion

The hypothetical model that was proposed earlier

(Lluch-Belda et al., 1991) suggested that the Sebastin

Vizcano sardine stock has been the primary one, at least

during the interval of available data. Unfortunately, no

data are available for this specific area regarding scale

deposition rates, but the ~100 yr (about 1860 to 1950)

record for the Soledad Basin shown by Holmgren-Urba

and Baumgartner (1993, their figure 5) shows permanent

existence of sardine scales. It has been shown that sar-

dine schools from Sebastin Vizcano move annually

south to the Magdalena Bay area, where this core was

obtained (Alvarado-Castillo et al., 1995). The existence

of continuously spawning sardine schools at this area has

been recognized since the 1930s (Tibby, 1937; Ahlstrom,

1954; Hernndez-Rivas et al., 2000). Further, Ahlstrom

(1954) described it as a compact area of intense year-around spawning. We have shown in Figs. 10 and 11 how

this stock expands its spawning area northward and south-

ward, at the same time as temperatures warm up or cool

down.

This does not seem to be the case for the southern

California Bight nor the Gulf of California, where the

cores collected at the Santa Barbara Basin and off

Guaymas show multidecadal gaps with no sardine scales

and wide abundance fluctuations along the full series

(Holmgren-Urba and Baumgartner, 1993). While the

biomass attained in the southern California Bight during

population growth periods appears to be much larger thanthat of the Sebastin Vizcano area, its large variations

demonstrate the sporadic colonization of a highly pro-

ductive and changing area, at least for California sardines.

Radovich (1982) demonstrated the way in which sar-

dine schools of southern origin spawned in the southern

California Bight, thus maintaining a higher population

level. Beyond the most recent collapse, which was aided

by a fishery, earlier plummeting population events in

southern California and northward of it have certainly

occurred and the role of the Sebastin Vizcano area must

Fig. 11. Relative abundance of yearly averaged California sardine eggs per line in the CalCOFI grid, 19511997. Circles are

proportional to log (number of eggs). Same data gridded in background. Left axis: CalCOFI grid lines. Bottom axis: years.

Map at right shown for reference. Upper panel: yearly standardized and averaged SST anomalies.


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have been similar. The explosive growth of the northern

populations is based on the capability of the southern one

to expand, at least during the early stages of the process.

On the other hand, the Gulf of California appears to

have been reinvaded rather recently, during the cold pe-

riod of the 1960s (Rodrguez-Snchez et al., 2001), but

not for the first time; there are sardine scales in the gulfcore during the late 1700s (Holmgren-Urba and

Baumgartner, 1993; their figure 2).

5. Conclusions

The available information confirms the existence of

two temperate-tropical transition areas, one in the west

coast of the Baja California peninsula (the Sebastin

VizcanoPunta Eugenia region) and the other in the

central Gulf of California.

Both regions include Biological Action Centers that

maintain high productivity thoughout the entire year, al-

lowing sardine stocks to persist.The central area appears to be the refuge of the pri-

mary stock of sardine, which gives rise to the northern

stock, which in turn grows to larger size and expands

poleward. Moreover, the central stock appears to have

originated the Gulf of California stock in relatively re-

cent times.

We found grounds to further support the hypothesis

previously proposed by Lluch-Belda et al. (1991).

Acknowledgements

Data from the CalCOFI database used in this paper

was kindly supplied by Dr. Paul Smith (Scripps Institu-

tion of Oceanography); his continuous interest, orienta-tion and encouragement is profoundly appreciated. Many

individuals shared with us their published and unpublished

information: Roberto Flix-Uraga, Martn Hernndez-

Rivas, Angel Jimnez-Illescas, Casimiro Quinez-

Velzquez, Rubn Rodrguez-Snchez and Ricardo

Saldierna R., all at CICIMAR-IPN, were particularly help-

ful. This work had support from the Instituto Politcnico

Nacional (CGEPI 20.05) and the Centro de

Investigaciones Biolgicas del Noroeste, S.C. (EP3.1).

Lluch-Belda holds a fellowship from COFAA-IPN.

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Documents

Baja California’s Biological Transition Zones: