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Paralabrax nebulifer population
genetics: "How connected are populations?"
 [photo
by Debbie
Karimoto]
 Barred
sand bass are distributed from Bahia Magdalena (Mexico) to Santa Cruz, CA
(not commonly found north of Point Conception) [Enlarge map...in
a new
window] Figure by G. Benavides
 [Enlarge cladomap...in
a new
window]
Figure by G. Benavides (tree based on MT
Craig et al., 2001)
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Paralabrax
nebulifer: Determining population
structure |
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Each year during mid spring to late summer, barred
sand bass aggregate offshore, usually over sand flats, to spawn.
This occurs throughout its range. |
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Barred sand bass larvae remain in the plankton for just under
a month, after which time they begin to settle out in the benthos.
Presumably, the longer the planktonic duration time, the higher
the dispersal capacity, but this has not been the case for many
marine organisms. |
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Physical factors, such as strong temperature gradients and complex
oceanic currents, may be sufficient to prevent breeding populations
from mixing with one another.
Off the coast of California and Baja, the net movement of water
throughout the year is south. While there is seasonal change
in oceanic
hydrodynamics, the California
Current flows southward, especially during sand bass spawning
period. |
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A net southward movement of larvae is expected.
This suggests that northern barred sand bass populations are
contributing disproportionately more larvae (and hence adults)
to southern populations. If this is the case, then northern populations
act as genetic sources while southern populations act as sinks.
Alternatively, seasonal changes in ocean current dynamics during
the spawning period may complicate dispersal patterns. This may
producing genetically isolated populations or homogenized populations
across the species range (panmixia) |
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To determine connectivity between breeding populations
of barred sand bass, it is necessary to use DNA markers with
sufficient resolution to show dispersal patterns. The DNA markers
must be polymorphic
enough (having many forms) to act as genetic 'fingerprints'.
One such class of DNA fingerprints are called microsatellites. The
goal
of my study is to use multiple sets of microsatellite markers
as independent lines of evidence to assess population connectivity. |
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If there exists asymmetric gene flow between
breeding populations of barred sand bass, then the microsatellite
data should exhibit significant spatial portioning of genotypes
across sampling locations. If complex oceanic
current patterns act to homogenize breeding populations by increasing
dispersal capability among all populations, then there will not
be significant spatial partitioning of genotypes; there will
be no population structure. |
Photo of Bob Halal holding a record
barred sand bass (6.2 kg) [photo appears on Jeff
Spira's website]
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Paralabrax
nebulifer: Ecology and geography |
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Paralabrax
nebulifer is a nearshore, benthic
marine fish belonging to the family Serranidae. The common name is barred
sand bass. Barreds are found from Punta Magdalena (Mexico) to just north
of Point Conception, USA (see map) |
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In general, adult and subadult sand bass
occur over sandy flats and rocky outcrops. There, they feed on invertebrates
and other benthic fish (especially midshipmen) Barred sand bass are sedentary
fish and usually don't wander far off from where they settled (that is, they're
philopatric) |
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Sand bass share a geographic range with
three other serranids: kelp bass (P. clathratus), spotted bay bass
(P. maculatofacsiatus), and gold spotted bass (P. arbitrates),
which is restricted to Mexican waters.
In San Diego Bay, it is not uncommon
to find juvenile to young -of-the-year sand, kelp, and bay bass
living together. |
Three sympatrically occurring congeners: barred
sand bass (top), spotted bay bass (middle), and kelp bass (bottom). These
fish were sampled from
San Diego Bay. [Photo by Larry
Allen]
Unidentified serranid larvae from CalCOFI Lines surveys (1951 to 1984)
Larvae were found as far offshore as 400 kilometers, but rarely north
of Point Conception. [Enlarge map...in
a new
window] Figure by G. Benavides
The California Current predominantly flows southwards during the
spring season. [Enlarge map...in
a new
window] Figure by G. Benavides
While the net flow is southwards during the summer, coastal eddies and
gyres are more prominent throughout barred sand bass range. [Enlarge map...in
a new
window] Figure by G. Benavides
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Paralabrax nebulifer:
Significance of study |
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The successful management of barred
sand bass is crucial on economic, ecological, and evolutionary fronts.
Taking into account the concerns brought by these three E's
should provide crucial information on how to effectively manage this
important species. |
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First, sand bass have consistently been among
the top 10 sought after fish by recreational anglers, especially
aboard commercial
passenger fishing vessels, or CPFVs. Along with the revenue generated
from California State fishing licenses, the sport-fishing economy
(CPFV day passes, tackle shops purchases, other related expenses)
benefits greatly from stable populations of this species. From a
purely recreational perspective, the sport-fishing industry provides
non-monetary benefits as well. |
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Much to their detriment, barred sand bass return to the same spawning
localities year after year. As a result, anglers maximize their returns
by focusing most of their fishing effort over these well-defined
areas. Incidentally, catch-per-unit-effort (CPUE) spikes up during
August, when barred spawning activity is at its highest. In the 1990s,
annual catches for
barred sand bass were upwards of half a million individuals.
While sand bass
have not received special protective designation, there may be real ecological
consequences
of
consistently removing half a million fish per year, especially during their reproductive
season. |
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From an evolutionary perspective, there are several
benefits from studying gene flow dynamics.
First, identifying source
populations is crucial if there exists asymmetric gene flow.
In the case of barred sand bass, most of the fishing pressure occurs
within the California Bight, which is the northern extent of sand
bass range.
Second, by identifying real genetic populations (versus
sampling populations), a more effective management strategy can be
implemented that takes into account the presence of unique breeding
populations (also called evolutionary significant units or ESUs)
Identifying ESUs is also crucial in designing marine protected areas
(MPAs) that
address real biological needs and concerns.
Third, understanding the processes that create and maintain genetic
diversity in marine systems not only satisfies our curiosity about
our world, it also provides us with an ethical framework with which
to properly conduct our activities, and in addition, it also allows
us to examine the serious impact humans have had on our natural resources. |
UCSC - Ecology and Evolutionary Biology
Long Marine Lab - Center for Ocean Health
100 Shaffer Road
University of California
Santa Cruz, CA 95060
Phone: 831.459.1282
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