credit: Gregorio Benavides (2003)

Gregorio Benavides

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 a new window] Figure by G. Benavides

[Enlarge a new window] Figure by G. Benavides (tree based on MT Craig et al., 2001)


Fish profile
  Paralabrax nebulifer: Determining population structure
Each year during mid spring to late summer, barred sand bass aggregate offshore, usually over sand flats, to spawn. This occurs throughout its range.
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.
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.
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)
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.
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]

News & Features
Paralabrax nebulifer: Ecology and geography
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)
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)
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 a new window] Figure by G. Benavides

The California Current predominantly flows southwards during the spring season. [Enlarge 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 a new window] Figure by G. Benavides

Fish profile
  Paralabrax nebulifer: Significance of study
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.
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.
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.
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