Population Connectivity

One of the goals within marine science is to understand what influences the abundance and distribution of marine populations. A vital part of this puzzle is to determine how populations are connected in the oceans. The principal method for connections is through drifting planktonic larvae produced by stationary populations of animals, (e.g. some fish, corals and invertebrates). The larvae drift on ocean currents for days to months, some traveling hundreds of miles before they settle down to begin a more sedentary life style (a process called recruitment). This process is extremely important to the stability of marine populations.

We have, however, a limited understanding of the spatial scales over which marine populations are connected by larval dispersal. Our lack of knowledge represents a huge obstacle to any comprehensive understanding of the population dynamics of marine animals and their related management. For example, if one reef serves as the primary source of fish larvae for many “downstream” reefs along a coast, that site might be essential to protect as a marine reserve, so it can continue to sustain the other populations.

Balanus Etchemendy

Tracking Marine Larvae

Understanding how marine populations are connected and replenished by larvae is vital for successful management and conservation of marine species, but tracking marine larvae is extremely difficult for a number of reasons:

  • They are too small to tag.
  • Larvae may rarely recruit to the location where they were spawned.
    • Although genetic studies can match up a parent with its offspring, finding the parent is like looking for a needle in a haystack.
  • Larvae do not drift passively in the ocean.
    • Larvae can influence the direction and distance that they move by controlling their depth in the water. Complex ocean currents move in different directions at different depths, and this directly affects larval transport.

PISCO research is helping to fill in the blanks of how marine populations are interconnected using research into the recruitment, genetic connectivity and microchemistry of larvae, and oceanographic models.

PISCO oceanographic models are developing coupled biological-physical models to evaluate connectivity and larval dispersal within the California Current Large Marine Ecosystem (CCLME).  These efforts are being employed in the establishment and evaluation of Marine Protected Areas in California.

Through PISCO genetic research we have determined that levels of connectivity, as measured by levels of genetic difference, are irregular among species, and that conventional explanations (that connectivity varies chiefly with pelagic larval development time) are not very accurate.

Using microchemistry techniques PISCO has been able to build a record of source signatures taken from hard parts of animals (e.g., a fish otolith) that accrete layers of calcium carbonate containing geographically distinct levels of trace elements. By running detailed ocean circulation models “backwards” from heavy recruitment events, we can generate predictions of source populations that can be validated with microchemistry.

 

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