Alan ShanksUniversity of OregonOregon Institute of Marine BiologyCharleston, Or 97420

Coastal Oceanography, Larval Behavior, and the

Cross-shelf Transport of Larvae

1. Our goal is to prevent the introduction of invasive speciesarriving at our coast in ballast water.

2. Ideally we don’t want any individual of an invasive speciesto reach the coast, but

3. Really what we need to worry about is the establishmentof self-sustaining populations of invasive species.

So what does it mean for a population to be Self-sustaining?

First, most organisms reproduce sexually. They have to beclose enough together to mate. If they settle too far apart, they may survive and grow, but they will not reproduce. Obviously, the population is not sustained. This is the Allee effect.

It is not enough that organisms get to shore, they have to settle at densities high enough to over come the Allee effect.

From an oceanographic perspective, we need to look at transport mechanisms that carry larvae to shore, but they also probably have to concentrate them at the same time.

Second, to be self-sustaining, the larval phase has to work - many larvae are pelagic. When their pelagic development ends they return and settle back into the adult population. There must be a successful migration between habitats.

1. Most of the organisms we are dealing with are estuarine.2. Many are estuarine dependent and the entire life cycle -

adult and larvae - is confined to the estuary.3. Others are estuarine dependent, but the larval stage is

exported, development occurs over the shelf, and the larvaehave to migrate back to an estuary to complete the life cycle.

4. Some of the organisms will be coastal and their developmentwill be similar to the estuarine dependent species with larvae developing away from the shore and then migrating

back to shore to settle.

•Larvae do not have to behave like water.•Larvae are generally not passive tracers.•Larvae swim.•Few larvae swim as fast as horizontal currents, but essentially all

swim at least as fast as vertical currents.•Many can make extensive vertical migrations. •Larvae can control their horizontal movement, but they do it by controlling their depth - they act like balloonists.

•When we look at how oceanography may affect the movement of larvae we have to think about the possible coupled affects of oceanography and larval swimming behavior.

•Larvae of species that complete their life cycle within an estuary will likely display behaviors designed to retain them in an estuary (e.g., tidally timed vertical migrations).

•Coastal and open ocean oceanography are very different from that in an estuary.

•If these larvae are released in the ocean outside an estuary they will likely behave incorrectly given the oceanographic context and fail to migrate to shore.

•The larvae of many species may even be killed by ocean salinity.

•What about the larvae of estuarine dependent species whose larvae are exported from the estuary, develop over the shelf and then migrate back to the estuary (e.g., blue crab).

•Or coastal species whose larvae develop in the coastal ocean and then migrate back to shore (e.g., green and lined shore crabs).

•To complete their life cycle, the larvae of these species will probably have evolved behaviors that exploit coastal oceanography to increase their chances of returning to shore.

•There are a number of features of coastal oceanography that are universal - they are present on nearly all coasts.

•These types of larvae are the most likely to migrate to shore from an offshore ballast water exchange.

Onshore larval transport over the continental shelf is dominated by the:

1. The internal tides - (a) large internal waves (solitons) and (b) broken internal waves or internal bores

2. Upwelling relaxation events.

The Internal Tide

The waves are non-linear or solitons. The propagating wavesgenerate currents aboveand below the wave..

Large internal wavesride on any densityinterface in the watercolumn.

Large tidally generated internal waves can both transport and concentrate larvae.

Waves propagate at 50 to 20 cm/sec - waves can cross the shelf in less than a day.They travel from the shelf break all the way to the beach, and larvae are transported all the way into the surf zone.

Large internal waves of expression can break forming an internal bore. These transport water shoreward.

Larvae can be transported shoreward within the bore. The bore ultimately hits shore so larvae in the bore may be transported to the beach.

Upwelling Fronts Relaxing to Shore

Lucifer faxoni Blue crab megalopae

Spionid polychaetelarvae

Bivalve larvae

Over the continental shelf, there are at least three mechanisms of onshore larval transport that are both apparently pretty efficient and that will concentrate larvae.

The currents causing transport are present on essentially all continentals shelves the world over.

My Conclusion:

Because of the higher probability of shoreward larval transport over the shelf, ballast water should not be exchanged anywhere over the continental shelf.

Flow in the Southern California Bight is characterized by numerous eddies, which can retain larvae. In addition, tidal currents flowing around the Channel Islands and associated banks generate internal tidal waves that propagate across the Bight to shore.

Throughout the Bight, there appears to be good mechanisms of retention and shoreward larval transport.

The Southern California Bight is a poor location for ballast water exchange.

Onshore transport from the Open Ocean to the Shelf or Shore:

1. Classic wind driven upwelling.2. Wind driven surface currents (e.g., Langmuir Cells)3. Upwelling jets and eddies.4. Movement of large estuarine plumes.

In areas that experience sustained upwelling favorable winds, the source of the upwelled waters is the continental slope.

Vertically migrating animals found off the continental shelf and over the slope may enter these waters and be drawn up onto the shelf during upwelling.

How deep would larvae have to migrate? Probably somewhere between 50 and 150 meters.

Are larvae strong enough swimmers to make this migration? Many crustacean larvae have swimming speeds > 2 cm/sec. They can swim to 100 m depth in a few hrs.

Do larvae swim so deep? Yes, larvae of a variety of species do.

For example, larvae of the Dungeness crab make a diurnal vertical migration to > 70 m depth and it looks like the upwelling generated by the spring transition transports their larvae onto the shelf from the open sea.

When the wind blows on water it generates Langmuir circulation cells. Larvae or flotsam at the surface in Langmuir cells move just about down wind and at about 3% of the wind speed.

These winds may transport larvae toward shore.

To be transported, the larvae just have to remain at or near the surface.

In the winter, sustained winds from the west push Vellela vellela and glass fishing floats from the open ocean all the way to the coast.

As the upwelling season progresses, perturbations develop in the coastal currents. These take the form of jets and eddies.

Larvae may be carried away from shore by jets and onto the shelf by eddies.

Unfortunately, the location of jets and eddies varies in time and space making it difficult to design ABWEA to avoid their affect.

Plume

windPlume

wind

Estuarine Plumes as Larval Transporters

Upwelling winds push an estuarine plume away from shore. Large plumes (e.g., Columbia River and San Francisco Bay) can extend far offshore.

Downwelling winds push the plume shoreward and up against the coast where it travels north hugging the shore.

Larvae from estuarine species released at sea into plume water during ballast water exchange may perceive the plume as an estuary.

If they behave as if they are in an estuary (e.g., swim up to avoid the high salinity water) they could remain concentrated at the surface in the plume.

When the plume is pushed back to shore during downwelling, the larvae may be transported all the way to the coast where they will be in an ideal location to be pulled into an estuary by the flood tide.

My Conclusion:

1. If ballast water contains larvae that vertically migrate, do not exchange water over the continental slope.

2. I am not sure what to recommend with respect to wind driven surface transport.

3. Because we cannot predict the location of topographically generated jets and eddies it is difficult to make general recommendations.

4. Do not exchange water near any location where estuarine plumes may be present. This would hold whether the plume was located off the shelf or on the shelf.

Current Regulations:

Exchange beyond 200 nm miles. Good distance - nicely conservative and should do the trick.

Exchange beyond 50 nm and at least 200 m depth. The distance is OK, probably safe in nearly all cases. I would increase the depth to 1000 m.

Why?First, because at 200 m the exchange is right next to the shelf where it would be easy for them to be transported onto the shelf.

Second, organisms that settle at this location settle into deep water where they will be unlikely to survive.

For ABWEA I would suggest:

Yellow - Avoid ballast water exchange over the continental shelf and slope. Further, avoid exchange within the Southern California Bight.

Blue - Avoid ballast water exchange adjacent to large estuaries.

Washington

Oregon

California