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The Decline of Seagrass Meadows

Zostera! Eelgrass, Zostera marina, is a flowering, marine vascular plant that remains submerged all the time. This is quite a feat for vascular flowering plants, and only a few dozen species world wide are capable of growing completely submerged in a marine environment. Eelgrass creates and extremely important habitat, its upright structures and complex root system create a 3-D living space for many different types of animals. It is (or was) the dominant habitat forming SAV (submerged aquatic vegetation) throughout much of the coastal waters in the northeastern United States. Unfortunately, for various reasons, eelgrass meadows have seen drastic declines, and in many locations eelgrass only exists in a mosaic of small patches. This is extremely bad news as many of the important, and formerly important, commercial and recreational fisheries of the northeast US are dependent on Zostera at some part of their life cycle as a nursery and foraging ground. Some of the species are finfish like tautog, bluefish, fluke, winter flounder, porgies, while others are shellfish such as blue mussels, hard clams, oysters, bay scallops, and blue crabs. Many of the aforementioned species support or once supported vibrant fisheries. Many of those fisheries have collapsed, also for various reasons. However, is it possible there is a link between the crash of the fisheries, the decline of Zostera and the failure for recovery on both ends?

Bay Scallop on Eelgrass

Argopecten on Zostera! Bay Scallops, Argopecten irradians , have developed a very close relationship with eelgrass, Zostera marina. As larvae, they are passively transported, and tend to settle in eelgrass meadows when the current is dampened by the 3D structure of the seagrass. This same 3D structure provides post-set juvenile scallops a spatial refuge from predation. Even as larger juveniles and adults, scallops are capable of, and have been shown to, actively select eelgrass habitats.

Other species also use eelgrass

grass shrimp A number of other species utilize eelgrass as a habitat. Included are grass shrimp, like the Palaemonetes pugio, other decapods such as blue crabs, bivalves such as hard clams, gastropods (snails), and numerous fish species, including winter flounder, tautog and cod.

Priming the larval pump!

So anyone following my blog knows that I was actively involved in the bay scallop restoration efforts in Long Island.  To refresh, scallop populations supported a vibrant fishery in NY until the mid 1980s, when populations crashed due to the first occurrence of a brown tide bloom, and recurrent brown tides pushed scallops to the brink of local extinction.  The brown tide has not occurred in the Peconic Estuary since 1995 (although it still occurs on Long Island waters), so in 2006, restoration efforts started to help jump-start local scallop populations in the Peconics.

Commercial bay scallop landings and Brown tide occurrence

Commercial bay scallop landings and Brown tide occurrence

These efforts sought to boost spawning stock and concentrate high densities of scallops in close proximity to enhance fertilization and reproductive success.  The idea was that low population densities of adults were limiting reproduction, which was subsequently limiting larval supply and recruitment.  The restoration efforts sought to boost adult populations by establishing spawner sanctuaries using an array of lantern nets or by high density on bottom restoration.  You can watch a number of videos on these efforts here, or watch the Fox News piece below:

<script type=”text/javascript” src=”http://video.foxnews.com/v/embed.js?id=1301157980001&w=466&h=263″></script><noscript>Watch the latest video at <a href=”http://video.foxnews.com”>video.foxnews.com</a></noscript>

The restoration efforts had been very successful – every year we see higher numbers of scallop spat than the year before (despite the same effort), and the results have translated to scallops on the bottom and to the fishery.  Recently, our group was able to publish some of our findings in Marine Ecology Progress Series.

We were able to demonstrate at all sites annual increases in the mean spat per bag – that means, each year post-restoration, we saw greater numbers of baby scallops in our collectors.  This occurred not only in our basins where we actively did restoration, such as Orient Harbor and Hallock Bay, but also in nearby basins with no active restoration efforts, such as Northwest Harbor.

A figure from our MEPS paper, illustrating the annually increasing densities of scallop spat in Orient Harbor. The cross denotes the site of a large lantern net spawner sanctuary.

In fact, scallop spat abundance increased up to 3000% of pre-restoration levels.  This was despite that none of the environmental parameters had changed from the 10 years prior to restoration beginning to the 5 post-restoration years we examined for this study.  Environmental variables could possibly influence the amount of larvae produced and larval survival.  However, temperature, chlorophyll (a proxy for food), nitrogen, monthly rainfall and salinity were not different between the 2 time periods.  This suggests that the restoration efforts played an important role in helping to increase the larval availability.  In essence, we “primed the larva pump!”

Environmental variables for the pre-restoration period (1996-2006) and the post-restoration period (2007-2010).

Obviously, we were expecting these results and were very excited that we were able to eliminate other possibilities of increased larval supply.  Additionally, the dates of peak settlement for the most part lined up with our estimates of spawning dates and settlement.
This doesn’t mean much, however, if it isn’t translating to the bottom, since we collect these scallops in spat collectors hanging in the water column.  Many sources of mortality, but primarily predation, can occur from the time the scallop settles on the bottom to the time it can spawn and then contribute to the fishery.  I focused most of my dissertation research on habitat and predation on scallops.  Some of the cool things from that research suggests that patchy seagrass might not be detrimental to scallop populations and that an invasive species might be a suitable alternative habitat.  So, despite limited seagrass in our restoration estuary, we have seen increases in scallops on the bottom.

 

On bottom increases in scallop densities post restoration

On bottom increases in scallop densities post restoration

And in many of the basins, these increases in on bottom densities in the fall correlates with the increases in spat fall during the spring of that same year.

Relationship between seed scallop densities in the fall on the bottom in Orient Harbor and the spat per bag landings from the spring.

Relationship between seed scallop densities in the fall on the bottom in Orient Harbor and the spat per bag landings from the spring.

We are currently preparing this data for a manuscript, showing the subsequent increases in on bottom densities, fishery yield and the economic benefits of the restoration efforts.  Hopefully, the success of this project and the information gathered will help other restoration efforts on Long Island, such as the Shinnecock Bay Restoration Project (which I have blogged about ) and elsewhere.  I am hoping to turn some of the things I learned with this project (and the many various side projects) to my oyster work here in NC, although I also plan to keep working with scallops.

More to come, so stay tuned!

3 comments to Priming the larval pump!

  • Cochlodinium kills

    Hmmm…..so we publish our publications on our blogs these days?

  • Martin

    Thank you for posting this online. I am a natural resource officer in a Cape Cod town, and this is very interesting to our shellfish propagation team. We don’t get any research journals, so there is no telling when we would hear about this. We are buying scallop seed this summer, so these ideas will help.

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