The Seascapes

The Seascapes

Wednesday, August 18, 2010

Methods of sampling the bottom communities in the Seascapes

Egg capsules of longfin inshore squid captured on videotape
of the seabed in the New York seascape using the camera 
sled described below.   
Many of the larval fish, crabs and molluscs we collect in our tucker trawl on plankton cruises are probably just passing though the seascapes off Rockaway, New York and Seabright, New Jersey like all the ships traveling inbound and outbound from New York Harbor.  However some may settle out of the plankton to become small juveniles that use the seascapes and adjacent areas as nursery grounds.  Th­e New York Seascape is often supplied with water flowing from the east along the south shore of Long Island and Block Island Sound, while the New Jersey seascape usually receives surface water flowing out of the Hudson-Raritan river estuary, as well as deeper offshore water flowing inshore when winds from the south drive coastal upwelling.  Since the sources of water to the seascapes are often so different, the communities of animals occupying bottom habitats could reflect differences in larvae in the water masses.  If the bottom communities are different there are other plausible explanations too.  For example larvae settling as juveniles could suffer much higher predation mortality in one of the seascapes, while some older animals could be residents in one seascape rather than the other because oceanographic barriers restrict their movements.  Answering these questions requires that we measure differences in the rates animals are eaten by predators or the pathways over which they move in the ocean. These processes are very difficult to measure in the sea. Like all things ecological these and perhaps other processes are probably going on simultaneously to greater or lesser degrees.  In our study we chose to answer the easiest question first. Are there differences in the juvenile and adult communities in the seascapes that are related to the supply of baby fish and crabs to them? Knowing the answers to this questions should allow us to ask the more difficult ones more specifically and with more nuance in the future.

New Jersey and New York seascapes.  Dark brown patches are
fine sand and mud, light brown patches are sandy based on
classified side scan sonar imagery.  Depth  contours are shown
in the closeup image of the New Jersey seascape.  Blank areas
in the seascapes are located where the rocky reef and other hard
structures prevented us from towing the video sled and two
meter beam trawl on the bottom.
Are there differences in the bottom communities of fish and invertebrates in the two seascapes?  We performed trawl and underwater video surveys of bottom habitats in 2008 and 2009 to answer that question.  We were careful to choose seascapes with similar bottom types on either side of the mouth of the Hudson Raritan river estuary.  Choosing similar bottom habitats allowed us to estimate the relative contributions of fine scale variation in the seabed characteristics as well as broader scale circulation patterns to making animal communities different between and within the seascapes. We used maps of bottom depth and sediment type based on sidescan sonar to divide up the bottom so that each seascapes had equal proportions of fine sand and mud, medium sand and hard bottom habitat patches in both  “shallow” and “deep” areas (Lathrop et al. 2006).   The surface area of the seabed in each of the seascapes was ~ 95 km^2 (~37 miles ^2).   Shallow areas ranged in depth from 10 to 20 meters. The deep areas ranged from 20 to 30 meters (~100 feet) which was as deep as we could sample using the boat and gear available to us.  Within the deep and shallow areas, fine and medium sand and hard bottom habitats made up about 1/3 of the surface area of the bottom.

The video camera sled with a low light Deep Sea power
and light camera (the black thing in the center) and a
conductivity temperature and depth probe mounted at the
top of the frame. The length of chain at the bottom of the sled
is a "tickler" that flushes the critters out into the field of view
of the video camera. (see the videos at the end of the post).
The two meter beam trawl which we towed immediately adjacent
to the track of the video sled to capture live animals for verifying
video imagery and to characterize the food webs in the seascapes
We towed a video camera sled and a 2 meter beam trawl to sample fish, invertebrates and bottom habitats in the seascapes.  We limited our sampling to the patches of fine and medium sand because we could not tow our gear over more complex hard bottom. We conducted the surveys during the late Spring (Late June), Mid Summer (Late July through early August) and the Autumn (late September through early October) to identify the seasonal changes in the habitat associations of fish and invertebrates. During each survey we randomly selected two patches in the two types of soft bottom habitat (fine and medium sand) in each depth zone (shallow and deep) within the seascapes (New York and New Jersey).  We randomly selected 2 sites within each "habitat patch" for towing the gears. Thus we sampled animals and their habitats at 32 locations in each survey (2 sites x 2 patches x 2 sediment types x 2 depth zones x 2 seascapes x 3 seasons x 2 years = 192 sites total).  After measuring the vertical structure of the water column at each site with a conductivity, temperature and depth profiler (CTD) we towed the video sled at speed of about 1 knot (0.51 meters per second) for 10 minutes over ~ 300 meters of bottom. Our sled which was designed after Spencer et al (2005) and equipped with a Deep Sea Power and Light low light video camera, allowed us to identify associations of fish and invertebrates with fine scale characteristics of the seabed, including sizes and wavelengths of sand waves and the presence and absence of burrows and pits made by animals (see video below).  We also attached a CTD probe that measured temperature, salinity, oxygen and PH to the sled, and used our ADCP to measure current speeds over the bottom during every second of tow. These instrument allowed us to relate the presence and absence of animals on video to changes in the characteristics of overlying bottom water as well as to the structure of the seabed itself.  After hauling back the video sled we immediately towed a two meter wide beam trawl (with a 3 mm mesh net) parallel to, but 10 to 20 meters to one side of, the sled track. We counted and measured all of the animals collected in these trawls.  The live animal samples allow us to confirm the identities and sizes of the animals we collected in the video sled images.  They also allowed us to perform dietary analysis to see whether the food webs in the seascapes were different. In the next few posts we will try to summarize some analyses of differences in the juvenile and adult bottom communities.

The video immediately below of the sea scallop swimming was collected in the deep portion of the New York Seascape, while the videos of lobster (middle video) and windowpane flounder (bottom video) were collected in the New Jersey Seascape.  There were many lobster burrows in the dredge spoils from the deepening of the port of New York that were deposited in the north east corner of the New Jersey seascape.  At the start of the middle video of the lobster the sled also passes over a large rock crab sitting in a burrow.

Spencer, M. L., A. W. Stoner, C. H. Ryer, and J. E. Munk. 2005. A towed camera sled for estimating abundance of juvenile flatfishes and habitat characteristics: Comparison with beam trawls and divers. Estuarine, Coastal and Shelf Science 64:497.

Lathrop, R. G., M. Cole, N. Senyk, and B. Butman. 2006. Seafloor habitat mapping of the New York Bight incorporating sidescan sonar data. Estuarine, Coastal and Shelf Science 68:221.

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