General patterns in the bottom communities in the seascapes

Length frequencies of spotted hake collected in beam trawl surveys of the New York & New Jersey seascapes in 2008 & 2009.  Fish less than 1 year in age (age-0) were represented by individuals less than 70 millimeters in length.  These age-0 fish were only collected in New Jersey.

Shell height frequencies for sea scallops collected in beam trawls towed in the New York & New Jersey seascapes in 2008 & 2009.  At least 2 height/age classes of scallops were represented in our collections and scallops were consistently more abundant off New York.


Fancy statistics are useful but if a data summary and simple graphs don't reveal a few intriguing trends, no amount of statistical hokus pocus will make the data interesting.  So in an effort not to lose the forest for the trees below is a general summary table that I admit is a little difficult to read (The fancy statistics will come later). The table lists the percent occurrence and mean abundance of species we collected in 2 meter beam trawls in the two seascapes during 2008 and 2009 using the methods described earlier. 


Species richness and patterns of age & size
Over the two years we collected 34 fish species and 19 invertebrates. Based on the animals lengths, 5 fish and 3 invertebrates were represented by more than one age class including an early juveniles less than a year old. Animals less than 1 year old are labeled age 0 in the table. For example age 0 spotted hake were represented in our trawl collections by fish less than 70 millimeters (mm) long. We collected at least two age classes of sea scallops; the youngest less than 40 mm in shell height. In addition northern sea robin, four spot flounder, black sea bass, windowpane flounder as well as rock crabs and long fin inshore squid used habitats in at least one of the seascapes as early juvenile nurseries.


The dominant fish species we collected were little skate, age 1+ spotted hake, butterfish, smallmouth flounder and gulf stream flounder. These last two species were among the 7 flatfishes occurring in our trawl samples. The butterfish we collected were all young juveniles. The most common invertebrates were seven-spine bay shrimp, sea stars, age 0 rock crabs, sand dollars, and spider crabs.


General differences between Seascapes
 Fish



A number of fish species appeared to be more common in the New Jersey seascape.  These included little skate, age 1+ spotted hake, bay anchovy were more common in New Jersey than New York while age-0 spotted hake collected exclusively in New Jersey over the two years. This suggests that larval delivery mechanisms and/or survival rates of newly settled spotted hake might make the New Jersey habitats more suitable nurseries. Butterfish and sand lance were also more abundant in New Jersey. Sand lance were rare in beam trawls, but these skinny little fish that live in sandy burrows were commonly captured on our underwater video and were dominant prey of the skates we collected in New Jersey during the early summer survey of 2008. The predators are always better samplers than we are. Gulfstream flounder, Red hake, age-0 searobin and striped searobin were more abundant in 2009. During that year the gulfstream flounder and red hake were more common in New York.


Invertebrates

Among invertebrates sevenspine bay shrimp and spider crabs were more common in the New York seascape. Age-0 rock crabs, which were very important prey for many of the animals we collected, were also slightly more common in New York. Sand dollars were more abundant in New Jersey in 2008 while age-0 longfin inshore squid were more abundant in that seascape in 2009.


Sea stars were consistently more abundant in our New Jersey collections. These animals are important predators of young sea scallops.   In the plot above of scallop shell heights, the smallest year class in 2008 is visible as a strong second size mode in 2009.  All year classes of sea scallops were more abundant in New York than New Jersey.  This might indicate that the settlement and survival of this 2008 cohort was high in New York. Differences in encounter rates of sea star predators with sea scallop prey in the two seascapes may be partially responsible for the differences in scallop abundance we observe.  This is just the kind of hypothesis we can test in field experiments to identify the seascape characteristics that effect the dispersal, growth and survival of animals that use the areas as nurseries. (Thanks to Jessica Lajoie for helping to get this information together)

A brief pelagic interlude

Densities measured with CTDs along the 4 transects on which we 

collected depth stratified plankton samples with a tucker trawl from 

August 9-12, 2010.   

I am trying hard to focus on the analysis of differences in fish and invertebrate bottom communities in the seascapes to follow up the last post about our bottom sampling methods.  But sometimes fun stuff happens.  First, Chip Haldeman from RUCOOL plotted up the density data we collected with CTDs during our plankton cruises last week. His waterfall plots are just too cool not to share.  Notice the highest density water (darkest red; cold & salty) in the head of the Hudson Shelf Valley in deep water off New Jersey.  The Steven's institute NYHOPs model showed a slug of cold salty water off the mouth of the estuary which could have welled up along the shelf valley. We had strong upwelling conditions all of last week. We tried to sample the plankton in the cold water on August 11th along the New York transect oriented southwest to northeast in the plot above. The positions of our tucker trawl tows on the NYHOPs model temperatures on the 11th are shown immediately below.

 Positions of the tucker trawls on NYHOPs forecast 

temperatures on August 11, 2010 showing the cool water
off the mouth of the estuary. This transect is also shown
stretching southwest to northeast in the plot above. 

RUCOOL used our CTD data to decide how to ballast a robot glider launched today off Sandy Hook.  The glider is to fly from Sandy Hook south to Cape May, New Jersey in a zigzag pattern from the near shore to 40 km offshore.  The glider will provide Steven's institute with temperature and salinity data to better tune the NYHOPs model for near shore forecasting.  It is also equipped with a dissolved oxygen and other optical sensors that will be used in the State of New Jersey's water quality monitoring program. This is exactly the kind of model tuning and habitat condition data we need to do our seascape work better.

At the last minute RUCOOL asked us if we could help with a vessel to launch the glider.  This was invitation for real fun.  Below are some pictures of the robot glider launch and a pod of porpoises that we saw on the way home. The mission of glider RU-16 over the next few weeks can be followed here.

Location where we launched the RU-16 glider on 8/20/2010 overlaid upon the surface salinity forecast from the NYHOPs model.  NYHOPs indicated that we launched the robot on the estuarine plume front which is why we may have had some buoyancy issues with the glider.

Launching the "bird" from the Research Vessel "The Torch".  Highlands New Jersey is in the back round on the right.

The "bird" at the surface.  The glider has a satellite telephone in its tail so the COOL room can upload instructions and download data to the robot anywhere in the world.

The pod of porpoises we saw on the way home about 1/2 mile off Sandy Hook.

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.

Tucker trawl and plankton catch, August 10, 2010

Recent sea surface temperatures from satellites

Looks like upwelling along NJ and LI coasts after some recent days of westerly and earlier southerly winds. Come aboard for our August ECOS cruise starting tomorrow, Monday August 9.