17 th Annual MDNR Shellfish Survey of the Md. Coastal Bays June 2011

Transcription

1 17 th Annual MDNR Shellfish Survey of the Md. Coastal Bays June 2011 A Benthic Travelogue Mitchell Tarnowski and Robert Bussell MDNR Shellfish Division

2 Recent Origins of Hard Clam Populations in the Maryland Coastal Bays

3 1908 Oyster Bar Chart of Upper Chincoteague Bay The following chart from the Yates survey indicates that oysters did not inhabit the coastal bays above South Pt. Prior to the opening of the Ocean City Inlet, these waters were too brackish to support oysters. Anecdotally, oysters on these northernmost bars were occasionally subjected to mortalities due to freshets. Thus hard clams, which require much higher salinities than oysters, were confined to lower Chincoteague Bay. Chincoteague Inlet in Virginia provided the necessary salinity for the southern Maryland hard clam populations, but its influence probably did not extend too far above the state line.

4 Oyster Bars

5 Ocean City Inlet In August 1933, a ferocious storm broke through the barrier island at Ocean City, inundating the coastal lagoons with ocean water. With the addition of this second inlet, salinities rose throughout the coastal bays. The inlet was quickly stabilized with stone jetties to provide a harbor of refuge for ocean-going vessels and to benefit commercial fisheries, particularly shellfisheries. Thus the Maryland coastal bays were altered from a largely brackish water ecosystem to one of elevated salinities.

6 O.C. Inlet

7 Hard Clam Landings Because there were no hard clam surveys at the time, commercial landings are used as a surrogate for trends in population size (recognizing the attendant caveats). The following graph illustrates two points: The rapid growth of the population after the inlet opened The Maryland hard clam population has been exploited since its inception The graph ends when the first surveys were conducted in 1952/53.

8 Hard Clam Landings - Worcester Co. 300 O.C. Inlet Landings (lbs x 1000)

9 Hard Clam Densities in Other States The following graph illustrates hard clam densities in other states. Note that the surveys were conducted at various times with different techniques. The Maryland population, based on the 1952/53 surveys, compares poorly to other states. The 1952/53 surveys are the baseline against which present-day densities are measured. This suggests that despite a rapid initial increase, further growth of the population was suppressed by exploitation, both commercially and recreationally. In fact, two estimates in the late 1950 s and late 1960 s indicate that recreational harvesting accounted for 50% of the total hard clam catch.

10 Hard Clam Densities From Various Regions (>30 mm in shell length) Clams per sq. m RI NY NJ MD VA NC SC GA MDNR Shellfish Div.

11 Methods The Hydraulic Escalator Dredge (HED)

12 Looking Forward on the Survey Boat From left to right: water intake pipe (barely visible behind muffler), water pump driven by diesel engine (in box), outlet hose across deck, hydraulic escalator dredge. The head of the dredge is lowered to the bottom through a system of cables, pulleys, and hydraulic winch. Using a 30 in. wide dredge, the tow distance was standardized at 250 ft. using the odometer function of a handheld gps unit. 225 randomly-selected stations stratified by embayment or tributary were sampled in 2011.

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14 The Business End of the Dredge Water is pumped to a manifold across the front of the dredge. A series of nozzles on the manifold jets water into the bottom ahead of the dredge, softening the substrate for the forward edge of the dredge to easily slip though. The cutting depth of 4-6 in. is maintained by runners on the side of the dredge. The catch is floated onto the stainless-steel mesh conveyor and up to the rear of the boat where it can be collected. For the survey, all macro-molluscs were collected, counted, and measured and other organisms as well as habitat were noted.

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16 A Benthic Travelogue

17 A Rich Mosaic of Habitats The Maryland coastal bays possess a wide variety of habitats which are readily appreciated with the HED. Some sampling gear types such as standard dredges or trawl nets integrate the samples over distance, while others such as grabs give a snapshot of a very small area. In contrast, the conveyor feature of the HED gives a picture (more like a movie) of the spatial relationships of the species to their habitat as well as to each other, both on and in the substrate of the bays. The following series of slides illustrates the major habitats.

18 Sand Sand substrate is found predominantly along the eastern side of the coastal bays adjacent to the barrier islands, with occasional small lenses along the western shore. The sample comes up free of substrate, which washes through the open-mesh belt of the escalator. In addition to the hard clams, the photo shows some worms, drift macroalgae, and sea cucumbers (the round objects).

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20 Mud While people generally think of mud bottom as a goopy wasteland, this is actually true in only a few areas of the Maryland coastal bays, such as the St. Martins River. As the following slide shows, many muddy areas are teeming with organisms and the structures they build, such as these worm tubes. Hard clam densities are generally lowest in mud substrate.

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22 Shell Bottom Shell is an increasingly scarce commodity in the coastal bays. With the demise of subtidal oyster populations, shell is no longer being replenished, and the remaining shell is slowly getting buried by sediment. The shell provides hard substrate for a number of epibenthic species such as serpulid worms, as can be seen in the photo by their orangey calcareous tube. Locals refer to this as worm coral. The blackened or gray portions of the shells are where they were buried in the substrate. Oyster shell on the surface is usually a golden-brown color.

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24 Shell as Refuge In addition to supplying hard substrate for epibenthic organisms, shell also provides a refuge from predators for infaunal species. The shell acts as a physical obstacle to many foraging predators such as blue crabs and cownose rays. Consequently, hard clam densities are usually higher in shelly bottom. Again, notice the clusters of serpulid tubes, many of which have been buried.

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26 Drift Algae The coastal bays are home to an abundant variety of drift macroalgae (unattached seaweeds). They often get snagged and accumulate on seagrass beds, where they compete with the grasses for sunlight. Macroalgae can provide habitat to many small and juvenile organisms. In addition, macroalgae offer substrate to which colonial epibenthic organisms such as sponges and bryozoans (in orange) can attach.

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28 Macroalgae Mats In certain years or places macroalgae can become quite dense and abundant, becoming a potentially deadly nuisance for benthic organisms, robbing them of oxygen when it blankets the bottom and starts to decay. In the following photo, a dense mat of Chaetomorpha sp. has clogged the head of the dredge. Fortunately, this was the only such incident this survey, although in past years this species has occasionally been extremely abundant, to the point where the survey vessel got entangled and stuck in a mat (a Sargasso Sea re-enactment in Chincoteague Bay.

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30 Submerged Aquatic Vegetation Submerged aquatic vegetation (SAV) or seagrasses are rooted, vascular plants, in contrast to macroalgae. There are two species of SAV in the coastal bays: eelgrass (Zostera marina) and widgeongrass (Ruppia maritima). Most of the seagrass beds in the coastal bays are found in shallow, sandy substrate on the east side of the bays, as well as some limited areas in the western portion, notably around Mills and Tizzard Islands. After explosive expansion in the 1980 s and 1990 s, SAV acreage has leveled off and in some areas has retreated. Seagrasses offer habitat to a vast assortment of organisms such as the burrfish nestled in a bed of eelgrass in the photo.

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32 The Seagrass Refuge Seagrass meadows provide a refuge to juveniles of many invertebrate and fish species, as well as more specialized adult animals such as seahorses and pipefish. The photo shows a 4.5 in. flounder, although numerous smaller flounder (1.5 2 in.) were observed in the grassbeds, as well as abundant juvenile blue crabs in the in. carapace width range.

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34 The Dark Side of Eelgrass Eelgrass continuously sloughs off shoots, particularly in the summer. These discarded shoots can accumulate in thick mats on the bottom, where they decay, turning the bottom anoxic and smothering the organisms under them. This is indicated in the photo by the blackened shoots and substrate. Note that there are no living organisms visible. The dead and decaying seagrass can form windrows along the shoreline, blocking access to the water s edge for mating horseshoe crabs, terrapins, and foraging shorebirds. These windrows can also accumulate on marsh grasses, killing them along the margin of the marsh, which accelerates erosion, as at Big Bay Point. Seagrasses also change the nature of the substrate, turning sand into organic-rich, poorly oxygenated muck.

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36 Bay Islands A rapidly diminishing habitat in the coastal bays are islands, on which erosion is taking its toll. These are especially important to the avian fauna of the region as roosting and nesting areas, where they are free of disturbances and predators (except other bird species). The photo shows only two small tumps remaining of a once substantial island Robins Marsh.

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38 Changing Land Forms in Chincoteague Bay The nautical chart from 1880 delineates the extent of Robins Marsh (upper left portion of the bay) back then. Compare with the previous photo. Also notice the existence of an inlet at Green Run Bay (lower right). During the 1860 s and 70 s when the inlet was open, the oyster cultivation industry thrived in Chincoteague Bay, shipping out thousands of bushels of oysters to Philadelphia, New York, and even Europe. After the inlet closed in the late 1870 s, oyster aquaculture continued, but never again to its previous extent. That is why great expectations were placed on the 1933 inlet, which unfortunately never panned out for the oyster industry.

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40 Other Species As mentioned previously, although the main target of the shellfish survey is the hard clam, data on other molluscan species are collected as well. As an example, the photo shows a wide range of sizes of the surf clam Spisula solidissima observed at a station in Sinepuxent Bay. An oceanic species that prefers cooler waters, surf clams are found in the coastal bays within a few miles on either side of the O.C. Inlet, where the summer bay temperatures are moderated by the waters of the Atlantic. This is a commercially valuable species which is used in chowders, clam strips, minced canned clams, etc. on a mass-produced basis.

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42 Brittle Stars The most unusual find of this year s survey was a couple of brittle stars in Sinepuxent Bay within a few miles of the inlet. These were a first for us in 17 years of extensive sampling. The arms are about 6 in. long. As the name implies, this species is extremely fragile and readily sheds its arms, as my colleague discovered while trying to arrange one for a photo. Despite their fragility, the brittle stars came up intact, demonstrating the utility of the hydraulic escalator dredge as a sampling gear.

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44 Results Hard Clam Populations

45 2011 Hard Clam Densities by Bay All of the bays except Assawoman showed increases in hard clam densities over the previous year. The highest average hard clam density by a wide margin was in Isle of Wight Bay, followed by Sinepuxent and Chincoteague Bays. As in past years, the St. Martin River had the lowest average density, which was unchanged from the previous survey there in These densities are well below those of the 1952/53 baseline surveys, with Isle of Wight Bay approaching the closest.

46 2011 Maryland Hard Clam Survey MDNR Shellfish Div. Clams/Sq M Assa IoW StM Sin New Chin

47 A Closer Look at Chincoteague Bay The following time-series shows that Chincoteague Bay hard clam densities have essentially been flat-lined over the past 18 years at levels well below the historical baseline. In recent years the trend has been a slight but persistent decline to alarmingly low levels, even after the ban on mechanical harvesting went into effect. Fortunately, this trend was reversed in 2011 with a modest but statistically significant increase in hard clam densities.

48 Chincoteague Bay Hard Clam Densities MDNR Shellfish Div Dredge Ban Clams per sq. m

49 Hard Clam Recruitment in Chincoteague B. Recruitment is defined as the percentage of clams mm in length in the population. This size range represent the sublegal-size clams that can be reliably retained (and counted) on the escalator mesh. Historically, hard clam recruitment has been poor in Chincoteague Bay. The late 1990 s and early 2000 s experienced relatively good recruitment, but this only resulted in a slight, unsustained increase in densities (see previous graph). The number of commercial harvesters, using efficient hydraulic escalator dredges, increased substantially during this period.

50 Chincoteague Bay Hard Clam Recruitment MDNR Shellfish Div. % Clams mm

51 2010 Recruitment in Chincoteague B. The preceding graph showed a welcomed improvement in recruitment in Chincoteague Bay in 2010 over the previous several years. The following photo is of a sample with an atypically high density and percentage of sublegal clams for Chincoteague Bay in 2010, but nicely illustrates what good recruitment looks like.

52 Chincoteague Bay - Oct. 2010

53 A Closer Look at Isle of Wight Bay Hard clam densities were fairly stable from the mid s to the late 2000 s at levels well below the 1953 baseline. In the year before the ban on mechanical harvesting and the three years since then the trend has been a steep increase in hard clam densities.

54 Isle of Wight Bay Hard Clam Densities MDNR Shellfish Div Dredge Ban Ban Clams per sq. m Not Sampled

55 Isle of Wight Bay Clam Recruitment Recruitment has been relatively good (>20%) since 1997 with two exceptional peaks. As in Chincoteague Bay, the first recruitment peak occurred around the turn of the millennium, but the population showed no change in densities. The more recent recruitment event around the time of the ban on mechanical harvesting has been accompanied by the sharp rise in hard clam densities.

56 Isle of Wight Bay Hard Clam Recruitment MDNR Shellfish Div. % Clams mm Dredge Ban Dredge Ban

57 Sand Vs. Mud Isle of Wight Bay stations were examined according to roughly two equal substrate groupings: mud and sand/sand-mud mixture. Hard clam densities in the mud stations were very low, similar to other bays. In contrast, stations with sand or sand-mud mixtures had extraordinarily high densities, exceeding the 1953 baseline study.

58 Isle of Wight 2011 Hard Clam Densities by Substrate Type MDNR Shellfish Div. Clams per sq. m Mud San d

59 Historical Substrate Distribution in IoW The results of the stratification by substrate analysis led to the question of what type of bottom was sampled during the 1953 survey. As it turns out, all of the 1953 stations were in areas of sand or sand-mud mixtures. What is remarkable is that the substrate of Isle of Wight Bay back then was categorized as entirely sand or sandmud mixtures. This is in contrast with today, when a large expanse of pure mud exists in the northwestern portion of the bay.

60 Wells 1954

61 Bay Scallops

62 Bay Scallop Surveys In addition to the standard shellfish survey, directed surveys targeting bay scallops are conducted. These are a follow-up to a scallop restoration project initiated by MDNR Shellfish Program in 1997/98, when 1.4 million were planted in Chincoteague Bay. Since then, scallops have been found throughout the Md. Coastal Bays from the Virginia line to Delaware. Unfortunately, scallops have slowly disappeared from the bays. They were still abundant behind Ocean City in 2008, but only one was found last year, in Assawoman Bay off the sewage-treatment plant. This year, no bay scallops were found during our surveys for the first time in about 15 years.

63 MDNR Shellfish Div.

64 Remnant Bay Scallops Just as I was about to write off this species, I received an from a MDNR colleague stating he knew where some scallops could be found in Sinepuxent Bay. After about 45 minutes of poking around in a small eelgrass bed (the preferred habitat of bay scallops) he turned up three individuals. Investigations in other nearby likely areas found no additional scallops. It seems likely that the near-term future of this species in the coastal bays is tenuous at best.

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66 The Fascinating Bay Scallop Bay scallops are very active for bivalve molluscs. By clapping their valves or shells (as seen in the photo), they can create jets of water that allows them to swim away from predators. Their light but sturdy shell and its ribbed configuration also assists in swimming. They have 18 pairs of blue eyes arranged along the margins of the valves. The reddish-brown object inside the scallop is the gonad. Bay scallops are hermaphrodites, simultaneously possessing both male and female sex organs, generally an unusual condition in bivalves.

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68 Oysters

69 Oysters in the Coastal Bays Intertidal oyster populations can be found in northern the northern bays above the Verrazano Bridge and in the southernmost portion of Chincoteague Bay. They are usually found on anthropogenic structures such as riprap, wooden pilings, bridge supports, concrete curbing (see photo), and even old tires.

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71 Subtidal Oysters Only one small subtidal (constantly submerged) oyster population is known to exist, and has been monitored since The population has a wide size-range of oysters, and has experienced regular recruitment until this year, when no spat were found. Samples are taken to the Cooperative Oxford Lab for disease analyses. Dermo disease is present but at much lower levels than in Chesapeake B. Either (or both) MSX or the closely related SSO disease is found at comparable levels to MSX-enzootic waters in Chesapeake B. SSO (seaside organism) was first discovered in Chincoteague B.

72 Chincoteague Bay Oysters Dermo Disease Prevalence % Infection Chincoteague Bay Oysters MSX/SSO Disease Prevalence % Infection

73 Acknowledgments

74 Angel Willey MDNR Coastal Fish Program Robert Bussell MDNR Shellfish Monitoring Program

75 Special Thanks to Capt. Bill Ryan (l.)

76 The End