Coastal & Estuarine Science News (CESN)
Coastal & Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries & Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bimonthly basis.
September 2005
Contents
By-catch Reduction Device Protects Terps
Sediment Slurry Application Helps Delta Marsh Rise to the Challenge
Poor Water and Sediment Quality: Accessories to the Long Island Lobster Crash?
Of Wakes and Dredging in the Intracoastal Waterway: Compensatory Effects?
By-catch Reduction Device Protects Terps
The public was first introduced to the issue of by-catch in offshore fisheries by dramatic video of dolphins caught in tuna nets and seabirds drowned on long lines, but by-catch is a serious problem close to shore as well. For example, one of the leading causes of decline in some east coast diamondback terrapin populations is by-catch in crab pots. Researchers recently speculated that in some parts of Chesapeake Bay, eel pots might pose a similar danger to the terrapins when they are fished in abundance in terrapin habitat. In addition to quantifying the problem, the researchers tested the effectiveness of a low-tech, low-cost by-catch reduction device (BRD). They found that terrapin by-catch can be high in the tidal creeks they studied: up to 0.458 terrapins were caught per pot per day, or nearly a terrapin every other day. Multiplied by the large number of eel pots deployed, this by-catch could spell serious trouble for the terrapins.
The BRD they designed and tested, consisting of a PVC ring attached to the opening of the cloth funnel at the trap's entrance, allows the eels to enter but not the terrapins. This simple device completely eliminated terrapin by-catch but did not reduce catch of eels, good news for eel fishermen. Because the device is inexpensive (less than a nickel apiece), easy to install and has no effect on eel catch, it seems to be a practical solution to a sticky problem. Installation of the BRDs would be simple during trap construction, requiring no extra hardware. "Retrofitting" existing traps would require removal and reattachment of the funnel rigging, but could be made easy by incorporating a small opening the BRD.
Source: Radzio, T. A. and W. M. Roosenburg. 2005. Diamondback terrapin mortality in the estuarine eel pot fishery and evaluation of a by-catch reduction device. Estuaries 28(4): 620-626. (View Abstract)
Sediment Slurry Application Helps Delta Marsh Rise to the Challenge
As sea level continues to rise, coastal wetlands, among the most productive and "endangered" ecosystems on earth, are being lost in low-lying areas through inundation by the rising sea. Nowhere is the problem more severe than in the mother of all U.S. coastal wetlands, the Mississippi Delta, where not only is the ocean rising, the land is also sinking. As a result, the rate of marsh loss in the Delta is 77 km2 per year (between 1978 and 2000). To combat the inundation and subsidence, restoration projects often add sediment to the eroding areas to keep ahead of the problem. One method of sediment addition was recently evaluated in the Delta: piping a slurry of 85% water and 15% sediment onto the marsh surface, which can be applied as a thin layer of sediment over a very large area (up to 1000 m from the end of the discharge pipe). The effect of varying amounts of added sediment on vegetation and soil condition was examined at one site over seven years. Overall, the most beneficial and longest-lasting effects were observed when moderate amounts of sediment (with a resultant increase in elevation of 2 to 12 cm) were added to the marsh. At this level, there was a greater percent cover of Spartina alterniflora (60-70% cover) and better soil condition (bulk densities of 0.4 - 1.0 g cm-3 and undetectable concentrations of the phytotoxin hydrogen sulfide) than in areas not receiving any additional sediment (which had ~30% vegetative cover, ~0.2 g cm-3 bulk densities, >1.0 mM H2S). These positive effects were observed even seven years after the slurry application. Areas receiving a higher "dose" of new sediment (12-20 cm) exhibited a short-lived growth pulse but this effect, likely due to the high nutrient content of the added sediment, faded after about three years.
The moderate sediment addition seems to have been most successful because it countered the effect of sea level rise without increasing the marsh elevation above that to which Spartina is best adapted. The sediment-slurry method has a lot of promise, but the authors suggest that additional long-term, appropriately-scaled studies are needed to test the method further.
Source: Slocum, M. G., I. A. Mendelssohn, and N. L. Kuhn. 2005. Effects of sediment slurry enrichment on salt marsh rehabilitation: Plant and soil responses over seven years. Estuaries 28(4): 519-528. (View Abstract)
Poor Water and Sediment Quality: Accessories to the Long Island Lobster Crash?
In the late 1990s, the Long Island lobster fishery seemed to defy gravity. Despite increasing fishing pressure, landings were on the rise and setting records. The boom came to an abrupt halt in the summer and fall of 1999, when a catastrophic die-off of lobsters in western Long Island Sound devastated the fishery. The culprit was identified as a previously unknown protozoan parasite. But western Long Island Sound is notorious for seasonal hypoxia and its associated impacts on sediment and water quality. Local researchers wondered, did water and sediment quality contribute to the lobsters' vulnerability to the parasite?
When the lobster mortality reports first started rolling in, the researchers mobilized a sediment survey in the Sound using an "inverted periscope" camera system, called a sediment-profile imaging camera (SPI). The SPI takes photos of the upper 15 to 20 cm of sediment in order to characterize the sediment quality based on its color (which indicates whether there is aerobic activity taking place) and classification of the benthic organisms observed. In 2000 they coupled SPI surveys with water quality measurements (dissolved oxygen, sulfide, and ammonium) in near-bottom waters. The 1999 and 2000 SPI surveys revealed poor sediment quality at most of the western sites sampled: sediments at the sediment/water interface were black, indicative of hypoxic/sulfidic conditions, and benthic fauna was limited to opportunistic polychaete worms. In 2000 they found that the sulfidic sites were accompanied by bottom water DO levels of <2mg/L and significant levels of sulfide and ammonium. DO concentrations generally increased from August through November. The release of sulfide and ammonium into bottom waters from anaerobic sediments, combined with low DO and abnormally high water temperatures that summer, may have battered the lobsters' immune systems and increased their susceptibility to the deadly pathogens. The timing of these observations, with the worst conditions present in the late summer/early fall, coinciding with the lobster die-offs, also supports the environmental effects hypothesis.
Source: Valente, R. M. and C. Cuomo. 2005. Did multiple sediment-associated stressors contribute to the 1999 lobster mass mortality event in western Long Island Sound, USA? Estuaries 28(4): 529-540. (View Abstract)
Of Wakes and Dredging in the Intracoastal Waterway: Compensatory Effects?
It is rare, probably even impossible, for an estuarine system to be influenced by a single stressor alone. Instead, multiple disturbances have interactive effects that differ from each effect by itself. A reminder of these complexities came with a recent study of the interaction of two factors that have each been studied extensively individually: disposed dredged material and boat wakes.
While wakes can coarsen grain size by preferentially eroding fine-grained sediment, dredged material disposal tends to deposit fine-grained material, thus having the opposite effect. Changes in sediment grain size lead to changes in the assemblages of organisms that live there (infauna), but the compensatory effects of dredge disposal and wake could hypothetically result in similar infaunal communities in areas subject to both effects and areas affected by neither. On the other hand, organisms living on blades of seagrasses (epifauna) might be affected by boat wakes but not by grain sizes. To tease these relationships apart, the author studied infaunal and epifaunal assemblages in the Atlantic Intracoastal Waterway in Bogue Sound, North Carolina, which has dredged material disposal islands affected by and not affected by boat wakes, and a non-dredge disposal area with no boat wake impacts. As hypothesized, infauna differed between sites with and without dredge disposal, but not between dredge deposition sites that differed in boat wake activity. Epifaunal assemblages differed between sites having and lacking wake impacts (mostly due to lower abundance of a gastropod and the slipper limpet at wake-exposed sites).
The message for coastal managers: remember to keep your eyes open to all of the potentially interacting influences on a given site - compensatory effects can mask individual impacts.
Source: Bishop, M. J. 2005. Compensatory effects of boat wake and dredge spoil disposal on assemblages of macro-invertebrates. Estuaries 28(4): 510-518. (View Abstract)
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