NOAA Sea Grant Program: Ecological Characteristics and Carrying Capacity of Suspended Shellfish Culture Systems

Purpose or Scope:

This project addresses high priority 2015 goal to: "Identify gaps in current understanding of shellfish ecology specific to West Coast ecosystems and pursue research to fill those gaps; and to gain a clear understanding of the ecological impacts associated with: Mussel culture - suspended raft and longline, Mytilus galloprovincialis and Mytilus trossulus and carrying capacity - modeling of intensively cultured estuaries."

Approach:

This project was designed to address water quality and ecological aspects of suspended or off-bottom shellfish culture including: 1) assessing bivalve growth and meat yield against water quality parameters including suspended materials in the water column; 2) measuring a suite of physical, chemical and biological variables within the culture system with concurrent physiological measurements of feeding and biodeposit production; 3) examining differing growout locations and culture methods; and 4)coordinating with an on-going large-scale tidal flow and nutrient modeling study to evaluate the potential carrying capacity of intensively cultivated suspended bivalves in an entire farming area. Study sites were located at commercial mussel farms in Penn Cove (Mytilus trossulus) and Totten Inlet (M. galloprovincialis), Puget Sound, Washington. Subcontractors studied mussel feeding and biodeposition, sedimentation effects and fouling communities, and the productive carrying capacity.

Progress and Results:

During the first year (2001-02), multiple observations were made of water currents, water chemistry, phytoplankton, mussel growth, seston, fouling, and fish utilization at a commercial mussel culture facility in Totten Inlet, Washington. During the second year (2002-03), experiments were continued in Totten Inlet and initiated at a second facility in Penn Cove, Washington which cultivates two species of mussels.

Water currents are decidedly lower within the mussel culture system than at control stations, current direction fluctuates rapidly on the down-current end of raft units (dependent on tidal direction), and varies in speed and direction with water depth.

Phytoplankton densities decrease markedly as water flows through the mussel culture system due to uptake by the mussels and removal as "pseudo-feces." However, within 3 to 6 meters downstream (in the tidal current) plankton levels recover to the same densities seen at upstream control stations. Water clarity increases as it moves through the raft system, with the clearest water in the center to down stream end of the raft. Clarity decreases rapidly as water moves downstream and mixes in eddies created by raft system.

Mussel growth (shell length and weight) mirrors the seasonal trends in phytoplankton densities, and was significantly greater on the portion of the raft system facing the entrance of the inlet. However, there was little variation in mussel growth within individual raft units - mussels grew at about the same rate in the middle of the raft as around the edges.

Sediment deposition attributed to mussel culture is largely limited to the footprint of the raft units with little or no sediment effects outside the immediate culture area.

Tides travel bidirectionally, northeast and southwest, along the length of Totten Inlet. The mussel raft is anchored in place parallel to the shoreline along this axis. As currents approach the raft on incoming neap tides, a portion of this water is deflected along the sides of the raft similar to the way air passes around an airfoil. Current velocities increase on the sides of the raft and to a lesser extent below the length of the mussel lines at a depth of 6-10 meters. Eddies form along the down current edge of the raft system where water merges. Horizontal movement of water passing through the center of the raft structure is slowed by the drag of the mussel lines to 2.2 cm/sec at a 2.5 meter depth. Vertical movement within the raft is minimal moving at most 0.5 cm/sec in either direction.

Phytoplankton: Phytoplankton in southern Puget Sound displayed trends typical of cold temperate regions with chain-forming diatoms blooming in spring, sustained to a lesser degree throughout the summer and giving rise to dinoflagellate species during periods of prolonged stratification particularly in mid to late summer. Samples collected at a 2.5 meter depth from the north buoy and north boom of the mussel raft on incoming tides revealed diatom blooms peaking in May followed by several smaller blooms throughout the summer. Chaetoceros spp. were the dominant species in the peak spring bloom comprising 91% of total phytoplankton. Three smaller blooms were identified in mid-June, late June and early August with Chaetoceros spp. (48%), Chaetoceros spp. (73%) and Eucampia (78%) as dominant species respectively.

Mussel Raft Influence on Phytoplankton: As water passes through the suspended raft system, feeding mussels markedly reduce the amount of plankton within the water column. On incoming tides, total phytoplankton concentrations, taken from the center of the raft at a depth of 2.5 meters, were significantly lower (71%, p=.006) than those taken two meters outside the north (leading) edge of the raft at the same depth. Phytoplankton concentrations at the south edge were 25% lower than the north edge as the currents passing along the sides of the raft merge together mixing plankton deficient water with plankton rich water. No difference was found between phytoplankton concentrations located 100 yards beyond the trailing edge of the raft at the south buoy and those taken from the leading edge of the raft. While fewer samples were taken during outgoing tides, trends appeared similar with phytoplankton depletion occurring within the raft (p=.016) and repletion beyond the trailing (northern) edge.

Chlorophyll and Water Clarity: Chlorophyll, a photosynthetic pigment contained in most plants, is an indicator of phytoplankton abundance. Measurements taken from around and within the mussel raft every 30 minutes throughout the season mirror cell count data determined using microscopy. On incoming tides, average chlorophyll concentrations at the leading edge of the raft were much higher than those found in the center. Chlorophyll abundance immediately beyond the trailing edge was greater than the center, but slightly less than measurements taken prior to passing through the raft.

Water clarity readings further underscore both the changes in chlorophyll and phytoplankton concentrations that result from high density suspended culture. Clarity was significantly greater in the center of the raft and strongly correlated with both plankton concentrations and chlorophyll values. Also affecting the water quality was the seston intake by the mussels. The average amount of seston ingested per mussel, per hour, was 8.3 mg and mussels ingested as little as 3.21 mg per hour in September to as high as 20.4 mg per hour in May.

Nutrients: Totten inlet nutrient data was consistent for both years, as 2003 data showed similar trends to the 2002 data. All reference station data, except that of ammonium, mirrored closely to data collected at 70m north or south of the rafts. Phosphate levels were variable from sample date to sample date. Levels both increased and decreased in some instances as the water moved through the rafts. Overall, post-raft levels either stayed the same or were reduced from pre-raft exposure levels. Nitrate levels consistently increased as water moved into the center of the rafts, then recovered to pre-raft levels as water approached 6 feet past the raft.

Nitrogen dioxide levels increased minimally or not at all in the center of the rafts. In either occasion, levels were back to pre-raft levels shortly after water exited the raft. Ammonium levels proved to be the most sensitive as they were higher at the reference station compared with the North buoy and other stations inside and surrounding the raft. Levels increased even more as water entered the rafts, levels then went back down to pre-raft levels 70m past the rafts. Silicic acid data was not consistent as levels both decreased and increased as water entered the rafts.

Penn Cove nutrient data proved to be more difficult to analyze as current flows varied greatly throughout the farm and tide cycle. Phosphate and Nitrogen dioxide levels did vary from sample date to sample date but a slight increase was seen inside the culture system on a few instances. Nitrate levels were variable as well but did not show significant change inside the culture system. As seen in Totten inlet, ammonium levels increased inside the culture area compared to the reference site. Silicic acid levels where similar to Totten observations as they were not uniform as levels were both higher and lower in the reference site on occasion.

Mussel Feeding Rates: Mussel feeding rates increased logarithmically with a rise in phytoplankton concentration suggesting that mussels are more efficient at removing phytoplankton from the water column when plankton abundance is high.

Mussel Feeding Selectivity: Results indicate that mussels feed indiscriminately on centric and pennate diatoms as they move from the north edge of the raft to the center during incoming tides. Percent decline of common species ranged from 56-79%. Percent decrease for several dinoflagellate species appeared lower. Akashiwo sanguineum and Protoperidinium spp. experienced a 26% and 43% decline respectively indicating possible feeding selectivity.

A number of presentations about project results have been made at regional and national meetings. The presentations listed below can be viewed in PDF format (file sizes are indicated).

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