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Identification of Important Habitats in Coastal New Hampshire

Chapter 4. Softshell Clam

Softshell clam, Mya arenaria, are harvested recreationally in coastal New Hampshire, and are ecologically important as filter feeders and as a food source for other invertebrates, fishes, and birds. The following tables are components of a model to map clam habitat. Most of the information was compiled by Brown et al. (unpub.) from the other sources listed; the model was adjusted to fit conditions occurring at known clam beds in Great Bay and the Hampton/Seabrook Estuary. Known clam beds were digitized from maps by New Hampshire Fish and Game (Nelson et al. 1981, 1982), Normandeau Associates (1995) and from sites drawn on base maps for us by R. Langan (Jackson Estuarine Research Laboratory).

The model operates on four parameters: substrate, salinity, temperature, and depth. The model indexes the relative suitability of each environmental parameter on a 0 to 10 basis, with 10 being optimal and 0 being unsuitable. These suitability index values are combined by computing their geometric mean for each grid cell in the study area. Thus, optimal habitat for any life stagewould occur where the index values were the maximum for each of the four inputs; no value is attributed to areas where any condition is completely unsuitable.

Suitability is calculated for each season, to accommodate annual changes in salinity and temperature. Habitat values for reproductive and for larval/spat stages were computed as the most favorable conditions which occur in either spring or summer. Habitat values for adult and juvenile stages were computed as the geometric mean of suitability index values for all four seasons, since clams cannot escape persistently unfavorable conditions. The extent of reproductive habitat then was reduced in order to correspond to areas having adult habitat values of at least 2.5 (out of a possible 10). Since habitat for larvae and spat appeared to be widely abundant, overall habitat was regarded as the maximum value from either reproductive or adult habitat maps (Figures of Spawning Habitat, Adult Habitat, Larval Habitat, Combined Life Stages).

SUBSTRATE PREFERENCES

Sources: Brown et al., unpub., Fefer & Schettig 1980, Newell and Hidu 1986.

SubstrateSuitability Index: 0 to 10 scale; 0 = unsuitable, 10 = optimal condition

SALINITY PREFERENCES

Sources: Brown et al., unpub., Fefer & Schettig 1980, Newell and Hidu 1986, Stickney 1959.

Salinity (ppt) Suitability Index: 0 to 10 scale; 0 = unsuitable, 10 = optimal condition

TEMPERATURE PREFERENCES

Sources: Brown et al., unpub., Fefer & Schettig 1980, Kennedy and Mihursky 1971, Newell and Hidu 1986.

Temperature (C) Suitability Index: 0 to 10 scale; 0 = unsuitable, 10 = optimal condition

DEPTH PREFERENCES

Sources: Brown et al., unpub., Fefer & Schettig 1980, Newell and Hidu 1986, Stickney 1959.

Depth (feet, mlw*) Suitability Index: 0 to 10 scale; 0 = unsuitable, 10 = optimal condition

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