ResourcesIdentification of Important Habitats in Coastal New HampshireChapter 9. American ShadAmerican shad (Alosa sapidissima) is an anadromous clupeid fish,
ranging from Florida to the St Lawrence River. It formerly was abundant
enough to be of major importance as a food source; dams and degradation
of water quality in spawning rivers have greatly reduced historic runs.
We mapped shad habitat using a combination of occurrence (known use) information
and habitat relationships to environmental features. SPAWNING HABITAT Shad ascend freshwater tributaries and spawn in slow-flowing sections
of rivers (Scott and Scott 1988). Spawning occurs at water temperatures
< 23 degrees C. (Scott and Scott 1988) and > 10 degrees C. (Leim
1924, Williams and Daborn 1984 cited in Scott and Scott 1988). Shad spawn
in Canadian tributaries between May and July (Scott and Scott 1988). Because
of the paucity of other information on spawning habitat requirements of
American shad, we mapped spawning habitat only from known occurrences
(D. Grout, New Hampshire Fish and Game Department). These areas were scored
as 10 (0 - 10 scale) since they in fact must meet shad environmental requirements
(Figure of Spawning Habitat). LARVAL HABITAT Shad eggs are pelagic and are carried downstream by the current (Weiss-Glanz et al. 1988). Incubation time of eggs is dependent on water temperature (Scott and
Crossman 1973, Marcy 1976). Eggs cease developing at water temperatures
of 7 degrees C. and abnormalities occur at 22 degrees C. (Leim 1924).
Eggs and larvae survive in water at salinities between 7.5 and 15 ppt,
but mortality occurs when salinity reaches 22.5 ppt (Leim 1924). The salinity
regime in which eggs hatch successfully remains unclear (Weiss-Glanz et
al. 1988). At hatching, larvae are approximately 7 mm total length (TL)
and are planktonic (Marcy 1976), and grow to 12 mm TL when the yolk-sac
is absorbed (Jones et al. 1978). Transformation to the juvenile stage
occurs at 25 - 28 mm TL at 2 - 3 weeks of age (Jones et al. 1978). Distribution of larval habitat was mapped only within rivers where spawning
is known to occur. Habitat was regarded as those reaches having salinities
< 15 ppt during the spring season (April - June). These were given
a score of 7.5 (0 - 10 scale) since actual presence of larvae has not
been documented (Figure of Larval Habitat). The
paucity of information on other environmental requirements prevented development
of a more rigorous model. JUVENILE HABITAT Juveniles occur in natal rivers during summer (Weiss-Glanz et al. 1988).
Decreasing water temperature is the stimulus for downstream movement of
juveniles into brackish water and finally to the sea (Weiss-Glanz et al.
1988). Because of the paucity of information on juvenile habitat requirements,
we mapped early juvenile habitat as only those sections of rivers where
they have been collected (D. Grout, NHF&G). These areas were scored
as 10 (0 - 10 scale) since they in fact must satisfy shad environmental
needs (Figure of Juvenile Habitat). Clearly, environmental
requirements and tolerances change as juveniles migrate from freshwater
nursery areas to the sea. COMBINED HABITATS The above spawning, larval and juvenile habitat information were combined
into a grid-cell coverage of overall habitat for American shad, using
the maximum value from any of the stages on a cell by cell basis (Figure
of Combined Life Stages). |
