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Landscape-Level Mapping of Fish for
Pennsylvania Gap Analysis
WAYNE MYERS, DAVID ARGENT, JOSEPH BISHOP, JAY STAUFFER,
JR.,
AND
ROBERT CARLINE
Pennsylvania State University, University Park, Pennsylvania
As many species living in streams and rivers of Pennsylvania are
imperiled, our gap analysis commitment included a landscape-level
consideration of fish fauna. Our primary concern has been to as-
cribe fish habitat to sectors of landscapes that are large enough to
be evident in regional mapping but small enough to inform envi-
ronmental and conservation analyses across landscapes. In light of
exploratory work in New York and Missouri, we considered stream
reaches to be inappropriately fine scale with respect to both map-
ping and effort. Other research work originally conducted for pur-
poses of hydrology and fisheries provided a basis upon which to
build habitat models at somewhat coarser scale for Pennsylvania.
Small watersheds constitute a next level of scale above stream
reaches that can serve for purposes of landscape segmentation rel-
evant to both hydrology and aquatic organisms. Small watersheds
serve the purpose of associating aquatic habitat with portions of the
landscape having most direct influence. Small watersheds also have
the advantage of being mappable as area features rather than linear
features, thus providing a tessellation. It has been fortuitous in Penn-
sylvania that the Water Resources Division of the U.S. Geological
Survey undertook to digitize watersheds of all named streams within
major river basins in the region. These data for the basins were
integrated and harmonized in the Office for Remote Sensing of Earth
Resources (ORSER) of the Environmental Resources Research In-
stitute (ERRI) at Penn State University with funding provided by
the Pennsylvania Department of Environmental Protection. Some
further editing was required for purposes of gap analysis, mostly to
resolve issues along the borders of the state. Our final version of
this layer consists of 9,855 polygon units representing the respec-
tive small watersheds.
Geomorphology controls development of drainage networks and
character of streams, with influence extending also to physical prop-
erties (e.g., turbidity) and chemical properties of water. In order to
capture such differentiation of streams because of geomorphology,
a layer of physiographic provinces and sections was overlaid to
assign each small watershed as one of 16 physiographic sections.
In Pennsylvania, these sections correspond to subregions of the in-
teragency ECOMAP effort.
Drainage divides constitute zoogeographic barriers to movement
of organisms that are wholly aquatic. The segregating effect of a
drainage divide depends on the instream linkage and whether down-
stream or upstream passage would traverse inhospitable conditions
for a particular species. Collection records in Pennsylvania estab-
lish that major drainage basins are effectively separate domains for
at least some species. It was decided in this regard to recognize
eight basin partitions: Erie, Allegheny, Ohio, Monongahela,
Susquehanna, Delaware, Potomac, and Genesee. Accordingly, each
small watershed was assigned one of these basin designations as an
attribute.
Stream order can serve as a surrogate for stream size, which re-
flects macrohabitat for fish species. A special geographic informa-
tion resource for Pennsylvania is a digital file of all blueline streams.
This file originated with digitizing by the Pennsylvania Department
of Transportation, but extensive editing and topological adjustment
was performed by ORSER at Penn State University. The stream
file was displayed over the small watersheds, and each watershed
was interpretively assigned to one of four size classes. First-order
(headwater) and second-order streams comprise a small-stream
class. Third- and fourth-order streams comprise a medium-size
class. Fifth- and sixth-order streams constitute a large-size class.
Seventh- and eight-order streams are combined with lakes as the
fourth size class.
A digital elevation model was used to calculate the median slope
for each small watershed, with coding in three classes: low (<1%),
medium (1% to 3%), and high (>3%). This variable serves to sepa-
rate fish habitat along the longitudinal axis of a stream, as some
fishes occupy streams of low gradient whereas others prefer higher
gradients. Low-gradient streams typically have sand, silt, and clay
substrates. High-gradient streams typically have cobble, boulder,
and rock substrates. Medium-gradient streams often have a hetero-
geneous mix of substrate types.
A land cover layer was used to assign a human disturbance class
for each small watershed. Human disturbance was considered toEach class for a variable was cast as a separate field (column) in a
spreadsheet for habitat modeling. Models were developed as a pro-
file for each species (row) in terms of these fields. Basin and physi-
ographic fields were coded as either 1 or 0 for presence or absence,
respectively. The size, gradient, and disturbance characteristics were
designated in terms of primary habitat (1), secondary habitat (2), or
unsuitable (0). Each fish species was categorized as to its highest
frequency of occurrence for a given stream size, which was desig-
nated as primary habitat. If secondary habitat or stream sizes were
determined, they were added to the profile as situations where the
fish may occur but with lower frequency.
A large digital database of fish collection records for Pennsylvania
was instrumental in developing and validating the profile models.
An extra field was added for each of the small watersheds to indi-
cate whether it had been sampled. Records from over 20,000 col-
lection events from 1950 to 1999 were used in the analysis.