7143

• UCREFRP Catalog Number: 7143
• Author: Inskip, P. D.
• Title: Habitat Suitability Index Models
• USFW Year: 1982.
• USFW - Doc Type: Northern Pike.
• Copyright Material: NO

Drop in water level during embryo and fry stages (V2~. Northern pike are
vulnerable to even slight water level changes during the incubation and nursery
periods because they usually spawn in water less than 0.5 m deep and often in
water as shallow as 0.1 to 0.2 m. Declining water levels can strand the
embryos, fry, and even adults. Embryos are particularly vulnerable because of
their immobility. Newly hatched fry become inactive after an initial burst of
activity and are, therefore, likely to be stranded by a decline in water
level. Once the fry begin to feed, they can probably follow a slowly receding
shoreline, provided that the pathway from the spawning area to summer habitat
is not blocked. The greater mobility of older fry is the reason for the
difference between curves A and B in the suitability index graph. Curve A has
a steep slope between 0.1 and 0.5 m because this is the depth range in which
most spawning occurs.
Percent of midsummer area with emergent and/or submerged aquatic vegeta-
tion or remains of terrestrial plants (bottom debris excluded) (V,~.
The extent
of vegetative cover is an important component of habitat suitability for
northern pike for several reasons. Rooted macrophytes supplement planktonic
production and, presumably, increase the food supply for northern pike.
Vegetation provides a refuge from predation for young pike and hunting cover
for pike of all ages. Complete vegetative cover, however, is probably sub-
optimal. Northern pike are. often associated with the vegetation-open water
interface (Reighard 1915). These presumed hunting stations would not be
available if the water body were totally covered with vegetation. Furthermore,
decomposing vegetation can deplete dissolved oxygen during winter in shallow,
ice-covered lakes and rivers with low discharges. It is assumed that optimal
habitat would have extensive submerged and/or emergent aquatic vegetation
interspersed with open water. At the present time, the model does not contain
an interspersion variable.
Logarithm (base 10) of TDS in surface waters durin midsummer (V4Z. Low
levels of TDS usually indicate low fertility, while-high concentrations can
cause ion regulatory or osmoregulatory stress. A positive relationship between.
TDS and food supply for northern pike is assumed for TDS values between 0 and
80 ppm. Habitat suitability is assumed to remain constant for TDS levels
between 80 and 800 ppm and to decrease at higher levels, with zero suitabi]ity
for TDS values > 3,500 ppm.
TDS is not a reliable indicator of food supply in most rivers and streams.
This variable should be excluded from the riverine model unless TDS is
> 800 ppm.
Least suitable pH in spawning habitat during embryo and fry stages (VSO.
Northern pike are tolerant of a wide range of pH values but pN can be a locally
or regionally important component of habitat suitability. There is no evidence
of any adverse effects over the pH range of 6.0 to 9.0. Northern pike repro-
duction is markedly impaired when pH falls below 5.0. An upper limit is not
well defined, but pH levels higher than 9.0 appear to interfere with reproduc-
tion. Embryos and .fry seem to be more sensitive to pH than do other life .
history stages.
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