Habitat suitability index models: juvenile Atlantic croaker (Revised)

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lJ. S. Fish and Wildlife Service
National Wetlands Research Center
Biological k?pOrt 82( 10.98)
JUNE 1985
HABITAT SUITABILITY INDEX MODELS:
JUVENILE ATLANTIC CROAKER
(Revised)
Fish and Wildlife Service
U.S. Department of the Interior
This model is designed to be used by the Division of Ecological Services in
conjunction with the Habitat Evaluation Procedures.
This is one of the first reports to be published in the new "Biological
Report" series. This technical report series, published by the Research and
Development branch of the U.S. Fish and Wildlife Service, replaces the
"FWS/OBS" series published from 1976 to September 1984. The Biological Report
series is designed for the rapid publication of reports with an application
orientation, and it continues the focus of the FWS/OBS series on resource
management issues and fish and wildlife needs.
Biological Report 82(10.98)
June 1985
HABITAT SUITABILITY INDEX MODELS:
JUVENILE ATLANTIC CROAKER (Revised)
Robert J. Diaz
Department of Invertebrate Ecology
Virginia Institute of Marine Science
College of William and Mary
Gloucester Point, VA 23062
and
Christopher P. Onuf
National Coastal Ecosystems Team
U.S. Fish and Wildlife Service
1010 Gause Boulevard
Slidell, LA 70458
Performed for
National Coastal Ecosystems Team
Division of Biological Services
Research and Development
Fish and Wildlife Service
U.S. Department of the Interior
Washington, DC 20240
This report should be cited as:
Diaz, R.J., and C.P. Onuf. 1985. Habitat suitability index models: juven-ile
Atlantic croaker (revised). U.S. Fish Wildl. Serv. Biol. Rep.
82(10.98). 23 pp.
First printed July 1982 as FWS/OBS-82/10.21; revised June 1985.
PREFACE
The habitat use information and habitat suitability index (HSI) model in
this report on juvenile Atlantic croaker is intended for use in impact assess-ment
and habitat management. The model was developed from a review and
synthesis of existing information and is scaled to produce an index of habitat
suitability between 0 (unsuitable habitat) and 1 (optimally suitable habitat).
Assumptions used to transform habitat use information into the HSI model, and
guidelines for model applications, including methods for measuring model
variables, are described.
This model is a hypothesis of species-habitat relationships, not a
statement of proven cause and effect relationships. The model has not been
field-tested, but it has been applied to four hypothetical data sets which are
presented and discussed. For this reason, the U.S..Fish and Wildlife Service
encourages model users to convey comments and suggestions that may help,
increase the utility and effectiveness of this habitat-based approach to fish:
and wildlife management. Please send any comments or suggestions you may have
on the croaker HSI model to the following address.
National Coastal Ecosystems Team
U.S. Fish and Wildlife Service
1010 Gause Boulevard
Slidell, LA 70458
iii
CONTENTS
Page
PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
ACKNOWLEDGMENTS ............................ vi
INTRODUCTION .............................
Life History Overview
SPECIFIC HABITAT REQUIREMENTS : : : : : : : : : : : : : : : : : : : : :
Temperature ...........................
Salinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Food . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Substrate ............................
Turbidity ............................
Water Depth ...........................
Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dissolved Oxygen .........................
HABITAT SUITABILITY INDEX (HSI) MODEL .................
Model Applicability .......................
Model Description . ... .. ....
Suitability Index (SI) Graphs for Habitat Variables
...................
Component Index Equations and HSI Determination .........
Field Use of the Model ......................
Interpreting Model Outputs ....................
ADDITIONAL HABITATS MODELS ......................
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
ACKNOWLEDGMENTS
Development of the habitat suitability index model and narrative for
juvenile Atlantic croaker was monitored, expertly reviewed, and constructively
criticized by Michael Weinstein, Virginia Commonwealth University, Richmond;
John Lunz, Environmental Laboratory, U.S. Army Corps of Engineers, Vicksburg,
Mississippi; and Brenda Norcross, Virginia Institute of Marine Science,
Gloucester Point. Thorough evaluations of model structure and functional
relationships were provided by personnel of the U.S. Fish and Wildlife
Service's National Coastal Ecosystems Team (NCET) and Western Energy and Land
Use Team (WELUT). Model and supportive narrative reviews were also provided
by representatives of the National Marine Fisheries Service and by Regional
personnel of the U.S. Fish and Wildlife Service. Finally, funding for model
development and publication was provided by the U.S. Fish and Wildlife
Service.
vi
ATLANTIC CROAKER (Micropoqonias undulatus)
INTRODUCTION
The Atlantic croaker is an important commercial and recreational species.
In the 1940's, the foodfish catch of Atlantic croakers was concentrated in
Chesapeake Bay; in the 1950's and early 1970's, the catch was concentrated in
the Gulf of Mexico; and in the late 1970's, the catch was concentrated in the
South Atlantic States (Wilk 1981). Industrial and recreational catches of
Atlantic croakers have been concentrated in the Gulf of Mexico, where the
Atlantic croaker is the most important species of bottomfish for industrial
uses (Knudsen and Herke 1978), and has ranked first, second, or third in
number caught by recreational anglers,
1981).
depending on survey year (Nakamura
Today, Virginia or Delaware is considered to be the northern extent of
the species. During climatically warmer periods, such as the 1930's and
1940's, the croaker extended its range north at least to New York, where it
was commercially fished. The southern extent of its range is Argentina.
Life History Overview
Croakers spawn in the fall in marine waters. Spawning grounds are not
clearly defined and can range from tidal passes and the mouths of estuaries to
Continental Shelf depths of at least 54 m (177 ft) (Pearson 1929; Hildebrand
and Cable 1930; Hoese 1965; Fruge and Truesdale 1978; Johnson 1978; Etzold and
Christmas 1979). Eggs are pelagic, and upon hatching, the larvae and
postlarvae move into estuaries. Actual mechanisms for larval transport into
the estuarine nursery grounds are unclear and may be a combination of both
passive current transport (Weinstein et al. 1980a; Norcross and Austin 1981;
Miller et al. 1984) and active swimming (Pearson 1929).
Once recruited from nearshore marine waters in the fall and winter,
larvae 10 to 18 mm (0.4 to 0.7 inches) total length (TL) move up the estuary
to areas of brackish water (Bearden 19641, where the transition to juveniles
occurs at a size range of 18 to 30 mm (0.7 to 1.2 inches) TL. Juveniles then
take up residence in their estuarine nursery areas. Juveniles are also common
in tidal riverine habitats (Raney and Massmann 1953).
Juveniles are abundant in estuarine nursery areas as early as
in
September
some areas,
August,
but as late as March in others; they remain through June to
depending on location and year (Parker 1971; Chao and Musick 1977;
Yakupzack et al. 1977; Copeland et al. 1984). Growth of juveniles in the
nursery areas is rapid, as much as 35 mm (1.4 inches) TL per month (Knudsen
and Herke 1978). Most emigrate at around 100 mm (4 inches) TL. Emigration can
be either direct to open coastal waters (Parker 1971; Yakupzack et al. 1977;
Knudsen and Herke 1978) or gradual, with larger individuals occurring closer
to the mouth of estuaries in more saline waters (Haven 1957; Bearden 1964).
Reported sizes of Atlantic croakers after one year of life range from 100 to
250 mm (4 to 10 inches) TL (Knudsen and Herke 1978) but estimates of 120 to
1
180 mm (5 to 7 inches) are most common. Reported sizes after two years of life
range from 200 to 310 mm (8 to 12 inches) (Johnson 1978). A validated method
of age determination based on microscopic examination of scales yielded
estimates of approximately 160 mm and 280 mm (6 and 11 inches) mean total
length at age I and age II, respectively (White and Chittenden 1977).
Atlantic croakers mature by the end of their first year of life south of
Cape Hatteras at lengths of 140 to 180 mm (5.5 to 7 inches) and seldom survive
longer than one or two years. North of Cape Hatteras maturity occurs a year
later, at lengths greater than 200 mm (8 inches), and individuals may live for
several years (Johnson 1978). Fecundity is 350,000 to 500,000 eggs per female
of 350 to 500 mm (14 to 20 in) length (Powles 19811, but must be much less in
the typically much smaller reproductive females south of Cape Hatteras.
SPECIFIC HABITAT REQUIREMENTS
The estuarine nursery areas for Atlantic croaker populations differ
considerably among locations, apparently in response to tidal range. Where the
tidal range is less than 0.5 m (20 inches), shallow open water areas are used
at the landward extremities of large bays (Parker 19711, as are shallow
creeks, ponds, and lakes intimately associated with marsh (Parker 1971;
Yakupzack et al. 1977; Knudsen and Herke 1978; Copeland et al. 1984). Where
the tidal influence is stronger, large numbers of small juveniles have been
collected from small tidal streams in the spring (Turner and Johnson 1974, in
South Carolina); however, according to most other reports, shallow areas are
avoided and juvenile croakers are concentrated in the deep, main channels of
estuaries as in the Delaware River (Thomas 19811, Chesapeake Bay (Haven 1957),
and the Cape Fear River (Weinstein 1980b). Apparently, shallow areas become
less suitable for juvenile croakers as daily fluctuations of water level
increase. Despite this major difference, the basic life requisites of water
quality and cover seem to be similar throughout the range of the Atlantic
croaker.
Temperature
Croakers tolerate wide ranges of temperature. Juvenile croakers have been
caught at water temperatures ranging from 0" to 36°C (32" to 97OF) (Parker
1971) and grow over a range from 6" to 32°C (43" to 9OoF) (Johnson 1978). In
general, the early life stages of the croaker are most cold tolerant and
adults are least cold tolerant (Johnson 1978). Since croaker recruits
immigrate to their nursery grounds from their spawning grounds in winter, very
low temperatures may be the major cause of larval and young juvenile
mortality. This climatic influence affects a whole region (Norcross and Austin
19811, and it, rather than habitat factors, may often control the abundance of
croakers.
Temperature also has been suggested to be an important localized habitat
variable when considered in terms of the wide variation and duration of
temperature extremes. The tolerance of juvenile croakers to rapid changes in
temperature (thermal shock) is limited by their thermal history, with an
increase of 17°C (31°F) incapacitating croakers acclimated at 18°C (64OF) and
an increase of 9°C (16°F) incapacitating croakers at 33°C (91°F) (Copeland et
2
al. 1974). These findings indicate that temperature variation in the spring
should not play as great a role as would temperature variations in summer when
overall temperatures are higher. In both spring and summer, temperature
variation in deep water habitats is less than it is in shallow areas. This
factor is consistent with the concentration of juvenile croakers in deeper
areas in most reports from estuaries with large tidal ranges. Large tidal
fluctuations expose shallow water to the extreme temperatures of alternately
exposed and submerged tidal flats.
Salinity
The tolerance of croakers to salinity is impressive. The species has been
found in waters ranging from 0 ppt (Johnson 1978) to 70 ppt (Simmons 1957);
however, this is an extreme range that includes all life stages combined.
Highest numbers of juveniles are associated with salinities in the oligohaline
and mesohaline range (0.5 to 18 ppt) (Parker 1971; Kobylinski and Sheridan
1979; Weinstein 1979; Weinstein et al. 1980b). As croakers grow, they are more
likely to be found at higher salinities (Parker 1971, Chao and Musick 1977,
Sheridan 1979). The one exception to this pattern was in Barataria Bay,
Louisiana (Rogers 19791, and may have resulted from the mortality of croakers
incidental to the inshore shrimp fishery in the higher salinity parts of the
study area. Rogers (1979) noted the high frequency of large catches of
croakers prior to the shrimping season and the rarity of large catches during
the shrimping season, just when large croakers should have been most abundant.
Shrimping did not extend into the low salinity sampling areas.
Stability of the salinity regime within croaker nursery areas may also be
a factor in controlling croaker distribution. Gerry (1981) found croakers most
abundant in habitats where salinity fluctuations were the least. Juvenile
croakers tend to avoid areas of fluctuating salinity (Herke 1971; Gerry 1981).
Rapid changes in salinity, on the order of 5 ppt/h, affect the behavior of
juvenile croakers while changes of 1 ppt/h do not (Perez 1969). Avoidance of
fluctuating salinity may in part be a reason that croakers in some strongly
tidal areas seem to prefer deeper tidal creeks over shallow flats and marsh
creeks, since the magnitude of salinity change should be less in deeper water
for a given period.
Croakers forage for a variety of organisms on and in the surface layers
of sediments (Darnell 1961; Parker 1971; Diener et al. 1974; Stickney et al.
1975; Chao and Musick 1977; Overstreet and Heard 1978; Etzold and Christmas
1979; Kobylinski and Sheridan 1979; Sheridan 1979; Weinstein 1979; Schwartz
1980). Mysids, decapods, amphipods, copepods and polychaetes form the bulk of
the croaker diet. At times, mollusks, finfishes, and detritus are also
consumed in large quantities.
Polychaetes and copepods (calanoid and harpacticoid) are the major
dietary components for small croakers. As fish grow into young adults, 120 to
180 mm (5 to 7 inches) TL, their diet includes more fish. The reported
consumption of detritus by all sizes of croakers may be incidental and of
little nutritive value. Stickney and Shumway (1974) found croakers to lack the
3
ability to digest cellulose. Detritus in the guts of croakers, therefore, is
most likely a byproduct of bottom feeding over unconsolidated mud of high
organic content.
Substrate
Substrate quality, in terms of dominant substrate type and organic
content, plays an important role in determining juvenile croaker distribution.
Sand and hard substrates are not suitable at all for juvenile croakers. Mud is
most suitable (Chittenden and McEachran 1976; Kobylinski and Sheridan 1979;
Weinstein 1979). Bottoms where juvenile croakers occur most abundantly usually
are covered with large quantities of detritus (Bearden 1964; Kobylinski and
Sheridan 1979). This suggests that there is a positive correlation between
occurrence of juvenile croakers and the amount of organic matter in the
surface sediments. Weinstein et al. (1980b) found highest ,abundances of
juvenile croakers in areas of high organic content, up to 33%. Croakers do not
use organic-rich sediments directly; however, the organic content of sediment
may determine habitat suitability for their prey and, therefore, indirectly
for croakers themselves.
Turbidity
Juvenile croakers tend to be found in highly turbid runoff areas (Bearden
1964; Parker 1971; Kobylinski and Sheridan 1979) and in the low salinity,
maximum turbidity zone of estuaries (Weinstein et al. 1980b). It is typically
an area of high sedimentation where salt water flocculates and traps much of
the alluvial load brought into the estuary (Nichols 1972). Highly turbid
areas, in general, also tend to have high organic loads which may cause an
increase in food availability to croakers. Turbidity does not pose any feeding
problem to croakers since they are morphologically adapted for tactile feeding
(Chao and Musick 1977). Based on data of Livingston (19841, croakers in
Apalachicola Bay were abundant at sites with turbidities exceeding 15 Formazin
Turbidity Units (FTU) during the period of residence and were rare at lower
turbidities. No comparable data are available for other estuaries; however,
less detailed information for St. Andrew Bay, Florida, indicates that croakers
occur in substantial numbers at turbidities as low as 3 FTU (Ogren and Brusher
19771, which is lower than the lowest measured at Apalachicola Bay.
Consequently, high turbidity may be optimal but low turbidity does not appear
to exclude croakers.
Water Depth
The abundance of juvenile croakers is not consistently related to depth.
In areas of small tidal fluctuations, such as the Gulf of Mexico Coast and the
North Carolina sounds, juvenile croakers are densest in shallow peripheral
areas (Parker 1971; Ogren and Brusher 1977; Yakupzack et al. 1977; Kobylinski
and Sheridan 1979; Copeland et al. 1984). In Lake Pontchartrain, an area of
low tidal fluctuation, young croakers were only caught offshore and were
heavily concentrated in deep channels in November and December; from January
on, however, they were caught in inshore areas (Suttkus 1955). In areas of
greater tidal fluctuation, such as Delaware Bay, Chesapeake Bay, and the Cape
Fear River Estuary, juvenile croakers were concentrated in deep channels and
4
were rare in shallow areas (Haven 1957; Weinstein et al. 1980b; Thomas 1981;
Weinstein and Brooks 1983); however, in South Carolina, large numbers of
juvenile croakers also have been caught in small marsh creeks subject to large
tidal fluctuations (Turner and Johnson .l974).
Cover
Juvenile croakers occur over bare, soft muddy bottoms. Structural cover
does not appear to be a habitat requirement for croakers. Behavioral and
morphological adaptations of the Atlantic croaker for feeding are directed
toward the exploitation of the surface layers of soft muddy bottoms and are
not useful where vegetation or rocks replace or interfere with access to a
soft bottom. In addition, other functions commonly associated with structural
cover are served by other features of habitat. Protection from visual
predators may be provided by high turbidity (Parker 1971). Observations of
higher incidences of scarring on juvenile menhaden (Brevoortia patronus) in
clear than in turbid estuaries are consistent with this hypothesis (Kroger and
Guthrie 1972). The occurrence of small juveniles in areas and seasons of low
salinity also may reduce vulnerability to predators. For example, juvenile
croakers appear to be a preferred prey of striped bass (Morone saxatilis),
when available (Dove1 1968). In Chesapeake Bay in the winter, striped bass
congregate in areas of 21 to 22 ppt, while juvenile croakers occur in highest
concentrations at salinities less than 20 ppt. Although this pattern of
distribution could be the result of physiological preferences for low
salinity, the pattern also is consistent with an effect of predation. Either
losses to predators virtually eliminate croakers at salinities greater than 20
ppt, or adaptations that restrict young croakers to low salinities have
evolved as a means of predator avoidance. Finally, the function of structural
cover for protection from the rigors of the physical environment appears to be
served by areas of deep water, where necessary.
Dissolved Oxygen
Juvenile croakers are abundant in conditions that often result in low
concentrations of dissolved oxygen. They occur in areas with highly organic
sediments, high concentrations of suspended solids, and high water
temperatures. When dissolved oxygen concentrations drop, most fish, including
croakers, will leave an area (Markle 1976; Chao and Musick 1977). Although the
tolerance of croakers to low dissolved oxygen is not specifically known,
oxygen concentrations below 3 mg/l are limiting to other species and oxygen
concentrations that do not drop below 4.5 mg/l have highest suitability
(Doudoroff and Shumway 1970; Hoss and Peters 1976). Limiting conditions may be
reached, especially in deep habitats during the summer, when biological and
chemical oxygen demand are high, and thermal or salinity stratification
prevents mixing of the water column.
HABITAT SUITABILITY INDEX (HSI) MODEL
Model Applicability
This model is developed for juvenile croakers. Factors that influence the
successful survival and recruitment of larvae from coastal marine waters are
complicated and outside the influence of the estuarine system (Norcross and
Austin 1981; Miller et al. 1984). Adult croakers do not usually occur in
oligohaline areas and are less tolerant of low temperature than juveniles. In
addition, adults occur in coastal marine waters. Consequently, the model is
not applicable to adults. Also, the model is not applicable where
environmental contaminants seriously affect habitat quality.
Geographic area. The geographic areas covered by this model are the
southeast Atlantic coast and the Gulf of Mexico coast. The basic life
requisites of water quality and cover seem to be similar throughout the range
of the Atlantic croaker, except that in some locations deep creeks and
channels are heavily utilized while in other locations shallow areas are
strongly preferred. The HSI model attempts to account for this major
difference in nursery habitat between locations but assumes other variables to
be operating similarly in all areas.
Season.
conditions,
The HSI model is designed to evaluate spring and summer
because they are the most critical. Some of the variables pertain
to environmental conditions that occur only during these seasons.
Cover types. Croakers typically use estuarine and nearshore marine
habitats. Spawning occurs in the marine habitat and near the transition to the
estuarine habitat. The estuarine habitat is used as the nursery ground. This
model is intended only for the estuarine habitat and applies to areas of
Estuarine Unconsolidated Bottom (ElUB), and to a lesser extent to Estuarine
Intertidal Unconsolidated Shore (EZUS), according to the classification of
Cowardin et al. (1979).
Minimum habitat area. The minimum habitat area is that area of contiguous
suitable habitat that is required for croakers to develop and reproduce
successfully. No minimum habitat size requirements for the Atlantic croaker
have been identified in the literature.
Verification level. Three biological experts outside the U.S. Fish and
Wildlife Service were identified to review and evaluate the croaker HSI model
throughout its development. These experts were John Lunz, U.S. Army Corps of
Engineers, Vicksburg, Mississippi; Michael Weinstein, Virginia Commonwealth
University, Richmond; and Brenda Norcross, Virginia Institute of Marine
Science, Gloucester Point. Ideas and suggestions from these experts were
incorporated into the model-building effort. Additional comments from users
and U.S. Fish and Wildlife Service field offices and new sources of data have
been used in revising the model.
Model Description
Overview. This HSI model for the juvenile Atlantic croaker considers
water quality and cover life requisites in the estuarine habitat. The
relationship of habitat variables, life requisites, and life stage to the HSI
is illustrated in Figure 1.
The two basic life requisites used in the model are not independent.
There is a great deal of overlap and correlation between the habitat variables
and life requisites. For example, turbidity in estuarine systems is directly
related to both salinity and depth (Nichols 1972). The grouping of habitat
variables into water quality, cover, and food is primarily for the development
of the HSI and is not intended to imply that water quality and cover
variables, for example, are mutually exclusive.
Water quality. The value of the water quality component is determined by
turbidity, dissolved oxygen, salinity, and temperature. Turbidity (V,> is
positively correlated with the abundance of juvenile croakers. Suitability is
assumed to increase as the logarithm of turbidity as measured by a turbidity
meter from 2 to 20 Formazin Turbidity Units (FTU) and remain optimal at higher
turbidities. Alternatively, turbidity can be measured as the concentration of
suspended solids. Suitability is assumed to increase as the loqarithm of the
concentration of suspended solids from 2 to 20 mg/l and remain optimal at
higher concentrations.
The dissolved oxygen variable -- the minimum summer concentration of
dissolved oxygen (V, > -- is assumed to be limiting at 2 mg/l and optimal above
5 mg/l. Adjustments have been made to the range cited in the previous section
for fishes in general to account for behavioral responses that can buffer
croakers from unsuitable conditions on the bottom and also to account for
limitations in a HEP sampling program for detecting the actual minimum
concentration.
The salinity variables -- mean spring (March to May) salinity near the
bottom (V,) and mean summer (June to September) salinity near the bottom (V, >
-- are based on seasonal relationships between catch of juvenile croakers per
unit of effort and salinity in South Carolina estuarine areas (Miglarese et
al. 19821, supplemented by and checked for consistency with more general
compilations of abundance versus salinity (Haven 1957; Parker 1971; Chao and
Musick 1977; Kobylinski and Sheridan 1979; Weinstein et al. 1980a; Ross and
Epperly, in press). The essential features of the salinity variables are (1)
although low salinity areas are most suitable, areas that are fresh throughout
the year are unsuitable in most locations, and (2) higher salinities are more
acceptable in the summer than in the spring. In the spring, salinities of 0 to
15 ppt are most suitable and salinities greater than 24 ppt are unsuitable. In
the summer, salinities of 6 to 26 ppt are most suitable and salinities less
than 1 ppt are unsuitable.
Variables V, and V, together account for all these factors in most
locations; however, the Barataria Bay and its marsh system in Louisiana are an
exception. There, not only were freshwater areas used by croakers in summer as
well as spring (Rogers 19791, but also catches in the spring were positively
7
Habitat variable Life requisite Life stage Habitat
“1 Turbidity
“2 Dissolved oxygen
“3 Salinity in spring
"4 Salinity in summer
03 Juvenile Estuarine HSI
Figure 1. Relationship of habitat variables and life requisites to the HSI for juvenile Atlantic croaker.
correlated within the salinity range of 0 to 10 ppt. In most locations,
highest abundances in the spring occur anywhere between 0 and 15 ppt, with no
consistent trend within that range. This discrepancy is not caused by a direct
response to salinity but by the low accessibility of freshwater parts of the
Barataria Bay system to small juveniles entering from the Gulf of Mexico. The
brackish zone is so broad and the water connections across it are so indirect
that relatively few croakers get to the fresh part of the system. The
freshwater zone is so extensive that the fish are not concentrated within it.
In most other estuaries, low salinity fringes may be much more accessible,
because the distances are small or currents exist that transport young
juveniles close to suitable low salinity areas (salt wedges up drowned river
estuaries, wind-driven circulation in large bays and sounds). To account for
these differences, an alternative form of the spring salinity variable (V,) is
provided for the Barataria Bay system, in which the low salinity end is really
surrogate variable for accessibility from the Gulf of Mexico. This
zlternative should be evaluated for use in other areas with broad (10 to 100
km, 6 to 60 mi) brackish and freshwater zones, and perhaps applies to other
large marsh-lake-bayou systems of coastal Louisiana.
Although changes in salinity of 5 ppt per hour have been shown to alter
the activity of croakers in a way that would reduce local abundance (Perez
1969) given sufficient time, the amount of time during which the conditions of
rapid salinity change occur in any area is assumed to be small and the effect
is ignored. Temperature undoubtedly is important in setting the northern limit
of the Atlantic croaker's geographic distribution, influencing year-class
strength, and determining the timing of entry and exit from nursery areas, but
is not assumed to make a difference among areas within a region. Although
rapid increases in temperature have been demonstrated to be harmful to
croakers in the laboratory, the minimum increase causing observable effects
-- 33'to 40°C at l"C/min (91° to 104°F at 1.8OF/min) (Copeland et al. 1974) --
is extreme under natural conditions. Since thermal tolerance is greater at the
temperatures that are usually encountered in estuaries, temperature change is
ignored as a possible determinant of habitat suitability.
Food/cover. The value of the cover component is determined by depth and
substrate; however, habitat suitability is related to the depth variable (V, 1
differently, depending on tidal range. In regions of weak tidal influence,
shallow areas closely associated with marsh are most suitable, and shallow and
deep open water areas are progressively less suitable. In regions of stronger
tidal influence, the main stem channels of drowned river estuaries can be
heavily utilized by juvenile croakers, and deep areas are commonly considered
to be most suitable (Haven 1957; Weinstein 1979). However, Chao and Musick
(1977) showed that croakers were concentrated in shoal areas rather than in
the channel of the York River Estuary, Virginia in spring and summer, and
Turner and Johnson (1974) collected very high densities in marsh creeks near
Charleston, South Carolina in the spring. In view of the variety of situations
which juvenile croakers are abundant in regions of strong tidal influence,
i: depth variable is incorporated in assessing habitat suitability for these
areas.
Soft muds are regarded as the most suitable substrate type (V,> in all
areas; half sand, half silt and mud are intermediate in suitability; sandy
9
bottoms are low in suitability; and shell, gravel, or rock bottoms or seagrass
beds are unsuitable. Although Bearden (1964) and Kobylinski and Sheridan
(1979) note that juveniles occur in high densities in areas with large quantities of detritus, the only report of a positive correlation with
sediment organic content was for tidal creeks of the Cape Fear River Estuary,
North Carolina (Weinstein (1980b1, sites regarded as only of minor importance
as nursery habitat for croakers in this system (Weinstein 1980a). In an
analysis of 51 primary nursery areas in Pamlico Sound, North Carolina, Ross
and Epperly (in press) found no correlation between catch per unit of effort
and sediment organic content, even though the substrates of the sites also
were described as covered with detritus. Apparently, areas in Pamlico Sound
are suitable over a broad range of sediment organic content, at least as low
as 2%, and suitability is limited by other factors. This finding is consistent
with inferences drawn from comparisons of nursery utilization by juvenile
croakers and another bottom-feeding sciaenid, the spot (Leiostomus xanthurus),
in one of the Pamlico Sound nurseries. Miller et al. (1984) documented higher
productivity of croakers than spots, even though the latter were more abundant
initially and were more abundant in deeper areas where the biomass of benthic
invertebrates was greater. They attributed this outcome to greater predation
in the deeper areas. Consequently, no indicator of food availability, such as
sediment organic content or benthic biomass, has been incorporated in the
model.
Suitability Index (SI) Graphs for Habitat Variables
This section provides graphic representations of the relations previously
described between the habitat variables and estuarine (E) habitat suitability
for the Atlantic croaker. An SI value of 1.0 indicates optimal conditions and a value of 0 indicates unsuitable conditions. Data sources and assumptions
associated with documentation of the SI graphs are listed in Table 1.
Habitat Variable
ElUB Vl Mean turbidity during 1.0
E2US March through
September.
Z 0.8
0
f 0.8
Suitability Graph
v ’ ’ ’ 11 I”‘1 , , r
1 2 5 10 20:
FTU or mgll
10
Habitat Variable
ElUB
EZUS
% Minimum concentration
of dissolved oxygen
during July through
September.
;
0+
a
z
2
a
Suitability Graph
mgll
ElUB
EZUS 5 Mean salinity during
March through May.
(Use dotted line for
estuaries with broad
brackish and fresh
wetland zones.>
PPt
ElUB
EZUS
v4 Mean salinity during
June through September.
PPt
11
Habitat Variable
ElUB
E2US
ElUB
E2US
V5 Depth category:
1) Shallow areas
closely associated ;
with marsh. 0
s
2) Open water <2 m
deep.
3) Open water >2 m
deep.
(Use area-weighted mean when
more than one depth category
occurs in an evaluation area.)
‘6 Dominant substrate
type:
1) >75% mud. ;
0
2) 25% to 75% mud. S
*r c
3) >75% sand, shell, z
z
or other hard material. Q
.f=
4) Seagrass beds or a
mostly rock and shell;
no soft material.
Suitability Graph
l.O-OS-I
0.8-
0.4-
I
OS-o-
o-,
1 ‘2 3
Class
1.0 L
0.8-
O-8-
0.4-
OS*
I
O-O +
1 2 3 4
Class
12
Table 1. Data sources and assumptions for Atlantic croaker suitability indices.
Variable and source Assumption
4 Bearden 1964
Parker 1971
Kobylinski and Sheridan 1979
Livingston 1984
Vz Doudoroff and Shumway 1970
Hoss and Peters 1976
Chao and Musick 1977
v3 Parker 1971
Rogers 1979
Weinstein et al. 1980b
Miglarese et al. 1982
Ross and Epperly, in press
V, Parker 1971
Weinstein et al
Miglarese et al
Ross and Epperl:
% Parker 1971
Yakupzack et a 1
Sheridan 1983
1980b
: 1982
y, in press
. 1971
Miller et al. 1984
‘6 Chittenden and
Kobylinski and
Weinstein 1979
Ross and Epper
McEachran 1976
Sheridan 1979
y, in press
High turbidity levels are posi-tively
related to the abundance of
juvenile croakers.
Low levels of dissolved oxygen
are not suitable.
In the spring juvenile croakers are
caught at salinities from 0 to 24
ppt. Salinities of 0 to 15 ppt are
most suitable, except in Barataria
Bay. There, abundances are posi-tively
correlated with salinity
from 0 to approximately 5 ppt.
In the summer fresh water is un-suitable.
Salinities from 6 to 26
ppt are most suitable. Salinities
greater than 30 ppt are low in
suitability.
In regions with small tides only,
shallow areas closely associated
with marsh are most suitable,
shallow open water is intermediate
in suitability, and deep open
water is least suitable.
Soft mud is most suitable. Sandy
mud is less suitable. Hard and
coarse substrates and seagrass
beds are unsuitable.
13
Component Index Equations and HSI Determination
The HSI equation considers two life requisite components: water quality
and food/cover. Water quality comprises turbidity, dissolved oxygen, and
salinity. Food/cover comprises depth and substrate type. To obtain an HSI for
the Atlantic croaker, the SI values for habitat variables and components must
be combined as follows:
Component Equation
Water quality (WQ)
Coastal Louisiana SI "s or (SIvlxSIv2 xSIv )lh whichever is lower
3
Other locations SI SI or (SIV1xSI~xSIV3xSIV4) l/4
V,’ V+’
whichever is lowest
Food/cover (FC)
Tidal range CO.5 m SI
v5
or SI
v6
whichever is lower
Tidal range >0.5 m SI
v6
HSI = WQ or FC, whichever is lower
Low SI values for either of the sa1init.y variables are assumed to be
limiting factors on the water quality component SI value. Partial compensation
for low values of the other variables is assumed to occur. Therefore, for
areas of coastal Louisiana with broad brackish and freshwater wetland zones,
the water quality component SI is determined by sprinq salinity or the
geometric mean of the three water quality SI values, whichever is lower. In
other locations, the water quality component SI is determined by
salinity, Sumner salinity, or the geometric mean of the four water qua
values, whichever is lowest.
soring
ity SI
The food/cover component is determined differently for regions of
and weak tidal influence. Where the tidal range is less than 0.5
inches), the lower of the SI values for the depth and substrate variab
strong
m (20
es is
assumed to be limiting. Where the tidal range exceeds 0.5 m (20 inches), the
food/cover component is determined by substrate alone.
The relative importance of the water quality and food/cover components to
the potential of a particular habitat to support the Atlantic croaker is not
known. The model assumes that either component can act as a limiting factor.
Therefore, the HSI for juvenile Atlantic croakers in estuarine habitats is
determined by the value of whichever component -- water quality or food/cover
-- is lower.
14
Table 3. Suggested methods for field measurements of variables used in the
croaker HSI model. a Variable Methods
h
13
v4
V5
aDetails for
Examination ef
Turbidity can be measured directly in Florazin
Turbidity Units (FTU) with a turbidity meter. FTU are
equivalent to Jackson Turbidity Units (JTU) in older
procedures. Since most turbidity in estuaries is from
suspended solids, turbidity also can be measured as
mg/l of suspended solids. FTU are approximately
equivalent to mg/l. Water samples for turbidity
determinations should be collected within approximately
30 cm (1 ft) of the bottom when possible.
Dissolved oxygen within approximately 30 cm (1 ft) of
the bottom can be measured using Winkler titration or
an oxygen meter.
Salinity within approximately 30 cm (1 ft) of the
bottom can be measured by titration, refractometer, or
salinity meter.
Same as for Vs.
Define evaluation area on topographic maps or
navigation charts. Open water is more than 30 m (100
ft) from the nearest shore or the nearest emergent
vegetation in areas of flooded marsh. Depth can be
determined from bathymetric charts or by direct
measurement. Use an area-weighted mean when more than
one depth category occurs within an evaluation area.
Substrate type is defined as the amount of coarse or
fine sediment in the top 5 cm (2 inches) of a core.
Substrate type is determined by sieving a known weight
of sediment through a 0.063 mm sieve (Tyler series No.
250). The material retained on the sieve is the sand or
coarser fraction from which the percentage of sand or
coarser material can be calculated. What goes through
the sieve is mud (silts and clays). The percentage of
mud is 100% minus the previously calculated percentage
of coarser material retained on the sieve.
water quality methods can be found in Standard Methods for
Water and Waste Water (Anonymous 1981).
16
should be used to check the suitability of extrapolations from other sources.
Suggested methods for measuring model variables are given in Table 3. Sources
of data should be documented.
Average values have been used for some model variables. The literature
suggests that the tolerance of the Atlantic croaker to changes in temperature
and salinity depends on the rapidity of the change. Variables of rates of
change in temperature and salinity have been excluded from this model for
reasons discussed previously. It should be recognized that these variables
cannot be ignored in applications involving discharges of hot or fresh water
directly into estuarine areas.
these cases.
Guidance should be sought to adapt the model in
This model has been written for the common case of estuarine residence by
juvenile croakers in the spring and summer. There are exceptions for which the
model should be modified. For instance, use of the upper Barataria Bay system
is so low by June (Rogers 1979) that only the spring salinity variable should
be applied. Low dissolved oxygen may make the area unsuitable in the summer,
judging from reports of fish kills attributed to the die-off of algal blooms
and low oxygen in nearby areas of coastal Louisiana (Yakupzack et al. 1977).
However, unsuitability in summer did not prevent heavy utilization earlier in
the year.
Interpretinq Model Outputs
The proper use of the HSI is one of comparison. This model can be used to
compare different habitats or the same habitat through time. The higher HSI
should correspond to the area that could potentially support more juvenile
Atlantic croakers. The accessibility of an area to larval recruits is an
important determinant of the level of utilization of some areas that may be
highly suitable in all other regards. In the case of Barataria Bay and similar
areas, the salinity variable is a correlate of accessibilty. In other areas,
it has not been possible to incorporate accessibility as a factor in this HSI
model; calculated HSI values may not be correlated with long-term estimates of
population density in these cases.
ADDITIONAL HABITAT MODELS
This model is a revision of an earlier habitat suitability index model
for juvenile Atlantic croakers in this series. The revision incorporates new
information about habitat requirements and responds to comments received since
the first printing. Additional comments were solicited from field offices of
the U. S. Fish and Wildlife Service in the geographic range of the Atlantic
croaker. This revision supersedes the original version, except for evaluations
involving thermal or freshwater discharges. Variables in the original model
may apply in these cases.
17
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19
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Haven, D.S. 1957. Distribution, arowth. and availability of iuvenile croaker,
Micropoqon undulatus, in Virginia.-Ecology 38:88-97-i -
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fourteen teleostean fishes at Beaufort, North Carolina. Bul
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i da1 marshes
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1
history of
. U.S. Bur.
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HOSS, D.E., and D.S. Peters. 1976. Respiratory adaptations: fishes. Pages 345- 356 in M. Wiley, ed. Estuarine processes. Vol. I. Academic Press, New
York7
Johnson, G.D. 1978. Develooment of fishes of the mid-Atlantic Biaht. Vol. IV:
Carangidae through Ephippidae. U.S. Fish. Wildl.
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marked juvenile Atlantic
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Micropoqonias
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and P.F. Sheridan. 1979. Distribution, abundance, feeding,
fluctuations of spot, Leiostomus xanthurus, and croaker,
undulatus, in Apalachicola Bay, Florida, 1972-1977.
Sci. 22:149-161.
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in clear and turbid estuaries. Mar. Fish. Rev. 34:78-80.
Livingston, R.J. 1984. The ecology of the Apalachicola Bay system: an
estuarine profile. U.S. Fish Wildl. Serv. FWS/OBS-82/05. 148 pp.
Markle, D.F. 1976. The seasonality of availability and movements of fishes in
the channel of the York River, Virginia. Chesapeake Sci. 17:50-55.
20
Miglarese, J.V., C.W. McMillan, and M.H. Shealy, Jr. 1982. Seasonal abundance
of Atlantic croaker (Micropoqonias undulatus) in relation to bottom
salinity and temperature in South Carolina estuaries. Estuaries 5:216-223.
Miller, J.M., J.P. Reed, and L.P. Pietrafesa. 1984. Patterns, mechanisms and
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and juveniles. Pages 209-225 i2 J.O. McCleave, G.P. Arnold, J.J. Dodson,
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H. Clepper, ed. Marine Recreational Fisheries 6. Sportfishing Institute,
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caught with a trawl in the St. Andrew Bay system, Florida. Northeast Gulf
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21
Ross, S.W., and S.P. Epperly. In press. Utilization of shallow estuarine areas
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22
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23
5
I-
;0272 -10,
R E P O R T D O C U M E N T A T I O N .~L~=‘ORT NO.
PAGE Biological Report 82(10.98) '.
4. Title and Subtitle
3. Recipient’s Accession NO .
5 . Repon Date
June 1985
Habitat Suitability Index Models: Juvenile Atlantic Croaker
(Revised)
6.
7. Author(s)
Robert J. Diaza and Christopher P. Onufb
8. Perbrmlng Orgpnizatton Rept. No.
9. Addresses of Authors 10. ProiectlTarklWork Unit No.
a Dept. Invertebrate Ecology bNational Coastal Ecosystems Team 11. Contract(C) or Grant(G) No.
Virginia Inst. Marine Science U.S. Fish and Wildlife Service (0
College of William and Mary 1010 Gause Boulevard
Gloucester Point, VA 23062 Slidell, LA 70458 (G)
12. Spanspring Organization Name and Address National Coastal Ecosystems Team 13. Type of Repoe .% Period Covered
Division of Biological Services
Fish and Wildlife Service
15. Supplementary NIX.5
U.S. Department of the Interior / 14.
Washington, DC 20240
Revision of FWS/OBS-82/10.21, published July 1982.
16. Abstract (Limit: 200 words)
A review and synthesis of existing information were used to develop an estuarine habitat
model for juvenile Atlantic croaker (Micropogonias undulatus). The model is scaled to
produce an index of habitat suitability between 0 (unsuitable habitat) and 1 (optimally
suitable habitat) for estuarine areas of the Atlantic and Gulf of Mexico coasts. Habitat
suitability indices are designed for use with Habitat Evaluation Procedures previously
developed by the U.S. Fish and Wildlife Service. Guidelines for juvenile Atlantic croaker
model applications and techniques for estimating model variables are described.
17. Document Analysis a. Oexnptorr
Habitability
Fishes
Estuaries
b. Identlf!err/Open.Ended T e r m s
Habitat
Habitat Suitability Index
Atlantic Croaker
Micropogonias undulatus
c. COSATl F i e l d / G r o u p
Lg. Availability Statement
Unlimited
19 Security Class (This Report)
/ Unclassified
I 20. Security Class CThis Page)
Unclassified
z,. No. of P ‘es
vi + 2_'3____-
22. price
(See n..i,l3r,1-7u-Jn.‘o.D,> OPTIONAL FORM 272 14-1
*U.S. GOVERNMENT PRINTING OFFICE: 1985- 5 7 3 - 2 6 0 (Formerly NTIS-3;)
Department 0, Commerce
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U.S. FISH AND WllDllFE SERVICE
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