Riverine discharge and estuarine fish nurseries first annual report for the ichthyoplankton survey of the Little Manatee River, Florida

[From the U.S. Government Printing Office, www.gpo.gov]

RIVERINE DISCHARGE AND ESTUARINE FISH NURSERIES:

FIRST ANNUAL REPORT FOR THE ICHTHYOPLANKTON SURVEY
OF THE LITTLE MANATEE RIVER, FLORIDA

Ernst B. Peebles and Susan E. Davis

Department of Marine Science
University of South Florida

November 15, 1989

Funds for this project were provided by the Florida Department of
Environmental Regulation, Office of Coastal Management using funds made
available through the National Oceanic and Atmospheric Administration under
the Coastal Zone Management Act of 1972, as ammended. Local administration of
this work was conducted through contracts with the Southwest Florida Water
Management District. Project manager: Michael S. Flannery.

RIVERINE DISCHARGE AND ESTUARINE FISH NURSERIES:

FIRST ANNUAL REPORT FOR THE ICHTHYOPIANKTON SURVEY
OF THE LITTLE MANATEE RIVER, FLORIDA

Ernst B. Peebles and Susan E. Davis

Department of Marine Science
University of South Florida

November.15, 1989

TABLE OF CONTENTS

Page No,

EXECUTIVE SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

INTRODUCTION . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . 1

OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Sampling Procedu-j;e . . . . . . . . . . . . . . . . . . . . . . . . . 3
Laboratory Procedure . . . . . . . . . . . . . . . . . . . . . . . . 4
Statistical Treatment of Fish Abundance, Length, and
Seasonality . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Analyses . * * * * , * * , * , , * * * * * , * , * * , , * * -* * , , I
Assessment of spawning ground vulnerability . . . . . . . . . 7
Assessment of nursery ground vulnerability . . . . . . . . . 7
Impacts of riverine discharge on nursery grounds . . . . . . 8

RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Spawning Vulnerability . . . . . . . ... . . . . . . . . . . . . . . 9
Spawning location . . . . . . . . . . . . . . . . . . . . . . 9
Spawning season . . . . . . . . . . . . . . . . . . . . . . . 11
Nursery Habitat Vulnerability . . . . . . . . . . . . . . . . . . . . 12
Riverine Discharge and Nursery Grounds . . . . . . . . . . . . . . . 12

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . .. . ... . 19

EXECUTIVE SUMMARY

The abundance of fish eggs and young fishes in the tidal portion of the
Little Manatee River was monitored on a regular basis during 1988.
Collections with fine-meshed (0.5 mm) nets were made twice each month at six
locations ranging between fresh water and the open waters of Tampa Bay.
Approximately 74,000 fish representing 68 species were collected. Fish
eggs and newly-hatched fishes (larvae) were concentrated in the higher
salinities at the mouth of the river and in Tampa Bay. The status of bone
development in the tail region of the larval fishes was used as index of
relative age. The different species were ranked according to the abundance of
their various larval age classes. Species which were most abundant during
their youngest stages were considered to have been spawned in closer proximity
to the study.area than species which were most abundant as older stages. This
analysis indicated that species such as spotted seatrout (speckled trout),
sand seatrout (silver trout), silver perch, kingfishes (whitings) and
anchovies (glass minnows) spawned nearby, probably near the the mouth of the
Little Manatee River. Although this general area may provide suitable
spawning conditions for a wide variety of fishes, it is not considered to be
an exclusive spawning location for Tampa Bay fishes. Previous surveys
indicate that the aforementioned fishes also spawn in similar habitats outside
of the study area. The spawning seasons were different for the different fish
pecies, but spawning activity was generally highest during the period from
March through August.
Within specific age classes, correlations between fish length (age) and
,salinity of capture were tested for statistical significance. This analysis
indicated that the young of several species migrated upstream to low salinity
areas of the river during the first few weeks of life. Percentages of total
abundance concentrated in low salinity (<18 O/oo) areas were calculated. More
than 759, of the total abundance of young menhaden, anchovies, spot, sand
seatrout, and several other fishes was found to be concentrated in these low
salinity areas. One or more of these species was found to be occupying low
salinity areas of the river at any time of year. In addition to simple
documentation of the low-salinity concentration phenomenon, abundances in low
salinity areas were tested for statistical difference from abundances in
higher salinity areas. This test proved significant for several species, most
notably young anchovies and menhaden. Previous research has indicated that
snook and red drum (redfish) also concentrate in low salinity areas during the
first few weeks of life.
Increased freshwater discharge had variable effects on the distribution
of young fishes. Some persisted in the upper river during strong discharge
periods, while others moved closer to the mouth of the river. Correlations
between young fish abundance and flow rate of the river were not found.
However, most of the "concentrating" species increased in abundance in the
river channel during periods of rapid salinity reduction. This could reflect
either migration into or out of the river. Likely causes of migration are
local shifts in food resources and limits on the physiological adjustment
capabilities.of the young fishes during periods of rapid salinity reduction.
Since small', suspended organisms (such as copepods) were not abundant in
low salinity areas, it is probable that the concentrations of young fishes

were feeding on organisms associated with the bottom and edges of the river
and its backwaters. In these areas, particulate organic matter, such as
decaying leaves, may be the foundation of the food web. A significant
correlation was found between flow rate of the river and the importation of
particulate organic matter into nursery areas.

INTRODUCTION

This report represents one component of a comprehensive study of the
estuarine portion of the Little Manatee River in west-central Florida. In
regard to municipal development, the Little Manatee River is the least
impacted of the rivers which empty into Tampa Bay. Documentation of the
river's biological and water quality characteristics is presently being
conducted in the form of a multifaceted database collection effort. This
report and reports on fisheries habitat (Haddad et al. 1989), phytoplankton
(Vargo 1989) and zooplankton (Rast and Hopkins 1989) represent the biological
portion of the study during 1988.
Tampa Bay is part of a nearly continuous coastal estuarine system
ranging from Chesapeake Bay to the western Gulf of Mexico. Species which are
associated.with estuaries comprise the vast majority of the fisheries yields
in most, if not all, of-this region (Skud and Wilson 1960, Gunter 1967).
General degradation of the estuarine environment and reductions in the
productivity of important estuarine-based fisheries have increased the demand
for estuarine management (Mansueti 1961, Copeland 1970, Odum 1970, Fischer
1970).
The use of estuaries as nursery grounds is a principal characteristic of
estuarine dependence, yet the relationship between freshwater discharge and
the abundance of young estuarine-dependent fishes is incompletely understood.
Some researchers have associated increased freshwater discharge with enhanced
fish nursery production (Gunter and Hall 1963), while others have not
(Kobylinski and Sheridan 1979, Weinstein et al. 1980,.Rogers et al. 1984). In
extreme cases, such as in the case of the totoaba (Cynoscio macdonaldi) of
the Colorado River estuary, reductions in freshwater discharge have resulted
in population collapse.
Because of the Little Manatee River study area's position on the
salinity gradient, the data provided by the present survey are well-suited to
the study of nursery grounds used by estuarine-dependent fishes. Analyses in
this report are directed toward assessment of the potential impacts of
freshwater discharge on these nursery grounds.

OBJECTIVES

1. To provide a record of the use of the tidal portion of
the Little Manatee River by the early stages of coastal
fishes.

2. To identify the fishes whose spawning and nursery grounds
are most likely to be impacted by changes in the quantity
and/or quality of riverine discharge.

3. To identify the seasons when impact on spawning and
nursery habitat would be greatest.

4. To initiate an investigation into the relationship
between the quantity and/or quality of riverine discharge and
short-term changes in the distribution of the early stages of
estuarine-dependent fishes.

METHODS

Sampling Procedure

Plankton collections were made at six fixed 1 ocations at approximately
two-week intervals from January 1988 through January 1989 (total of 24
collections). The six stations covered an 11-mile (17.7 km) transect between
a very low salinity area and the open waters of Tampa Bay (Fig. 1). Some
characteristics of these stations are given below (depth and salinity are
presented as means with ranges in parentheses).

Sta. Mile Depth Salinity Description
W (0/00)
1' 8.8 1.9 0.8 Braided channel, sloughs, marsh, hammocks
(1.0-2.5) (0.0-4.5) Dominant vegetation: Juncus, Typha,
Acrostichum, Serinoa, Myrica, Juniperus,
Pinus. Sabal. Alternanthera

2 6.4 2.3 2.9 Channel bordered by low bluffs on left
(1.5-3.0) (0;0-11.5) bank; marsh-rimmed embayment on right.
Dominant vegetation: Juncus. Serinoa. Sabal,
Pinus, Quercus, Casuarina

3 4.4 1.7 6.7 Channel bordered by low bluffs on right
(1.3-2.0) (0.0-19.0) bank; low, marshy ground on left.
Dominant vegetation: Juncus, Schinus,
Serinoa, Sabal, Pinus, Rhizophor
Phramites

4 2.2 2.3 10.8 Channel bordered by municipal development on
(1.5-3.3) (0.0-22.5) right bank, mangrove swamp on left.
Dominant vegetation: Rhizophora, Avicennia,
Ouercus

5 0.0 1.8 16.6 Braided channel bordered by municipal
(1.3-3.0) (1.0-30.0) development on right bank; mangrove swamp
on left - oyster growth near shoreline.
Dominant vegetation: Rhizophora, Halodule,

6 -2.2 2.9 25.0 Open bay with sand bottom.
(2.0-3.5) (13.0-34.0)

All collections were made at night on an incoming tide. The plankton
samples were taken-sequentially, rather than simultaneously (a single boat was
used). In order to keep tidal phase as similar as possible during the
collections, the upper (1-3) and lower (4-6) stations were sampled on
consecutive nights. There was no established order in which the two groups of
stations (upper and lower) were sampled, except that each night's sampling
progressed upstream, in the direction of tide travel.
Sampling gear consisted of a 0.5 meter mouth diameter, 505 um mesh,
conical (3:1) plankton net equipped with a digital flowmeter. Two three-step
oblique tows (bottom-midwater-surface), one surface tow and one bottom tow
were made at each station (24 samples per collection, total of 576 samples).
Fishing depth of the weighted net was controlled by adjusting the length of
the tow line while keeping line angle as constant as possible. The position
of the winch on the gunnel caused a controlled "crabbing" by the boat, which
directed propeller turbulence away from the towed net. The net was towed at
approximately 2 m/s for five minutes, resulting in a tow length of about 500 m
and an average volume-filtered of about 75 mi. Samples were fixed in sodium
borate-buffered 6-10% formalin in saline.,
Immediately preceding net deployment, water column samples taken with a
Niskin bottle were filtered through 35 um mesh. The analysis of the . I
zooplankton from these samples is found in the report by Rast and Hopkins
(1989). Before and after net deployment, salinity, temperature, and dissolved
oxygen were determined electronically at surface, bottom, and at one-meter
intervals between surface and bottom.

Laboratory Procedure

After allowing a few days for tissue fixation, the oblique samples were
rinsed with water and transferred to 70% ethanol in de-ionized water. At this
step it was discovered that one of the oblique samples had not been properly
preserved, so it was replaced with one surface and one bottom sample taken at
the same location and time (the remainder of the surface and bottom samples
were stored, and there are presently no plans for processing them).
-Dissecting microscopes were used to separate fishes from invertebrates for
later identification, measurement, and enumeration.
Identifications were based on descriptions found in: Fahay (1983),
Lippson and Moran (1974), the U.S. Fish and Wildlife Service's series on fish
development in the Mid-Atlantic Bight (e.g., Fritzsche 1978), Moser (1983),
Houde and Fore (1973), Hettler (1984), Hogue et al. (1976), Peters (1981,
1983, 1985), Watson (1983), Fable et al. (1978), Houde et al. (1970), Carr
(1942) and Fuiman et al. (1983). The taxonomic nomenclature used follows
Robins et al. (1980). Each identified taxonomic group (species, in most
cases) was subdivided by stage of development. The divisions were based
primarily on the status of bone development in the caudal region of the young
fish (Fig. 2), specifically:

preflexion stage - period between hatching and notochord
flexion; tip of notochord is most distal osteological
feature.
flexion stage - period during notochord flexion; upturned

notochord or_4arostyle is most distal osteological feature.
postflexion stage - = postlarval stage; period between
completion of flexion and juvenile stage; hypural bones are
most distal osteological feature.
juvenile stage - period beginning with the approximate size at
which median fin meristic characters are complete, or in
some cases, when dense pigment fields appear (such as the
development of silver pigmentation in clupeiform fishes);
ends with attainment of sizes known.to be sexually mature.,

The abundance of fish eggs and dominant inverte brate groups (mysids,
amphipods, cumaceans, ostracods, etc.) were estimated after subsampling, if
neccesary, with a box-type plankton splitter. This re-examination of the
sample provided a means for quality control for the fish/invertebrate sorting
process. Although the fishes in the vast majority of samples-were counted
directly, the plankton splitter was used on a few occasions when anchovy
catches were very high. In these samples, all other fishes were removed and
counted directly, rather than splitting them along with the anchovies * The
data were recorded using an electronic spreadsheet and were then sorted into
taxonomic subfile@ to facilitate analysis with a microcomputer.

Statistical Treatment of Fish Abundance, Length, and Seasonality

The three principal biological variables associated with the
ichthyoplankton collections are fish abundance, fish length, and season of
collection. In regard to fish abundance, each plankton sample hasa total
catch (number of individuals) and an estimated density (number of individuals
per filtered volume) associated with it. The analyses included within this
report require descriptions of various combinations of abundance data from
individual plankton samples. The frequency distributions for these
combinations tend to be kurtose and positively skewed (Fit. 3).
Cumulative catch, average density and median density were evaluated as
descriptive parameters for fish abundances that were based on combinations of
samples. As a particular taxon is encountered more frequently during
sampling, the first-order trend in the cumulative catch tends to be linear.
However, a stepwise second-order trend was often found to be superimposed upon
this, and sometimes had a large impact on apparent abundance, particularly at
small sample sizes. Similarly, the strong influences of values in the tails
of combined density distributions often caused average densities to be much
more variable than median densities at small sample sizes. Since different
taxa,are found in different abundances in the estuary, sampling resolution
cannot be equal for all taxa. Therefore, the potential for sample-size
dependence of cumulative catch and average density, along with the discussions
of Steel and Torrie (1980) and Sokal and Rohlf (1981), argue against the use
of these parameters in describing abundances with skewed frequency
distributions. The remaining parameter, median density, is generally less
variable at low sample sizes, but loses sensitivity when zero densities are
more numerous than nonzero densities. In such cases the median is zero
regardless of the magnitude of the nonzero values. To overcome this problem,
the median from nonzero.densities was weighted by the proportion of zero

densities in the collections. In other words, just as a nonzero average
density can be corrected to include zero values by multiplying the nonzero
average by the ratio of nonzero sample size to total sample size, the nonzero
median can be corrected to produce a positive number even when zero densities
dominate. This is expres sed as.

f
A Dm ---
n

where RM is the median density from the nonzero samples, f is the number of
nonzero-samples, and n is total number of samples. The value A is referred to
as an abundance "index" rather than a median. Single sample (instantaneous)
densities are also reported in several cases.
The exponential nature of larval mortality results in length-frequency
distributions that are often similar in shape to those for abundance
frequency, although an ascending limb, which represents escapement through the
mesh or an absence of small larvae, is sometimes present (Fig. 4). Gear
selectivity and migration into or out of the study area may affect both ends
of the collected length range. For each sample, an estimate of median
standard length (using an ocular micrometer for specimens <10 mm standard
length) was made for each developmental stage of each fish taxon collected.
This number was based on a maximum of 20 specimens from each stage.
The spawning seasons of estuarine and estuarine-dependent fishes are
variable on both an interspecific and interannual basis. Instantaneous
densities were plotted over time as an indication of spawning season (Appendix
I). Assuming that spawning activity is directly reflected by the abundance of
newly-hatched larvae, then Appendix I indicates that spawning intensity often
undergoes a rapid increase during the first part of the season, and then
diminishes gradually. A more concise representation of spawning season was
included in a catch table (Table 1) as seasonal ranges and modal values of f.
Modes were used to locate peak spawning months since median determinations
would not apply to year-round spawners and would be sensitive to the
disjunction in the ordination of the months (e.g., when a species spawned
between November (month 11) and March (month 3)). Seasonal trends in
abundance (as A) were also plotted for some taxa.

Analyses

Analysis 1: Assessment of spawning ground vulnerability

Spawning locations which were closest to riverine discharge were assumed
to be the most sensitive to changes in the quantity and/or quality of
discharge. The relative abundance (as A) of the different developmental
stages was used as a means of.gauging the proximity of spawning locations.
The collected taxa were divided into categories based on the relative
abundance of their preflexion, flexion and postflexion stages. For example,
species which were most abundant at the preflexion stage were presumed to have
spawned in greater proximity to the study area than those which were only
present as postlarvae. Within each of these categories, taxa which otherwise
had similar stage distributions were ranked by the abundance of their dominant
stages. The interpretation of this secondary ranking is somewhat limited by
the variability in the total egg productions of the different species. In
this procedure, is also possible that mesh extrusion of species with very
small preflexion stages could result in an underestimation of spawning ground
proximity.
In addition to spawning ground proximity, the season when the greatest
number of species were spawning was identified (species-specific information
on spawning season is included in Table 1). In this procedure, the most
abundant larval stage of each species was assigned a single month of peak
occurrence (from Table 1). The frequency distribution for these months was
plotted for the inshore-spawning (discharge-sensitive) and offshore-spawning
(discharge-insensitive) assemblages in order to identify the seasons with the
highest number of spawning peaks. This was supported by a seasonal plot of
the number of species collected.

Analysis 2: Assessment of nursery ground vulnerability

Nursery habitat vulnerability was assumed to be proportional to degree
of dependence on low salinity habitats. A systematic design for categorizing
this dependence is diagramed by-Figure 5. The principle classification factor
used in this categorization is the salinity occupied by different
developmental stages. In order to identify trends in salinity occupation,
instantaneous densities were plotted against salinities at capture for the
various collected stages (Appendix II). These density-salinity distributions
were represented numerically by density-weighted average salinities at capture
(Table 3). Among the frequently collected taxa, trends in salinity occupation
allowed identification of those which were dependent on low sal 'inity habitats.
The higher part of the salinity range was not always sampled. It is
therefore possible that the weighted salinitied at capture for species which
spawn in high salinities (e.g., >30 O/oo) are underestimates of the true
salinity where the earliest stages of these species were concentrated. At the
other end of the length range, the abundances of certain postlarvae and small
juveniles may be underestimated due to gear avoidance or microhabitat
specializations which would make them unavailable to the gear.
Spearman's rank correlations between salinity at capture and fish length

were also used as tndicators of upstream migration against the salinity
gradient. The significance of negative correlations between salinity and
length is limited to adequately sampled, slowly migrating stages (as opposed
to fast-moving juveniles), since growth during the migration is neccesary for
statistical significance. Therefore, although significance in these tests was
considered to be evidence of migration, lack of significance was not
interpreted as lack of migration.
The concentration of young estuarine-dependent fishes in low salinity
waters was identified b@ ranking the ratios of abundance (&) in 0-18 O/oo to
abundance (A) in 18-36 /oo. The abundance index takes the difference in
sampling intensity between the two salinity ranges (186 vs. 102 samples) into
consideration. Density differences in the two salinity ranges were tested for
significance by using Mann-Whitney U-tests. An adequate sample size from both
salinity categories is neccesary for significance; therefore, as with the
correlations, lack of significance was not interpreted as lack of
concentration.

Analysis 3: Impacts of riverine discharge on nursery grounds

As a preliminary investigation into this relationship, the abundance
data for species which are most dependent on low salinity habitats (as
determined by Analysis 2) were compared with trends in salinity and freshwater
discharge. The relationship between discharge and the availabilty of food
resources, including river-borne organic matter and crustacean zooplankton,
was examined. The data for flow rate and water chemistry used in this '
analysis were provided by the Southwest Florida Water Management District.

RESULTS AND DISCUSSION

over 74,000 fish representing approximately 68 species were collected by
the plankton gear during 1988. Catch statistics are provided in Table 1. As
is typical of many estuaries in southeastern North America, the
chthyoplankton community was dominated by anchovies (Engraulidae), gobies
(Gobiidae), and drums and seatrout (Sciaenidae).
i

Spawning Vulnerability

Spawning location

Fish eggs were concentrated in the higher salinities near the mouth of
the river and in Tampa Bay. The most commonly collected egg type (50
collections, approx. 48,000 eggs) was identified as belonging to either the
Order Perciformes and/or the Order Pleuronectiformes (Fahay 1983), but could
not be reliably identified beyond this level. This egg type is spherical,
ranging in diameter from 0.6-0.9 mm, and had a density-weighted salinity of
capture of 29 O/oo. The Gobiid and Blenniid eggs expected to occur in the
study area have adhesive filaments and adhesive discs, respectively, and are
not likely to be collected by plankton nets (Fritzsche 1978, Peters 1981,
1983). The remaining taxa which have eggs that are similar to those collected
and which were concurrently abundant in the study area at the earliest larval
stages include: Menticirrhus spp., Cynoscio nebulosus, OligoRlites saurus,
Trinectes maculatus, Bairdiella chrysoura, Cynoscion arenarius, and Achirus
lineatus.
A second unidentified egg type (22 collections, approx. 1100 eggs) is
also spherical, but ranges in diameter between 1.0 and 1.5 mm, and sometimes
has segmented yolk. The density-weighted salinity at capture was 26 O/oo for
this type. Based on the concurrent abundance of small larvae of species with
eggs matching this description, identities may have included: Pogonias cromis,
02isthonema oglinum, and ChaetodiRterus faber. -
The eggs of Anchoa mitchilli, Anchoa heRsetus, Gobiesox strumosus, and
an unidentified Tetraodontiform fish@ had density-weighted average salinities
ranging from 22 to 26 O/oo. The eggs of Anchoa mitchilli were far more
abundant than all other types of identified eggs (20 collections, approx. 2100
eggs). The eggs of Menidia beryllina were associated with vegetation
collected from fresh water at station 1.
The results of spawning ground proximity analysis (Analysis 1) are given
in Table 2. Comparison of the Tanking in Table 2 with existing literature
shows that the species in the first three categories tend to be considered
inshore spawners (within Tampa Bay and/or the Little Manatee River), while the
species in the fourth and fifth categories tend to be considered bay-mouth or
offshore spawners.
Most, if not all, of the species indicated by minus signs in Table 2 are
included within one of the larger taxonomic divisions indicated by plus signs.
Therefore, the position of the more inclusive taxa is a better indication of
the status of these species in Table 2. Assuming that species compositions
are proportionally similar in the preflexion and later larval stages, then:

the taxon Anchoa sRp. is dominated by A. mitchilli; the taxon Cobiidae is
dominated by Cobiosoma spp. and Microgbius gulosus; the taxon Blenniidae is
dominated by Chasmodes saburrae; and the taxon Prionotus spp. is dominated by
Prionotus tribulus. All but one of the preflexion and flexion-stage larvae
included in the taxon Clupeidae were associated with catches of Opisthonema
op-linum and Harengula Jaguana, and are assumed to be one or both of these
species.
The advanced development of the newly-born young of the live-bearers
Synanathus louisianae and S. scovelli resulted in their classification as
juveniles, yet the occurence of very small individuals was considered evidence
of "spawning" in intermediate-to-high salinity areas.
Fundulus spp. fell into the offshore spawner group because this genus,
like the offshore spawners, invaded the study area at the postflexion stage.
However, the majority of Fundulus were collected from the upstream end of the
study area after heavy rains, and it is likely that they were invading from
fresh water rather than from a seaward direction. As adults, Fundulus
seminolis was particularly abundant near station 1.
LeDisosteus platyrhincus (the probable identity of LeRisosteus sp.),
Notemigonis crysoleucas, Ictalurus catus, Elassoma okefenokee, Lepomis
auritus, L. macrochirus, and Micropterus salmoides are freshwater fishes, most
of which occasionally invade brackish or tidal fresh water at the juvenile and
adult stages. Richmond (1940) reported spawning by Levomis auritus in
brackish water. The live-bearers Gambusia affinis, Heterandria formosa and
Poecilia latipinna are freshwater fishes with a high degree of salt tolerance,
and undoubtedly give birth within.the study area.

NOTE:
The evidence provided by egg distribution and by Table 2 indicates that
many species spawn close to or within the study area, but does not indicate
the proportion of total spawning that is taking place there. Total spawning
effort varies from species to species. For example, the impact of riverine
discharge on a large clupeid stock which spawns intensively off of the mouth
of Tampa Bay and farther offshore, but has a small percentage of eggs washing
into Tampa Bay, is going to be less than the impact on a particular gobiid
stock which spawns locally among the oyster beds at the mouth of the Little
Manatee River.
The spawning grounds of estuarine-dependents are sometimes widely
displaced from the low salinity areas used later as nursery habitat. However,
a rigorous definition of estuaries as places where salinity falls below that
of seawater would include the spawning locations of a large percentage of
coastal marine fishes. Within this percentage, most fishes, such as seatrout,
make an effort to spawn in intermediate-to-high salinity areas that are
relatively stable in a physicochemical sense (Peebles and Tolley 1988).
Others, such as many species of killifish and silversides, spawn locally in
unstable, low-salinity backwaters. Members of the former group tend to be
highly motile, usually belong to families composed mostly of marine species,
and often occupy higher salinities after they leave their low-salinity nursery
grounds. These fishes are included in the estuarine-dependent category of
Figure 5. Members of latter group, categorized as estuarine in Figure 5, tend
to belong to families with many freshwater representatives, and may undergo
most or all of their life histories in low salinity habitats.

Spawning Season

Figure 6A indicates the period from March through August as having the
greatest number of species undergoing peak spawning activity in or near the
spawning area. This period coincides with temporal peaks in egg abundance and
total number of species collected (Fig. 7). The abundance of eggs and small
larvae in the study area-during this period suggests that there is relatively
high potential for riverine discharge impact on the spawning activity of the
following fishes:

bay anchovy (Anchoa mitchilli)
skilletfish (Cobiesox strumosus)
silversides (Menidia spp.)
kingfishes (Menticirrhus spp.)
spotted seatrout (Cynoscio nebulosus)
sand seatrout (Cynoscio arenarius)
silver perch (Bairdiella chrysoura)
Florida blenny (Chasmodes saburrae)
highfin blenny (Luginoblennius nicholsi)
frillfin goby (Bathygobius soporator)
naked goby (Gobiosoma bosci)
code goby (Gobiosoma. robust
clown goby (Microgobius gulosus)
wormfish (Microdesmis sp.)
bighead searobin (Prionotus tribulus)
hogchoker (Trinectes maculatus)
lined sole (Achirus lineatus)

Peak activity for bay-mouth and offshore spawners was distributed throughout
the year (Fig. 6B). Fishes which utilize the study area as a nursery ground,
but whose spawning activity is not likely to be impacted by riverine discharge
include:

menhaden (Brevoortia spp.)
snook (Centropomus undecimalis)
pigfish (OrthoRristis chrysoptera)
sheepshead (Archosargus probatoceRhalus)
pinfish (Lagodon rhomboides)
spot (Leiostomus xanthurus)
red drum (SciaenoRs ocellatus)
striped mullet (Mugil cephalus)

Nursery Habitat Vulnerability

Figure 8 illustrates the decreasing salinity of capture for several
fishes as they migrate toward low salinities. Generally, migration upstream
was first detected at the flexion stage, but appeared to be stronger at the
postflexion stage (Table 3). Table 4 provides abundance values for
comparisons between 0-18 and 18-36 O/oo. In Figures 9 and 10, the percentage
of overall abundance found in the lower salinities is used as a gauge for
identifying some of the species which not only disperse into, but congregate
in low salinities at the postlarval or early juvenile stage. Figures 9 and 10
also show that for most species, concentration occurs during the early
juvenile, rather than postlarval, stage.
"Concentrating" species have life history stages where most or all of
the individuals in a particular stage occupy or, in the case of diadromous
fishes, pass through low salinity habitats. Snook and red drum are
concentrating species which are locally abundant in the upper estuary as
juveniles. Snook migrate toward low salinity areas at the postlarval stage
(Tolley et al. 1987) and concentrate as early juveniles in freshwater and
oligohaline areas, usually in association with emergent or shoreline
vegetation (Gilmore et al. 1983). In the Tampa Bay area, over 90% of the
juvenile red drum in the 15-59 mm length range collected by Peters and
McMichael (1987) were found in backwaters with mean surface salinities ranging
from 7-21 O/oo. During the two years prior to the onset of plankton sampling
for the present study, the first author (EBP) participated in the FDNR Marine
Research Institute's seine and stop-n.et surveys of the Little Manatee and
Alafia rivers. It is clear from these surveys that species such as snook, red
drum and striped mullet are abundant in the study area as very small
juveniles. The absence of these species in the plankton collections could be
attributed to net avoidance, insufficient sampling resolution during peak
ingress periods, or failure to sample specialized habitats used by postlarvae
and early juveniles.
Young estuarine-dependent fishes are usually present in low salinity
habitats for at least several months after arrival from the spawning grounds,
resulting in year-round use of these nursery areas: young red drum arrive in
autumn; peak arrivals of postlarval menhaden occur in winter; young spot and
black drum arrive from late winter to early spring; and most of the remaining
fishes arrive from late spring through early summer. Peak spawning by the bay
anchovy is in late spring and early summer, but juveniles are abundant in low
salinity areas throughout the year. Many of the young gobies and hogchokers
remain in low salinity habitats until maturity, and then return to higher
salinities for spawning.

Riverine Discharge and Nursery Grounds

Two principal features of the discharge regime for 1988 were a
spring/early summer drought and a late summer/early fall rainy season, which
included a period of exceptionally heavy rain in early September (Fig. 11).
Three cycles of salinity reduction and increase were evident: the first was
caused by the passage of cold fronts in January, February and March; the
second by the late summer thunderstorm season, and the third by the passage of

a cold front and tropical storm Keith through the area in November (Fig.12).
For taxa which had an affinity for low salinity areas, simple.
correlations between seasonal abundance and seasonal salinity or flow rate
were not evident. There was evidence that the rate of change of salinity and
flow influenced abundances, however. Juvenile bay anchovies (Anchoa
mitchilli) were collected year-round, but were most abundant during the three
periods when salinities were falling the fastest, and were least abundant
during the drought and during periods of extremely high discharge (Fig. 15).
Postlarval menhaden (Brevoortia spp.) were not present during the second
salinity cycle, but were most abundant during salinity reductions of the first
.and, to a lesser extent, the third cycles (Fig. 16). Juvenile hogchokers
(Trinectes maculatus) were not collected during the first salinity cycle, but
appeared to be most abundant during or soon after the salinity reductions in
the second and third cycles (Fig.,17). Juvenile sand seatrout (Cynoscion
arenarius) were collected during the second salinity cycle only, and also
appeared to be most abundant during a period.of rapid salinity reduction (Fig.
18). These fluctuations in abundance may simply represent salinity-related
migrations superimposed on the seasonal spawning activity of the adult stocks.
Both emigration from and immigration tothe upper river could result in
increased abundance in the channel where the nets weretowed. Most of the
captures.of burrowing fishes, such as juvenile speckled worm eels (Myrophis
Punctatus) and adult pink wormfish (Microdesmis longipinnis), were associated
with freshets in 1988 and 1989.
In regard to spatial distribution and short-term salinity fluctuation,
postlarval and juvenile menhaden and juvenile hogchokers persisted in the
upper river during freshets (Figs. 19, 20, 22). Postlarval and juvenile spot
were somewhat less persistent, but remained in the river during freshets
(Figs. 23, 24). Juvenile bay anchovies, sand seatrout, and gobies (Gobiosoma)
were found closer to the mouth of the river during freshets (Figs. 26, 28,
30). This displacement was even more pronounced during the postlarval stages
of these species (Figs. 21, 25, 27, 29). As suggested by Rogers et al.
(1984), theadvection associated with freshets probably inhibits the upstream
migration and retention of small, relatively weak-swimming seatrout,
hogchoker, and goby postlarvae, while having a lesser effect on larger, more
robust forms, such as postlarval menhaden. However, variable physical impacts
would not explain why juvenile bay anchovies move farther downstream while
postlarval menhaden persist in the upper river. It therefore appears that
biological factors play a role in displacement. Likely biological influences
include differences in osmoregulatory capability and differences in the
response of unique food resources to freshwater discharge.
It is clear that many fish species concentrate in low salinity areas of
the river (Tables 3 and 4; Figs. 8-10, Appendix II). In eurybaline species,
the energetic cost of enduring rapidly-fluctuating, reduced salinities is
compensated through reduced interspecific competition for the abundant food
resources found in low salinity areas, and possibly through exposure to a
reduced number of predatory species.
In general, organisms which might serve as potential food for young
fishes were removed from the water column during high discharge periods. This
was particularly true for copepod nauplii and copepodites (primarily Acartia
and Oithona, Rast and Hopkins 1989), although these organisms primarily serve
as food for the younger stages found outside of the river. Among estuarine@
dependent fishes, it is common for principal food sources to shift from

copepods to mysids,_ amphipods, polychaetes, and organic detritus as the young
fishes grow larger-(Darnell 1961, Carr and Adams 1973, Chao and Musick 1977,
Peters and McMichael 1987). Mysids (Neomysis) and gammarid amphipods were
widely distributed along the salinity gradient (Figs. 31, 32). Although
mysids were often very abundant in low salinities, abundance was generally
correlated with salinity (abundance, A, vs. salinity at station 3: r-0.59,
d.f.=22, p=0.003), and was reduced during peak discharge periods (Fig. 33).
Amphipods were fairly abundant in the water column during the drought, and
peaked at the start of the rainy season. However, during and after the period
of sustained high discharge, water column amphipod abundance dropped to very
low levels and did not return to pre-rain concentrations (Fig. 34). Other
small crustaceans which might serve as food for young fish (cumaceans, decapod
zoeae, ostracods, cladocerans, and isopods) tended to be most abundant in
higher salinities.
Organisms in the water column of low salinity areas of the river do not
appear to be reliable sources of food for young fishes. When food is abundant
in the water column, it is probably exploited in an opportunistic manner.
Benthic food resources, which were not monitored during this study (except as
vertically migrating benthos and tychoplankton), are a more likely source of
food, particularly for demersal species.
The sedimentary accumulation of organic detritus is a distinctive
characteristic of the area of the Little Manatee River where many young
estuarine-dependents congregate. Imported organic matter is often a dominant
energy source for streams and even large rivers (Berner 1951, Fisher and
Likens 1972). Teal (1962) estimated that 45 percent of the net production of
a temperate salt marsh is exported to estuarine waters. The importance of
allochthonous organic matter as a seasonally-buffered source of nutrition for
young estuarine-dependent fishes was discussed by Darnell (1961, 1967). The
microbial protein content of decomposing, naturally-shed vegetable matter
(leaves and sticks) may increase for months, but a variety of factors, such as
species of plant, determine the rate of decomposition and the quality of the
material as food (Kaushik and Hynes 1971). The distribution of many benthic
animals is correlated with detritus distribution (Egglishaw 1964). Some
invertebrates, such as copepods and amphipods (and probably mysids and
polychaetes), are able to assimilate much of the microbial protein ingested as
detritus, which allows these organisms to function as trophic intermediates
between detritus and those fishes which do not eat detritus directly (Hargrave
1970, Heinle et al., 1977). An intermediate trophic position may also be held
by protozoans such as flagellates or tintinnids (Robertson 1983).
The discharge-associated peaks in particulate organic carbon is evidence
of the importation of energy into low salinity nursery habitats (Fig. 35).
Levels of both particulate and dissolved organic carbon were correlated with
the flow rate of the river (POC: r-0.70, d.f.- 22, p<0.001; DOC: r-0.73,
d.f.-9, p-0.02). In addition to flow rate, cycles in the accumulation of
organic matter on land (such as leaf litter) undoubtedly play a role in the
importation of organic carbon into the Little Manatee River. Much of the
organic carbon in freshwater streams is dissolved leaf leachate which may
contribute to particulate deposition through precipitation as solids (Fig.
36). The sources of allochthonous particulate and dissolved organic carbon in
the Little Manatee River range from riparian vegetation to the terrestrial
communities of the entire watershed, depending on the extent and intensity of
freshwater runoff.

The impact of organic matter importation on eutrophication is dependent
on the type of organic matter being imported. Naturally7shed plant material
is often rich in ofganic carbon, but very poor in nutrients (Fenchel and
Harrison 1975). Bacterial decomposition of leaf litter is dependent on
nitrogen and phosphorus concentrations in the water, such that heterotrophic
bacteria actually remove nutrients from the water column during decomposition
(Kaushik and Hines 1971, Fenchel and Harrison 1975). On the other hand,
remineralization of nutrient-rich organic matter, such as sewage, releases
nutrients into the water column, thereby enhancing eutrophication.
The watershed encompasses an area that is comparable in size to Tampa
Bay, and most of the runoff from this region is funneled through the study
area (Fig. 37), so there is great potential for terrestrial and wetland
subsidization of nursery ground productivity. Backwaters and deep holes in
the channel collect organic debris in a manner analogous to that of ponding
areas in freshwater streams. Many of these areas were scoured out by the
September flood, leaving sand where detrital bottoms once were, Under most
flow conditions, however, only fine particulate organic matter is exported
farther downstream, so that downstream areas depend increasingly on
autochthonous sources of organic matter, and the influence of freshwater
discharge on organic matter availability becomes less noticeable. In these
higher salinity areas, cycles in mangrove, seagrass, macroalgae and
phytoplankton growth play a greater role in the availability of secondary, as
well as primary, production.
There is no reason not to assume that upper estuarine nursery areas
benefit from energy importation in the same manner as freshwater ecosystems.
Two seasonal cycles are immediately evident as regulators of this process:
organic matter accumulation in the watershed and physical flushing of organic
matter into the upper estuary. Reduction in either capacity could result in
decreased food resources for important fish nursery grounds. However, the two
factors are interdependent in the sense that the timing of the two cycles
relative to each other may beas important as their magnitude. It is likely
that, in some cases, sustained flushing is excessive in relation to the
watershed's finite supply of organic matter.

CONCLUSIONS

The spawning activities of fishes which were present as eggs or newly-
hatched young in the Little Manatee River study area are considered to
be the most likely to be impacted by changes in the quantity or quality
of freshwater discharge. The following fishes are in this category:

FISH PERIOD OF SPAWNING
VULNERABILITY

bay anchovy (Anchoa mitchilli) April-December
striped anchovy (Anchoa heRsetus) May-June
skilletfish (Gobiesox strumosus) January-December
tidewater silverside (Menidia beryllina) January-December
Gulf pipefish (SyLignathus scovelli) January-December
chain pipefish (Syngnathus louisianae) January-December
Atlantic bumper (Chloroscombrus chrysurus) July-September
leatherjacket (OligoRlites saurus) May-August
kingfishes (Menticirrhus spp.) April-October
spotted seatrout (CyLioscio nebulosus) April-August
sand seatrout (Cynoscio arenarius) March-September
silver perch (Bairdiella chrysoura) April-September
black drum (PoRonias cromis) March-April
Atlantic spadefish (Chaetodipterus faber) May-July
Florida blenny (Chasmodes saburrae) January-December
feather blenny (EUsoblennius hentzi) April-July
highfin blenny (LiMinoblenniu nicholsi) April-November
frillfin goby (Bathygobius soporator) April-October
naked goby (Gobiosoma bosci) March-December
code goby (Gobiosoma robust May-August
clown goby (Microgobius gulos May-January
wormfish (Microdesmis sp.) April-August
bighead searobin (Prionotus tXibulus) April-August
hogchoker (Trinectes maculatul) April-November
lined sole (Achirus lineatus) April-November
blackcheek tonguefish (S3Mphurus plagiusa) May-August

A wide variety of fishes undergo peak spawning activity during the
period from March through August.

2. Migration from moderate-to-high salinity spawning grounds into low
salinity areas of the Little Manatee River occurred during the
postlarval and early juvenile stages.

3. During the survey, the majority of the estuarine-dependent fishes
which concentrate in low salinity areas (>75% abundance in <18 O/oo)
did so during the early juvenile stage. The young of fishes which
concentrate in low salinity areas are likely to be impacted by changes
inthe quantity or quality of riverine discharge. The following fishes
are in this category:

FISH PERIOD OF
INGRESS/INITIAL CONCENTRATION

menhaden (Brevoortia spp.) November-August
bay anchovy (Ancho mitchilli) January-December
skilletfish (Gobieso strumosus) January-December
rainwater killifish (Lucani parva) May-October
tidewater silverside (Menidia beryllin March-December
snook (Centropomus undecimalis) May-September
mojarras (Eucinostomus spp.) January-December
sand seatrout (Cynoscio arenarius) March-September
spot (Leiostomus xanthurus) December-April
,red drum (Sciaenops ocellatus) September-November
naked goby (Gobiosoma bosci) March-December
lined sole (Achirus lineatus) April-November
hogchoker (Trinectes maculatus) April-November

Most of these fishes remain in low salinity areas for at least several
months following ingress and initial concentration.

4. Concentrations of dissolved organic carbon, amphipods, and the young of
several species of fish increased at the beginning of high discharge
periods, but fell to relatively low levels during periods of sustained
high discharge.

5. The distributional response of young fishes to increased freshwater
discharge depended on developmental stage and species. During high
discharge periods, postlarval menhaden and juvenile hogchokers persisted
in the upper river, postlarval and juvenile spot were widely distributed
in the river, and juvenile bay anchovies, sand seatrout, and gobies
(Gobiosoma) were most abundant in the lower river. The postlarvae of
the latter group were concentrated in higher salinities than the early
juveniles during high discharge periods.

6. Postlarval and early juvenile food sources (copepods, mysids, and
amphipods) were inconsistently present in the water column of low
salinity nursery areas,, and are strongly reduced in the water column
during periods of sustained discharge (see also: Rast and Hopkins 1989,
Vargo 1989). Benthic food resources probably play a greater
role as food for young fishes than do water column resources.

.17

7. Energy in the form of dissolved and particulate organic carbon is
imported into low salinity nursery habitats. The quantity of energy
that is imported is largely dependent on cycles in both freshwater
runoff and terrestrial accumulation of organic matter.

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