Making restoration priorities for estuarine nurseries:

An example using red drum in Tampa Bay Shannon D. Whaley, Florida Fish and Wildlife Conservation Commission

Mary C. Christman, University of Florida/MCC Consulting

James J. Burd, Florida Fish and Wildlife Conservation Commission

A “Fish Eye” View of Tampa Bay:

Proportional map of the population of larger

juveniles of red drum (51mm –100mm SL)

(averaged across years)

Study area

Tampa Bay (Figure 1), located on Florida’s peninsular

gulf coast, is the largest estuary in the state, with an open

water area of approximately 1,000 km2. The major fresh-

water sources for Tampa Bay are the Hillsborough, Alafia,

Little Manatee, and Manatee rivers which flow into the

bay from the north and east. In southwestern Florida,

there is increasing interest in using rivers to provide

freshwater to a growing local human population. Despite

increasing urbanization of the watershed, Tampa Bay re-

mains an important destination for recreational fishing

and boating for visitors and the local population.

Monitoring juveniles

of red drum

Juvenile fish data included in this analysis were collected

on a monthly basis during daylight hours from 1996 to

2008 by the State of Florida’s Fisheries-Independent

Monitoring Program (FIM; Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute). Juveniles were collected

with a 21.3-m center-bag seine (3.2-mm stretched mesh) that targeted smaller-sized fishes (generally 15 – 100mm standard length) and was de-

ployed in water depths of ≤ 1.8-m (rivers) or ≤ 1.5-m (estuary). To select sample sites, the Tampa Bay study area was divided into 12 geographic

zones. For this analysis, we used eight of the 12 geographic zones that had the most consistent monthly monitoring data for the study period. Each

selected grid cell was subdivided into 100 micro-grid cells and locations for seining were selected randomly from these micro-grid cells. The number

of grid cells selected within each zone depended on the size of the zone and the proportions of habitat within the zone. Typical monthly samples sizes

are 3 – 6 cells per zone.

Although this program monitored a large variety of nekton in Tampa Bay, this analysis covers juvenile life stages of red drum (Sciaenops ocellatus).

To examine potential ontogenetic shifts in habitat use which may result in differing spatial distribution, we divided the juveniles into two size ranges

(15mm - 50mm SL and 51 – 100 mm SL). Although sampling is monthly, we included data for the months over which juveniles of red drum were

most abundant in the estuary. Peak season of juvenile abundance was defined as October, November, and December for smaller juveniles of red

drum (15mm – 50mm) and January, February and March for larger juveniles (51mm – 100mm). Data from these months were combined to calcu-

late an annual relative abundance for each zone and bay-wide. If a grid cell was sampled more than one month during the time period, the cell data

were averaged for use in the zone and bay estimators of abundance. The usual stratified random sample estimator (Cochran, 1977) was used to ob-

tain annual relative abundances where the catch per unit effort per grid cell (total number caught/area swept) was multiplied by the total water area

in the grid cell to obtain an estimate of grid cell abundance.

Characterizing freshwater inflow

To examine the influence of freshwater inflow patterns, we used a USGS classification in which stream flow for each month is compared with

monthly data collected in a 30-year reference period (1961-1990). Stream flow is ranked for the reference period then classified as above normal (>

upper quartile), normal (between lower and upper quartiles), or below normal (< lower quartile) (http://water.usgs.gov/nwc/). We characterized

freshwater inflow conditions during the 12-month period prior to and including the peak abundance season for each size range for the 13-year study.

For each 12-month period, we calculated the proportion of months that were classified as above normal streamflow (%Year Wet), and proportion of

months below normal streamflow (%Year Dry) for the Tampa Bay watershed.

Legend

A - Old Tampa Bay

B - Middle West

C - hillsborough Bay

D - Lower West

E - Lower East

K - Alafia River

L - Little Manatee River

M - Manatee River

±

E

A

C

B

D

M

L

C K

Flo

rida

Gulf ofMexico

0 5 Miles

±Legend

A - Old Tampa BayB - Middle WestC - Hillsborogh BayD - Lower WestE - Lower EastK - Alafia RiverL - Little Manatee RiverM - Manatee River

A

E

C

B

D

M

L

C K

Legend

FIM Zones

A - Old Tampa Bay

B - Middle West

C - hillsborough Bay

D - Lower West

E - Lower East

K - Alafia River

L - Little Manatee River

M - Manatee River

0 5 Miles

±

Results

Spatial distribution examined across sampling zones revealed the relatively narrow distribution of red drum centered in the riverine zones during most years. Riverine areas (Zones K, L, and M com-

bined, Figure 1) comprised less than 2% of the total study area. However, the largest proportion of population (annual range: 40% - 96%) of both size classes of red drum were concentrated within

these major rivers for almost the entire 13 years of the study (Figure 2, Figure 5b,c). In particular, Zone K (Alafia River, <1% of the study area) contained the most red drum juveniles surviving to the

larger size class in seven of the 13 years (annual range: 35% to 71% of population).

Temporal patterns showed that both the smaller juveniles and larger juveniles of red drum exhibited the most restricted distribution in 2008 (11% and 2% of grid cells, respectively) and lowest abun-

dance, during the drought that followed the major red tide event (Figure 5b, c). The maximum distribution of the smaller red drum occurred in 2002, a period of heavy freshwater inflow, when they

were found in 41% of the sampled grid cells. The larger juveniles also reached a maximum distribution of 14% of grid cells during a similar period (2002 - 2005).

Abundance of both the smaller and larger juveniles of red drum were strongly and positively correlated with high freshwater inflow (% Year Wet). The abundance of only the larger juveniles were nega-

tively correlated with low inflow conditions (% year Dry, Table 1).

Significant correlations between spatial distribution of the larger red drum juveniles and freshwater inflow metrics (% Year Wet, % Year Dry) suggest that freshwater inflow limits the quantity of habitat

occupied by these larger juveniles during most years. The larger juveniles generally distributed themselves over a broader area in the wetter years, and constricted their distribution during periods of

low freshwater inflow. The smaller juveniles also increased in abundance with increasing high inflow (% Year Wet), yet they did not consistently increase their distribution in these heavy inflow peri-

ods. In addition, periods low inflow did not consistently result in reduced abundance and spatial distribution.

Figure 3. Density Equalizing Cartogram (Gastner and Newman 2004) of sampling zones in Tampa

Bay. The shape of each zone was changed so that it’s size is equal to the population abundance of juve-

nile red drum (51- 100mm SL). The zones covering the Alafia River, Little Manatee River, and Mana-

tee River represented a very small proportion of the study area (2%) but contained the majority of the

juvenile population.

Restoration-related Conclusions

Lower portions of the rivers are important nursery areas for juveniles of red drum. These areas should be targeted for restoration projects with goals to benefit nursery habitat of red drum, a socio-economically important fishery species.

Annual variability in freshwater flowing into Tampa Bay affects the extent of occupied habitat, and also is closely related to size of the juvenile population in Tampa Bay. Therefore, restoration efforts with goals to enhance red drum nurseries should also include manage-

ment of freshwater inflow.

Figure 1. Map of Tampa Bay study area showing the eight sampling

zones used in this analysis.

Photograph of Fisheries-Independent Monitoring staff

collecting small fish using the 31-m bag seine.

Red drum, Sciaenops ocellatus % High flow % Low flow

smaller juveniles abundance 0.71** -0.51 (15 - 50 mm SL) distribution 0.53 -0.41

larger juveniles abundance 0.68* -0.76**

(51 - 100 mm SL) distribution 0.80** -0.87***

Table 1. Spearman's rank correlation coefficients re-

lating abundance estimates and spatial distribution

to the percent of the year above normal stream flow

(% High flow) and percent of year below normal

stream flow (% Low flow). * p <0.05 ** p <0.01

*** p <0.001

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

6,000,000

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Figure 5. Stream flow (a) Bar height represents total population abundance for larger (b) and smaller juveniles (c) of red drum. Each bar is stacked representing the abundance within each zone (see maps

for zonal color codes).

Stream flow

Percent of year with above

(wet) and below (dry) aver-

age streamflow

Red tide

17% 25%

58%

8% 0%17%

42% 50% 58%

25%0% 0% 0%8%

33% 25% 33%

92%50%

25%0% 0% 8%

50%92% 67%

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Wet

Dry

Alafia acid spill

Abundance of

large juveniles

(51– 100mm SL)

Abundance of

small juveniles

(15—50mm SL)