A discussion of the effects of recent, intense red tide blooms on the biology and ecology of spotted seatrout and other studied species in the Tampa Bay estuary.
A protracted and intense red tide (Karenia brevis) bloom affected Tampa Bay and surrounding waters during 2005 (see also Red Tides in Florida). Originating south of Tampa Bay, the bloom was first detected at medium to high levels at the mouth of the bay on June 10, 2005, moving into the lower bay by July 6 . The medium to high levels as indicated by pink and red dots correspond to cell counts greater than 100,000 cells per liter, levels consistently associated with fish mortalities.
These elevated cell counts persisted within Tampa Bay through the beginning of October 2005. Two marine finfish biology research projects documented the bloom effects on spawning populations of three species of fish found within Tampa Bay: (I) a study monitoring a known spotted seatrout (Cynoscion nebulosus) spawning site at a Tampa Bay pass; and (II) a hydrophone survey conducted throughout Tampa Bay to monitor spawning activity based on male courtship calls for spotted seatrout, sand seatrout (Cynoscion arenarius), and silver perch (Bairdiella chrysoura).
Fish images © Diane Rome Peebles. Illustration provided for viewing purposes only.
A resident aggregation, or gathering, of spawning spotted seatrout at a gulf pass in Tampa Bay has been monitored for the past five years. In 2005, it was monitored by recording male spotted seatrout courtship sounds over the spawning season using long-term acoustic recording systems (LARS) as part of a collaborative research project with David Mann at the University of South Florida (www.marine.usf.edu/bio/fishlab/). To hear these spawning sounds, please review the hydrophone survey article. Each of these six LARS are indicated by the speakers in the figure below and recorded 10 seconds of sound every 10 minutes. In addition, the site fidelity of individual fish was monitored using ultrasonic tags. Eighteen remote receivers (VR2s) were installed in the gulf spawning site. The amount of receivers as well as their positions was determined by the range the receivers were capable of detecting a tagged fish (70 meters). Range bubbles were positioned over the pass area in order to effectively cover the spawning site. Any tagged fish swimming through the spawning site would be detected by the receiver array.
Prior to the 2005 red tide bloom, 31 tagged fish were consistently relocated by the 18 permanent receivers covering the spawning area. These fish were tagged after the spawning season had begun in early to mid May 2005. Although survival of the implanted fish was high (31 of 32) and fish were relocated regularly in the early part of the spawning season, by July 12, 2005, no implanted fish were ever located again even though spotted seatrout spawning does not typically end until mid-September.
Courtship sound production, as recorded on the LARS at Site 6, showed a similar decrease beginning at the end of June. As of July 17, 2005 only a handful of fish were heard at this site, compared to the usual large aggregation sound which has been documented at this site throughout the spawning season in previous years.
Preliminary monitoring of the gulf passspawning site in 2006 indicates the spawning population of spotted seatrout has not repopulated the area following the massive mortality/evacuation observed in July 2005. The same gear (experimental gill net) and protocol were used in 2006 collections compared to 2001and 2002. The catch per unit effort (CPUE) for 2006 was 0.5 spotted seatrout per net set. It was 4.7 seatrout per set in 2001 and 2.3 seatrout per set in 2002. The gear used to collect fish for implantation, a 400-foot center bag haul seine showed a similar decrease in 2006. In May of 2005, the CPUE was 10.6 seatrout per set compared to 0.1 seatrout per set in April and May of 2006. Lastly, we sampled five sites in the Boca Ciega area in 2001, 2002, and 2006. CPUE for data pooled over May-July and 2001 and 2002 was 2.4 seatrout per net set compared to 0.7 CPUE for 2006 May-July.
A mobile hydrophone survey using a stratified random sampling design, or a random sampling design of a specific study area, was conducted in Tampa Bay in 2004 and 2005 to map spawning locations of spotted seatrout, sand seatrout, and silver perch. Spawning locations were found by listening for the different courtship sounds of these three fish (see hydrophone survey article for details). Red tide effects from the 2005 bloom were evident in the sand seatrout spawning population. Significant differences in the amount of sand seatrout spawning between the years was observed in the lower and middle areas of Tampa Bay, areas affected by the bloom in 2005. Upper Tampa Bay, an area that was not affected by the bloom because of low salinity (average salinity was less than required threshold for red tide at 24 parts per thousand (ppt)), did not demonstrate a difference in seatrout spawning between 2004 and 2005. Significant monthly differences in sand seatrout aggregation detections was observed in 2005 in the lower and middle areas of Tampa Bay but not in the upper bay. No monthly differences occurred in 2004 for either the upper bay or the lower/middle bay areas.
When only 2005 data are mapped according to sand seatrout aggregation detections before and after the red tide enters Tampa Bay, there is an obvious absence of aggregations in the lower bay and a reduction in the middle bay following the red tide. When only the lower and middle area of the bay is considered in 2005, there is a significant decline in the number of sand seatrout aggregations present before and after the red tide event.
Tampa Bay sand seatrout spawning locations were significantly impacted following the 2005 bloom. Although spawning location data were also collected for spotted seatrout and silver perch, spawning differences between years attributable to red tide were not as pronounced as were observed with sand seatrout. The small sample size of spotted seatrout aggregations in both years (2004: n=134; 2005: n=91) makes it difficult to detect the bloom signal. In both years there were significant monthly differences.
However, when annual differences were examined for the different areas of the bay, there was a significant decline in the middle/lower bay in 2005 but not in the upper bay. Bloom effects were not detectable with silver perch since the end of their spawning coincided with the mid-July bloom event. For both 2004 and 2005 there were significant monthly differences in the detections of silver perch aggregations regardless of the area in the bay because after July there is normally relatively little spawning . Additionally, annual differences in detecting an aggregation was significant for both areas of the bay, making it difficult to assess red tide's affect on this species.
These two studies offer unusual opportunities to assess spawning population alterations of various fish following an episodic event. Although red tide is known to cause fish mortalities, there is relatively little information reporting the impact or degree of changes to populations. Attempts simply to count fish are often efforts in futility as fish carcasses quickly decompose, sink, are consumed by shore birds and wildlife or are easily displaced by wind and currents.
Long term research projects already in place can provide valuable information when an event such as red tide occurs since there is data before, during, and after the event. Both research projects described above indicate that the 2005 red tide bloom caused severe impacts to the spawning populations of both spotted and sand seatrout. However, as both of these species mature at a relatively small size and age, it is likely they will be able to recover quickly. Continued monitoring at the pass spawning site will be imperative in addressing the time required for populations to recover.