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post-red-tide

Status of fish populations in Sarasota Bay post-red-tide

Dec 21, 2010 No comments By Elizabeth Berens McCabe

By Elizabeth Berens McCabe, MS, Chicago Zoological Society

Natural disturbances such as harmful algal blooms can greatly affect the community dynamics of estuaries and nearshore

environments. In Florida, red tides are a type of harmful algal bloom caused by the dinoflagellate Karenia brevis. This naturally

occurring alga produces brevetoxins (PbTx), lethal neurotoxins which affect the respiratory system of vertebrates, including fish,

manatees, turtles, dolphins, and seabirds. Recent work suggests PbTx persists in the food web, cycling through low trophic level

fish and benthic organisms to top predators. Community and species specific responses to K. brevis blooms and PbTx likely

function on different scales based on the temporal and spatial nature of the bloom, as well as bloom severity. Possible responses

include changes in fish abundance, species composition, growth and mortality rates, and community structure. Along the west

coast of Florida, K. brevis harmful algal blooms occur regularly in estuarine and nearshore ecosystems, with varying degrees of

severity and longevity. It is imperative to identify common and species specific response patterns to fully understand the impact

of a bloom.

In 2004, the Sarasota Dolphin Research Program initiated a multi-species fish survey in Sarasota Bay to investigate distribution

and abundance of dolphin prey fish, in addition to fine-scale habitat selection and prey selection in bottlenose dolphins. We

regularly catch, measure, count, and release fish from June-September and January-March using our 183 m purse seine sampling

gear. Since this survey began, two red tides have occurred (Jan-Nov 2005 and Aug-Oct 2006) in Sarasota Bay and vicinity. 

Using data gathered from our fish survey, our work has shown that these two red tide events corresponded with massive fish

kills, sharp decreases in the abundance of living fishes, and changes in fish community structure (Gannon et al., 2009, Marine

Ecology Progress Series 378:171-186). Densities of some fish species were reduced by as much as 94%. Three months

following the cessation of the K. brevis bloom, PbTx levels in the viscera of some fish were still >1,400 ng/g. In fact, PbTx has

been demonstrated to accumulate in tissues of living specimens and has been detected in fish tissues more than a year following a

red tide bloom.

Considering the frequency of red tides in the Sarasota Bay region, an important finding of our work is the speed at which the fish

A Sarasota Bay resident dolphin feeding on

pompano in January, 2010.

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community apparently recovers from red tide disturbance. Analysis of our summer data from 2004-2009 in the seagrass habitat

showed that fish abundance in Sarasota Bay was significantly lower during 2005 (red tide occurred Jan-Nov 2005) compared to

all other years based on catch per unit effort (i.e., the number of fish caught per net set, CPUE). Fish abundance in the seagrass

habitat rebounded in 2006 despite a brief K. brevis bloom that occurred Aug-Oct 2006. When the CPUE data were analyzed by

month, fish abundance in August was significantly lower in 2005 compared to all other years, and in September, abundance was

significantly lower in 2005 compared to 2006, 2008, and 2009. We did not detect any significant differences in CPUE across

years in June and July. Our highest fish densities in the seagrass habitat were recorded in summer 2008, after a nearly two-year

absence of red tide. After standardizing for differences in fishing effort between years, we were not able to detect significant

differences in overall species richness across years, nor did we detect significant differences when the data were analyzed by

two-month periods across years. Shannon-Weaver diversity indices measure biodiversity, taking both species richness (the

number of species present) and evenness (how evenly individuals are distributed among the species present) into account. When

Shannon-Weaver diversity values were analyzed, diversity differed significantly in 2005 compared to all other years. At the

species level, CPUEs were analyzed for significant differences across years and then within each month across years. Several

species CPUEs were significantly lower in 2005 compared to all other years, including the CPUEs of such important dolphin

prey as pinfish, pigfish, spotted seatrout, and silver perch. However significance was not consistent. After the 2005 and 2006

red tides, catches of most species rebounded by 2008, however, recovery varied by species. For example, post-red tide CPUE of

pigfish was similar to or exceeded pre-red tide levels by 2006 (i.e., CPUE did not significantly differ between 2004 and 2006). 

In contrast, spotted seatrout and silver perch did not reach levels similar to or exceeding pre-red tide abundances until 2008 (i.e.,

CPUEs did not significantly differ between 2004 and 2008).

In addition to K. brevis harmful algal blooms, a bloom of the diatom Pseudo-nitzschia pseudodelicatissima was confirmed in

Sarasota Bay in 2008. Toxic Pseudo-nitzschia spp are known to produce domoic acid (DA), a neurotoxin that can cause amnesic

shellfish poisoning in humans. Throughout 2009 and early 2010, low to moderate levels of Pseudo-nitzschia spp persisted.

Detectable levels of PbTx and DA occurred in SB bottlenose dolphins and fish (Twiner et al., in review, PLoS ONE). PbTx and

DA appear to persist in the food web, cycling through low trophic level fish and benthic organisms to top predators. An

understanding of the possible synergistic effects of both toxins is lacking.

To date, we have completed 1049 purse seine sets in 5 distinct habitats since 2004. We have sampled 132 different species and

420,968 individual fish. This past summer and winter we focused our efforts on the seagrass habitat because it is considered one

of the most productive habitats in our study area. We completed 30 purse seine sets last winter despite unusually cold

temperatures and 40 sets last summer. Our total winter catch consisted of just 45 species and 2528 individual fish. The average

number of fish caught per set in the winter was 84.27 fish and 9.10 species per set. Our summer catch was comprised of 66

different species and 35,878 individual fish, an average of 896.95 fish and 14.95 species per set. Fish are known to move to

deeper, warmer waters as temperatures drop. Our winter sampling targeted shallow water seagrass beds, therefore, our winter

catch results likely reflect a temporary change in overall fish distribution rather than a dramatic drop in fish abundance in

Sarasota Bay.

While fish abundance in Sarasota Bay appears to have recovered from the severe red tide events in 2005 and 2006, important

questions remain regarding (1) the importance of temporal and spatial scales of red tide blooms to the fish community’s

response, (2) the fitness consequences of red tide to individual fishes, (3) the differences among species regarding their responses

to red tide, and (4) the economic and resource-management implications of red tide to fisheries. Long data sets are needed to

answer these questions due to natural variation and the complex interactions that occur in nature. We plan to continue

monitoring fish abundance and species composition in Sarasota Bay and hope to address some of these questions in the near

future.

We thank the many interns and volunteers who worked on this project. This work would not be possible without them. The

Batchelor Foundation, Disney Worldwide Conservation Fund, NOAA’s Fisheries Service, Harbor Branch Oceanographic

Institution’s Protect Wild Dolphins Program, and Florida’s Fish and Wildlife Research Institute provided funding for this work. 

This research was authorized by the Florida Fish and Wildlife Conservation Commission (Special Activity License nos. 03SR809, 04SR-809, 04SR-809a) and by Mote Marine Laboratory’s Institutional Animal Care and Use Committee (protocol nos. 06-

10-DG1, 07-10-DG1, 09-09-RW2).

Related posts:

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Ecology, Population Structure and

Dynamics

Elizabeth Berens McCabe

About the author

Elizabeth Berens McCabe, MS, is a Research Associate-Ecologist with the SDRP. My interests include foraging ecology, habitat

selection, population and community dynamics, and the ecological effects of hazardous algal blooms (HABs) on bottlenose

dolphins and fishes. Since 2004, my work has focused mainly on quantifying the abundance and distribution of prey available for

resident bottlenose dolphins in Sarasota Bay, Florida. Data from this work have allowed us to look at prey selection and habitat

selection in bottlenose dolphins as well as the ecological effects of HABs, predominantly Karenia brevis red tides, on bottlenose

dolphins and nearshore marine fishes.

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