fish kill

(Florida Department of Environmental Protection)

fish kill

(Florida Department of Environmental Protection)

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Light micrograph of Karlodinium veneficum isolated from the Chesapeake Bay. The location of the ventral pore and apical groove are marked. (Allen Place, UMCES)

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Summary of fish kills in Maryland waters in 2007, along with quantification of karlotoxins and screening for harmful algal species. (Allen Place, UMCES)

Fish Kills

Whether toxic or noxious algal species dominate a bloom or alternatively, occur at low but harmful levels within a phytoplankton community, their presence often affects other trophic levels, resulting in ecosystem dysfunction, public health risk, and enormous economic losses. The devastating effects of HABs are frequently seen on the west coast of Florida where the proliferation of the toxic dinoflagellate Karenia brevis  can result in massive fish kills, closure of shellfish beds due to NSP and skin and respiratory irritation to humans at the seashore. These blooms are responsible for the loss of millions of dollars to the commercial and recreational fisheries and tourist industries.

Adverse effects ranging from reduced growth and reproduction to mass mortalities may lead to increased occurrence and severity of disease, significant losses in harvestable resources, or to spoiled or contaminated products - outcomes that result in substantial economic damage and adverse health consequences for consumers.  Aquaculture operations are particularly vulnerable to toxic and harmful algal events because of the inability to move or protect certain kinds of stock when a HAB event threatens. 

Pictured here are fish killed by red tide blooms, washed ashore either to accumulate on beaches or in small quiet coves near residential homes. These fish pose a health hazard as they rot and decay, and birds such as pelican, seagulls, cormorants, and possibly marine mammals can become intoxicated by eating dead or dying fish.


Impacts of Prymnesium parvum or "Golden Algae"

Prymnesium parvum, commonly referred to as "golden algae," is one of the most problematic HAB taxa in the U.S. and has caused fish kills in Texas each year since 2001.  Pymnesium parvum has been documented in more than 25 lakes and rivers in five of Texas’ major river basins, with most toxic blooms occurring in winter months and in brackish inland waters. Fish kills have included game fish such as largemouth bass, smallmouth bass, striped bass, catfish, crappie, and rainbow trout, as well as threatened species such as blue suckers and Rio Grande darters. Pymnesium parvum has also been confirmed in New Mexico, Colorado, Wyoming, North Carolina, South Carolina, Georgia, Arkansas, and Alabama and has been suspected in Oklahoma and Nebraska.  In New Mexico, P. parvum has entered into stream environments where it endangers the survival of the Pecos bluntnose shiner, which is listed as a threatened fish species.


Impacts of Karlodinium veneficum (D.Ballantine ) J. Larsen- The Small Athecate Dinoflagellate that Kills Fish

This small gymnodinioid dinoflagellate has been associated with toxic activity every since it discovery in the 1950s, and has seen its name changed multiple times over the last 60 years (see Place et al. 2012 for taxonomic synonyms). The ichthyotoxicity of karlotoxins can be traced to its targeting the gills of fish, and especially the chloride cells responsible for osmoregulation in fish (Deeds et al., 2006).  We view fish kills as collateral damage to blooms of K. veneficum which typically occur in shallow, stratified systems with high cell abundances (> 10,000 cells/ml) and can occur in situations with both low and high fish biomass (Deeds et al., 2002; Kempton et al., 2002;  Hall et al., 2008). It should be noted that large blooms of K. veneficum (approaching 1,000,000 cells/ml) in Maryland have also been reported with no observed fish mortalities.

A significant fish kill attributed to K. veneficum occurred in September 2005. On September 26, Maryland Department of Natural Resources (DNR) field personnel estimated a few hundred fish dead along the upper and middle Corsica River while conducted monitoring efforts. By September 29, Maryland Department of the Environment (MDE) and DNR estimated 30,000-50,000 fish had died. 15 species of fish were identified by MDE - the most abundant species was menhaden but many gizzard shad (Dorosoma cepedianum), hickory shad (Alosa mediocris), sunfish Lepomis spp., catfish, white perch (Morone americana) and yellow perch (Perca flavescens), carp (Cyprinus carpio) and killifish (Fundulus sp.) were also observed. Karlodinium was measured at concentrations up to 56,000 cells/ml; PCR screening for any of the ten of the most likely possibly toxic algae species found in Maryland waters only detected Karlodinum; no Pfiesteria species were found. Two of the three water samples contained sublethal but still harmful karlotoxin levels at 229 and 410 ng ml-1. The third sample contained nearly twice the lethal concentration of toxin (1809 ng ml-1). Stressful to lethal levels of dissolved oxygen were located in parts of the Corsica River throughout the week resembling the conditions described earlier for the Swann River. However, fish were observed in some areas dying when dissolved oxygen conditions were measured well above critical levels but algal toxins were present. Necropsy only found gill tissue deterioration consistent with laboratory findings on effects of karlotoxins  (Deeds  et al.  2006). The combined effects of low or no dissolved oxygen availability in some regions and karlotoxins at lethal and sublethal levels are considered to have combined to produce the kill (Adolf et al. 2015).  Several fish kills also occurred in Maryland waters in 2007 (summarized in Table at right).

The most recent fish kill in Maryland occured in November 2015 on the Middle River and its tributaries.  An estimated 201, 789 fish died, with 99% being freshwater species.  The final cause of the fish kill was concluded to be gill damage caused by karlotoxin exposure, exacerbated by osmotic stress resulting from high salinities.


Adolf, J. E., Bachvaroff, T. R., Bowers, H. A., Deeds, J. R., and Place, A. R. (2015).  Ichthyotoxic Karlodinium veneficum in the Upper Swan River estuary (Western Australia): synergistic effects of karlotoxin and hypoxia leading to a fish kill.  Harmful Algae 48, 83-93.doi:10.1016/j.hal.2015.07.006

Deeds, J.R., Terlizzi, D.E., Adolf, J. E., Stoecker, D., and Place, A. R. (2002). Hemolytic and Ichthyotoxic Activity from Cultures of Karlodinium micrum ((Dinophyceae) Associated with Fish Mortalities in an Estuarine Aquaculture Facility. Harmful Algae 1: 169-189.

Deeds, J. R., Reimschussel R., and Place, A. R. (2006). Histopathological effects in fish exposed to the toxins from Karlodinium micrum (Dinophyceae). Journal of Aquatic Animal Health 18; 136-148.

Hall, N. S., Fensin, E., Litaker, W., Adolf, J. E., Place, A. R. and Paerl, H. (2008). Environmental Factors Contributing to the Development and Demise of a Toxic Dinoflagellate ( Karlodinium veneficum ) Bloom in a Shallow, Eutrophic, Lagoonal Estuary. Estuaries and Coasts 31; 402-418.

Kempton, J. W., Lewitus, A. J., Deeds, J. R., Law, J. McH., Wilde, S.B., and Place, A. R. (2002) Toxicity of Karlodinium micrum (Dinophyceae) associated with a fish kill in a South Carolina Brackish Retention Pond.  Harmful Algae 1: 233-241.

Place, A. R., Bowers, H. A., Bachvaroff, T. R., Adolf, J. E., Deeds, J. E., and Sheng, J. (2012). Karlodinium veneficum - The little dinoflagellate with a big bite.  Harmful Algae. 14; 179-195

Last updated: July 11, 2016