Extracted from:

Donald F. Boesch, Donald A. Anderson, Rita A. Horner, Sandra E. Shumway,Patricia A. Tester and Terry E. Whitledge. 1997. Harmful Algal Blooms in Coastal Waters: Options for Prevention, Control and Mitigation. NOAA Coastal Ocean Program, Decision Analysis Series No. 10, Special Joint Report with the National Fish and Wildlife Foundation, February 1997.

Site developer's note: genus and species names of algae are indicated in red, rather than standard underlining (not a webpage option unless a link...) or italics, in order to make the names easier to find and read.

Algae which do not cause illness in humans -
but are harmful to fish

Catastrophic losses of cultures and wild fish sometimes occur due to species of phytoplankton that do not cause illnesses in humans. Blooms of the raphidophyte flagellate Heterosigma akashiwo have caused kills of pen-reared salmonids in Washington in 1989 and 1990, and wild fish in 1994. Losses to the fish growers, including higher insurance rates as well as lost production, are about $4-5 million per year when blooms occur. The mechanism by which Heterosigma kills fish is not known, but may involve an ichthytoxin (R.A. Cattalico, presentation to the Panel, 1996), or production of a superoxide hydroxy radicals or hydrogen peroxide (Yang et al. 1995). Heterosigma blooms cause problems at high cell densities, usually exceeding 107 cells/l. Blooms often start in shallow back bays of Puget Sound and spread into the sound, carried by tides and currents. This species is a vertical migrator, usually occurring in surface waters during the day and at depth during the night. Vertical stability of the water column is probably an important factor in maintaining blooms

Fish kills are also caused by the diatoms Chaetoceros convolutus and C. concavicornis (possibly also C. danicus) which do not produce a toxin, but have long setae with short secondary spines. Chains of cells apparently become lodged between secondary lamellae in the fish gills and cause blood hypoxia as a result of mucus production. Chaetoceros blooms kill at low cell densities, sometimes as low as 104 cells/l. These diatoms may be restricted to near-surface waters or mixed throughout the water column depending on local hydrographic conditions. Most fish growers have their own phytoplankton monitors who sample at the pen sites on a daily basis from April through September. They also rely on reports from other phytoplankton monitoring programs. Economic losses are about half a million dollars per event.

UNIVERSITY OF MARYLAND College of Ag & Natural Resources Department of Veterinary Medicine Aquatic Pathobiology Center