The New England Aquarium maintains a "schooling" exhibit consisting of approximately 3,000 blue-back herring (Alosa aestivalis). Fish are housed in a semi-closed, bare bottomed, 2,700 gallon saltwater system. Water quality is maintained with biological and mechanical filtration, ozonation, and foam fractionation.

Mortality rates in this exhibit usually range from 0-3 fish per week. In August 1994, mortalities from the exhibit suddenly increased to as high as 60+ fish per day. All water quality parameters, including dissolved oxygen, were within normal limits, although some animals displayed signs of respiratory distress (elevated RR). Necropsies revealed emaciated animals whose gills were pale and heavily infested with trematodes. Hematocrits of trematode infected fish were significantly lower than those of wild caught animals. Sequential trial treatments, consisting of both formalin and freshwater baths, proved ineffective. Trial treatments of Praziquantel baths, ranging from 1-9 ppm, appeared to effect the shedding of the trematode.

In the exhibit tank, three serial, 3 hour treatments of Praziquantel wore administered. Each treatment consisted of reducing the salinity to 20 pot, dosing Praziquantel at 3 ppm, followed by subsequent tank bottom vacuuming. A reduced mortality rate in the system was noted after the first treatment (ave. 7 fish per week) with a return to normal mortality rates after the subsequent two treatments. During the treatment period of 51 days, sub- of the population showed that hematocrits were increasing, approaching those of wild caught animals. Additionally, numbers of trematodes infecting the gills of sampled fish were reduced to zero.