Abstract

The Minnesota Zoological Garden opened a new marine education facility in the Spring of 1997. Part of the exhibit consists of a 220,000 gallon mixed teleost fish and shark exhibit. All of the fish in the exhibit tank had gone through at least 30 days of quarantine in holding tanks at our facility prior to being placed in the exhibit tank. No significant morbidities or mortalities occurred during the quarantine period.

The exhibit tank was filled with chlorinated city water on 20 February 1997. The tank's filtration system consists of six 8-foot diameter vertical sand filters. The filtered water then goes though an ozone tower for elimination of microorganisms. Instant Ocean saltwater mixture was added to the filtered city water on 10 March 1997. The original fill water was never replaced prior to placing animals in the tank, although it was run through activated carbon prior to introduction of fish.

On 8 May 1997, the exhibit contained 54 jacks, 6 triggerfish, and 11 red snappers, 2 green moray eels, 2 southern stingrays and 4 sandtiger sharks. Most of the animals had been in the exhibit for several weeks and all appeared clinically normal. On this day, contractors had done some seal coating in an adjacent marine mammal exhibit area, which was accessible to the shark exhibit through two doorways. At 0800 hr on 9 May, jacks were noted to be swimming frantically and jumping from the water surface. The sharks were swimming slowly and intermittently resting on the bottom. An odor of paint fumes was noticeable and a metallic sheen was noted on the water surface. Two sharks were dead by noon. Even though the remaining sharks were moved back to their quarantine tank in another building and given supportive care, they died by 1500 hr. The two stingrays were also moved to the same quarantine tank and survived. It was not possible to remove the other fish from the tank. Of these, 36 jacks died, but all triggerfish, snappers and moray eels survived.

Blood samples were obtained from two moribund sharks prior to death. Complete post mortem exams were performed on all sharks and several of the jacks shortly after death and were unremarkable except for gill congestion. Several volatile organic compounds (VOC's ) were detected in the shark blood samples as well as liver samples from the jacks. VOC's were also noted in water samples and included: acetone, tetrahydrofuran, 2-butanone, 1,1,1-trichloroethane, methyl isobutyl ketone, toluene and 1,2,3 trichloropropane. Histopathologic examination of all dead sharks and all jacks examined showed acute toxic change in the gills and death was attributed to this process. Because toxic organic compounds were detected in the fish and water, and all other water parameters were normal, the source of the toxic exposure was most likely VOC fumes coming from the adjacent exhibit.

Water VOC levels were periodically monitored over the next week and had decreased substantially by 13 May; however, acetone and tetrahydrofuran were still detectable at low (parts per billion) levels. Two sandbar sharks were placed in the exhibit tank on 14 May. These sharks died within 4 hr of being placed into the tank. Histopathologically, gill lesions in these animals were very similar to those seen in the animals, which had died on 9 May.

In conclusion, it is essential to recognize the sensitivity of at least some species of fish to relatively low levels of organic compounds. This factor needs to be taken into account when devising construction schedules and planning public openings of aquatic facilities. In this way, facilities can be completely constructed and systems thoroughly analyzed prior to introduction of animals, which may be sensitive to a wide variety of environmental factors.

(Reprinted with permission: 1999 Proceedings of the American Zoo and Aquarium Association.)