THE VIRUS AND THE WATERSHED
IPNV is a double-stranded, non-enveloped RNA virus which primarily infects salmonid fishes (trout and salmon), but can and does infect a much wider variety of finfish in both fresh and salt waters, including eels, striped bass, menhaden (‘mossbunker’) several kinds of flatfish, & others. In addition, it may infest (if not infect) shrimps, crabs, crayfish, mussels, clams, and even microscopic worms called rotifers.
IPNV has no health consequences to humans, whether directly or indirectly ingested. This virus is worldwide in distribution, found on virtually every continent except Australia and Antarctica .In the U.S., many Eastern and Midwestern States have IPNV in their waters, especially through the Appalachians, Ohio, Pennsylvania, and the Northeast.
Viral transmission occurs in several different ways. Vertical transmission occurs from infected parents to offspring because the virus is present in large amounts in the fishes’ reproductive fluids. “Horizontal” transmission occurs when fish ingest contaminated waters and/or foodstuffs. “Vector” transmission can take place by the introduction of contaminated nets and equipment to ‘clean’ environments. Because this virus is able to withstand the acid of digestive-system passage, a wide variety of both avian and mammalian predators and visitors to watersheds are also “horizontal” vectors via their droppings .In fact, visiting waterfowl may be the biggest ‘culprit’ in spreading disease. Unlike most viruses, IPNV can survive for extended periods in moist environments outside a natural host (up to 4 months) This very tenacious “bug” is also highly resistant to iodophors and ultraviolet light frequently used as ‘disinfectants’.
The virus is mostly lethal to young fry, especially those under 5 grams in weight. Survivors of the initial infection, once old enough to be immunologically competent, usually are able to feed and grow normally, but become chronic carriers and ‘reservoirs’ of virus. Also unlike most epidemic diseases, when IPNV is introduced into a watershed it behaves as a so-called “point-source” epidemic, rather than a “propagative” one—that is, a great number of fish fall victim all at once, rather than the affected number increasing over time.
Once established in a watershed, the virus becomes endemic, with periodic outbreaks followed by periods of relative quiescence. There is also significant seasonal variability in measured viral concentrations in the affected waters. Reduction of the virus from a watershed is mostly a matter of hope that by reducing the fish population, the viral “load” will attenuate over time. However, this does not take non-piscine hosts into account.
Fish farms and hatcheries are particularly prone to disease, because in their crowded conditions only about one-one-thousandth of the amount of virus needed to infect a free-swimming adult may suffice to infect a crowded fish tank or other enclosure.
THE CRSPP HATCHERY
The 19th-century hatchery sits athwart the Connetquot river in such a way that the stream essentially flows directly through the hatchery complex. There are 11 holding ‘ponds’ in addition to the ‘hatch house’ where fertilized eggs are incubated. Ponds are netted-over to keep out predators or waterfowl. Nine of these are concrete-walled, grave-bottomed enclosures with natural ground-water spring seeps. This makes them presently impossible to ‘dry out’ and/or disinfect. All the ponds (‘raceways’) are supplied exclusively by rver-water. Only the hatch-house itself has a potentially usable well-water supply.
One of the raceways is open at its lower end to permit fish to access it from the lower river. Trout which have been imprinted from birth with the distinct chemical signature of the river but which have ‘gone to sea’ into the Great South Bay and survived to maturity will return at spawning-time to this pond. Thus, over many decades, the CRSPP hatchery has been able to utilize these uniquely robust fish to improve the genetic patrimony of their fertilized eggs.
Egg-rearing technique has altered little in over a century. It is a far cry from the semi-automated systems in use in modern facilities, and natural mortalities are far in excess of those occurring in up-to-date hatcheries. Nonetheless, the survivors have been vigorous and plentiful. The operation has therefore been both a boon to the fishery and a historical window for the visiting public.