Abstract

Over the last 20 yr, increasing mass mortality and diseases in aquatic organisms in the United States are being considered as another “sign of declining ecosystem health.” Causal factors typically associated with disease and mortality are poor water quality, toxicants, other environmental perturbations, infectious agents, and genetic susceptibility. Despite the ubiquitous nature of harmful algal blooms in marine environments, the extent to which biotoxins are involved in mortality and disease processes in fish (and other aquatic organisms) has only recently been realized. This paper will discuss a series of fish disease and mortality events in the Gulf of Mexico, eastern USA, and Salton Sea, California, will provide specific examples of the difficulties and challenges associated with current diagnostic approaches, and will hypothesize multifactored causal scenarios.

Harmful Algal Blooms and Biotoxins

Harmful algal blooms (HABs) cause massive fish kills and animal mortalities, shellfish poisonings, and respiratory irritation in humans.²⁷ For example, at least 37 species of toxic microalgae, including 11 ichthyotoxic species, are known in the Gulf of Mexico coastal waters.^10,27,29 The sudden appearance of toxic, planktonic blooms (e.g., red tides) that lead to acute, mass mortalities of fish have been historically documented.²⁷ Traditionally, HABs are known for their lethal, fast-acting effects on aquatic organisms, but recently the potential role of HABs and their associated biotoxins in chronic or sub-lethal disease events has been identified.^10,11

In the summer of 1980,⁷ winter/spring of 1993/1994¹⁰ and again in the summer of 1997 (unpublished data), heavy mortalities of tropical reef fish were reported along Florida’s southeast coast and along the Florida Keys. In each case a chronic disease syndrome affected mostly adult herbivores or omnivores. In most cases, diseased fish had lesions or shallow ulcerated body sores, fin or tail rot, and a heavy mucus coating on the body surface. Numerous protistan parasites and bacterial infestations were detected. The widespread nature and distribution of disease amongst different fish species with a commonality in specific feeding strategies suggested that the potential pathogens observed were not the principal cause of the disease syndrome but were secondary invaders of fish whose health had already been compromised.¹⁰ Biotoxins should be considered as a strong possibility in the role of primary stressors leading to chronic toxicity and immunosuppression. Tropical reef fish are potentially exposed to numerous biotoxins through their diet, either through direct consumption of toxic macroalgae such as Caulerpa, or through the consumption of toxic microalgae such as cyanobacteria or dinoflagellates that are epiphytic on macroalgae, seagrass, sponges, corals, or sand. Thus, many herbivorous or omnivorous fish species are at risk of dietary exposure to biotoxins that may be a predisposing factor in fish disease either directly or up the food chain. The seasonal dynamics in population abundances of these organisms over a widespread area might correlate with the sporadic and isolated incidences of this tropical reef fish disease syndrome. In many ways, this scenario may be linked to the tropical food poisoning in humans known as ciguatera that is also common in the same areas and may involve the same toxins. Traditionally, ciguatera toxins produced by benthic dinoflagellates such as Gambierdiscus toxicus are passed up the food chain without affecting fish health. There are strong indications that this may not necessarily be the case.

From 1980–1989, at least 50% and 69% of fish kills in the Gulf of Mexico and South Atlantic respectively were attributed to low dissolved oxygen.¹⁷ It is possible that many of these kills were associated with harmful algal blooms caused by small, ephemeral dinoflagellates that were not, until recently, recognized as being toxic. Such fish kills would have been attributed to low dissolved oxygen associated with the bloom rather than direct ichthyotoxicity. Recently, a series of fish kills, ulcerated fish disease events, and public health threats have highlighted the enigmatic life strategies of small (<25 :m), toxic dinoflagellates such as Heterocapsa, Gymnodinium, Gyrodinium, Pfiesteria^4-6,13,28,29 or a new cryptoperidiniopsoid. Ulcerative disease syndrome (UDS) in estuarine fish has been documented from the late 1970s until the present, from New York to Florida.^{1,8,16,18-20,22} While a clearly defined pathology and the presence of opportunistic pathogens characterize the disease,¹⁹ it was unclear what environmental factors were involved. Only recently has an association between UDS and P. piscicida been implicated.²¹ When fish are experimentally exposed to Pfiesteria piscicida, toxins cause hemorrhaging and sloughing of the skin epithelium.²¹ Ulcers are caused by the proliferation of opportunistic pathogenic fungi or bacteria (e.g., Aeromonas hydrophila) that invade the damaged external layers of the fish skin. The basic pathology of UDS appears to be similar in all areas surveyed, but the opportunistic pathogens associated with the disease^9,18-20 may vary because most invading microbial flora are typical estuarine inhabitants that are ususally normal component of fish surface, gills, and intestinal tract.² In the wild, at least 25 species of estuarine fish are affected by UDS.¹⁹ UDS is common in low to moderate salinities but not all fish species in these salinity ranges are affected.²⁴ Degraded water quality conditions in certain estuarine waters (UDS occurs in inshore areas of low to moderate salinity up to 25 ppt)²⁰ are thought to be associated with disease outbreaks, but no definitive cause-effect has been shown. There does appear to be strong circumstantial evidence for a correlation between the presence of UDS and P. piscicida, but this needs to be verified in the field and investigated with respect to the distribution of other potentially toxic dinoflagellates.

Many dinoflagellates have superficially similar morphologies that can lead to misidentification and inaccurate reporting. For example, we have recently identified as many as 10 potentially toxic dinoflagellate species at fish kill or disease events.^28,29 The potential for several species of dinoflagellates to co-occur in estuaries makes specific identification of these organisms critical because management strategies may vary depending upon the risk of toxin exposure to animal resources or to the public. Along the eastern seaboard, we have consistently documented the presence of the cryptoperidiniopsoid at sites where fish lesion events are occurring. Along with Pfiesteria piscicida, the role of the new cryptoperidiniopsoid in the potential initiation of fish lesions needs to be examined.

Pathogens

Mass mortalities of a single species of free-ranging marine fish over a broad area would typically be associated with a species-specific infectious pathogen. However, such cases are rarely documented. In the fall of 1995 and summer 1996, a mass mortality of the hardhead catfish, Arius felis, was recorded throughout the Gulf of Mexico and Florida Atlantic from Jacksonville, Florida to Galveston, Texas (Landsberg et al., unpublished data). Millions of (mostly) adult fish were estimated to have died. Initial investigations revealed the presence of amoebae and non-specific ciliate protists on the gills of fish. External characteristics of affected fish showed marked, bloody lesions of the mouth, barbels, and nares, as well as petechiae on the body. By light microscopy, intranuclear, eosinophilic rhomboidal or diamond-shaped inclusion bodies were typically noted in 3.5-m histologic sections of the posterior kidney (n=27) with some inclusion bodies present in the liver and spleen. Transmission electron microscopy (TEM) of infected posterior kidney typically revealed viral arrays (R. Reese, FDEP, unpublished data) in the nucleus of tubular epithelial cells. Viral arrays were detected by TEM in the majority of fish examined from throughout the Gulf of Mexico (n=15). The presence of the virus in dead and moribund fish and its absence in control fish suggests a strong, circumstantial but etiologic relationship with the hardhead catfish mortality. The absence of marine catfish cell lines and limited funds have precluded identification of the virus and further investigation of this event.

In August 1997, a widespread mortality of fish, particularly tilapia, occurred in the Salton Sea, California. Examination of a small sample (n=23) revealed heavy infestations of the parasitic dinoflagellate Amyloodinium ocellatum on the gills of 95.6% of the fish. A. ocellatum is recognized as a persistent pathogen that causes serious mortalities in aquaculture facilities and in aquaria.^12,23,25 Under such conditions, where fish are confined and overcrowded, and apparently also in the Salton Sea where the ecosystem is closed, parasite levels can build up to extremely high levels. Healthy marine fish held in aquaria can die after only 12 hr of exposure to Amyloodinium. This parasite is global in distribution and infects a wide range of fish hosts, including over 100 in North America alone.^14,15 In the wild, however, the number of parasites per fish is typically very low and fish do not usually die from infestation. The life cycle of Amyloodinium consists of three stages: a trophont that feeds by attaching to the gills and skin of fish, an encysted tomont that develops after the trophont detaches from the fish, and motile dinospores that are released after the tomont divides.³ The parasite impairs both respiratory function and osmotic balance, and can suffocate the fish when present in high numbers.²⁵ Diagnosis is based on finding the attached trophont stage in gill scrapings. Since the life cycle can be completed in less than 1 wk at high temperatures and highly saline conditions,²⁶ like those present in the Salton Sea, massive and lethal infestations could develop rapidly. Now that the parasite is present in the Salton Sea and apparently is able to reproduce rapidly without control, Amyloodinium might be associated with persistent, chronic die- off of fish during the next few years. High salinities are optimal for this parasite, but it does not tolerate freshwater or low salinity conditions. Along with other environmental stressors and bacterial pathogens, Amyloodinium represents a further threat to an ecosystem already in distress.

The above examples highlight some emerging issues with respect to fish health in the United States. Many interactive environmental factors may contribute to disease processes in free-ranging fish species. Environmental conditions or anthropogenic inputs such as nutrients, contaminants, or the results of water management policies may ultimately influence ecologic systems and indirectly lead to epizootics. It is essential to document the interaction of causal factors in the development of disease in fish populations by investigating environmental cofactors or potential initiators of stress.

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