Gary D. Marty

Relationship of farm salmon, sea lice, and wild salmon populations.

Increased farm salmon production has heightened concerns about the association between disease on farm and wild fish. The controversy is particularly evident in the Broughton Archipelago of Western Canada, where a high prevalence of sea lice (ectoparasitic copepods) was first reported on juvenile wild pink salmon (Oncorhynchus gorbuscha) in 2001. Exposure to sea lice from farmed Atlantic salmon (Salmo salar) was thought to be the cause of the 97% population decline before these fish returned to spawn in 2002, although no diagnostic investigation was done to rule out other causes of mortality. To address the concern that sea lice from fish farms would cause population extinction of wild salmon, we analyzed 10–20 y of fish farm data and 60 y of pink salmon data. We show that the number of pink salmon returning to spawn in the fall predicts the number of female sea lice on farm fish the next spring, which, in turn, accounts for 98% of the annual variability in the prevalence of sea lice on outmigrating wild juvenile salmon. However, productivity of wild salmon is not negatively associated with either farm lice numbers or farm fish production, and all published field and laboratory data support the conclusion that something other than sea lice caused the population decline in 2002. We conclude that separating farm salmon from wild salmon—proposed through coordinated fallowing or closed containment—will not increase wild salmon productivity and that medical analysis can improve our understanding of complex issues related to aquaculture sustainability.

Evaluation of Five Diagnostic Methods for the Detection and Quantification of Myxobolus Cerebralis

Diagnostic methods were used to identify and quantify Myxobolus cerebralis, a myxozoan parasite of salmonid fish. In this study, 7-week-old, pathogen-free rainbow trout (Oncorhynchus mykiss) were experimentally infected with M. cerebralis and at 7 months postinfection were evaluated with 5 diagnostic assays: 1) pepsin–trypsin digest (PTD) to detect and enumerate spores found in cranial cartilage, 2) 2 different histopathology grading scales that provide a numerical score for severity of microscopic lesions in the head, 3) a conventional single-round polymerase chain reaction (PCR), 4) a nested PCR assay, and 5) a newly developed quantitative real-time TaqMan PCR. There were no significant differences (P > 0.05) among the 5 diagnostic assays in distinguishing between experimentally infected and uninfected control fish. The 2 histopathology grading scales were highly correlated (P < 0.001) for assessment of microscopic lesion severity. Quantification of parasite levels in cranial tissues using PTD and real-time TaqMan PCR was significantly correlated r = 0.540 (P < 0.001). Lastly, 104 copies of the 18S rDNA gene are present in the M. cerebralis genome, a feature that makes this gene an excellent target for PCR-based diagnostic assays. Also, 2 copies of the insulin growth factor–I gene are found in the rainbow trout genome, whose detection can serve both as an internal quality control for amplifiable DNA and as a basis to quantify pathogen genome equivalents present in quantitative PCR assays.

Histopathology of swimbladder noninflation in walleye (Stizostedion vitreum) larvae: role of development and inflammation

Abstract Noninflation of the swimbladder is a major obstacle in culture of many fish larvae including walleye (Stizostedion vitreum). 79 walleye larvae, 4–19 days posthatch, were serially sectioned for histologic study of swimbladder development. All walleye had a swimbladder and vascular rete. Inflation of the swimbladder began at 6 days posthatch, coinciding with the time of yolk sac depletion and initiation of feeding. From 7 to 11 days posthatch, macrophages infiltrated 11 of 18 (61%) noninflated and 3 of 16 (19%) inflated swimbladders. From 12 to 19 days posthatch, macrophages infiltrated 16 of 16 (100%) noninflated and 0 of 14 (0%) inflated swimbladders. Organic debris and large numbers of bacterial rods filled the noninflated swimbladder of a 13 day old larvae examined by electron microscopy. In larvae with noninflated swimbladders, the pneumatic duct was patent and its diameter remained fairly constant (25–45 μm) through 19 day posthatch, but the pneumatic duct atrophied in larvae with inflated swimbladders. During the interval of swimbladder inflation, 6–12 day posthatch, ostia of the common bile duct and pneumatic duct occupied the same lumen of the undifferentiated foregut; we hypothesize that surfactant-like secretions from the common bile duct affected fragmentation of large ingested air bubbles for transfer into the relatively small-diameter pneumatic duct. After 12 day posthatch, however, the pyloric sphincter developed and separated the common bile duct in the intestine from the pneumatic duct in the dorsal wall of the stomach. We conclude that swimbladder noninflation in walleye results from ingestion of bacteria and organic debris into the swimbladder 6–11 day posthatch, and that differentiation of the foregut prevents inflation after 12 day posthatch.

Age-dependent changes in toxicity of N-nitroso compounds to Japanese medaka (Oryzias latipes) embryos

Abstract Three different developmental stages of embryonated eggs of Oryzias latipes were exposed to 2-h pulses of a number of different concentrations of MNU, MNNG, and DENA. Lethality and teratogenic endpoints were assessed through 24 h posthatch. MNU (≥2.5 mM) and MNNG (≥0.75 mM) exposure at the multicell stage was lethal, but the same exposure during early organogenesis was largely teratogenic. Four days before hatching, embryos were very resistant to MNU and proceeded to hatch normally; 7.5 mM MNU was required to significantly reduce the percent normal hatch. Effects of DENA were equivocal and only seen at the multicell stage. In order to differentiate between potential changes in chorion permeability, and toxicity per cell, the same three stages were exposed to a series of radiolabeled compounds of varying hydrophobicity, and apparent uptake measured. There were no differences in uptake rate or equilibrium concentration between any of the compounds at the different stages. These results suggest that, as in mammals, certain stages of development are more susceptible than others, and that these differences are a result of embryo sensitivity rather than chemical bioavailability. The most sensitive indicator of exposure was posthatch inflation of the swimbladder. Other terata included pericardial edema, anisophthalmia, and partial rupture of the chorion.

Retrospective analysis: bile hydrocarbons and histopathology of demersal rockfish in Prince William Sound, Alaska, after the Exxon Valdez oil spill

Demersal rockfish are the only fish species that have been found dead in significant numbers after major oil spills, but the link between oil exposure and effect has not been well established. After the 1989 Exxon Valdez oil spill in Prince William Sound, Alaska, several species of rockfish (Sebastes spp.) from oiled and reference sites were analyzed for hydrocarbon metabolites in bile (1989-1991) and for microscopic lesions (1990 and 1991). Biliary hydrocarbons consistent with exposure to Exxon Valdez oil were elevated in 1989, but not in 1990 or 1991. Significant microscopic findings included pigmented macrophage aggregates and hepatic megalocytosis, fibrosis, and lipid accumulation. Site differences in microscopic findings were significant with respect to previous oil exposure in 1991 (P=0.038), but not in 1990. However, differences in microscopic findings were highly significant with respect to age and species in both years (P<0.001). We conclude that demersal rockfish were exposed to Exxon Valdez oil in 1989, but differences in microscopic changes in 1990 and 1991 were related more to age and species differences than to previous oil exposure.

Failure of population recovery in relation to disease in Pacific herring

Following an estimated 60% decline in population abundance in early 1993, recovery of the Pacific herring Clupea pallasii population of Prince William Sound, Alaska, USA, has been impaired by disease. Comprehensive epidemiological study from 1994 through 2002 validated an age-structured assessment (ASA) model of disease and population abundance; from 2003 to 2006, the impact of disease was modeled by analyzing only 2 lesions: ulcers and white foci in the heart. The ASA model identified increased natural mortality since 1993 that can be explained by (1) epidemics associated with ulcers (prevalence about 3%) and the North American strain of viral hemorrhagic septicemia virus (VHSV Type IVa; prevalence up to 14%) in 1994 and 1998 and (2) relatively high prevalence of the mesomycetozoean Ichthyophonus hoferi from 1994 through 2006, including epidemics with the greatest sample prevalence in 2001 (38%, by histopathology) and 2005 (51%, estimated histopathology prevalence). Fourteen other parasites occurred at prevalence > 10%, but none were considered significant contributors to fish mortality. We predict that if natural mortality after 1994 had returned to background levels that best fit the model from 1980 to 1992 (0.25 yr(-1)), population biomass in 2006 would have been 3 times the best estimate, despite relatively poor recruitment since 1994. In conclusion, disease information can be used to explain and predict changes in populations that have confounded traditional fisheries assessment.

Notes: Oxygen Consumption by Larval Japanese Medaka with Inflated or Uninflated Swim Bladders

Abstract Inflation and regulation of swim bladder volume enables fish to maintain position within the water column with minimal energy cost. Larvae whose swim bladders fail to inflate must move to stay suspended and would be expected to use additional energy to maintain position. As a relative measure of extra effort expended by fish with uninflated swim bladders, oxygen consumption rates at 26°C were compared for larval Japanese medaka Orvzias latipes with inflated and uninflated swim bladders. During the first 24 h after hatching, oxygen consumption rates were significantly higher for larvae with uninflated swim bladders (1.75 ± 0.13 μg O2·h−1·larva−1) than for those with inflated swim bladders (0.92 ± 0.05 μg O2·h−1·larva−1); after 96 h, oxygen consumption rates had decreased for all larvae to about 0.6 μg O2·h−1·larva−1. For larvae held in sealed containers for 72 h without access to free surface air, the volume of air-saturated water required to maintain an adequate concentration of dissolved oxygen ...

Piscine Orthoreovirus from Western North America Is Transmissible to Atlantic Salmon and Sockeye Salmon but Fails to Cause Heart and Skeletal Muscle Inflammation

Heart and skeletal muscle inflammation (HSMI) is a significant and often fatal disease of cultured Atlantic salmon in Norway. The consistent presence of Piscine orthoreovirus (PRV) in HSMI diseased fish along with the correlation of viral load and antigen with development of lesions has supported the supposition that PRV is the etiologic agent of this condition; yet the absence of an in vitro culture system to demonstrate disease causation and the widespread prevalence of this virus in the absence of disease continues to obfuscate the etiological role of PRV with regard to HSMI. In this study, we explore the infectivity and disease causing potential of PRV from western North America—a region now considered endemic for PRV but without manifestation of HSMI—in challenge experiments modeled upon previous reports associating PRV with HSMI. We identified that western North American PRV is highly infective by intraperitoneal injection in Atlantic salmon as well as through cohabitation of both Atlantic and Sockeye salmon. High prevalence of viral RNA in peripheral blood of infected fish persisted for as long as 59 weeks post-challenge. Nevertheless, no microscopic lesions, disease, or mortality could be attributed to the presence of PRV, and only a minor transcriptional induction of the antiviral Mx gene occurred in blood and kidney samples during log-linear replication of viral RNA. Comparative analysis of the S1 segment of PRV identified high similarity between this North American sequence and previous sequences associated with HSMI, suggesting that factors such as viral co-infection, alternate PRV strains, host condition, or specific environmental circumstances may be required to cause this disease.

Piscine reovirus, but not Jaundice Syndrome, was transmissible to Chinook Salmon, Oncorhynchus tshawytscha (Walbaum), Sockeye Salmon, Oncorhynchus nerka (Walbaum), and Atlantic Salmon, Salmo salar L.

A Jaundice Syndrome occurs sporadically among sea-pen-farmed Chinook Salmon in British Columbia, the westernmost province of Canada. Affected salmon are easily identified by a distinctive yellow discolouration of the abdominal and periorbital regions. Through traditional diagnostics, no bacterial or viral agents were cultured from tissues of jaundiced Chinook Salmon; however, piscine reovirus (PRV) was identified via RT-rPCR in all 10 affected fish sampled. By histopathology, Jaundice Syndrome is an acute to peracute systemic disease, and the time from first clinical signs to death is likely <48 h; renal tubular epithelial cell necrosis is the most consistent lesion. In an infectivity trial, Chinook Salmon, Sockeye Salmon and Atlantic Salmon, intraperitoneally inoculated with a PRV-positive organ homogenate from jaundiced Chinook Salmon, developed no gross or microscopic evidence of jaundice despite persistence of PRV for the 5-month holding period. The results from this study demonstrate that the Jaundice Syndrome was not transmissible by injection of material from infected fish and that PRV was not the sole aetiological factor for the condition. Additionally, these findings showed the Pacific coast strain of PRV, while transmissible, was of low pathogenicity for Atlantic Salmon, Chinook Salmon and Sockeye Salmon.

Systemic iridovirus from threespine stickleback Gasterosteus aculeatus represents a new megalocytivirus species (family Iridoviridae).

Megalocytiviruses have been associated with epizootics resulting in significant economic losses in public aquaria and food-fish and ornamental fish industries, as well as threatening wild fish stocks. The present report describes characteristics of the first megalocytivirus from a wild temperate North American fish, the threespine stickleback Gasterosteus aculeatus. Moribund and dead fish sampled after transfer to quarantine for an aquarium exhibit had amphophilic to basophilic intracytoplasmic inclusions (histopathology) and icosahedral virions (transmission electron microscopy) consistent with an iridovirus infection. Phylogenetic analyses of the major capsid, ATPase, and DNA polymerase genes confirmed the virus as the first known member of the genus Megalocytivirus (family Iridoviridae) from a gasterosteid fish. The unique biologic and genetic properties of this virus are sufficient to establish a new Megalocytivirus species to be formally known as the threespine stickleback iridovirus (TSIV). The threespine stickleback is widely distributed throughout the northern hemisphere in both freshwater and estuarine environments. The presence of megalocytiviruses with broad host specificity and detrimental economic and ecologic impacts among such a widely dispersed fish species indicates the need for sampling of other stickleback populations as well as other North American sympatric marine and freshwater ichthyofauna.