Atlantic Menhaden mortality updated report

Jan Lovy, Ph.D. Research Scientist in Aquatic Animal Health 

Jan.Lovy@dep.nj.gov 

Atlantic Menhaden mortality- Liberty State Park, 05 JAN 2021: This is an updated report on results from an ongoing Atlantic Menhaden mortality investigation in New Jersey. 

This report has been updated on 15 Mar 2021. 

Summary: Chronic mortality of Atlantic Menhaden, occurring in November and December 2020, was investigated off the coast of New Jersey. Fish exhibited abnormal swimming behavior that included swimming in tight circles near the water surface, cork-screw swimming, and more rarely lethargy at the water surface. Moribund fish were collected and examined by gross necropsy, histopathologic evaluation, virus isolation using cell culture assays, and genetic evaluation for bacteria. No viruses were isolated from the samples. Consistent histopathologic lesions occurred in the fish that suggest significant disorders in the kidney, spleen, liver, and brain. A common finding included hematopoietic cell degeneration in the kidney and spleen, perivascular cuffs of cellular necrosis in the liver, and hemorrhage with degenerative cellular changes in the brain. Two of the fish had mats of bacteria and cells engorged with bacteria in the hindbrain. Genetic evaluation of the brain tissue using Illumina sequencing for bacterial 16S rDNA indicated high levels of a bacterium consistent with a Vibrio sp., with the closest identity to V. anguillarum in all the samples. Though this could not be accurately quantified, the levels of Vibrio sp. sequence were much higher in the brain than in the kidney, and considerably higher than any other bacterium detected. These results suggest that infection with Vibrio sp. is a factor in mortality and may contribute to the neurologic signs in the fish. Further studies are currently being conducted to confirm this and to evaluate other possible etiologies.

Background on Atlantic Menhaden mortality events: Atlantic Menhaden are found from Nova Scotia, Canada down to northern Florida on the Atlantic coast. They are a highly migratory fish that overwinters and spawns in the early winter in southern locations (around North Carolina). In New Jersey, mortality occurs nearly annually in this species, likely because of numerous factors. Some of these mortalities are attributed to acute hypoxic events, as previously described (Smith 1999) An Atlantic Menhaden mortality pattern that is unique from acute hypoxic events has been occurring nearly annually off the coast of New Jersey, though often in the spring months. These events are characterized by chronic mortality lasting weeks to over a month, with fish exhibiting uncoordinated and erratic swimming behavior. This fall and early winter in 2020, a particularly large mortality event with these characteristics occurred. The intent of the current investigation was to determine the cause for this mortality by histopathologic, microbiologic, and genetic methods.

Mortality observations and notes: Chronic mortality specific to Atlantic Menhaden occurred in November and December 2020 in northern New Jersey. The fish kill spanned Newark Bay, Raritan Bay, Navesink River, and the Hudson River. Reports indicated that hundreds of fish were impacted daily in each of the areas. The mortality event likely affected tens of thousands of fish over the two-month period, though there was no way to adequately estimate the numbers of dead fish. Reports from the Bureau of Marine Fisheries (NJDFW) reported that a very large school of Atlantic Menhaden was holding off of northern New Jersey (near Monmouth County) and that there was considerable predation of the fish, particularly by Bluefish, dolphins, and a report of a humpback whale. Another observation made by the NJ DEP Bureau of Water Compliance and Enforcement in the Hudson River was the presence of industrial vessels, including a sludge tanker that appeared to kill or stun fish in its travel path. With Atlantic Menhaden often occurring near the water surface, large vessels may stun fish or suck them into their propellers. The DEP Bureau of Water Compliance and Enforcement witnessed this occurring resulting in fish that were chopped up and some which were stunned by the vessels. It is possible that diseased fish were unable to avoid these vessels making them more vulnerable to being struck.

 Mortality reports confirmed similar fish kills in Atlantic Menhaden in New York, Connecticut, and Rhode Island. Erratic and disoriented swimming was reported in the affected fish. Following a report of large numbers of dying fish around Liberty State Park in Hudson County, New Jersey, we investigated this area on December 1st, 2020. Hundreds of dying fish were observed, showing disoriented swimming behavior. Fish were swimming in tight circular patterns on the surface, corkscrew swimming, and occasionally were lethargic hanging near the water surface. This chronic mortality had similar characteristics to prior mortality events that occurred in the spring within this region. The water temperature at time of investigation (around noon) was 11.5°C and dissolved oxygen was 9 mg/L. After several hours of walking along the coast a total of 30 adult Atlantic Menhaden, showing the described behavioral signs, were captured by dip net. The fish were transported on ice in a cooler to the Pequest Aquatic Animal Health Laboratory for necropsy. The fish remained on ice and were necropsied approximately 20 hours after collection from the field.

Fish data and gross necropsy findings: The average total length of fish (n = 30) was 29cm (range of 24.5 – 33cm) and average weight of 229g (range of 176.1 – 312.9g). Fulton’s body condition index (kfactor) was calculated for each fish (weight/length3 x 100) to determine body condition. The mean k-factor was 0.924 with a range of 0.798 to 1.184 (n = 30). Our previous  studies indicate the normal k-factors for Atlantic Menhaden in this region to be around 1.0. The mean k-factor herein is slightly depressed from baseline, though 12 fish had poorer condition that was in the range of k = 0.8 - 0.9, while the 18 other fish were closer to having a normal body condition factor (0.9-1.18). These body condition values do not appear to support starvation, as consistently lower values would be expected in starved fish. External lesions on the fish included light hemorrhage on the skin and at the fin bases, which was observed in 18 out of 30 fish. Also noted was infection with anchor worm, Lernaeenicus radiatus, a copepod parasite that penetrates the skin and anchors into the muscle of the fish, which occurred in 18 out of 30 fish and ranged from 1 – 7 parasites/fish. Infection with the parasite was associated with ulceration and muscular degeneration at the infection site. Some of the skin lesions may have been related to parasites that had dropped off the fish. Due to the common occurrence of this parasite in these numbers on healthy fish, the mortality is likely unrelated to L. radiatus. A hemorrhage in the eye occurred in 6 out of 30 fish and was severe in one (Figure 1). No other external lesions were noted. Internally, the organs and viscera appeared dark due to congestion or hyperemia. The gastrointestinal tract was generally empty, contained fluid, and the gall bladder was enlarged, indicating that fish were not actively feeding. This is consistent with the lower k-factors. The most prominent finding during gross necropsy was moderate to severe congestion and hemorrhage in and around the brain (Figure 2). This occurred in 21 out of 30 fish.

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Conclusions: The results herein demonstrate that this mortality event was consistently associated with a pathologic process involving multiple organs. This disease process consistently was associated with severe brain lesions causing hemorrhage, cellular degeneration, and cell sloughing into the ventricle, particularly in hindbrain. Histology of two samples indicated bacteria were associated with the lesions. In both cases the bacteria were detected in the hindbrain. It is possible that histology underrepresented these lesions, as hindbrain was not always present in the section. The genetic results are suggestive that the brain lesions are associated with a Vibrio sp. bacterium, based on presence of  high levels of this bacterial sequence in all brain samples. Further work is being done in collaboration with the USGS Eastern Ecological Science Center (Dr. Luke Iwanowicz) to confirm these findings. Future work will aim to develop a quantitative real-time PCR assay to better quantify the abundance of this Vibrio sp. and to amplify a larger portion of the 16S rDNA and other genes to better identify this Vibrio species. The sequence results place it closest to Vibrio anguillarum, but this is only based on a small portion of the 16S rDNA. Vibrio anguillarum is a known fish pathogen that causes disease in cultured and wild marine fish. Obtaining bacteriologic cultures directly from brain will be important when investigating these mortality events in the future. Other histopathologic lesions in the internal organs included degeneration of hematopoietic/lymphoid tissue in the kidney and spleen and cuffs of cellular degeneration around blood vessels in the liver. The cause for these lesions is unknown and may be related to another etiology, particularly since levels of the Vibrio spp. are very low in these tissues. Samples have been processed for transmission electron microscopy (TEM) and in the future will be evaluated to better describe these cellular changes and to determine their etiology. Once these findings are available, this report will be updated. The pathologic changes along with the presence of microscopic inclusions described herein could indicate infection with a rickettsia-like organism. For example, piscirickettsiosis in salmonids affects similar organs and replicates within macrophages and hematopoietic cells. Further, pathologic changes including perivascular necrosis in liver and lesions in the brain have been described for piscirickettsiosis (Arkush and Bartholomew 2011). The genetic results indicated the presence of a Wolbachia sp. rickettsia in the kidney/spleen of fish. This is not known to be a pathogen of fish and is best known as a rickettsial endosymbiont of invertebrates. Other rickettsial sequence was not detected in the genetic analysis. TEM results should aid in determining if rickettsia is associated with these lesions or if they have a different etiology. Previous menhaden mortality events that were chronic in nature and caused abnormal swimming behavior, like observed herein, were presumed to be related to an IPN-like virus (aquabirnavirus) that reportedly causes spinning disease (Stephens et al. 1980). Based on Stephens et al. (1980) cytopathic effects (CPE) occur in BF-2 and CHSE-214 cell lines. CPE was not observed in either of these cell lines in the current investigation, thus this virus is unlikely in the current mortality event. Future evaluation of these samples with TEM will aid in ruling out the potential for a viral etiology in these samples. The findings herein suggest that a neurologic infection with a Vibrio spp. is another cause for neurologic whirling behavior, thus must be considered as a rule-out when investigating these mortalities. The histopathologic lesions in the brain and the association of a Vibrio sp. bacteria helps to explain the abnormal swimming behavior of the fish. Hemorrhage was severe in the affected fish and they occurred primarily in the optic tectum and the medulla oblongata. Of note is the hemorrhage and damage done to the medulla oblongata and sloughing of cells into the 4th ventricle. Also, when sections were present of the medulla oblongata, that is where bacteria were detected by histology. The medulla oblongata is known to control motor function and the autonomous nervous functions. Perhaps the damage caused by bacterial infection in this region can explain the abnormal and uncontrolled swimming behavior of the affected fish. Future investigations of these mortality events should ensure bacterial cultures are taken from the hindbrain.