Aquaculture and Fish Health, sole test

Biggest challenge facing aquaculture industry

Increasing production to meet demand, in a sustainable manner.

Most commonly reared finfish species

Carp, tilapia, catfish, and Atlantic salmon (the most important in North America, mainly occuring around Vancouver and the Atlantic coast).

Controversy with open cage culture

Atlantic salmon are farmed on the Pacific coast, which provides opportunities for escape or pathogen exchange that can negatively affect the wild ecosystems.

Life cycle of the wild Pacific salmon

Smolts (<150g) grow in streams for 1-2 years, and then move to the ocean for 1-2 years. Once they have reached adult size (4-6kg), they return to their birth stream to spawn and die.

Life cycle of the farmed Pacific salmon

Fries are born in a freshwater hatchery, then move to a seawater growing cage. Mature salmon go to a processing plant or the broodstock facility, where they will either be harvested or produce roe.

Benefits of sea cage farming

Allows for natural temperature, salinity, and oxygen levels, and constant water flow changes. On-site monitoring is essential.

Risks of sea cage farming

Pathogen transfer between wild and farmed species, predation by wildlife, mortality due to weather events, or defouling of equipment such as the net.

Salmonid damage from jellyfish

Use nematocysts to puncture the gills and release toxins, which are cytopathic to fish cells.

Values assessed by on-site monitoring

Fish behaviour, feeding, size and growth rates, fouling of net and algal impacts. Biweekly, gross gill lesion scoring and sea lice counts should be conducted on a selection of the population.

Poor performers

Smolt unable to properly transition from freshwater to saltwater, which appear stunted and eventually die due to excess salt intake.

Sea lice management

Medicinal intervention has been reduced due to resistance, so mechanical treatment and handling is now important. Generally, the number of acceptable lice is based on risk to wild fish.

Major disadvantages of semi or full containment system

Not economically viable for full growout, and the closed environment encourages replication of parasites and disease that get in.

Cause of increased mass mortality events in aquaculture

Due to increasing coastal temperatures, worsened lice infections.

Most important step in treating fish illness

Correcting the environment – reducing stress is crucial, as if done early enough it can prevent disease from occuring, and in morbid fish can help treat infection.

Illness pathway 1

Opportunistic, caused by an environmental shift creating the perfect conditions for biofilm formation. Bacteria must colonise and multiply on the host to cause clinical disease.

Investigating aquatic mortality events

Environment and its history is extremely important – examples include temperature changes, suboptimal water circulation, cage fouling, or administration of Abx in feed.

Pathogen bloom

When a pathogen modifies the gill / skin surface, causing changes in mucus, a build up of dead cells, and inflammation. This makes the surface more vulnerable to opportunists.

Bacterial gill disease - cause

Caused by a variety of filamentous bacteria which induce clubbed gills, especially Flavobacterium columnare and Flexibacter maritimus.

Bacterial gill disease – pathology

See large numbers of filamentous bacteria on the gill lamellae, which leads to epithelial hyperplasia (and eventually fusion), hydropic degeneration, necrosis and edema.

 Bacterial gill disease – treatment

Flowthrough Chloramine-T, usable by special permit. However, environmental modification remains important.

Mouth rot

Mainly affects fish recently entered into seawater. Caused by Flexibacter maritimus, leading to ulcerative stomatitis.

Biggest risk for amoebic infection

Often secondary to bacterial infection (as amoeba feed on them).

Amoebic / proliferative gill disease – cause, pathology

Caused by Neoparamoeba peurans. Induces hyperplasia of the epithelial / mucus cells, and migration of leukocytes to associated lesions. Mortality is mostly associated with osmoregulatory failure.

Amoebic / proliferative gill disease – risks, treatment

Usually occurs in water warmer than 10C, and in fish in their first year with low resistance. Treated by freshwater or hydrogen peroxide bath, but must be repeated so can be tricky.

Management of active disease

Corrective measures (take off feed, reduce density, increase flow, remove fouled material), and treatments (chemical treatment via bath or feed additives).

Three most major pathogens associated with pathway 1

Flexibacter maritimus (mouth rot), Neoparamoeba peurans (amoebic gill disease), Flavobacterium columnare (warm water fish illness) or Flavobacterium psychrophilum (bacterial coldwater disease).

Five steps of pathway 1

1: multiplication in environment.

2: gains entry onto / into host.

3: multiplication, colonisation of host.

4: modified surface allows for other infections to colonise.

5: pathogens attract others, such as amoeba being attracted to bacteria.

Illness pathway 2

Covert infections, often highly virulent but subclinical due to the fish having recovered or never shown signs (low dose), reactivated by host stress. Can be prevented by vaccination.

Phases of classical virulent pathogen progression

1: infection and dissemination.

2: targeted cell proliferation, mortality.

3: carrier animals, recrudescence.

Detecting viruses in fish populations

Cell culture, testing (ELISA) repeated to determine disease state / progression. Cell line growth often conducted, for example with chinook salmon (CSCL).

Infectious pancreatic necrosis virus  (IPNv)– pathology

Causes acute gastroenteritis and pancreatic destruction in salmonids under 4-6 months old (or trout under 4 mos) who have recently entered seawater. Appears very similar to infectious hematopoetic necrosis virus (IHNv), should be differentiated histologically.

Infectious pancreatic necrosis virus – diagnosis, treatment

Salmon will show exopthalmia and a distended abdomen. Histologically, appears as cell damage to the exocrine pancreas. Detected with ELISA, PCR, CHSE cell line. Vaccines are available, and eggs can be disinfected.

Furunculosis – cause, treatments

Caused by Aeromonas salmonicida, a non-motile intracellular bacteria which lives in furuncles of both wild and farmed salmon. Can be treated with Abx, vaccines, and AMR.

Furunculosis – vaccines

Most common is Forte Micro, which covers Aeromonas as well as several Vibrio species, and comes in liquid + oil adjuvant form. Used to prevent furunculosis and vibrosis by injecting healthy salmonids. Rare side effects are melanisation of the injection site and adhesions.

Bacterial hemorrhagic septicemia – environmental factors, treatment

Stress and rapid changes in environment are usual triggers to enduce carriers to shed, and turns commensal Aeromonas and Vibrios into opportunistic pathogens. Treated with Oxytet.

Bacterial hemorrhagic septicemia – pathology

Ascites, splenomegaly and renomegaly, colitis, with widespread hemorrhages in the muscles, viscera, fat, and swim bladder. May see intravascular colony formation histologically.

Furunculosis – pathology

There are acute and chronic forms, the latter of which is more likely to cause furuncle formation. Histologically, shows as focal bacterial colonies surrounded by necrosis.

Winter ulcer disease – signs, cause, treatments

Presents as skin discolouration, progressing to worsening ulcerative patches. Caused by Moritella viscosa, mainly in cold water <10C. Results in highest use of Abx in salmon aquaculture (high mortality, meat downgrading). Vaccines have recently been developed.

Bacterial kidney disease – cause, signs, spread

Caused by Renibacterium salmoninarum, an intracellular G+ pathogen of salmon. Fish will be lethargic, darkened and have greyish, granulomatous lesions on the kidney. Grossly appears similarly to nephrocalcinosis. Can transmit horizontally and vertically.

Bacterial kidney disease – treatment

Renogen vaccine is protectant, culling infected broodstock for prevention is common. Oxytet / Fluorphenicol not particularly effective.

Kudoa thyrsites - signs

Multivalvulid, myxozoan parasite that benignly infects the somatic muscle of fish until 24h post-mortem, where it causes major proteolysis and liquefaction of the musculature, grossly appearing as pale pitting.

Management of Kudoa thyrsites

Monitoring prevalence and abundance of parasite (abundance more important at harvest). Exposed fish will develop resistance to reinfection with high numbers. April – December is infectivity window, most commonly when smolts move into saltwater, so biosecurity is important.

Enteric red mouth – cause, signs

Mostly in trout, caused by Yersinia ruckeri, inducing erythema and hemorrhage around the mouth as well as serious bacteremia – hemorrhage, congestion, inflammation, necrosis of the kidney, spleen and liver.

Enteric red mouth – useful for diagnosis / treatment

Diagnosis: Biochemical analysis, PCR

Treat: environmental modification, furunculosis Abx, Ermogen vaccine.

Illness pathway 3

Spillover, where pathogens pass from wild to farmed fish, amplify, and spillback, where pathogens pass from farmed to wild fish.

Infectious hematopoetic necrosis virus (IHNv) – pathology, signs

Enzootic to PNW, causes covert infection in adult salmonids but severe disease in salmonids under 6 months by inducing necrosis of the interstitial hematopoetic tissue in the liver, spleen, and pancreas. This presents as nearly 100% mortality with abnormal behaviour, exopthalmia, hyperpigmentation or bloody fin discolouration, distended body cavity, pale gills and organs, petechial hemorrhages, and skeletal abnormalities in the survivors.

Infectious hematopoetic necrosis virus (IHNv) – epidemiology, diagnosis

Has been extirpated in farmed salmon in BC due to vaccination (except Mainstream Canada, who don’t vaccinate fish on West Vancouver Island). The virus decays due to UV light and requires a certain quantitiy for injectivtiy. Must be differentiated from IPNv by cell culture, PCR.

Infectious salmon anaemia virus (ISAV) – signs, vaccine status

RNA virus similar to Orthomyxoviridae, primarily affects Atlantic salmon. Causes lethargy, pallid gills, low hematocrit and potentially mortality or chronic infection (+- carriers). Vaccines being variably tested.

Genetic resistance to infectious salmon anemia virus (ISAV)

HE-HPR gene determines virulence. Virulent forms have a deletion mutation in HPR. Avirulent strains seem to follow the course of virulent ones, and are only found in the gills.

Infectious salmon anemia virus (ISAV) – pathogenesis

Targets macrophages and endothelium, causing a degeneration of the liver sinusoids and a subsequent congestion and ischemic necrosis of liver tissue. Erythrophagocytosis by infected macrophages reduces hematocrit, causing anemia.

Infectious salmon anemia virus (ISAV) – pathology, diagnosis

Grossly, appears as ocular hemorrhage with exopthalmia, ascites, a darkened and enlarged liver, splenomegaly, and gut congestion. Histologically, appears as multifocal hemorrhagic hepatic necrosis (+ hematopoietic kidney). Diagnosed with cell lines, PCR, SK-1.

Infectious salmon anemia virus (ISAV) - transmission

Native to the Atlantic Ocean. More common in seawater than fresh, and may be passed via ectoparasites (i.e., L. salmonis). Usually transmits via cohabitation or contaminated equipment / staff, as the virus sheds through the skin, mucus, and excretions. The fish ingests it through the gills or by corpophagy. Mussels may inactive the virus.

Infectious salmon anemia virus (ISAV) – control

Has caused major devastating outbreaks before in Norway, Chile. Ban use of seawater hatcheries and movement of fish between seawater sites, disinfect waste, separate fish by years, and require health certificates for aquaculture farms.

Illness pathway 4

Wild thing complex life cycles, requiring interaction with wildlife (often aquatic birds, the definitive hosts) and leading to infection in mature farms.

Establishment, management of wildlife-related illness

Environment must be suitable for wildlife such as birds and snails to visit. Draining is necessary to avoid nutrient loading, algal growth, and snail / worm establishment. Takes 2-3 years of pond use before trematodes and myxoans become problematic.

Diplostomum spathaceum – histopathology, life cycle

Causes lens fluke. Will see multifocal, white oblong structures around periphery of eye lens that when cut open will contain metazoan parasites. Intermediate host is snail, bird is final host.

Snail management – indirect and direct

Indirect: controlling aquatic weeds (reduce nutrient levels / light, mechanically or chemically remove, stock pond with species that consume them).

Direct: reduce snail population (yearly treatment with molluscicides or copper sulfate, introduction of snail eating fish such as sunfish).

Bird management

Involves habitat changes, use of exclusion barriers such as netting or lines, changes to shoreline characteristics such as increasing angle (prevents heron wading), use of scaring devices. Lethal methods and culling are discouraged.

Myxobolus cerebralis – life cycle, prevention, pathology

Causes whirling disease in trout. Intermediate host is oligochaete worm, which can be eliminated by concrete raceways. Induces progressive destruction of cartilage around auditory capsules (leads to tail chasing, blackened tails).

Henneguya ictaluri – life cycle

Causes proliferative gill disease (hamburger disease) in catfish. Has an actinospore stage, released by oligochaete worm Dero digitata, and a myxospore stage which develops in the fish’s gills. Birds and fish can be definitive hosts, and are permanently infected.

Henneguya ictaluri – pathology, control

Significant gill damage occurs due to continous actinospore stage exposure. Chemical treatments for actinospores can be done (must be repeated). Empty ponds may use single batch or multibatch chemicals that are lethal to parasites, but also to fish (so require cooldown time). Buffalofish are sometimes used as biological control vectors.

Good species choices for new owners

Cyprinids (carps): hardy and active.

Poecilidae (live-birthers): fun, easily adaptable.

Anabantoids (gouramies): tolerable of many water conditions.

Biotope tank design

Recreates a specific niche natural setting, i.e. a shallow Amazonian stream. Generally, this is performed by more advanced hobbyists.