I used to go fishing in Vancouver with my dad until just a few years ago we found the fish full of cancers. Purpose and effect on the audience?

Don't eat the fish :)

Explanation:

Fish with disease/ Cancer can show a variety of signs. If you notice your pet fish having any unusual disease signs, contact your veterinarian for further advice. These are some of the most common signs of disease seen in pet fish:

Appearing disoriented, such as swimming upside down

Leaving food uneaten

White spots on fins or body

Discolored gills

Trouble breathing such as gasping at surface of water

Bulging eyes (one or both)

Mucus accumulation on the body

Rubbing on hard surfaces

Unusual isolation from the group of fish (if housed with others)

Skin lesions/sores anywhere on body

Bloating

Crooked back or other changes in fish shape or size

Many of the following signs found above are commonly associated with a variety of diseases found in fish. Below are some common diseases found in pet fish:

Physical injury

Parasites such as white spot disease or “Ick”, nematodes, anchor worms, fish lice, gill maggots, monogenean flukes

Bacterial infections such as columnaris disease and mycobacterial infections

Fungal infections such as Saprolegnia, Branchiomyces, and Fusarium.

Finrot or ulcer disease: rotting of the fins caused by bacterial or fungal infections

Hole-in-the-head: holes in the front or side of a fish’s head. The cause for this disease is still unknown but several factors are believed to play a role

Fish pox: caused by a fish herpes virus resulting in gray, pink or white wart-like growths (common in Koi fish)

Velvet or coral fish disease: a parasitic disease that  results in dusty and slimy scales (common in tropical fish)

Ammonia or chlorine poisoning resulting from poor water quality