Fish Slime

Every time I land a fish, my mind and body go through a wave of emotions:

SHOCK - “Wow, do I really have a fish on? Is it just a snag or can I begin the battle to the net?”

EXCITEMENT - “I wonder what kind it is? How big it is?”

FEAR - “I hope I am not hurting it. How can I catch and release this fish as quickly and safely as possible?”

I have always been told that my mind over-analyzes my actions, however in this case, I think it serves as a great reminder.  A reminder to every angler that how we handle our fish can severely influence their life after.  We have all felt it. The soft and slick body of a trout as we try to release its fragile jaw from the fly. Have you ever questioned what that slickness really is? Or does?

Fish mucus, often referred to as “fish slime,” is produced in the goblet cells of the epidermis layer of the fish (outermost layer of skin).  Humans also produce mucous, however, ours serves a different function in the body. Human mucous is produced in the goblet cells of the mucous membranes that line our respiratory, intestinal, urinary and reproductive passages. The coating serves as a lubricant for the passage of materials, while also keeping the areas healthy by maintaining a moist environment. As in humans, the mucus created by fish goblet cells also serves to maintain conditions, however, it is present externally on the surface of the fish.

The mucus, present in all fish, is a combination of compounds that aid in protecting the fish, much like our skin protects us. Each community of fish has its own chemical make-up within the mucus, because factors that are produced are specific to the species and environment in which it lives. The mucus is a mechanical defense against parasites and pathogens, blocking them from entering the body. There have also been studies that show osmoregulatory and locomotive functions (Van Oosten 1957, Rosen and Cornford 1971). Osmoregulation is crucial for fish to maintain osmotic pressure within their cells while in the water. By regulating the concentrations of ions flowing in and out of the body, the fish can keep a correct balance between its internal concentrations and the concentrations found in the water column. Without the regulation, these ions would accumulate and become toxic for organs such as the kidneys and their skin. This mucus allows proper release and uptake of ions while also providing a barrier for unwanted molecules. Because the chemical make-up of the mucus varies along genetic and environmental conditions, research is very interested in looking at specific compounds that aid in protecting some of the most economically important species, such as the rainbow trout.

One study took mucus from the coat of rainbow trout and isolated the compounds to see what protective properties it holds. They found that along with an overall mechanical defense, the mucus specifically contained the proteins lysozyme and protease ,which function to destroy gram-positive bacteria and clear any suspension of gram-negative bacteria, respectively. (Hjelmeland et al 1983). Because the mucus contains a high diversity of compounds, it is able to specifically target and destroy invaders, keeping the skin of the fish free from infection and disease.

As an active scientist, I question how these natural defenses may be able to serve in a broader purpose. Sunscreen is a compound that is necessary for humans to protect our skin from UV-A and UV-B rays from the sun. I highly recommend the use of sunscreen, however, it is also important to note that many of the sunscreens we use contain chemicals that are harmful to aquatic ecosystems.  Science is making strides to compensate for this.  One study showed that when combining fish mucus with a compound found in crustacean shells they were able to produce a “natural sunscreen” that doesn’t cause damage to our aquatic ecosystems - marine reefs in this scenario. They found that in conjunction, the resulting material was photoresistant (sun did not damage it), thermoresistant (heat/cold did not damage it), and that it had a high absorption of both UV-A and UV-B radiations (Fernandes et al 2015).

There is another substance that is present in water bodies with predator fish species that could potentially be mistaken for this mucus. Fish slicks are used by seasoned anglers as a way to locate where predatory fish (for example speckled trout or redfish)  have been feeding. Because both the mucus and slicks are compounds of “oily” nature, meaning they don’t mix well with water, it is good to note that the shiny surface you are seeing is not the result of mucus. Fish slicks can be created by two ways: 1) the predator fish eats smaller fish and “burps” up digested prey in the form of oil or 2) as the fish are feeding, small bits are missed and are left in the water column. In either scenario, the oil bands together (because of their hydrophobic, “water hating” nature) and floats to the surface (because of density differences). Some experienced anglers can actually smell the slicks and have noted scents of watermelon or fresh cut grass.

So why should this matter to people who fish?

We want to keep our fish populations healthy and sustaining so that we can continue enjoying the sport. Any small acts that we can do as stewards of the land to maintain populations will help our natural resources team and will diversify our populations in the long run.

Some ways we can alter our fishing habits to protect fish:

It’s OK to photograph your fish, but catch and release as QUICKLY as possible.

Use a rod size that matches the size of fish you expect to catch.

Use larger tippets for landing fish faster.

Try not to drag the fish up on shore. Have a net with a small mesh or rubber basket.

Use barbless hooks.

Wet your hands before handling fish so that you don’t strip the oils and slime.

Keep the fish in the water as much as you can with it facing upstream to increase oxygenated water flow over the  gills.

When holding fish, do not squeeze and keep your fingers away from the gills.