Plastic baits will usually pass through both an intestinal system and a true stomach of any fish large enough to ingest them.
A U-shaped stomach with a slightly enlarged cavity, as in salmon and trout;
A Y-shaped stomach (imagine a Y on its side) as found in eels;
No stomach, as is the case of cyprinids.
The size and shape of the stomach relates to the type of food items that the species eats, so fish that feed upon large prey items have tubular cavities, known as lumens, capable of holding large food items and stretching without causing disturbance to other organs. Species that primarily feed on small items tend to have no true stomach, as they do not need to store food. Instead, they possess a very long intestine, designed to process food items as they pass through it.
Digestion usually starts in the buccal (throat) cavity, where food items may be crushed by the pharyngeal teeth and coated
But what of plastic and hooks? Both are non-biodegradable and tend to pass through the digestive tract intact, modified slightly perhaps by chewing and acid/enzyme attack, but largely untouched.
The presence of nonbiodegradable waste, particularly plastic, in the aquatic environment is big news. David Attenborough’s Blue Planet II alerted the wider public to the extent of the problem, and when you realise that at current rates of disposal there will be more plastic in our oceans than fish by 2050, you begin to appreciate the scale we are dealing with.
It’s not just the marine environment, a recent Angler’s Mail news story (March 27, 2018) highlighted the fact that Manchester’s River Tame was found to contain over 500,000 particles of microplastic per square metre – the highest concentration ever found. All our rivers are contaminated by microplastics and, ultimately, they all drain into the sea and affect every creature in the food chain, from plankton to apex predators.
The size and texture of artificial baits is such that they will usually pass through both an intestinal system and a true stomach of any fish large enough to ingest them in the first instance. Indeed, look at the size and texture of some items that a carp will excrete whole, such as tiger nuts or crayfish claws, and you realise that a small artificial bait is unlikely to cause a problem.
But it is not as simple as that. Small plastic baits may cause problems with smaller species, we don’t yet know the fate of microplastics, and there is a likely chemical influence, too. What we do know is that studies of fish in the North Sea in 2014 found no relation between the condition factor of the fish and the presence of ingested plastic particles, but it’s early days.
Dealing with deep hooks
In respect of hooks, laboratory experiments on the impact of deep hook removal from an oesophagus showed that fish were often able to discard a hook after a time, rather than it being dissolved away. In terms of deep hook management, fish in which the line was cut and the hook left in place tended to show a much lower mortality rate than fish that had been unhooked before return.