Text: Alan Bulmer Lead image: Grant Bittle (https://www.catchfishing.pro/)
Recently I was contacted by a good friend and asked if I could help explain a feeding phenomenon he had observed. It was a spring tide in Dorset and millions of helpless seaweed fly maggots were being washed into the ocean with every crashing wave. Bass were clearly visible gorging themselves on the maggots and it appeared that they were the only fish species taking advantage of the bounty. My friend did very well and ventured back the following evening to have another crack at the feeding bass. However, when he arrived he quickly discovered that, while the maggots were still there, the bass were not. They had been replaced almost completely by mullet. Why was this? Where did the bass go and why were there different species feeding exclusively on the seaweed fly maggots on consecutive evening tides?
I suggested that perhaps it was down to the fact that the bass had gorged themselves to the point where their stomachs were distended and were then unable to feed again until they had digested their prey. My friend didn’t think this was the most likely explanation as he believed that fish digestion typically only took 12 hours. I wasn’t convinced and set out to try and find more about how long it takes fish to digest prey and if this could affect feeding behaviour.
One of the things that was quickly apparent is that there is not a lot of information on fish digestion outside of farmed fish, probably because it is not an easy thing to measure accurately in wild fish.
The first useful article I discovered was a Food and Agriculture Organisation publication entitled “Digestion in Teleost fishes” (FAO Digestion in Teleost Fishes). This is a fascinating insight into digestion and it contains lots of useful information. For example, the gut length of herbivorous fish is generally much longer than carnivores and omnivores. This is believed to be because herbivores sometimes ingest a lot of indigestible material (e.g. mud) and need a longer gut to “separate the wheat from the chaff”. If the gut was small then they would get full and stop feeding before they were able to get enough essential nutrients from what they’d eaten.
The FAO publication also stated that for skipjack tuna gastric emptying time and total passage time at 23 – 26 degrees Celcius is typically about 12 hours with the intestine being maximally filled about five hours after eating and empty after 14 hours. This supported what my friend thought but bass are definitely not skipjack tuna.
Another key discovery was that digestion rate is influenced by the size of the meal and temperature. Large meals are typically digested faster on a unit weight basis than small meals. Digestion rate is also heavily influenced by temperature, especially when the water temperature was colder or warmer than the normal temperature range the fish encountered. This is because digestive enzyme activity is temperature dependent.
Temperature also influences several processes which are directly or indirectly related to food demand and feeding activity. Except for some representatives of the tuna family mentioned above, fish are exothermic animals that do not regulate their body temperature and body temperature fluctuates with that of the environment (with a difference of about 0.5 degrees Celcius).
What this means is that as the temperature cools fish slow down and feed less as a result of thermal dependent metabolism and its influence in two areas:
- Low temperature slows metabolism so the drive to feed (i.e. the drive to obtain energy) lessens.
- Muscle function (“twitch”) slows under cooler, less optimal conditions so fish cannot swim as fast. (Some species of tuna can however keep muscle temperature quite high with heat exchangers so they can swim fast in cool waters)”
Fish metabolism and muscle function slows over winter so the desire to feed and chase prey diminishes. This explains the reduced incidence of visible fish activity and the shorter windows where fish feed actively. What this means is that fish may still be present at favoured locations but they become more opportunistic in their feeding behaviour. Click on the following link to read more:- WINTER FISHING TIPS
Next I stumbled onto a paper from the Shirshov Institute of Oceanology in Russia entitled “Duration of gastric digestion in fishes” by V.B. Tseitlin. Tseitlin looked at a lot of the reported digestion data for warm and cold water fishes and used this to develop a mathematical formula to predict digestion time. For classification purposes he specified that warm water fish dwell at temperatures ranging from 15-30 degrees Celcius and cold water fish from 2-20 degrees Celcius.
What the formula showed for warm water fish is that if the water temperature was optimal (20 degrees Celcius) then digestion times ranged from 20 – 63 hours, depending on the size of the fish and how much food was eaten. See Graph One below. The formula makes sense as you’d expect most fish species to digest their prey more slowly than a fast moving predator like skipjack tuna and that it takes less time to digest smaller amounts of food.
GRAPH ONE: Digestion time for warm water fish (20 C)
However, if the water temperature dropped by 5 degrees (i.e. to 15 C) then digestion time for warm water fish increased from hours to days. See Graph Two below.
GRAPH TWO: Digestion time for warm water fish (15 C)
Things get even more interesting for warm water fish if the temperature drops 10 degrees Celcius below the optimal range (i.e. from 20 to 10 C) then digestion time increased from hours to weeks. This is a phenomenally slow time to digest food but remember the fish is living outside its optimal range and is unlikely to be very active. See Graph Three below.
GRAPH THREE: Digestion time for warm water fish (10 C)
Apparently the same holds true for cold water fish but in reverse. Increasing the temperature slows digestion rates as physiologically cold water fish are adapted to digest prey at lower temperatures, especially 10 degrees Celcius. At the optimal temperature (10 C) digestion times were identical but increase the temperature to 20 degrees Celcius and digestion times get very long as their enzymatic systems are not designed to operate at elevated temperatures which makes it difficult for them to digest prey.
The actual increase in digestion times for individual warm and cold water species has not been quantified and in reality the actual numbers are not that important. It is the observed increase in digestion time with changing temperature and the sheer magnitude of the increase that is. Digestion times go from hours into days and days turn into weeks the further the temperature moves from optimal.
If Tseitlin’s formula is correct then this may explain why the bass mentioned at the start of this article didn’t feed at the same time on two consecutive days. They were smaller fish and if the water temperature was slightly below optimal then it could take several days for them to completely digest a stomach completely full of maggots. Ditto for mullet. It may just be that the two groups of fish ended up taking turns to feast on the abundance of seaweed maggots.
There is no doubt that digestion time influences feeding behaviour. Tseitlin’s formula elegantly explains why many fish seem to feed heavily on a single day and then go torpid for a couple of days, especially in late summer or mid winter when water temperatures are sub optimal.
The other thing that needs to be taken into consideration with Tseitlin’s formula is that is a generalization. Not all fish are created equal and there is a marked difference in metabolic rate between individual fish within a particular species. This influences how often fish feed and also makes some fish harder to catch than others. The “fast” active fish need to feed more often than “slow” sedentary fish and feeding frequency will be influenced by how quickly they digest what has been eaten. Simon Blanford wrote an excellent article on the scientific experiments proving this for MidCurrent and I summarized it in the following AANZ piece:- DO FISH LEARN TO AVOID FLIES & LURES?
It is important to know the water temperatures in the areas you fish as it is very likely that it could affect digestion rates and feeding behaviour. For example, sea temperature in NZ typically ranges from 15 degrees Celsius in winter to 21 degrees Celsius in summer. Daily fluctuations are fairly minimal unless there is a significant drop in air temperature and cold wind to stir up the water surface. Fog can also drop surface water temperatures quickly and affect feeding behaviour. If you want to find out more click on this link:- UNDERSTANDING FOG AND HOW IT AFFECTS FISHING
If the water temperature does drop quickly, even if only by a degree Celsius or two, then it will slow digestion rates and the desire to feed.
Fish are keyed into taking advantage of prey availability. If there is a massive increase in the availability of food then those fish with the highest metabolic rates will feed hard out and continue to eat to the point where there stomachs are fully distended. They will not feed again until their prey has been significantly digested. This can take a long time if the temperatures are sub optimal. It does not mean that they will not take a lure of fly, just that the chances of them doing so aggressively are diminished. If you had phenomenal fishing at a particular spot and time don’t expect it to be the same the following day. Similarly, if nothing is happening a spot which often produces then it could just be that the fish there fed heavily in the 12 – 24 hours before you arrived and are preoccupied digesting the fruits of their labour.
River water temperature fluctuations in NZ are much more significant on a day to day basis and can range from 7 to 23 degrees Celsius between winter and summer. It is not unusual for river temperatures in small – medium sized rivers to drop by more than 3 degrees Celsius over a couple of days in some areas and with this drop comes an increase in digestion time. Fish that have been feeding steadily will quickly lose the desire to feed if the temperature drops by this much as it takes longer to digest their stomach contents.
Back in the day fly fishermen targeting trout and salmon, especially in Europe and USA, used to measure and record water temperature and use it to predict the hatch of the fly species that the fish fed on. I can remember discussing this with my father and his fishing friends as a child. Their biggest frustration was that “sometimes it is useful but often it is not”. This is probably because simply measuring water temperature fails to take into account the changes in fish digestion brought about by river water temperature fluctuations. The flies may hatch but if trout stomachs are full, and they are still digesting flies eaten during a major hatch in the preceding 24 – 48 hours, then there is no guarantee that they will opt to take advantage of the latest bounty. Some have suggested that flies rely on this to survive and that this is one of the reasons they hatch together in such large numbers, especially if the water temperature is dropping.
This was brought home to me twice recently when I was fishing for trout in a specific back eddy on the Tongariro river. On the first day I could do no wrong and hooked plenty of trout. On subsequent days I struggled to hook a fish, even when I could see them. On both occasions I started fishing as the river was dropping and clearing after a flood and I’m picking the trout were initially feasting hard out on dislodged caddis. Once they were satiated they stopped feeding to digest the nymphs. After the initial day I watched several big trout just sitting motionless in the shallows and even when I gently dropped a nymph well upstream and drifted it in front of them they did not react. Were these fish just too full and digesting food from their last feeding orgy?
In this case the best option was probably to keep on moving to other spots given I wasn’t catching anything. It may be possible to hook the occasional fish if you persist at a favoured spot or one that was “hot” yesterday but don’t expect to get the same results. It is hard to leave big fish that are clearly visible and not actively feeding but it is wise to do so. Return a couple of hours later. The situation may have changed.
The problem is that anglers can never know precisely how full the fish are and their digestion rates. The latter can be guessed but the former is a mystery. We may know they fed heavily yesterday if we were there but not if we weren’t. If they did eat a large quantity of food yesterday and the water temperature is sub-optimal then the likelihood of them feeding strongly again today is not high. The only way to work the patterns out is to spend time on the water and record everything observed in a fishing diary. Even then though, there is still no cast iron guarantee you can predict accurately what is going to happen. Isn’t not knowing what the day will bring one of the things about fishing that draws us back?
- Duration of gastric digestion in fishes. V.B. Tseitlin. Marine Ecology – Progress Series. Vol. 2: 277-280. 1980.
- Food & Agriculture Organisation – Chapter 1. Digestion in Teleost Fishes (see link above).