Barometric Pressure vs Fish Action

Author: JayPenfold

Category: Technique & strategies

Created on Monday, 24 October 2011 17:42

Written by Jay Penfold

Recently I have been trying to key in important bite triggers to make the most of what is often limited kayak fishing time, trying to determine among other factors of tide, moonphase and my own mood, whether a session out is warranted or not. Since raising the question of 'Barometric Pressure vs fish activity' in a recent forum post, I have started to look at the effect of barometric pressure upon the feeding activity of fish in general.  It's turned what I believed to be true on it's head and given me some vital clues as to why some days are better than others. It all comes down to the gravity like effect of air pressure and the building blocks of marine ecosystems. It's a domino effect and one that after listening to many anecdotes, I have found some science to give some idea as to what's going on.

Everything that you will find in any given body of water will either sink, float or suspend, ie. sit neutrally buoyant. Anything that finds itself in the water column weighs less underwater and is affected more by air pressure than objects found out of the water.

Air pressure is like gravity. Higher pressures are like stronger gravitational fields, Lower pressures are like walking on the moon. As the air pressure rises, it's like a weight descends on the water column.  As the air pressure drops, it's like gravity lifts it's heavy hands.  Low pressure is associated with cold, unstable weather. High air pressure is associated with warm, stable conditions. Predicting which way the pressure will go isn't always as simple as looking at a chart and watching the MSLP gradient bars on your weather chart. Some folks (the older arthritic types such as myself) claim to be able to feel it in their joints. A rapid fall may cause fluid to move in the joints an induce some sensation. Air pressure can be extremely localised depending on your geography and microclimate.  One thing that is for sure is that ahead of a cold front or a thundestorm is that the pressure will drop, and the gradient will be steep.

Let's first look at what happens during a sudden fall in air pressure, say a cold front approaching or a Summer thunderstorm rolling across the land. Most of the bigger fishes are superbly well equipped to deal with these changes, even sudden ones, inflating or deflating their swim bladders to accomodate the shift in air pressure. On the other hand, the tiny zooplankton that the baitfish feed on are far less able to correct for the rapid fall of air pressure with their swim bladders and become unstable. Phytoplankton rely on light to create food (much like any green land plant) so rise higher in the water column to get access to better light as the water visibility (and therefore light penetration) drops.

It is often possible to see water quality alter, to become darker and the visibility drop ahead of a pressure drop as sediment and micro organisms start to rise from the bottom, released from the weight of the air pressure. This is a key for the baitfish and invertibrates to move in, feed heavily and in numbers. Where the bait is, predators will naturally follow. Predatory fish take advantage of the sudden increase of activity and feed agressively. This is a time to match the hatch with which ever baitfish species are prevailing at the moment and mix up your retrieves/trolling strategy to a more agressive one.

Now let's examine what happens when the pressure is steady. Normal 'stable' fair weather air pressure is considered to be around the 1016 millibar mark. At this level, when the barometer is steady, everything finds an equilibrium, just like walking on land. All of the swim bladders of the resident critters are functioning normally, sediments have settled or washed into stable positions with the tide. Usually the sun is shining, it's a nice day to be out and the fishing is steady.  When other factors like tide and moon phase are taken into account, activity might not be 'hot' but the fishing is likely to be ok.

When the barometer starts to rise rapidly, the pressure (therefore the 'gravitational weight') increases forcing anything light to the bottom.  Larger fishes will seek out deeper holes or deeper parts of the estuary geography as the increased 'weight' puts pressure on the swim baladder, chasing the baitfishes that persue the tiny building blocks of the ecosystem down with the rising pressure. Bite activity may slow, targets are better sought in deeper water at this time. It has been widely observed that bite/feeding activity heightens during a gradually rising barometer.

Tools for the job? Up until recently, I had a fantastic old-school brass barometer on my wall, passed down from my late grandfather, until one of my children destroyed this wonderous antiquity. Now I rely upon the Bureau of Meteorolgy website which has this excellent barometric trend chart for a weather station nearby.  There are many excellent digital weather stations available online for the same cost as a traditional style one. There are also small hand held digital and analogue versions available that fit in a pocket and some even rated to IPX4 water resistance designed for fishermen. I have a Weatherzone.com app installed on my android smartphone that at least gives me the latest barometric observation which I can check at regular intervals to keep an eye on conditions.

Of note also, fishes residing in shallow water columns are more succeptable to this phenomenon than those living in deep ones. For example, a Flathead in a couple of meters of water is more succeptible to this than a Mulloway under 50m of water.

The bar dropping isn't always a good thing however, with the bonuses already discussed comes some cons. A cold front approaching may affect the watery way by delivering dropping water temperatures, some sloppy water via wave action and of course once that thunderstorm does arrive, a wet backside and the chance of a lightning strike through a graphite rod!