Research theme: Netting methods

Netting is the main method used to catch fish in lakes and estuaries. If we are interested in a particular fish species and want to know more about it, we can target certain areas when netting. For example:

• If we need to catch brown trout, we set nets in open water (pelagic zone) or on the surface of the lake, where they may be feeding 
• If we wish to find tench, we set nets at the bottom of the lake
• And for pike, the weeded margins

Often, we simply want to learn about the whole fish population in a lake or estuary, so we set a variety of nets in different areas, and at a range of depths. By covering a variety of habitats within the lake or estuary, we should find a good mix of fish.

Fish captured in the nets are removed to be measured and weighed. Netting requires personnel to go out on the water on a boat; therefore, health and safety measures are very important. It is crucially important that the necessary health and safety equipment is worn and that all equipment is in good working condition. Furthermore, staff should be trained so that they are aware of any possible risks, for example, high winds and tides.

Netting should be linked to an understanding of the life-cycle of the target species.  In most circumstances, sampling should be done at the end of the growing season when young fish are large enough to be identified and are strong enough to withstand the stress of capture.

Disinfection of equipment is necessary after each survey to prevent the spread of alien invasive species and other organisms to uninfected waters. Read more about Biosecurity here.

Methods

At Inland Fisheries Ireland, we use a number of different netting methods. See examples below:

• Monofilament gill nets: European standard gill nets with panels ranging in mesh size to catch different sized fish. These can be set on the bottom of the lake or left floating on the surface. 
• Braided gill nets: Made up of random panels with larger mesh sizes to catch bigger fish. These can be set on the bottom of the lake or left floating on the surface. 
• Fyke nets: Long conical or tubular nets used to catch bottom dwelling fish in lakes and estuaries. They are usually set close to the water’s edge.
• Beach seine nets: Long nets used to catch fish in the shallows of estuaries. This net is held on the bank at one end and fed out of the back of a boat at the other. The resulting loop is pulled into the shore and a bag in the middle captures the fish.
• Beam trawl: Used to sample fish at the bottom of deep open waters. The net is pulled along the bottom of the bed using a boat and scoops up any fish.
• Coghill nets: Consisting of "cone-shaped" chambers, one inside the other, which only allow the fish to move inwards.These nets are usually attached to fixed structures in flowing water, for example, at "eel weirs".

After fishing, we “process” our catch. Here, we typically identify what species it is, and we take various measurements from each fish, such as its length, weight and a scale sample for ageing back in the lab. Fish are returned the lake or estuary afterwards.  Live fish are returned to the water whenever practical or when the likelihood of their survival is considered to be good.  Samples of fish are retained for further analysis.  Fish are frozen immediately after the survey and transported back to the IFI laboratory for later dissection.

What We Learn

Netting surveys give us lots of useful information about fish. If we catch different species, we learn about “biodiversity” or “species richness”. This is a measure of the number of different species present and, usually, the more the merrier! Unfortunately, however, in Ireland’s lakes, we have lots of introduced species, so this is not always a very good thing.

The number of individuals of any one species captured tells us about “abundance”. High numbers of individual species can indicate to us that their populations are fit and healthy. This can also be used to gauge the overall health of a system when certain, more sensitive (to pollution) species are present, such as the Atlantic salmon or brown trout. If there are more pollution-tolerant species present, including many of the coarse fish species, or if one of these species dominates, it could indicate impacts on the system caused by pollution or habitat degradation.

The size and age of fish also gives us interesting information. If we catch lots of brown trout, and they range in size from small to large, we can say something about the health of the whole population. Fish grow very predictably with age, and in general, the bigger the fish, the older it is. A range of sizes tells us that there are a number of age groups present and that the population has good “recruitment”, or is replenishing itself well for the future.

The size of a fish at a certain age in one natural unpolluted lake or estuary can be different to a fish of the same age in another. This happens because some waterbodies are more “productive” than others. A productive lake has a better food supply, usually because there is a better supply of nutrients at the bottom of the food chain, which results in more organisms and food for fish to eat at the top. Other lakes are naturally unproductive and have smaller fish, with slower growth.  Unfortunately, nutrients can also come from human activities and favour certain fish species, or even none at all.

A more reliable way to age a fish than simply measuring its length is to examine its scales. Fish ageing using scales or other parts of a fish’s body (for example, a gill cover bone or an otolith) helps us to determine the age of a fish in years and gives us a more reliable picture of the whole population. More information of fish ageing techniques can be seen here.

Beach seining involves deploying a long fine-meshed net to surround fish living in the shallow marginal (littoral) areas. This is usually close to the beach and areas mostly influenced by tide. Here, we catch lots of young thick-lipped grey mullet and sea bass, as well as large numbers of sprat and gobies.

Fyke nets are long cylindrical or cone-shaped nets which are set along the bottom and close to the shore. These nets capture bottom-dwelling (benthic) fish, such as young cod, eels, flounder and rockling.

Beam trawls are bag-shaped nets, deployed from a boat and pulled along the bed for a short distance. They scoop up bottom-dwelling fish, typically in deeper, open water areas where the bed type is suitable. Flat fishes, such as dab, flounder and young plaice, are commonly recorded using this method.

Watch our video about lake monitoring by the National Research Survey Programme below: