Native Aquatic Sampling Techniques
Seining is an active method of catching fish that uses a net connected by two poles. Seining often targets species that hang out in the water column. This technique also works well in stock tanks or isolated pools where a single seine haul can sample the entire area. The net has weights on the bottom to keep fish from escaping underneath and floats on the top to keep fish escaping over the top of the net. Seining typically occurs during daylight hours in waters that have a smooth bottom without large obstacles for the fish to hide in. The lengths and mesh sizes of seines vary.
A variation on this is called kick-seining and is done in swift moving water. The seine is set up downstream of the area to be sampled after it is in place with the lead line firmly on the ground a person upstream disturbs the substrate with their feet chasing fish downstream into the net.
Gill netting is a passive capture technique where a fish swims into the net and passes only partway through the mesh. When the fish struggles to free itself, the twine slips behind the gill cover and prevents escape. The Arizona Game and Fish standard for gill nets are 150 feet long and 6 feet tall. The net has six panels with increasing mesh sizes starting with ½ inch to 3 inches. The top end of the net is called a float line and has rope that is buoyant to keep the net vertical in the water column. The bottom of the net is called the lead line and has weights along the line to keep the bottom of the net on the lake bottom or suspended towards the lake bottom. Gill nets are typically set during daylight hours, fished overnight, and retrieved the following day. In most cases gill nets are set at the bottom and oriented perpendicular to the shore. These nets target fish that can be found along the shoreline. Some gill nets are set float ing along the surface with buoys along the top of the float line. In these cases floating nets target fish that swim in the open water like gizzard shad.
Hoop netting consists of different size hoop nets that have an opening on at least one side to allow fish or other aquatic species to enter. Hoop nets are usually baited with dog food or fish food. Nets are generally set for a minimum of two hours to overnight. If set in an area with sensitive species an air pocket is left for any air breathing aquatic species enter the net. Hoop nets vary in mesh size and are used to sample deeper water, large pools and for cryptic species. In systems were there are large numbers of non-native crayfish longer set times can cause detrimental interactions between natives and crayfish.
Another active method electrofishing uses of electric fields in water to capture or control fish is a valuable sampling technique used by the Arizona Game and Fish Department. Electricity is passed through the water, by two types of electrodes, the anode (positive) and the cathode (negative). When applying the correct current, fish will be stunned for a few seconds, netted, and put into a holding tank or bucket for observation. The Department uses three different methods of electro-fishing equipment: boat, canoe/raft, and backpack.
The backpack unit is the smallest of the three types of electrofishing devices and is used primarily in wadeable streams. Similar to the other electrofishing units, the backpack electroshocker has both an anode and cathode. The anode is a probe with a control switch that is placed in the water once the unit is turned. The cathode is either another probe or a cable that drags behind the person operating the shocker. Sites selected must begin at designated coordinates and end at a predetermine distance. Each sample site within the stream is traversed moving upstream using a sweeping motion and alternating back and forth to cover both sides of the stream. Typically two other individuals accompany the person shocking to net and hold fish for measurements.
A canoe electroshocker is used primarily for non-wadeable rivers and streams. The canoe or raft is fixed with metal plates or cables (cathode) on each side and a metal sphere (anode) that drops off the front of the canoe into the water. Similar to the boat eletrofisher a generator and control box produce and control the electricity that enters the water. Shocking sites are conducted downstream during the day alternating from bank to bank throughout a site. The person in front of canoe or raft operates a foot pedal that puts the electricity into the water and collects fish with a dip net. The person in the back steers the canoe or raft along the selected site collecting any fish missed by the person in the front. Additionally a chase canoe or raft can follow closely behind capturing additional missed fish.
Boat electro-fishing is conducted primarily on reservoirs or larger still standing waters. The electro-fishing boat is rigged with two booms which act at anodes, extending out from the front of the boat with arrays or a metal ball that is lowered into the water. A generator is used for the power source and a control box is needed to take AC power from generator to electrodes. Two netters stand on the bow of the boat. One operates the trolling motor and the other operates a foot pedal that applies electricity to the water. Typically electro-fishing is conducted at night as fish tend to move into the shallows making them easier to target.
There are many standardized techniques used to survey fish while snorkeling. The most commonly used one for The Department is transect counts. One or two people swim upstream side by side and count the number of each fish species seen for a predetermined length. This technique works well for systems that have high water clarity and low conductivity which may make other techniques less effective. This technique is also frequently used for detection and removal of non-native fish.
This method works well for fish that hang out near the top of the water column such as spinedace or topminnow and for frogs and tadpoles. A dip net is swept through a short length of water where the target species has been visually observed, usually this is done quickly.