Fish Sampling

SAMPLING SITES
The same sampling sites are used each year. All sampling occurs between the 3rd week of July and Labor Day. Lakes are sampled in the following order: Trout, Allequash, Crystal, Big Muskellunge, Sparkling, Crystal Bog, Trout Bog, Wingra, Fish, Monona, Mendota. Sites for fyke nets, trammel nets and night seining sites were chosen by random process in 1981 for the Northern Highland State Forest lakes (Trout, Big Muskellunge, Allequash, Crystal, Sparkling). Sites for Lake Mendota were chosen in 1981, and for the other Madison lakes (Monona, Fish, and Wingra) in 1995. In 1998, all the Northern Highland lake sampling sites were recorded and archived as GPS coordinates. In 1999, all the Madison lake sites were recorded and archived as GPS sites. Prior to 1998 and 1999, fyke and trammel net sites were found each year by reference to lake maps, local landmarks, and stake locations. Gill nets are placed near the deep-hole, which is marked by a buoy and GPS coordinates, on each lake. In the bog lakes (Trout Bog and Crystal Bog), which are sampled with only fyke nets and minnow traps, there are no fixed sites; nets are placed equal distances apart around the entire circumference of the lake in approximately the same locations each year.
 
NIGHT SEINING
Night seining is conducted to achieve relative abundances of small fish species such as minnows, darters, sculpin, and young gamefish species on a yearly basis. Seining is most effective on small fish at night, due to reduced net avoidance, and is one of the most effective methods of catching small fish species.
The seine used is 12.2 m long by 1.2 m deep, consisting of two 5.5 x 1.2 m wings surrounding a 1.2 x 1.2 x 1.2 m central bag. The wings are made of 6.4 mm stretch measure knitted delta-strength nylon mesh, and the bag is of 3.2 mm delta strength nylon mesh. The entire net is tarred. The two wings and the opening to the bag have weighted foot ropes and buoyed head ropes. The seine is pulled via two PVC or steel poles on either end of the seine.
Prior to 1997, seine sites consisted of 100 meters of shoreline. This was subdivided into 3 seine hauls, each covering 33 meters. Stakes were placed to mark the beginning and end of each haul, with the first stake lettered A and the fourth (final) stake lettered D. In 1997, seine hauls were reduced to 2 hauls of 33m each. The final 34m of the 100m site (stakes C-D) is now used as an alternate seine site in the event of difficulty (snag, twisted net) in one of the first two hauls. Our convention is that the first haul (identified as "site #-1") is the one segment at the left end of the site, as one faces the site from the lake. The day crew working the lake will have marked the location of these stakes using green 12-hour chemical light sticks. There are 6 seine sites per lake for a total of 18 hauls per lake prior to 1997; starting in 1997, there are 12 hauls per lake.
The seine crew approaches the site from the lake by boat in such a way as not to pass over the area to be seined. The seine is deployed using as little light as possible. An 8m length of rope is tied between the poles as a guide for the maximum spread of the seine. Two people, working 8 meters apart when possible, pull the seine on a course parallel to the shore line. The outside or deep person should be 8m from the shallow person (max rope length) or as deep as they can be without overtopping their waders (just below chest height). The inside or shallow person keeps as close to shore as possible without steeping onto dry land. When the shallow person is about 8 meters from the end of the haul he/she moves very slowly, allowing the deep person to swing around toward the chemical light stick; both seiners should reach the light at the same time. The seine is quickly landed by crossing the poles and drawing the lead line together. The lead line is kept on the lake bottom while the wings are drawn in. When the bag reaches the poles it is picked up by the 4 corners. Fish are collected from the bag and processed before the crew goes on to the next haul.
 
TRAMMEL NET
The trammel net is used to sample fish species present near the bottom at the thermocline/substrate interface. This area is utilized by a number of fish species, and is an important area of the lake due to the large change in temperature in a relatively short distance. As in the terrestrial environment, the thermocline acts as an ecotone and several fish species which require very different physical environments may exist in relatively close proximity. So achieving yearly fish abundances in this habitat is also important in determining long term trends in fish abundances.
The trammel net used is 30.5 m long and 1.1 m deep. It consists of two outer nets of 170 mm square 32 kg test mesh multifilament nylon with an inner panel of 51 mm stretch mesh 9 kg test multifilament nylon. The three nets are connected at the leaded foot line and the buoyed head rope.
The trammel net is set on the bottom, along a line perpendicular to the shoreline and crossing the thermocline. This can generally be accomplished by setting the shallow end in about 3 meters of water, and running the net out perpendicular to shore. Fish are picked out of the trammel net as it is brought back into the boat. The trammel net is set by the day crew at two sites in each lake, and fished for approximately 24 hours at each site.
 
FYKE NETS
Fyke netting is a very common method of sampling a wide size range of fishes which use littoral zone habitat. At different times of the day and/or season, many different fish species utilize the littoral zone area for feeding, digesting, and mating purposes. Sampling the abundances of fish species in this area, thus, is also very important in determining yearly changes in fish abundances. To monitor yearly changes in littoral fish abundances, fyke nets are deployed at six sampling sites in all 11 LTER study lakes.
A separate set of three fyke nets of similar dimensions are used for the Northern Highland lakes and the Madison lakes. For the Northern Higland lakes, each fyke net is approximately 12 m long and consists of two rectangular steel frames 90 cm wide by 75 cm high and 4 steel hoops, all covered by 7 mm delta stretch mesh nylon netting. An 8 m long by 1.25 m deep leader net made of 7 mm delta stretch mesh nylon netting is attached to a center bar of the first rectangular frame (net mouth). The second rectangular frame has two 10 cm wide by 70 cm high openings, one on each side of the frame's center bar. The four hoops follow the second frame. Throats 10 cm in diameter are located between the second and third hoops. The net ends in a bag with a 20.4 cm opening at the end, which is tied shut while the net is fishing. New nets of the same dimensions were purchased for the Northern Highland lakes in 2000. Fyke nets for the Madison lakes are 10 m long (including lead) with 1 rectangular aluminum frame followed by 2 aluminum hoops. The aluminum frame has the dimensions 98 cm wide x 82 cm tall, and is constructed of 2.5 cm tubing, with an additional center vertical bar. The hoops are 60 cm in diameter and constructed of 5 mm diameter aluminum rod. The single net funnel is between the first and second hoops and is 20 cm in diameter. The lead is 8 m long and 1.25m deep, constructed from 7mm delta stretch mesh.
Each fyke net is set in shallow water perpendicular to shore such that the net mouth is covered by about 1 meter of water when possible. When the net is properly set, the lead is perpendicular to shore, vertical and not twisted, the mouth of the net is upright and facing shore, and all the hoops are upright. When the net is pulled in, the hoops and frames are gathered together and lifted into the boat. The net is positioned over a live well with the net mouth upward. One frame at a time is lifted and any fish present are shaken down into the next chamber, until all the fish are in the bag, which is emptied into the live well. Three fyke net sites are set per day (for two days), each with a single net in the middle of a 100m site, for a total of 6 fyke net sites per lake.
Due to the soft bottom, and small size of the bog lakes, minnow traps and fyke nets are the only gear used to sample the fish community of these systems. The fyke nets are suspended by placing floats at the apex of each hoop, and on the top of the opening frames. This is done to prevent the nets from sinking into the soft sediments at the bottom of the bogs.
 
CRAYFISH AND MINNOW TRAPS
There have been introductions of exotic crayfish species in recent years into many north temperate lakes. Monitoring yearly abundances of crayfish species is important in determining the status and extent of the invasions.
Crayfish traps are set on all lakes except the bog lakes (Crystal Bog and Trout Bog). Minnow traps are set only on the bog lakes. Prior to 1998, five traps were set at each fyke net site. Starting in 1998, three traps are set per site. Thus, prior to 1998, thirty traps were set on each lake (covering 6 sites.) As of 1998, 18 traps are set on each lake.
Minnow traps and crayfish traps are set in shallow water (approx 1 m), 2 traps on one side, and 1 trap on the other side of the fyke net lead. Minnow traps are baited with 1 slice of bread per trap to attract minnows inhabiting the bogs. Crayfish traps are baited with 120 g of liver. Traps are fished for approximately 24 hours . Crayfish are identified to species and returned to the lake. Minnows caught in either the crayfish or minnow traps are identified to species, measured for total length.
Minnow traps used are galvanized steel two piece traps, 44.5 cm long by 30.5 cm maximum diameter with 2.5 cm diameter openings at the ends. The mesh size is 6.4 mm on a side. Crayfish traps are identical, but the opening hole of both sides of the trap has been forced to 5 to 7 cm.
 
GILL NETS
In most lakes, there are species of fish which inhabit the pelagic (open water) zone. These fish species can have a large impact on lake ecosystem dynamics when they occur in abundance. To monitor yearly changes in the abundance of pelagic fish species, we sample the deep basin of eight of the LTER lakes with vertical gill nets.
Our gill nets are a set of 7 nets, each in a different mesh size, hung vertically from foam rollers, and chained together in a line. Each net is 4 m wide and 33 m long. From 1981 through 1990 the nets were multifilament mesh, in stretched mesh sizes of 19, 25, 32, 38, 51, 64, and 89 mm. In 1991, the multifilament nets were replaced with monofilament nets of the same sizes. One side of the net is marked in meters from top to bottom. Stretcher bars have been installed at 5 meter intervals from the bottom to keep the net as rectangular as possible when deployed. The bottom end is weighted with a lead pipe to quicken the placement of the net and to maintain the position of the net on the bottom.
Gill nets are set at the deepest point of all LTER lakes except Crystal Bog, Trout Bog, and Fish Lake. The nets are set for two consecutive 24 hour sets. The nets are set in a straight line, each connected to the next, and anchored at each end of the line. Once the nets are in position, they are unrolled until the bottom end reaches the bottom, and then tied off to prevent further unrolling. The nets are pulled by placing each net onto a pair of brackets attached to the side of the boat and rolling the net back onto its float; the fish are picked out as the net is brought up, placed in tubs according to depth. The fish are processed when the net is completely rolled up and before it is redeployed.
 
ELECTROFISHING
We use a boom style electrofishing system to sample the littoral zone fish community. Prior to 1997, four electrofishing transects were done on each lake. In 1997, the number of transects was reduced to 3. The same transects are used each year. Each transect consists of 30 minutes of current output, with the boat moving parallel to shore in 1-2 meters of water at a slow steady speed. We use the DC pulse system, with 240 volts at 3-5 amps. Two crew members in the bow of the boat dip up all stunned fish, placing them in the live well for processing at the end of each transect. Transect lengths vary depending upon the size of the lake. If the end of a transect is reached before 30 minutes has elpased, time is paused while the electrofisher loops back to the start of the transect. The transect is then repeated for the remaining time. In 1999, dip nets were standardized to 10 foot poles attached to 18in. x 20in. tear drop shaped hoops. The nets are made of 7 mm stretch mesh.

PROCESSING THE CATCH
For all collecting methods, the fish are processed as follows. Each individual fish is identified to species. If it cannot be positively identified, after it is processed, it is preserved in 10% buffered formalin or 95% ethanol for later identification. The total length of the fish (measured from nose to end of caudal fins pinched together) is measured in mm. Prior to 1997, the weight (g) of the first 5 fish of each species in each 10 mm size category was also measured, using the appropriate Pesola spring balance (fish weight registering in the middle range of scale). A tally sheet was used to record how many fish in each size category had been measured. Starting in 1997, 2 fish are weighed for each fish species in each 5mm size category. Also in 1997, data recording switched to an electronic system which tallied measured fish. For yellow perch, rock bass, and cisco, a scale sample is collected from each weighed fish. This is removed from the left side of the fish, above the lateral line and below the origin of the dorsal fin. Scale samples are stored in scale envelopes and labeled with a unique ID number, the date the scale was taken, a lake ID number, the species code, land length and weight. For gill net catches, the depth at which each individual is caught is also recorded.
Fish from all gear (except gillnets) are held in live wells during processing. Fish are sorted by species into buckets, processed as quickly as possible, and returned to the lake. Fish from the gillnets are very rarely alive. If alive, they are usually badly damaged when the nets are raised.
 
PROTOCOL CHANGES
 
  • 1983     Discontinued fykenets and trammel nets on Lake Mendota until 1995
  • 1984     Discontinued crayfish on Lake Mendota until 1995. Only gillnet and seines on Lake Mendota.
  • 1995     Resumed sampling Lake Mendota with full suite of sampling gearr
  • 1995     Began sampling Lakes Wingra, Monona, and Fish
  • 1997     Two fish are weighed for each fish species in each 5mm size category. Previously, five fish were weighed for each fish species in each 10mm size category
  • 1997     Data recording switched from manual field sheets to an electronic system
  • 1997     Changed from 4 to 3 electrofishing runs per lake
  • 1997     Changed from 18 to 12 seine hauls per lake
  • 1998     Changed from 30 to 18 crayfish / minnow traps per lake
  • 2004     Discontinued crayfish / minnow traps on southern lakes
Protocol type: 
field
Protocol format: 
Process