Fish are an important part of the biota of most aquatic systems. However, their mobility often makes it difficult to estimate the sizes of natural populations. This exercise will familiarize you with some local lentic and lotic fishes, and introduce to you a method used commonly to evaluate and compare the sizes of fish populations.

The relative abundances (in terms of both numbers and biomass) of planktivores, benthivores, and omnivores will be greater than that of piscivores in Lake Winona and the Mississippi River during autumn.

During autumn, many fishes prefer the shallower, nearshore habitats over the open water areas. Fish inhabiting these shallower portions of Lake Winona and the Mississippi River will be collected, and abundances of the various species will be compared.

Fish will be collected with a boat-mounted electrofisher. Electrofisher operation and safety will be discussed prior to beginning collecting. The electrofisher creates an electrical field in the water around the bow of the boat, and fish within the field are stunned temporarily and can be netted. The smaller fish may be less affected by the electrical current than the larger fish, and may be more difficult to capture.

The most accurate method of determining the size of a fish population would involve catching, or in some way counting, the entire population. However, this is usually either impossible to carry out in practice or, at least, destructive to the population. Instead, either a mark-and-recapture procedure or a method that makes use of a catch-per-unit-effort measure is used frequently.

The mark-and-recapture technique is based upon the premise that recognizable (marked) organisms released to the population will later be recaptured in numbers proportional to their abundance in that population. The size of the natural population can be estimated from the proportion of marked to unmarked organisms in random samples obtained from the entire population (Remember that fun General Ecology lab with brown trout?).

Any method that makes use of the catch-per-effort (CPE) measure will not provide the same information as the mark-and-recapture method. However, CPE, usually expressed in terms of fish collected per time spent collecting, can be used as a measure of the relative abundances of different species of fish within the same system, or of the same species between systems.

In Lake Winona and the Mississippi River, fish will be collected from nearshore habitats for a known period of time (the electrofisher has a timer to monitor the electrofishing period). All fish captured will be identified, counted, and weighed (wet weight biomass) before being returned to the water.

1) Calculate the relative abundance of each species of fish collected in terms of CPE with respect to number and biomass. Perform separate calculations for Lake Winona, Bartlet Lake, and Sam Gordy Slough. You should combine the two samples from Bartlet, and the two samples from Sam Gordy, but perform separate analyses for east and west Lake Winona (four total systems).

2) Combine the CPE's for fish of the same trophic status (see below), and compare the relative abundance of piscivores to the abundances of the other groups. Once again, do this separately for Bartlet, Sam Gordy, and east and west Lake Winona.

3) Do shoreline fish assemblages differ among the four systems in terms of total fish CPE and trophic status CPE?

4) Calculate a Simpson diversity index for the total fish community from each of the 4 systems (4 systems = 4 Simpson diversity values). Does any system appear to have higher or lower fish diversity than the other systems?

5) Calculate a Bray-Curtis community similarity index to compare the fish commuities of each of the 6 possible site pairs. Do the systems have similar or different communities? Reminder: a Bray-Curtis value <0.6 indicates significant differences between communities.


GENERAL TROPHIC CATEGORIES FOR LAKE WINONA AND MISSISSIPPI RIVER FISHES

Benthivore
Mooneye
Buffaloes (bigmouth, smallmouth)
Redhorses (shorthead, golden)
Suckers (spotted)
Minnows/shiners (emerald, spotfin)
Bullheads (black)
Catfishes (channel)
Sunfishes (bluegill, green, pumpkinseed, rock bass)
Yellow perch
Darters
Freshwater drum

Piscivore
Silver lamprey

Gars
Bowfin
Northern pike
White bass
Largemouth bass
Smallmouth bass
Walleye
Sauger
Crappie (black)

Omnivore
Gizzard shad
Carpsuckers (quillback)
White sucker
Common carp
Goldfish
Fathead minnow
Bluntnose minnow

Planktivore
Paddlefish

Return to Limnology Lab

Neal D. Mundahl
Winona State University
Winona, MN 55987-5838