3 | What's in your landing net?

Fish make up more than half of the known vertebrate species in the world. Currently, there are about 50,000 known species of fish, with new species being identified every year! Despite the fact that freshwater rivers and lakes represent only about 1 percent of the world's water, more than 41 percent of all fish species are found there. Some fish, less than 1 percent, live in both fresh and marine environments. There is a higher proportion of fish species in freshwater environments because these environments are often more isolated, which leads to a higher rate of speciation.

Fish Anatomy 101
Historically, all bony fish were placed in the class Osteichthyes. However, bony fishes are now divided into two classes, the Sarcopterygii (lobe-finned fishes) and Actinopterygii (ray-finned fishes). About 350 million years ago, actinopterygians became dominant in freshwater and are currently the most successful class of fish, found in both freshwater and marine environments. They are also the most diverse group of all the vertebrates.

Most fish are ectotherms, meaning that their body temperature is strongly dependent on the environment. Consequently, water temperature is one of the most important physical factors affecting the behavior, physiology and distribution of fish. Fish are sometimes classified as either cold-water or warm-water species. Cold-water species, such as the brook trout, have optimal development and growth in cold temperatures. They can withstand cooler temperatures, but warmer water can be lethal to them. In contrast, the bluegill sunfish lives best at warmer temperatures.

Comparing common trout and salmon species
Related site: Trout and Salmon Identification Guide

Chinook Salmon (Oncorhynchus tshawytscha)
This Pacific and Arctic coast species was intentionally introduced to all of the Great Lakes. This is the largest of the Pacific salmons growing to a length of 35 inches (90 cm). In breeding males, the upper and lower jaws are so deformed into a hook shape that they are unable to close their mouths.

Coho Salmon (Oncorhynchus kisutch)
This species is not native to the Great Lakes, but has spread rapidly after successful stocking efforts in Lake Michigan in the mid 1960s. Initially stocked to exploit and reduce alewife populations, this species was also seen as a lucrative sport species and stocking has continued by the Great Lakes states and the province of Ontario.

Brown Trout (Salmo trutta)
Likely the first salmonid species to be introduced to the Great Lakes, brown trout are now found in all of the lakes.

Lake Trout (Salvelinus namaycush)
This native deepwater predator has been an important part of the commercial and sport fisheries. Its populations in the Great Lakes have suffered as a result of pollution and predation by the non-native sea lamprey.

Rainbow Trout, or steelhead (Oncorhynchus mykiss)
There are two primary forms of this species: the "rainbow trout," which is smaller, darker and lives in streams and rivers; and the "steelhead," a larger, silvery form that lives in large bodies of water such as lakes or the ocean. The rainbow trout is another Pacific coast species that has been intentionally introduced to all of the Great Lakes with reasonable success.

Siscowet, or fat lake trout (Salvelinus namaycush siscowet)
Though related to the lake trout, the siscowet looks quite different and has an extremely high fat content. It's found only in the deep, cold waters of Lake Superior.

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Graphics: Fish anatomy, Canada's Aquatic Environments (University of Guelph, Ontario); Chinook and Brown Trout, copyright ©Joseph Tomelleri; Siscowet, Minnesota Sea Grant.

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