Like invertebrates, fish have many adaptations for life in moving water. They do, however, face a slightly different set of challenges, since the adults are larger and typically higher on the food chain. This means they must consume more biomass to obtain adequate energy for growth and survival. Rarely is the sit-and-wait-for-something-to-tumble-into-my-mouth approach successfully used by fish. They tend to be more aggressive in seeking food and also mating. Fish also have greater mobility than most insect larvae, which allows them to forage, investigate, and escape. Certainly these capabilities are an advantage, but the current could really ruin their day if they do not plan accordingly. There is a definite correlation between the shape of the fish in cross-section and the ability to resist current. Some fish actually swim in running water, not just withstand the current. These tend to be round in cross-section. The salmon and trout of New Hampshire exhibit this body shape. These fish are piscivorous, eating other fish, and generally this body shape supports speed and maneuverability. They also have well-developed teeth.
Another common adaptation to running water is to conduct life's business from the stream bottom. Benthic organisms have many physiological as well as behavioral adaptations to support life near the stream bed. Benthic feeders, like the suckers and catfish, scavenge whatever is available among the bottom debris. These fish have morphological adaptations to lessen floatation, such as a reduced swim bladder, or the swim bladder located toward the rear, effectively pointing them downward. Their mouths are often circular in shape and pointed downward.
There are numerous other feeding mechanisms exhibited by fish in general, and modified slightly for a lotic existence. Filter feeding fish seek plankters. These types of fish will have a terminal mouth, and fine sieve-like gill rakers for trapping drifting organisms. Examples of these are the sunfish. Their body shape is laterally flattened which is best suited for "sidling" - that is, residing close to or behind rocks or trees or other protective objects. This helps them avoid the strong currents.
Those fish that are surface feeders, seeking primarily insects, will have dorsal eyes, and an upturned mouth. Their swim bladder may be located closer to the front to help buoy them up. As one might expect, these fish are not well-represented in running water habitats, although killifish are found in streams and feed in this manner.
Habitat considerations for stream fish include temperature and related oxygen levels. Anglers often classify fish by the temperature regimes in which the fish thrive. In New Hampshire, the distinction of cold water and warm water fish is widely accepted. Generally, if the ambient water temperature is greater than 700 F, the habitat is considered a warm water fishery. Here you could expect to find species such as large mouth bass, pickerel, yellow perch and catfish. Cold water habitats, with ambient temperatures less than 700 F, would support all of the trout species.
Other habitat considerations are the availability of spawning areas, resting areas, current velocity, substratum, turbidity, and the availability of food. All of these factors are directly or indirectly dependent on the geology and land use in the watershed. For example, in regions of high agricultural activities where soil erosion occurs, turbidity may exclude the existence of many fish species. If it is a region with abrupt changes in elevation, the current may be too great to allow certain species to dwell. Often, the type of habitat sought for spawning is vastly different than what the species typically seeks for feeding and growth stages. The brook trout, for example can easily exist in bedrock or boulder subrates. However, gravel must be available somewhere along the stream reach for spawning beds. The most drastic example of this contrast in habitat requirements at different life cycle stages is the american eel. These guys move from fresh water to the sea to spawn....and not just any sea. They make a remarkable journey to the Sargasso Sea.
In New Hampshire there are 65 species of fish, excluding those which are solely marine. The distribution of these throughout the state is surprisingly regional. For example, the tesselated darter and silvery minnow only occur in the Connecticut River Basin. The northern redbelly and finescale dace only occur in the Androscoggin and northern Connecticut River Basins. The only place you can locate a redfin pickerel is in the Merrimack River Basin. This regionality speaks to the specificity of habitat, and more so about the impact of land cover on stream communities.
Fish for Biomonitoring
Since the goal of biomonitoring is to use the living fauna as representatives for the ecosystem, fish are obvious spokesmen. When considering the distribution of energy in stream habitats, the major players are 1) the plants - in streams this is mainly attached algae and rooted stream-side macrophytes, 2) the microscopic fauna - this includes bacteria, protozoans and the like, 3) macroinvertebrates, and 4) fish. There are occasional other visitors such as mammals, amphibians, and reptiles, but for purely aquatic forms, the four categories listed are the major players in the energy game. Any of them could be candidates for biomonitoring, and each group offers advantages. In New Hampshire, the invertebrates and fish are targeted for a number of reasons. Why we use invertebrates is explained in the Common Stream Invertebrates section.
Why fish? Under "normal" natural environmental conditions, fish populations tend to be quite stable. As with invertebrates, some fish are indicators of existing conditions. If an entire population of a sensitive species crashes, and no natural phenomenon can explain this (such as drought), then some pollutant source must be considered. Also, fish live longer. This allows pollutants to bioaccumulate, and possibly manifest in observable ways, like tumors and skin lesions. Fish are also relatively easy to collect and to identify to species level. Possibly the most pertinent reason that fish make good biomonitoring organisms is that they are consumed by humans. While the entire population of fish may represent a couple of trophic levels, somewhere at the top of that food chain could be… you.