Activity and distribution
The ectothermic nature of fishes makes them affected by and therefore highly responsive to seasonal fluctuations in temperature. At temperate and polar latitudes, food availability, vegetative cover, turbulence, oxygen availability, and water clarity all vary greatly among seasons. Ice cover on high-latitude lakes leads to oxygen depletion and winterkill conditions; thermal stratifi cation at lower latitudes creates analogous summerkill conditions. Hence fishes in these habitats characteristically move into and out of shallow, nearshore zones with the progression of the seasons. For example, a typical pattern in a lake in the northeastern United States finds the shallow regions devoid of vegetation and fishes in early spring after ice melt and until the surface water warms above about 10°C. With warming water, minnows, catfishes, pickerel, sunfishes, black basses, killifishes, and Yellow Perch move into nearshore regions. At water temperatures of around 15°C, sunfishes and many others spawn and vegetation growth is apparent. In late spring and early summer, as temperatures exceed 20°C, vegetation is well established and fish are distributed throughout the littoral zone, including deeper portions such as drop-offs down to the thermocline. In late summer and early fall, as temperatures fall below about 15°C and plants begin to die back, fishes first move from the deeper littoral zones to the shallower regions. As temperatures fall below 10°C and vegetation becomes sparse, fishes abandon nearshore regions presumably for deeper water. If periods of warm weather occur during the fall, fi sh will reoccupy and then abandon the shallows as the water warms and recools.
The fauna contains resident (topsmelts, surfperches, gobies, flatfishes) and seasonal (anchovies, mullets) species. Seasonal movements in and out of the bays are strongly linked to changes in temperature, salinity, and the productivity of macroalgae. In Puget Sound, Washington, which is relatively protected from winter storms, rockfishes (Sebastes, Scorpaenidae) school in midwater and move down a few meters to slightly deeper water in the winter. Benthic species remain in kelp bed and reef areas year round (Ebeling & Laur 1985; Horn & Allen 1985; Ebeling & Hixon 1991; Stephens et al. 2006).
On the Atlantic coast of North America, common names imply seasonal cycles of movement and abundance. Summer Flounder (Paralichthys dentatus, Bothidae) spend warmer months nearshore along the coastline and in bays. They migrate offshore in the fall to deeper (30–200 m) water to spawn. In contrast, Winter Flounder (Pleuronectes americanus, Pleuronectidae) migrate to deeper water in the summer and then return to bays as the water cools; they also spawn in winter. Other species undergo seasonal movements that differ by individual age. Adult Tautog (Tautoga onitis, Labridae) move offshore in the fall as water temperatures drop below about 10°C, while young Tautog and Cunner (Tautogolabrus adspersus) move from grass and algal beds that are dying back to other shallow habitats that provide greater shelter before these fishes enter a winter torpid state. A pattern in many temperate marine environments is a dependence on algae as a refuge or as an indirect or direct food source. As colder months approach and algal beds cease productivity and lose their “above-ground” parts, many species abandon these regions for deeper waters or waters that will provide cover during months of low food production (Bigelow & Schroeder 1953b; Olla et al. 1979; Rogers & Van Den Avyle 1982).
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