The Changing Seasons: Freshwater Fish Must Deal With Different Challenges

The dramatic changes that are part of the annual cycle in many tropical rivers provide challenges and opportunities for the fish that live in them. The floods bring an annual explosion of nutrients and fish exploit this opportunity by timing their breeding to coincide with them. In fact, this strategy of breeding when the conditions are at their most suitable for the fry is the crux of all fishe's annual cycles.

How do fish of temperate zones time their breeding seasons?

Some of the subtropical species kept by aquarists, including White Cloud Mountain minnows (Tanichthys albonubes), use different cues to determine the best time to breed. North and south of the tropics, day length changes throughout the year; the days are longer in summer than in winter. In addition, water temperatures are higher at certain times of year. The fish use these as cues to decide when to breed. The typical pattern among subtropical and temperate fish, such as sunfish, is to breed during spring.This allows the fry to take advantage of the increase in food associated with spring and summer, in conjunction with the higher temperatures that allow them to grow large enough to survive the harsh winter.

Do ail fish have breeding seasons?

Breeding seasons are not for everyone. A better strategy for fish that live in stable environments can sometimes be to breed throughout the year, spreading the risk by producing multiple broods of young. This pattern is seen in many wild Central American cichlids, for instance. However the energetic costs of breeding are extreme, even more so if, like many cichlids, parental care is provided. So, although breeding may occur all year round to a greater or lesser extent, there are peak breeding periods. For wild Nicaraguan
convict cichlids, this occurs during the dry season, roughly from January to May. Some fish species, including the guppy, have lifespans that may be measured in weeks rather than years and this negates the point of a breeding season. Female guppies therefore produce young every four weeks or so.The number and size of the offspring in each batch is determined by the conditions — plenty of food especially allows larger broods.

What cues trigger spawning in lake fishes?

Although lakes undergo less obvious seasonal changes than rivers, major transformations are occurring beneath the water surface, even in the huge Rift Lake systems. During the cooler months from June to August, a southeasterly wind known as the "mwera" blows. It creates currents in the waters of Lake Malawi and causes nutrient-rich waters to arise from the depths. This upwelling of nutrients boosts the food chain from algae to zooplankton and upwards and provides excellent conditions for larval fish to feed. As a result there is a peak in fish spawning at this time. A second peak in the fishe's breeding season occurs at the end of the rains in April and May, possibly for similar reasons, as parental fish aim to time their breeding efforts to coincide with peak growth conditions for the young.

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