Density‐Dependent Effects and the Regulation of Crucian Carp Populations in Single‐Species Ponds

Crucian carp (Carassius carassius) is often the only fish species present in small ponds of northern Europe. Such populations are typically at high densities and consist primarily of small, relatively slender, and short—lived fish; populations in multispecies assemblages in lakes display opposite traits. Field and laboratory experiments have implicated predation as an important mechanism structuring crucian populations in multispecies assemblages. To investigate the role(s) of density dependence in structuring pond populations, we manipulated densities of crucians stocked into four sections of a subdivided natural pond in Finland. Mortality of stocked fish during the 3—mo experiment was low (0—11%) and unrelated to density. Growth rates of crucians were both size and density dependent. Larger fish did not noticeably grow in either high— or low—density sections, but growth of smaller size classes was 22% greater in low—density sections. Lower densities and smaller sizes of zooplankton, especially of inshore cladocerans that are an important food resource for smaller crucians, indicated that resource limitation caused by exploitation competition was an important density—dependent process restricting growth of small fish in high—density sections. Crucians rely on reserves of glycogen, stored in the liver and muscle, to fuel the over winter anerobic metabolism that allows them to maintain populations in ponds where winter hypoxia eliminates other fishes. Liver size was significantly larger and glycogen levels were 82% higher in our low—density pond sections than in high—density sections. At higher, but not lower, densities, relative liver size and glycogen levels decreased with body size; combined with the lack of growth of large fish, this indicates that density—dependent energy limitations contribute to the short life—spans and rarity of large fish that characterize pond populations. An increase in relative body depth can be induced in crucians by the presence of piscivorous fish, reducing vulnerability to predation. It has been proposed that increased body depth incurs a substantial energy cost during swimming and should be restricted to specific size classes in populations sympatric with predators. However, in our experiment, crucians of all sizes became significantly deeper bodied, which contributed to higher condition factors in low—density vs. high—density sections. This morphological change, occurring in the absence of piscivores, suggests that energy benefits (accumulation of overwintering reserves) can override any proposed costs for this sluggish, but metabolically unique, fish. Crucian carp successfully reproduced in all sections during the experiment, but growth and recruitment of 0 + fish (young of year) were strongly and negatively related to the numbers of older fish present. The observed density—dependent recruitment was likely caused by starvation—induced mortality and cannibalism; reduced growth observed in 0 + fish would also be expected to cause higher fist—winter mortality, reducing further recruitment at higher densities. Density dependent has a variety of regulatory effects on pond populations of crucian carp that contribute to their presence and resilence.