Invasive gibel carp ( Carassius gibelio ) outperforms threatened native crucian carp ( Carassius carassius ) in growth rate and effectiveness of resource use: Field and experimental evidence

Abstract Invasive alien species pose a serious threat to biodiversity. They frequently compete with native species for resources, resulting in the decline or extinction of the latter. Native crucian carp (Carassius carassius) faces a severe decline in European waters and has become critically endangered in many European countries. Possible ecological mechanisms of the rapid decline of the crucian carp attributable to resource competition with the invasive Carassius gibelio were identified. A field study was combined with a controlled experiment to compare (i) standard length–weight relationships and growth patterns in 12 Czech and Swedish ponds in which the species occur alone or in syntopy, and (ii) individual growth and food utilization under the same conditions. The growth increment of C. gibelio was greater than that of C. carassius under the same experimental conditions (mean increase in weight: C. gibelio, 21.7%; C. carassius, 5.2%; increase in standard length: C. gibelio, 6.3%; C. carassius, 2.0%), suggesting that C. gibelio uses food resources more efficiently than C. carassius does. Moreover, larger C. carassius individuals did not grow, whereas growth of C. gibelio individuals was largely independent of standard length. Field data suggested that C. gibelio grew faster and were heavier than C. carassius of the same standard length in four Czech ponds where they occurred together. Individual weight‐at‐length declined in C. carassius in the presence of C. gibelio, especially in more dense populations, whereas the weight‐at‐length of C. gibelio remained similar. Taken together, these results imply that C. gibelio has better competitive abilities for food than C. carassius and provide novel support for the long‐standing hypothesis that the introduction and spread of C. gibelio causes the decline of C. carassius in European waters.