Among birds and mammals most growth is determinate, which means that growth ceases after an individual matures. Other vertebrates show indeterminate growth, meaning that growth continues throughout the lifespan of an individual, although at a constantly decelerating rate. For these animals, which include fi shes, older individuals are generally larger, all other factors being equal. The last statement is crucial, as age is not the only factor affecting the size of a fi sh; age is not even the most important factor in wild populations. Other factors, both intrinsic (e.g., genetic and maternal effects) and extrinsic (e.g., temperature, food supply, water quality, photoperiod, predator fauna) are important as well. Thus we will fi nd differences between individuals when comparing species, populations, individuals within populations or within cohorts and clutches (see Wootton 1990; Fig. 8.1), in other words, size at a certain age is a plastic characteristic in fi shes. Body size and temperature are generally regarded as the two most important variables affecting nearly all biological rates and times, especially

1 Department of Zoology, Stockholm University, SE-106 91 Stockholm, Sweden, and Tomtebogatan 10, 113 39 Stockholm, Sweden.