Growth rate

7.2.4 Growth rate

Energetic constraints associated with parental care alone have been reported to negatively influ- Growth rate is of major importance to life-history ence future growth and fecundity (e.g. Balshine- evolution, notably in organisms that continue to Earn 1995; Wiegmann and Baylis 1995). Energetic grow after attaining maturity, because of its deter- losses might also effect survival and fecundity mination of body size at age and because of the costs if reproduction is associated with immune positive associations that can exist between body system deficiencies, leading to an increased risk size and various metrics of fitness, such as fecun- of infection or parasitism. Although direct links dity (Wootton 1998), egg size (Kamler 1992), sur- between reproduction and infection risk or para- vival (Hutchings 1994), fertilization success

(Hutchings and Myers 1988; Thomaz et al. 1997; Jones and Hutchings 2001, 2002), parental care (Sargent et al. 1987; Wiegmann and Baylis 1995) and migration distance (Schaffer and Elson 1975). Once the physiological minimum reproductive size has been attained, an individual’s maturation strategy is predicted to depend not only on the consequences to present and future survival of maturing at various ages but also on the conse- quences to present and future fecundity.

A trade-off between present and future fecundity, effected by declines in growth rate con- comitant with maturity, formed the basis of initial predictions of how growth rate might influence life history (Gadgil and Bossert 1970; Schaffer 1974a,b; Bell 1980). Schaffer (1974b), for example, predicted that environments that allowed for increased growth during potentially reproductive ages should favour delayed maturity and increased reproductive effort. Although Schaffer and Elson’s (1975) positive association between age at maturi- ty and growth rate at sea in Atlantic salmon ap- peared to support this hypothesis, the correlation did not differ significantly from zero upon reanaly- sis (Myers and Hutchings 1987a).

Nonetheless, there is reason to believe that Schaffer’s (1974b) predictions hold merit. Hutch- ings (1993a) suggested distinguishing the growth rate experienced during the juvenile stage from that experienced during the adult stage. He pre- dicted that increases in juvenile growth rate rela- tive to adult growth rate should favour increases in reproductive effort and reductions in age at matu- rity. These predictions were supported by empiri- cal data on Newfoundland populations of brook trout (Hutchings 1993a). Fox (1994) tested this hypothesis and reported that pumpkinseed sun- fish introduced into a previously fishless pond ex- perienced a higher ratio of juvenile to adult growth rate, matured at an earlier age and had a signifi- cantly higher GSI relative to those in the original population. Positive associations between the ratio of adult to juvenile growth rate and age at maturity also exist for Newfoundland populations of ouananiche (S. salar; Leggett and Power 1969) and Swedish populations of brown trout (S. trutta; Näslund et al. 1998).

Reductions in age at maturity with increases in individual growth rate have been repeatedly docu- mented in fish (e.g. Alm 1959; Hutchings 1993a; Fox 1994; Trippel et al. 1995; Godø and Haug 1999). The capacity of individuals to respond in such

a manner, and the rapidity with which these changes can occur within populations, relative to generation time, suggests that this life-history response is phenotypically plastic, the magnitude of which would depend on the shape of the norm of reaction for age at maturity and on the magnitude of environmental change in growth rate. Two ex- amples are gadids reported by Trippel et al. (1997) and yellowtail flounder, Pleuronectes ferruginea, by Walsh and Morgan (1999). Using empirical age-specific survival, fecundity and growth rate data for Newfoundland populations of brook trout, Hutchings (1996) explored the individual conse- quences to fitness of maturing at various ages in response to changes in growth rate. The resultant fitness functions supported the prediction that a reaction norm describing a negative association between growth rate and age at maturity can represent an adaptive response to environmental change. However, for one population, fitness was maximized by maturing as early in life as possible, regardless of growth rate. Selection for such a flat, non-plastic reaction norm can be expected when the probability of realizing the fitness benefits of delayed maturity (increased fecundity for females, increased access to mates for males) is relatively low (see Section 7.5).