9780521861724htl 1..2

Ecological applications of life-history analysis

Fishing-induced evolution

Life-history theory has mostly been used to predict body-size variation within

species (e.g. Roff, 1992 ; Stearns, 1992 ; Berrigan & Koella, 1994 ; Day & Rowe,

2002 ). This will be important for the structure and functioning of aquatic

ecosystems if the species plays an important role, for instance as a keystone

predator. A prominent example of selection for smaller size and earlier repro-

duction comes from fisheries (Law, 2000 ; Jennings & Reynolds, this volume).

However, when analyzing fisheries data, a major challenge is to separate

genetic variation from plasticity and genotypeenvironment interaction. As

well as evolutionary shifts in age and size at maturity, changes in these matura-

tion traits can also result from changes in growth rates if stock depletion due to

fishing leads to abundant food for the remaining fish, hence quicker growth

(Lorenzen & Enberg, 2002 ), which can accelerate the optimum onset of maturity

(Day & Rowe, 2002 ). However, a new statistical method has been devised for

disentangling these different sources of phenotypic variation, which estimates

from fisheries data the probabilities of maturing at each relevant age and size

(Heino, Dieckmann & Godø,2002a, b). This technique has now been used to

10

12

Age class

allopatric fish

sympatric fish

3 4

15

Growth (mm/yr) 20 57.6

63.7

34.9

38.3

81.9

77.8

92.7

97.7

25

30

35

40

–0.04

–0.02

0.00

0.02

0.04

0.06

0.08

–0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bluegill density (relative biomass)

3.5

IGR

Figure 3.2Left: growth increments for different age classes of pumpkinseed sunfish in the wild;

allopatric fish (solid symbol) are from a lake without bluegill sunfish; sympatric fish are

from a lake with bluegill present. Numbers above or below symbols are mean sizes (mm)

of fish for that age. Lines cross between ages 2 and 3, when pumpkinseed begin to feed

mainly on snails, and when their growth rate is no longer constrained by interspecific

competition. Right: instantaneous growth rates (IGR) for six pumpkinseed populations

reared in enclosures. Solid symbols and solid lines represent allopatric populations; open

symbols and broken lines represent sympatric populations. There is a significant decrease

in growth rate as the density of bluegill increases, and the sympatric pumpkinseed grow

faster than all allopatric pumpkinseed under all densities of bluegill. Thus, growth rate

shows countergradient variation. Figures are from Arendt and Wilson ( 1999 ).

40 D. ATKINSON AND A. G. HIRST