identify a shift in maturation of northern codGadus morhuatowards earlier ages
and smaller sizes preceding collapse of the fishery in the late 1980s and early 1990s
(Olsenet al., 2004). Such techniques are timely, asthere is growing concern about
the consequences of fisheries-induced evolution, which may ultimately lead to
lower sustainable yields (Law, 2000 ;Conover&Munch, 2002 ;Walshet al., 2006).
Even where increased mortality occurs across a wide range of sizes and ages,
selection is often expected to act against those genotypes that are slow to
mature, hence typically at a large size, because the evolutionary response of
juveniles to harvesting (on whom mortality selects earlier breeding) is often
more sensitive than that of adults (on whom mortality selects delayed breeding)
(Ernande, Dieckmann & Heino, 2004 ). However, the more obvious selection
against larger adults comes from fisheries targeting larger fish (Law, 2000 ;
Ernandeet al., 2004).
Genetic shifts in mature size of fish under mortality selection are most clearly
seen where controlled experiments have been performed to test selection pres-
sures and rates of evolution, but these tend not to be from commercial fisheries.
One such system is provided by Trinidadian guppiesPoecilia reticulatafrom streams
containing sites that pose either high- or low-predation risks (Reznick & Ghalambor,
2005 ). In the high-predation environments studied by Reznick and colleagues
predation on guppies appears to be spread across all size classes, whereas in low-
predation environments small individuals suffer greatest predation. A combination
of experiments under identical rearing conditions (‘common garden’ experiments),
predator introductions and transplantation between sites have demonstrated rapid
evolution of smaller size at maturity in response to increased predation, and the
reverse response when predation risks are reduced (Fig.3.3).
An artificial selection experiment on body size of the Atlantic silversideMenidia
menidiafound that populations where large fish were selectively harvested
showed substantial declines in fecundity, egg volume, larval size at hatch, larval
viability, larval growth rates, food consumption rate and conversion efficiency,
and willingness to forage (Walshet al., 2006). This experiment clearly demon-
strates how selection for size affects numerous other traits that correlate with
size, and that these changes generally reduce the capacity for population recov-
ery. However, it is suggested here that in real fisheries, where populations have
age structure, these maladaptive shifts in life histories may be subject to counter-
selection pressures because fish can be large not just because they grow fast, but
also because they have managed to survive long enough to achieve large size.
Rapid growth in the Atlantic silverside is associated with decreased swimming
performance and higher susceptibility to predation (Walshet al., 2006). Therefore,
if slower growth is associated with higher survival, fishing of large individuals
may not just remove fast growers (as in the experiment of Walshet al., 2006)but
also slow ones (good survivors), which will provide a counter-selection pressure
on traits associated with capacity for population recovery.
LIFE HISTORIES AND BODY SIZE 41