Harvest management - Ecological Applications: Toward a Sustainable World - page 201

late 1970s. It is not surprising that later surveys showed that these and other North
Atlantic cod stocks were very seriously depleted as a result of overfi shing. This
species reaches sexual maturity around the age of 4 years, but the cod have been so
heavily exploited that now some 1 year olds are harvested and almost all 2 year
olds are taken each year, leaving only 4% of 1 year olds to survive to age 4 (Cook
et al., 1997).
Indigenous harvesters have long had their own 'regulations' to reduce the chance
of overexploitation. In their harvest of moi ( Polydactylus sexfi lis ), a fi sh associated
with sandy beds near the shore, Hawaiian customary fi shers take only intermedi-
ate-sized fi shes, leaving both juveniles and large females. Thus, they went a stage
further than the approach of increasing the mesh-size of nets, which, while reducing
the numbers of larger individuals taken, nevertheless invariably capture the largest
individuals in the population. The good sense of the Hawaiian strategy has recently
been reinforced by the discovery that large females of some fi sh not only produce
exponentially more offspring (something known for a long time) but also each of
their eggs is more likely to achieve adulthood. The black rockfi sh ( Sebastes melan-
ops ), off the coast of Oregon, USA, is a long-lived fi sh that produces live young.
Bobko and Berkeley (2004) noted that, as usual, bigger fi sh produce more eggs to
be fertilized. However, the proportion of these that are in fact fertilized is itself
greater in larger females and, in addition, larvae produced by older females grow
more than three times as fast and survive starvation more than twice as long as do
larvae produced by younger females (Figure 7.11).
We do not know how widespread is this effect of fi sh size on the quality of off-
spring, but where it occurs how could the knowledge be incorporated into fi sheries
management? Scuba divers and spear fi shers could take it on board, because these
recreational fi shers can select the individuals they target. The same would be true
for crabs and lobsters caught in pots, since large individuals can be freed unharmed.

Fig. 7.11 Relationship
between maternal age
and (a) growth rate of
larval black rockfi sh in
laboratory experiments
and (b) time by which
50% of larvae have died
due to starvation.
Larvae from older
mothers grow substan-
tially faster and survive
much longer. (After
Berkeley et al., 2004.)

(a)

0.08

0.06

0.04

0.02

0

(b)

14

12

10

8

6

4 4

6 8 10 12 14 16 18

Maternal age (yr)