9780521861724htl 1..2

turnover time of the exploited community (1/P:B) was almost twice as fast as that

of the unexploited community, falling from 3.5 to 1.9 years.

Comparisons such as these are useful for comparing the magnitude of fishing

effects in different ecosystems and for assessing the relative impacts of fishing

on a range of community and ecosystem properties. However, such static

descriptions cannot be used to guide management and to modify fishing to

achieve desirable community structures, such as those that provide good yields

of target species while ensuring that large and vulnerable species do not go

extinct.

Trophic level

5

4

3

12345

Body mass (log 10 )

6

(a)

(b)

0

–1

–2

12345

Body mass (log 10 )

1

Biomass (log

) 10

4.0 4.2 4.4 4.6 4.8 5.0

Trophic level

5.2

TE = 0.150

TE = 0.125

TE = 0.100

Figure 14.2The

relationship between

trophic level, as estimated

with nitrogen stable isotope

analysis, and body mass for

North Sea fishes in 2001 (a);

95% confidence intervals for

the mean reflect among-site

variation, and the slope of

this relationship implies a

mean predator–prey mass

ratio (PPMR) of 390:1.

Predicted slopes of

abundance–body mass

relationships for the

unexploited community

(b) were calculated from

PPMR and estimates of

transfer efficiency (TE) of

0.100–0.150. The observed

slope of the abundance–body

mass relationship in 2001 is

shown with a broken line.

From Jennings and

Blanchard (2004).

276 S. JENNINGS AND J. D. REYNOLDS