approximately measured as stock estimates, while the ratio (P/B)i can be a sum across
age classes of year-on-year growth of individuals: weigh samples and age specimens
from otolith growth rings or from guesses. This product represents all the growth
“output” of the prey.
(^) On the right are fisheries catch (Ci), plus net biomass change (Ei) due to prey
emigrating from and immigrating into the region modeled (perhaps an unfished
reserve, perhaps the whole Mediterranean Sea), plus biomass accumulation in the
region (stock increase, BAi), plus the sum over all predators, j, of their biomass times
their consumption per unit biomass ([Q/B]j) times the proportion of Q taken from prey
species i (DCij). Again, Bj and [Q/B]j are more readily estimated than Q directly.
(Beware: many papers on this subject are printed with subscripting errors: i for j, j for
i.) Generally, such models are simplified by assuming, at least for some period of
interest, that emigration is zero or balanced by immigration (Ei = 0) and that the prey
stock does not change (BAi = 0). Given N-linked equations of this type representing
major stocks (or guilds) in a food web, with estimates or guessed values for most
quantities, up to N missing quantities can be estimated. That generates a static picture
of the transfer of organic matter (and energy) through that food web.
(^) Almost all enthusiasm for Ecopath models comes from fishery scientists. They have
estimates of sundry stock biomasses from fishery returns. Most species they work
with can be aged (to give P/B from size vs. age data), have identifiable gut contents,
and often remain identifiable in the diets of their predators. For example, in the
northern Benguela current off Namibia, sardine, anchovy, hake (two species), horse
mackerel, snoek (Thyrsites atun), several tunas, several sharks, and more, are all
fished and fur seals are regularly culled. These harvests produce catch records of
variable quality for most of these species that are engaged with each other and in the
wider pelagic community as predators and prey. A static Ecopath model of this trophic
interaction scheme with 32 trophic groups (equations) was created by Heymans and
Sumaila (2007). The main stocks and their interactions can be sketched as an
Ecopath-like food web (Fig. 9.12).
Fig. 9.12 Generalized diagram of trophic connections among major ecosystem
components in the northern Benguela ecosystem off Namibia. Diet characterizations
are from discussions in Heyman and Sumaila (2007). Phytoplankton grazing, heavy
dark lines. Zooplanktivory, dotted lines. Predation on clupeids and other small pelagic
fish, thin solid lines. Upper-level predation, dashed lines. Arrows point in the
direction of nutrition transfer. Vertical lines in the center column imply potential prey
switching and/or competition. Numbers in ovals are approximate annual production
tonnages (millions of metric tons).