the other hand, some fishery scientists are thinking that substantially more refined
identification of stocks is in order (Hauser & Carvalho 2008). It is likely that the
advantages of studying animals with close attention to their species and population
relationships transfer completely to fisheries work. It is not fruitful to lump together
animals that grow fast with others that grow slowly, or animals which spawn once
with those reproducing repeatedly, because each will have different demands for
management. That is true even if it is unavoidable that we catch them in the same
nets. If fishing for one impacts the other, we just have to account for that. It remains
useful to analyze the interaction of fishing with stocks separately from the interaction
of fisheries with their markets, and then to tie the two together. Nevertheless, the unit
stock concept is often rather wishfully applied, and exploitable organisms grouped
together for fishery management are often not the neat interbreeding subpopulations
defined in the ideal case.
(^) David Cushing (1995 and earlier books) was the modern champion of the unit stock
concept, although thinking on a definition of stocks goes back over a century.
According to older definitions, a stock was simply those fish of a given kind, or even
of a general class like “flatfish”, accessible to the boats fishing out of a given port, or
from a cluster of ports grouped for statistical summary of landings. More recently, an
ideal unit stock has been seen as a reasonably strict breeding group of a biological
species, just one kind of fish, squid, or shrimp, etc. The notion that there are such
groups arises partly because a large number of species participate in seasonal
migrations to restricted spawning locations in the sea that are remarkably consistent
from year to year. It is these mating meetings that define subpopulations with a
countable (at least in principle) number of individuals, distinctive both in terms of size
and age structure, with enough homogeneity in behavior and other aspects of biology
to make them manageable under harvest. Such stocks have persistent proportions of
variant genotypes (as represented by blood types, other protein variants, or DNA
sequences), and they vary in those proportions from stocks with mating migrations at
different times or sites. Thus, genetic markers can help to define the limits of unit
stocks, keeping always in mind that proportions of genotypes in population samples
are required to decide if a group of fish (or port landings) is in one stock or another.
Similar proportions do not imply any very great level of genetic intermixing, since
only a few individuals switching their mating site or time will provide some gene
flow.
(^) Cushing offers the term hydrographic containment as one explanation for the
recurrence across species, even phyla, of mating conventions. The North Sea plaice
(Pleuronectes platessa), for example, gather over a modest region in the southwest
corner of the North Sea (Fig. 17.1a) to spawn. Individuals travel long distances to
reach this site. The nearly certain reason that selection has promoted this site as the
place for plaice to mate and spawn is that the most suitable juvenile rearing habitat is