“see” the fish in the sea is endless. Current recommendations are to manage by
“integrated assessment” (Maunder et al. 2006) and even more broadly focused
“ecosystem-based management”, mentioned below. Assessments have CPUE as an
input, but age structure; reproductive biology; estimated recruitment; growth
dynamics; distribution patterns; predator and prey ecology; shifting oceanography;
and more, are taken into account. The better-managed fisheries in the United States, in
particular that on the Eastern Bering Sea pollock stock (Ianelli 2005), are regulated in
light of fisheries-independent trawl and acoustic surveys, and exploitation is regulated
to well below estimates of maximum sustainable yield (considered below). In
addition, fishing boats in the pollock and several other US fisheries are required to
have agency observers aboard, ensuring greater integrity in catch and bycatch
reporting. Elaborate mathematical/statistical models are typical stock assessment
tools, albeit requiring both explicit and implicit assumptions.
Recruitment (ΔB/Δt = R(B,A) + . ...)
(^) Fish are considered to be “recruited” to a fishery when they are large enough to be
captured by its methods. Thus, size at recruitment sometimes can be chosen by the
fishers or by fishery managers (usually by decree). For example, the pots used to
capture Dungeness crab on the northwest coast of the USA have an escape port just
smaller than the legal size, a minimum width of carapace. Many small crabs visit the
pots then leave again, and undersized crabs found by crabbers retrieving their pots are
tossed back to the sea. Females are all thrown back, too. Thus, recruits are quite
strictly defined as male crabs larger than the legal size. For bottom-trawl fisheries,
mesh size can be adjusted, and pre-recruits slip through the openings – at least that is
the intention. Recruitment rate (numbers of young entering the catchable stock per
time) depends upon a list of variables, and understanding how those factors control
the rate is a major scientific challenge for fisheries biologists. Variables include the
number of eggs spawned, hatching success, survival to recruitment age or size,
retention throughout development within a suitable habitat, and the unused carrying
capacity of the habitat available for new recruits. Survival of larvae depends upon
temperature, speed and direction of currents, availability of sufficiently nutritious
food of the right size, and the activity of predators and parasites. Carrying capacity
can be mostly filled by large numbers of older individuals, which may inhibit
recruitment – sometimes by cannibalism. Survival and growth of potential recruits are
reduced by competition with larger and tougher, older individuals.
(^) Usually, recruitment is taken to be a quasi-parabolic function of stock size (Fig.
17.9a), referred to as a “spawner–recruit” curve. At the low end of the scale (often an
“experiment” forced by over-fishing, but which can also be a result of climate
degradation relative to the requirements of the fish) there are indeed very few recruits