Discussion paper regarding the
biological basis for regulating rod numbers
to manage fisheries effectively1. Fishing effort: how many rods
should be permitted?1. Basic principles · Regulation is required to prevent
overfishing. · Effort may be regulated by
controlling numbers of anglers, their
efficiency and the length of the fishing
season. and the length of the fishing
season · Efficiency may be regulated by
controlling methods, such as types of bait and
approach (eg legering vs spinning), and number
of rods. · There may be rules to say what, if
any, fish may be kept, and what fish must be
released. rods 2. Basic distinctions between life
history characteristics of the foray
species The primary distinction among species, with
regard to regulating fishing effort, is
probably not whether they are "game" or
"coarse" fish, but whether cohorts of the
species tend to be vulnerable to the fishery
over successive or single years. In this
respect, salmon tend to be vulnerable over a
single year when they return from the sea to
spawn, whereas trout, cyprinids and pike are
vulnerable over multiple years. Of course, some
salmon spawn several times as anadromous fish
and may be taken by the fishery on multiple
occasions, but this is a relatively rare
occurrence and in the present context the
species can be classified as single spawners.
Spawning as salmon parr is irrelevant to the
arguments. In the case of multiple spawning
species, large, old fish tend to be
particularly valued and may often be the main
determinant of the quality of a fishery. 3. Single spawners Principles of fisheries regulation are well
established for single spawners. There tends to
be a bottleneck in the freshwater stage of the
life cycle such that there is often an excess
in numbers of adults above that required to
spawn and saturate rearing capacity. This
excess can support a sustainable fishery. In
theory, it is possible to manage populations
efficiently by cropping below the minimum
number of adults needed to saturate rearing
capacity, but there is also a strong argument
for incorporating large safety margins for
uncertainties in stock size due to inter-annual
variations. Generally, the catch of salmon by
rod-and-line is estimated to be of 5-20% of the
stock. There has seemed to be little need to
regulate numbers of rods permitted per angler
because multiple rods with set baits are
generally not efficient methods for catching
salmon, with the exception of harling. It
appears that, historically, exploitation has
generally been easily accommodated leaving an
abundance of spawners. In principle, an
increase in angling efficiency would be of
concern only when stocks are particularly weak,
as is the case at present. 4. Multiple spawners In the case of multiple spawners, each
cohort is vulnerable to the fishery for several
years before the fish are of a size that is
often of real value. During each year of
growth, a fraction of the cohort is killed due
to angling and the catchibility of a further
fraction may be reduced due to captured (and
subsequently released) fish learning to avoid
baits in the future. In the context of the
present discussion, fish that have learned to
avoid baits can be deemed to be dead to the
fishery. Even with a "no-killing" policy, some
angled fish die. Some fish suffer inadvertent
lethal damage from hooking. Other fish die due
to the physiological exertions of being
captured. This process, which can be delayed,
is well-known to biologists and can be
accounted for by a lethal build up of poisonous
lactate in the muscles. Other fish are likely
to succumb to predation when in a weakened
condition after having been caught and released
but separated from the shoals that normally
provide them with protection. A simple steady-state model can be used to
demonstrate the way that the interaction among
fishing effort, fishing mortality expressed as
the percentage of captured fish that die (F)
and other annual natural mortality (M) can
influence the catch of fish of different age
classes in multiple-spawning fish. It is
assumed (1) that eggs are spawned in excess of
a carrying capacity determined by a bottleneck
early in life; (2) that there are no
interactions among year classes; (3) F and M
are constant throughout life. Variation in
fishing effort can be through regulating either
or both numbers of rod/angler and numbers of
anglers. Note that doubling the number of rods
is not necessarily the same as doubling the
number of anglers. M is set at 0.1 (that is
10%) per annum in the example shown. In each of
the graphs attached, the series of lines shows
values for cohorts within the population with
the youngest recruited to the fishery at the
top and the oldest at the bottom. Type I response When F is relatively low, as shown for a
value of 0.01 (Fig. 1a), it has little
influence on year class strength, that is, the
lines are nearly flat. Catch per unit effort
decreases with age of the fish because
predominantly M reduces the size of the cohort
from one year to the next. Under these
circumstances, it makes good sense to increase
fishing effort because there is then an
increase in the total catch of fish from all
cohorts (Fig. 1c). This type of response (Type
I) is probably typical of many carp
fisheries. Type II response When F is increased, in the example shown to
0.04 (Fig. 2), there is a marked decrease in
year class strength with increase in fishing
effort and this effect increases with age (Fig.
2a). Catch per unit effort shows similar trends
to those for cohort strengths. The total catch
of young fish increases with fishing effort.
However, the catch of older large fish
increases to a maximum and then declines as
fishing effort increases. Type III response When F is increased further, for example to
0.1 (Fig. 3), increase in fishing effort
results in an increase in the total catch of
small fish. However, any increase in effort
inevitably reduces the total catch of large
fish. The response of the relationship between
total catch and fishing effort is curvilinear.
Therefore, a unit increase in effort from 2-3,
has a greater reduction in catch than an
increase from 1 to 2. 5. Managing fishing
effort on multiple-spawning fish The requirement for regulation of fishing
effort depends on the values of F and M. For
Type I fisheries, there are significant gains
to be made from permitting multiple rods
because catch per rod and total catch increase.
In the case of Type II fisheries, a balance
must be struck between fishing effort and catch
of older year-classes. For example, for the
model depicted in Fig. 2, if 100 anglers
constitute 1 unit of fishing effort, they could
increase their catch of old large fish by using
sufficient rods to double their efficiency.
However, any further increase in the numbers of
rods per angler would actually reduce their
total catch of old fish due to the increased
mortality on small fish. For Type III
fisheries, any increase in angling effort has a
cost in terms of the numbers of large fish
captured. 6. National versus local
management In the case of Type I fisheries there is
clearly no need to regulate fishing efficiency
of individual anglers except to minimise the
chances of increasing F, for example, through
deep-hooking fish that take baits
simultaneously on multiple rods. Arguably, this
may best be achieved at a local level taking
into account specific fishery conditions and
experience. Alternatively, or additionally, a
national maximum number of rods could be set
and fisheries could impose further restrictions
locally. In the case of Type III and some Type II
fisheries, there is a dilemma that permitting
the use of multiple rods will reduce the total
catch of large fish. Note that this is not just
fish per rod, but fish per angler.
Unfortunately, this effect is
counter-intuitive: it can seem on the face of
it that everyone can increase their catch rate
by increasing the numbers of rods that they
use, but this is not the case. Of course, on a
fishery where nearly everyone uses a single
rod, the individual who uses two rods can
increase his/her catch rate without the major
reductions in densities of older cohorts that
result when many anglers use two rods. There is a case for national regulation of
angling effort through numbers of rods
permitted in order to maintain the quality of
Type II and III fisheries. The problem with
local management is that fisheries might be
forced to allow multiple rods in order to
compete effectively in a market driven by
individual desire to maximise catch and an
intuitive perception that multiple rods must be
better than one. If this case is worthy of
taking further, then issues of classifying
fisheries into Types need to be considered. John Armstrong FRSFL 1 September 2004 |