Recruitment
Forgive me for momen-
tarily bringing math into a
bass fishing magazine, but
one of the most basic ways
scientists describe the stabili-
ty of bass populations in any
lake, river or pond is the
BIDE Method (illustrated on
the previous page), where
the net population change is
equal to the following:
(B)irths – (D)eaths +
(I)mmigrations – (E)migra-
tions
Imagine there’s a pond in
your neighborhood with 100
bass living in it. Over a year,
the pond experiences 50
births (B), 40 deaths (D), has
10 bass dumped in via a
bucket from an adjacent
pond (I) and loses 20 bass
that swim down the creek
outlet to the neighboring
reservoir (E). The net change
in this example would be
zero, which biologists would
call a stable population.
Although simplistic, this
example accurately portrays
the basic idea behind the
study of population dynam-
ics. The goal is to understand
how a population responds
to natural and human stimu-
lus. The more complex the
system, the more complicat-
ed the dynamics can get. Add
in additional factors such as
angler harvest, seasonal
migration, and climate or
geochemical changes and
you can quickly understand
why folks get college degrees
to study this.
Imagine trying to
describe the overall popula-
tion status of a cosmopolitan
species (one with worldwide
distribution) such as
Atlantic bluefin tuna, or
smallmouth bass in a lake
the size of Erie or Michigan.
To do so, you’d need to
bring in some additional
concepts, some of which are
detailed in the following: Biologists often use the
term “recruitment” in lieu of
“births” for fisheries studies,
because counting the num-
ber of fish spawned is almost
impossible due to the vast
numbers of eggs fish pro-
duce. Because the majority of
these fry will not survive, it
makes more sense to talk
about “recruitment,” or the
number of young that reach
a specific measurable age.
So, in the above pond exam-
ple, although the 100 bass in
the pond certainly dropped
thousands of eggs, we only
got 50 to measurable size, so
that’s the number used in the
calculation.
Length-at-Age
This is essentially a rep-
resentation of a popula-
tion’s growth rate. By meas-
uring a number of fish of a
given species out of the
same body of water, you can
establish the average length
of a population at specific
ages. Compare that across
several bodies of water and
you can make management
decisions.
Example: A 2-pound bass
in Florida must be 3 or 4
years old, whereas a 2-pound
bass in a Canadian Shield
lake is probably closer to 7 or
8 years old. Even in a single
region, habitat and forage
availability can cause lakes or
rivers in close proximity to
have very different length-at-
age numbers for the same
species, which could be
either good or bad, depend-
ing on your perspective and
management goals.
Stunting
When recruitment is high
and resources (food and
habitat) are limited, bass
growth slows, resulting in
decreased length-at-age. This
creates a condition where
bass are abundant in number
but smaller than normal for
their age. Scientists call the
condition “stunting,” and it’s
fairly common across the
country. Stunting happens
most frequently in smaller
ponds and lakes and can be
combatted by increasing
angler harvest or predation.
You may have seen DNR
managers remove the size or
bag limit on a lake. It’s a
good bet they’re trying to
combat stunting.
Carrying Capacity
In any system, whether a
tiny pond or one of the Great
Lakes, there are limited
resources available. There’s
only so much suitable habitat
and a specific amount of prey
biomass. Carrying capacity is
the theoretical maximum
population size a specific
system can support.
Biologists use this number to
help set harvest quotas, as
well as make management
decisions about size limits
and closed or open season
dates.
Maximum Sustained Yield
This describes the theo-
retical largest harvest rate
for a given species in a fish-
ery that will allow the
species to perpetuate indefi-
nitely. By understanding
growth rate and recruitment,
and by having a rough
approximation of the total
population, biologists can
estimate the amount of har-
vest the fishery can sustain.
This is the calculation many
regulators use to set quotas
on commercial harvest as
well as bag limits on recre-
ational fisheries.
Bass Math
HOW TO USE THIS
KNOWLEDGE TO CATCH
MORE (OR BIGGER) BASS
State DNRs and management organizations are publicly funded.
Because of that, their data, reports and other information are all public
record and available for angler research. Lots of reports are easily found
on DNR websites. Spend some time reading harvest data, creel surveys,
fisheries reports and other information on your local waters. Combing
through this data can provide critical information to help you catch
more and bigger bass. Look for which lakes have higher populations,
higher length-at-age data and more. Many reports also include data on
available forage species, which can help narrow down location and
presentation info.
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