Understanding Abundance Estimates in Fisheries Management

Abundance Estimates
R
e
c
r
u
i
t
m
e
n
t
Immigration
N
a
t
u
r
a
l
M
o
r
t
a
l
i
t
y
F
i
s
h
i
n
g
M
o
r
t
a
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i
t
y
Emigration
P
o
p
u
l
a
t
i
o
n
N
u
m
b
e
r
s
 
Common Abundance Estimates
 
CPE/CPUE (relative density)
 
Depletion/Removal (estimate of N
0
)
 
Mark-Recapture (estimate of N
0
)
 
Abundance -- CPE
 
2
Catch-Per-Unit-Effort
 
Consider …
Lake A – 4 nets, 40 total fish
Lake B – 2 nets, 30 total fish
Which lake has a higher density of fish?
 
Consider …
Stream A – 50 fish in 25 minutes of shocking
Stream B – 60 fish in 35 minutes of shocking
Which stream has a higher density of fish?
Abundance -- CPE
3
Catch-Per-Unit-Effort
 
CPUE or CPE
 
Calculated as
(Catch / Effort)*scaling factor
 
Examples
50 Lake Trout in 1600 m of gillnet set for 2 nights
50 Brook Trout in 1600 s of electrofishing
Abundance -- CPE
4
Catch-Per-Unit-Effort
Abundance -- CPE
5
 
 
 
Gillnet
 
Longlines
 
Electrofishing
 
Trawling
Catch-Per-Unit-Effort
Abundance -- CPE
6
 
Define
C
t
 = catch at time t
f
t
 = fishing effort at time t
q = catchability coefficient
proportion of population captured w/ one unit of effort
As long as one unit of effort is small (discuss more later)
 
What is …
q*f
t
q*f
t
*N
t
q*N
t
 
Thus, CPE measures 
relative
 abundance
Abundance -- CPE
7
C
t
/f
t
 = q*N
t
 
CPE directly estimates
abundance
“Things seem as they
really are” – i.e., if CPE
de/increases then
abundance de/increases
proportionately
CPE Measures Relative Abundance
 
CPE Measures Relative Abundance
 
ONLY …
of portion of stock vulnerable to the gear
if q is constant (see readings)
 
Abundance -- CPE
 
8
Abundance -- CPE
9
Non-Constant q (Hyperstability)
 
C
P
E
 
o
v
e
r
e
s
t
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m
a
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s
a
b
u
n
d
a
n
c
e
“Things seem better
than they really are”
Common when fisheries
are good at searching
and finding fish
Abundance -- CPE
10
Non-Constant q (Hyperdepletion)
 
C
P
E
 
u
n
d
e
r
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s
t
i
m
a
t
e
s
a
b
u
n
d
a
n
c
e
“Things seem worse
than they really are”
Common when fish get
sequentially harder to
catch (e.g., “spook”)
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Explore the concept of abundance estimates in fisheries management, including natural mortality, recruitment, population numbers, fishing mortality, immigration, and emigration. Learn about common abundance estimation methods like CPE/CPUE, depletion/removal estimates, and mark-recapture techniques. Understand how density of fish is determined in lakes and streams, investigate catch-per-unit-effort calculations, and grasp the significance of CPE in measuring relative abundance. Get insights into how fishing effort and catchability coefficients influence abundance estimates. Discover the impact of non-constant q values on abundance overestimation.


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  1. Abundance Estimates Natural Mortality Recruitment Population Numbers Fishing Mortality Immigration Emigration

  2. Common Abundance Estimates CPE/CPUE (relative density) Depletion/Removal (estimate of N0) Mark-Recapture (estimate of N0) 2 Abundance -- CPE

  3. Catch-Per-Unit-Effort Consider Lake A 4 nets, 40 total fish Lake B 2 nets, 30 total fish Which lake has a higher density of fish? Consider Stream A 50 fish in 25 minutes of shocking Stream B 60 fish in 35 minutes of shocking Which stream has a higher density of fish? 3 Abundance -- CPE

  4. Catch-Per-Unit-Effort CPUE or CPE Calculated as (Catch / Effort)*scaling factor Examples 50 Lake Trout in 1600 m of gillnet set for 2 nights 50 Brook Trout in 1600 s of electrofishing 4 Abundance -- CPE

  5. Catch-Per-Unit-Effort GillnetLonglines Electrofishing Trawling 5 Abundance -- CPE

  6. Catch-Per-Unit-Effort Define Ct = catch at time t ft = fishing effort at time t q = catchability coefficient proportion of population captured w/ one unit of effort As long as one unit of effort is small (discuss more later) What is q*ft q*ft*Nt q*Nt Thus, CPE measures relative abundance 6 Abundance -- CPE

  7. CPE Measures Relative Abundance Ct/ft = q*Nt CPE directly estimates abundance Things seem as they really are i.e., if CPE de/increases then abundance de/increases proportionately CPEt Abundance (Nt) 7 Abundance -- CPE

  8. CPE Measures Relative Abundance ONLY of portion of stock vulnerable to the gear if q is constant (see readings) 8 Abundance -- CPE

  9. Non-Constant q (Hyperstability) CPE overestimates abundance Things seem better than they really are Common when fisheries are good at searching and finding fish CPEt Abundance (Nt) 9 Abundance -- CPE

  10. Non-Constant q (Hyperdepletion) CPE underestimates abundance Things seem worse than they really are Common when fish get sequentially harder to catch (e.g., spook ) CPEt Abundance (Nt) 10 Abundance -- CPE

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