Fisheries Techniques, 3rd Edition

Chapters 6 & 7

Outline Today

Goals – Fish sampling gear (2 lectures)

1) Electrofishing

2) Passive and active gear

Accompanying lab: Thursday Sep 28

Capture Gear in Fisheries (& WL)

Which is the ‘active’ method?A. …we’ll eat like kings

B. …keep the lights in their eyes and you can bag them

Passive Techniques—Advantages

Passive Techniques—Disadvantages

Float Line

Lead Line

Entanglement

Gill nets

Trammel netsFloat Line

Lead Line

Entanglement

Problems with gill nets and trammel nets

Figure 7.2 – Mesh

Factors influencing

Gill Net CPUE

Passive Angling gears

SOPs

Mouth

Cod endLotic

Lentic

“wing or lead net”

Entrapment

Light traps Larval fish

Pot Gears

Active Advantages

Disadvantages

Electrofishing

Angling

Seining

Throw and cast nets

Toxicants?

TrawlsMouth

Cod end

Bottom Trawls

Midwater Trawls

Dredges

Encircling Nets

Nemo’s last scare

Float Line

Lead Line

OthersCast net

Gig

OthersCast net

Gig

Data and data analysis

Fisheries stock assessments

Data Relative abundance

Size structure

Growth

Mortality

Recruitment

Collecting specimens

Accessory data: diet, age (scales)

CPUE INDEX of density

Index of density trends

CPUE

Den

sity

Assumed pattern of CPUE vs. Density

A. Inversely proportional

B. Directly proportional

C. Exponential

D. Flat line (no slope)

CPUE

Den

sity

CPUE INDEX of density

Index of density trends

What else can affect relationship?CPUE

Den

sity

BOX 6.4

Techniques Book

CPUE Best solution:

rigid sampling protocol from time 1 to time x

Factors affecting gear choice

Things to consider

Usually want to be quantitative

And Representative

Factors influencing gear choice

All gear is selective Good – for business

Bad – for science

Results in…

…Bias—unknown shift from truth So standardize to detect changes

Factors influencing gear choice

Fish size / species

“Recruitment to the gear”

Pond fyke nets catch crappies not bass?

Factors influencing gear choice Environmental influences

Weather

Habitat – e.g., tree stumps and seining

Flow / turbidity

Crew experience

Equipment type and crew experience interaction

Animal behavior Know their biology/ecology

Recruitment to the gear

Factors influencing gear choice OBJECTIVES

INFERENCE

CONSISTENCY

Choose your gear properly

Defend your methods

Decision Tree

for ACTIVE

Gear Choice

Figure 7.8

Does NOT

substitute for

your thinking !

y = 2.11x - 14167

R2 = 0.8795

y = 1.58x - 10625

R2 = 0.8795

6000

8000

10000

12000

14000

16000

18000

6000 8000 10000 12000 14000 16000 18000

Acoustic Abundance

Ro

ten

on

e A

bu

nd

an

ce

1 day

3 day

Verification