lecture 2 - fishing methods and gear.pdf

7/28/2019 Lecture 2 - fishing methods and gear.pdf

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Fishing gears and techniques (Chap. 5)

Types of gear and methods used:

Dependent on species targeted Active and passive gears Must account for fish behavior Choice of gear depends on intended market

Traps

Encourage entry and preventescape

baited or unbaited barrier or fence traps (pound nets) eels, menhaden, crabs, structure-

oriented fish (sea bass)

Hook and line

Hand-held J -hook vs. Circle hook commercial and recreational shark, halibut, deep water reef fishes

Trolling towed behind boats to catch

PELAGIC species simulating school of bait fish tuna, mackerel, dolphinfish


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Longlines set near surface for pelagic fish (e.g. tuna, sharks)

set near seafloor for demersal fish (e.g. halibut)

typically in open ocean up to 100km long, fishing to 400mdepth

Stationary Nets

Gill nets used in shallow water for mackerel,mullet, shad,

flounder, spot used in deeper water for demersal fish such as sharks very selective fishing gear (see Box 5.4)


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R e

l a t i v e

F r e q u e n c y

( % )

0

5

10

15

20

Red Drum Total Length (mm)300 350 400 450 500 550 600 650 700 750 800

0

5

10

15

Fall 1997n = 229

Spring 1998n = 369

Stationary Nets Trammel nets small mesh panel between 2 panels of larger mesh very effective for benthic species

Towed nets and dredgesOtter trawls otter boards (trawl doors), floats demersal or pelagic species fishing speed, gear weight, scope


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Towed nets and dredges

Beam trawl Horizontal beam with shoes, heavy Tow speed vs. gear weight Shrimp, flatfish

Towed nets and dredgesDredges Similar to beam trawls Chain bag and tooth plates Scallops, clams, gastropods


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Towed nets and dredgesDredges Most destructive to benthic habitat, but Bycatch of non-target species is low

Surrounding netsBeach seine efficient for juvenile fishes inshore small predators, bait fishes

Surrounding netsPurse seine efficient gear for pelagics Large catches (>20 tons) 1 or 2 boats to set net Fish located with sonar or

helicopter then...


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Aggregation devices

Take advantage of fishaggregation behavior

Floating rafts anchoredoffshore ($$)

Sargassummats, oil rigs(free to fishers)

Advantages: reducesearch time

Disadvantages: high costsand short lifespan

Other techniques

spears diving (can be very efficient) poisons explosives

Effects of fishing

1) Non-target bycatch (unwanted spp. & sizes)

What happens to escaped/returned individuals? Assume that fish escaping gear are:

not damaged minimally stressed make a complete recovery

However, many potential negative effects: Injury and stress Increased vulnerability to predators and disease Growth & reproductive capacity impaired Depend on gear (trawl, gillnet, hook and line)


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Effects of fishing

Gear modifications (TEDs) to reduce bycatch Trawl Efficiency Devices Trash Eradication Devices Turtle Excluder Devices

Diamond mesh

Square mesh

More Effects of fis hing

2) Ecological impacts on marinecommunities

Trophic dynamics and competition (e.g.,effects of krill fishery on whale populations)

3) Damage to benthic habitats demersal trawls and dredges can assess physical damage, but hard to

determine long term impacts on communities may prevent settlement of benthic animals may reduce structural complexity for small fish


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Industrialization of fishing has led tolarger, heavier gear that is moredamaging to the substrate


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2001 workshop:

Prioritized impacts onbiological structure

Identified otter trawls andscallop dredges as mostdamaging gears

Otter trawl door

Scars fromrepeated trawling

in Narragansett Bay, RI

Maine scallop dredgeremovingbiogenic structure


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0.2

0.4

0.6

0.8

1.0

1.2

1.4

20 40 60 80 100

Largest 15 year decline (%)

P o p u l a t i o n r e c o v e r y a f t e r 1 5 y e a r s

Resilience to Collapse

Gear selectivity

A good catch index depends on: consistent (precise) and accurate catch estimates catch rates depend on sampling gear used

Factors to consider when choosing a gear: fish girth vs. mesh size gear avoidance capabilities fish behavior (schooling, habitat use, activity

rhythms) day vs. night gear deployment random sample?


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Gear selectivity

Example of a selectivity ogive:

Gear selectivity

trawl

gill net

retention of sea breamin gill nets

selection curves

lecture 2 - fishing methods and gear.pdf

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