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 prevent escape ¾ baited or unbaited ¾ barrier or fence traps (pound nets) ¾ 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

1 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 400m depth

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)

2 20 Fall 1997 n = 229 15

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0 Spring 1998 n = 369 15 Relative Frequency (%) 10

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0 300 350 400 450 500 550 600 650 700 750 800 Red Drum Total Length (mm)

Stationary Nets Trammel nets ¾ small mesh panel between 2 panels of larger mesh ¾ very effective for benthic species

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

3 Towed nets and dredges Beam trawl • Horizontal beam with shoes, heavy • Tow speed vs. gear weight • Shrimp, flatfish

Towed nets and dredges Dredges ¾ Similar to beam trawls ¾ Chain bag and tooth plates ¾ Scallops, clams, gastropods

4 Towed nets and dredges Dredges • Most destructive to benthic habitat, but • Bycatch of non-target species is low

Surrounding nets Beach seine ¾efficient for juvenile fishes inshore ¾small predators, bait fishes

Surrounding nets Purse seine • efficient gear for pelagics • Large catches (>20 tons) • 1 or 2 boats to set net • Fish located with sonar or helicopter then...

5 Aggregation devices

• Take advantage of fish aggregation behavior • Floating rafts anchored offshore ($$) • Sargassum mats, oil rigs (free to fishers) • Advantages: reduce search time • Disadvantages: high costs and 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? ¾ Assume that fish escaping gear are: • not damaged • minimally stressed • make a complete recovery ¾ 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)

6 Effects of fishing Gear modifications (TED’s) to reduce bycatch ¾Trawl Efficiency Devices ¾Trash Eradication Devices ¾Turtle Excluder Devices

Diamond mesh

Square mesh

More Effects of fishing

2) Ecological impacts on marine communities • 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

7 Industrialization of fishing has led to larger, heavier gear that is more damaging to the substrate

8 2001 workshop:

Prioritized impacts on biological structure

Identified otter trawls and scallop dredges as most damaging gears

Scars from repeated trawling in Narragansett Bay, RI

Otter trawl door

Maine scallop dredge removing biogenic structure

9 Resilience to Collapse

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P 20 40 60 80 100 Largest 15 year decline (%)

Gear selectivity A good catch index depends on: ¾consistent (precise) and accurate catch estimates ¾catch rates depend on sampling gear used

Factors to consider when choosing a gear: ¾ fish girth vs. mesh size ¾ gear avoidance capabilities ¾ fish behavior (schooling, habitat use, activity rhythms) ¾ day vs. night gear deployment ¾ random sample?

10 Gear selectivity

Example of a selectivity ogive:

Gear selectivity

trawl

gill net

retention of sea bream in gill nets

selection curves

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