Fish Behavior near Gillnets By. Ledhyane I Harlyan
Dept. of Fisheries and Marine Resources Management Fisheries Faculty, Brawijaya University
Tujuan Instruksional Khusus Mahasiswa dapat mengetahui: 1. Mekanisme tertangkapnya ikan (capture mechanisme) dan desain alat tangkap 2. Selektivitas gillnet 3. Tingkah laku ikan 4. Conservation issues
Gillnets Gillnets characteristic: simple and versatile (Plain sheet of webbing salvaged to frame ropes) UNLIKE: Mobile gears no need to be towed Baited gears no need the addition of bait Fixed gears no need fixed structures and more portable COMPLEXITY: The construction affects species and size selectivity
Gillnetting
From time to time.. 3000 years ago : Net-making 1800s : natural fibers (cotton&hemp) used for net material 1950s : synthetic materials tested invisible gillnet webbings
Types of gillnets:
Set gillnets : anchored to the bottom Drift gillnets : drift the current (either tied or not tied to the vessel Trammel nets : 3 layers of nets (mid with smaller mesh size and two outer nets with larger one) Fixed gillnets : hung onto stakes to form a wall in tidal or shallow waters
Capture Mechanisms
Gilled : caught with the mesh behind the gill
Wedged : caught by the largest part of the body
Snagged : caught by the mouth or teeth (other head region)
Entangled : caught by spine, fins or other parts of body due to struggling mechanism
Key design of gillnets
Net material and color (natural vs synthetic) Monofilamen visibility rendah hasil tangkapan tinggi dibanding multifilament karena mudah kusut
Twine diameter & number of filament Twine diameter visibility
and easily tangled
The deeper depths, the lower will be seen Jaring yg lebih tipis cenderung menangkap lebih banyak, namun memiliki selektifitas yang buruk karena pengaruh “elongation”
Key design of gillnets
Mesh size or opening (Menentukan ukuran terbesar ikan yang dapat tertangkap di jaring) Kasus: Mayoritas ikan yg tertangkap pada gillnet hanya 20% yang memiliki panjang optimum (modal length) selektifitas ukuran susah tercapai karena ukuran morfologi yg berbeda2.
Net dimension (length & height) Panjang jaring mempengaruhi peningkatan effort Tinggi jaring berperan dalam species selectivity
Key design of gillnets
Vertical and horizontal ratios Rasio panjang tali (headrope/footrope) terhadap panjang tali yang ditarik maksimal. Nilainya merupakan gambaran kekenduran jaring.
Gillnet selectivity
Ilustrasi penggunaan hanging ratio thd beberapa bentuk ikan
Fish Behavior 3 factors affect static gear:
Fish availability
Vulnerability
Mobility (horizontal and vertical migrations) vertical pasang surut, light level horizontal fish encountering gillnets
Fish behavior Temperature affects distribution, movement, swimming capacity localized fish concentration fish vulnerability Kasus: swimming capacity are lower in lower temperature Reduction of activity and swimming capacity due to temperature
Potential area due to temperature The change of fishing area due to:
-water temperature -soaking durations
When fish encounter the net..
Gilled fish swim forward, pulling net with them The smaller ones escape by squeezing through the meshes Tangled fish swim backward or alongside the net
Low visibility in water has 2 effects:
+ Fish can not detect the net (low contrast between the net and background) high possibility of being caught Less visible if: in smaller diameter of material, night time hours, deep water, turbid water, type of material
- Fish move slower reduces encountering to the net
When fish encounter the net.. Gillnet catch in relation with soaking time
The catch rate initially decreased after setting 6 – 10 h and increased between 10-20 h. Pengurangan ini krn pengurangan schooling sesaat setelah setting, apalagi setelah di jaring terdapat ikan yang tersangkut (ikan yg tersangkut berjuang utk lolos, posisi ikan di jaring adl entangled) Small fish passing through the mesh Catchability Big fish turned away + never contact the net
Baited gillnets
Idenya berasal dari tingginya hasil tangkapan jika gillnet di setting dalam waktu yang relatif lama Ikan yg tertangkap di awal waktu rendaman akan berperan sebagai umpan untuk menarik perhatian lebih banyak ikan (Eng å s et al.2000 )
Baited gillnets vs Non-baited gillnets
Ikan menghabiskan byk waktu di baited gillnets dan lebih byk bersinggungan dengan jaring tersebut Ikan berenang lebih lambat di baited gillnets Ikan lebih banyak tertangkap secara tangled dibandingkan secara gilled mayoritas ikan tidak berenang secara langsung (tegak lurus) terhadap baited gillnets
Ghost fishing and solutions Gillnets lost due to weather or sea conditions or conflict with other fishing gears or vessel traffic Discarded at sea without any control of fishermen Continue to fish for an extended period of time Additional mortality of fish and other organisms Ghost Fishing
Ghost fishing and solutions Gear designs and modification to eliminate ghost fishing OR Reduce the fishing capacity of ghost gear DE-GHOSTING Technologies The causes: introduction of synthetic materials the gillnets will continue to fish without control Increased use in deep water Using derelict gillnet in shallower waters overgrown by algae more visible fishing capacity reduced
Research topics: 1.
2. 3.
4.
Mengetahui faktor-faktor yang mempengaruhi tingkat aktivitas ikan dan kemungkinan ikan untuk menemukan alat tangkap pasif Kemungkinan untuk menemukan jaring yang dapat dilihat pada hasil tangkapan (gillnet) Area aktif alami dan lainnya, berapa banyak ikan di dalam area tersebut yang tertangkap karena aroma umpan dan tingkah laku mencari makan. Seberapa banyak ikan-ikan tersebut menuntaskan pencariannya atas makanan dan tertarik pada umpan dan berapa jumlah ikan yang tertangkap pada hasil akhirnya. (longline).
Prevention of ghost fishing
Use degradable float material to lose floatation reduce vertical profile
Retrieve the derelict gillnets Clean up the fishing ground
MATUR NUWUN NUWUN.. ..
Tipe material Jaring sintetis yang dibuat dari jalinan yang tipis akan meningkatkan hasil tangkapan jika dibandingkan dengan jaring alami (natural fibres)
Jaring alami menggunakan bahan pengawet untuk menghindari degradasi (kebusukan)
Adanya arus semakin menyebabkan ikan akan mendeteksi adanya aroma tersebut
The contrast of net material hung vertically
