Bycatch

Bycatch

1. What is bycatch?

   • The incidental or unnecessary catch of any undesired species
     • this could be species, age or sex related.
   • Individuals caught in this process could be unharmed, stressed, released with injuries or even killed.
   • been quoted as "the single greatest threat to marine mammals globally" - Read et al. (2006)
     • Affects many species around the globe including whales, dolphins, seals, porpoises, turtles & seabirds
   • wherever there is fishing there will be bycatch
     • There is very rarely any bycatch-free fishing

2. What species are vulnerable?

   • It is the PRIMARY threat to many endangered species around the world
     • e.g. Vaquita - with only around 30 individuals left in the wild, dropping every year
     • e.g. Hectors Dolphin-
     • E.g. Mediterranean Monk seal
     • also is the reason for the drop in the NA Right whale pop numbers
       • These are often entangled but are also hit and injured by ships
   • Fisheries bycatch has been attributed to population declines in many species including:
     • Leatherback Turtle -
     • Harbour porpoise
     • Wandering albatross
     • White-chinned Petrel
     • Mediterranean striped Dolphin
   • + MANY OTHERS

3. What are the main fisheries to cause bycatch?

   • Gillnets
     • This can be highly selective (mesh size)
     • Used to catch many species including bass, hake, salmon, ling , pollack etc.
     • Net sizes are very variable, from 15mm-250mm
     • We used to have nets that break under pressure
       • Now we have moved to nylon leading to much more bycatch
   • Purse Seine nets
     • This is the main method for catching Tuna
     • This method has a massive impact on bycatch of fish species in the US
     • Identify where the school of fish is, then place a net around the whole school
       • This can be done by spotting dolphin pods/ swarming seabirds
   • Trawls
     • Bottom trawls are more unspecific in catch
     • Two general types 1) Pelagc trawls & 2) Bottom trawls
       • Pelagic: take anchovies tuna shrimp & mackerel
       • Bottom: take Groundfish, squid, cod, halibut...
     • Bycatch consists of dolphins, whales and sharks, also young fish
   • Longlines
     • Long wires with hooks on
     • These can affect seabirds, shark, etc.
     • Can be set on the bottom or at the surface
     • Target: swordfish, halibut, toothfish

4. How can we mitigate bycatch?

   • spatio temporal closures
   • Visual alerts
   • Acoustic alerts
   • changes in fishing gear
   • reductions in fishing depth

5. Why are marine mammals so vulnerable?

   • Due to their long lifespan
   • They have a very low reproductive output and so only produce few calves every couple of years
   • Their iteroparous reproduction strategy