Bycatch of deep dwelling cetacean in gillnet fisheries of Pakistan
Gillnet being deployed for catching tuna and tuna like species is known to be marred with high bycatch of non-target species including cetaceans. Studies have indicated that small cetaceans mainly dolphins frequently get entangled and die in gillnets that are placed on the surface of the sea. However, introduction of subsurface gillnetting (placing net 2 m below sea surface) led to major reduction in the entanglement of cetacean. This mode of gillnet operation was adopted by entire tuna gillnet fleet in Pakistan which eliminated mortality of cetaceans in Pakistan. Studies have, however, revealed that subsurface gillnetting is not effective against deep dwelling cetaceans. Species belonging to family Delphinidae (Risso’s dolphin), Family Kogiidae (dwarf sperm whales and pygmy sperm whales) and Family Ziphiidae (Longman’s beaked whale, Mesoplodon sp. and Cuvier's beaked whale) were reported to get entangled in gillnets placed on both surface and subsurface of sea. These deep dwelling species dive to deep sea (possibly deeper than 300 to 500 m) to feed mainly on meso- and bathypelagic cephalopods, fish and crustaceans. It seems that while surfacing, these cetaceans cannot avoid gillnet placed on surface or even subsurface. The study further revealed that entanglement of Risso’s dolphin (Grampus griseus), dwarf sperm whales (Kogia sima) and pygmy sperm whale (Kogia breviceps) has an increasing trend since 2015 till 2019. Main entanglement of these species were observed during Pre-Southwest Monsoon Period (March and April) whereas limited entanglements were observed in other parts of the years. Study has further revealed that the entanglement of all deep dwelling cetaceans were more frequent between 1,000 and 2,000 m. The study also reports for the first time entanglement and release of Longman’s beaked whale Indopacetus pacificus, an unidentified species of Mesoplodon sp. and Cuvier’s beaked whale (Ziphius cavirostris) in subsurface gillnets deployed along Pakistan coast. The study also suggests that Murray Ridge and continental slope along Indus Swatch seems to be hotspot of deep dwelling cetaceans.