RELATIONSHIP BETWEEN DRIFTNET MESH SIZE AND BYCATCH RATES OF MARINE MAMMALS TAKEN FROM THE NORTH PACIFIC

The relationships between driftnet mesh size (M, mm) and bycatch rate (Y, number of animals per 1,000 tans) for northern fur seals, Callorhinus ursinus, and for small cetaceans (five species combined: Phocoenoides dalli, Lissodelphis borealis, Lagenorynchus obliquidens, Delphinus delphis, Stenella coeruleoalba and Mesoplodon sp.) were estimated from data collected in 413 driftnet fishing operations in the North Pacific during 1984-90. The driftnets were composed of 17 stretched-mesh sizes ranging from 33 mm to 197 mm and total fishing effort was 39,070 tans. Assuming that the theoretical number of marine mammals caught by driftnets follows either the Poisson or negative binomial distribution, the relationships were estimated as Y = 8. 81 X 10(-5)M2 for northern fur seals and Y = 5.29 X 10(-5)M2 for small cetaceans. The principal factor affecting mesh selectivity was the size of entangled body parts of the animals, the head for northern fur seals and the flukes for small cetaceans.