Cluster analysis used to re-examine fleet definitions of North Pacific fisheries with spatiotemporal consideration of blue shark size and sex data

Citation
Kinney MJ, Carvalho F, Kai M, et al (2021) Cluster analysis used to re-examine fleet definitions of North Pacific fisheries with spatiotemporal consideration of blue shark size and sex data. In: ISC 2021 2nd Shark Working Group Workshop. ISC/21/SHARKWG-1/Info-01, Online
Abstract

This study looked at re-examining the North Pacific fleets that have been used for previous assessments of blue shark by investigating the size and sex composition data from observer records, port and scientific samples in greater detail. Our goal is to provide information that can be used by the ISC shark working group to more appropriately define fleet structure for the assessment based on size and sexual composition of the catch. Ultimately, refining fleet structure within the model with greater consideration for the spatiotemporal characteristics of blue shark catch may help reduce model misspecification in future assessments. We analyzed nearly 600,000 individual records of blue shark size and sex information divided across 240 5 x 5° grid cells covering the North Pacific. A clustering approach was taken to discern areas with related size and sex compositions. Results suggested four distinct clusters, where Clusters 1 and 4 (made up primarily of smaller immature animals) predominate in the catch at higher latitudes (north of ~25°N), especially in the eastern and western edges of the North Pacific (waters nearer the coasts). While Cluster 2 (mature males and females) and Cluster 3 (mostly males, both mature and immature) predominate in a band from ~ 20°N to near the equator. During fall and winter (seasons 1 and 4) this band of mature animals expands north in central Pacific waters, loosely around Hawaii, as high up as ~40°N. We suggest that this work, along with several other studies carried out by various members of the ISC shark working group over the years, be used to better define the fleets used in future assessments of blue sharks in the North Pacific.