Comparison of biomass estimates from multiple stratification approaches in a swept area method for Pacific saury Cololabis saira in the western North Pacific

Hideaki Kidokoro,Toshihide Kitakado,Dharmamony Vijai,Hiroomi Miyamoto,Midori Hashimoto,Yasuhiro Ueno,Miyako Naya,Satoshi Suyama,Taiki Fuji

doi:10.1007/s12562-020-01407-3

Abstract

Fishery-independent surveys using sea surface trawl nets for Pacific saury Cololabis saira in the western North Pacific since 2003 have enabled the investigation of their annual distribution patterns and total biomass during June and July, prior to the main fishing season in Japan. We compared biomass estimates and their associated variances derived based on five different post-stratification approaches in a swept area method, and then observed that these approaches have little effect on biomass estimates and their precision, owing to well-organized survey designs and homogeneously allocated sampling stations. We were able to utilize decreasing biomass estimates for 15 years as an essential abundance index in the ongoing stock assessment. Notably, examination based on stratification along with longitudinal survey lines indicated that the estimated biomass had decreased in the western survey area, resulting in an eastward shift in the gravity center of Pacific saury distribution after 2010. We recommend biomass estimation in an east–west direction based on longitudinal stratification as an effective measure to develop population dynamics models which reflect westward migration into the fishing grounds around Japanese coastal waters, and to forecast the expected catch during the subsequent fishing period.

Highlights

Pacific saury Cololabis saira is a small pelagic fish broadly distributed across the North Pacific Ocean (Hubbs and Wisner 1980)
Our surveys indicate that Pacific saury are widely distributed across the western half of the North Pacific Ocean and preferably inhabit water with sea surface temperature (SST) of 7–15 °C during the survey period in June and July
Density was significantly associated with both longitude and SST, and both variables were expected to perform effectively in the process of area stratification

Summary

Introduction

Pacific saury Cololabis saira is a small pelagic fish broadly distributed across the North Pacific Ocean (Hubbs and Wisner 1980). Between April and July, it migrates towards the foraging ground located in its northern habitat (Fukushima 1979) and presents at least from the Kuroshio-Oyashio transition region to the subarctic region in the west of 165°W longitude during summer (Suyama et al 2016) It migrates westward, reaching the Kuril Islands and Japanese coastal waters between August and December (Suyama et al 2012; Miyamoto et al 2019). Fisheries Science fleets operate mainly within their domestic exclusive economic zones from August to December, most other members target Pacific saury mainly in the high seas west of 165°E from June to December (Fukushima 1979; Kosaka 2000; Huang 2010) This stock has been internationally managed since 2015 by an inter-governmental organization, the North Pacific Fisheries Commission (NPFC), to ensure its long-term conservation and sustainable use. An abundance index derived from fishery-dependent information might introduce biases in biomass estimates, because it does not decline in proportion to the decrease of true biomass owing to the concentration of fishing efforts in areas of high density, known as hyperstability (Hilborn and Walters 1992)

Methods

Results

Conclusion