Monthly Change in Food Size of Larval Walleye Pollock Theragra chalcogramma at the Initial Feeding Stage in Funka Bay and Surrounding Vicinity in Hokkaido

Abstract

Larvae of walleye pollock Theragra chalcogramma were collected by vertical hauls from the sea bottom to the surface with a NORPAC net from January to March 1991 in Funka Bay and surrounding vicinity in Hokkaido. Larvae were abundant in January and decreased in February and March. To clarify the food availability of walleye pollock larvae at the initial feeding stage, the size of food for larvae smaller than 7 mm in total length (TL) were measured. The total length of larvae collected in January ranged from 4.2 to 6.5 mm. Most of 4.2-5.1 mm TL larvae had empty stomachs. Food organisms were found from 7 larvae (4.8-6.5 mm TL). Among them, copepod nauplii were dominated (60.5%), followed by invertebrate eggs (25.6%) and copepodids (13.9%). The body width of Oithona nauplii in the stomachs ranged from 84 to 121 μm and Paracalanus nauplius was 103 μm. Larvae of 4.0 to 13.2 mm TL were collected in February. Six larvae (<7.0 mm TL) fed on copepod nauplii (67.6%), invertebrate eggs (20.6%), copepodids (8.8%), and diatom Coscinodiscus sp. (3.0%). Copepod nauplii in the stomachs were larger than 95 μm in width. In March, food organisms taken by 5 larvae (<7 mm TL) consisted of copepod nauplii (54.5%), invertebrate eggs (36.4%), and copepodids (9.1%). The width of copepod nauplii in the stomachs ranged from 156 to 176 μm. From these observations, it is considered that the primary food items of larval walleye pollock at the initial feeding stage are large-sized copepod nauplii (> 84 μm) in this area. © 1995, The Japanese Society of Fisheries Science. All rights reserved.