Catching the prey: Sampling juvenile fish and invertebrate prey fields of juvenile coho and Chinook salmon during their early marine residence

Marine diets of juvenile coho (Oncorhynchus kisutch) and Chinook salmon (Oncorhynchus tshawytscha) in the northern California Current are made up primarily of micronekton prey including juvenile fish, adult euphausiids, and large crab megalopae. However, these animals are seldom caught in the conventional plankton gears used to define juvenile salmon prey fields in ocean salmon programs. Four types of sampling gears with various mouth openings and configurations were examined for the ability to catch known juvenile salmon prey. Samples were examined for differences in species composition, relative biomass, length distribution, and taxonomic overlap with prey in the diets of salmon sampled concurrently. The herring, Marinovich, and Methot trawl nets generally caught juvenile prey fish such as hexagrammids, rockfish, cottids, and osmerids. These prey were in the 15–95 mm fork-length range, consistent with the type and size eaten by juvenile salmon. The bongo net sampled smaller invertebrate prey, which are rarely eaten by juvenile salmon, but instead are common prey of the juvenile fish that salmon consume. Overlap between prey fields and salmon diets was moderate for samples from the larger gear types but low for those from bongo nets towed in the same area. The fact that no gear matched exactly with coho and Chinook salmon diets was related to differences in catchability of the prey in different gears but may also in part be probably due to the high mobility of juvenile salmon, which enables these fish to consume food in locations distant from where they are sampled, and also to selectively feed in areas of high prey concentration. Based on our analysis, we recommend the use of micronekton gears with larger mouth openings and mesh sizes for better filtration rather than standard plankton gears (i.e., bongo nets) for direct estimates of available prey resources for juvenile coho or Chinook salmon. Sampling the abundance, size, and distribution of prey fields for juvenile salmon during their first summer in the ocean, a period of high natural mortality, may help us to better understand the mechanisms of bottom-up forcing on interannual changes in salmon mortality.