Shoaling Behavior as a Tool to Understand Microhabitat Use by Juvenile Chum Salmon, Oncorhynchus keta

Abstract

Density is most often used to compare estuarine microhabitats to determine if they are nurseries for juvenile marine fishes and invertebrates. But using only density fails to determine how organisms are using, or meeting life requirements, each microhabitat. Measuring in situ behavior of organisms is a tool that can complement density to better clarify how organisms are using estuarine microhabitats. Juvenile chum salmon, Oncorhynchus keta, is an example of an organism where behavior in nearshore estuarine microhabitats can be used to understand how they are using these microhabitats. Two estuarine microhabitats where juvenile chum salmon are found are the cobble/gravel beaches of the upper intertidal and eelgrass beds, Zostera marina, in the lower intertidal. The upper intertidal is about 2 – 3 times steeper than the lower intertidal. Eelgrass beds have long been thought to be particularly important because prey preferred by juvenile chum salmon are often found in higher densities in lower intertidal eelgrass beds. This study developed an in situ method of observing juvenile chum salmon shoaling behavior in nearshore microhabitats by video recording a 0.25 m2 quadrat nested inside a larger 8-m x 6-m rope grid divided into 2-m x 2-m squares. Shoaling behavior was used as a method to examine microhabitat use because predation and foraging are the two main factors that structure fish shoals. A number of quantitative metrics of shoal cohesiveness were developed and 4 of the 7 of these metrics produced significant results that indicated shoals were more cohesive in the lower intertidal than in the upper intertidal. From shoal movements and the depth distribution of fish in the upper and lower intertidal microhabitats it was discovered that there is a “stay shallow” mandate for juvenile chum salmon that likely developed as a way to avoid predators. This stay shallow mandate possibly limits microhabitat availability of the lower intertidal microhabitats when the tidal level is high. Also, because of the combination of differences in slope between upper and lower intertidal microhabitats and the stay shallow mandate, any interpretations of only density are likely to be flawed because fish concentrate in areas with the steeper beach slope. Possible explanations are increased predation in the lower intertidal or decreased foraging in the lower intertidal. One site, Tabook Point, in Dabob Bay had the lowest indices of shoal cohesiveness for both years as well as higher than expected density in the lower intertidal. This indicates that this site may be an important potential foraging ground during low tides.