Body size dependence of euphausiid spatial patchiness

Abstract

We analyzed size‐dependent variations in spatial patchiness of the eight numerically dominant euphausiid species in the California Current System (Euphausia pacifica, Nematoscelis difficilis, Nyctiphanes simplex, Thysanoessa gregaria, Euphausia recurva, Euphausia gibboides, Thysanoessa spinifera, and Euphausia eximia). Patchiness was measured by using a count‐based statistic using euphausiid densities and applied to 11 yr of detailed size‐specific enumerations from the California Cooperative Oceanic Fisheries Investigations program. We rejected the hypothesis of size‐independent patchiness for seven of the eight species. The most common pattern observed was a “U‐shaped” curve, showing elevated patchiness in the smallest size classes, a rapid decrease in patchiness of intermediate‐sized euphausiids, and a later increase in patchiness of adults following the onset of reproductive maturity. These size‐dependent changes parallel ontogenetic changes in spatial dispersion observed for some marine fishes. The initial descending limb of the patchiness curve appears to be caused by turbulent diffusion, while the later ascending limb of the curve is consistent with the onset of predator‐induced aggregation behavior. The patterns were surprisingly consistent across years and different reproductive characteristics (egg‐brooding vs. broadcast spawning euphausiids).