The timing and location of spawning for the Euphausiid Thysanoessa spinifera off the Oregon coast, USA

Thysanoessa spinifera eggs were sampled biweekly from 1997–2005 along a transect extending off the coast of Newport, OR, USA. T. spinifera eggs were typically found in greatest abundance at NH05, our shallower mid-shelf station, and in lowest abundance at NH25, our offshore, deep-water station beyond the shelf break. In most years small peaks in density of T. spinifera eggs were found in late winter (February–March) and/or spring (April–May) along with large, prolonged peaks in summer, from July–September. However, it was more common to find egg densities of <1 m−3 or to find no eggs at all (58–91% of sampling dates per year had densities <1 m−3 at NH05). We found that egg densities were significantly positively correlated with chlorophyll a concentrations during the winter and spring (r2=0.52 and 0.55 respectively, p<0.001), but not during summer. We did not find a significant correlation between egg densities and female densities.When winters were stormy, as in 1998, 1999 and 2000 the first eggs of Thysanoessa spinifera were not observed at any station until after upwelling was initiated later in the spring. However, in other years eggs were likely to be found earlier in the year if there were fewer storms, or winter or spring upwelling events that were not followed by a large storm. In most years, spawning continued until the upwelling season ended in the autumn, however this trend ceased in 2003–2005 and spawning was interrupted earlier in the season. Overall, we found that chlorophyll a peaks and egg peaks increased in magnitude in the later part of our study. We have concluded that T. spinifera is likely an intermittent spawner, whose ovaries are not constantly mature and prepared for spawning, despite the presence of ocean conditions that are suitable for spawning.