Oregon Sea Grant

Nearshore hypoxia within the Northern California Current (NCC) system is a seasonal phenomenon caused by coastal upwelling and occurs mainly during late-summer and early fall. The effects of low oxygen levels on fish and invertebrate communities, particularly during early-life history stages, however, are poorly known for this area. I investigated the effects of hypoxia on the density, community structure, vertical and horizontal distribution of fish larvae and juveniles, as well as body condition of juveniles, along the central Oregon and Washington coasts during the summers of 2008 - 2011. During this sampling period, bottom dissolved oxygen (DO) values ranged from 0.49 to 9.85 ml l^-1, and the number of hypoxic (e.g., < 1.4 ml l^-1) stations sampled was low compared to 2002 and 2006 (only 54 sampling stations for the ichthyoplankton study out of 493, and only 12 stations out of 90 for the benthic juvenile study). From the ichthyoplankton study, I found that the overall density of fish larvae increased as bottom-DO values increased; however, the effect on individual species density was limited. Between 44.65 ^oN and 46.00 ^oN (~Florence, OR – Astoria, OR), fish
larvae altered their vertical distribution when bottom-DO was low by rising in shallower water layers.

From the benthic juvenile study, I found that English sole (Parophrys vetulus), butter sole (Isopsetta isolepis), speckled sanddab (Citharichthys stigmaeus) and Pacific sanddab (Citharichthys sordidus) dominated the catch with annual variation in abundances. Species composition, abundance and length had strong relationships with depth. Species abundance for English sole (< 75 mm), speckled sanddab (<100 mm) and Pacific sanddab also increased with increased bottom-DO. However, the body condition of butter sole (< 75 mm) and of large speckled sanddab (≥100 mm) increased with decreased bottom-DO.

Overall my research elucidates important patterns of larval and juvenile fish distribution within the NCC during summer. In both studies I have found a limited effect of DO on abundance, distribution and community assemblages. Variables other than DO, such as depth, season and location, dominated the explained variance of the intervening multivariate and univariate analysis. However, due to the paucity of samples during hypoxic events, continued monitoring of nearshore larval and juvenile species over varying hypoxic conditions is necessary for understanding the impact of hypoxia on these communities and subsequent adult populations.