Growth rates and species composition of juvenile rockfish (Sebastes spp.) in Oregon’s nearshore and estuarine habitats

Graduation date: 2008Identification of critical habitat for all life stages of commercially exploited \ud fish populations is critical for effective management. Despite a clear need for basic \ud biological information on juvenile rockfish life history, there have been very few \ud efforts to describe distribution and habitat of this life stage, particularly along the \ud Oregon coast. This study investigated the relationships between habitat-type, species \ud composition and growth of juvenile rockfishes following settlement into nearshore \ud reefs and estuaries. By using a four-level classification system to prioritize and \ud identify Essential Fish Habitat, results of this study refine scientific knowledge of \ud EFH for Oregon’s nearshore rockfish species. \ud Four species of rockfish were collected during the summers of 2004 and 2005. \ud blue rockfish (S. mystinus), black rockfish (S. melanops), yellowtail rockfish (S. \ud flavidus) and widow rockfish (S. entomelas). Nearshore hand net samples were \ud dominated by blue rockfish. Estuary samples were almost exclusively black rockfish, \ud indicating that this species is common in Yaquina Bay. No black rockfish were \ud collected from our nearshore sampling sites in either 2004 or 2005, although this \ud habitat is listed as common for juveniles of this species. \ud Identification of critical habitat for all life stages of commercially exploited \ud fish populations is critical for effective management. Despite a clear need for basic \ud biological information on juvenile rockfish life history, there have been very few \ud efforts to describe distribution and habitat of this life stage, particularly along the \ud Oregon coast. This study investigated the relationships between habitat-type, species \ud composition and growth of juvenile rockfishes following settlement into nearshore \ud reefs and estuaries. By using a four-level classification system to prioritize and \ud identify Essential Fish Habitat, results of this study refine scientific knowledge of \ud EFH for Oregon’s nearshore rockfish species. \ud Four species of rockfish were collected during the summers of 2004 and 2005. \ud blue rockfish (S. mystinus), black rockfish (S. melanops), yellowtail rockfish (S. \ud flavidus) and widow rockfish (S. entomelas). Nearshore hand net samples were \ud dominated by blue rockfish. Estuary samples were almost exclusively black rockfish, \ud indicating that this species is common in Yaquina Bay. No black rockfish were \ud collected from our nearshore sampling sites in either 2004 or 2005, although this \ud habitat is listed as common for juveniles of this species. \ud Blue rockfish settlement was detected from April thru July in 2004, and from \ud May thru June in 2005. Peak settlement timing was in June of both years. Settlement \ud for black rockfish ranged from March thru July both years, with peak settlement \ud timing occurring in mid-May. Yellowtail and widow rockfish juveniles were rarely \ud encountered, with settlement occurring in April and July respectively. \ud Abundance, as estimated from catch per unit effort (CPUE) calculations, \ud varied among years and habitats. Black rockfish were more abundant in 2004 than in \ud 2005. Some of the highest densities were found around pilings, docks and other \ud anthropogenic structures: CPUE decreased within rock, eelgrass, and sand habitats \ud respectively. \ud Growth was not significantly different among habitats for either black or blue \ud rockfish. Growth rate differences were not significant among years for either black or \ud blue rockfish. However, growth differences were significant between species (black \ud 0.50 mm d-1 and blue .54 mm d-1 (two sample-t test, T stat = -3.19, df = 133 P-value \ud <.002). \ud Additionally, this thesis presents a tank validation experiment that investigated \ud the validity of traps as a quantitative measure of habitat quality. This trap validation \ud study showed that as habitat complexity increases, trap efficiency decreases. Trap \ud attractiveness is an important source of bias when used for juvenile rockfish \ud abundance estimation. This study demonstrates abundance estimates may be inflated \ud in areas of low habitat complexity in the natural environment, or under-estimate the \ud relative abundance of rockfishes within complex habitats. \ud This study identifies estuaries as Essential Fish Habitat for black rockfish \ud juveniles along the central Oregon coast, and nearshore reef as EFH for blue rockfish \ud juveniles. Results of this study also provide specific information on spatial and \ud temporal patterns of recruitment, habitat selection and growth, previously unknown for Oregon's nearshore Sebastes species