Assemblage Structure of Eastern Pacific Groundfishes on the U.S. Continental Slope in Relation to Physical and Environmental Variables

Abstract

We analyzed data from the National Marine Fisheries Service continental slope trawl surveys (1999–2002) to examine patterns in eastern Pacific groundfish assemblage structure in relation to longitude, latitude, depth, temperature, and interannual variation. The slope surveys spanned approximately 32–48°N and 200–1,200-m depths. We focused on the 26 most abundant species, which made up more than 95% of the catch in terms of biomass. Assemblage structure was strongly correlated with depth and latitude. For the most part, changes in assemblage structure appeared to be fairly continuous, although there were more-abrupt changes at approximately 500–600 m and in the vicinity of Point Conception and Cape Mendocino, California, and Cape Blanco, Oregon. There was also an interaction between depth and latitude; more northerly sites had deeper scores on the canonical axes than did more southerly sites of comparable depth. Shallower sites also showed more variation in assemblage structure with latitude than did deeper sites. We were able to identify five assemblages on the continental slope. Pacific grenadiers Coryphaenoides acrolepis, giant grenadiers Albatrossia pectoralis, deepsea soles Embassichthys bathybius, longspine thornyheads Sebastolobus altivelis, California slickheads Alepocephalus tenebrosus, and roughtail skate Bathyraja trachura formed a deepwater assemblage with a broad latitudinal range. Southern species that inhabited very shallow waters included splitnose rockfish Sebastes diploproa, spotted ratfish Hydrolagus colliei, English soles Parophrys vetulus, stripetail rockfish Sebastes saxicola, chilipeppers Sebastes goodei, and shortbelly rockfish Sebastes jordani. Shallow-water, midlatitude species included spiny dogfish Squalus acanthias, longnose skate Raja rhina, Pacific hakes Merluccius productus, rex soles Glyptocephalus zachirus, bigfin eelpout Lycodes cortezianus, and sandpaper skate Bathyraja interrupta. Darkblotched rockfish Sebastes crameri, arrowtooth flounder Atheresthes stomias, and Pacific Ocean perch Sebastes alutus were identified as a shallow, northerly assemblage. The final group—sablefish Anoplopoma fimbria, brown cat sharks Apristurus brunneus, aurora rockfish Sebastes aurora, Dover soles Microstomus pacificus, and shortspine thornyheads Sebastolobus alascanus—occurred at different depths over a range of latitudes and might be best described as a shallow–middepth and midlatitude–southern grouping.