Twenty-three bottom-trawl fish assemblages were identified from the relative biomass of
33 dominant species that occurred in the National Marine Fisheries Services' triennial
trawl surveys over the continental shelf and upper slope off California, Oregon, and
Washington from 1977 to 1992. The assemblages accounted for about 70% of the total
variation in species composition among 2,565 hauls. Although the assemblages persisted
over the 15-yr study period and occurred within broad geographic boundaries, some had
substantially different spatial distributions among surveys. The ability to differentiate
assemblages across five environmental variables (latitude, depth, surface and bottom
water temperatures, and surficial substrate) was low. The preponderance of hake-dominated
assemblages throughout the study area suggests that Pacific hake (Merluccius
productus) may play a large role in the dynamics of demersal fish communities off the
west coast of the United States. The same data were used to establish general features
regarding the abundance and distribution of the 33 dominant fish, and investigate
intersurvey regional variation in species composition. Flatfish were generally widespread
and at low density within areas of their occurrence. In contrast, rockfish were
comparatively less widespread, and were at higher density within areas of their
occurrence. Pacific hake, spiny dogfish, and sablefish were both widespread and
occurred in high density. The greatest amount of variation in species composition
occurred in the shallow shelf region off California, and the shallow and deep regions
between Cape Mendocino and Cape Blanco. These regions corresponded to areas with
the greatest amount of annual variation in upwelling. Contrary to upwelling, intersurvey
variation in surface temperature did not appear to correspond to variation in species
composition, but there was an unanticipated negative correlation between variation in
bottom temperature and variation in species composition. Species composition was
influenced in most regions by Pacific hake, spiny dogfish, and sablefish. A conceptual
model was developed to explore the relationship between regional changes in a species'
biomass, incidence, and density, and their potential affect on species composition.
Empirical examination of the model was difficult. Five of six flatfish species (Pacific
sanddab, rex sole, Pacific halibut, Dover sole, and English sole) exhibited a significant
positive linear relationship between incidence and log-transformed biomass which is
consistent with density-dependent habitat selection. There was evidence (albeit weak)
from patterns in the occupancy of substrate types by these flatfish, that marginal habitats
are associated with areas of mud for Pacific sanddab and areas of sand for rex sole,
Pacific halibut, and Dover sole. / Graduation date: 1997
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/34472 |
| Date | 25 April 1996 |
| Creators | Jay, Chadwick V. |
| Contributors | Sampson, David |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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