A Biogeographic Assessment off North/Central California

Analyses/Maps

Fish

National Marine Fisheries Service (NMFS) Demersal Trawls on the Continental Shelf
NMFS Shelf Trawls

Recently managers and scientists have begun to understand the importance of studying communities of species rather than just managing by individual species. This study was an initial assessment aimed at determining which species tended to be caught together, and where. Multivariate statistics were used to analyze species assemblages over trawlable habitats of the continental shelf between 55 and 500 meters depth off California. For an introduction to the continental shelf ecosystem, see section VII F in the Ecological Linkages Report. Sixty-one species were grouped into thirteen species assemblages, (Figure) and 883 trawls grouped into eight site groups (Table, Map). The average frequency of occurrence of species assemblages (percent occurrence calculated for each species and then averaged for each fish assemblage) within each site group was calculated to analyze the interaction between the species assemblages and site groups (Table). As with all data sets, the most significant result was the effect of depth on species assemblages. All attempts to isolate and remove the effects of depth in order to determine secondary effects were unsuccessful. Certainly secondary effects exist, but at the scale of this study, they were not discernible. Our results support previous results by Williams and Ralston (2002), Sullivan (1995), Field et al. (2002), Gabriel and Tyler (1980), and Matthews and Richards (1991), who found bathymetry to be an important factor in defining west coast demersal fish assemblages. Through this analysis, a large amount of information has been condensed to assemblages of co-occurring species, as well as groups of similar locations. A map is provided to visually portray the spatial arrangement of the results.

NMFS Shelf Trawls Species Assemblages

Table. Site group results for shelf trawl data. The number of trawls associated with each group as well as average depth ± standard deviation are provided. Different letters signify a significant difference using Tukey's pairwise comparison on log adjusted depth with overall a set at 0.001. Average frequency of occurrence of fish species assemblages (percent occurrence calculated for each species and then averaged for each fish assemblage) for each shelf site group are also provided. Numbers in bold represent influential species assemblages.
  Group 78 meters Group 93 meters Group 96 meters Group 119 meters Group 153 meters Group 268 meters Group 328 meters Group 415 meters
N 125 103 136 72 171 116 37 123
Depth±SD(m) 78±16a 93±19b 96±25b 119±37c 153±41d 268±52e 328±51f 415±48g
Rex Sole Assemblage 0.64 0.59 0.82 0.74 0.80 0.78 0.93 0.63
Pacfic Hake Assemblage 0.41 0.37 0.62 0.39 0.58 0.61 0.87 0.68
Shortspine Thornyhead Assemblage 0.20 0.23 0.28 0.32 0.39 0.64 0.96 0.83
Pacific Herring Assemblage 0.63 0.31 0.34 0.21 0.06 0.01 0.01 0.01
Halfbanded Assemblage 0.22 0.27 0.11 0.24 0.07 0.02 0.01 0.01
Pacific Sanddab Assemblage 0.90 0.91 0.88 0.82 0.55 0.13 0.10 0.03
Big Skate Assemblage 0.27 0.26 0.40 0.25 0.25 0.16 0.24 0.12
Chilipepper Assemblage 0.12 0.26 0.22 0.57 0.61 0.42 0.12 0.03
Darkblotched Assemblage 0.01 0.00 0.03 0.03 0.10 0.41 0.41 0.17
Blackgill Rockfish Assemblage 0.01 0.01 0.00 0.02 0.01 0.05 0.30 0.56
Canary Assemblage 0.02 0.10 0.06 0.21 0.15 0.04 0.01 0.00
Sharpchin Assemblage 0.01 0.01 0.02 0.05 0.13 0.23 0.10 0.01
Arrowtooth Flounder Assemblage 0.00 0.05 0.01 0.01 0.11 0.14 0.14 0.02