Table 5.7. Relative abundance (%) and total 

 numbers of fish collected at Tijuana Estuary 

 from 1986-1989 (from Nordby and Zedler 

 1991). 



important role in reducing goby populations 

 prior to this study (Nordby and Zedler 



1991). 



5.8.3 Benthic Invertebrate Responses 



Fifty-eight taxa of polychaetes and 

 bivalves were collected from September 1986 

 to June 1988 (Nordby and Zedler 1991). The 

 California ghost shrimp (Callianassa 

 californiensis), a decapod crustacean, was 

 also present. Through the sampling period, 

 there was both a decline in the numbers of 

 species and in densities. Bivalve densities 

 were highest in September 1986 (with 



~2500/m 2 collected at The E-W Channel). 



Jackknife clams (Tagelus californianus) 

 were initially dominant but declined in 1987 

 and again in 1988. Ghost shrimp were found 

 in low densities in 1986 but increased 

 thereafter. The false mya (Cryptomya 

 californica) was absent in 1986 but became 

 more common through time, along with its 

 commensal, the ghost shrimp. Littleneck 

 clams (Protothaca staminea) also increased in 

 abundance through the study. Mean sizes of 



the two dominant species (jackknife clams, 

 40.3 mm; littleneck clams, 11.2 mm) 

 suggested that most were 0-1 year old. Thus, 

 Nordby and Zedler (1991) concluded that the 

 benthic community was largely newly 

 recruited individuals. 



Highest bivalve densities were generally 

 at the E-W Channel, which is farthest from 

 the river of sewage. One other disturbance, 

 dune washover, complicated the picture. 

 Station TJE1 was buried with several cm of 

 coarse sand twice during the study and dredged 

 once to remove sediments. Turbid water 

 and/or mechanical removal were no doubt 

 detrimental to the bivalves. 



Capitellids and spionids were the dominant 

 polychaetes (Table 5.8). Greater numbers of 

 polychaetes were collected in 1988 than in 

 1987, especially at the Mouth station. 

 Capitellids and spionids, primarily Polydora 

 nuchalis and P. cornuta were the most 

 abundant in 1988; both are known to tolerate 

 pollution. Polychaete densities were low until 

 September 1988, when densities exceeded 

 4,000/m 2 (Nordby and Zedler 1991). The 

 dominant polychaete species are ones that 

 mature quickly; some capitellids become 

 reproductive within a month and may 

 reproduce year-round (Grassle and Grassle 

 1976). 



5.8.4 Cause-Effect Relationships 



Nordby and Zedler (1991) concluded that 

 salinity dilution was the primary cause of 

 species eliminations, density reductions, the 

 predominance of young animals, and 

 dominance by species with early reproductive 

 maturity and prolonged spawning periods. 

 Three reasons were given: 



First, historical streamflow data showed 

 that summer flows were rare or absent and 

 that large, old bivalves were abundant. Prior 

 to the 1978 flood, Hosmer (1977) measured 

 the sizes of the dominant bivalves and found 

 means of: 71 mm for Nuttallia nuttallii; 22 

 mm for littleneck clams and 27 mm for 

 jackknife clams. Of these species, Nuttallia 



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