followed by about 6 months of low flow. Flow 

 volumes were on the order of 5 to 20 MGD in 

 late 1986 and early 1987 but increased again 

 in late 1987. More recent years had flows of 

 approximately 13 MGD (IBWC, unpubl. data). 

 From comparisons of Tijuana Estuary and Los 

 Pefiasquitos Lagoon (Nordby and Zedler 

 1991), and from experiments with both 

 fishes and molluscs (Nordby and Baczkowski, 

 unpubl. data), it has become clear the the 

 nonsaline character of the sewage flows has 

 significant effects on the channel biota. The 

 organic matter (through its biological oxygen 

 demand) and contaminants are also important, 

 but salinity dilution alone can stress or kill 

 the native species of Tijuana Estuary. 



5.8.1 Sampling to Document Changes in the 

 Channel Community 



The Tijuana Estuary channel monitoring 

 program has three sampling stations in 

 Oneonta Slough (Nordby and Zedler 1991); 

 they reflect differences in channel 

 morphometry (width, depth and substrate 

 type) and distance from the mouth. The Mouth 

 station received sewage continuously and 

 directly from Tijuana River; the northern two 

 sites were diluted by sewage when incoming 

 tides forced river flows to spread laterally. 

 The Mouth station is just inside the ocean inlet 

 on a side channel paralleling the Tijuana 

 River. This is the shallowest site (< 0.5 m); 

 it has sloping banks 6-7 m wide with a sand 

 substrate. The E-W Channel station is about 

 1800 m from the mouth in a very old man- 

 made channel that runs east to west and links 

 the former sewage lagoons with Oneonta 

 Slough. This is the deepest site (usually about 

 1 m); it has eroding banks 10-11 m wide. 

 The substrate is a clay/mud mixture with 

 broken shell fragments in the upper 10-15 

 cm over a bed of coarse sand/gravel. The Sea 

 Coast Channel station is about 900 m north of 

 the mouth; it is 15 m wide and usually less 

 than 1 m deep; it has a sand substrate. 



Nordby and Zedler (1991) analyzed 12 

 quarterly samples of fishes (June 1986- 

 March 1989), 10 quarterly samples of 

 bivalves (Sept. 1986-Dec. 1989), and 7 

 quarterly samples of polychaete and other 



benthic species (June 1987-Dec. 1988) to 

 identify impacts of sewage inflows. 



5.8.2 Response of the Fish Community 



Over the three-year period, 21 species of 

 fishes representing 15 families were 

 collected. The dominants were arrow goby 

 (Clevelandia ios) 75%, topsmelt (Atherinops 

 affinis) 19%, and California killifish 

 (Fundulus parvipinnis) 3% The remaining 

 19 taxa added 3% to the total fish caught 

 (Nordby and Zedler 1991). 



During the study, the fish community 

 shifted from one co-dominated by topsmelt and 

 arrow gobies to dominance by arrow gobies 

 (Table 5.7). There were 14 species of fish in 

 September 1986 and only 6 species in 

 December 1988 and March 1989. The largest 

 drop occurred between September 1986 and 

 June 1987, when sewage inflows became 

 more frequent and voluminous. The number of 

 species declined with proximity to the mouth 

 (two-way ANOVA without replication, 

 p<0.05), with the Sea Coast Channel averaging 

 7 species; the E-W Channel, 6 species; and 

 the Mouth, 5 species. Fishes also declined in 

 size. Topsmelt averaged 188 mm, 121 mm, 

 and 68 mm for the three successive June 

 sampling periods (one-way ANOVA, p<0.001; 

 Nordby and Zedler 1991). 



The community shift also represents a 

 change in life history strategy toward short- 

 lived species with prolonged reproductive 

 periods. Whereas topsmelt are longer lived, 

 the arrow goby matures within one year 

 (Brothers 1975) and spawns from September 

 through June. 



Increased abundances of arrow gobies may 

 have been influenced by reduced predation 

 pressure from other fishes, e.g., California 

 halibut (Paralichthys californicus) diamond 

 turbot (Hypsopsefta guttulata), Pacific 

 staghorn sculpin (Leptocottus armatus), and 

 California killifish (Fundulus parvipinnis). 

 The longjaw mudsucker (Gillichthys 

 mirabilus), which may have been extirpated 

 by the 1984 nontidal drought, is also an 

 aggressive predator that may have had an 



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