late 1980's further damaged the topsmelt 

 population (Nordby and Zedler 1991, Chapter 

 5). In 1991, the estuary was dominated by 

 arrow gobies and killifish, with very low 

 densities overall (PERL, unpubl. data). 



3.6.4 Ichthyoplankton 



Estuaries are often considered essential 

 spawning and nursery grounds for many fish 

 species. In order to determine the value of 

 estuarine channels to these functions, 

 comparisons with shallow coastal waters are 

 necessary. Nordby (1982) compared the 

 ichthyoplankton communities of the estuarine 

 channels within Tijuana Estuary with those in 

 adjacent nearshore waters. In addition, 

 comparisons were made between large, main 

 estuarine channels and tidal creeks. 



Larvae of 28 taxa of fishes representing 

 19 families and more than 27 genera were 

 collected during the study period, and eggs 

 from 18 taxa were found (Table 3.7; see lists 

 by species Nordby 1982 and in Zedler and 

 Nordby 1986). 



There were distinct differences in the 

 ichthyoplankton assemblages collected from 

 each habitat. Estuarine larvae demonstrated 

 patterns of spatial distribution related to 

 channel morphometry and other channel 

 organisms which indicated spawning habitat 

 preferences. 



Tidal creek larval collections were 

 dominated by longjaw mudsucker (Gillichthys 

 mirabilis) and Atherinidae, presumably 

 topsmelt. The longjaw mudsucker is known to 

 prey upon and inhabit the burrows of the 

 yellow shore crab (MacDonald 1975). High 

 densities of these larvae were collected where 

 crab burrows were abundant. Atherinid 

 larvae and eggs were associated with 

 macroalgal mats, primarily Enteromorpha 

 species, that grow in the low tidal-velocity 

 creeks. Topsmelt attach their eggs to the algal 

 blades, and the juveniles and adults feed on the 

 abundant plant material (Allen 1980, Nordby 

 1982). 



A complex of three indistinguishable goby 

 larvae comprised of arrow goby (Clevelandia 

 ios), shadow goby (Quietula y-cauda) and 

 cheekspot goby (llypnus gilberti) dominated 

 main channel larval collections (61%) while 

 longjaw mudsucker made up 29% of the total. 

 The goby complex larvae were densest at the 

 sampling station closest to the mouth of the 

 estuary, apparently due to substrate 

 preference. The substrate at this site was 

 mud/sand compared to mud and clay/shell 

 substrates of the other main channel sampling 

 stations. The arrow goby was the most 

 abundant post-larval goby collected; they have 

 been reported to live commensally with ghost 

 shrimp (MacDonald 1975). Ghost shrimp 

 burrows were common in mud/sand substrate. 

 Thus, it appears that the distribution of goby 

 complex larvae is related to substrate and 

 interaction with another channel organism. 



Because of their position relative to the 

 mouth of the estuary, larvae of the goby 

 complex were tidally transported to the 

 nearshore habitat where they comprised 57% 

 of the total. These species spawn in low tidal- 

 velocity with associated fine sediments 

 (Brothers 1975); thus, it is doubtful that 

 spawning occurred in the nearshore habitat. 

 Conversely, eggs from nearshore-spawning 

 species, especially Sciaenidae and Pacific 

 sardine (Sardinops sagax caeruleus), were 

 imported to the estuary during flooding tides. 

 These two species comprised 69% and 1 2% of 

 main channel eggs and 70% and 11% of 

 nearshore eggs, respectively. However, very 

 few larvae from nearshore species were 

 collected within the main channels, suggesting 

 that most imported eggs are not retained until 

 hatching. The nearshore larval dominants, 

 queenfish (Seriphus politus), white croaker 

 (Genyonemus lineatus) and northern anchovy 

 (Engraulis mordax), were rarely collected 

 within the estuary. 



Tidal flushing appears to be a determining 

 factor in the distribution of ichthyoplankton 

 at Tijuana Estuary. The presence of post- 

 larval Sciaenidae within the estuary suggests 

 that some nearshore spawned eggs can hatch or 

 develop there. The transportation of goby 

 species to the nearshore environment is 

 probably fatal. Brothers (1975) measured 

 98% mortality for spawned cohorts of arrow 



55 



