As one of the largest (~1000 ha) and least-fragmented coastal wetlands in southern California, Tijuana River National Estuarine Research Reserve provides diverse habitats which support many plants and animals, including 24 sensitive species. PERL has been monitoring the vegetation, fish, and invertebrate communities along with water quality parameters since 1986. This long-term data set allows us to evaluate ecosystem responses to environmental changes (i.e., sewage spills, El Niño) and human modifications (i.e., sewage diversion, reduction of freshwater inflows). The current monitoring program includes quarterly sampling of fishes and invertebrates, annual sampling of vegetation communities, and continual water quality sampling using YSI dataloggers installed in two field locations (see section on Water Quality). For detailed information regarding are monitoring program and the data collected, please refer to PERL's annual reports, which are available on-line.

Fish and Invertebrates [Click here for map of sampling stations.]
Fish are sampled quarterly at three sites in TE; invertebrates are sampled at four sites (see map). To capture fishes, two blocking nets are stretched across the width of a channel. A bag seine is swept across the channel to the opposite bank; passes are repeated until depletion is reached. Benthic invertebrates are sampled by taking two series of cores (deep = 20 cm; shallow = 5 cm). These methods provide us with quantitative density estimates of fishes and invertebrates.

Dominant fishes at TE include:

California killifish (Fundulus parvipinnis)
Topsmelt (Atherinops affinis)
Arrow goby (Clevelandia ios)
Pacific staghorn sculpin (Leptocottus armatus)
Striped mullet (Mugil cephalus)
California halibut (Paralichthys californicus)
Diamond turbot (Hypsopsetta guttulata)
Longjaw mudsucker (Gillichthys mirabilis)

Some common invertebrates include:

Crabs

Hemigrapsus oregonensis
Pachygrapsus crassipes
Uca crenulata

Polychaetes

Capitellids (i.e., Capitella sp., Notomastus sp.)
Spionids (i.e., Polydora complex, Streblospio benedicti)

Amphipods

Corphium sp.
Grandidierella japonica

Gastropods

Acteocina sp.
Bulla gouldiana
Cerithidea californica

Bivalves

Macoma nasuta
Protothaca staminea
Tagelus californianus
Tellina carpenteri

Vegetation and Soil Salinities [Click here for map of sampling stations.]
During the time of peak biomass (late summer - early fall), soil salinities and vegetation parameters are measured at 12 monitoring stations. Four replicate stations are monitored in each of three habitat types: low-marsh, marsh plain, and high-marsh. The low-marsh is dominated by cordgrass (Spartina foliosa ) and the marsh plain is dominated by pickleweed (Salicornia virginica), which provides nesting habitat for the endangered light-footed clapper rail (Rallus longirostrus levipes) and the state-listed Belding's Savannah sparrow (Passerculus sandwichensis beldingi). the high-marsh at Tijuana Estuary is home to a number of species, including the endangered salt-marsh bird's beak (Cordylanthus maritimus spp. maritimus).

Some species observed at Tijuana Estuary include:

Low-marsh

Spartina foliosa (cordgrass)
Salicornia virginica (pickleweed)

Marsh plain

Salicornia virginica (pickleweed)
Jaumea carnosa (fleshy jaumea)
Trigolchin concinnum (arrow grass)
Distichlis spicata (salt grass)
Cuscuta salina (dodder)
Frankenia salina (alkali heath)
Limonium californicum (sea lavender)

High-marsh

Salicornia subterminalis (glasswort)
Salicornia virginica (pickleweed)
Frankenia salina (alkali heath)
Monanthochloe littoralis (shoregrass)
Cressa truxillensis (alkali weed)
Cordylanthus maritimus (salt-marsh bird's beak)

1. Fish Monitoring

Fishing sites (n=3) are located in the main channel directly in front of cells 2, 4, and 6. Two blocking nets are stretched across a section of the channel, so as to enclose an area of known size. Fish are caught using a beach seine, which is swept back and forth between the blocking nets and up onto the opposite bank. Passes are repeated until the number of fish caught approaches zero. All fish and mobile macroinvertebrates (i.e., crabs) are identified and counted, and later released. Size measurements (fish - total length; crabs - carapace width) are made on a subsample of the individuals collected. The data collected allows us to determine fish and crab densities, population size structure, and relative species composition.

Future sampling schedule:
March 2001
June 2001
September 2001
December 2001

Eight species have been collected during sampling from May-Dec. 2000: topsmelt (Atherinops affinis), arrow goby (Clevelandia ios), CA killifish (Fundulus parvipinnis), longjaw mudsucker (Gillichthys mirabilis), shadow goby (Quietula y-caua), cheekspot goby (Ilypnus gilberti), Pacific staghorn sculpin (Leptocottus armatus), and striped mullet (Mugil cephalus). From May to August 2000, fish densities in all cells increased thru time. In the fall and winter, densities in some cells began to decline; this corresponds with the general pattern observed in long-term monitoring (i.e., fish abundance peaks in summer). In December 2000 we observed extremely high densities, which can be attributed to a large influx of juvenile mullet (20-45 mm total length). Juvenile mullet have typically been rare in collections at Tijuana Estuary, except for during the 1997-98 El Niño event (see Williams et al. in press).

Table 1. Fish densities (individuals/m2) at Model Marsh fishing sites during the year 2000.

2. Invertebrate Monitoring

Benthic invertebrate cores are taken at the fishing sites noted above, and about 5 meters "upstream" in cells 1, 3, and 5. Three replicates cores are taken at each site, so that a total of 18 cores are collected on each sampling date (sampling schedule is the same as the fish monitoring -- see above). The core samples are taken back to the lab for processing, where each sample is stained in a Rose Bengal solution and sorted under a microscope. Once the animals have been picked out of the sample, individuals are identified to the lowest taxonomic level possible and enumerated.

Seventeen invertebrate taxa were identified in the May 2000 samples. Dominants include oligotchaetes and Spionids (Polydora complex and Streblospio benedicti -- typical colonizers of disturbed/newly excavated sites), and amphipods (mostly Corophium sp.). Insect larvae and nematodes were also fairly abundant. The highest total density of invertebrates was seen in cell 2, with cell 6 containing the lowest density (Table 2). Continued monitoring (see schedule above) will enable us to closely examine changes in these communities through time.

Table 2. Total density of invertebrates in May 2000 benthic cores.

Invertebrate sampling is done one day prior to each fish monitoring date to assure that benthic samples are collected before bottom sediments are disturbed (i.e., either by seining or trampling in adjacent areas). However, due to the lengthy nature of invertebrate sample processing, data from our subsequent sampling is not yet available.

3. Minnow Trapping Study

The vegetated marsh surface is an intertidal habitat that is available to fish only on high spring tides. Despite this limited availability, recent studies have shown that marsh access is very important for several resident fishes, providing rich foraging areas, and, ultimately, increased growth (West and Zedler 2000, Madon et al. in press). Because we know that the marsh surface is important to fishes, we need to know how to best provide this habitat for fish in restoration sites. Hence, we asked wheter tidal creeks facilitate fish movement onto marsh surface habitats (i.e., Will more fish be found on the marsh in areas with tidal creeks? Do fish in tidal creek habitats move further into the marsh interior -- farther away from the main channel?).

To help answer this question, we designed a minnow trapping study. On each sampling date, 10 minnow traps are baited and staked on the marsh surface in each cell (for a total of 60 traps). The figure below illustrates the trap placement, which is identical in the "with creek" and "no creek" cells. Traps are set prior to a high spring tide, and retrieved after the tide has receeded off the marsh surface. Due to the nature of our tidal regime in San Diego, some high spring tides occur during the night, in which case traps are set out the previous afternoon and collected the following morning. When trapping is conducted in the daytime, water level observations are also made (the time each trap first becomes inundated is note, as well as the time at which it is no longer inundated). As trap contents are emptied, the species identity and total length of each individual are recorded. Sampling is conducted once or twice monthly, from June 2000 to June 2001.

Five species have been collected in the minnow traps: CA killifish (Fundulus parvipinnis), longjaw mudsucker (Gillichthys mirabilis), yellowfin goby (Acanthogobius flavimanus), arrow goby (Clevelandia ios), and topsmelt (Atherinops affinis). Of these five species, killifish and mudsuckers are by far the most abundant.

Results collected so far don't indicate that fish are more abundant in cells with creeks. In fact, our data show just the opposite trend -- more fish have been collected in cells without creeks. However, a closer look at the data reveals an interesting pattern (see figure below). In cells with creeks, fish are evenly distributed across both the low and mid marsh. However, in cells without creeks, fish are much more abundant in the low marsh than in the mid marsh. This "crowding" of a greater number of fish into a smaller space may have other implications. It is probable that an increased density of fish will result in a more rapid depletion of food resources in these "no creek" low marsh areas. If food resources are limited, growth and reproduction will also be negatively affected. Continued sampling will show whether this trend remains consistent over all seasons and stages of marsh development.

In our water level observations, we also noted that low marsh traps placed in cells with creeks reamin inundated 30-50 minutes longer than traps placed in cells without creeks. Hence, the creeks appear to aid in retaining water for the maximum time possible (i.e., allowing fish maximum availability to intertidal habitats).

References
Madon, S. P., G. D. Williams, and J. M. West. In press. The importance of marsh access to growth of the California killfish, Fundulus parvipinnis, evaluated through bioenergetics modelling. Ecological Modelling.

West, J. M., and J. B. Zedler. 2000. Marsh-creek connectivity: Fish use of a tidal salt marsh in southern California. Estuaries 23(5): 699-710.

Williams, G. D., J. M. West, and J. B. Zedler. In press. Shifts in fish and invertebrate assemblages of two southern California estuaries during the 1997-1998 El Niño. Bulletin of the Southern California Academy of Sciences.