(Figure 6)
The wakasagi, H. nipponensis(McAllister, 1963), was first introduced into California in 1959 as a forage to salmonids in warm-water reservoirs (Wales 1962). At the time it was thought that wakasagi and delta smelt were the pond smelt, H. olidus(Pallus, 1814). The six original reservoirs where wakasagi were introduced are Dodge Reservoir (Lassen Co.), Shastina Reservoir ( Dwinell Reservoir, Siskiyou Co.), Freshwater Lagoon (Humboldt Co.), Spaulding Reservoir (Nevada Co.), Jenkinson Lake (Sly Park Lake, El Dorado Co.) and Big Bear Lake (San Bernardino Co.) (Figure 1). Since these initial introductions, a complete list of other locations where wakasagi were stocked is currently unavailable.
In 1994, wakasagi were observed in the Sacramento-San Joaquin Estuary in much larger numbers. Sporadic observations of wakasagi in the Estuary have been noted for at least the last 13 years (CDFG, unpublished data). Large populations of wakasagi are now known to exist in Oroville Reservoir and Lake Almanor on the Feather River and Folsom Reservoir on the American River (CDFG, unpublished data). Both Oroville and Folsom Reservoirs are used as major water storage and release facilities for both the State Water Project and the Central Valley Project which transfer vast amounts of water through the Estuary. Locations where wakasagi have been observed below these reservoirs include the lower American River (below Nimbus Dam), Cache Slough off of the Sacramento River, the Mokelumne River system and at the CVP and SWP fish salvage facilities in the south delta (Figure 2). With this invasion into the Estuary, the problem of identification of juvenile and adult Hypomesus spp. has become increasingly difficult.
At larval stages, speciation is very difficult (Figure 3) (see Wang 1991). Likewise, juvenile and adult stages identification can be difficult (Figure 4) unless specific guidelines are followed.
There are six species of osmerids that are present in at least part of the Estuary (Miller and Lea 1972, Moyle 1976). Of these, two are considered predominantly marine; the whitebait smelt, Allomerus elongatus(Ayres, 1854), and the night smelt, Spirinchus starksi(Fisk, 1913). The surf smelt, Hypomesus pretiosus(Girard, 1855), is mainly marine however, it can be found in brackish to fresh water. The longfin smelt, Spirinchus thaleichthys(Ayres, 1860), and the delta smelt, Hypomesus transpacificus(McAllister, 1963) are considered more euryhaline although longfin smelt have been observed from the Gulf of the Farallones to Sacramento on the Sacramento River while delta smelt have been observed from San Pablo Bay to the mouth of the Feather River on the Sacramento River. For identification of these marine and brackish water species, Miller and Lea (1972) is a good reference. The introduced wakasagi, Hypomesus nipponensis, has been found only in freshwater, however it may become established in brackish water as it moves to lower parts of the Estuary. It should be noted that the true "pond smelt", Hypomesus olidus, is not found in California, therefore, in order to avoid confusion the approved common name, "wakasagi", should be used for H. nipponensis (Committee on Names of Fishes 1991).
The three species of osmerids which are found in brackish water to freshwater portions of the Estuary are longfin, delta, and wakasagi smelt. Longfin smelt can usually differentiated from the two Hypomesus spp. by use of the following criteria (modified from Miller and Lea 1972, Wang 1991):
In McAllister's (1963) revision of the osmerid family, he used two main characteristics to differentiate delta smelt from wakasagi (at that time they were considered subspecies). These characteristics are fin ray counts and the number of chromatophores between the mandibles. In 1994, 283 Hypomesus spp. were collected from several reservoirs and the Estuary and then identified to species by electrophoretic analysis (Moyle, unpub.data). These smelt were then used for the meristic analyses presented here. The results are based on 177 smelt currently analyzed. When all 283 smelt have been analyzed, these results will be updated.
Fin Ray Counts
Fin ray counts on fishes identified to species by electrophoretic analyses suggest that the use of only fin ray counts for species identification is ambiguous for about 90% of the Hypomesus spp. observed in 1994 (Figure 5). Thus, extreme care must be taken at getting accurate counts (see Strauss and Bond (1990) for a description). The count ranges are as follows, the values in parentheses are reported by McAllister (1963):
Dorsal Fin 8-11 (9-10) 8-11 (7-9)
Anal Fin 15-19 (15-17) 14-17 (13-15
Pectoral Fin 10-12 (10-12) 11-14 (12-14)
Pelvic Fin 8-9 (8-9) 8 (8-9)
Delta Smelt Wakasagi
The use of the number of chromatophores that are present on the isthmus between the
mandibles appears to be the best method to differentiate delta smelt
from wakasagi, as illustrated in Figure 6.
(Figure 6)
Delta smelt had 0-1 chromatophore present on the isthmus which is consistent with McAllister (1963). The wakasagi measured so far ranged from 1 to 77 chromatophores (Figure 7). Only one wakasagi had less than 5 chromatophores and it was collected in Barker Slough. We are currently rechecking the identification of this fish. If only the wakasagi from the reservoirs are plotted against standard length (Figure 8) the correlation becomes quite strong (r2=0.87) indicating that wakasagi appear to increase the number of chromatophores as they increase in size. However, the wakasagi collected from the Estuary do not appear to be holding to this relationship (Figure 9). We are currently attempting to identify a mechanism as to how wakasagi are in essence "losing their spots".
Although there is slight overlap in the number of chromatophores at the isthmus between species this characteristic appears to be the best avenue for quick identification in the field with proper magnification. I have found that a 10X hand-held magnify lens (lupe) appears to give enough magnification to count the number of chromatophores on the isthmus. It must be noted that both species have chromatophores on the mandibles. For easier viewing, it is best to separate the mandibles from the isthmus by opening the mouth.
Delta smelt have a distict odor of fresh cucumbers. In field observations, wakasagi do not appear to have any cucumber odor. To smell a delta smelt click here!
The values reported in this notice are preliminary, however they are intended to
supply the researcher in the field the most up to date information on identification of
delta smelt from wakasagi. We are presently working on describing differences in body
shape between the two species. If you have any questions or would like to report the
presence of wakasagi in other parts of the Estuary, please contact me.
References:
